Sam's Laser FAQ, Copyright © 1994-2001, Samuel M. Goldwasser, All Rights Reserved.
I may be contacted via the Sci.Electronics.Repair FAQ Email Links Page.

  • Back to Sam's Laser FAQ Table of Contents.

    Solid State Lasers

    Sub-Table of Contents



  • Back to Sam's Laser FAQ Table of Contents.
  • Back to Solid State Lasers Sub-Table of Contents.

    Characteristics, Structure, Safety, Common Types

    Introduction to Solid State Lasers

    The Solid State (SS) Laser uses a solid crystalline material as the lasing medium and is usually optically pumped. SS lasers should not be confused with semiconductor or diode lasers which are also 'solid state' but are almost always electrically pumped (though in principle, optical pumping may be possible with some). For information on these, see the chapter: Diode Lasers.

    The original laser invented in 1960 was a solid state laser. It used a synthetic ruby rod (chromium doped aluminum oxide) with mirrors on both ends (one semitransparent) pumped with a helical xenon flashlamp surrounding the rod. The lamp was similar to what is used for indoor and high speed photography. The intense flash of blue-white light raised some of the chromium atoms in the matrix (the aluminum oxide is just for structure and is inert as far as the laser process is concerned) to an upper energy state from which they could participate in stimulated emissions (see the chapter: What is a Laser and How Does It Work? for a brief explanation if this isn't familiar to you. The result was an intense pulse of coherent red light at 694.3 nm - the first ever laser light in the world. Gas and semiconductor lasers followed closely behind but only the SS laser can claim to be first.

    It was found early on that these lasers could burst balloons and blow holes in razor blades and someone even attempted to coin a new measure of laser energy to be measured in 'Gillettes' based on how many razor blades could be holed at once. :) And, the popular notion that hand-held death ray weapons would soon follow are based on these sorts of demos of solid state lasers, not on whimpy gas lasers (though the carbon dioxide laser is actually a much more likely candidate being the classic heat-ray of science fiction)!

    SS lasers are used in all sorts of applications including materials processing (cutting, drilling, welding, marking, heat treating, etc.), semiconductor fabrication (wafer cutting, IC trimming), the graphic arts (high-end printing and copying), medical and surgical, rangefinders and other types of measurement, scientific research, entertainment, and many others where high peak power and/or high continuous power are required. A high energy pulsed YAG laser has even been used in rocket propulsion experiments (well, at least to send an ounce or so aluminum projectile a few feet into the air using just the pressure of photons!). The largest lasers (with the highest peak power) in the World are solid state lasers. Many of the laser projectors for light shows and for other laser displays use solid state rather than gas lasers like argon or krypton ion. And, that green laser pointer is a Diode Pumped Solid State (DPSS) laser.

    Common Types

    Modern solid state lasers are not all that much different from that original prototype. However, while ruby crystals have and continue to have their place, the majority of modern solid state lasers use neodymium (Nd) doped materials such as Nd:YAG (Yttrium Aluminum Garnet which is Y3Al5O12), Nd:YVO4, Nd:Glass, and others. These have a much lower lasing threshold than ruby as well as other desirable physical and optical properties. The strongest output wavelength of neodymium doped lasers is around 1,064 nm - near-IR and totally invisible.

    The exact wavelength of the strongest lasing lines depends on the actual host material but usually doesn't vary that much. In addition to Nd:YAG and Nd:YVO4 at 1,064 nm, examples that lase at slightly shorter wavelengths include Nd:LSB at 1,062 nm, Nd:Glass at 1,060 nm and Nd:YLF at 1,053 nm. However, the lasing wavelengths of some like Nd:NiNbO3 (niodymium doped lithium niobate, 1,084 nm and 1,092 nm) are longer and further away.

    Other materials include holmium doped YAG (Ho:YAG) or Ho:YLF. These lase at around 2,060 and 2,100 nm respectively. In the fiberoptic arena, erbium doped glass (Er:Glass) may be used in optical repeaters and amplifiers at around 1,540 nm. Er:YAG lases at 2,840 nm.

    Beyond these, there are not that many examples of widely used commercial solid state lasers though many other materials are capable of the population inversion needed for laser action. The workhorse by far is still Nd:YAG with Nd:YVO4 becoming increasingly important for low to medium power (up to a few watts) 1,064 nm and frequency doubled 532 nm (green) diode pumped solid state lasers.

    Power Output - Pulsed and CW

    Solid state lasers can be pulsed, CW (Continuous Wave), or quasi-CW. Note that while the peak power of the output pulses for a Q-switched laser may be many orders of magnitude greater than with the Q-switch disabled or bypassed, for the fundamental output wavelength (e.g., 1,064 nm for a Nd:YAG laser), the total energy (pulsed) or average power (quasi-CW) is generally lower. However, when frequency multiplication is used (e.g., 532 nm green from a Nd:YAG laser), total energy or average power may actually be greater with the Q-switch because frequency multiplication is a non-linear process and is much more efficient as the peak power increases. Depending on the application, the high peak power may more than offset the lower total energy or average power or vice-versa.

    Efficiency of Solid State Lasers

    Wall plug efficiency can vary from well under 1 percent for flashlamp and arc lamp pumped SS lasers to 25 percent or more for those pumped with laser diode pumped.

    (From: Anonymous (localnet1@yahoo.com).)

    A (laser) diode pumped Nd:YAG may have a 40% efficiency (operating multimode with good thermal control of the diodes), and the pump diodes themselves have about a 45% efficiency, resulting in a net 18% of efficiency from electrical power to the diodes to output beam power. However, at increased pump powers, thermal issues may cause the efficiency to decrease after a certain point. This decrease is power dependent, as well as resonator and pump assembly design dependent.

    Miniature to Monster

    Solid state lasers can range in size from something that could be built into a pistol shaped enclosure to fire a bug zapping laser pulse (a fraction of a Joule every few seconds) or the laser pointer mentioned above, to a football STADIUM sized monster. The National Ignition Facility is a solid state laser/amplifier array comprising over 7,000 optical elements, some more than a meter across, and producing megaJoule pulses to be used for inertial confinement fusion research and nuclear bomb physics simulation. (So they say - actually we all know that the laser engineers just want to be able to play with a very large toy! :)

    Unlike HeNe and Ar/Kr ion lasers, there is little standardization of solid state laser components. Laser rods come in all shapes and sizes - some not even rod-shaped :) with or without mirrors (for use with external mirrors and Q-switch optics). They are also relatively expensive as despite their deceptively simple appearance - partly due to the fact that they are a lot fewer of them than laser diodes or HeNe tubes. A price of $300 for a 75 x 5 mm Nd:YAG rod could be a bargain.

    The most common type of solid state lasers to have shown up on the surplus market are the laser head assemblies and pulse forming networks from some versions of the M-60 and M-1 tank rangefinders. Yes, if you come across a blown up M-60 or M-1 battle tank in your local junk yard, there may be a laser in there you can salvage! But don't worry, most of the time, you just have to take the laser. :)

    In fact, building a solid state laser if you have a Nd:YAG rod with integral mirrors in-hand is very easy - just add a linear flashlamp of with enough energy in close proximity wrapped in degreased aluminum foil! For small rods, a single-use (disposable) pocket camera flash will even work. See the paper: Micro-Laser Range Finder Development: Using the Monolithic Approach.

    My first contact with lasers was in the late 1960s when I inherited a student built ruby laser based on a design from Popular Science magazine. This used a ruby rod with integral dichroic mirrors about 1/4" x 3" (this is all from memory) and a linear flashlamp with an energy input of up to 400 W-s. Regrettably, I don't know if it ever worked - the lamp fired fine but I was too chicken to turn the capacitor voltage up to its maximum setting for fear of blowing up the flashlamp! Oh well. :( At least, shortly after that, our high school acquired a *real* 1 mW HeNe laser so I played with that some and used it to view the hologram that was part of an issue of, I believe, Scientific American. Not the same as exploding balloons or drilling holes in razor blades, however. :(

    The Laser Equipment Gallery has many detailed views of various solid state lasers from the M-60 Tank rangefinder to a high power arc lamp powered system putting out over 100 W CW.

    Microchip Diode Pumped Solid State Lasers

    While what comes to mind when the term "solid state laser" is mentioned will likely be that of a ruby or YAG rod pumped by a linear or helical lamp, modern CW lasers are more likely to be diode pumped (see the sections starting with: Diode Pumped Solid State Lasers) and small to medium power ones will use a small piece of Nd:YAG or Nd:YVO4 as the lasing medium. For example, Organization of Basic Green DPSS Lasers shows the major components of typical green laser pointers and a 100 mW green DPSS laser. Both of these use a chip of Nd:YVO4 (abbreviated as vanadate) to produce the 1,064 nm IR intracavity beam which is frequency doubled generating green light at 532 nm.

    Some people may only the first one to be a true microchip laser due to the small size of the lasing crystal but I include the other two since their designs are similar. However, in all cases, the only reason the lasing chip is so large in comparison to the active volume is due to manufacturing, handling, mounting, and thermal considerations. Thus, in principle, for the 100 mW green laser, a microrod say 1.2 mm long x 0.2 mm in diameter would be all that is actually required. But until laser chips are fabricated like computer chips and a way is found to get rid of the waste heat, much more material must be used. And, it is the thermal problems that ultimately limit performance - these tiny bits of lasing crystal are potentially capable of much more power output than can be obtained without them being damaged from heating. The smallest mass produced microchip laser crystals I know of are the CASIX DPM0101 hybrid vanadate-KTP module used in some green laser pointers: 1x1x2.5 mm. With cooling on all 4 sides, these may be capable of more than the small number of mW required for a pointer. The larger DPM0102 can generate over 50 mW intermittently at least (but the glue used to cement the two crystals may be damaged by the high intensity green light after awhile).

    Melles Griot's low to medium power high quality green DPSS lasers now use composite crystals similar to CASIX's but of their own design optically contacted, not glued, so there is no problem with high intracavity flux. They use optics to shape the pump beam and active TEC cooling so these are much better than laser pointers (and of course cost a lot more as well!). I was told that the cavity is something like 1.5 mm in length (unconfirmed) so this is even shorter than the DPM0101 but it probably has a cross-section more than 1x1 mm. The models currently available produce up to 20 mW but they have gone much higher in the lab. See the section: The Melles Griot 58 GCS Series Green DPSS L\ aser for more info.

    Unlike lamp pumped rod based side-pumped SS lasers which may use much of the volume of the laser rod, end-pumped DPSS lasers typically shape and focus the diode pump beam to a very narrow waist to boost the power density in the lasing crystal and to match the TEM00 mode volume of the cavity. This is an extremely efficient process compared to that of a lamp pumped laser. The typical conversion from diode pump light to IR laser output is over 33%. Compare this to a typical efficiency of 1% for a lamp pumped YAG laser. A DPSS laser may have a better than 10% wall plug efficiency for IR and frequency doubling efficiency (from 1,064 nm IR to 532 nm green) may exceed 50 percent.

    Since microchip lasers can use so little actual lasing material and the pump diodes are also very small, they can be very compact, and potentially mass produced and inexpensive. In addition to green laser pointers and low to medium power DPSS IR and green lasers based on YAG or vanadate, all sorts of other SS lasing materials can be used. Of particular interest for communications are erbium (Er) doped materials which lase around 1,530 nm, a wavelength which is optimal for fiber-optic cable.

    Microchip lasers also don't necessarily need high pump power. Depending on type, cavity design, and pump beam shape, a few mW of pump beam may be enough to exceed the lasing threshold and they have very high slope efficiency (percent increase in laser output versus increase in pump input) as well.

    What Ultimately Limits Output from SS Laser

    While much greater energy or power can generally be obtained from a given volume of a solid state lasing medium compared to a gas laser, it isn't unlimited. The same sorts of effects come into play as pump input is increased. Beyond a certain point, even with everything optimal, all that will happen is that your electrical and cooling bills will go up - assuming laser survives! The lasing medium will be doing as much as it can.

    (From: Doug Little (dmlittle@btinternet.com).)

    Like other lasing mediums, the output power from a YAG, ruby, or similar solid state rod will rise according to pump energy - but only up to the point where the active lasing medium is saturated (i.e. all the dopant ions are raised to the upper state). Beyond this point, no amount of extra pump energy will make any difference beyond generating unwanted waste heat. Also, a low-% doped crystal will reach this state more quickly, and will have a longer fluorescence period because the laser 'chain reaction' is inhibited by a reduced population of contributing ions - something like sticking carbon rods in a nuclear reactor to slow it down (well, that's how I like to think of it but feel free to flame, grill, or laser zap me if you think it's a bad analogy :-)

    (From: Sam.)

    Actually, I think it is an excellent analogy. Just think of all those mouse traps in the upper energy state! :)

    (From: Doug.)

    The saturation thing is a fairly obvious point, but it would be unfortunate to see enthusiasts building some huge 6-lamp device with a tiny pink ruby rod to find that they get the same output as they could achieve with 2 or 3 lamps! :-)

    It would also be nice to have a good clear explanation of doping percent differences and what effect this typically has on laser action. It can make a big difference when you are designing a laser that will work properly even with reasonably well known pump energies.

    (From: Sam.)

    Yes, the last item would be nice. Are you volunteering? :) However, realistically, where the laser rod is surplus, there probably isn't any easy way to determine the doping percent or control it!

    (From: Bob.)

    There are all sorts of things that limit the amount of output energy or power from a given size crystal including: damage threshold of the laser medium, energy storage capacity of the laser medium, thermal considerations, and optical considerations (such as self focusing and thermal lensing). You can scale any laser, but there comes a point where you have to make the laser bigger to get more energy. Look at the NOVA laser at Lawrence Livermore National Labs: The light starts out in a small laser rod that could be placed in the palm of your hand, then it gets amplified in a chain of laser amplifiers that take up the area of a football STADIUM!

    Solid State Laser Safety

    In addition to general laser safety issues (see the chapter: Laser Safety), solid state lasers present a number of unique - and potentially very serious safety risks - of their own:

    There are several potential hazards in dealing with the innards of electronic flash, solid state laser power supplies, and other xenon strobe equipment.

    1. The energy storage capacitor. Even on small pocket camera electronic flash units, these are rated at 100 to 400 uF at 330 VDC. This is 5 to 20 W-s which is enough to kill you under the right (wrong?) conditions. The xenon flashlamp power supplies in solid state lasers (often innocently called the 'pulse forming networks') may use capacitors that have a 10 times or higher uF rating and a 10 times or higher voltage rating. A 'medium size' SS laser might use 1,100 uF at 3,000 VDC - that's about 4,900 Joules. The energy in that capacitor bank is enough to instantly kill 80 adult humans based on a 60 J/person fatality threshold. You don't get a second chance at these energies and voltages - arm-to-arm contact across such a capacitor bank WILL be fatal - touch one of these and you will be but a puff of vapor in the wind...

      High voltage with high energy storage is an instantly deadly combination. Treat all of these capacitors - even those in tiny pocket cameras with the same respect as a loaded gun or stick of dynamite. Always confirm that they are fully discharged before even thinking about touching anything. On larger systems especially, install a shorting jumper after discharging just to be sure - these types of capacitors commonly recover a portion of their original charge without additional power input. In the case of an SS laser capacitor bank, it doesn't take a very large portion to be fatal. Better to kill the power supply than yourself if you forget to remove the shorting bar when powering up the unit.

    2. Systems that are line connected (no power transformer) have all the dangers associated with AC line power in addition to the large power supply and energy storage capacitors. More modern SS laser capacitor charging supplies are likely line connected high voltage inverters. Always use an isolation transformer when probing line connected systems. However, keep in mind that the power supply filter capacitors and energy storage capacitors remain just as deadly.

    3. Systems using power transformers such as neon sign or oil burner ignition types (as is likely with a home-built design) still have the danger of the raw high voltage AC being enough to kill. With the centertap of these transformers usually being grounded to the case, an isolation transformer, even if you have one that is big enough - may not be of much value.

    Some links:

    Reading and following these recommendations and heeding the warnings is especially important when working with high power solid state laser power supplies or xenon strobes of any kind.

    Comments on Surplus Solid State Lasers and Components

    The people who run some of the surplus outfits (on the Internet and elsewhere) are not necessarily knowledgeable enough about solid state (or other) laser technology to know exactly what they are selling. Some unscrupulous individuals may even consider it good business practice to offer junk at inflated prices with the expectation that when the completed laser doesn't operate as advertised, the buyer will assume it is due to a failure on their part (the buyer's) to assemble it correctly. The seller will, of course, claim that it had been tested before shipping - which is often a total load of something that smells bad. :) Not all the suppliers listed in the chapter: Laser and Parts Sources are equally reputable. I have attempted to identify the few that are particularly slimy but am unable to be as pointed as I would like since my information is limited and not from direct personal experience.

    For solid state lasers provided as kits of parts (which is probably the most common for types like Nd:YAG or ruby other than the M-60 rangefinder), be aware that the only type that can likely be made to work easily are those that are flashlamp pumped unless you have access to high power laser diodes of the proper wavelength. The rod must be optically polished and coated (HR, OC, or AR as appropriate) - you won't do that in your basement. See the section: Grinding and Polishing a Ruby Rod.

    Using broad band sources like halogen lamps or the Sun for pumping is extremely difficult due to the limited range of wavelengths that matches the lasing medium's absorption spectrum and the huge amount of waste heat. And, any claims about CW operation for some of these are often totally bogus as the physics simply prohibits it.

    (From: Chris Chagaris (pyro@grolen.com).)

    Commercial laser rods are typically finished with the following specifications: Ends flat to l/10 wavelength, ends parallel to ± 4 arc seconds, perpendicularity to the rod axis to ± 5 minutes, rod axis parallel to within ± 5º to [111] direction. These tolerances cannot readably be achieved by the home experimenter. All commercial laser rods also have anti-reflection coatings applied to their ends which must also be done professionally. If the mirrors aren't included or part of the rod itself, they will have to be purchased separately.

    The CW pumping of ruby is not impossible but nearly so, with terrible efficiencies. The pumping of ruby or Ti:sapphire to threshold is literally impossible using tungsten-halogen lamps as has been suggested by some uninformed individuals. Ruby's main absorption bands are located at 404 nm and 554 nm and Ti:sapphire's peaks at about 490 nm. Tungsten-halogen lamps have an emission maximum at 840 nm which is very far from the either of these crystal's absorption bands. Radiation output at the blue and green wavelengths is very poor in these types of lamps, hence another major problem.

    Finally, ruby has a very high excitation threshold, being a three-level system, despite its fairly long fluorescence lifetime of 3 ms (at 300K). In early experimental tests, a very small ruby rod (2 mm diameter x 50 mm length) was pumped by special capillary mercury arc lamps (good spectral match) and it took an input of 2.9 kW to produce a CW output of 1.3 watts. Only a small portion of the ruby was excited by the filament arc and laser action only occurred in 6 x 10-3 cm3. Using this data, the lamp input power per unit volume of active material to obtain threshold is about 230 kW per cubic centimeter.

    Other Sources of Information on Solid State Lasers

    Every book that deals with lasers will have some sort of mention at least of ruby and YAG lasers. However, for detailed technical coverage, the following is highly recommended and is generally considered the reference for anyone interested in solid state lasers:

    While portions are quite technical with many equations, much of it can be read and understood without a fancy college degree. The book has been published in several editions betweem 1976 and 1999. The earlier ones (which may be available at reasonable prices from used technical book sellers) are probably better for pulsed lasers as some material on this topic has been dropped in the latest (5th) edition in favor of more coverage of diode pumped solid state lasers.

    Some other relavent publications can be found in the chapter: Laser Information Resources.

    There are a number of Web sites with laser information and tutorials.

    Acknowledgements for SS Laser Information

    Must of the information which will eventually fill this and the following chapter originate from sources other than myself. I will be providing the basics but many of the nitty-gritty details depend on actual research and industrial experience. Wherever possible, I have attempted to identify the individual contributor. However, if you feel that there is something here you wrote without an acknowledgement, please let me know.

    Special thanks to Chris Chagaris (pyro@grolen.com and Wes Ellison (erl@sunflower.com) for their contributions to this document and their comments and additions to the chapters on solid state lasers and power supplies.



  • Back to Solid State Lasers Sub-Table of Contents.

    Basic Structure of Solid State Lasers

    Cavity Components

    Like most other types of lasers, the heart of the solid state laser is the laser cavity or resonator. In the following sections, we deal with the traditional direct output solid state laser. Frequency multipliers, OPOs, and other variations and enhancements will be covered later.

    The basic structure of the SS Laser hasn't changed in any fundamental way since its invention in 1960. A transparent rod (most common shape) doped with a small amount of impurity (the actual lasing medium) is optically pumped by a light source (most commonly one or more linear xenon flashlamps or an array of high power laser diodes) whose spectrum contains significant energy at wavelengths matching one or more of the absorption lines of the lasing medium. One or both mirrors are either an integral part of the laser rod or external. A Q-switch device is often included to compress and boost the energy in the output pulse (pulsed or quasi-pulsed lasers only) with some loss in total energy or average power at the fundamental wavelength. Additional devices such as an intra-cavity frequency harmonic generation crystal (most commonly, doubling - second harmonic generation or SRG) or external Optical Parametric Oscillator (OPO) may be added. Total output energy or average power may actually increase compared to CW operation due to the non-linear behavior of these processes.

    Properly selecting the cavity components and driving the pump source properly can make all the difference in terms of output pulse energy, beam quality, and stability.

    Power Sources

    Unlike gas or semiconductor lasers, most of which employ direct electrical excitation, the power supply for the optically pumped SS laser drives the light source - be it a xenon flashlamp, arc lamp, array of high power laser diodes. Thus, it isn't directly associated with the lasing medium as with those other types of lasers. Optical pump sources include:



  • Back to Solid State Lasers Sub-Table of Contents.

    The Solid State Laser Medium

    Shapes and Sizes

    The actual lasing medium for solid state lasers are the atoms of the dopant in a transparent crystalline or amorphous (glass) host material. Physically, this can be in the shape of a small block, a rod, a slab, or something else. For the smallest diode pumped solid state (DPSS) lasers - those which are part of a 3 mW green laser pointer or 20 mW laser module, a tiny block is all that is needed - perhaps 2 x 2 x 1 mm. However, the most common shape by far is the laser rod - typically 25 to 300 mm or more in length, and 3 mm to 25 mm or more in diameter. Output pulse energies from these range from a few milliJoules (mJ) to many Js per pulse or into the 100s of watts if CW or quasi-CW. Very large lasers may use slabs up to a meter across - usually as laser amplifiers rather than sources without outputs in the kJ or TeraWatt range!

    However, most of our attention will be devoted to the common rod shape for lamp pumped solid state lasers and "microchips" for diode pumped solid state lasers.

    Laser Materials - Ruby, Nd:YAG, Nd:YVO4, Nd:Glass

    The heart of any laser is the actual lasing medium which in the case of a solid state laser, is, well, a solid. :) Most of these consist of a crystalline or amorphous host doped with the actual active lasing material. The most common by far to date have been:

    Other important solid state lasing materials include:

    Some additional notes on the comparison of amorphous (glass) and crystalline lasing material:

    (From: M. C. D. Roos (roosmcd@dds.nl).)

    Straight out of my text-book (1975 and first edition):

    "Glass laser hosts are optically isotropic and easy to fabricate, posses excellent optical quality, and are hard enough to accept and retain optical finishes. In most cases glasses may be more heavily and more homogeneously doped than crystals, and in general, glasses posses broader absorption bands and exhibit longer fluorescence decay times. The primary disadvantage of glass are its broad fluorescence line widths (leading to higher thresholds), its significantly lower thermal conductivity (a factor of 10, leading to thermally induced birefringence and distortion when operated at high pulse repetition rates or high average powers), and its susceptibility to solarization (darkening due to color centers which are formed in the glass as a result of the UV radiation from the flashlamps). These disadvantages limit the use of glass laser rod for CW and high-repetition rate lasers."

    Comments on Nd:YAG, Nd:YLF, and Nd:YVO4

    (From: Chris Chagaris (pyro@grolen.com).)

    First Nd:YAG, Now Nd:YVO4, Next Nd:LSB

    (From: Juozas Reksnys (rexnys@uj.pfi.lt).)

    Nd:YAG, Nd:YVO4, and Nd:YLF are common in diode-pumped lasers. But, the most effective is the newly developed laser crystal Nd:LaSc3(BO3)4 or Nd:LSB. Nd:LSB has has absorption and radiation cross section similar to Nd:YAG but the bands are five time wider. The absorption coefficient of Nd3+ (10%at) in LSB is three times higher than Nd:YAG. LSB can be very heavily doped with Nd3+ (until 50%at), which provides record efficiency in the end-pumped configuration. This is a very high level in comparison with YAG (1.2% before luminescence quenching) or YVO4 3%, and 1.5% YLF. Furthermore, the saturation intensity of Nd:LSB is five times bigger than those of YAG or LSB.

    For example, using a microchip 0.5 mm thick and 2 to 3 mm in diameter, is is possible to obtain 0.5 to 50 mW of green output at 531 nm. Q-switch mode in such a microchip is possible with a Cr4+:YAG absorber. On LSB with KTP for SHG grown in Russia, BREMLAS is producing powerful green lasers with cubic inch dimensions. A 10 W green microlaser is under development.

    Exact Wavelengths of Neodymium Lasers

    The most common YAG and Nd:YVO4 (vanadate) lasers are usually listed as operating at 1,064 nm. However, this is not totally accurate and their wavelengths differ slightly.

    The wavelength for vanadate is more precisely 1,064.3 nm. There is also a weaker line at 1,342 nm.

    (Portions from: Juozas Reksnys (rexnys@uj.pfi.lt).)

    This most powerful lasing Nd:YAG line is composed from two lines 1,064.17 nm (strong line) and 1,064.4 (week line). At room temperature, the half-width of lasing line is 6.5 cm-1 which exceeds the distance of 2 cm-1 between two lines. Therefore, they are a joint line.

    The wavelength of this line depends on temperature. In the practical range of +/-60 °C, it linearly shifts to longer wavelengthes during heating by ratio 5x10-3 nm/deg. At 27 °C (300 °K), the center of the lasing line is at 1,064.15 nm.

    In addition to the common 1,064 nm wavelength, Nd:YAG has over a dozen other weaker lasing transitions between 1,052 nm and 1,444 nm.

    (From: Sam.)

    However, the vanadate and YAG wavelengths are close enough (0.15 nm) that a lamp or diode pumped YAG crystal can be used as an amplifier for the output of a vanadate laser in a (MOPA - Master Oscillator Power Amplifier) configuration since the gain bandwidth of YAG is about 0.5 nm.

    Comments on Nd:KGW and NYAB

    These are less common and not widely available materials. Castech is one supplier.

    (From: Bob.)

    KGW has a NICE broad absorption spectrum, that makes it a lot easier to work with than YAG BUT its thermal properties are poor.

    I have a paper titled "Generation of visible light with diode pumped solid state lasers" by Boller/Bartschke/Knappe/Wallenstein from 1993 that was published in "Solid State Lasers: New Developments and Applications" Edited by M.Inguscio and R. Wallenstein, Plenum Press, New York, 1993. This long paper (17 pages) focuses on NYAB. The authors state: "We report the so far highest 531 nm output power of 130 mW generated with 1.55 Watt of diode pumping."

    (From: Milan Karakas (mkarakas@vk.tel.hr).)

    I have a Nd:KGW rod 5 mm diameter x 50 mm long. This is a neodymium doped potassium-gadolinium tungstate single crystal. Complete data fo this rod may found at: Institute of Inorganic Chemistry Laser and Optoelectronics Crystals (Russia). This crystal has a Nd doping of 3% and operates at 1067.2 nm with 4 to 6% efficiency (Q-switched, 6.3 mm x 75 mm at 50 Hz), 3% efficiency CW, and 60% efficiency when diode pumped laser (quasi CW). The lasing threshold is extremely low - 0.2 - 1 J! I have not found reasonably priced optics for this laser (we may use optic for classic Nd:YAG, because wavelength is close) and pump source with low thermal emission (808 nm laser or LED). The rod was inexpensive - $209 USD including DHL shipping and duty.

    (From: Bob.)

    NYAB is a self-doubling (combined lasing and non-linear crystal) but it has a much lower doubling efficiency than traditional vanadate/KTP or YAG/KTP. The numbers I have seen are on the order of about 30 mW out for 1 W of diode pumping (efficiency is much higher with Ti:Saph pumping, but it's kind of inconvenient to have a such a laser pump a 100 mW 532 nm system, not to mention expensive. :)

    The following is from a 1990 paper so better performance is likely: "Work on diode-pumped self-doubling lasers is still in the early phases of development. The most attractive nonlinear gain medium is Nd:YAB, which is a dilute form of the stoichiometric neodymium compound neodymium aluminum borate (NAB). Diode-pumped Nd:YAB lasers with output powers in the milliwatt range have been demonstrated (reference 10.67)"

    Power Output versus Wavelength for Vanadate and YAG

    The absorption curve of Nd:YVO4 (vanadate) is quite broad compared to Nd:YAG (YAG). The following is from a graph in "Solid State Laser Engineering" by Koechner. (This actually deals with output power, not absorption, but since that is the desired effect, the numbers should be similar): For pumping with diodes, the wider wavelength range of vanadate is very significant because temperature tuning is a lot less critical and for low power DPSS lasers such as green laser pointers, thermal management isn't essential (though some variation may still be evident).

    Types of Nd:Glass

    (From: Chris Chagaris (pyro@grolen.com).)

    There are slight variations in the peak wavelengths for different types of Nd doped glasses. These differences are only very slight and should not be of great concern. The following are some glass types and peak emission wavelengths:

    Only a few types of Nd:doped glass are commercially available. Most common are silicates and phosphates. Doping concentration varies from 0.5% to 6%. Where the rods or slabs are from high energy fusion research I would assume that they are probably a fluorphosphate type that tends to minimize nonlinear effects, which becomes quite important in this particular application.

