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.

    Diode Lasers

    Sub-Table of Contents



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

    Basic Characteristics, Structure, Safety, Common Types

    Introduction to Diode Lasers and Laser Diodes

    Note: Throughout this document, we will use the terms 'laser diode' and 'diode laser' somewhat interchangeably although we will tend to use the term 'diode laser' when referring to a complete system or module. When a device is called a 'laser diode', this generally refers to the combination of the semiconductor chip that does the actual lasing along with a monitor photodiode chip (for used for feedback control of power output) housed in a package (usually with 3 leads) that looks like a metal can transistor with a window in the top. These are then mounted and may be combined with driver circuitry and optics in a 'diode laser module' or the common (red) laser pointer. A Variety of Small Laser Diodes shows some examples.

    Note: Most of the information on Diode Pumped Solid State (DPSS) Lasers has moved to the chapter: Solid State Lasers.

    Diode lasers use nearly microscopic chips of Gallium-Arsenide or other exotic semiconductors to generate coherent light in a very small package. The energy level differences between the conduction and valence band electrons in these semiconductors are what provide the mechanism for laser action. This is not the sort of laser you can build from scratch in your basement as the required fabrication technology costs megabucks or more to set up. You will have to be content with powering a commercial laser diode from a home-made driver circuit or using a pre-packaged module like a laser pointer. Fortunately, laser diodes are now quite inexpensive (with prices dropping as you read this) and widely available.

    The active element is a solid state device not all that different from an LED. The first of these were developed quite early in the history of lasers but it wasn't until the early 1980s that they became widely available - and their price dropped accordingly. Now, there are a wide variety - some emitting many *watts* of optical power. The most common types found in popular devices like CD players and laser pointers have a maximum output in the 3 to 5 mW range.

    A typical configuration for a common low power edge emitting laser diode is shown below:

    
                              +                                     +
                              o                                     o
                ______________|______________                _______|_______
         Laser |   P type semiconductor      |  Laser       |     P type    |
          beam |                             |  beam        |               |
       <=======|:::::::::::::::::::::::::::::|=======>      |ooooooooooooooo|
               |        Junction---^         |              |               |
         End ->|   N type semiconductor      |<- End        |     N type    |
       facet   |_____________________________|   facet      |_______________|
                              |                                     |
                              o                                     o
                              -                                     -
    
                         (Side view)                            (End view)
    
              |<----------------------- 1 mm ------------------------>|
    
    
    This configuration above is called a 'homojunction' since there is only one P-N junction. It turns out there are benefits to using several closely spaced junctions formed by the use of layers of P and N type materials. These are called 'heterojunction' laser diodes. There are many many more advanced structures in use today and new ones are being developed as you read this! For example, see the section: Vertical Cavity Surface Emitting Laser Diodes (VCSELs) for a description of one type that has the potential to have a dramatic impact in many areas of technology.

    The 'end facets' are the mirrors that form the diode laser's resonant cavity. These may just be the cleaved surfaces of the semiconductor crystal or may be optically ground, polished, and coated.

    For these types of integrated laser diodes, everything takes place inside the chip. Therefore, the output wavelength is fixed and determined by the properties of the semiconductor material and the device's physical structure. Or, at least that's the way it is supposed to work though with some, reflection of the laser light back into the chip can cause stability problems or even be used to advantage to frequency stabilize the output. There are also tunable diode lasers using external cavity optics to provide a continuous and in some cases, quite wide range of wavelengths without mode hopping. See EOSI's Tunable Laser Diode Systemsfor an example of one commercially available product. You really don't want to ask about the price! :-)

    There are also pulsed laser diodes requiring many amps to to reach threshold and providing watts of output power but only for a short time - microseconds or less. Average power is perhaps a few mW. These are gallium arsenide (GaAs) heterojunction laser diodes. They are not that common today but some surplus places are selling diodes like these as part of the Chieftain tank rangefinder assembly. They mention the high peak power output but not the low average power. :( Modern devices with similar specifications are also available. One manufacturer is Infineon Technologies AG.

    Electrical input to the laser diode may be provided by a special current controlled DC power supply or from a driver which may modulate or pulse it at potentially very high data rates for use in fiber optic or free-space communications. Multi-GHz transmission bandwidth is possible using readily available integrated driver chips.