    Manufacture of Synthetic Ruby Crystals

    (From: Christopher R. Carlen (crobc@epix.net).)

    Ruby rods for lasers are made synthetically. Aluminum Oxide (Al2O3) with a very small amount of chromium impurity is melted in an induction furnace. A seed crystal (perhaps a natural ruby or a chip off another synthetic crystal) is stuck into the melt on a rod then slowly withdrawn. A cylindrical rod of "ruby" crystal is formed and is slowly pulled out of the melt. This rod is then cut up and ground with diamond machining equipment to form the precisely shaped laser rod. The ends are polished to extreme levels and then treated with whatever optical coatings are desired, depending on the design of the laser (i.e., mirrors directly on the rod or external).

    (From: Mark W. Lund (mlund@moxtek.com).)

    There are several ways to do this. The first is the easiest, to pull from the melt. You can melt Al2O3 in molybdenum crucibles and pull a crystal directly from the melt. Even single crystal tubes and other shapes having a fixed cross section can be pulled using a technique called "edge defined growth." Unfortunately, because of the incredible temperatures that sapphire melts at any dopants that you might want to use vaporize, so you can't make red or blue material, only water-white material.

    If you want colored sapphire or ruby there are two more methods used. The first, Vernuile (sp?), uses a hydrogen-oxygen flame and drops powdered Al2O3 plus dopant through the flame. The flame melts the powder, which falls on the seed crystal and crystalizes. Because only the surface of the crystal is molten the dopant gets incorporated into the bulk. The crystals are called boules, and look vaguely like a pop bottle, with a small neck, opening up into a cylindrical crystal. The stresses are so enormous in these boules that when you snap the neck off the entire crystal breaks into several pieces along the axis of the boule. Most colored sapphire and ruby sold is made this way, including the watch jewels.

    The last method used commonly is flux growth. The Al2O3 is dissolved in a molten salt, usually lead oxide plus cryolite, in a platinum crucible. The crystals come out of solution as the melt is cooled just like sugar in hot water. These are the most desirable of the synthetic stones because they look more like natural stones after cutting, and the process is the most expensive.

    (From: Fred Perry.)

    Actually, Union Carbide in Washougal Washington makes synthetic Ruby and other colored variants of Al2O3 (sapphire) by the Czochralski method. I bought an nice big CZ 'ruby' gemstone from UC at CLEO a few years ago. You are right that it is hard to get dopants to dissolve in the pot; but this is more a limitation on max concentration and hence achieved depth of color than something that can't be done at all. UC in fact makes (sole source - patented) the 'ruby' laser rods that were discussed in another post this week. They are pink, not red.

    (From: Mark.)

    Hmm, whom am I going to believe, Fred, whom I have a lot of respect for, or me, whom I have to live with? CZ is usually the method of choice if you can grow a crystal, but I have never seen a paper or patent on CZ growth of colored sapphire. I can't imagine going through all the pain and cost of flux growth or Vernuile if you could pull it from the melt. The method of choice for lasers, by the way, was flux growth when I last looked. On the other hand, if anyone could do it it would be Union Carbide, and it has been a few years since I did search the literature.

    I can imagine that some kind of sealed high pressure CZ puller could drive the dopants back into the melt.

    Of course the dopant level of a ruby laser is much less than a gemstone. How do they grow titanium doped sapphire? Anyone know?

    (From: charlesk@vloc.com.)

    As Fred pointed out UC grows ruby by Czochralski as does VLOC (without patent violations, mind you) and we do it quite well as pointed out by the Rogers (Thanks for the recommend). Mark, Czochralski is the preferred method for ruby growth for lasers, has been for a while. Now Ti:Sapphire is a different story, probably due to the higher dopant levels as you surmised. Crystal Systems can probably answer that point.

    There is more information on the VLOC Web Site.

    Gem Stones from SS Laser Material?

    "Does Nd:YAG material yield interesting gems when cut? Is it actually considered a "gem"? I think I have a piece of scrap material left over after a bunch of laser rods were cut from it. It's interesting to show to people because it appears transparent to slightly yellowish under most fluorescent illumination, but becomes magenta/pink under full-spectrum illumination."
    (From: Chris Cox (ccox@slip.net).)

    Yes, and gem faceters like it. Nd:YAG is considered a man-made gem material. It will go almost clear under some more recent rare-earth fluorescent lamps (which confused me when I brought some home. ;-)

    There are many types of these crystals, which are referred to as "color change" materials in lapidary/gemstone circles.

    (From: Uncle Al (UncleAl0@hate.spam.net).)

    Ditto glassblowers' didymium glass lenses and the fabulous gem alexandrite, sunlight versus candle light or incandescent illumination (blue in sunlight, red in cool illumination).

    See: "Man-Made Gemstones" by Elwell.

    Laser crystals make more than passable gems if they are hard enough to retain facetting and especially if they are optically isotropic. Pale laser ruby doesn't look like much, but if you give it a megarad of Co60 gamma (piggyback on a medical sterilization) you get a superlative tawny orange. (Facet first, because warming to above 100 C gives F-center decay and an eerie deep red glow as it returns to pale pink).



  • Back to Solid State Lasers Sub-Table of Contents.

    Optical Pump Sources

    Solid state lasers are generally pumped optically. This may be from xenon flashlamps which are probably larger but otherwise basically similar to those in photographic electronic flash units; via continuous arc xenon, krypton, or other discharge lamps, or more recently - and constantly growing in popularity for many reasons - high power laser diodes or laser diode arrays.

    Flashlamps are the method of choice where high peak power is required. None of the alternatives can produce the short, high intensity, burst of light needed to pump a solid state laser for the generation of optical output pulses with peak power measured in Megawatts or more. While the xenon flashlamp is most common, other gas fills may be used to tailor the output spectrum to more closely match the absorption bands of the solid state lasing medium. However, none are really that great and most of the light ends up as waste heat that must be removed - one of the major limitations on maximum pulse rate.

    Arc lamps were used in the past where CW operation was required. However, a major difficulty with these was the need to remove kWs or 10s of kW of waste heat from the lamp, rod, and cavity components. Circulating water or oil was needed along with a separate 'chiller' unit for cooling. Arc lamps are rapidly being replaced by arrays of high power laser diodes which are at least 10 times more efficient partially because their output is at the precise absorption wavelength of the solid state lasing medium. They can usually be convection or force air cooled and operate from a regular 115 VAC outlet.

    Other types of light sources including the Sun and halogen lamps have been used where the physics permits (Nd:YAG, for example), but their efficiency is very small and the heat dissipation problems are significant. Due to the continuous spectrum produced by these sources, the percentage of light that matches the absorption bands of the solid state lasing medium is quite small. And, for the halogen lamp, at most 10 percent of the electrical input power ends up as visible light to begin with (the rest is IR or heat with a bit of UV).

    (From: Leonard Migliore (lm@laserk.com).)

    There are lots of CW-Nd:YAG lasers. Laser markers are, most commonly, CW-pumped Q-switched Nd:YAG lasers. The rod (or slab) is generally immersed in water, with illumination by arc lamps or diodes going through the water. They get very unhappy with even a momentary loss of cooling.

    The laser mode is quite sensitive to the amount of heat being pumped into the rod; they only work properly over a narrow range of lamp currents. I don't think you could get any output out of an air-cooled YAG rod before it cracked.

    Xenon Flashlamps

    Xenon flashlamps come in all shapes and sizes - from those less than 1 inch long used in disposable cameras to monsters a meter or more in length used to pump the Nd:Glass optics of the NOVA and NIF lasers.

    There are many possible configurations. Which one is used may depend on many factors including the type and shape of the lasing medium (rod, slab, etc.), cooling requirements, and cost:

    Interpreting Xenon Flashlamp Specifications

    Among the critical parameters determining the characteristics of a flashlamp are:

    See EG&G 1300 Series Linear Flashlamp Specifications and Links for detailed info on the other models.

    Here are some notes on the K factor and its relationship to flashlamp voltage and current:

    (From: Don Klipstein (don@misty.com).)

    For more, see the section: Flashlamp and Arc Lamp Manufacturers and References.

    EG&G 1300 Series Linear Flashlamp Specifications and Links

    Here are the specifications for the popular 1300 series linear low average power air-cooled EG&G (now Perkin-Elmer) flashlamps. These, their predecessors, or clones from other manufacturers are the most likely flashlamps to be found in medium to high energy pulsed lasers.

    This data used to be available on the EG&G, now Perkin-Elmer, Web site but for now at least, much of it is gone. If you want more info, request their CDROM which includes complete product specs as well as the EG&G technical papers that used to be at their Web site. This material is also available at Polytec PI France - Department Electro-Optique. A variety of useful information is available for driving flashlamps (and other topics) in the Perkin Elmer (formerly EG&G) Technical Library.

    Mechanical specifications:

                  Bore   Arc     Tube    Overall
      Flashlamp   Size Length  Diameter  Length
        Type      (mm) (in/mm)   (mm)    (in/mm)
    -----------------------------------------------
     FXQ-1300-1    2    1/25      4     3.56/90.4
     FXQ-1300-2    2    2/51      4     4.56/115.8
     FXQ-1300-3    2    3/76      4     5.56/141.2
    
     FXQ-1301-1    3    1/25      5     3.56/90.4
     FXQ-1301-2    3    2/51      5     4.56/115.8
     FXQ-1301-3    3    3/76      5     5.56/141.2
    
     FXQ-1302-2    4    2/51      6     4.56/115.8
     FXQ-1302-3    4    3/76      6     5.56/141.2
     FXQ-1302-4    4    4/102     6     6.56/166.6
     FXQ-1302-6    4    6/152     6     8.56/217.4
     FXQ-1302-10   4   10/254     6    12.56/319.0
    
     FXQ-1303-2    5    2/51      7     4.56/115.8
     FXQ-1303-4    5    4/102     7     6.56/166.6
     FXQ-1303-6    5    6/152     7     8.56/217.4
    
     FXQ-1304-3    6    3/76      8     5.56/141.2
     FXQ-1304-4    6    4/102     8     6.56/166.6
     FXQ-1304-6    6    6/152     8     8.56/217.4
    
     FXQ-1305-3    7    3/76      9     6.06/153.9
     FXQ-1305-4    7    4/102     9     7.06/179.3
     FXQ-1305-6    7    6/152     9     9.06/230.1
     FXQ-1305-9    7    9/229     9    12.06/306.3
    

    Electrical specifications:

    All lamps listed are filled to a xenon pressure of 450 Torr. They are designed for convection or forced air cooling. Water cooling is not recommended. Lamps may operate with either series or parallel triggering and are supplied with a trigger wire. Minimum flashing voltage parameters assume an unloaded trigger pulse.

                                         Maximum      Minimum         
                   Ko       Minimum      Average      Trigger       Explosion
     Flashlamp  Impedance   Flashing    Power (W)   Voltage (kV)    Energy (J)
       Type    (ohm-A^0.5) Voltage (V) Conv Forced Series Parallel T=100us T=1ms
    ------------------------------------------------------------------------------
     FXQ-1300-1   16.2        400        25    50     12      15      70     250
     FXQ-1300-2   32.4        500        50   100     12      15     140     500
     FXQ-1300-3   48.3        600        75   150     12      15     210     750
    
     FXQ-1301-1   10.8        400        35    70     12      15      90     300
     FXQ-1301-2   21.6        500        70   140     12      15     180     600
     FXQ-1301-3   32.4        600       105   210     12      15     270     900
    
     FXQ-1302-2   16.2        500       100   200     12      15     240     780
     FXQ-1302-3   24.3        600       150   300     12      15     360    1170
     FXQ-1302-4   32.4        700       200   400     12      15     480    1560
     FXQ-1302-6   48.6        900       300   600     15      20     720    2340
     FXQ-1302-10  81.0       1300       500  1000     15      20    1200    3900
    
     FXQ-1303-2   13.0        500       120   240     15      20     340    1040
     FXQ-1303-4   25.9        700       240   480     15      20     680    2080
     FXQ-1303-6   38.9        900       360   720     15      20    1020    3120
    
     FXQ-1304-3   16.2        600       225   450     15      20     600    1800
     FXQ-1304-4   21.6        700       300   600     15      20     800    2400
     FXQ-1304-6   32.4        900       450   900     15      20    1200    3600
    
     FXQ-1305-3   13.9        600       255   510     15      20     660    2160
     FXQ-1305-4   18.5        700       340   680     15      20     880    2880
     FXQ-1305-6   27.8        900       510  1020     20      25    1320    4320
     FXQ-1305-9   41.6       1200       765  1530     20      25    1980    6480
    

    Useful Equations for Flashlamp Selection and PFN Design

    These equations can be used to aid in identifying a suitable flashlamp given the energy and pulse width requirements of a laser, and to determine the capacitance, inductance, and voltage required of the pulse forming network (PFN). For additional design information and some handy programs, see the chapter: SS Laser Power Supplies.

    The following are from the EG&G (now Perkin Elmer) Linear Flashlamp Technical Brief, available by contacting Perkin Elmer.

    Explosion energy: The explosion energy is the energy input at which a particular flashlamp is likely to fail after (or during!) a single shot at a given pulse width. As can be seen, longer pulses result in much higher explosion energy values.

        u = k * d * l * (t1/3)1/2
    
    Where: Note mixed units!

    So, explosion energy goes up as the square root of the pulse width.

    Here are some approximate guidelines for lamp life versus input energy/explosion energy (Eo/u):

     Eo/u     Life Expectancy (Shots)
    ----------------------------------
     0.1           >106
     0.2           >105
     0.3           104 - 106
     0.4           1,000 - 30,000
     0.5           200 - 3,000
     0.6           50 - 300
     0.7           10 - 75
     0.8           4 - 20
     0.9           2 - 5
     1.0           1 or less
    
    Ko parameter:

    The design of the PFN would be trivial if the flashlamp behaved as a simple resistor. Unfortunately, it is a dynamic impedance with a value designated as Ko (units: ohms-amps1/2). The Ko parameter determines the voltage across the lamp as a function of current just like a resistor except that the effective resistance (ER) is a function of current. For example, at 1 A, the ER of the lamp is Ko; at 100 A, it is Ko/10, at 10,000 A, it is Ko/100, and so forth.

        V = Ko * |i|1/2
    

    Where:

                    l      p
        Ko = 1.28 * --- * (---)1/5
                    d      x
    

    Where:

    C, L, and V for optimal PFN design:

    Normally, it is desired that the circuit be critically damped. This puts the most energy into the flashlamp in the shortest time without undershoot. For a given flashlamp Ko value, there are unique values for C, L, and V given the desired flash energy and pulse width.

              2 * Eo * a4 * T2
        C = (-----------------)1/3
                     Ko4
    
             T2
        L = ----
             C
    
               2 * Eo
        Vo = (--------)1/2
                 C
    

    Where:

    Peak current:

    It is important to know the peak current since it affects the spectral output and to assure that it is within the ratings of the lamp.

                 Vo
        Ipk = ---------
               Zo + Rt
    

    Where:

    Comments on Flashlamp Drive Limits and Effects

    A couple of excellent references for the general design of flashlamp based systems are the "EG&G Linear Flashlamp Technical Brief" and the "EG&G Design Considerations for Triggering of Flashlamps". These documents are also useful for products from other manufacturers. See the section: EG&G 1300 Series Linear Flashlamp Specifications and Links.

    There is also general information on xenon flashlamps including guidelines for estimating appropriate voltages and energy levels for glass and quartz flash tubes on Don Klipstein's Flash and Strobe Page. Don's General Xenon Flash and Strobe Design Guidelines Page which also includes some basic design equations.

    And, of course, there is tons of xenon strobe information, handy circuits, and complete schematics in Sam's Strobe FAQ (also mirrored at Don's site, above, and other sites Worldwide).

    (From: Chris Chagaris (pyro@grolen.com).)

    The maximum energy that a flashlamp can withstand is referred to as the 'explosion energy' and it is the energy at which the flashlamp is most likely to fracture. This explosion energy is determined by a number of factors including the type of lamp, size, and current pulse width. If a flashlamp is indeed built for laser pumping it would be of quartz construction but could actually be a number of different models.

    For example, a new, EG&G, FXQ-1302-3 (4 mm bore x 76 mm arc length) flashlamp has an explosion energy of 360 joules for a 100 us pulse. As pulse width is increased, explosion energy rises.

    In other words, you cannot just buy any old flashlamp driver and expect it to operate your particular flashlamp. I would suggest building your own pulse forming network for your application. It is not overly difficult (although can be very dangerous) if you have some background in electronics. All the formulas to calculate what you'll require are in a booklet available from EG&G or in any good book that deals with solid-state lasers. Capacitors for operating such a small flashlamp are readily available at very reasonable prices.

    (From: Don Klipstein (don@misty.com).)

    The "EG&G Linear Flashlamp Technical Brief has a very general rule that has a fair chance of being good for most quartz flashtubes, even someone else's. As for glass? Stay below both half the quartz limit and the tube's regular ratings, and it will probably be OK. See the section: EG&G 1300 Series Linear Flashlamp Specifications and Links.

    And EG&G recommends staying below 30 percent of the explosion energy if you want the tube to have a reasonable life expectancy.

    For really short pulse width, the limiting factor is ablation - evaporation of the glass or quartz. The vapor decomposes in the arc and you get oxygen among whatever else. The oxygen really increases voltage requirements for flashing. If the electrodes get hot enough, they may react with the oxygen and may remove most of it, but then you may discolor the inner surface of the tube with oxide in addition to any discolorations from silicon or other decomposition products.

    I have been through this, and even did some damage to a quartz tube with just a few joules per flash. Heimann DGS0610 (10 mm arc length) does not like voltage much above 300 volts combined with a few joules of energy, nor 1.5 kV at even a fraction of a joule.

    Flashlamp Failure Behavior and Causes

    (Portions from: Don Klipstein (don@misty.com).)

    When a flashlamp fails, it may do so quietly or with a bang.

    Generally, only laser pump flashlamps or similar ones with a lot of flash energy for their size will likely die spectacularly. When lower power flashlamps such as those used in small to medium size photographic strobes crack, they tend to stay in one piece or sometimes break apart surprisingly quietly.

    As for failure modes due to abuse:

    About Flashlamp and Arc Lamp Polarity

    Polarity is critical! The anode and cathode are not the same due to materials properties and/or shape. Running a xenon lamp with reverse polarity at anywhere near its rated energy will result in almost immediate, and possibly violent failure. Even if the ends appear to be identical, check the manufacturers specs to be sure that they are identical - they probably aren't!

    (From: Don Klipstein (don@misty.com).)

    Some xenon flashtubes do have identical electrodes and can be operated in either polarity. If the flashtube is polarized, wrong-way operation usually shortens the life by sputtering or overheating the anode (being used as a cathode), or by having getter material evaporated from the normal cathode location, drift to what is being used as the cathode and, discoloring much of the tubing along the way - active metal vapors in discharge lamps tend to have some positive ions and will drift to the negative end.

    I have seen some flashtubes have difficulty flashing the wrong way. Usually an extra hundred volts can force an anode to work as a cathode.

    Arc lamps may have thermionic emission materials on their cathodes (but not flashlamps). Abusing an anode as a cathode will usually overheat it, often sputter it, and the arc can have an excessive voltage drop (and then conduct less current) which often leads to the arc being less stable, and the arc tube material can overheat around the anode being abused as a cathode. If the arc voltage rises more than the arc current decreases (common), then the whole lamp can overheat - but I think overheating will mostly be around what is being misused as a cathode. Then again, if the lamp discolors from sputtered electrode material then it can absorb light and overheat.

    Testing Flashlamps and Arc Lamps

    First, visually inspect the lamp for obvious damage or excessive discoloration (which would indicate a high mileage lamp or one that has been abused). Obviously, if the lamp is cracked, its use is questionable to say the least. :) Seriously, even if the gas hasn't leaked out/atmosphere leaked in, firing a cracked lamp could result in a shower of glass or quartz bits. Discoloration will reduce the light output somewhat but the lamp may still be usable.

    The simplest electrical test is to apply a current limited high voltage to confirm ionization. The required peak voltage will need to be greater than the trigger voltage for the lamp. An easy way to do this is with a neon sign or oil burner ignition transformer on a Variac. Current limiting is built in. An adjustable high voltage power supply with a few hundred K ohms of high voltage ballast resistance can also be used. Since very little current is required, almost any source of HV will do. The start voltage from a helium-neon laser power supply will be sufficient for smaller lamps.

    Start at 0 V and turn it up until the lamp fires. For a small (e.g., 2 inch) xenon flashlamp, this will typically be in the 4 to 8 kV range; for a medium size arc lamp, perhaps 10 to 15 kV; large ones may require 30 kV or more. The start voltage will depend on the gas type (xenon or krypton typically), fill pressure, tube inside diameter, and amount of use or abuse.

    At these low currents, the operating voltage is probably no where near what it would be at normal current but with this approach, if the lamp fires at all, it is most likely good. The appearance of the discharge at the gas pressure inside the arc lamps is similar to that of a plasma globe - streamers of lightning that move around in response to (internal) thermal gradients and possibly even (external) proximity to conductive materials like fingers. So, if you don't want to use the lamp for a laser, it could be powered from a little HV module and make an interesting display piece. :)

    It should be possible to do further testing of arc lamps using an ion laser power supply (but if running for more than a couple seconds, most excellent cooling will be required). This is left for the advanced course.

    Flashlamp and Arc Lamp Manufacturers and References

    Here are some of the major companies that supply lamps to the laser industry. Many also have a variety of useful technical information either on their Web sites or available in print.

    Continuous Arc Discharge lamps

    Arc lamps also use a gas discharge like a flashlamp but are designed to dissipate the tremendous amount of power needed for continuous operation. Power consumption will be in the kW or 10s of kW range. Thus, an arc lamp pumped laser will almost always have significant cooling associated with it - either high CFM forced air or liquid oil or purified (de-ionized) water with a separate chiller.

    The most common arc lamps for solid state laser pumping are the xenon and krypton variety. Specifications for a variety of arc lamps used to be available on the EG&G, now Perkin-Elmer, Web site but for now at least, much of it is gone. If you want more info, request their CDROM which includes complete product specs as well as the EG&G technical papers that used to be at their Web site.

    Arc lamp power supplies have a lot in common with ion laser power supplies: a relative low voltage (under 50 to several hundred VDC) at high current (many AMPs) and a high voltage trigger required for starting. (However, with their massive cathode - where much of the destructive energy is dissipated - no heated filament is used.) See the chapters starting with: Ar/Kr Ion Laser Power Supplies for general information on systems that are similar to those for arc lamps.

    High Power Laser Diodes

    As noted, most modern CW solid state lasers are pumped by high power laser diodes or laser diode arrays. This approach solves many of the problems associated with alternative pump sources like arc lamps, the main one being efficiency (or lack thereof) and the associated immense heat dissipation and power requirements.

    Modern laser diodes are quite efficient and can be designed to produce the precise wavelength needed to match an absorption band of the solid state lasing medium. For Nd:YAG, this is near-IR at 808 nm. These laser diodes are inexpensive (as these things go) at less than $10 a watt for small quantities in chip form. Arrays of diodes mounted side-by-side of 40, 100, or more total WATTs are commercially available. Multiple such laser diode bars may be arranged surrounding a Nd:YAG rod. Laser systems using several hundred watts of laser diode pump power producing 100 W of coherent 1064 nm output or perhaps 40 or 50 W of 532 nm frequency doubled green output are compact, can be plugged into a standard 115 VAC outlet, and require not special cooling.

    Power supplies (usually called 'drivers') for high power laser diodes must be designed for absolute current limiting and to compensate for the change in laser diode characteristics with temperature. These types of laser diodes do not have internal monitor photodiodes like their low power cousins so other techniques must be used to regulate output power. Needless to say, preventing damage to these expensive laser diode arrays during power cycling, from power surges, and many other possible dangers, is extremely critical. See the chapters starting with Diode Lasers for more information.

    And, if you are wondering... No, LEDs really can't be used since not even a truckload of those super bright Radio Shack LEDs can be focused to achieve the required power density. (Even the brightest produce at most a few mW compared to the minimum of 1/2 W or so used in the smallest DPSS green laser pointer. In addition, being incoherent, their spectral width is much greater than that of laser diodes for a given power, the electromagnetic field intensity is lower.

    Other Pump Sources

    In addition to flashlamps, arc lamps, and high power laser diodes, broad spectrum sources like incandescent or halogen lamps have been used as well as the Sun and even some light producing chemical reactions - but none of these is really very popular due to the pathetic efficiencies and need to remove huge amounts of waste heat (but see below). About all that can be said for such approaches is that they use inexpensive easy to use parts like the Sun. :)

    Lasers (predating laser diodes) have also been used where their output wavelength matched an absorption band of the target lasing medium. However, until the advent of the high power laser diode, such systems were very expensive, had terrible efficiency, and were probably only used for very specialized applications where there were no alternatives.

    Bob's Comments on Halogen Lamp Pumped Lasers

    (From: Bob.)

    I have seen a General Photonics laser that put out 5 W, with a 'few' kW of pump power - 2 or 3 or 4 - don't remember exactly how many. :) This was one HELL of a power hungry beast! The reason is that the emission spectrum is not matched to the laser rod. In theory, if you looked at the emission spectrum, you could shift it up or down by controlling the power to the lamp and thus the temperature. But I have no idea where to suggest one find a spectrum for an off the shelf lamp unless you happened to have a spectrophotometer to measure it. :)

    My first laser was built with a 3 mm by 60 mm YAG rod, 2 tungsten halogen lamps, an intracavity piece of lithium niobate, and focusing optic. The rod was cooled by a HUGE flow of forced air, and the laser could be run for 5 or 10 seconds at a time before it would overheat. The mirrors were set in homemade mounts using 8-32 screws - NOT what you would call fine adjustment. :) I used a HeNe laser for alignment, then hoped and prayed when it came time to do actual alignment with the thing running, as there was such little time. After about an hour of turning it on, then letting it cool for a minutes, I saw some flashes of green light. Surely no more than microwatts, but then, I was using a very crude, low power YAG in CW mode.... Still one heck of an accomplishment if I do say so myself. :)

    Links to Info on Solar Pumped Lasers

    Here are some sites related to solar pumping of lasers (mostly solid state lasers but there may be some others).

    (From: sarlock@twcny.rr.com.)



  • Back to Solid State Lasers Sub-Table of Contents.

    Resonator Mirrors

    Mirror Types and Alignment

    As with other types of lasers, the resonator in a SS laser (used as an oscillator rather than an amplifier) will require mirrors at both ends.

    The HR mirror may be dichroic, metal coated, or a corner or half-corner reflector, to name just a few possibilities depending on the lasing wavelength, presence of additional cavity optics (like a Q-switch), and application. The OC mirror will generally be either dichroic or resonant optic (like the one in the Hughes rangefinder. A resonant optic is basically a multiplate etalon with one of its peak reflectances adjusted to coincide with the lasing line). Both mirrors are likely planar so there are no focused regions inside the rod.

    Unlike low gain gas lasers, aluminized (metal coated) mirrors may have enough reflectance (greater than 95 percent) to be work in a solid state laser. However, in addition to the less than optimal reflectance for the HR, that missing 5 percent is due to absorption, not transmission. Thus, a significant percentage of the pulse energy inside the resonator will be deposited in the mirror coating as heat. So, the damage threshold for these metal coated mirrors is much lower than for dichroic or resonant types. In other words, at some modest peak pulse power, you may end up with a nice clear spot (or worse) where your mirror coating used to be. :(

    Where the laser operates at an IR (invisible) wavelength, it generally isn't possible (or at least not easy) to determine the characteristics of dicrhoic or resonant mirrors without test instruments. In fact, it may not even be possible to differentiate between the HR and OC by visual inspection! They may both appear very similar and virtually transparent to visible wavelengths. If you have an unmarked laser head, assume that the beam could emerge from either end unless one is obviously covered!

    On the two samples I've seem up close and personal, the little SSY1 and an old large quasi-CW Quantronix Model 114F-O/QS (see the descriptions later in this chapter), the OC had a slight green tint in reflection. The HR of SSY1 was pale blue in reflection and the HR of the Quantronix was pale yellow in reflection. The color of transmitted light in all cases was as expected, a very very pale complement of the reflected color (almost neutral clear). Given that the appearance of the HRs of the two lasers were almost complements of each-another for the same wavelength (1064 nm) suggests that it isn't really possible to determine anything about anything by just viewing the mirror colors of lasers producing invisible outputs. :)

    Due to the typically high gain of the lasing medium, and its relatively large diameter, mirror alignment may not be nearly as critical as with narrow-bore low gain gas lasers despite the mirrors very likely being planar. Thus, on a short resonator, it is quite possible for there to be absolutely no adjustments for mirror alignment - just a machined mating surface on the rod-side of the mirror mount.

    Note that SS lasers are often used as amplifiers rather than oscillators - the light makes a single pass through the lasing medium and is boosted in intensity. In that case, there are no mirrors at all!

    Using Gas Laser Mirrors in a Solid State Laser?

    While this may be possible in some cases, there are a number of factors to consider and it is probably not recommended:

    Lasing Without Mirrors

    For people accustomed to the high quality high reflectivity mirrors and precise mirror alingment needed to get anything out of many gas lasers (like HeNe and Ar/Kr ion), it might come as a surpise that very little is needed for many types of solid state lasers. The gain of a laser rod may be on the order of 10 or 20 percent per inch as opposed to less than 1 percent per inch for a HeNe laser tube. Thus, it doesn't take much to form a suitable cavity - the reflectance of a SS laser OC can be quite low and still produce an output under the right conditions.

    (From: Bob.)

    Has anyone seen a Nd:YAG crystal lase off of 2 un-AR coated faces before? This hapened to me last night when I was fooling around with some optics, well OK, I did have the help of a 250 W, 808 nm laser diode array. That might have had something to do with it. :)

    I have seen it in flash lamp pumed systems. That's why so often you see a pulsed amplifier rod with the ends cut with a wedge, so that you don't have two parallel faces that are normal to the rod axis. But I have never seen it in a CW pumped scinerio, especially as one that is so 'photnically sloppy' All I was doing was holding the rod in front (by hand) to see if the spontanious emmision would be too bright to look at with my infra red viewer. Good thing I didn't look down the axis with my finderscope first, istead of looking at the barrel, the convertor tube in a find-r-scope isn't cheap to replace.

    (From: Paul Pax (phpax@azstarnet.com).)

    I've seen flashlamp pumped Nd:Glass rods lase from the two AR coated faces before, the face were even at an angle to the rod axis. Got a fair amount of power out, too. I guess if you've got enough pump power, it doesn't take much feedback.



  • Back to Solid State Lasers Sub-Table of Contents.

    Q-Switching

    What is Q-Switching?

    A Q-switched laser is a type of (or modification to) a pulsed laser which shortens it output pulse width, boosts peak output power, and improves the consistency of the output from pulse to pulse. However, there will likely be a reduction in total energy in the output when a laser is operated in Q-switch mode.

    With a normal pulsed laser, the pumping source raises the active atoms of the lasing medium to an upper energy state. Almost immediately (even during the pumping) some will decay, emitting a photon in the processes. This is called spontaneous emission.

    If enough of the atoms are in the upper energy state (population inversion) and one of these photons happens to be emitted in the direction so that it will reflect back and forth between the mirrors of the resonator cavity, laser action will commence as it triggers other similar energy transitions and additional photons to be emitted (stimulated emission). However, the resulting laser pulse will be somewhat broad and of random shape from pulse to pulse.

    The idea of a Q-switched laser is that the resonator is prevented from being effective until after the pumping pulse and most of the atoms are in the upper energy state (the population inversion in as complete as possible). Its so-called Q is spoiled by in effect disabling one of the mirrors. This can be accomplished mechanically by simply rotating the mirror or an optical element like a prism between the mirror and the lasing medium, or electro-optically using something like a Pockel's cell (a high speed electrically controlled optical shutter) in a similar location. With the cavity not able to resonate (mirror blocked or mirror at the wrong angle), there can be no buildup of stimulated radiation. There will still be the spontaneous emission but this is a small drain on the upper energy state.

    At a point in time just after the pumping is complete, the Q is restored so that the resonator is once more intact - the mirror has rotated to be perpendicular to the optical axis, for example. At this instant, with a nearly total population inversion, laser action commences resulting in a short, intense, consistent laser pulse each time and the pump energy is used more efficiently. Peak optical output power can be much greater than it would be without the Q-Switch. Because of the short pulse duration - measured in nanoseconds or picoseconds (or even less), peak power of megawatts or gigawatts may be produced by even modest size lasers - with truly astounding peak power available from large lasers like those found at Lawrence Livermore National Laboratory.

    With a motor driven Q-switch, a sensor is used to trigger the flash lamp (pump source) just before the mirror or other optical element rotates into position. For the Kerr cell type, a delay circuit is used to open the shutter a precise time after the flash lamp is triggered.

    Q-Switched lasers are very often solid state optically pumped types (e.g., Nd:YAG, ruby, etc.) but this technique can be applied to many other (but not all) lasers as well.

    A somewhat related process, called cavity dumping, is sort of the opposite of Q-switching: The intra-cavity power is allowed to build to a maximum at which point an electro-optic device is pulsed to cause what is in the cavity to go elsewhere. Thus, a pulse roughly 2*L/c (L is the length of the cavity and c is the speed of light) long is dumped from the cavity.

    WARNING: With their extremely high peak power, these are nearly always Class IV lasers! Take extreme care if you are using or attempting the repair of one of these.

    CAUTION: For some lasers which run near their power limits, if the cavity is not perfectly aligned, it may be possible to damage the optical components by attempting to run near full power in Q-Switched mode. Perform testing and alignment while free running - not Q-Switched (rotating mirror set up to be perpendicular or shutter open). Use a CCD or other profiling technique to adjust for a perfectly symmetric beam before enabling the Q-Switched mode.

    Mechanical Q-Switching

    The simplest type of Q-switch uses a rotating mirror or prism to form one end of the optical cavity. A sensor triggers the pump source (flashlamp) just before the mirror or other optical element rotates into position such that the resonator mirrors are parallel to each other. This is fairly easy to do with ruby as the lasing medium with its long (3 ms) fluorescence lifetime. However, the corresponding value for Nd:YAG is only 230 us. Thus, only 100 to 200 us is available once the flashlamp fires and sensing the position of a rotating optical element to this precision would be somewhat more difficult. For example, at 6,000 rpm - 100 rps, 100 microseconds is 1/100th of a full rotation.

    Mechanical Q-switches aren't found that often if at all in modern equipment. In addition to the difficulties in timing, having any high speed, wear prone, low reliability moving parts in a high tech laser is just bad form. :) The only common pulsed laser I know of with a mechanical Q-switch is the popular M-60 Tank rangefinder (which isn't exactly modern).

    Alternatives to motors are electromagnetically or piezo-transducer wobbled or vibrated optical elements.

    The following comments relate to mechanical Q-switching of a Nd:YAG laser. Since the fluorescence lifetime of YAG is less than 1/10th that of ruby, the difficulty of implementing a mechanical Q-switch are greatly increased.

    (From: Bob.)

    It may not be as easy to use a rotating Q-switch with YAG, but it certainly can be done. I have seen both a flashlamp pumped system by Litton that was used by the military (presumably part of a REALLY non-eyesafe rangefinder) and a medical laser that was arc lamp pumped from a European company. For the modern laser amateur, perhaps a mirror mount with a piezo transducer under one axis would work better than a rotating prism. But that would require one to be electronic saavy to build a driver.

    (From: LaserguruChris (laserguruchris@aol.com).)

    Believe it or not the chopping does work somewhat for YAG Q-switching although crude and inefficient. I managed to do this with a CVI YAG max model 95 laser in an attempt to get green out of it. The green power increased from a pathetic 70 uW to about 3 mW average power (still poor since it gave about a couple watts CW at 1,064 nm but better then nothing. :-) With the doubler taken out you could focus the beam enough to make little sparks where it hit. The wheel 1 mm holes cut in the edge separated by 2 mm and was spinning at 55,000 rpm. It is probably extremely difficult to get true Q-switching this way, what you will most likely get is a Q-switch pulse with a CW level "tail".

    Electro-Optic Q-Switching

    Rather than having something as crude as a moving part like a rotating mirror or prism, an electronically activated optical element like a Kerr cell is mounted between the end of the laser rod and associated cavity mirror. A delay circuit is then used to open this electronic shutter a precise time after the flash lamp is triggered.

    Optical/Passive Q-Switching

    Fully optical or passive Q-switch techniques exploit the non-linear properties of certain materials called "saturable absorbers". Below a specific threshold, these block the (incoherent) light and prevent laser oscillations from building up in the resonator just like a rotating mirror or prism that isn't aligned with the cavity or a Kerr cell in the off state. However, at some point, the material becomes transparent and the laser pulse can form. The result is a very short (possibly sub-nanosecond) length output pulse. The saturable absorber then returns to its absorbing state and if the pumping continues, the process will repeat generating a series of short pulses. This can be used to provide a means of easily controlling the pulse rate of a diode or arc lamp pumped laser with excellent consistency of pulse energy (since the Q-switch only activates when its threshold is reached). Since no complex mechanical and/or electronic systems are involved, this is a nice approach, especially for compact lower power systems. However, I don't know how the overall efficiency compares to that of an active approach.



  • Back to Solid State Lasers Sub-Table of Contents.

    Frequency Changers

    Frequency Multiplication

    To obtain visible beams from Neodymium doped lasers requires some process to convert the 1064 nm beam to a shorter wavelength. One way of doing this is to pass the beam through another crystal which responds in a nonlinear way to the intense IR radiation. From basic system theory, one learns that doing anything nonlinear to a pure sinusoidal signal will result in frequency components in the output at multiples (harmonics) of the original. There are a number of materials which have this property and are suitable for use with the 1064 nm wavelength.

    Solids state lasers may use frequency multiplication to generate the second harmonic (double or SHG), third harmonic (triple or THG), forth harmonic (quadruple or FHG), and even higher harmonics, though conversion efficiency generally goes down with increasing multiplication factor. The basic doubled solid state laser uses a three step process to obtain green 532 nm light from electrical power:

    However, just pointing an IR laser at a KTP crystal is not an efficient way of getting this to work. The laser and doubler crystals are usually both inside the cavity (this is called intra-cavity doubling). The mirrors are designed to have high reflectivity in the IR and to be transparent at 532 nm. So, 1064 nm IR photons bounce back and forth until they are finally convinced to double their frequency and become 532 nm green photons. They then find it easy to escape from the prison. :-) See the section: Anatomy of a 3 mW Green DPSSFD Laser.

    See the sections starting with: Diode Pumped Solid State Lasers for more information on this specific technology.

    High energy pulsed lasers can be frequency multiplied directly and it is possible to buy an external unit to place in the beam to do this. However, this isn't practical for low power CW lasers.

    Monolithic DPSSFD Lasers

    (From: Bob.)

    Monolithic laser systems, typically small DPSS doubled Nd:YAG or Nd:YV04 systems can be made in one or two ways: They can be assembled or they can be 'grown' in a single boule and sliced up to form microcavity lasers. One must keep in mind that the cavity is very small in these lasers - on the order of a couple of millimeters. Thus, they are very insensitive to misalignment of both the optics and the SHG crystal. A small cheap DPSS (not even a monolithic DPSS, but one with discrete components) may have the optics glued to an assembly or otherwise simply held in place. Gone are the fine adjustments of the traditional laser cavity. All monolithic DPSS systems are low output power, so cooling is not a huge concern. If the system is cooled, however, obviously all the optical elements are at the same temperature. This is completely contrary to the norm found in higher power DPSS systems, where the KTP (or other SHG) is normally at an elevated temperature and the lasing crystal is simply at a stabilized room temperature.

    Selecting Type and Size of SHG Crystal

    (From: Bob.)

    The required type and size of a the non-linear crystal depends on your application.

    If you want to do frequency doubling (SHG - Second Harmonic Generation) of a CW or quasi-CW beam them a KTP crystal with a 3 x 3 mm aperture will suffice up to about 70 or 80 W of extracted green output power. If you are looking for higher powers use a 5 x 5 mm crystal and a respectively bigger beam waist. This will give you enough room for outputs of several hundreds of watts, and is the crystal size used in the current record holding laser for most green output power.

    If you are thinking of using a SHG crystal for a pulsed laser, KDP would actually be your best bet. As a general rule of thumb with a electro-optically Q-switched laser, you want the spot size on your SHG no smaller than your output beam diameter. As it is extremely expensive to get a large KTP crystal, KDP is often used, and with high power pulsed lasers, the lower nonlinear coefficient is not noticed.

    The damage threshold for a normal KTP crystal is 100 to 500 megawatts per square centimeter. The efficiency increases as the power density increases, so the power output at the second harmonic increases exponentially as the power density increases. However, although it is true that the damage threshold is very high in terms of power, it is much lower in terms of energy. Damage can occur at tens of Joules per square cm. That's one reason why large doubled YAGs like the Laserscope systems can't be gated with the Q-switch driver. At high repetition rates, the first pulse supression goes isn't effective in those lasers, so the energy goes up in the first pulse eating the optics, normally starting with the KTP.

    LBO has a much lower nonlinear coefficient for 1064 nm SHG than KTP. However, it also have a much higher damage threshold. LBO is normally only used in systems that either (1) use very high powers (i.e., 100 W class lasers) or (2) need one of the optical properties of the crystal, such as the small angular acceptance angle. Since LBO has a lower nonlinear coefficient, it requires the use of a much longer crystal.

    (From: Skywise (skywise@cwixmail.com).)

    My copy of Casix's "Crystals and Materials Laser Accessories" lists beta Barium Borate as having been used to generate second, third, fourth, and fifth harmonics of Nd lasers. But KTP is more efficient than BBO for this purpose.

    Discussion of the Harmonic Generation Process

    (From: Martin Poyser.)

    My thoughts so far are as follows: Assume that the input to the non-linear crystal is a sinusoid with a frequency of several 1014 Hz (for visible light). Then assume that the transmittance of the crystal as a function of the instantaneous value of the input signal (i.e., the value of the input signal, which varies between +peak_amplitude and peak_amplitude) is not a straight line, as normal, but rather, a curve, perhaps resembling a log curve, or, curving with the opposite sign, an exponential curve. Then, the signal emerging from the crystal would be distorted, and no longer a pure sinusoid. Then, taking the Fourier transform of the output signal, it would no longer approximate a delta function at the frequency of the input signal, but would contain other components, including harmonics of the input signal.

    Is this anywhere near the mark, or is it a different process entirely?

    (From: Doug McDonald (mcdonald@scs.uiuc.edu).)

    This is correct.

    (From: Martin.)

    If my guess is anything like correct, it would seem valid to predict that the doubling should not be particularly frequency-specific, so one should be able to use the crystal to double (or triple, etc.) any visible/IR/UV wavelength more or less equally. Yet, I have not heard of this being done. I have not heard of doubling the output of an 808nm pump diode directly (without YAG, etc.) to get UV. This suggests to me that there is strong wavelength-dependence. If so, why?

    (From: Doug.)

    Now the tricky part. You are thinking like radio frequencies. At readio frequencies, the non-linear element (e.g. diode) is small compared to a wavelength. At optical frequencies it is not. Consider a yagi antenna that has each element nonlinear. It won't work for the second harmonic at all, and will have a vastly different spatial pattern for the third harmonic.

    The point is that the non-linear signals from different parts of the crystal have to add up in phase, and this is tricky to arrange because the speed of light is different for different frequencies. You CAN use most doubler crystals for different wavelengths, you just have to tilt them. KTP can't be due to a quirk, and this quirk is why it is so efficient.

    (From: Martin.)

    Also, why do these doubling crystals need such high input power to "get going"? Is it simply that the non-linearity becomes more noticeable as the amplitude of the input signal increases, and the transmittance curve deviates further from linearity?

    (From: Doug.)

    Yes.

    (From: Martin.)

    Is there a well-defined threshold amplitude (or input power) at which things suddenly start happening, or does the amplitude of Nth harmonic light increase gradually with increasing input power?

    (From: Doug.)

    Thre is no threshold - the output of a doubler crystal is quite "quadratic" up to saturation.

    (From: Martin.)

    Finally, are tripling/quadrupling crystals made specially as such, or are the same crystals used as for doubling, with the required harmonic selected by intra or post-resonator filtering?

    (From: Doug.)

    They have to be cut at different angles to get the phase right

    (From: Bob.)

    First of all, within reason a frequency doubler can be found for any wavelength in question. it's really only a matter of optical transparency at the wavelengths in question, as well as the nonlinear coefficient. There has been some direct doubling of diode laser light. Spectra Diode Labs doubled roughly 900 nm light to get 450 nm. But in order to do this they needed to take the output of a beam conditioned laser diode to a tapered laser amplifier to get a high quality beam that could be manages optically and focused int he SHG crystal. The only reason why you don't see this happen a lot is because the light coming from a laser diode is a PAIN to deal with, and for efficient SHG you need a very good quality beam (at least compared to most laser diodes). The nonlinear efficiency of ANY material increases with power squared - that is the reason why efficient SHG requires a lot of power. People who have been hit in the eye with a high power pulsed YAG laser have reported seeing a flash of green light (normally the last thing they have seen with that eye). This is because the vitreous humor acts as a nonlinear element at very high powers. All materials have a nonlinear coefficient, it's just like an index of refraction, but in this case it is power, as well as wavelength dependent. An ultra high power laser can cause air to act as a nonlinear medium. There is no magic power threshold that SHG processes start at. MOST harmonic crystals are cut for phase matching of a particular process, such as doubling, at a particular wavelength. It is possible to use a 'generic' crystal for any nonlinear process, but efficiency suffers dramatically. A 3rd harmonic crystal is cut for mixing of fundamental and the second harmonic from a doubling crystal. a 4th harmonic crystal is cut for phase matching the SHG process of the 2nd harmonic of a laser, and so on.

    (From: Jo.)

    KTP is not suitable for doubling to the blue spectrum (it can't be phase matched below about 500 nm). Normally, KNBO3 is used to double the 946 nm line of a Nd:YAG or the 914 nm line of Nd:YVO4. But the efficiency of these lines is poor (10% in comparison with the 1,064 nm emission). Some companies make blue lasers by direct doubling a 980 nm diode. But this is not easy, because you need a very good beam quality which requires a single mode diode - not available at high power. The other problem is that you have to use extra-cavity doubling (since you can't get inside normal diodes!). With cheap multimode diodes, there is no way to do the needed beam shaping. You can build a nice green laser using 980 nm diodes and Yb:KGW (1,025 to 1,045 nm; greater than 50% efficiency) and doubling with KTP. KGW has its major absorption wavelength at 980 nm - the problem is that it is not cheap (starts from $1,000/crystal). I am currently constructing a blue laser (457 or 473 nm) and I think there will be some news around the blue lasers next year. Many companies are developing new materials for powerful blue lasers.

    Phase Matching for Harmonic Generation

    To achieve any useful efficiency (or anything at all) from KTP (or any other non-linear crystal), it must be oriented very precisely with respect to the incident fundamental beam. Complex mathematics is needed to fully analyze or predict this behavior but there is a relatively simple way to explain it if one is willing to accept certain aspects of E/M and materials interactions without proof. Based on these simplifications, it will be possible to understand why the orientation of KTP and other non-linear crystals is so important in achieving any reasonable performance. :)

    We deal mainly with second harmonic generation which is a special case of two beams interacting in a non-linear material to produce the sum of their frequencies. In this case, they are actually two components of the same beam (with a single frequency) so the result is also half their wavelength. However, the same basic explanation also applies to any non-linear optical process including optical mixing of two beams (for example, one at the fundamental and the other at the second harmonic to produce the 3rd harmonic), OPOs (Optical Parametric Oscillators) and OPAs (Optical Parametric Amplifiers) to generate a pair of new wavelengths from a single input, and so forth.

    Non-linear optical processes can be either elastic (optical energy conserving) such as harmonic generation or inelastic (which deposits some energy in the material) such as Raman or Brillouin scattering. We only deal with the elastic case here.

    The following is based on material from "Solid State Laser Engineering" by Walter Koechner (5th edition) and has been greatly simplified. See that book for all the gory details.

    Non-linear optical processes are based on the response of the dielectric material at the atomic level to the electric fields of an intense light beam. The propagation of a wave through a material produces changes in the spatial and temporal distribution of electrical charges as the electrons and atoms respond to the electromagnetic field, mostly a displacement of the valence (outer) electrons from their normal orbits. This perturbation produces electric dipoles whose macroscopic manifestation is polarization. For small field strengths, the polarization is proportional to the electric field. In the non-linear case, reradiation comes from dipoles that do not faithfully reproduce the electric fields that generates them. The (electrical) polarization wave resulting from this non-linear behavior includes both the original frequency or frequencies as well as the sum and difference frequencies (as with an electronic mixer or other non-linear device).

    Whether a given material reacts in a linear or non-linear way depends on its basic composition and structure, the intensity and orientation of the incident wave(s), and their intensity. For each material, there is a parameter (which is a function of orientation) known as the "non-linear coefficient" which determines how sensitive it is for these processes. (However, just having a high non-linear coefficient doesn't necessarily make a given material suitable for anything if its damage threshold or some other property isn't favorable.)

    The non-linear effect makes the conversion efficiency proportional to the square of the input power until the material saturates. (Unlike a lasing process, there is no threshold, just very low effeciency at low power levels.) Thus, high peak power is needed to achieve the best performance. This explains why pulsed lasers can be easily frequency doubled using external non-linear crystals but intracavity frequency doubling is much more effective for CW lasers.

    Second harmonic generation should be viewed as a two step process:

    1. First, the input at the fundamental frequency produces a polarization wave in the crystal at the second harmonic as a result of the non-linear behavior of the material. This polarization wave has a phase velocity and wavelength in the crystal which is determined by the index of refraction for the fundamental wave, n1.

    2. Second, the energy is transferred from the polarization wave at the second harmonic to an E/M wave at the second harmonic - the useful output. The phase velocity and wavelength of the second harmonic are determined by the index of refraction for the second harmonic, n2.
    For efficient energy transfer, these two waves must remain in phase but for real world materials, n1 and n2 are generally not equal due to optical dispersion and the E/M wave will lag behind the polarization wave. Over very short distances, this will be slight and the waves will be substantially in phase. Output power will flow back and forth between the polarization wave and the E/M wave along the length of interaction based on this phase mismatch. One half the period is termed the "coherence length" (not to be confused with the coherence length of a laser to which it is NOT related). However, this coherence length is typically very small resulting in insignificant amounts of second harmonic power - no more than is produced after one coherence length no matter how long the crystal is.

    The typical dispersion values in the visible and near-IR of most crystals limits the coherence length to about 10 um. Thus, the maximum output power will be very small. (If one could stack 10 um thick sections of such a crystal in alternating fashion, it is possible to get around this limitation but such a structure would be prohibitively expensive if it could be fabricated at all. However, this is the concept of "Periodically Poled" non-linear crystals - a topic for a future discussion).

    One way around the limited coherence length is to take advantage of the natural birefringence of some materials. Birefringence results in slightly different index of refraction values depending on the direction of polarization of the wave in the crystal. If it was possible to arrange the orientation of the crystal such that the incident (fundamental) and output (harmonic) beams propagated in just the proper directions such that n1 would be exactly equal to n2, the coherence length, and thus the power output, could be greatly increased. It turns out that materials like KDP, KTP, LBO, BBO, and LiNbO3 have suitable dispersion characteristics for phase matching to be accomplished and high enough non-linear coefficient, damage thresholds, and other properties to make them useful for harmonic generation as well as other non-linear optical processes like optical mixing, OPOs, OPAs, etc.

    The math is quite hairy and involved :) but from a practical perspective, unless you are manufacturing the crystals, there is no need to worry about the precise angles as the supplier takes care of cutting them so that the orientation in your optical cavity will be something reasonable - usually close to the crystal axis being parallel to the resonator axis, and at either a 90 degree or 45 degree orientation around its axis.

    The "Type" of phase matching relateds to the polarization directions of the input and output beams:

    Critical versus non-critical phase matching relates to the angle of the incident and harmonic beams with respect to the crystal's optical axis: Green (532 nm) DPSS lasers use Type-II critical phase matching for the KTP. The KTP crystal has to be oriented at a 45 degree angle to the polarization direction in which the vanadate produces stimulated radiation for optimal performance (Type-II) and the KTP crystal's optical axis is at some non-90 degree orientation to the optical axis of the laser (critical phase matching at 21 degrees for flux grown KTP or 26 degrees for hydrothermal KTP. Of course, this isn't generally apparent to the user since the manufacturer cuts the KTP so that its faces will be close to perpendicular to the optical axis of the laser at the optimal phase match orientation.) See: DPSS Laser showing Type-II Phase Matching of KTP. The polarization of the pump diode and vanadate are both vertical when the vanadate is oriented for maximum absorption. The 1,064 nm light from the vanadate is effectively a pair of orthogonal components (at +45 degrees and -45 degrees to the vertical) which interact in the Type-II phase matching process. The resulting 532 nm second harmonic is rotated 45 degrees with respect to the 1064 nm input. The walk-off angle is 4.5 mR which ultimately limits the length of interaction - thus a really long KTP crystal won't be particularly useful. The non-linear coefficients for Type-I interactions in KTP are too low to be useful but some other materials (like KDP) can be cut for either Type-I or Type-II phase matching, and/or critical or non-critical phase matching depending on the application.

    Intra-Cavity Versus External Frequency Multiplication

    (From: Bob.)

    The NIF, NOVA, and all the lasers previous to them in the Inertial Confinement Fusion (ICF) programs around the world use external frequency conversion. That's the only way multiply the wavelength of the light from a Master Oscillator - Power Amplifier (MOPA) type laser (you can't multiply the wavelength and than pass the light through the amplifiers because there is no amplifying medium that works at the shorter wavelengths). But here again, the flux is orders of magnitude greater even extra cavity than one deals with for CW sources, especially compared to DPSS lasers. The ICF lasers are run so 'hard' that the optical flux is near the damage threshold, in some cases upwards of 100s of megawatts per square centimeter. As all nonlinear processes have efficiencies based closely on power squared, obviously a 100 MW beam will be doubled much more effectively than a 5 watt beam inside a DPSS laser 5 mm in diameter. That is why solid state lasers get doubled intra-cavity - it greatly increases the power flux at the crystal. In fact, a lot of intra-cavity doubling schemes rely on focusing the beam down at the crystal to make the flux even higher. This is something that would be difficult with diodes due to their poor focusability resulting from their large divergence.

    Converting a Medical Laser YAG to SHG Operation

    (From: Curt Graber (cgraber@fwi.com).)

    The re-engineering of a medical system is something that is a daunting task typically only for those somewhat versed in NLO systems However with perseverance and much time invested YOU should succeed in the quest and more importantly increase your understanding and knowledge of the NLO systems and their unique quirks......

    Any arclamp (CW pumped) YAG system is a viable option for a L-fold or Z-fold NLO system , medical systems are great base line systems for many reasons - they are plentiful, cost effective, and well engineered.

    The subsystems needed to typically convert on of these units are as follows:

    1. NEOS or other brand Q-switch RF driver and module capable of 25+ watts of RF output and AR coated for 1064 and 532 nm (higher power units such as 50 to 100 watt pump lasers and higher will require more RF power to "hold off" the intracavity energy.

    2. 3-axis KTP oven and TE-driver assembly to allow the heating of the crystal to it's unique peak efficiency temperature which will be used to tune the crystal specifically.

    3. A KTP crystal - 3 x 3 x 5 mm is typical however other dimensions are available as well , a side note on the crystals is that ALL KTP material varies in performance - the best material is only available from Litton Airtron. This is highly grey tack resistant and capable of rather high power because of it's inherent high damage threshold. On the other hand the cheap KTP material from overseas suppliers is not capable of high power (typically less than 1 watt)/

    4. Optics and optics mounts needed for a complete system

    Optical Parametric Oscillators

    Optical Parametric Oscillation (OPO) is a nonlinear process in which a single input laser beam or 'pump' beam is converted into two lower-energy beams known as the 'signal' beam and the 'idler' beam. This nonlinear process enables a fixed wavelength laser beam to be converted into other wavelengths.

    The wavelengths/frequencies of the three beams must satisfy:

                          1                 1                  1
                    -------------- = ---------------- + ---------------
                     Lambda(Pump)     Lambda(Signal)     Lambda(Idler)
    
    or equivalently:
                   Frequency(Pump) = Frequency(Signal) + Frequency(Idler)
    
    Energy is conserved since this also says that the sum of the energies of the Signal and Idler photons must equal that of the Pump photon. (The energy of a photon is proportional to its frequency.)

    Unlike lasers using frequency multiplication to obtain shorter wavelengths where the frequencies of the pump and output are related by small integers (SHG=2, THG=3, FHG=4, etc. - see the section: Frequency Multiplication of DPSS Lasers), with OPOs there is NO explicit requirement that the wavelengths of either of the resulting beams be related directly to the wavelength of the pump beam as long as they satisfy the equations, above. Thus, it is possible to implement a laser capable of being continuously tuned over a wide range of wavelengths - as much as several um - by adjustments only of the OPO (not the pump laser).

    Also note that while we use the term 'pump' to describe the input source, an OPO is NOT a laser in itself - there is no stimulated emission taking place, just conversion of wavelengths through non-linear optical processes.

    In current OPO devices, the wavelengths that can be generated are limited by the availability of nonlinear materials that can simultaneously satisfy the phase-matching, energy conservation and optical transmission conditions.

    The output wavelengths of current OPO's are controlled with angle or temperature tuning of the refractive indicies. Tuning by angle results in restricted angular acceptance and walk-off, which restricts the interaction length and reduces the efficiency of converting small pulse energy beams. Temperature tuning is generally restricted to relatively small wavelength ranges.

    For two examples of this technology, see the Periodically Poled Lithium Niobate Optical Parametric Oscillator and OPOTEK's Patented Ring Oscillator Design.

    SNLO - A Program to Help in Selecting Non-Linear Crystals

    SNLO (Select Non-Linear Optics), developed and maintained by Arlee Smith (arlsmit@sandia.gov), is a Windows (NT or 9X) based program with several functions to assist in selecting a nonlinear crystal and in modeling nonlinear frequency conversion processes in those crystals. Over forty crystals are contained in the database used by SNLO to calculate angular, spectral, and temperature acceptance bandwidths, phase matching angles, quasi-phase matching periods, group velocity dispersion, effective nonlinear coefficients, and parametric gain. SNLO enables you to use these quantities to model various frequency mixing processes from femtosecond to CW, including OPO's and OPA'S with group velocity and diffraction.

    SNLO is free and may be downloaded from the:

    Non-Linear Optical Effects in Liquids?

    (From: Bob.)

    You can get non linear processes to happen in various liquids. I have seen water operate as a non-linear medium before. but the problem is that the efficiency of using such materials is so ridiculously low that you need e very powerful laser to see any results. You can forget about doubling any 800 or 900 nm laser diode. If on the other hand you would like to double a high power Ti:Sapphire laser, yup you can do it. Only problem is since water and other liquids do not have a rigid structure, you can't exactly phase match whatever non-linear process you want. So what ends up happening is a lot of non-linear processes at the same time with varying (low) efficiencies and you get something along the lines of continuum generation - white light.



  • Back to Solid State Lasers Sub-Table of Contents.

    Diode Pumped Solid State Lasers

    What are DPSS Lasers?