    However, unlike LEDs, laser diodes require much greater care in their drive electronics or else they *will* die - instantly. There is a maximum current which must not be exceeded for even a microsecond - and this depends on the particular device as well as junction temperature. In other words, it is not sufficient in most cases to look up the specifications in a databook and just use a constant current power supply. This sensitivity to overcurrent is due to the very large amount of positive feedback which is present when the laser diode is lasing. Damage to the end facets (mirrors) can occur very nearly instantaneously from the concentrated E/M fields in the laser beam. Closed loop regulation using optical feedback to stabilize beam power is usually implemented to compensate for device and temperature variations. See the sections on CD and visible laser diodes later in this document before attempting to power or even handle them. Not all devices appear to be equally sensitive to minor abuse but it pays to err on the side of caution (from the points of view of both your pocketbook and ego!).

    In their favor, laser diodes are very compact - the active element is about the size of a grain of sand, low power (and low voltage), relatively efficient (especially compared to the gas lasers they replaced), rugged, and long lived if treated properly.

    They do have some disadvantages in addition to the critical drive requirements. Optical performance is usually not equal to that of other laser types. In particular, the coherence length and monochromicity of some types are likely to be inferior. This is not surprising considering that the laser cavity is a fraction of a mm in length formed by the junction of the III-V semiconductor between cleaved faces. Compare this to even the smallest common HeNe laser tubes with about a 10 cm cavity. Thus, these laser diodes would not be suitable light sources for high quality holography or long baseline interferometry. But, apparently, even a $8.95 laser pointer may work well enough to experiment in these areas and some results can be surprisingly good despite the general opinion of laser diode performance.

    Even if not as good as a helium-neon laser in the areas of coherence and stability, for many applications, laser diodes are perfectly adequate and their advantages - especially small size, low power, and low cost - far outweigh any faults. In fact, these 'faults' can prove to be advantageous where the laser diode is being used simply as an illumination source as unwanted speckle and interference effects are greatly reduced.

    As noted, not all laser diodes have the same performance. See the section: Interferometers Using Inexpensive Laser Diodes for comments that suggest some common types may indeed have beam characteristics comparable to typical HeNe lasers. And, for short range applications, see: Can I Use the Pickup from a CD Player or CDROM Drive for Interferometry?. Also see the section: Holography Using Cheap Diode Lasers.

    The following sites provide some relatively easy to follow discussions of the principles of operation, construction, characteristics, and other aspects of laser diode technology:

    Examples of Common Laser Diodes

    A Variety of Small Laser Diodes" shows those typically found in CD players, CDROM drives, laser printers, and bar code scanners. These were scanned at 150 dpi. The laser diodes on the left are from CD players, CDROM drives, and laser printers. The one in the middle is also from a laser printer. The components of the diode laser module on the right are from a bar code scanner. The actual laser diode is mounted at the rear end of the aluminum block and the single element plastic lens is all that is needed to provide a reasonably well focused beam.

    The closeups below were scanned at 600 dpi - laser diodes (at least the small ones we are dealing with) are really not this HUGE! These two laser diodes can also be found in the group photo, above.

    The Closeup of laser diode from the Sony KSS361A Optical Pickup shows a type that is found in many CD players and CDROM drives manufactured by Sony. The actual laser diode is inside the brass barrel shown in the photo of the optical pickup. The front of the package is angled so that the exit window (anti-reflection coated) is also mounted at what may be the Brewster angle, probably to further prevent stray reflections from the window's surfaces from feeding back into the laser diode's cavity or interfering with the detected signal. (At the Brewster angle, light polarized parallel to the window is totally reflected and light polarized perpendicular to it is totally transmitted. The output of these edge emitting laser diodes is polarized. See the section: What is a Brewster window?.)

    The Closeup of Typical Laser Diode shows one that is from a laser printer. It was mounted in a massive module (relative to the size of this laser diode, at least) which included the objective lens and provided the very important heat sink. In some high performance laser printers, a solid state Peltier cooler is used to stabilize the temperature of the laser diode. The low power laser diodes in CD and LD players, and CDROM and other optical drives (at least read-only types) get away with at most, the heat sink provided by the casting of the optical block - and many don't even need this being of all plastic construction.

    Differences Between LEDs and Laser Diodes

    (From: Don Stauffer (stauffer@htc.honeywell.com).)