    Diode Pumped Solid State (DPSS) lasers are a variation of the well known ruby, Nd:YAG, or other type of optically pumped crystal based laser where a high power IR laser diode or array of laser diodes provides the excitation instead of a flashlamp or other intense light source. Note that many DPSS lasers are designed for SHG (Second Harmonic, 532 nm green for a YAG lasing medium) or higher frequency multiplication but this is not inherent in the DPSS designation.

    While DPSS lasers generally can't achieve the same peak power as their flashlamp pumped cousins, they are capable of high CW or average power and are much more efficient (20 times or more) than flashlamp or arc lamp pumped SS lasers. Applications for DPSS lasers include most of those previously handled by lamp pumped SS lasers as well as many new ones where size and efficiency are important. These include most of those for lamp pumped SS lasers like materials processing as well as graphic arts, and entertainment (light shows, laser TV, etc.), and many more. Output power ranges from a few mW for green DPSS laser pointers to more than a kilowatt for industrial DPSS lasers - and that upper limit is climbing as you read this. :)

    In more detail:

    The most common DPSS lasers use a crystal doped with Neodymium (Nd) - either Nd:YAG (Yittrium Aluminum Garnet which is Y3Al5O12),) or Nd:YV04 (Nd:Yttrium OrthoVanadate) which lases at around 1064 nm - in the near IR region of the spectrum. Compared to many other solid state laser mediums, Nd, has a relatively low pumping threshold (in a relative sort of way) so it requires less pumping energy and is thus more efficient compared to ruby, for example.

    However, the 1064 nm output is invisible and therefore somewhat boring. :-)

    Pieter Ibelings' Laser Page shows photos of a typical DPSS laser fiber coupled module using an Nd:YLF crystal pumped by a 2 watt 795 nm laser diode.

    Frequency Multiplication of DPSS Lasers

    To obtain visible beams from Neodymium doped lasers requires some process to convert the 1064 nm beam to a shorter wavelength. One way of doing this is to pass the beam through another crystal which responds in a nonlinear way to the intense IR radiation. See the section: Frequency Multiplication for an introduction.

    The DPSSFD approach is used in many modern high power visible lasers producing up to 10 or more watts of output. For many applications, this solid state alternative is rapidly replacing bulky, cumbersome, high maintenance, power hungry, argon ion lasers. The same performance that used to require a 230 VAC three-phase 30 A feed can now be obtained from a laser that plugs into an ordinary 115 VAC outlet.

    Single high power laser diodes (0.5 to 1 watt or so) have made the compact green laser pointer possible. Until direct injection blue and green laser diodes become commercially available at affordable prices, this is the technique used to create green laser pointers (except possibly for a rare green HeNe laser pointer - don't how many of there were ever produced). (See the section: Availability of Green, Blue, and Violet Laser Diodes?.) However, compared to red laser pointers, these things eat power so getting significant operating time from a set of batteries is quite a challenge. Typical power requirements may be: 400 mA at 3 V - tough on AAA batteries!

    Check out the following for some basic info on DPSS lasers:

    End-Pumping Versus Side Pumping of a DPSS Laser

    Low power DPSS systems shine the diode light through the crystal from the rear (through the cavity HR mirror which will be AR coated for the pump wavelength). This allows the pump beam to be shaped so that it closely matches the desired mode volume in the laser crystal. But end-pumping isn't very scalable. The light focused on to the crystal is normally absorbed very quickly (if the pump wavelength matches an absorption band as it should), so if pump power density is too high, thermal fracture of the crystal will occur.

    With side-pumping, laser diodes are arranged radially around the rod, just as in a flashlamp or arc lamp pumped system. Such lasers have been built as large at 1,000 WATTS of IR and 300 WATTS of green out of a single rod. The maximum for an end-pumped system is around 30 W of IR and 10 W of green. There are some high power lasers that are end pumped, but the light isn't focused too sharply. Such a system is rather lossy, but is quite compact.

    Comments on the Materials in DPSSFD Laser

    (From: Bob.)

    Nd:YVO4 is considered better by some, but only for the lower power applications (<10W) because it has a lower threshold and a broader absorption spectrum, thus ensuring better coupling of the pump energy. It also has better birefringant properties. But on the other hand, it has less thermal conductivity than YAG and is not as physically strong a material and thus has a lower maximum pump power. Also, since it is a new crystal, individual and lot quantity quality control is a concern. And yes, it does lase at 1064 nm.

    Nonlinear crystals can be made up of just about anything. All other things being equal, the nonlinear coefficient determines how good a doubler it is. At high enough energy densities, you can get air to act as a nonlinear medium (on an interesting note, for the unfortunate few that has witnessed this, the vitreous humor in the eye can act as a frequency doubler for high power YAG pulses). There are other issues behind why a particular crystal might be chosen including acceptance angle, the spectrum of efficient transmission through bulk crystal, damage threshold, etc.

    Finally, there are several different ways of producing a frequency doubling crystal, noncritical, and critical phase matching. In critical, the physical properties of the crystal are specified for efficient doubling at a particular frequency (i.e., the crystal is cut in a particular orientation for efficient doubling). In non critical, either a tuning angle or temperature is used to provide efficient frequency doubling at a particular wavelength. More recently, quasi-critical phase matching has been demonstrated in periodically poled crystals, but to my knowledge, this technique has not yet made it to the main stream commercial laser product yet.

    Why Laser Diodes are Not Frequency Doubled Directly

    Since reasonably high power laser diodes operating at around 1060 nm (similar to what is produced from Nd:YAG or Nd:YVO4 pumped by 808 nm laser diode) are commercially available, why aren't these used to pump the KTP frequency doubling crystal directly? It would seem that this could save cost and dramatically boost efficiency by bypassing the intermediate step.

    I have heard of some special cases where direct doubled diode lasers have been built. However, I don't have any specific knowledge if this has been done commercially. Certainly not for the common 532 nm green wavelength.

    In fact efficiency would be terrible. Here are several reasons why:

    (From: Bob.)

    There are just too many variables in determining output efficiency of an extra-cavity doubler. These include the length of the crystal, if it is pulsed (either low rep rate or quasi-CW), if you have an external cavity, the type of kTP you use, if it's temperature controlled, and the size of the beam. I took a piece of plain old ordinary KTP a few minutes ago and put it in front of a 30 W YAG laser, with a 4 mm diameter beam, and I got about only 170 mW out. not great efficiency!!!

    A First Generation Green Laser Pointer

    The earliest green DPSSFD laser pointers were built with discrete components (vanadate including HR, KTP, OC, etc.) mounted on what is esssentially a microminiature optical bench shown in Organization of Basic Green DPSS Lasers (upper left). More modern cost reduced versions may combine many of these into a hybrid module (upper right) and some may also include the pump diode. With such a degree of integration, if any part goes bad, it would be impossible to replace. Those with discrete separate components are more fun because (with a lot of work), the crystals and optics can be removed and remounted on home-built adjustable mounts for your lasing pleasure. :) (Although I call them "first generation", some fancy models being sold today use exactly the same construction probably because the beam quality can be better controlled with the discrete optics.)

    The DPSS laser module is almost certainly of Far East origin. One supplier of modules that from outside appearance look physically identical is Enlight Technologies, Inc., (I doubt Enlight is the actual manufacturer though they may be relatives - the text on their Web site appears to have been written by someone whose native language isn't English!) This particular DPSS module would be one of the "-B" versions of the PGL-VI Series. Another supplier is DeHarpporte Trading Company and they even state that the crystals (at least) come from China. (Take the "Green Laser Diode Modules" link at the bottom of the page.) And another is Changchun New Industries Optoelectronics Tech. Co. Ltd..

    The power source is a CR2, 3 V, lithium battery. A regulated pulsed driver produces a squarewave output at about 4.5 kHz (at 3 V input). Using a laser diode simulator (2 silicon diodes and a 0.1 ohm resistor in series), I measured a peak current of about 0.3 A or an average current of 0.15 A. However, since the test circuit isn't quite the same as a real laser diode, these values may not be quite accurate (if anything, the current would be slightly high). There is a pot to adjust laser diode current. The squarewave frequency varies slightly with battery voltage but as far as I can tell, the duty cycle and diode current remain constant which means that the perceived output of the pointer will also have a constant average brightness, though it will of course, be pulsed. I don't know whether this pulsed driver was used to boost efficiency or as a means of better control of diode current or both. I am attempting to reverse engineer the driver but am not optimistic given the (apparently) house numbered IC for which specs are likely not available.

    A very detailed sequence of photos of all the blood (green) and guts of one of these first generation units can be found in the Laser Equipment Gallery (Version 1.47 or higher) under "Dissection of Green Laser Pointer". See Internal Organs of Green DPSS Laser Pointer for an annotated photo of the major components.

    Refer to Edmund Scientific L54-101 Green DPSS Laser Pointer for a detailed diagram of a pointer with the identical DPSS module as the one in the photos (also described in more detail in the section: The Edmund Scientific Model L54-101 Green Laser Pointer).

    Here are the specifications as best as I can determine them) for the major components:

    Green pointers using this DPSS module output either a CW or high duty cycle (typicall 50 percent) pulsed beam depending on the drive to the laser diode. However, some other models operate quasi-CW using a passive Q-switch (sometimes called FRQS - Free Running Q-Switch). Both these techniques (or a combination) are used to achieve higher efficiency in the lasing and non-linear frequency doubling process. Speaking of which....

    Techniques to Improve the Efficiency of a Green DPSS Laser Pointer

    Many, if not most (newer at least) green DPSS laser pointers are designed to produce a pulsed output beam. There is no functional advantage to the pulsed system (it's actually less desirable since the spot breaks up into dots when swept over a screen) but it can be made much more efficient reducing the need for thermal management and extending battery life at the same perceived brightness for these current hogs. Quasi-CW pointers may use either a pulsed pump diode, a passive Q-switch (sometimes called FRQS - Free Running Q-Switch), or both, to improve the efficiency.

    CAUTION: Pulsed operation of the laser diode assumes that it is rated to accept the peak current - don't assume you can modify a CW green laser pointer for higher efficiency by installing a pulsed driver - it may just blow the laser diode! Thus, I really don't suggest attempting any modifications to an existing pointer.

    1. Pulsing the laser diode generates more average pump power for the same average electrical input power because of the characteristics of the laser diode - the output is essentially 0 until the lasing threshold is reached and increases linearly with current after that.

      For a typical pump diode with a lasing threshold of 75 mA and a slope efficiency of 0.6 W/A, pulsing at 300 mA with a 50 percent duty cycle instead of a constant 150 mA will result in 50 percent more average pump power (three times the peak power) although the electrical input power will be very nearly identical since the voltage drop (around 2 V for a typical diode) doesn't vary much with current (150 mA * 2 V being equal to 300 mA * 2 V * 0.5). The actual output power will be about 135 mW peak (67.5 mW average) compared to the CW power of 45 mW.

    2. Pulsing the laser diode will also result in higher efficiency in the 1,064 nm lasing process because the higher peak pump power means that the lasing threshold of the vanadate will be less of a factor. Like the pump diode behavior, 1,064 power is 0 until the lasing threshold is reached and then increases more or less linearly with increasing pump power.

      For a typical laser pointer cavity, the lasing threshold may be 20 mW so if the pump power is pulsed at 135 mW instead of being run CW at 45 mW, the peak intracavity power will be 4.6 times greater and the average intracavity power will be 2.3 times greater for the same electrical power input!

    3. Pulsing the laser diode will also result in higher efficiency because the peak power inside the cavity will be greater for the non-linear 1,064 mm to 532 nm conversion process. As with the increased laser diode and vanadate lasing efficiency, using pulsed drive in the example above, doubling the intracavity power will more than double the 532 nm power thus resulting in a net increase in output even with the 50 percent duty cycle. With a factor of 4.6 as in the example above, doubling efficiency will be much much greater.

    4. Using FRQS will further increase the peak power inside the cavity and result in much greater efficiency in the 1,064 mm to 532 nm conversion process. FRQS uses a saturable absorber in the form of either an additional optical element or possibly just a coating so no additional electronics are needed.

    It's quite possible for these techniques to improve overall efficiency by a factor of 5 or more with the disadvantages (depending on which ones are used) being a more expensive pump diode, a more complex driver, the cost of the saturable absorber for the FRQS, and the somewhat less desirable pulsed beam.

    (There may also be a perceptual advantage to quasi-CW operation where at certain repetition rates - just under the flicker fusion frequency of human vision - they will appear slightly brighter for a given average power. However, I'd be surprised if any manufacturer actually deliberately took advantage of this effect - I would expect the flicker to be annoying at the very least.)

    It may be possible to tell which type of pointer you have by the duty cycle of the beam. Although the frequency of pulsed drive and the FRQS could be similar (several kHz), a duty cycle that is large (e.g., 25 percent or more) is likely the result of pulsed drive since a higher cost diode is needed to handle the peak power and pushing this too far makes it very expensive. Q-switched output pulses would be very narrow compared to the pulse rate - probably only a few ns - which for all intents and purposes, would appear as singularities. :) And, if both a pulsed pump diode and FRQS are used, there may be a mixture of spot sizes. Two green pointers I've tested that used the same DPSS module but not the same drivers both pulsed with about a 50 percent duty cycle but at widely different frequencies - 300 Hz and 4.5 kHz (neither used FRQS).

    The Edmund Scientific Model L54-101 Green Laser Pointer

    I was given one of these, almost new, for the express purpose of taking it apart. The safety/info sticker says it is actually from B&W Tek and is a model BWGP, serial number 305. (Edmund Scientific is just a reseller of these types of products. Their current stock number is CR30541-02.) But, although B&W Tek has many models of green DPSS lasers, no pointers are currently listed on their Web site so they apparently don't sell pointers to end-users, only as an OEM for sale through distributors.

    Partly, this endeavor was intended to help with a research project on microchip lasers and I suspect partly because it's stability and output were not that great and the owner wanted an excuse to get a new one. The original cost was $495! Heck, it's only Government money. :) When I got it, the output would vary from less than 1 mW to as much as 2.5 to 3 mW apparently at random. (It is rated 1 to 3 mW but I would have expected the power to more consistent.) At first I thought this was just the natural behavior of an inadequately thermally controlled DPSS system but then discovered that physical pressure on the laser diode contacts on the end of the DPSS module itself affected output power. (The specs for the DPSS laser modules used in these things does state a power variation of up to 30%. See the section: A First Generation Green Laser Pointer for links to suppliers.) So, I suspect it was a combination of both causes and I would have to get inside the DPSS module itself in any case.

    This pointer looks like a fat silver pen in two sections with gold trim. It's powered by a 3 V lithium battery. They claim a battery life of 2 to 3 hours when operated continuously. I'm not sure I believe that. Manufacturers often specify a battery life which assumes the duty cycle of usage is less than 50 percent as it would be in an actual pointing application as opposed to its use as an expensive cat teaser. What a concept. :)

    The construction details are shown in Edmund Scientific L54-101 Green DPSS Laser Pointer. (Contrast this to the simplicity of a Typical Red Laser Pointer!, also shown next to one-another in Comparison of Red and Green Laser Pointer Complexity.) The diagram should help make sense of the discussion below.

    A bit of somewhat gentle bending and twisting separated the battery holder rear section from the front section containing the laser diode driver sticking out its end and the actual DPSS laser module. The front gold plated bezel could also be unscrewed revealing the collimating lens on a screw mount glued in place. That was the easy part. Getting into the actual DPSS module would be much tougher. I'm beginning to sense something very familiar at this point though.

    Next, I wrapped a wire around the two terminals for the laser diode (to prevent damage from ESD, etc.) and unsoldered the driver board.

    After some careful scraping of Epoxy from the edge and threads of an aluminum retaining ring and scraping additional Epoxy to free it from the plate it holds in place (so that wouldn't turn), I was able to get apart without destroying anything physically using my custom made pointer retaining ring removal tool - bent piece of sheet steel with two prongs to fit the slots in the ring. :)

    And guess what? As I suspected from outward appearance of the collimating lens and rear of the module, the guts are identical to those shown in the "Dissection of Green Laser Pointer" found in the Laser Equipment Gallery (Version 1.74 or higher). Not just similar, but identical. See Internal Organs of Green DPSS Laser Pointer for an annotated photo of the major components. What's interesting is that this sample is a current model pointer while the dissected one was quite old (as these things go). Thus, I assume but don't know for sure that since the DPSS module is the same as those in green pointers from other sources, B&W Tek must buy the DPSS modules from the Far East and install them in a case with their own driver board and safety label. However, they won't even give you the time of day if asked about these pointers but just direct you to the original seller for info or repairs.

    I was hoping at this point to repair whatever was causing the erratic power and reassemble the pointer but that wasn't to be. At some point, the diode seems to have gotten damaged so the pointer's output is almost non-existent with the original diode and driver; with a fiber coupled 808 nm pump, it still works. How exactly the pump diode died I don't know for sure. I thought at first that the external power supply I was using might have had too much ripple and the driver couldn't cope with that. However, I later tested the driver on the same power wupply with a laser diode simulator as a load (a pair of silicon diodes in series with a 0.1 ohm resistor). While monitoring the output current on an oscilloscope, I couldn't detect anything amiss despite trying low and high input, switching the supply on and off at random, etc. So, I now tend to doubt it was an electrical problem. Another possibility is that contamination got on the diode facet when I opened it up. Whatever the cause, the pointer was happily outputting 3 mW when over the course of 10 seconds or so, the output dropped to under 1 mW and has been going down hill from there.

    Then, the vanadate fell off - bad glue job so I am now reattaching that.

    The OC was mounted way off-center indicating that either the diode was off center and/or the vanadate was slightly tilted - I'ms leaning toward the latter and expect that to be reduced at least with my new glue job. As the vanadate/KTP assembly is rotated, there are positions where there is a lot of green (with my fiber coupled pump) but it doesn't come out the front indicating that the alignment is way off.

    With hints from the dissection photos (thanks Dave), I have not had to use a hacksaw for anything! The front optics screw off and the entire DPSS module was a not so hard to remove press fit in the outer casing.

    The only reason I can see that they still make these things with discrete optics is that there is more control over beam quality with the separate spherical OC than with a hybrid CASIX type crystal. However, since at least one company, Melles Griot, now sells high quality DPSS lasers using composite crystals of their own design, there are ways around this (probably by careful shaping of the pump beam).

    But the overall mechanical quality looks quite good for some things (e.g., all the retaining rings and screwed together parts fit perfectly) and poor for others with some hand-filed parts (like a spacer ring) and sloppy tolerances for the vanadate and KTP plates inside the barrel.

    For the step-by-step procedure to take this to bits without a hacksaw, see the section: Disassembling a Green DPSS Laser Pointer.

    Anatomy of a 3 mW Green DPSSFD Laser

    Green laser pointers and green diode laser modules have many features in common. Since direct injection green laser diodes are not yet commercially available, they are both based on DPSSFD laser technology and are therefore much more complex inside than their red counterparts. In addition, in part because of this complexity, corners can't be cut as much with green pointers as is the case of red pointers compared to generally higher quality red diode laser modules. Thus, the high cost of these things appears to be justified.

    For more details and all the gory details of a step-by-step disassembly procedure performed on a green DPSSFD laser pointer, see the section: A First Generation Green Laser Pointer, above. Having the photos referenced there in front of you (preferably in a separate browser window) may also help to clarify some of the fine points in the following explanation.

    I don't know whether the unit described below was a green laser pointer or a green diode laser module. However, for the same output power, the structures should be very similar.

    Note: The manufacturer of this particular device shall remain anonymous for obvious reasons as you shall see below. :-) Suffice it to say that it was from a well known company and cost about $450 new - ouch!

    (From: Steve Roberts (osteven@akrobiz.com).)

    I was sent a diode pumped doubled laser of 3 mW power level for dissection as it was virtually dead. See the section: "Failure analysis of 3 mW DPSSFD green laser" for a discussion of what went wrong. What follows is a summary of the construction details of this device:

    I am thoroughly convinced this would have to be the easiest green laser to build, orders of magnitude less then argon, however it will lack some in power. However much care is needed with thermal management and diode current, but just about anybody could make one of these out of stock copper and aluminum with not much more then a drill press and a file and chopsaw and dremel tool. It was probably the most alignment insensitive laser i have ever seen.
                _     _____ 
        HHH<   [_]   [_____]   )|      ()   |)    ||
       Diode Nd:YVO4   KTP   Mirror   Lens Lens Filter
    
    
    So you have the pump diode at one side, effectively shining in the end of the laser cavity, this is referred to as "End Pumping" as opposed to "Side Pumping". The laser light bounces back and forth inside the Nd:YVO4 and KTP between the coatings on the outside end of the Nd:YVO4 and the BK7 mirror.

    Nd:YVO4 is what lases. Neodymium is the lasing material, the YVO4 is the crystalline host material. Potassium Titanyl Phosphate is the nonlinear medium for doubling. In this case it is placed inside the cavity as tremendously high field strengths are needed for doubling to work. Your Lexel-88 argon ion laser may do two W of output, but floating inside the cavity is as much as 3 to 4 KILOwatts of laser light one of the reasons a lasers optics must be very clean, a larger HeNe laser has as much as 40 W of laser light in the cavity, a typical small barcode tube has about 10 W inside.

    The laser is based on an approach called intracavity doubling. Other DPSSFD lasers may just shoot the coherent beam from a high power YAG crystal at the KTP crystal outside the cavity. The National Ignition Facility laser currently under construction (Winter 1999) at Lawrence Livermore National Laboratory uses the latter approach (for 1.8 MJ, 500 Terawatt pulses!) Of course, its final stage frequency multiplier crystals are just bit larger. They use KDP (Potassium Dihydrogen Phosphate) for doubling or KD*P (Potassium Dideuterium Phosphate) for tripling. Each slab is about 2 FEET across cut from ingots weighing over 500 pounds! And, there are 192 of them since there are 192 beams in all. :-) Not to mention the over 7,000 other large optical components in the NIF! If your are curious, see: NIF Optics for details.

    (From: Sam.)

    Without any optics to shape and focus the output of the pump diode onto the Nd:YVO4 crystal, I bet a lot of the pump power is wasted. Better DPSS lasers typically have a collimating lens, prisms, and focusing lens between the laser diode and crystal. For example, see the "80 mW Green DPSSFD Laser" under "Miscellaneous DPSS Lasers" in the Laser Equipment Gallery (Version 1.74 or higher).

    Failure Analysis of a 3 mW Green DPSSFD Laser Module

    See the section: Anatomy of a 3 mW DPSSFD green laser for a summary of the construction of this compact 3 mW laser.

    (From: Steve Roberts (osteven@akrobiz.com).)

    Well it's like this, the driver was fine, the pump diode was consuming the right amount of current, and judging from the lasing mode, something internal was way misaligned since a 100 uW YAG pointer is a wonderful toy but not of much use, I decided that a educational exploration was in order to further the cause of potentially inexpensive but bright green lasing in NE Ohio and Arizona. My conclusions:

    DPSSFD lasers are one hell of a lot easier to build then argons, by a couple of orders of magnitude!!!

    The autopsy required destruction of the shell, the heatsink fins unscrewed revealing a set of four small screws to remove the core of the module. This was a problem because they were bonded down with spotwelds and everything was coated in a thick glop of TorrSeal. Torrseal for those of you who don't know, is a ultra high vacuum compatible cement used for fixing leaks in vacuum and laser systems, you put torrseal on it, its bonded forever, I don't know of any solvents that will touch it. It does not outgas and is for all practical purposes a non conductive metal when hard. Its hard as diamond . SO I drilled out the screws.

    Several ingenious traps were built in to prevent disassembly, such as left hand threads, threading the diode module barrel with 80 tpi microthreads and then screwing it past the mating female threads so it could not be blindly rethreaded and removed etc.

    So what killed the laser?

    The Nd:YVO4 crystal had a thermal microcrack right where the diode pumped it! Nd:YVO4 is sensitive to heat. As far as I can tell, something caused the Nd:YVO4 surface to craze, having half a watt focused at it should have been fine, I think we can write it off to poor quality crystals and design.

    According to someone who manufactures these things, for every winner he produces, he gets three to five low grade units, and that's what drives the costs up. Supposedly this has improved over the past few years.

    The failure was most likely due to the design. If they would have used silicone instead of TorrSeal (rigid Epoxy) to hold the crystal to the copper disk it probably wouldn't have propagated the cracks with the heat-up and cool-down cycles.

    The Melles Griot 58 GCS Series Green DPSS Laser

    The 58 GCS are compact self-contained green DPSS lasers listed as suitable for applications including reprographics, spectroscopy, and diagnostic instrumentation.

    The most notable feature of these lasers is that they are based on a composite crystal rather than separate vanadate, KTP, and OC mirror. However, the crystal is optically contacted rather than glued (like the CASIX DPM0101 or DPM0102) so there is no problem with damage from high intracavity flux.

    Here is a summary of what I was told about the design of this DPSS laser:

    See: Melles Griot DPSS Lasers for general specifications and more product information.

    Steve's Comments on the Quality of Green Laser Pointers and Small DPSSFD Lasers

    (From: Steve Roberts (osteven@akrobiz.com).)

    A lot depends on quality control and who's doing the buying. I bought a few "duds" or off spec greens (less than 5 mW) to keep costs down and to use for experimentation. This resulted in some eye opening lessons. The thermal management is very critical with Nd:YVO4, for example, cooling a cheap pulsed pointer at the diode-end with a can of component cooler easily results in a doubling of the output power. In general, you want to heat the KTP, cool the Nd:YVO4 and temperature tune the diode so that its wavelength, which varies greatly with temperature, matches the peak adsorption of the lasing medium. I had one reject that did 100 uW or so of green at room temperature, take it down 15 °C and it did 3 mW. The alignment did not need to be changed, it was already picked well for both temperatures. The caveat is that just because power comes up, it does not mean mode quality goes up. As you chill a cheap DPSS laser, all sorts of stray beams show up and the divergence broadens.

    With a commercial module, there is more flexibility in constructing the alignment structure in the laser. Most pointers have the optics glued down on sleds and the optics are tweaked as the glue dries, as opposed to the 70 TPI screws used for KTP and OC adjustment on better grade units. A low cost YAG must be tweaked under its operating conditions and probably has its best beam quality over a 5 °C range around room temperature. Cost control is the issue here, so the pointers maker sacrifices.

    I am surprised they are still making pulsed units. When they make the pulsed pointers, they are doing it to keep the diode temperature down while pushing it harder to compensate for lower quality parts and poor alignment. KTP quality control in crystals priced less then $200 is nothing to write home about, and $35 KTP crystals are even worse. The brass and aluminum used in constructing these things in the pen form is by no means thermally stable. My friend who makes systems got 3 low power KTP for every 1 he got that met specs. Manufacturers have improved on this recently, but if your going to do green, buy the matched and graded pairs of KTP-Nd:YVO4 that have already been tested together.

    If you are thinking of buying one, limit your search to larger domestic manufacturers like Meshtel Intelite or B&W Tech, so you can have a warranty that is enforceable. And, get one that is on spec, not on sale. Save up the $$$ and buy a diode laser module that is more then 5 mW (laser pointers are limited by CDRH rules to 5 mW max). Quality control improves as power and cost goes up. The lasers are often built from the same design, then graded for power. So a 5 mW unit is one the alignment tech could not tweak up to 10 mW. They tend to gracefully degrade from undercooling and handling, so carefully adhere to the manufacturers spec on the input power.

    A better unit will state a larger power supply input range. Beware of those that want to see exactly 3.3 V and no more. Not only is 3.3 V hard to generate but such a spec is often a warning about the drive electronics - or lack thereof. Also watch out for units that have a positive case polarity, so you can't have the case ever touch ground. This may make using the unit in a lab or in a projector more difficult.

    Increasing Output Power of Small DPSSFD Laser

    (From: Steve Roberts (osteven@akrobiz.com).)

    Start by chilling the laser, but DO NOT increase the current, As you chill the diode, you shift the point at which it will run away and blow up to a lower level. If you don't get decent gains in power with chill alone, something is way wrong with the operating point of the laser. You might find you only have to chill it 10 to 25 °C below room temp, much more then that may shift the pump diode wavelength away from the adsorption wavelength of the YAG crystal and power will also drop. This assumes the structure of the laser will keep it aligned as its chilled - some cheap ones wont. A quick test can be done with a can of component cooler, if the case is sealed so that the spray wont hit the optics.

    You should see dramatic gains with just cooling the existing laser in its case without ripping it open and modifying it for the TE. If this is the case, just get a decent sized TE and a big heatsink for the TE and strap it on to the existing heatsink.

    Ideally you'd chill the diode, slightly chill the Nd:YAG crystal to compensate for the additional pump power, and heat the KTP doubler, but attempting to do this is not worth it on a 10 mW system to begin with.

    (From: Anonymous (localnet1@yahoo.com).)

    Basically cooling a DPSS laser will help prevent damage by heat. BUT it can actually reduce your output power. Many DPSS manufacturers, especially the ones who make inexpensive modules, use diodes that are a bit short in wavelength and are expected to heat up a bit, since they only have ambient cooling. Then, the output wavelength of the diode matches the absorption peak of the lasing medium (e.g., YAG). So, if you cool the module, even to room temperature, the output may be reduced. For maximum output, you should most likely increase the current, in conjunction with active cooling, but the cooling should be regulated so that the assembly does not get too cold.

    If you maintain cooling efficiency (and that is sometimes hard to do in a small package) diode life of large diodes (i.e., bars, which have very efficient cooling packages) asymtotically approaches zero with higher currents. At rated current, lifetime may be 5,000 hours; at 200% rated current, lifetime (of my vendor's diodes) is around 500 hours; and at 300% rated current, lifetime is normally less than several tens of hours - or it may fail immediately or after a day). As you can see, even at 200% rated current the diode will still have a fairly decent life, although no where near what a diode run at rated current will last.

    What it comes down to in the end is how you define the term 'significantly'? If you are an experimenter or hobbyist, and only turn your laser on from time-to-time for a few hours, a decrease in lifetime of a few thousand hours should be acceptable. However, if you are building a device for regular use, that may not be. Unfortunately, there is no way to tell what increasing the current will do for certain. But if the increase isn't extreme, it shouldn't cause catastrophic failure of your assembly. However, your other optics may not be able to handle the increase in power. The only way to find out if they will is to try.