    One can think of an LED as a laser without a feedback cavity. The LED emits photons from recombining electrons. It has a very broad spectrum.

    When we add a high Q cavity to it, the feedback can be high enough to trigger true laser action. Most laser diodes have the cavity built right into the device but there are such things as external cavity diode lasers.

    The addition of the high Q cavity cuts down drastically the number of modes operating (in fact, it is almost improper to speak of mode structure with an LED. The result is that the emission line narrows drastically (more monochromatic) and the beam narrows somewhat spatially. One can still not easily get true single mode lasing with normal diode lasers, however, so the line will not be as sharp as a gas laser, nor the beam as narrow.

    For more info, see the section: How LEDs Compare to Laser Diodes - Wavelengths, Spectrum, Power, Focus, Safety.

    Comparisons of Diode Lasers with Other Types of Lasers

    While a laser diode is a true laser and not just a glorified (and expensive) LED, there are major difference compared to a gas or solid state laser - not all of them bad.

    (From: Don Stauffer (stauffer@htc.honeywell.com).)

    Yes indeed, a diode laser is a true laser. That being said, looking at matters quantitatively, it is harder to make a diode laser with a very narrow line emission than a gas laser or large crystal laser. Adding cavity length to a laser in general acts to narrow the line (in spectral space, though a higher Q cavity does tend to narrow beam in space also). It is possible to use a larger, high Q external cavity with a laser diode to increase its coherence.

    (From: David Schaafsma (drdave@jnpcs.com) and Rajiv Agarwal (agarca@giascl01.vsnl.net.in).)

    A couple of minor points:

    High Q cavities narrow the spatial profile only if they are confocal - planar high Q cavities (as in diode lasers, and especially vertical-cavity diode lasers) are prone to problems with walk-off and the mode must be confined physically.

    In a gas laser, you also start with a much narrower fluorescence line and thus the gain spectrum is limited spectrally. Diode lasers (being band-to-band or excitonic semiconductor transitions) have much broader fluorescence spectra.

    The typical edge-emitting diode laser actually lases in quite a few fundamental modes (especially when operated using its own facets as the cavity) and though each lasing mode is "monochromatic", the overall spectrum really isn't. External cavities are really the only way to obtain approximately single mode operation from an edge-emitting diode laser.

    VCSELs are usually true single mode devices. The reason you can get away with lengthening the cavity in a gas laser is that you don't need to worry about lowering the free spectral range because the gain bandwidth is small.

    DFB or DBR lasers achieve very similar results and have Side mode suppression ratios better than 30 db. These lasers have been the mainstay of Optical fiber base telecom for a while now.

    DFB Lasers are use for long haul telecommunications network - the kind used by say Sprint (>1GB for up to 25 miles) for their phone networks between cities. These have been for Trans-Atlantic cables (TAT) between US and Europe. LEDs are used more for FDDI type application between computers (~100Mb and less than 1 mile).

    (From: Vishwa Narayan (vishwa.narayan@ericsson.com).)

    While LEDs are quite popular in Datacom applications (read short distances), Telecom applications typically use DFBs, either directly modulated for low speeds (e.g., OC-3 155 Mb/sec) or externally modulated for high speeds (e.g., OC-48 2.5 Gb/sec). Distances can typically range over tens of kilometers, to hundreds of kilometers with optical amplification, sans repeaters.

    Diode Laser Safety

    Despite their small size and low input power, diode lasers may still represent a significant hazard to vision. This is especially true where the output is collimated and/or invisible (near IR), and/or higher power than the typical 3 to 5 mW. At least you don't have to worry about getting zapped by any high voltage (as in a HeNe or argon laser).

    One should never look into the beam of any laser - especially if it is collimated. Use an indirect means of determining proper operation such as projecting the beam onto a white card, using an IR detector card or tester (where needed), or laser power meter.

    With both of these, the beam from the bare laser diode is highly divergent and therefore less of a hazard since the lens of the eye cannot focus it to a small spot. However, there is still no reason to look into the beam.

    For IR laser diodes in particular, especially if you are considering selling a product:

    (Portions from: Steve Roberts (osteven@akrobiz.com).)