    Also don't forget, lasers are complex creatures. Increasing the current a little can cause your output to decrease due to thermal problems, or a small increase in current could double it. DPSS systems are nonlinear, so output power often jumps with small increases in current, up to the point where efficiency starts to 'fold-over' due mainly to thermal problems

    What is the Difference Between Green and Blue DPSSFD Lasers?

    (From: Bob.)

    Well, a little, and a lot, depending on how you look at it. Green lasers are doubling the 1064 nm transition of Nd:YAG or Nd:YVO4, or some other similar host medium. The 946 nm line is what is being doubled in blue lasers, and 473 nm light is the result. Often, the choice for a Non-Linear Optical (NLO) crystal is different for the two lines. KTP is the crystal of choice normally for green, and LBO for blue. Also, the 946 nm line has a much smaller cross section for emission. This means lower efficiency and the 1064 line and even the weak 1319 nm line will try to compete with it, stealing energy. On top of that, the 946 line is self absorbing making the device a lot trickier to generate (like ruby, this is a case where the laser medium is actually somewhat opaque to the frequency of light the laser is trying to operate at, where as YAG is almost perfectly transparent at 1064 nm).

    So, they start out with pretty much the same structure: High power laser diodes at 808 nm pump a Nd host which lases at 948 nm, and this is inter-cavity doubled. But upon closer examination there are a lot of differences between the mechanisms operating in each laser.

    For some of the reasons mentioned above, the brightest commercial source for 473 nm light that I know of is limited to 400 mW, where as you can get a 10 W CW, or higher 532 nm DPSSFD laser with a pulsed beam. (Actually at least 10 times this now. --- Sam.)

    (From: Jo.)

    The doubling crystal is KNBO3 (KN). Temperature stabilization is a big problem for blue DPSS laser. We use modules where YAG and KN are bonded together. The modules are coated ready to use. With TE-control on both the crystal module and laser diode, a very stable beam is possible at about 5 to 15 mW. I think there will be better materials and components next year. Many companies (we too) are working at developing blue lasers.

    You can try a KTP-crystal. For extra cavity doubling, output power will maybe not be very high. Better to use the KN crystal. This will cost about $220 at Goldbridge, which is a manufacturer in China or Taiwan. I'ms also developing a range of blue and green lasers. Currently, I get 160 mW CW green when pumping a Nd:YVO4+KTP using 1.5 W of pump power at 808 nm. At the moment I'ms working at temperature control for better stability.

    Comments on Quality and Cost of DPSS Lasers

    (From: Bob.)

    Here is a bit of my philosophy on DPSS lasers. There are basically three levels of performance and cost:

    1. Great engineering and top quality components. These lasers approach the theoretical limits for efficiency and output power.

    2. Good engineering and good components. These lasers are a realistic tradeoff in cost versus power.

    3. Sloppy engineering and questionable components. These lasers are very cheap, but have a short lifetime, and put out less power as a comparable unit of types (1) or (2).
    An example of the category (1) laser would be the DPSS units made by Coherent and Spectra-Physics. For example, Coherent's Compass(tm) series DPSS lasers put out a well regulated 200 mW of power. This is achieved with a 2W pump diode ran at a hair under 50% of its maximum rated power. The current 'lab' efficiency record is about 280 mw out of a 1 W pump. In order to get these efficiencies, complex optics and engineering are needed. These lasers are VERY expensive. The 200 mW unit sells for around $20K last time I checked.

    There are plenty of lasers that fall into category (2) on the market. I feel The primary reason these lasers do not achieve the maximum possible performance is that they often have a small linear cavity, as this is a much easier to design unit, than the complex geometries in a more pricey unit. Micro chip laser use a very rugged design, but by it's inherent nature, is even less efficient than short linear cavities normally are (i.e., cavity lengths on the order of 5 cm and under). This simple design creates a HUGH cost saving in assembly though.

    Here is an idea of how the Coherent laser is assembled. Keep in mind it is a ring laser, with two mirrors, a KTP crystal, and a piece of vanadate that diffracts the beam by about 30 degrees. The components are all held in long tweezer-like tools during alignment. The baseplate of the optics etc. is held below it. Then, the laser is pulsed (as there is no thermal contact between the optics and their heat sinks). The components are aligned by using 5 axis positioning equipment, holding each tweezer - literally in mid air. When the optimal alignment is optimal, an optical cement is used to form their mounts. This is not an automated process. From set up to gluing take about 2 days. Not volume work here!

    Also, another reason why the category (3) lasers do not reach the expected output power is that they are only being pumped with inadequate power. DPSS output is NOT a linear function. If the laser has good thermal management, a laser putting out 100 mW at 1.4 W input power may put out 200 to 250 mW with 2 W of pump power. But, in order for the laser to be turned up in current in such a fashion, requires that it has been designed to operate at such power levels. Thermal lensing plays an important role in DPSS design at pump powers in this regime. If the laser wasn't designed to operate stably at the higher pump power, no amount of extra diode light will increase it's efficiency.

    Finally, for category (3) lasers, the single largest reason I am so skeptical is the rated life time being much lower than industry standards. This leads me to believe the diode is being run 'hot' which leads to high levels of uncertainty of its life time, and also suggests that its assembly is not the best. If a manufacturer is going to use poor quality diodes, or run the diodes past their recommended power, there is no reason for them to use good quality optics.

    DPSS Laser Efficiency

    Compared to lamp pumped solid state lasers, DPSS lasers are the fabulously efficient and better than many common light bulbs. :) Here are couple of typical examples, though no general rules:

    On CASIX's web site they list some examples of conversion statistics when using an 808 nm laser diode to pump a Nd:YVO4 (vanadate) crystal:

    1. Output of 278 mW at 1064 nm with pump power of 730 mW, a conversion efficiency of 38% from 808 nm to 1064 nm.

    2. Output of 2.5 mW at 532 nm with a pump power of 50 mW using intra-cavity KTP doubling, a conversion efficiency of 5% from 808 nm to 532 nm.

    (From: Matthijs Amelink (matthijs27@hotmail.com).)

    In the book "Solid State Laser Engineering" (5th edition) by Walter Koechner there's an example on page 365 for a diode pumped Nd:YAG laser:

    There's an entire chapter on doubling efficiency which can't be denoted in one figure.

    (From: Bob.)

    The record efficiency for a green DPSS laser stands at about 12% electrical power in to green output. Under optimal conditions, you will get a bit over 1/2 a watt of green for 2 W of 808 nm pump power, but if you are making a home-built system and not paying megabucks for the hardware, getting a few hundred mW would be doing a very good job.

    Doping Levels of Nd:YAG, Nd:YVO4, and Ruby

    Here are some typical doping levels for various size crystals used in DPSS lasers. (Source: CASIX which has substantial information on the materials properties of common laser and non-linear crystals.)

    Nd:YAG:

       Dimensions (mm)    Doping (%)
    ----------------------------------
      4 x 50 (rod)           1.0
      3 x 5 (cylinder)       1.0
    
    
    Nd:YVO4:
      Dimensions (mm)    Doping (%)
    ---------------------------------
        3 x 3 x 0.5          3.0
        3 x 3 x 1            1.0
        3 x 3 x 3            1.0
        3 x 3 x 5            1.0
        4 x 4 x 4            0.7
        4 x 4 x 7            0.5
        3 x 3 x 12           0.5
    
    From another source:
      Dimensions (mm)    Doping (%)
    ---------------------------------
        3 x 3 x 1.2          2.0
        4 x 4 x 4            0.5
    
    Here's one for doping of chromium in a large ruby crystal:
       Dimensions (mm)     Doping (%)
    ----------------------------------
     19 x 194 (cylinder)      0.03
    

    Pump Polarization Considerations for DPSS Lasers

    Lasing crystals like Nd:YVO4 (vanadate) have a preferred orientation for pump absorption and output a polarized beam. Regardless of pump polarization, the output polarization is unchanged so that only impacts the cavity design and orientation of subsequent components like KTP. This section addresses some of the issues with respect to pump polarization.

    Using vanadate as an example, the ratio of the absorption coefficients in the two axes is about 4:1. For convenience, let's arbitrarily choose the high absorption direction to be vertical (V) corresponding to the Z axis of an a-cut crystal) and the low absorption direction to be horizontal (H). This would be the case for maximum absorption when pumped directly with a laser diode mounted with its fast axis vertical. The absorption length or distance into the crystal where a given percentage of incident light is absorbed will vary by the same 4:1 ratio. This has implications for total absorption, beam shaping and transverse mode matching, single versus multiple longitudinal mode operation, and distribution of thermal load:

    The bottom line is that if you have an existing laser and don't know which way to orient the crystal, select the one that works best. :) If you're designing a laser and need to select a crystal, all these factors need to be taken into consideration.

    LED Pumped DPSS Laser?

    There are some mighty powerful LEDs being produced these days. How about using one or more of them instead of laser diodes for the pump source?

    For the purposes of Nd:YAG or Nd:YVO4 lasers, what would actually be needed is more correctly called an IRED - Infra-Red Emitting Diode. But I will simply call them LEDs as that's what we all know and love. Yes, an LED pumped green laser pointer would still be a laser pointer but would contain only one laser instead of two! :)

    Unfortunately, for the most part, this isn't practical. Here are some of the problems:

    One advantage LEDs would have is in robustness. They aren't as easily damaged by current spikes or ESD!

    For datasheets of some typical super high power LEDs, see: Roithner's Diverse LED Page.



  • Back to Solid State Lasers Sub-Table of Contents.

    Additional Information Related to Solid State Lasers

    Laser and Optics Crystalline Materials

    Here is a list of crystals used in laser, optics, and some other related applications, (Source: Superconix, Inc.).
     Crystal Type    Chemical Formula        Typical Applications
    -----------------------------------------------------------------------------
     Nonlinear       BBO                     SHG, THG, OPO
                     LBO                     SHG, THG, OPO
                     KTP                     SHG
                     LiNbO3                  SHG, OPO
                     KNbO3                   SHG, THG
                     ADP                     SHG, THG
                     KDP                     SHG, THG, FHG
                     KB5                     SHG (UV)
    
           (SHG = Second Harmonic Generation, frequency doubler,
            THG = Third Harmonic Generation, frequency tripler,
            FHG = Forth Harmonic Generation, frequency quadrupler,
            OPO = Optical Parametric Oscilator.)
    
     Laser Medium    Nd:YAG                  1064 nm Lasing
                     Nd:YLF                  1053 nm Lasing
                     Nd:YAP                  1079-1340 nm CW Lasing
                     Nd:YVO4                 1064 nm High Efficiency Lasing
                     doped GGG               High Efficiency Lasing
    
     Photo Reactive  BaTiO3                  Self-Pumped 2 Beam Conjugator
                     KNbO3                   Photo Reactive Effect
                     SBN                     Photo Reactive Effect
    
     Acusto-Optic    TeO2                    Modulator and Switch
                     PbMoO4                  Modulator and Switch
                     LiTaO3                  SAW Device
    
     Electro-optic   BSO                     Modulator
                     BGO                     Modulator
    
     Infrared        NaCl                    Window and Lens
                     MgF2                    Window and Lens
                     BaF2                    Window and Lens
                     LiF                     Window and Lens
    
     Semiconductor   GaSb                    Light Source, Detector, Solar Cell
                     InP                     Detectors, Photoelectric IC
                     GaAs                    Microwave, Laser, Photoelec. Devices
                     GaP                     Color LED
                     Si                      Integrated Circuits
                     Ge                      Integrated Circuits, IR Windows
    
     Oxides          LaAlO3                  High-Tc, Magnetic, Ferromag. Films
                     SrTiO3                          "                 "
                     Al2O3                           "                 "
                     ZrO2                            "                 "
                     CaNdAlO4                        "                 "
                     MgO                             "                 "
                     MgAl2O4                         "                 "
    
     Piezo-electric  Quartz                  Piezoelectric Oscillator and UV Window
    
     Calcite         CaCO3                   High Excitation Polarizer
    
     Magneto-optic   Tb:Glass                Visible and Near IR Isolator
    
    Some of these require special handling and storage, and protection once installed in the equipment. For example, ADP or KDP are hydroscopic (water absorbing) so protection is critical. However, KTP and LBO crystals are not hydroscopic and thus less susceptible to damage from environmental conditions.

    Additional information can be found at:

    There are many others. Here is one that may be useful - CDA:

    (From: David Van Baak (dvanbaak@calvin.edu).)

    CDA is Cesium Dihydrogen Arsenate, CsH2AsO4, and is an isomorph of the well-known nonlinear crystal KDP, potassium dihydrogen phosphate, KH2PO3. My only reference to CDA is a paper in JOSA B vol. 4, July 1987, pp. 1072 ff. which gives refractive indices but not damage thresholds.

    (From: William Buchman (billyfish@aol.com).)

    You cannot just take any host crystal and dope it with any dopant. The crystal lattice spacing has to be able to accept the dopant. YAG accepts neodymium well enough, but corundum does not. In fact, it does not like to hold a lot of chromium either. Yttrium is like a rare earth in the sense that adding any additional protons to the nucleus produces a rare earth element. That is why Nd can fit albeit not all that well.

    Types of KTP and Growth Methods

    The Czochralski growth process commonly used for ruby, YAG, and many other crystals is not suitable for KTP because the KTP material (KTiOPO4) decomposes upon melting (about 1,150 °C). Thus, other techniques must be used. Two are common: flux and hydrothermal:

    There are actual physical differences in the resulting crystal, mostly due to the fact that the flux method changes the precursors a bit. Flux grown material is physically softer and more prone to damage, both from laser light and mechanical handling, and is also more susceptible to grey tracking (a type of ion migration, reversible by annealing). Hydrothermal KTP is more efficient, especially at the highest powers. The phase matching angle for Type-II interaction is also slightly different for the two types: As measured in the X axis from the XY plane, it is 21 degrees for flux grown and 26 degrees for hydrothermal (since the crystals will have been cut appropriately before you see them, this really isn't much of a concern for laser builders though). To provide an idea of cost: A prime quality flux grown 3 x 3 x 5 mm KTP crystal is a few hundred dollars while a hydrothermal crystal of the same size would be about $1,000 MORE. The KTP on the Sterling Resale Optics Web site would go for about $2,000 new. Litton/Airtron/Synoptix is the only company I know of making hydrothermal KTP. I don't think it's because of patent issues but rather that no one else wants to invest the capital in making the stuff.

    There are may references on growth techniques and characteristics of KTP. Crystal Associates KTP References Page has an extensive list, I don't know how many are useful or comprehensible to more than a half dozen Ph.D.s in the World though. :)

    Pump Range, Output Wavelengths, and Thresholds for Various Materials

    The following data has been tabulated from various sources by Stephen Costaras (stevecs@chaven.com).

    The data is interpreted as follows: The threshold values for a particular material are the energy input needed at a particular temperature (noted in degrees Kelven) at the listed region of pump wavelengths in order to lase rod 2" to 3" long and 0.3" to 0.5" in diameter of the specified material. I used a resonant cavity from an old ruby laser that was modified to allow n incoming pump beam. I remember it was a real pain to get even illumination without much loss/feedback.

    For example, in order to lase CaWO4:Nd3,+ you need a pump laser with output wavelength at 570 to 600 nm with at least 3 J of output power at room temperature (295K).

    Tests were done at three temperatures: Room temperature of 295K (~72F), 77K using liquid nitrogen to cool the materials, and 20K for some.

      Material   Temp. (K)  Pump Region (nm)  Wavelength (nm)   Threshold (J)
    --------------------------------------------------------------------------
     BaF2:Nd3+
                    77         570 - 600          1060.0          1600.00
    
     CaF2:Ho3+
                    77         400 - 660          2092.0           260.00
    
     CaF2:Nd3+
                    77         560 - 580          1045.7            60.00
                    77         700 - 800          1045.7            60.00
    
     CaF2:Tm2+
                    20         280 - 340          1115.3           450.00
                    20         390 - 460          1115.3           450.00
                    20         530 - 630          1115.3           450.00
                    77         530 - 630          1115.3           800.00
    
     CaMoO4:Nd3+
                    77         570 - 590          1067.0           100.00
                   295         570 - 590          1067.3           360.00
    
     CaWO4:Ho3+
                    77         440 - 460          2046.0            80.00
                    77         440 - 460          2059.0           250.00
    
     CaWO4:Nd3+
                    77         570 - 600          1057.6            80.00
                    77         570 - 600          1063.3            14.00
                    77         570 - 600          1064.1             7.00
                    77         570 - 600          1065.0             1.50
                    77         570 - 600          1066.0             6.00
                   295         570 - 600          1058.2             2.00
                   295         570 - 600          1065.2             3.00
    
     CaWO4:Tm3+
                    77         460 - 480          1911.0            60.00
                    77        1700 - 1800         1916.0            73.00
    
     LaF3:Nd3+
                    77         500 - 600          1039.9            75.00
                    77         500 - 600          1063.1            93.00
                   295         500 - 600          1063.3           150.00
    
     PbMoO4:Nd3+
                   295         570 - 590          1058.6            60.00
    
     SrF2:Nd3+
                    77         720 - 750          1043.7           150.00
                   295         780 - 810          1037.0           480.00
    
     SrF2:Tm3+
                    77        1700 - 1800         1972.0          1600.00
    
     SrMoO4:Nd3+
                    77         570 - 600          1059.0           150.00
                    77         570 - 600          1061.1           500.00
                    77         570 - 600          1062.7           170.00
                    77         570 - 600          1064.0            17.00
                    77         570 - 600          1065.2            70.00
                   295         570 - 600          1057.6            45.00
                   295         570 - 600          1064.3           125.00
    
     SrWO4:Nd3+ 
                    77         570 - 600          1057.4             4.70
                    77         570 - 600          1060.7             7.60 
                    77         570 - 600          1062.7             5.10
                   295         570 - 600          1063.0           180.00
    

    Color of Common Lasing Crystals

    The actual color can vary quite a bit depending on dopant concentration and any co-dopants that may be present.

    Mark's Comments on Ruby Laser Pump Power and Cavity Reflectors

    (From: Mark Folsom (folsomman@redshift.com).)

    I built a little ruby laser with a pair of straight xenon pump lamps. I found that I needed a very large amount of pump energy to get to threshold. We could get big pulses out of Nd:YAG or Nd:Glass with a pair of capacitors smaller than my thumb, but the ruby required caps the size of beer cans. Low Chromium ion concentrations make for lower thresholds. I would also caution you that the lamp should be close to the rod or else the cavity should have highly reflective ends. Elliptical reflectors have very different magnification near side versus far side, so focusing extended objects gives very different results than you would suspect by ray tracing a line source. Using a small eccentricity in your ellipse can help minimize this effect. We always got better results with close-coupling in a cylindrical-segment cavity than in an elliptical cavity.

    Pulsed Ruby Laser Questions and Comments

    Here are some assorted questions along the lines of: "I have such-and-such a ruby rod and want to make a laser".
    "I have 2 ruby rods - one is 3" long, 1/4" diameter, other is 3.5" long, 1/8" diameter. The 3" one is high quality (got from a university), and the 3.5" one is "dodgy", but i would like to try and get it lo lase. The 3" one is almost clear, and a HeNe laser beam going through it is barely affected (looks the same brightness after going through). The 3.5" rod is almost opaque, and decreases the brightness of a HeNe beam quite a lot. The 3" one will allow the beam created to go through it more easily, but the 3.5" rod will give off more light when excited.

    How does this opacity affect the use of the rod in a laser? Will the 3.5" one need more input/give higher output?"

    (From: Chris Chagaris (pyro@grolen.com).)

    I think that I am familiar with the ruby rods that you have. The 3" polished rod should "lase" without any problem in a suitable cavity. The fact that the sides of the rod are polished will affect the pump light distribution in the rod and would tend to cause some central focusing, especially in an elliptical pumping cavity. This is a quite complicated phenomenon which depends on many factors besides the cylindrical surface finish of the rod, including optical thickness and pumping geometry.

    Your other "dodgy" rod with the matt finished cylindrical surface will give a more uniform pump light distribution under certain circumstances. The most important parts of the laser rod is of course the quality of the ends. The ends must be precisely polished to a high degree and anti-reflection coated for best performance in a laser cavity. You are likely assuming that the unpolished rod is "giving off" more light, but this is mainly the effects of diffusion from the unpolished ends. This rod will not work in any type of laser cavity unless the ends have been suitably polished and over-coated. This would be very difficult to achieve oneself without the proper facilities and equipment.

    "I have an old ruby laser, with everything except the power supply. The flashlamp has a 3" arc length, and about 3 or 4 mm diameter. I have been told that for optimum performance, i will need a capacitor (or bank) rated 1,200V at 300-400uF (BIG!!!) does anyone know what the minimum capacitor might be to still cause lasing action? (3"ruby rod, 1/4" diameter)."

    (From: Chris Chagaris (pyro@grolen.com).)

    I have a ruby laser of the same dimensions, but I have no way of knowing if our flashlamps are equivalent. Anyway, my capacitor has to be charged to about 90 joules in order to achieve laser action (with a pulse width of 250 us). With a 1,200 volt charge on the capacitor this would mean a capacitance of least 125 uF. Don't forget the proper pulse forming network.

    John's Notes on Er:YAG Lasers

    (From: Johnathan Leppert (leppertjle@excite.com).)

    We had a medical Er:YAG given to us awhile ago which was still mostly functional. Wasn't diode pumped, but the research I did on it to refurb it to maximum op, I remember a few things. The upper state pumping is at 970 nm. Suitable diode pumping can be had with InGaAs diodes with around 30% to 40% diode to Er:YAG efficiency. In this mode it usually runs at 2,937 - 2,940 nm.

    The above is considering pulsed. If you want CW, you should pump at 790 nm with something like AlGaAs diodes (I think...???). This gives you something closer to 3 um, but isn't quite as efficient.

    I'ms not sure about this crystal's thermal properties, but I do know the laser we had was very sensitive toward room temperature. In the morning, when the air system was becoming stable after standby at night, sometimes we couldn't even start it. In the afternoon, when the room was fixed at 75.4 F, it worked better. I'ms not sure if this was because there was something wrong with it, or what, but it was even more sensitive than my open-air Diode + Nd:YVO4 + KTP 532 nm laser, which was very poorly thermally managed.

    CW Ruby Laser?

    It seems that few people are content with a pulsed laser when they realize that most of the time, nothing is happening and they figure that much greater output power should be possible. Well, at least for ruby, it is extremely difficult and any claims to the contrary (from suppliers of laser parts) should be taken for what they're worth - which isn't much. :)
    "Having spent some time with gas and 4 level (YAG) laser systems, I am contemplating CW pumping a ruby with linear Arc tube and a Q-Switch. I realize that being a three level laser, it is significantly less efficient than YAG etc. The ruby I have is 6" long and 5/8" in diameter.

    Has anybody any experience with doing this and what sort of input power is needed/possible?"

    (From: Curt Graber (cgraber@fwi.com).)

    I'd hate to see that Massive pretty ruby rod thermal dynamically explode but if you do put this together use a video camera so we can all get a glimpse of the death and funeral. I read somewhere that a lab did have moderate success with a cw ruby cavity however they used an incredibly small rod and were pumping it with other than a arc lamp (huge heat and waste energy), anybody else have an opinion?

    (From: Steve Roberts (osteven@akrobiz.com).)

    Yeah, the only CW ruby laser I have ever seen data on was a liquid nitrogen (LN2) cooled little cube of ruby pumped by a 5 watt argon ion laser. The output power was very low even with LN2 for cooling. You're gonna blow that rod into smithereens. Ruby doesn't shed heat well, nor does it like CW pumping. You might find yourself depopulating the storage with your arc light as fast as you can store the energy, so no lasing. Try YAG instead. You'd get a 10 fold increase in output power anyways.

    I did see a color picture of a cryo cooled ruby cube pumped by a focused large-frame argon ion laser, maybe early seventies, but I don't have a reference. They did have the beam paths shown in smoke - you could just see the faint red beam in the picture.

    (From: William Buchman (billyfish@aol.com).)

    I am not familiar with this particular configuration.

    You do, however, have to distinguish between pink ruby, the standard ruby laser material, and red ruby. Pink ruby can operate as a three level laser at room temperature. Cryogenic may be used to transfer heat. Red ruby, on the other hand, can operate on satellite lines at somewhat longer wavelength as a four level laser. It requires a cryogenic temperature to achieve population inversion.

    Four level lasers have relatively low thresholds so as not to have the final laser state be the same as the ground state. This greatly lowers the threshold. Neodymium lasers are an example. There are other such crystals. Calcium fluoride doped with uranium is one such. It has to be cooled, however to thermally separate the final laser state from the ground state.

    For ruby lasers, it is necessary, neglecting various degeneracies, to excite more than half the ions from the ground state in order to exceed threshold and keep it there. It can be done. It was done for the first ruby laser. Keeping the concentration of chromium ion down helps. That is why ruby has 0.01% to 0.02% while neodymium runs at 1% if that much can be kept in the crystal structure.

    However, ruby does not run truly CW like a well behaved electronic crystal oscillator. The optical oscillator squegges in a series of pulses like a self excited super regenerative detector.

    (From: Chris Chagaris (pyro@grolen.com).)

    The original experiments on CW ruby pumping were done with high-pressure long-arc, mercury vapor lamps. Maximum input power was 560 W/cm. A small one inch long by 0.079 inch diameter ruby rod was pumped periodically at a maximum repetition rate of 110 Hz and a maximum output energy of 2 watts was obtained. I would doubt that you could continuously pump such a large rod as you have described.

    A CW ruby laser was indeed built and reported by V. Evtuhov in a 1967 article in the Journal of Applied Physics. From: "Solid-State Laser Engineering" (Koechner):

    "A CW-pumped ruby laser, which used a rod 2 mm in diameter and 50 mm in length, generated an output of 1.3 watts at an input of 2.9 kW. Only a small part of the crystal's cross-section was excited by the filament arc, and lasing action occurred only in the small volume of 6 x 10-3 cm3. Using this value, the lamp input power per unit volume of active material required to obtain threshold is approximately 230 kW/cm3. The main reason for the poor efficiency was the low absorption of useful pump light by the small lasing volume."

    A capillary mercury arc lamp was used as the pump source, operated at 200 atmospheres. These types of high pressure mercury arc lamps produce a spectral output which coincides very well with the absorption spectrum of ruby.

    (From: Michael Andrus (andrus@ccountry.net).)

    Ruby is self limiting so even if you used an arc lamp you have a semi-CW beam. As others have said cooling these lasers is a chore. I have a ruby operating at 1 pulse every 5 seconds and it gets HOT. If you liquid cooled a small rod you could build a power supply that could run a flash lamp at 50 Hz which would be far more efficient than an arc lamp. Your PSU would have to be in the kW range though. If it is high poer you need go with YAG, but if you want visible, try a YAG pumped KTP.

    (From: William Buchman (billyfish@aol.com).)

    A CW ruby laser was indeed reported by Bell Labs. I think it was first published in Apllied physics letters, possibly by Nassau and Boyd.

    The laser was made from a single piece sapphire grown so that one cylindrical portion was doped to form ruby while attached to it was a clear sapphire cone. It was end pumped so that light was collected by total internal reflection in the cone. I believe that the rod was immersed in liquid oxygen which in turn was cooled by liquid nitrogen. The light source was an arc lamp. It operated on the same transition as room temperature ruby. The low temperature did narrow the linewidth thereby lowering the threshold. The laser was run barely above threshold.

    (From: Stephen Swartz (sds@world.std.com).)

    When I was a graduate student in the University of Colorado's JILA program, we actually built a cw ruby laser in the late '80s. No published work came out of it but I can tell you the laser rod was about 1-2 cm long AR coated on on side an brewester cut on the other. The pump source was a 10 watt argon-ion laser and to make the thing work the ruby crystal had to be cooled to liquid nitrogen temperature. This has the effect of thermally depopulating the upper part of the ruby's ground state so the laser can act like a 4 level laser. Efficiency was not too great but it did glow a pretty cherry red. This type of laser has been published in the literature several times but I can't think of where just now.

    What is a Zig-Zag Crystal?

    (From: Bob.)

    A zig-zag crystal is a rectangular crystal with both the ends polished as well as two of the sides. Conventionally, the top and bottom are used for cooling and the pump light enters the crystal from the polished sides and the lasing mode zig-zags through the crystal using total internal refraction. Obviously, the pump beams and the lasing mode occupy the same 'plane' in the crystal.

    What makes these things such a pain to work with is the fact that you have to not only have cavity mirrors aligned to the lasing mode, but you have to have the mode 'entrance angle' aligned to what the pump crystal was engineered for, so that it exits the crystal at the proper angle after X number of bounces.

    The only good thing about this design, other than the rather simple way pump light is coupled into the crystal, is the fact that the thermal lensing is unaxial, and can be fairly easily predicted and compensated for. The zig-zag was one of the first commercially available diode pumped systems. Spectra-Physics came out with what they called a "TFR", tightly folded laser, referring to the zig-zag nature of the lasing crystal.

    Comments on Erbium Lasers

    (From: William Smith (frostybeard@hotmail.com).)

    Er:Glass lases at 1.535 um and Er:YAG lases at 2.94 um.

    From what I have seen, mirrors for Er:YAG are frightfully expensive, I suppose due to the long wavelength compared with Nd:Yag at 1064 nm. Maybe CO2 mirrors would work with Er:YAG if they are not extremely wavelength-specific. CO2 mirrors can be found for a reasonable price. The CO2 mirrors I have seen have been gold or copper, so they would work. Get one that is extremely reflective (as close to 100% as possible) for the mirror and one that is about 80 % reflective for the output coupler, if you have an Er:YAG.