    You need to take a close look at the CDRH rules, because there is no blink reflex in the IR. IR diode lasers are considered much more dangerous and therefore are in a higher class. CDRH has a curve of power versus wavelength that is used for determining safety classes. The only way a IR laser gets less then a IIIb rating (read: dangerous) is if the beam is totally enclosed or of very low power. Go to CDRH, call them and request a manufacturers' packet by mail. It's huge and confusing, but covers the requirements for products using IR laser diodes such as 3-D scanners, perimeter sensors, and so forth.

    Typical Laser Diodes

    The most common laser diodes on the planet by far are those used in CD players and CDROM drives. These produce a (mostly) invisible beam in the near infrared part of the spectrum at a wavelength of 780 nm. The optical power output from the raw laser diodes may be up to 5 mW but once it passes through the optics, what hits the CD is typically in the .3 to 1 mW range. Somewhat higher power IR laser diodes (up to about 30 mW) may turn up in surplus WORM (Write Once Read Mostly) or other optical drives.

    Visible laser diodes have replaced helium-neon lasers in supermarket checkout UPC scanners and other bar code scanners, laser pointers, patient positioning devices in medicine (i.e., CT and MRI scanners, radiation treatment planning systems), and many other applications. The first visible laser diodes emitted at a wavelength of around 670 nm in the deep red part of the spectrum. More recently, 650 nm and 635 nm red laser diodes have dropped in price.

    Due to the nonuniformity of the human eye's response, light at 635 nm appears more than 4 times brighter than the same power at 670 nm. Thus, the newest laser pointers and other devices benefitting from visibility are using these newer technology devices. Currently, they are substantially more expensive than those emitting at 670 nm but that will change as DVDs become popular:

    Laser diodes in the 635 to 650 nm range will be used in the much hyped DVD (Digital Video - or Versatile - Disc) technology, destined to replace CDs and CDROMs in the next few years. The shorter wavelength compared to 780 nm is one of several improvements that enable DVDs to store about 8 times (or more - 4 to 5 GB per layer, the specifications allow up to 2 layers on each side of a CD-size disc!) the amount of information or video/audio as CDs (650 MB). A side benefit is that dead DVD players and DVDROM drives (I cannot wait) will yield very nice visible laser diodes for the experimenter. :-)

    Like their IR cousins, the typical maximum power from these devices is around 3 to 5 mW. Cost is in the $10 to $50 for the basic laser diode device - more with optics and drive electronics. Higher power types (10s of mW) are also available but expect to spend several hundred dollars for something like a 20 mW module. Very high power diode lasers using arrays of laser diodes or laser diode bars with power output of WATTs or greater may cost 10s of thousands of dollars!

    Laser Diode Construction

    A rough diagram of a laser diode of the type found in a laser pointer or CD player is shown below. This is in no way to scale. The size of the overall package will typically be 5 to 10 mm overall but the actual laser diode chip will be less than 1 mm in length.
    
                    ___
                   |   |          Metal case
                   |   |_______________________________
                   |                                   \
                   |    _____________________________   |
                   |   |                             |  |
         LD -------:===:------------------+          |  |
                   |   |__                |          |__|
                   |   |  |___      ______|______    :  :
                   |   |  |   |    |             |   :  :
         PD -------:===:----+ |<---|:::::::::::::|============> Main beam
                   |   |  |___|____|_____________|_  :  :          (divergent)
                   |   | Photodiode  Laser diode   | :__:
                   |   |\__________________________| |  | Protective window
        Com -------+   |          Heat sink          |  |
                   |   |_____________________________|  |
                   |                                    |
                   |    _______________________________/
                   |   |
                   |___|
    
    
    The main beam as it emerges from the laser diode is wedge shaped and highly divergent (unlike a helium-neon laser) with a typical spread of 10 by 30 degrees. External optics are required to produce anything approaching a parallel (collimated) beam. A simple (spherical) short focal length convex lens will work reasonably well for this purpose but diode laser modules and laser pointers might use a lens where at least one surface is aspheric (not ground to a spherical shape as are with most common lenses).

    In the case of a sample I removed from a dead diode laser module, the surface facing the laser diode was slightly curved and aspheric while the other surface was highly curved and spherical. The effective focal length of the lens was about 5 mm. It appeared similar to the objective lens of a CD player - which was perhaps its original intended application and thus a low cost source for such optics.