    I have an Er:glass rod, so I have been finding out a lot about Erbium lasers. I just ordered a mirror and output coupler from Alkor Technologies (Russia).

    Erbium is a good first solid state laser project. Erbium is a so-called eye-safe laser, since the longer wavelength does not penetrate the eye to the retina. However, it can burn your cornea! So be careful. You have no hope of seeing any light from it. The wavelength is too long.

    Use a xenon strobe light at about 1 Hz to pump it, unless you are q-switching it. Then you can pump it with a higher repetition rate. You can also use laser diode pumping at 980 nm. It is not a good idea to pump glass CW, because glass is not a good thermal conductor and can be rather easily damaged by heat.

    Natural Ruby Crystal for Solid State Laser?

    Aside from the problem of finding a large enough chunk of natural ruby at a price affordable by a small country, the optical quality and doping level are not likely to be acceptable. Despite the inflated prices for gemstones, their uniformity and freedom from impurities may be quite poor. The doping level of natural ruby is also likely to be much higher than desirable for a laser crystal. This would result in a much higher threshold, if it can be made to lase at all.

    So, your spouse can rest easy that her rock won't be recycled into a hobbyist laser. :)

    Choice of Lamp Pumped or Diode Pumped Solid State Laser

    (From: Martin Sharp (martinsharp@btconnect.com).)

    Lamp pumping is the older technology, and uses inert gas filled lamps, which have to be replaced every 500 to 2,000 hours. Diode pumped lasers use laser diodes to excite the YAG crystal. Diodes last much longer, lifetimes seem to be quoted at 30,000 hours or so. But they are far more expensive than flashlamps.

    The two big advantages of Diode pumping are in electrical efficiency and laser beam quality. Lamps generate a lot of heat and the overall efficiency is low. This means you would be looking at a three-phase electrical supply, and a water chiller for the system. With diode pumping it is possible to get a laser which will mark metal that can run off single phase and is air-cooled.

    Because of the heat generated by the lamps, the YAG crystal distorts and the resultant laser beam has a lower beam quality - this means that it cannot be so easily focussed to a small spot. Again the diode pumped laser does not suffer this problem.

    So what about power? Well a typical lamp pump laser will generate 75 to 100 W of continuous light output power. For many metal marking applications you would use the laser in CW mode (continuous). For some metals, and other materials, you need to pulse the beam using a Q-switch. Each pulse has a very high power, but is low in energy. The product of the pulse energy and pulse frequency (typically 1 to 20 kHz) gives you the average power, and this will be less than the CW power say 50 to 75 W (or less) because of the pulsing.

    Now, to achieve these powers, the laser beam is run with a multi (transverse) mode beam, again a beam quality factor. Multimode beams don't focus as well as single mode beams (i.e., they have a larger focal spot size). Why is focal spot size important? The laser power divided by the focal spot area is a measure of intensity, and the higher the intensity, the crisper, darker and possibly faster the laser mark will be. You can increase the intensity by increasing the power, or by decreasing the spot size. So reducing the spot size is important, and this can be achieved by increasing the beam quality. This can be done in a YAG laser by putting apertures inside the laser, and this converts the laser beam to single mode, hence better beam quality.

    However the power is reduced. So the 75 W multimode laser above becomes say a 10 or 12 W single mode laser, but the resultant increase in intensity may be beneficial.

    Now diode pumped lasers inherently have better beam quality anyway, but their raw power is still limited (but increasing). Even at lower powers they may produce better marks.

    Thermal Lensing in SS Lasers

    (From: Bob.)

    Thermal lensing is a generic term used to describe the effects of heat on a laser medium. With reference to DPSS lasers, there are two main ways that thermal lensing can be evident. With an end-pumped arrangement, you can get the face of the laser crystal to bulge outward due to localized heating, and thermal expansion (remember you may not be using a high power pump diode, but it is focused well, meaning you have some pretty substantial power densities at the pump beam waist). Interestingly, this effect can be alleviated by sandwiching the DPSS crystal between two pieces of sapphire, and applying a high pressure to the crystal faces. The sapphire does not absorb the pump light to any extent, and since sapphire is a very strong, hard crystal. It does not allow the vanadate, YAG, etc., to deform significantly.

    The second kind of thermal lensing is the more classical type of lensing that refers to the temperature gradient in the laser rod or crystal. Since the inner part of the laser crystal is heated by pump light, and only the outer surface is cooled, there is a temperature gradient formed that causes a weak change in the index of refraction. This can by symmetrical around the axis of a rod, where you get spherical thermal lensing, or in the case of a cube of vanadate, you can get cylindrical thermal lensing, if the cube has heatsinking on two sides.

    So, when is thermal lensing a problem? Unfortunately, there is no simple answer to that question. it's a case by case sort of thing, all depending on host, pump power, pump power density, cooling, ambient temperature, resonator design, and the list goes on and on. But thermal lensing is not always a bad thing, sometimes it is actually used in the design process of a laser and required for proper operation. For example, the newer Continuum pulsed YAG lasers are set up to run with a certain amount of thermal lensing. The lasers normally have an adjustable rep rate from 1 to 50 Hz. When the user wants to run the laser at 1 Hz, the flashlamps still fire at full power, at 50 Hz, but the q-switch is only gated once a second. this is because the laser was designed to run stably with the thermal load that would be present when the laps were firing at 50 Hz. that particular laser would NOT operate properly with reduced thermal lensing if the lamps were fired less often (that's normally the reverse of how things happen - normally less lensing is good, more is bad).

    Which brings up a related question:

    When determining the focal length of a Nd:YAG rod while operating under a heat load from the lamps/diodes, I normally use a secondary YAG laser and simply measure the focal length of the rod. I remember seeing a reference to making focal length measurements by using various placements of the rod in a laser cavity but don't recall the procedure. (I'ms trying to help a buddy out who wants to measure the focal length of a rod, but doesn't really have any type of 'tools'.)

    (From: A. E. Siegman" (siegman@stanford.edu).)

    You may be thinking of journal articles that calculate the Gaussian mode spot size, mode stability and other mode parameters in a YAG laser using standard ABCD techniques and treating the thermal focusing in the rod as an equivalent thin lens at the center of the rod.

    There have been several of these published but I believe they were all concerned with evaluating the effects of thermal focusing on the laser mode, rather than on using this the other way, to measure the thermal focal length. One result from these is that there can be special locations within a laser cavity where the mode size is to first order independent of the thermal focusing.

    The rule of thumb for thermal focusing in YAG rods is about 0.5 to 1 diopter (f in meters = 1/d) per kW of power into the pump lamps, more or less independent of rod dimensions -- right?

    If your friend can measure laser performance with, say, a flat/flat cavity over a wide enough pump power range that the cavity goes unstable and the laser goes out -- or just carefully measure mode spot size versus pump power in any cavity -- I'd think it would be possible to deduce the focal length versus pump power to reasonable accuracy by comparing mode theory to experimental results.



  • Back to Solid State Lasers Sub-Table of Contents.

    Micro Laser Rangefinder Using Disposable Flash Pumped Nd:YAG and OPO

    See the paper: Micro-Laser Range Finder for a complete description (along with cool graphics and photos) of this system. It uses the flash unit from a Kodak 'MAX' single-use (disposable) 35 mm camera (slightly modified to permit control from a PIC as the pump source for a passively Q-switched Nd:YAG rod with an OPO to generate an eye-safe beam. See the section: Power Supply for Micro Yag Laser (uYAG) for information on the power supply.



  • Back to Solid State Lasers Sub-Table of Contents.

    Florod LCP Laser Trimming System

    This is a pulsed doubled Nd:YAG laser system (532 nm) designed for trimming of semiconductors (e.g., blasting holes in on-chip resistors, zener diodes, etc.). The LCP consists of a laser head which attaches to a microscope (mounted on a semiconductor probe station) and has a mount for a CCD camera. Photos of the laser head and power supply can be found in the Laser Equipment Gallery under "Florod YAG Lasers".

    The laser head consists of the flashlamp and YAG rod in a sealed box, the KTP doubling crystal (outside the cavity), servo controlled apertures (variable slits) for X and Y spot size, and a servo controlled attenuator to adjust pulse energy. Note that this attenuator approach is much simpler and more consistent than alternatives using adjustable capacitor voltage or pulse duration control.

    The YAG rod is probably about 50 mm long by 3 or 4 mm in diameter, AR coated both ends. The mirrors are glued to the cavity box (non-adjustable). The servos are the types used for RC models but work fine in this application. :) Optics are glued to precision X-Y adjustable mounts. A fiber optic light pipe cable introduces a targeting beam for viewing the dimensions of the laser spot into the light path via a 45 degree mirrors which is transparent to the laser beam but reflects an adequate portion of the white light beam.

    Energy into the flashlamp is about 28 Joules (66 uF at 850 V provided by three 200 uF photoflash capacitors in series). Triggering is via an SCR and EG&G trigger transformer to an external electrode on the flashlamp. I don't know what the exact maximum output energy is but for this application, less than a mJ is adequate once the beam is focused to a spot of a few um.

    The controller consists of analog knobs for X and Y aperture and laser power (operating those RC servos, all with digital readout), single shot or 1 pulse/second select, and a knob for the targeting light source brightness via a phase control dimmer. Nothing particularly high tech!

    Although the output of the YAG rod is clearly dangerous (it is probably a few 10s of mJ) and the final green output may even be hazardous to vision, the system has a Class I rating (unconditionally safe) because everything is fully enclosed under normal operation. There are two head interlocks: A magnet on the cover and dual reed switches on the optics chassis prevent operation if the cover is removed and a tilt sensor prevents operation if the head isn't within 20 or 30 degrees of vertical. There is also an interlock connector on the rear of the power supply and the firing control is a momentary SPDT (foot) switch. The interlocks interrupt primary power to the high voltage transformer (the rest of the system continues to function) but the firing switch controls logic inputs.



  • Back to Solid State Lasers Sub-Table of Contents.

    Hughes Rangefinder Ruby Laser Assembly

    Description and Specifications

    This pulsed ruby laser was used as part of the time-of-flight rangefinder in the M-60 battle tank to determine target distance. The official designation is apparently "AN/VVG-2 Rangefinder, Fire Control" or "AN/VVG-2 Laser Rangefinder (LRF)" should you care. (The photomultiplier based return sensor and computation unit that go ith it rarely, if ever, appear surplus. At least I haven't seen them.) A typical sample of this unit is shown in Hughes Ruby Laser Assembly with Parts Labeled [photo provided courtesy of Dave (Ws407c@aol.com)]. Some versions of this laser obtained surplus may differ slightly, lacking the collimator, for example. Additional photos, description, and comments on the ruby laser assembly and various home-built power supplies may be found in the Laser Equipment Gallery under: "Hughes Rangefinder and Home-Built Ruby Lasers". Complete power supply schematics are provided in the section: Wes's Ruby Laser Power Supply (WE-SP1). Adjustment information can be found in the section: Aligning the Hughes Ruby Laser.

    The following specifications have been confirmed by Chris Chagaris (pyro@grolen.com). However, there could be other variations with slightly different part values so double checking what you have would not be a bad idea! And, of course, if you replace the PFN with one of higher energy, the values for output pulse energy will be greater (unless the flashlamp explodes).

    Notes on the Hughes Q-Switch

    (From: Doug Little (dmlittle@btinternet.com).)

    The pulse generated by the Q-switch's magnetic pickup looks a little like this:

                /\
            ___/  \    ___
                   \  /
                    \/
    
    If you build your trigger circuit carefully and make sure you connect the magnetic pickup the right way around (rising or falling edge) you can minimize any unwanted delay between pickup and trigger. You can then of course introduce an artificial and adjustable delay of your own for optimization purposes. A suitable circuit is shown in Q-Switch Trigger Circuit for Hughes MS-60 Ruby Laser and described in the section: Doug's Q-Switch Triggering Circuit for Hughes MS-60 Ruby Laser (DL-ST1).

    There are some important things to realize when you try to set up your own timing circuit:

    1. The pickup produces a weak signal (I don't remember the value, but it's only about 2 or 3 volts). You won't be able to drive a standard Schmitt trigger directly with it, so a transistor or other amplifier may be needed. I used a BC547 transistor with a resistor and it worked great. You could use an op-amp, but that might just be overkill.

    2. The pickup is timed precisely to trigger the flashlamp without any artificially introduced delay. This severely limits your ability to tune timing in both directions. All you can do is increase the time between detect & fire - you can't decrease it.
    There are two ways around the second problem. One is to run the motor backwards, giving you a whole rotational period of about 2 ms to play with. Being a mechanical motor, this is a lot of time to wait before discharging the lamp without expecting some sort of speed fluctuation. The longer the delay, the less accurate the prism's final (flash) position becomes in terms of motor speed!!! I have strobed the prism with a super-bright LED using a very short on-time of several us and I can say that a 2 ms delay results in a slightly wobbly prism, instead of a preferred rock-solid one. The second solution involves the motor/Q-switch mounting platform. If you loosen the hex bolts you can rotate the whole unit about 5 to 10 degrees in either direction. This affects timing quite a bit and gives you the opportunity to buy back a few 10s of us.

    WARNING! Adjusting the Q-switch platform may kill your laser's alignment and you will have to go through the whole horrible process of adjusting the optics with a reference laser and it can take hours. I know because I did it myself. If your laser is already aligned, you may want to think very hard before you go adjusting those hex bolts!

    (From: Randy Smith (randysmith@adelphia.net).)

    I too have one of these ruby laser units that I am trying to get running. To start off with, there needed to be some sort of timing control unit to synchronize the flashlamp with the spinning mirror. I built such a device using an 87C552 micro, with a 4 digit thumb switch control to allow for an arbitrary offset from TDC (top dead center), entered in degrees. The jury is still out as to the functionality of this unit, but it does look good on a scope and also, when used to drive a small laser diode, it can be used to view the instantaneous position of the mirror. I will find out for sure this coming weekend, when I test it in operation with the laser.

    Comments on the Hughes Ruby Laser

    (From: Paul (sinebar@bellsouth.net).)

    I finally got the thing to work but I had to step up the power input to the flash lamp. I simply added a second 150 uf cap in parallel with the other to get a total input of about 216 Joules. I charged both up to 1,200 volts. I used the Doug Little's Q-Switch Trigger Circuit for Hughes MS-60 Ruby Laser to synchronize the flash lamp discharge with the Q-switch (See the section: Notes on the Hughes Q-Switch. I ran the motor CCW at 36,000 RPM and adjusted the Q-switch prism to be about 1/8th turn past the pickup when the lamp fires. This seems to give the best results. It blows the ink off a page. Next, I'ms going to see what it will do metal. :)

    I figure that with only the original 150 uF or so cap producing at most 126 Joules, at 1,300 volts max, it is probably just barely at the lasing threshold with an optimally timed and aligned Q-switch. The military techs had a device for this unit that tuned the Q-switch without firing the flash lamp. If one had that device then you could probably get it to work with just one cap. Also if it had a real OC instead of just a clear optical medium I think that would help a lot.

    (From: Sam.)

    Yes, we know that the use of a dichroic OC reduces the lasing threshold significantly. Wes Ellison actually got the laser operating without the Q-switch using an OC from some other ruby laser.

    Note and Caution About the Resonant OC

    The following refers to what is inside the block at the output of this laser, not the Q-switch itself.

    (From: heru_kuti@yahoo.com.)

    The Hughes ruby laser Q-switch mirror block is composed of a beam diverging optic, a spacer, and a "mirror" that is slightly concave. If all three optics are aligned perfectly it will give about 33% reflective power.

    The OC for a Q-switched laser is typically 30% while it is 70% for lasers that operate long pulse mode. The one used in the rangefinder is the most durable of all being totally devoid of any coatings whatsoever.

    However, if you disassembled it, there is no practical way to realign its optics and you have probably ruined it unfortunately. :-(

    Increasing Repetition Rate of Hughes Ruby Laser?

    "I purchased one of the Hughes rangefinders (two, actually, if I can find the other one...), and have been looking at what might optimize the output. It appears that simmer pulse operation, with 600 V square wave pulses with a duty cycle such that one pumps for the length of a rotational period without killing the tube would do the trick. IGBTs would do the switching - the question is how to trigger the tube without a serial transformer in the existing cavity. The best idea I have would be to use an insulated wire externally as the trigger - has anyone tried this and made it work?"
    (From: Chris Chagaris (pyro@grolen.com).)

    How exactly do you intend to "optimize the output"? I get the impression that you wish to optimize repetition rate by utilizing a pseudo-simmer mode circuit. You must realize that this laser was designed to operate at a low repetition rate and must do so for a number of reasons. The original flashlamp contained in this laser is an EG&G, FX-103C-3 which is the predecessor of their FXQ-1302-3. With the design of this cavity employing only convection cooling of this original lamp, the maximum average power is rated at only 20 watts. At an input energy to the lamp of let's say 100 joules (somewhat above minimum for laser operation) your pulse repetition rate would be limited to one pulse every five seconds. With such a slow repetition rate I cannot see the justification for employing a simmer mode of operation. Since there are no active means of cooling the ruby rod, this could also present a problem, as ruby does not dissipate heat very well and the likelihood of damage from over-temperature is great if this system were to be operated much above its design limitations. With the configuration of this particular laser cavity (semi ellipse) the use of an external trigger wire for successful firing would be highly unlikely. The flashlamp is in intimate contact with the grounded aluminum base of this reflector to aid in the cooling of the lamp. A wire of any kind would interfere with this contact and of course would serve no purpose as the current would just flow to ground. A wire with enough insulation to protect against the very high voltage pulse (10 kV or more) would be very impractical.

    (From: Sam.)

    I agree with Chris 100% that boosting the repetition rate isn't really viable. As far as triggering, an alternative to series triggering is parallel triggering which can easily be extended to multiple trigger sources. See the section: Basic Structure and Characteristics of SS Laser Power Supplies. EG&G discusses simmer mode in their Design Considerations for Triggering of Flashlamps.

    (From: Chris.)

    In more detail, there are two points to consider in answering this question:

    1. The first and most important is calculating the damage threshold of the xenon flashlamp in regards to repetition rate and power input. All flashlamps have 'average power limitations' that are based on the type of lamp (glass to metal seal) and the method of cooling. Average power input is calculated by:

                        P(avg) = E x f
      
      Where:

      • E = Energy in joules
      • f = Flash repetition rate in pulses per second.

      The flashlamps that one may find in the MS-60 rangefinder ruby lasers are either the original EG&G lamp, FX-103C-3 or the replacement EG&G flashlamp, FXQ-1302-3. Since this ruby laser's cavity is not actively cooled (merely convection cooled) the maximum average power rating for these lamps are 20 watts and 150 watts respectively. Consider an input of 100 joules to this first lamp. This would limit repetition rate to one pulse every five seconds. This same input to the replacement lamp rated at 150 watts would give you a safe maximum pulse rate of 1.5 pulses per second. Of course an increase in pump energy to the lamp would decrease the maximum safe repetition rate.

    2. The second point to consider is the capacitor charging power supply. As with any cap charging circuit it takes a certain amount of current to charge a capacitor in an allotted time. I = CV/t, again. :-) The PFN that is commonly available contains a capacitor of about 160 uF. Charging this capacitor to store 100 joules will require a maximum voltage of about 1,120 volts. To be able to charge this capacitor to this level, at a repetition rate equal to the average power rating of the convection-cooled FXQ-1302-3 lamp, would require a power supply capable of providing at least 1120 VDC at about 270 mA.
    Ruby was never meant to be pulsed at a great repetition rate. Another problem that one would face at high repetition rates is the overheating of the ruby rod, which does not dissipate heat too well (unlike YAG). This can permanently damage the ruby crystal.



  • Back to Solid State Lasers Sub-Table of Contents.

    A Small Nd:YAG Laser - SSY1

    Description and Specifications

    SSY1 is a small Nd:YAG laser head which includes a flashlamp with external trigger, cavity reflector, and optics, all in a package about 25x25x105 mm (not including mounting feet). See SSY1 Laser Head Assembly for a photo. This laser was used in the rangefinder for the M-1 tank.

    This unit is available from Meredith Instruments along with a matched pulse forming network (see the section: Pulse Forming Network 1. (Meredith has also been auctioning these and other items on eBay.) New SSY1s and parts may also be available from Anderson Lasers, Inc. and elsewhere. I constructed a capacitor charger and external trigger circuit. See the section: Sam's AC Line Power Supply for SSY1 (SG-SP1). An alternative design which runs from low voltage DC is described in the section: Sam's Inverter Power Supply for SSY1 (SG-SI1).

    For initial testing, figuring it would be real effort to get it lasing, I used my trusty IR remote control tester for detecting the beam. Big mistake. :( The first shot sent the photodiode off to photodiode heaven (or wherever faithful photodiodes go when they die). Its output just stayed on! I should have used the IR detector card available from Radio Shack (and elsewhere).

    OK, so go to plan B. :)

    I placed a piece of black coated paper in front of the laser and fired off a few shots. No effect except for a bright blotch of white light from the flashlamp. (Maybe I didn't examine it closely enough.)

    Next, I tried a small lens approximately focused on a piece of black coated paper. To make sure any effect wasn't just due to spill from the flashlamp, these were positioned about a foot from the laser head. Immediate gratification! The moderately focused output beam easily obliterated the black coating on the paper. This was accompanied by a very nice 'snapping' sound and white or yellow incandescent plume when hitting the black coating, and a more muted sound after the black stuff had vaporized. When carefully focused, it will make nice tiny holes in aluminum foil (the incandescent plume is green-blue in this case) and other thin materials, and mini-craters on thicker objects. I've heard of people driving this laser with much higher energies to blasting holes in razor blades (see below). However, it is all too easy to blow up the laser components when doing this - the flashlamp and Q-switch are most susceptible to damage or destruction.

    I don't have any way of actually measuring the energy of the beam but let's just say it is definitely not something to be taken casually, as far as eye safety is concerned! My wild off-the-top-of-the-head guestimate would be at least 10 mJ, probably 20 or 30 mJ, though it may be as high as 50 to 100 mJ. Hopefully, someone will eventually measure the output pulse energy! The Nd:YAG rod is probably capable of much greater energies but that flashlamp doesn't look all that sturdy so I'ms not about to push my luck, at least not yet. :)

    The lasing threshold is about 7.5 J - less than the energy of the electronic flash in a typical pocket camera! This low value is no doubt due to both the cavity and optics design - and the optimal pulse length from the PFN. Thus, using one of those cheap flash units (or just its power supply) directly probably wouldn't work at all as the duration of the flash pulse would be way too long with insufficient peak intensity. (The unit described in the section: Micro Laser Rangefinder Using Disposable Flash Pumped Nd:YAG and OPO is based on a much smaller Nd:YAG rod - about 1/8th the volume.)

    Here are the specifications, as best I can determine:

    Note: The dimensions are from my memory or lack thereof - I haven't measured them since getting SSY1 to lase, corrections welcome.

    WARNING: Despite its small size, this is a Class IV laser. While SSY1 probably won't set anything on fire unless you fire it at an explosive or have a natural gas leak, this laser is quite capable of doing serious damage to vision. Treat it with respect! Cover the HR mirror aperture (I used black electrical tape) since there may be some leakage from there which is invisible and enclose the output beam path so that backscatter can't hit anything of importance (like your eyes).

    Variations in Performance of SSY1s

    Depending on how much use or abuse any given SSY1 has had in a past life, there could indeed be significant differences in output energy for the same input energy. I don't know to what extent this may happen for samples that appear to be in perfect condition but it should be possible to identify obvious problem areas. Inspect the Q-switch dye cell, flashlamp, rod ends, and optics for dust, dirt, discoloration, mottling, and physical damage. If the dye cell appears anything but perfect, it probably should be removed as its condition will only deteriorate with use. Discoloration of the flashlamp will reduce the amount of light to the rod but unless it is very severe, won't prevent the unit from working but will just increase the lasing threshold slightly. Unlike a HeNe laser with its low gain, a spec of dust won't kill lasing but a careful cleaning probably won't hurt. For anything beyond light dusting, use proper laser mirror cleaning procedures to avoid damaging the optics.

    I've now tested 3 of these babies - 2 that appear to be in original condition and another with the Q-switch removed and the AR coating gone from one end of the rod. (I've also used the mirrors from an SSY1 to construct the resonator for another YAG cavity, see the section: Mini YAG Laser using SSY1 Optics and SG-SP1.) The two intact units produce about the same output energy. The other one lases but probably at slightly lower energy. It still smokes black tape (possibly better than the other ones) but won't penetrate aluminum foil. The sound it makes when focused on a target is also softer. However, I don't know to what extent these differences are due to the lack of a Q-switch versus the missing AR coating It's probably a combination of both but the reduced effect on thermally conductive aluminum foil and softer sound would be consistent with the longer, lower peak power pulse produced without a Q-switch. Perhaps at some point in the future, I will swap rods with an original SSY1 to separate out the effects of the missing Q-switch and AR coating.

    Comments on the SSY1 Nd:YAG Laser

    (From: Moses Clark (itek@sybercom.net).)
    1. I have fired the laser using a 120 uF, 900 V homemade PSU.

    2. I could generate smoke from carbon paper (what a benchmark!).

    3. The manufacturer of the cavity is Kigre Laser (according to Jeff Myers who works there). They supplied it to Hughes aircraft. The laser is a Huges Aircraft M-1. It was originally used on the M-1 Tank and was later integrated into some hand-held laser rangefinders. The Nd:YAG rangefinder on the M-1 is now being replaced with a 1.54 um Er:Glass eye-safe rangefinder made by Litton Laser Systems.

    4. Kigre also sells a passive Q-switch, a dye filled saturable absorber cell) as an add-on to the cavity for $150.00. (This or one similar to it is part of SSY1. --- Sam.)
    I am trying to build a laser rangefinder using this laser.

    (From: Ivan (sinebar@bellsouth.net).)

    I got my small YAG laser working using the PFN from Meradith Instruments and a power supply based on the SG-SP1 schematic. Even without a lens it will burn a spot on a black target.

    (From: Sam.)

    The laser described below sounds similar, perhaps even smaller than SSY1:

    (From: Erbium1535 (erbium1535@aol.com).)

    The South Carolina State Museum in Columbia uses a Nd:YAG laser to pop a balloon inside a balloon in their Townes exhibit. (C.H. Townes was born in Greenville, South Carolina.) The laser, manufactured by Kigre, Inc. in Hilton Head, SC is a Q-switched MK-367 unit and is described on the Kigre MK-367 Nd:YAG Laser System Page. The actual laser is approximately 0.6 x 0.8" x 4" in size and emits a 17 mJ pulse pulse with s duration of less than 4 ns. They also offer a frequency doubled green version. The MK-367 was originally developed for the ophthalmic surgical market, specifically as a photo disrupter for posterior capsulotomy. The power supply is approximately 4" x 4" x 1.5" and operates from 12 VDC.

    The laser is somewhat unique in that it is permanently aligned, utilizes a ceramic exoskeleton for stability, and a positive branch confocal resonator design for high beam brightness. Kigre has sold more than a thousand of these miniature lasers for various applications including medical, industrial, rangefinding, and pyrotechnic ignition. The MK product line has been around for more than 15 years, so these lasers sometimes find their way to the used laser discounters. New ones are still available and cost about $3,600. If you do come across one of these, be very careful as it is a very powerful Class IV laser! (Yes, but the SSY1 is potentially an even more powerful Class IV laser! --- Sam.)

    Shawn's High Energy Experiments with the SSY1 Laser Head

    WARNING: The passive Q-switch will not survive the abuse inflicted by high energy operation for very long - it is a high failure item even under normal conditions. The mirrors apparently tolerate it better but these also may degrade after awhile. And, even if the flashlamp doesn't explode, the pulse repetition rate must be very low so as not to exceed its average power ratings and to limit heating of the rod and the entire assembly. Using PFN1 (36 uF, 900 V), MTBF may be in the 100K shots range; this may drop by several orders of magnitude with ultra-high energy operation!

    (From: Shawn West (west007@libcom.com).)

    I've taken a different approach than the others and am pumping it with a long pulse, about 2.5 ms. With my long pulse I have put a 0.020 inch diameter hole in a 0.004 inch thick razor blade. I've punched holes through aluminum foil of different thicknesses too. I've back calculated the energy required to punch the holes in the razor blade and the two aluminum foil experiments. The calculations show that it would have taken 1.7 to 1.8 Joules to melt and vaporize the metal in each case (if I did my calculations right). When I hit the razor blade with 800 volts on the capacitor (360 Joules) I was able to punch a 0.024 inch diameter hole in the 0.004 inch thick blade. My calculations, which again could be wrong, show that it would have taken about 2.5 joules to do this. These calculations do not include the amount of reflected energy or the energy conducted away from the material. I have also sparked the air using a short focal length (about 1.5 cm) lens. I'ms using a 1,120 uf capacitor with approximately a 0.15 ohm ESR.

    My inductor is 820 uH with a resistance of about .15 ohms. It is from Parts Express (part #266-760, about $23). The inductor is wound with an effective 12 gauge copper foil and has an air core. I'ms using a piezo-electric igniter from a gas grill to flash the tube.

    I have also used a 270 uF capacitor and a 80 uH inductor (ESR of 8 or 9 milliohms). However, the longer pulse PFN put out more energy (more destruction to the target) than the short PFN when the caps were charged with the same energy. This could have been due to the ESR differences of the two capacitors or the higher current density with the shorter pulse PFN exciting the shorter wavelengths of the xenon (i.e. not exciting the 800 nm hues as well to mate with the Nd absorption). I'ms trying to keep the current density in the flashlamp below 4000 A/cm2 to favor the 800 nm absorption band of the Nd:YAG crystal. I also wanted to pump out a lot of energy. This forced me into a long pump pulse.