    Due to the nature of the emitting junction which results in a wedge shaped beam and unequal divergence (10 x 30 degrees typical), a laser diode is somewhat astigmatic. In effect, the focal length required to collimate the beam in X and Y differs very slightly. Thus, an additional cylindrical lens or a single lens with an astigmatic curvature is required to fully compensate for this characteristic. However, the amount of astigmatism is usually small and can often be ignored. The general beam shape is elliptical or rectangular but this can be circularized by a pair of prisms.

    The light from these edge emitting laser diodes is generally linearly polarized. You can easily confirm this even with a simple laser pointer by reflecting at about a 45 degree angle from a piece of glass (not a metal coated mirror). Rotate the pointer and watch the reflection - there will be a very distinct minimum and maximum with the elongated shape of the beam at close range being aligned with the glass and perpendicular, respectively. For the advanced course, determine the Brewster angle. :)

    For addition information, see the section: Beam Characteristics of Laser Diodes.

    The beam from the back end of the laser diode chip hits a built-in photodiode which is normally used in an opto-electronic feedback loop to regulate current and thus beam power. Note that the photodiode is likely mounted at an angle (not possible to show in ASCII) so that the reflection does not interfere with the operation of the laser diode.

    CAUTION: Some complete modules may use the reflection from external optics along with an external photodiode for power stabilization as it is more accurate since the actual output beam is sampled. For these, one should never attempt to clean or even focus the lens when operating near full power as this may disturb the feedback loop and damage the laser diode.

    Interpreting Laser Diode Specifications

    Here are the major parameters that are listed in manufacturer datasheets for small (i.e., 5 mW) laser diodes. This is for the Sony SLD1135VS visible laser diode, typical of those found in newer laser pointers and small diode laser modules. Most of the same parameters are used for high power laser diodes but those types generally don't include the internal monitor photodiode. And, of course, actual values will be quite different.

    Note: Some of the symbols below are not exactly what is found in the datasheet so they can be represented in ASCII. However, the meaning should be obvious.

           Parameter        Symbol       Conditions        Min   Typ.   Max   Unit
     ------------------------------------------------------------------------------
      Threshold current      Ith                                  30     40    mA
      Operating current      Iop          Po = 5mW                35     45    mA
      Operating voltage      Vop          Po = 5mW               2.2    2.4    V
      Wavelength           lambdap        Po = 5mW               650    660    nm
      Radiation angle
        Perpendicular      theta_|_       Po = 5mW         22     30     40   Deg.
        Parallel           theta||        Po = 5mW          5      7     12   Deg.
      Positional accuracy  dx,dy,dz       Po = 5mW                     +/-150  um
      Angular accuracy
        Perpendicular       phi_|_        Po = 5mW                      +/-3  Deg.
        Parallel            phi||         Po = 5mW                      +/-3  Deg.
      Differential eff.      nD           Po = 5mW        0.3    0.6    0.9  mW/mA
      Astigmatism            As           Po = 5mW                 7     15    um
      Monitor PD current    Imon     Po = 5mW, Vr = 5V    0.05   0.1    0.25   mA
    
    Descriptions of the parameters are provided below: The datasheet will also of course include pinout and package info which I have omitted here.

    What About High Power Visible Laser Diodes?

    It is possible to buy visible laser diodes capable of a half watt or more:

    "I was just browsing Meredith Instrument's site, and noticed that they have 635 nm diodes rated at 500 mW. Has anyone ever dealt with these things? Looking around on the site, it appears I could put together a half watt red diode laser for under $600, or a 250 mW one for under $400. Is there some catch to using these? The whole setup would be cheaper than a 25 mW HeNe laser".
    Yes. Aside from the ease with which one of those pricey diodes can be blown out due to improper drive, the beam quality is no where near that of even a cheap HeNe laser. It is multimode and very non-circular and astigmatic. The latter can probably be dealt with using some (expensive) optics. However, multimode operation means that these are unsuitable for applications like holograpy or interferometry.

    (From: Frank DeFreitas (director@holoworld.com).)

    I have a 500 mW laser diode from Polaroid. 660nm I believe. It needs the heftier driver that Meredith offers - the one that can put out 1000 mA or so. The laser diode is gain guided/multi-mode, rather than index guided/single (mono) mode -- so you can pretty much forget any application that would call for any type of coherency or high contrast fringes.

    The output beam profile is basically a line. It is very similar to taking a standard HeNe beam and sending it through a cylindrical lens. (However, on the other hand, I'm wondering if a cylindrical lens would actually help it when used in the other dimension. Or at least bring it to a spot which could be collimated utilizing secondary optics in the path.)

    I'd also like to point out that it's not a diode to play around with. The optical output at 500 mW is not going to knock any missles out of the sky, but will certainly warrant caution when working with the beam. The beam is much more powerful than it appears at 660 nm due to the eye's reduced sensitivity at that wavelength compared to HeNe 632.8 nm.

    And Those Really High Power Laser Diodes?

    You may have read about truly high power laser diodes - those putting out WATTs, 10s of WATTs, or even 100s of WATTs from a one diode or an array (bar) of diodes in a single package, or multiple laser diode bars. These are usually near-IR emitters, often at 808 nm. Solid State Diode Pumped (DPSS) lasers are driven by these light sources with some providing upwards of 1,000 WATTs (and the upper limit is climbing as you read this). Also see the section: Diode Pumped Solid State Lasers.

    About those laser diode bars:

    (From: Walter Skrlac (Walter.Skrlac@t-online.de).)

    "Bars are a 10 mm wide chip with typically 16 to 24 emitters, each emitter being about 150 microns wide and emitting up to 2 watts of power per emitter. The highest power for solid state laser pumping is 40 watts from a 19 emitter bar. Almost all bars are a single chip, multiple emitter device. I do know that in the beginning days of bars, Siemens produced a 5 watt device consisting of 5 separate 1 watt laser diodes mounted in a row 10 mm long. The individual laser diodes are connected in parallel so you can't switch them individually."

    The good news is that this technology is developing very rapidly.

    The bad news from our perspective is that there are no really low cost sources, new or surplus, for these diode lasers as far as I know at the present time.

    For example, a 1 W 808 nm laser diode is currently (Spring 2000) $300 in the Lasershop catalog.

    Actually, it isn't necessarily the diode itself that is so expensive. A 1.5 W 800 nm diode chip goes for about $10 when they are purchased in reasonably large quantities. However, these are only about 0.5 mm on a side and maybe 0.1 mm thick. Mounting means using low temperature solder and flux to bond the chip to a large heat sink and copper strip (for the two connections - no monitor photodiode, that function must be performed externally). The soldering is best done on a hot plate (to raise the temperature of the heat sink and chip to almost the melting point of the solder), with a fine tip iron for the last few degrees. They have an HR and OC side, and a top and bottom, and thus orientation matters. So, if you have access to a surface mount rework station with a stereo microscope, a steady hand, infinite patience, and don't sneeze much (which will blow your chips away to never be found again), you could try your hand at the mounting. I have a couple of these diode chips so once I get up the nerve to try this, I will report on success or failure.

    The better way to deal with these laser diodes is to have them already mounted on a heat sink. But now we're talking about $100s for a single unit. But, for a number of reasons, the best type of high power laser diode to get is probably a fiber coupled module. Then you don't have to mess with beam shape issues, the diode is safely tucked away out of harm, and the fiber output can easily be adapted to your favorite crystal shape. Some power is lost in the coupling but it appears as though the specs I've seen are similar for the bare diode assembly and fiber coupled module. Of course, the cost for such a module now appoaches that of a nicely equipped PC. :) For more info, see the section: Anatomy of Fiber Coupled Laser Diodes.

    Apparently, laser diode bars/assemblies of much higher power are now available at roughly equivalent prices if you multiply $10/W by the number of watts (but I don't know for what quantities these prices apply). Check out Industrial Microphotonics Company as one possible supplier. The IMC Products Page lists a wide variety of really interesting items but unfortunately doesn't have any prices. Bars can be connected in series to ease the power supply requirements enabling them to be driven with lower current at higher voltage (e.g., a 4 bar configuration would use 8 V at 50 A instead of 2 V at 200 A). With individual chips on a common heat sink, this really isn't an option.

    Note that most high power diode lasers are near IR - often around 800 nm for pumping DPSS lasers and optical communications. High power visible laser diodes are much less common and usually limited to less than a W at 670 nm. Not that this is terrible. :)

    If you have your heart set on one of these for your birthday, all I can suggest at the present time is to keep track of what is available surplus and to check with the manufacturers listed in the chapter: Laser and Parts Sources. If this is for some sort of academic project with a legitimate research objective, you may be able to obtain a cosmetic reject or one that doesn't quite meet specs by persistent pleading with one of the laser diode manufacturers. Or, if you can deal with the bare chips, it may be possible to beg a few from one of the companies that produces DPSS laser systems since they use them by the carload, and when purchased by the carload, the cost goes way down.