    I spoke to Jim McMann (sp?) from Perkin Elmer (EG&G) about the flashlamp in mid-December, 1999. His phone number is 1-800-950-3441. At that time, he thought the flashlamp was an FXQG-264-1.4. From what I have found out since then, there are two EG&G flashlamps that could have been used for the SSY1. The first is the FXQG-264-1.4. This flashlamp is made from titanium doped quartz that cuts off UV wavelengths below about 225 nm. The second is the FXQSL-559-1.4. This flashlamp is made from cerium doped quartz that cuts off UV wavelengths below about 320 nm. I don't know which one was originally used.

    Both of these have a 1.4 or 1.5 inch arc length, and are probably xenon filled to 500 Torr (though I have not been able to verify the fill pressure). The ID was 3 mm and the OD was 5 mm. If you calculate Ko with a 1.4 inch arc length, you get:

                   1.28 * (1.4 * 25.4)        500
             Ko = --------------------- * (---------)0.2 = 15.5
                            3                 450
    
    
    Using a 1.5 inch arc length results in a Ko of 16.6 which is what I measured it to be.

    For the more conservative arc length of 1.4 inches with a 3 mm bore, the explosion energy for the flashlamp = time.5 * 90 * arc length in inches * bore in mm = 378 * time.5. (Time is in milliseconds.)

    I designed this to run from 300 volts (50 joules) to 800 volts (360 joules). My damping factor (alpha) ranged from 1.03 at 300 volts to 0.8 at 500 volts to 0.63 at 800 volts. I think at about 560 volts the current density in the flashlamp was about 4,000 A/cm2. The explosion energy with a 2.5 ms pulse is about 590 joules and at 800 volts I was running at about 60% of the explosion energy. I normally run at about 560 volts where alpha = 0.76, at 30% of the explosion energy (about 177 joules), and the current density is about 4,000 A/cm2 in the flashtube (the approximate maximum current density for which the 800 nm line is strongly excited). When I was hitting the razor blade and the aluminum foil the capacitor was charged to 700 volts (274 joules - about 46% of the explosion energy). The maximum pulse rate is about once every 45 seconds. Right now my charger is running from 120 Vac but I plan to make this portable and run from 12 volts with a pulse rate capability of about once every 30 to 40 seconds.

    I have not removed the Q-switch to see the effect yet.

    (From: Sam.)

    Well, that's certainly impressive!

    I assume that with the Q-switch, you are actually getting a series of short pulses of a few dozen mJ each. My quick off the top of my head calculation for output energy using the Q-switch would be 25 to 50 times 20 or 30 mJ which is in the .5 to 1.5 J range so your calculations of output energy may not be far off. This laser would probably also do nicely with an arc lamp if you could cool it somehow. :)

    (From: Shawn.)

    My scope is getting calibrated now, but when I get it back I'll check the reflected light to see I am getting a bunch of pulses or a long continuous pulse with a steep front end (maybe even a spike on the front end of the pulse). Does this Q-switch have a self terminating bleaching effect independent of incident power or does it remain bleached as long as the power is above a certain threshold?

    (From: Sam.)

    I don't know for sure but assume that it returns to its non-bleached state immediately after the laser pulse and until the spontaneous emission (not the incident flashlamp power) exceeds the threshold again. Not knowing the exact composition of the dye used here, I can't say what the exact time is. For the rangefinder, the likely objective would be one intense pulse for each firing of the flashlamp so there would be no need to select one that recovered quickly but they do exist.

    (From: Greatest Prime (FishyBill@mediaone.net).)

    The nickel complex BDN in toluene has a recovery time of about 1 ns. (Actually, you can make it in a number of ways. One is to dissolve BDN in methyl methacrylate and polymerize it. You have to watch out the active catalysts do not destroy the dye.) This allows for multiple pulsing. Other dyes and solvents tend to shorten the recovery time. That is what makes mode locking possible at a pulse repetition rates of more than 100 MHz. However, repetitive operation of dye Q-switched lasers is more complicated than merely considering recovery time of the dye. There usually are long term thermal effects of considerable importance.

    (From: Sam.)

    It might be possible to test the SSY1 laser for multiple pulsed operation by firing the flashlamp with a longer than normal pulse. Once the first Q-switched output pulse depletes the upper energy state, the Q-switch should revert to its non-bleached condition. If the flashlamp is still on, the cycle should repeat. Doubling the flashlamp pulse duration from 100 to 200 ns while maintaining approximately the same flashlamp light intensity should be enough and this can probably be done safely (for the flashlamp and dye cell at least for a few shots to perform the test) by doubling the values of the PFN capacitor and inductor. I've heard of rangefinder lasers similar to the SSY1 failing in a way that results in multiple output pulses - this may be a way to experiment with this mode! Diode pumped solid state lasers take advantage of this effect to generate a series of very short pulses with very consistent energy between pulses and a rate determine by the pump input.

    One way to determine the pulse shape or pattern would be to fire the focused laser beam at a rotating disk with a piece of black paper or carbon paper glued to its front surface. The shape of the burn mark or pattern of spots should reveal whether it is lasing CW for the duration of the input pulse or pulsing at a regular rate as would be expected if the Q-switch were active the entire time. A 75 mm diameter disk rotating at 3,600 rpm would result in a linear velocity of about 1.4 mm/100 us for this laser oscilloscope. :)

    (From: Shawn.)

    I noticed that my divergence is significantly greater with the long pulse (2.5 ms) versus the short pulse (approximately 400 us). Do you have any thoughts on why this could be happening? How much more energy do you think I could get out if I removed the Q-switch?

    When I was using the short pulse PFN I could discolor a black piece of cardboard about 2.5 feet away with the spot size only growing slightly (perhaps a few mm in diameter). However, with the long pulse PFN, I placed a piece of black cardboard about 3 inches from the output coupler (and hit it) and then moved it back 4 inches (about 7 inches from the output coupler) and the diameter grew by about 2 mm. At about 1 foot from the output coupler I can't discolor the black cardboard with the long pulse PFN.

    (From: Sam.)

    That's interesting and could indicate that the dye does remain bleached after the initial pulse. Or, the dye bleaches from the center out which would restrict the area of lasing when Q-switched.

    (From: Shawn.)

    Are you thinking that if the dye bleaches from the center out in combination with the applied pulse duration, then the Q-switch will effectively clip the higher order modes letting only TEM00 to oscillate. However, with a long pulse, the dye possibly remains bleached over the whole rod diameter which permits the higher order modes to oscillate creating the high divergence. Maybe I should pull the Q-switch and insert an aperture into the cavity to clip the higher order modes?

    (From: Sam.)

    As far as total energy, if the Q-switch is not participating after the initial pulse, than it won't make much difference. However, if the dye bleaches and recovers quickly, then perhaps it could be significant.

    (From: Shawn.)

    I use a cheap 660nm laser pointer to bore sight the laser. When I get the laser pointer lined up I can see the "orbit" reflections that seem to surround the fundamental spot. However I thought with a plano-plano cavity the reflected spots tend to follow a line from the fundamental or follow a slight curve (i.e., not surround the fundamental spot). Could this cavity be a near hemispherical or a plano-plano cavity? If this is a near hemispherical cavity could that explain why the center of the q-switch would bleach first?

    (From: Sam.)

    I thought it was a plano-plano cavity but didn't check carefully. Just look at the reflections from the optics of something distant and see if they look flat. :)

    Shining a laser pointer into it you also have reflections from the rod ends and the Q-switch to confuse things. I'll have to check...

    I just went and used a HeNe laser reflected off the mirrors with a piece of paper to block the reflections from the rod ends and Q-switch (so they wouldn't confuse things). The mirrors appear to be planar as far as I can tell but this still isn't conclusive since I was just kind of holding the thing steady and trying to view the reflected spots.

    It does look as if the rod ends and/or Q-switch is ground on a slight angle because without the paper, there is a distinct far off-axis spot.

    (From: Shawn.)

    I noticed that far off axis spot too when I'ms bore sighting it with the laser pointer. Do you think it would be worth it to put an aperture in the cavity and how big of an aperture do you think would be good to use? What is confusing me is that the output of the side of the rod closest to the flashlamp seems to put out more energy and I am trying to envision the optimal location for the aperture (i.e., should the aperture be placed off centerline toward the flashlamp side).

    (From: Sam.)

    The fact that you get more energy off-center suggests (at least to me) that the cavity is indeed planar. A cavity with curved mirrors would tend to homogenize the distribution I would think.

    What are you hoping to accomplish with an aperture? Obtain a TEM00 beam? That may not be possible from such a short cavity. There's a magic number for a given cavity configuration to determine if a TEM00 beam will be produced (sorry, I don't have the equation or the value for this laser) but I bet it would require a rather narrow beam.

    (From: Shawn.)

    I was just hoping/dreaming to be able to project the unmanipulated beam further. I think you are right again about the planar cavity. A near hemispherical cavity should have more energy in the center.

    (From: Sam.)

    Well, you can still expand/collimate it and that will help but if you were after HeNe-like beam quality, not likely. :)

    (From: Shawn.)

    I fixed my divergence problem. I remember when I got the laser, I illuminated the bore and noticed a slight star-burst pattern that seem to be coming from the Q-switch. Yesterday, I noticed the star-burst getting more pronounced. I guess my higher energy pulse must have aggravated the existing imperfection. So, I removed the Q-switch. My divergence problem has gone away. I'ms assuming that the imperfection in the Q-switch was dampening the oscillations in the center of the laser rod. The beam now grows about 0.1 to 0.15 inches in diameter over a 3 foot distance.

    Before, when I charged my capacitor up to 700 volts (about 275 joules) I could only put about a 0.020 inch diameter hole in a 0.004 inch thick razor blade. Now, without the Q-switch I can put a 0.033 inch diameter hole through the same razor blade. If you just ratio the changes in volume the output energy has increased by over 2.5 times.

    (From: Sam.)

    Yes, I've heard that the dye based passive Q-switch is one of the items that fails most often (the other being the flashlamp). So, it may have been slightly bad to begin with but your super power pulses might have really done it in!

    For those who haven't yet begun to abuse SSY1, it is probably best to remove the Q-switch dye cell before attempting to run at much higher energy input than the 15 J max of PFN1. To do this, detach the rod/flashlamp assembly from the resonator frame (make a note of the direction in which it is installed). At one end you can see an AR coated end of the YAG rod (I think there is a screw at that end which holds the rod in place). At the other end is the Q-switch dye cell (slightly larger diameter than the rod) which is held in secured with some tan or brown adhesive which has to be removed to free it. There is a tiny fill hole where some adhesive was forced in on the side - using a drill bit in your hand to remove what's in there may also be needed. Take care to avoid scratching or breaking the dye cell - you may want to replace it at some point in the future (and that dye cell originally cost something like $200!).

    Without the Q-switch, the output will not be as short a pulse but may actually result in more total energy (though less peak power).

    (Several months pass.)

    I have now built everything into a portable self contained unit (including the laser pointer target designator) that could operate from a 12 VDC source. A pushbutton must be held in to charge the caps but there is an overvoltage cutoff to prevent accidental overcharging. There is an LCD readout for capacitor voltage. Of course, the most important part of this rig is my pair of 1,064 nm laser safety goggles!

    I've fired well over 2,000 shots with my SSY1 setup and there appears to be no decrease in output power (based on the diameter of hole through a razor blade). The Q-switch has long since died and was removed about 2,000 shots ago. :) My max pulse rate is about 1 shot every 45 seconds. EG&G says that I am driving the flashlamp properly. I bought a couple extra flashlamps just in case.

    I've made a sort of hodgepodge laser power meter. I sliced a piece of carbon from a carbon zinc battery anode. The slice is 0.239" diameter (6.071 mm) by 0.065" thick (1.651 mm). I epoxied a thin piece of plastic to the back of the carbon disk to act as an electrical insulator for a Fluke k-thermocouple junction. The thermocouple junction was epoxied perpendicular to the flat surface of the disk. I used an 805 nm laser diode to "calibrate" the disk. The laser diode is calibrated. I set the laser diode to put out 1 watt. I put the carbon disk in front of the laser diode aperture and turned on the laser for different durations as measured by an oscilloscope. I took several measurements while measuring the delta T and time duration for each exposure to the laser diode. Approximately 2 minutes elapsed between each measurement. My data is shown below:

        Test   Tinitial   Tfinal    Delta T   Pulse Duration   MC calculated
          #    (Deg C)    (Deg C)   (Deg C)     (seconds)      (Joules / C)
       ----------------------------------------------------------------------
          1      23.8       30.0       6.2         1.56            0.252
          2      24.1       31.2       7.1         1.67            0.235
          3      24.2       27.8       3.6         0.92            0.256
          4      23.8       28.2       4.4         1.11            0.252
          5      23.7       26.1       2.4         0.58            0.242
          6      23.5       34.1      10.6         2.50            0.236
    
    Energy into the sensor in Joules = time duration in seconds since the power input is 1 W. The average MC comes out to be 0.246 J per Deg C.

    It took about 10 seconds for the temperature to stabilize. I guess that the thermocouple wires were not bleeding away the heat too fast.

    I charged up the capacitor for the SSY1 to different voltages and fired it into the sensor which was about 1 foot away. I have a laser pointer with a cross hair diffractive lens that bore sights the laser and is aligned to perhaps 1 to 2 mm. The following are the test results:

      Vcap   Tinitial  Tfinal Delta T Calc Eout    Flashlamp Energy   Efficiency
     (Volts)  (Deg C) (Deg C) (Deg C) (Joules)  (Joules, from Pspice)     (%)
    -----------------------------------------------------------------------------
       350     24.4     27.0    2.6     0.64           57.1               1.1
       400     23.7     28.3    4.6     1.13           73.6               1.5
       450     23.9     29.9    6.0     1.48*          91.9               1.6
       500     23.9     31.7    7.8     1.92*          112.0              1.7
       500     24.0     31.3    7.3     1.80*          112.0              1.6
       550     24.0     32.2    8.2     2.02*          133.8              1.5
       600     23.8     33.6    9.8     2.41*          157.3              1.5
    
    * Smoke came from the sensor during these measurements!

    The flashlamp energy was calculated by the Pspice simulation. The following are some of the things that were not considered in the measurements:

    1. I'ms not sure if the entire SSY1 output beam was hitting the carbon disk a foot away. The disk is about 6mm in diameter and the beam at the output coupler is about 4mm.20

    2. When smoke came from the disk (as indicated by a * above), I'ms not sure how much energy was actually being lost due to vaporizing some of the surface of the sensor.

    3. I'ms just guessing that the absorption of the 805 nm laser diode is about the same as the 1.06 um SSY1.
    I'ms looking for a larger diameter piece of carbon so that I can expand the beam without vaporizing spots on the surface.

    (From: Sam.)

    Cut, file, or grind one of your carbon rods to create some slices length-wise. Sand them smooth and butt the long edges together to form a larger surface area. Yes, I know this will be messy!

    You're getting me interested in trying this stunt. I have a pair of 1,800 uF, 450 V computer grade electrolytic caps. Yes, I know, not laser caps, but at with relatively discharge pulse, might survive. With the caps in series, at 800 V, they would provide about 288 J; at 900 V, about 360 J. Or, better yet, I should run them in parallel which would be slightly less efficient but would eliminate any issues of voltage balancing, reduce the stress on the flashlamp, and the air-core inductor would only need to be about 200 uH. I have plenty of thick wire to wind it.

    I would remove the Q-switch before the first shot so that it would live to pulse another day. :) I also have some other mirrors with cosmetic defects which I might substitute as well. The same capacitor charger I used originally with SSY1 would work fine here though I might have to beef up the current limiting resistor's wattage a bit. :)

    As I mentioned, the air core inductor I used was from parts express. It was about 2.5 inches in diameter and about 2 inches long. It was wound with copper foil 2 inches wide and used insulation between each layer. However, here is a formula for the inductance of a coil whose length is greater than 0.4 times its diameter:

                                            d2 * t2
              L (Inductance in uH) = ---------------------
                                      (18 * d) + (40 * b)
    
    Where: So, here are some options for 820 uH: You can see why the inductor from parts express was so attractive.

    (From: Sam.)

    Nah, that's cheating. :) I found a 3 inch diameter form during a walk in the park - from a Hallmark(tm) party ribbon or something - perfect. Extrapolating from the tables above, a 200 uH inductor would require about 50 turns. I actually wound 55 turns in 5 layers using #14 insulated solid building wire. This isn't exactly magnet wire but the insulation is still rather thin so it packs nicely. The 55 turns should yield a bit more inductance - perhaps 250 uH - resulting in a slightly longer pulse. So much the better - it will be easier on the flashlamp.

    I located the pair of 1,800 uF, 450 V caps and confirmed that their ESR is still unmeasurable (0.0 ohms) but I will probably need to reform them since they are quite old. I even have a preliminary power supply design. See the section: Sam's High Energy AC Line Power Supply for SSY1 (SG-SP3) and stay tuned for exciting developments.

    Microsim Pspice Program for SSY1

    (From: Shawn West (west007@libcom.com).)

    I put together a Microsim Pspice simulation that accurately models the flashtube characteristics (with a given Ko) that agrees with measured results.

    Based on the simulation, the amount of energy that actually makes it to the flashlamp terminals is about 75% of the capacitor stored energy for my PFN setup. So for my previous % of explosion energy numbers you can multiply by 0.75 to get the real % explosion values. So, for worst case (800 volt = 360 joules stored on the capacitor) only about 270 joules make it to the flashlamp which gives a % explosion energy of 270 / 590 = 45% rather than the theoretical maximum of 60% as previously stated.

    The Microsim Pspice files (ASCII text) for the flashtube follow. You can change Rctrl from 1u to put the reverse diodes in the circuit or a 1M resistor to take the diodes out to see if you would be getting any negative ringing current. Resr is the ESR for the capacitor and Rind is the resistance of the inductor. You can set the capacitance, inductance, Ko, and the initial capacitor's voltage in the PARAMETERS box. You can use Rsense to display the flashtube current. Vtube is the voltage across the flashtube. The energy line integrates the tube voltage x tube current to arrive at the energy that makes it to the flashtube to gauge the efficiency of your circuit. For the energy line 1 volt equals 1 joule. The key for proper simulation is to know the proper C, L, Rind, and especially Resr.

    See the OrCad/PSpice Web Site for info - there may be a demo version of Pspice which would have enough capability to run this simulation.

    Other SSY1 Tidbits

    (From: Wayne Verish (wverish@aol.com).)

    Just when I thought I had run out of things to point my little Yag laser at I decided to try a tuft of steel wool (no soap please!). The result was surprising! With the voltage cranked up to 900 volts, and the output focused through a simple hand lens the shot ignited a small portion of the steel wool, which then rapidly proceeded to consume the entire pad! This will be interesting to capture on video or digital camera.

    Tired of smoking carbon paper with your SSY1? Try steel wool if you dare. Also a great way to blast holes in those pesky free CD rom disks you get in the mail!



  • Back to Solid State Lasers Sub-Table of Contents.

    Mini YAG Laser using SSY1 Optics and SG-SP1

    I was given a cute little YAG cavity including a 50 mm long 3 mm diameter Nd:YAG rod with AR coated ends and a flashlamp similar to that used in SSY1. Coupling from the flashlamp to the rod is probably better than in SSY1 - there, a noticeable percentage of the flashlamp light escapes out the sides of the white powder covered non-reflective transparent cavity. This one has a highly polished elliptical reflector.

    So, I built a resonator using some scrap aluminum with a pair of simple adjustable mirror mounts with SSY1 mirrors (from a defunct SSY1 - whereabouts of rod and flashlamp unknown). I mounted a 20 mm focal length focusing lens in front of the OC mirror.

    The original flashlamp was apparently supposed to be triggered using a series pulse technique. I prefer external triggering so I added a fine wire running along the side of the flashlamp for the trigger transformer I use with PFN1. Triggering works great. :)

    Alignment was the real pain with all the weak reflections (HR, OC, and rod, front and rear, and both mirrors were ground with wedge). But, I was able to smoke black electrical tape when focused on the first try using the SSY1 PFN1 at 800 to 900 V (approximately 10 to 15 J input energy). I'ms sure alignment isn't optimal though. The output energy seems similar to that of SSY1, maybe a bit better on black tape (nice wisps of smoke), not quite as good on aluminum foil. Without a Q-switch, the output pulse is longer and of lower peak power which may be the main factor. (The behavior does appear similar to that of SSY1 with its Q-switch removed.) I will really need to have some sort of energy meter though to optimize alignment. This could possibly be done with just a photodiode centered on the optical axis with the beam spread out (to reduce peak power to the photodiode) monitoring the current on a scope.

    Summary of specifications:



  • Back to Solid State Lasers Sub-Table of Contents.

    Quantronix Model 114F-O/QS Quasi-CW YAG Laser Head

    Description and Specifications

    This is a high power liquid cooled flashlamp pumped Nd:YAG laser from the early 1980s (my sample is dated 1981). Although the head looked like it had fought a losing battle with one of those M-1 tanks, the rod, cavity reflector, and optics appear to be in pristine condition. Considering the general condition of this laser and the nasty hole in the cooling chamber lid (that isn't supposed to be there) in particular, the fact that the most delicate components came through totally unscathed must rank as one of the minor miracles of the past Millennium. :) However, the flashlamp appears to be up to air due to overheating and breech of the solder seal (not physical damage). This was confirmed by testing with both a flyback based high voltage generator and HeNe power supply. It was most likely that failure which resulted in the the laser being removed from service. This unit originally was Q-switched - unfortunately the Q-switch had been scavenged - perhaps by the crew of that M-1 tank. :)

    Photos of a Quantronix 114 (in slightly better condition) can be found in the Laser Equipment Gallery (Version 1.71 or higher) under "Quantronix YAG Lasers".

    Here is a general description though specifications are somewhat sparse:

    Chris's Comments on the Quantronics Laser

    (From: Chris Chagaris (pyro@grolen.com).)

    It looks as though you have got the makings of a nice project. A 'bashed up' laser is better than no laser at all. :-) At least the most important components survived. If you could provide me with the number on the arc lamp, perhaps I could uncover what it actually is. Typically a krypton arc lamp of 70 mm arc length and a 5 mm bore (EG&G, FK-125-C2.75) filled to 2 atmospheres would operate at 100 volts at 30 amps. With this typical input power of 3 kW, coolant flow rate should be at least 120 cm3/s.

    The conical and heimspherical electrodes are common. The pointed cathode is to help maintain arc stability.

    There is a similar EG&G Krypton arc lamp (FK-111-C3) which has a 7 mm bore with a 75 mm arc length rated at 6,000 W with liquid cooling. Electrical characteristics are 112 VDC at a whopping 56 A. Wall loading is 145 W/cm2 as opposed to the smaller 5 mm bore lamp of 110 W/cm2. However, average lamp life is only 40 to 60 hours, whereas the FK-125-C2.75 should last from 400 to 600 hours with proper cooling.

    Sam, where's your sense of adventure? :-) I think an attempt to refurbish this laser as an arc lamp-pumped CW type would be fascinating. Consider the cost of a new flashlamp, the likely necessity to install a new OC of a lesser reflectivity for successful pulsed operation, and the need of a PFN, as opposed to the challenge of building a phase-controlled arc lamp power supply. The design and construction of a PSU such as this strikes me as something that would be right up your alley. I have recently acquired a 6 inch arc length, krypton-filled arc lamp and have considered the construction of such a supply myself. Of course, the lamp that I have will require about 40 amps at 150 VDC! I've got a 10 kW isolation transformer. So there's a start. :-)

    Interesting that the OC reflects green. I would tend to agree with you that this laser was not likely doubled. The OC for SHG would normally reflect close to 100% of the fundamental wavelength and transmit about 100% of the harmonic. This being the case, I would doubt such an optic would appear to reflect green.

    (From: Sam.)

    Geez I dislike even working on the power supplies for little air-cooled argon ion lasers with their current-hog requirements let alone 40 A at 150 V!! :)

    It is definitely not a green YAG and I don't even know if intra-cavity doubling had been introduced in those days.

    (From: Chris.)

    As far as the Q-switch is concerned, I would expect that it was not a simple mechanical system like the one on the Hughes MS-60 ruby laser. I would tend to doubt that a rotating prism Q-switch would be used in-line. Usually if a mechanical Q-switch was going to be used in-line, it would be a rotating HR mirror at one end of the resonator. A roof prism is most often the rotating element in such a system because of its retro-reflecting properties, which assures alignment in one direction, while the rotation of the prism brings in alignment in the other direction.

    Mechanical Q-switches tend to be rather slow as compared to electrooptical and acousto-optical Q-switches and judging from the rated pulse width achieved by this laser, I doubt that a mechanical Q-switch would be able to achieve that 50 ns pulse duration.

    Ed's Comments on the Quantronics Laser

    (From: Ed Xavier Gonzalez (ohlaser@flash.net).)

    The power supply/heat exchanger on my 116 requires 208VAC 3 phase to crank the silly thing up. Admittedly, Quantronix did over design the power supply for worldwide use, so the transformers and control circuitry are a bit over-kill. The important point, however, is the fact that a lot of juice gets sucked up generating a clean initial pulse to jump start the krypton lamp and then maintain the 25-35 Amps DC to keep it going. Also, the water for the cooling needs to be kept VERY clean (as you may already know). The micron and de-ionizing filters basically make de-ionized water from store bought steam-distilled, ozonated water. Any particulates in the water stream when the lamp is running is a sure guarantee that the flowtubes and the lamp jacket are going to get coated and cooked!

    Be careful YAG rod assembly. Some of the original flowtubes were uranium doped quartz to stabilize the UV into visible wavelengths. Just a word of caution.

    The endplates you describe as "polished gold plated brass caps" are now gold plated nickel, since brass has a tendency to contaminate the DI-coolant and turn stuff green. Not good for the flowtubes or the lamp and crystal.

    The Q-switch on my 116 is an 25 W RF driven Acousto-Optical model from IntraAction Corp. My guess is the 114 was probably driven the same way.

    Anything in the DI-coolant stream should be nylon or stainless steel. No brass, bronze or anything else. The DI-water will pull "tons" of metal ions out of the fittings and put them into the coolant. Also (and this one is a real stretch), under no circumstances should the DI-water be consumed internally! It would literally take the calcium out of your blood-stream and in enough quantities could kill. Sounds strange, doesn't it: Ultra pure water will kill you! Takes the elemental ions right out of your system, or so I've been told. We'll have to leave that experiment untried!



  • Back to Solid State Lasers Sub-Table of Contents.

    Coherent Model 315M Green DPSS Laser

    General Description of C315M

    This is a very high quality single frequency 100 mW green DPSS laser manufactured by Coherent, Inc.. It is one of their Compass series: Coherent DPSS Laser. The datasheet may be found at: Compass 315M CW Green Laser. Being single frequency (single longitudinal mode), the coherence length should be extremely long and ideal for holography and interferometry. However, unlike the 532-200 described in the next section, it does not use a ring cavity but a more conventional Fabry-Perot type.

    Photos of its construction (and dissection) can be found in the Laser Equipment Gallery (Version 1.86 or higher) under "Coherent Diode Pumped Solid State Lasers".

    These lasers are now showing up on eBay and elsewhere for around $500 but some caution is advised before buying a dozen. In addition to the pump diode, there are three (3) sets of TE coolers (a pair for the pump diode, one for the KTP, and another pair for the overall cavity) that need to be controlled independently for optimum performance. It may be possible to power just the pump diode and its TEC but depending on the particular unit, the output power and stability may be substantially reduced. In the unit for which the photos were taken, it happened that full output power was produced without even bothering to cool the diode (at least for long enough to take the pics - definitely not advised for continuous operation!). However, getting decent output power is not guaranteed without tuning the temperature of the KTP. In fact, there may be little or no green output at all for some samples!

    Also note that this laser uses a small YAG rod (not vanadate) with a seprate HR mirror and a very small KTP crystal. None of these is particularly useful for a home-built DPSS project so buying one of these lasers just to salvage parts is probably ill-advised. In addition, while the pump diode is in a nice package with a GRIN lens on its output, it is not set up for a very small pump beam spot as would be required in a typical home-built green DPSS laser using a (relatively thin) vanadate crystal. The optics (HR and OC) are also matched to the C315m's cavity configuration. Thus, any home-built using these parts would have to retain the cavity design so best to just leave it intact!

    After examining the photos, one may be inclined to believe that these lasers utilize advanced alien technology and would require alien repair people for service. But, without the alien warranty information, we don't know where in the Universe to direct inquiries. :) They are definitely unconventional in a variety of respects - at least compared to the typical 100 mW-class green DPSS laser. As is obvious from the photos, the cavity itself is not even visible without somewhat destructive disassembly. If you have a C315m, I don't advise going inside the case at all but if your curiosity gets the better of you, limit your adventure to removing the top cover in as dust-free an environment as you can find (and only if you are willing to take a significant risk of the laser never working again). The best way to remove the cover is by using a utility knife at one corner carefully prying it up. It is also possible to set the laser on a hot plate and gradually bring up the temperature until the low temperature solder used to seal the top cover to the case melts. However, all internal components are also soldered in place (yes, soldered, not glued or screwed except for the pump diode) and should their solder melt, remounting and realignment of some of them would be virtually impossible without a total tear-down and access to the alien manufacturing line. ;) I have no idea at what temperature relative to the cover the component mounting solder melts - it might even be the same temperature with the cavity TEC driven during the final manufacturing step of installing the cover to prevent everything else from falling to pieces!

    The warmup is similar to other DPSS lasers. Not as bad as others but the same "fluffing and pulsing" of the output as the unit warms through the temp cycle. The C315M using the Coherent, Inc. controller turns on with a nice smooth time delay ramp (0 to maybe 50 mW) then the fluffing/pulsing until it is stable and BRIGHT! :)

    Should you acquire a 315M without the controller module (which itself goes for something like $2,000 new) and want to power it, here is some info on the wiring as determined during the dissection. Use at your own risk!

    Versions of the C315M made after some time in 1997 have what is almost certainly a digital running time meter consisting of a PIC12C508, and 24C021 EEPROM, and (32,768 Hz probably) crystal. Info on the PIC and related parts can be found at: PIC Programmer 2, 16C84, 12C508, etc. Page. Older versions are functionally identical in other respects but lack the time meter. The PIC and EEPROM are powered only when the laser diode is actually being driven, both keeping track of on time and outputting a serial bit stream so the time can be read out.

    The following includes contributions from Bob (no email) Dave (ws407c@aol.com), and Mike Harrison (mike@whitewing.co.uk).

    Coherent 315M Internal Connector Pinout

    Note: Pinout now corrected based on marking on cable. Pin 1 is at right near large amber LED. I am still not sure if this matches Coherent documentation but for now, it's the official labeling!

                                   Connection            Signal Direction
      Pin   Internal Function       X for yes  PCB Function   <- or ->  Controller
     ------------------------------------------------------------------------------
       1                                   LD Current Control    <-
       2  Jumpered to pin 6                LD Current/Limit      ->
       3  LD anode (case, +)            X  Protection/LED enable <- LD+ drive
       4  LD cathode (-)                X                        <- LD+ drive
       5  LD thermistor                 X  10k pullup to +5      -> LD temp sense
    
       6  Jumpered to pin 2                LD Temp. Setpoint     -> LD temp ref.
       7  Temperature sensor common        RES Temp. Setpoint    -> RES temp ref.
       8  RES thermistor                X  10k pullup to +5      -> RES temp sense
       9  Lower LD TEC+                 X                        <- L LD TEC+ drive
      10  Lower LD TEC- (16 ohms 9-10)  X                        <- L LD TEC- ret.
    
      11  Factory use only, heater under 4th stop
      12  Upper LD TEC+                 X                        <- U LD TEC+ drive
      13  Upper LD TEC- (16 ohms 12-13) X                        <- U LD TEC- ret.
      14                                   KTP Temp. Setpoint    -> KTP temp ref.
      15  KTP TEC+                      X                        <- KTP TEC+ drive
    
      16  KTP thermistor                X  10k pullup to +5      -> KTP temp sense
      17  KTP TEC- (1 ohm 14-16)        X                        <- KTP TEC- ret.
      18  RES TEC+                      X                        <- RES TEC+ drive
      19  RES TEC- (30 ohms 18-19)      X                        <- RES TEC- ret.
      20                                   LEDs Return           -> LED control
    
      21  Factory use only, heater ???     LED Power             <- DC input
      22  BP thermistor                 X                        -> BP temp sense
      23  Common                        X  Temp sensors, setpoint circuitry, PD.
      24  PD Anode                      X  Output Power Setpoint -> Power sense
      25  PD Cathode                    X  +5 for pullups, setpoint circuitry, PD.
    
      26  Factory use only, heater under 3rd stop.
      27  Factory use only, heater under OC mirror.
      28  Factory use only, heater common.
      29  Factory use only, heater under 2nd turn mirror.
      30  Factory use only, heater under 1st turn mirror.
    

    Schematic of C315M Laser Head PCB (Common Wiring) shows the circuitry associated with laser operation and A HREF="315cct1.pdf">Schematic of C315M Digital Running Time Meter shows the circuitry only present on newer versions of the PCB. The two PCBs are shown in Older Version C315M Laser Head PCB (Note: PCB in photo is rotated 180 degrees from normal orientation) and Newer Version C315M Laser Head PCB. There may be minor differences in component values depending on PCB revision. It wouldn't surprise me if some resistors are select-on-test. It would be useful to compare values on a few units. The letters on the presets seem mostly fairly obvious but some may be German as Germany is where these are manufactured:

    The LD, KTP, and RES temperature sensor pullups are on the PCB. It is almost certain that the set-points are where the output voltage of each temperature sensor with pullup is equal to the output voltage of the corresponding adjustment. So, if we know the behavior of the thermistors, we can predict the correct temperatures for each components. Even if we don't, the settings will enable the correct temperatures to be maintained either manually by comparing the adjustment and sensor output, or with a closed-loop controller. Based on experience with the Coherent 532, the temperature for the sense or setpoint will be approximately: 20*(2.5-V)+25 °C.

    Pads U and O - may be "Under" and "Over" for KTP temp - adapting to cases where the optimal KTP temperature is outside the standard range?

    Description and measurements for each of the laser head signals

    Voltages in [] correspond to measured values after 10 minute warmup for a C315M that meets specs in an ambient temperature of about 68 °F with a small fan keeping the heatsink cool (24 V mini fan running at 12 V). Voltages may be quite erratic during warmup. All voltages are positive with respect to pin 23 (Common) unless otherwise noted (for individual components).

    Additional notes

    Pinout of DB25 Relative to Laser Head on Controller

    This is needed if a cable needs to be constructed to the Coherent power unit.

    (View looking toward connector):

               1   2   3   4   5   6   7   8   9  10  11  12  13
               o   o   o   o   o   o   o   o   o   o   o   o   o
    
                 o   o   o   o   o   o   o   o   o   o   o   o
                14  15  16  17  18  19  20  21  22  23  24  25
    

    Factory Alignment of the C315M

    All components inside the laser head that must be precisely aligned are either mounted via low temperature solder directly to a large ceramic plate, or soldered to a raised ceramic platform which is itself soldered to the large ceramic plate. Resistance heaters under each component enable its solder to be selectively melted to enable precise positioning, then frozen in place almost instantly when current is removed. Access to the terminals for each heater are via a combination of edge pins (most of which don't exit the laser head) and/or contact strips which may be probed during the manufacturing process.

    In the factory during final assembly, there were probably a forest of computer controlled multi-axis positioners and current driver probes for adjusting and tweaking alignment.

    CAUTION: Under no circumstances should significant current be applied between any pins or pads associated with the solder melt heaters. Alignment will be lost forever!

    Measurements of TEC I/V Characterstics

    The following tests were done with a bench supply and are very approximate since the I/V relationship may change by 10 percent or more depending on actual TEC temperature and this changes while attempting to take readings!:

          TEC       - Pins +   0.25 A   0.5 A    1.0 A    1.5 A    2.0 A
     --------------------------------------------------------------------
         Cavity     18 to 19      3 V   4.7 V      8 V     10 V      -
          KTP       15 to 17    0.6 V   1.2 V      -        -        -
        Upper LD    12 to 13      -     1.2 V    2.5 V    3.5 V      5 V
        Lower LD     9 to 10    2.8 V   4.7 V    7.5 V     12 V
    

    In all cases, heating the specified item was accomplished with the polarity shown because it was easier to check for cooling on the bottom plate. (I couldn't confirm polarity for the KTP - too small - but based on the arrangement of pads on the TEC, should be the same.) The fact that some voltages are equal at the same current is just a coincidence.

    Using the Coherent Controller

    (From: Dave (ws407c@aol.com).)

    For those lucky enough to get or have a Coherent power unit, here are the connections on the 15 pin connector as you are looking at it on the controller unit: Controller Wiring for Lasing.

    The Coherent PDF data sheet says "output power is variable via analog controller or optional digital controller". I am unable to determine the difference and *think* the controller that I have is the digital one. My serial number ends with a B and could be a clue.

    Please use these jumpers with caution.

    Here is a diagram of how the 15 pin connector connects to a PC board that came with the unit. This PC board has a RJ-45 connector and also controls the AOM unit: Partial Schematic of Controller Internal Wiring.

    There seems to be nothing "analog" about the input on my controller unit.

    The lone AND gate seems to be a "warmed up" indication and a HIGH is sent to it upon warmup. The other 4 AND gates in a group are unknown and could possibly be fault flags or feed back's to the manipulations on the 2 TRI-state inputs driven by the 26C32A.

    The DATA pin as I call it sends pulses in unison with some of the fluttering during warmup and holds HIGH after warmup. This pin terminates without connection on the PC board.

    It seems that power is changed when the 2 inputs are manipulated before lase switch is switched on.

    An interesting note: When the laser is switched to standby mode it continuously sends out in easily visible data packets, the total hours (most likely) via the IR led on the back of the head. This can easily be seen with the aid of an IR sensor card or IR detector circuit. It seems if this repeating data stream is recorded then run the laser for a specific amount of time the code could be figured out. :)

    Powering the C315M with a Third-Party or Home-Built Controller

    To properly drive this laser head without the Coherent power supply, the following will be required. (The "basic version" is open loop for power output and may have less long term stability than the "optimized version".):

    1. A constant current laser diode driver for the pump diode with an output of about 1.5 A maximum current will suffice for the basic version. For the optimized version, modulation capability will be needed to handle optical feedback.

    2. A controller for the pump diode TECs for cooling and temperature tuning of its wavelength. Hopefully, a single controller will suffice for the upper and lower TECs in series. A controller that maintains a constant temperature using thermistor sensor feedback will suffice for both versions.

    3. A controller for the KTP TEC which will adjust its temperature to maintain peak output power/minimize pump diode current. A controller that maintains a constant temperature using thermistor sensor feedback will suffice for the basic version. Modulation capability (can be done via the sensor input) may be needed for the optimized version.

      Since the KTP TEC is very small, a simple op-amp circuit can be used here. A suitable circuit is shown in Low Power TEC Controller. This circuit is derived from the design used in the Coherent Compass 532 laser (see the next section). I have made minor simplifications but retained the original 1% resistor values - the nearest 5% values should be just fine. R1, R2, and R3 can be eliminated if the KTP Temperature Setpoint output on the laser head PCB is used. Even this circuit is probably somewhat more complex than necessary but the total cost should be under $10 even if you lost the keys to your junkbox. :) The "Offset" input may be useful later when power optimization is implemented. Note that this circuit is only suitable in its current form for very small TECs - typically these are less than 1 cm square. However, if wasted power isn't an important consideration, a pair of power buffers can easily be added to drive larger TECs. CAUTION: Circuit copied quickly - errors are possible! Use at your own risk.

      CAUTION: Make sure that any driver circuit limits average energy into the TEC particularly in the direction which results in heating of the low (thermal) mass KTP to avoid damage to it or even an unsightly melt-down. The circuit, above, has protection for this but other power sources including expensive commercial ones must be set up to stay within safe limits (which may be more conservative than necessary if just based on a maximum current).

    4. A controller for the cavity TECs which is also getting rid of the small amount of waste heat from the KTP TEC. A circuit that maintains a constant temperature using thermistor sensor feedback will suffice for both versions. Where a good heatsink is used and ambient temperature is modest (e.g., 20 to 25 °C), this shouldn't require very much power. In fact, just providing a constant current may be adequate if the temperature of the baseplate is nearly at ambient.

    5. A large heatsink with enough forced-air cooling to maintain the temperature of the laser baseplate near ambient. Note that good heatsinking is needed not only to allow for proper heat dissipation but to avoid major interactions between the control loops of the two large sets of TECs, which may lead to instability.

    A professional implementation of the basic version of this system could always be put together for under $500 using off-the-shelf modules from Wavelength Electronics. The WLD-3343 laser diode driver and either the WTC-3243 or WHY-5640 temperature controller would be suitable and cost under $150 each in single quantities. Using one of these modules (or an equivalent from another well-known companie) would be a good investment for at least the pump diode which is very easy to destroy. I would think twice about using cheapie laser diode drivers for use with this expensive laser. They may have little or no protection and tend to fail shorted. TECs are much tougher to damage than laser diodes and with care, any decent commercial or home-built controller, or even a simple constant current or constant voltage supply, may be adequate at least for testing.

    For the optimized version, feedback will be required to control pump diode current and fine adjustment of KTP temperature. LD and cavity TECs should still run in constant temperature mode. In the Coherent Compass 532 laser (see the next section), KTP temperature is controlled by a secondary feedback loop to peak output power and pump diode current is maintained at a level which provides the spec'd output power. We don't know whether the C315M has this level of sophistication or just uses optical feedback to regulate pump diode current but runs the TECs in constant temperature mode.

    Powering the C315M for Testing 1

    It should be possible to safely power the C315M laser head an approach optimal conditions but everything will be done manually. While there may be little correlation among samples of the C315M particularly for pump diode and KTP temperature, starting with the values listed in the section: Coherent 315M Internal Connector Pinout won't hurt. Just realize that differences can be quite large - even the difference between heating and cooling of the pump diode or KTP.

    This procedure may take awhile to converge but doesn't require knowing the original factory settings on the laser head PCB (which doesn't even need to be present). A faster procedure which takes advantage of this information is provided in the next section.

    1. Find a nice large heatsink, wipe some thermal grease on your 315M laser head, and smush the C315M onto the heatsink. A fan is desirable to maintain heatsink temperature as constant as possible.

    2. Hook up 4 adjustable power supplies - 1 each for the upper and lower pump TECs, the KTP TEC, and the cavity TEC. I expect that the upper and lower pump TECs can be wired in series (with possibly some loss in efficiency) and driven from a single power supply. Put enough resistance in series with the KTP TEC to limit maximum current to about 100 mA.

    3. Fire up (not literally!) the pump diode using a proper laser diode driver and increase current until there is some green light but not to more than 1.5 A. DOUBLE CHECK polarity before applying power to the pump diode!!!!

    4. Slowly increase current to the upper and lower diode TECs in the cooling direction (positive to the right-hand pins). Increments of 100 mA are probably reasonable. Or, start at 0.929 V and 2.268 V across the respective TEC and let it stabilize for several minutes. Then, adjust from there. Wait a minute or two between each increment for the temperature to stabilize. If the intensity of the green light decreases, you're probably cooling when you should be heating or vice-versa. Locate the point where green is maximum.

    5. Do the same with the KTP TEC either starting from 0 V or using the intial value of about 200 mV. CAUTION: It's possible to destroy this TEC along with the KTP crystal and any hope of ever getting the laser operational again, especially with excessive heat. Limit current voltage to 200 mV or current to 100 mA to be safe. As above, adjust for maximum green light. If the intensity of the green light decreases, you're probably cooling when you should be heating or vice-versa. The KTP TEC thermal response should be quite rapid.

    6. Set the voltage on the cavity TEC to around 1.165 VDC which is the value Dave found for the unit he tested. (Don't worry about being exact.) This will interact with the KTP TEC setting and is just a starting point.

    7. Increase pump diode current to produce more green light if desired but don't exceed about 1.8 A through the diode or 2.8 V across the diode since we don't know what the diode's maximum ratings are.

    8. Then go back and re-tweak the diode TECs. You will need to keep re-tweaking everything for maximum green and to keep the diode safe.

    9. You will need to KEEP AN EYE on diode voltage and current while you are running this without automatic control (but hopefully, you are using a diode driver that will do this for you!).

    Powering the C315M for Testing 2

    The following procedure should converge on the proper settings for close to optimal power output quite quickly because it uses the factory settings for the temperatures of the pump diode, KTP, and cavity. However, the laser head PCB must be present with all adjustments at their original factory settings.

    In addition to a proper laser diode driver, three adjustable power supplies for the TECs (0 to 3 VDC at 1 A or so for the diode and cavity TECs; 0 to 500 mV at 100 mA for the KTP TEC) and a low current 5 VDC supply for the laser head PCB will be required.

    A DMM or VOM will be needed to check factory settings and monitor temperature sensors and laser diode current.

    1. Wire up a cable for the laser head. Connections will be required for the pump diode (2 wires), all the TECs (8 wires), temperature sensors (3 wires), the photodiode output voltage (1 wire), and the +5 and common (2 wires). Including power to the LED with its return connected to common is also a good idea so the LED will come on whenever the system is powered. Also including connections for each of the setpoint pots (5 wires) and the LD Current Control (1 wire) will make making measurements easier.

      Connect a 10k ohm resistor between the +5 V input and the RES Temp Sense pin (this signal does not have a built in pullup).

    2. Apply +5 VDC between pins 25 (+5) and 23 (Common) and measure the outputs of each of the temperature setpoint pots with respect to pin 23. Record these values. A voltage greater than 2.5 V means that the specified component needs to be cooled relative to 25 °C (positive current into/voltage on the TEC+ pin); a voltage less than 2.5 V means it needs to be heated (negative current into/voltage on the TEC+ pin). Based on the thermistors used in the Coherent 532 laser, the temperature sensitivity of the Temp Sense outputs would be about -50 mV/°C near 2.5 V (but these things aren't perfectly linear!). Thus, a decrease of 100 mV represents a change of +2 °C. However, I do not know for sure that the same thermistors are used for the C315M.

    3. Find a nice large heatsink, wipe some thermal grease on your 315M laser head, and screw the C315M to the heatsink. A fan is desirable to maintain heatsink temperature as constant as possible. Although some of the photos show a heatsink the same size as the laser head, one that is much larger will be better especially if airflow is limited.

    4. Attach an adjustable power supply to the pump diode TECs (suggest 0 to 5 V, 1 A max). The polarity of the power supply is determined by the setpoint value as described above. My feeling is that the upper and lower diode TECs can be connected in series. This may result in some loss of efficiency but will probably work. This is the most critical of the TECs since it needs to keep the pump diode happy as well as wavelength tuning to around 808 nm to maximize absorption by the YAG crystal.

    5. Attach an adjustable power supply to the KTP TEC (suggest 0 to 500 mV, 100 mA max or more easily, 0 to 5 V with a 50 ohm, 2 W current limiting resistor in series). CAUTION: Excessive current into the KTP TEC, particularly in the heating direction, can ruin it. Don't neglect the current limiting!

    6. Attach an adjustable power supply to the RES (cavity) TEC (suggest 0 to 5 V, 1 A max).

    7. While monitoring the voltage between the LD Temp Sense and LD Temp Setpoint pins, very slowly increase voltage to the LD TECs. As the LD heats or cools as appropriate, the voltage difference should decrease if your polarity is correct. "Very slowly" means perhaps 100 mV, wait 2 minutes, then 200 mV, wait another 2 minutes, and so forth. There is a significant amount of thermal inertia in the LD TECs - be patient. Once the voltage difference between the sense and setpoint pins is fairly small (perhaps 10 mV), continue on with the procedure.

    8. Using your proper laser diode driver, power up the pump diode until there is some green light but not to more than 1.5 A for now. DOUBLE CHECK polarity before applying power to the pump diode!!!! If there is no green output even at 1.5 A, don't panic yet. Go back to the previous step and readjust the LD TEC voltage - the additional power dissipation of the pump diode will have changed its required setting. Even if there is still no green output when you get back to this step, there is still no need to panic, the KTP temperature may be way off.

    9. While monitoring the voltage between the KTP Temp Sense and the KTP Temp Setpoint pins, slowly increase voltage to the KTP TEC. As the KTP heats or cools as appropriate, the voltage difference should decrease if your polarity is correct. "Slowly" means perhaps 10 mV, wait 10 seconds, then 20 mV, wait another 10 seconds, and so forth. There is some amount of thermal inertia in the KTP TEC but not nearly as much as the LD. Once the voltage difference between the sense and setpoint pins is fairly small (perhaps 10 mV), continue on with the procedure.

    10. There should definitely be some green light at this point. If there isn't even at 1.5 A of laser diode drive current, something is wrong - power down and determine what it is.

    11. While monitoring the voltage between the RES Temp Sense and RES Temp Setpoint pins, very slowly increase voltage to the RES TECs. As the cavity heats or cools as appropriate, the voltage difference should decrease if your polarity is correct. "Very slowly" means perhaps 100 mV, wait 2 minutes, then 200 mV, wait another 2 minutes, and so forth. There is a significant amount of thermal inertia in the RES TECs - be patient. Once the voltage difference between the sense and setpoint pins is fairly small (perhaps 10 mV), continue on with the procedure.

    12. Now go back and tweak each of the three TEC voltages to reduce the difference between the associated sense and setpoint voltages. Once the LD and KTP temperatures have stabilized close to these values, conditions should be near optimal. The RES temperature isn't nearly as critical but will interact with the other two.

    The next set of steps will attempt to maximize output power within safe limits.

    1. With +5 VDC applied to the PCB, also apply +5 VDC to the LD Current Control (pin 1) and measure the voltage on pin 2. We *believe* that this value is the pump diode current limit setting. It will probably be in the 2 to 2.5 V range meaning 2 to 2.5 A. DO NOT EXCEED THIS VALUE of current to the pump diode under any circumstances, even for 1 nanosecond! In fact, I would not recommend exceeding 2 A on any unit until we know more about the ratings of the pump diode.

    2. Measure the voltage on the Photodiode Anode (pin 24, Vpd). The actual output power in mW is equal to (Vpd/2.5)*100 assuming the PD pot (P6) is at the original factory setting.

    3. After confirming that the three TECs are maintaining their temperatures near the optimum values, *slowly* increase pump diode current while measuring Vpd. On a typical C315M, the rated output occurs at around 1.8 A of pump diode current. If your laser achieves this, congratulations! If it isn't even close, go back and attempt to figure out what's wrong. Realize that some of these are used lasers and may not be able to achieve rated output under any conditions. Even if your laser is able to run at full power, there is nothing wrong with running it at reduced power as this will greatly extend pump diode life. However, the factory temperature setpoints will no longer be quite correct.



  • Back to Solid State Lasers Sub-Table of Contents.

    Coherent Model 532-200 Green DPSS Laser

    The 532-200 is a very high quality (and very expensive, about $38,000 new!) green diode pumped solid state laser with an output power of up to 200 mW. It is a single frequency (single longitudinal mode) laser with a coherence length of greater than 150 meters! It uses a Nd:YAG rod in a small unidirectional ring cavity. Photos of this laser and the interior of the cavity can be found in the Laser Equipment Gallery (Version 1.86 or higher) under "Coherent Diode Pumped Solid State Lasers".

    Here are the specifications (from the user manual):

    (High power versions are those with a maximum rated output power of 200 mW or more.)

    The following chart lists the signals on the external HD15 pin interface connector. The most important signals for confirming proper controller operation are LDI, and LD and KTP Temp. If these values agree with those printed on the cavity sticker but output is low or non-existent, the problem is likely with the pump diode, crystals, or optics inside the cavity.

     Pin       Function                  Description
    ----------------------------------------------------------------------------
      1        Interlock Return          Jumper 1 to 2
      2        Interlock
      3        EO
      4        LD Temp                   LD Temperature (°C) = V * 20 + 25
      5        Analog Ground
      6        Ground
      7        CDRH 5 VB Supply
      8        Alighment Mode            (Not implemented)
      9        Fan On (to pin 5)
     10        LTPWR-
     11        KTP Temp                  KTP Temperature (°C) = V * 20 + 25
     12        LDI                       LD Current, 1 V/A
     13        LDIM                      LD Max Current, 1 V/A
     14        Output Adjust/LDI Fault
     15        Power Meter/DPSS OFF/Interlock Fault
    



  • Back to Solid State Lasers Sub-Table of Contents.

    The Transverse Green DPSS Laser

    These are among the least expensive medium power green DPSS lasers available. However, the low cost (under $1,000) comes at a price (no pun...). The heart of the laser - the pump diode and vanadate - are glued together making any repair or modifications difficult if not impossible. More fundamentally, there is no beam shaping of the pump diode output so the output beam may not have a nice circular Gaussian profile even if it is TEM00. See: Transverse Industries Co., Ltd Laser Products Page for complete specifications of their TIM622 series of green DPSS lasers (as well as other laser gadgetry).

    I consider the laser described in the sections starting with: Reconstruction of an 80 mW Green DPSSFD Laser to be of a more sophisticated design, even if it does have all sorts of quality and manufacturing problems.

    (From: Mike Poulton (tjpoulton@aol.com).)

    I just took delivery of my newest laser - a 50 mW 532 nm DPSS module, Transverse model TIM622. They have several models available at other power ratings. I got it from a guy named Scott Smith, of PWS in Fresno, CA (scott93727@aol.com). He imports them from Transverse Technologies in Taiwan. These are not the highest quality lasers in the world, but they are a very good deal at $995. The rated specs are: 50 mW, 532 nm, less than 2 mR divergence, 100:1 polarization, TEM00, and 2,000 hour life. That last one is what gets me - they should last a lot longer than 2,000 hours. However, they are warranted for 2,000, so they will probably go for awhile after that. It came with complete test data sheets, indicating that it greatly exceeds these specification - 70 mW, 0.3 mR divergence. The pump diode is rated 2 W and is being run at 1.1 W, so it really should keep going for awhile. The beam profile is a bit sketchy - it's sort of a slightly skewed Gaussian, but hey - it's pretty good, and it should do holography with no problem. I don't think you'll find 50+ mW of green light (especially not at such high apparent intensity - 532 is really bright), guaranteed for at least 2 K hours, for less than that price anywhere. Most argon tubes won't do more than 5,000 hours or so, and they degrade over time, weigh a lot, and require massive amounts of power. This thing is likely to keep above 50 mW until the bitter end, it uses less than 24 W of input, and it weighs about a pound. Another big advantage is the small size (3.5" x 4" x 5.75") and 12 VDC power. I love it!

    Note that I'ms not affiliated with his company in any way, and I have no long-term experience with these lasers - I'm just real happy I got something this bright for about a thousand bucks!

    (From: Lynn Strickland (stricks760@earthlink.net).)

    How's the power stability over time? How repeatable is the power at turn-on? (Does it come on at 70 mW sometimes, 55 mW sometimes, etc.)?

    Any idea of the percent optical noise (i.e., does it have the green noise problem?).

    Lifetime limiting factor is probably the pump diode. How does he guarantee 2,000 hours, when (I assume) the thing doesn't have an hour-meter?

    (From: Mike.)

    I have not measured the power stability or repeatability yet, but I will in the next couple weeks. Visually, it looks consistent in both respects - but that doesn't say much. I have not measured optical noise, but I will do that, too. I just got the thing yesterday afternoon, so I haven't had time to do anything but a single power reading yet. The pump diode is rated 2 A and is being run at 1.16 A, if the spec sheet is real (it's from the diode manufacturer, not Transverse). I wondered the same thing you are, though: How do you guarantee lifetime if it has no hour meter?

    (From: AESLasers (aeslasers@aol.com).)

    Yeah, those died anywhere from 50 hours to a few hundred hours. Seems to me the person looked at it, and said the diodes were crap. $995 isn't a good deal for a boat anchor, and what good is a warranty if you can't collect on it? I don't think you want to fly to Taiwan to make sure it gets fixed. Back to the old adage "you get what you pay for".

    (From: Mike.)

    Quite true - especially for one that only weighs a pound (it would be useless as a boat anchor!).

    Well, I knew I was taking a risk when I ordered it. This is an insurance-funded replacement for a 25 mW, 488 nm argon laser that was damaged in an incident involving toxic mold (long story, don't ask). That argon had 2,500 hours on it but was still doing fine until the microbes hit. This DPSS unit (which is sitting beside me right now, illuminating the wall) is far more fun. I essentially got it for $600, since that's what I paid for the argon to begin with. It's real hard to believe this is a lemon, but I guess I'll find out in a couple months. If and when it checks out, I think I'll probably just disassemble it for parts and educational value rather than attempt to get it repaired by the manufacturer. If it's the pump diode that goes out, I may be able to replace it with a 1.2 W fiber coupled unit I have and a GRIN lens. It's frequency is tuned for YLF (797 nm), but I'ms sure I'd get a few tens of mW out of it at 532 nm - assuming the original diode can be removed without messing with the vanadate and KTP. If all else fails, it sure is a neat little fan-cooled case! Oh, well.

    (From: Bob.)

    These units ARE the same as what people have complained about on the USENET newsgroup alt.lasers. These systems are rather poorly put together, and are prone to failures and rapid degradation of performance over time. Other people who have used numbers of them have had many fail well before the rated 2,000 hours. I wouldn't buy a bunch of um till you see how long it lasts if I were you.

    (From: Joachim Mueller (JoachimMueller@swol.de).)

    It is not possible to remove the vanadate-chip because it is glued to the diode (see the photos this model or one similar to it at the bottom of JM Laser Display - New DPSS Laser, not one of JM Laser's). I had 2 of the units from a friend with dead diodes and he wanted me to repair them. There is no way to that! You will definitely break the chip or damage the coatings. Maybe you are lucky and the thing runs long time. Because they drive the diode with only half of the power, lifetime should be longer.

    If you want to see beam characteristics, unscrew the collimator lens at the front. Then you have a large spot and can see if it is a Gaussian shape. You don't need to measure noise. It is clear that such a laser (and nearly all low-price DPSS) have more or less noise and also large power fluctuations (10 to 20%). But this is not important for a show device.

    Maybe you can put an hour-meter to the thing and later let us all know, what lifetime it reached (hopefully more than 1,000 hours).

    (From: Hays Goodman (HGood501@aol.com).)

    This is my first laser, so I thought I'd relate briefly my experiences with the Transverse DPSS laser.

    I've probably operated my 60 mW model a total of around twenty hours or so, and so far I am extremely pleased with the performance. Warm-up time is heavily temperature-dependent. I received the laser in the dead of winter, and it was often reluctant to fire up until as much as a twenty-minute warm up time, then the beam would stabilize. Now that summer is here and indoor ambient temperatures have probably risen by fifteen degrees or so, warmup is in the several minute range before the beam really settles down. Power also seems greater (I don't own a power meter), but it's definitely brighter at 75°F ambient as opposed to 60°F ambient.

    On a purely aesthetic level, the beam is lovely. Solid and extremely bright, even when reflected and somewhat diffused. With a light layer of smoke in the air, even large "cones" are spectacularly visible. With the beam projected in open space, going out a thousand feet or so produces a beam about closed-fist diameter; I intend to experiment with external collimation to see if this can be improved. The beam visibility definitely makes safety a bit less onerous, since beam paths are easily detected in low-light conditions. Best of all it's light, small, easily transported and quiet, the dual fans making only a slight sound like a computer tower.

    I can't speak to lifetime yet, but to this point it's been a dream.



  • Back to Sam's Laser FAQ Table of Contents.
  • Back to Solid State Lasers Sub-Table of Contents.
  • Forward to SS Laser Testing, Adjustment, Repair.


    Sam's Laser FAQ, Copyright © 1994-2001, Samuel M. Goldwasser, All Rights Reserved.
    I may be contacted via the Sci.Electronics.Repair FAQ Email Links Page.