    Keep in mind that obtaining the diode is only a small part of the problem. These devices are exceeding fussy about drive and cooling - even much more so than the wimpy little laser diodes found in CD players and laser pointers! However, if reasonable precautions are taken and they aren't run near their maximum ratings, actually blowing them out totally isn't that likely.

    And, needless to say, at these power levels, your eyes (and flammable objects) don't get a second chance - laser safety must be at the top of your list of priorities.

    And Those High Power Pulsed Laser Diodes?

    You may have seen offers of IR laser diodes with 9 W or 14 W or much higher too-good-to-be-true power ratings from various surplus companies. These are pulsed ratings and the power rating is peak. Such laser diodes have been available surplus as part of the laser rangefinder from the Chieftain tank. Since they are actually not that expensive to buy new as these things go (maybe $20 to $100), you can get them from manufacturers like Infineon - possibly even as free samples. Unfortunately, while they have nice peak power ratings, the average power ratings are typically only a few mW as they must be run at a very low duty cycle - typically 0.1 percent (1 part in 1,000) or less. Furthermore, the most common wavelengths are between 850 and 910 nm and these aren't much use for most laser projects (though wavelengths from 780 to 980 nm are available). There isn't any realistic possibility of efficiently frequency doubling these to visible (though a few blue photons might be possible if focused into a KTP crystal at a funny angle) and the wavelength isn't useful for pumping common solid state laser crystals. However, they would be suitable for rangefinder or similar applications.

    These laser diodes come in plastic packages that look much like LEDs and thus there is no real possibility of decent cooling. Therefore, power dissipation is one of the major limiting factors. It may be possible to use a lower peak current with a longer pulse width than what's specified in the datasheet as long as the average power dissipation rating isn't exceeded. However, with the high threshold current, this probably doesn't provide much benefit. And, no guarantees of any kind with laser diodes!

    The following assumes a device rated at 16 W peak power, 100 ns max pulse width, 0.1% max duty cycle:

    (From: Roithner Lasertechnik" (office@roithner-laser.com).)

    The absolute limit is the heat stress of the LD chip inside. Under normal conditions, the laser will emit a pulse of the rated 16 W, 100 ns at 10 kHz (200 ns at 5 kHz is the absolute limit) - which is highly recommended for an expected long lifetime of several khours with usual chip degradation. Take this integrated V x I (voltage x current) thermal heat stress as a final constant. If you run with a higher frequency than the rated, but with a shorter pulse width, still never go higher than this constant. If you go higher, the laser pulse power will go down rapidly due to overheating of the LD chip (still reversible, LD is not yet blown) but overall lifetime is shortened. Keep in mind, that the rise and fall time of this LD is typically 1 ns, so you will get the next limit soon.

    Vertical Cavity Surface Emitting Laser Diodes (VCSELs)

    Most laser diodes up till now (as well as most of those discussed in this document) are edge emitters - the beam exists from the cleaved edge of the processed laser diode chip. These are also called Fabry-Perot (FP) diode lasers since the cavity is essentially similar to that of a conventional gas or solid state laser but formed inside the semiconductor laser diode chip itself. The mirrors are either formed by the cleaved edges of the chip or (for high performance types like those that are very stable or tunable) one or both of these are anti-reflection (AR) coated and external mirrors are added.

    VCSELs, on the other hand, emit their beam from their top surface (and potentially bottom surface as well). The cavity is formed of a hundred or more layers consisting of mirrors and active laser semiconductor all formed epitaxially on a bulk (inactive) substrate.

    This approach provides several very significant technical advantages:

    There are also numerous manufacturing and cost advantages: VCSEL technology is in its infancy and its potential is just beginning to be exploited. Quite possibly, VCSELs will become the dominant type of laser diode in the future with capabilities so fantastic and costs so low as to be unimaginable today. There is some technical information at the following sites:

    For a general review article, see: "The Ideal Light source for Datanets", K.S. Giboney, L.B. Aronson, B.E. Lemoff, IEEE Spectrum V.35 (2) p. 43, Feb 1998.

    If you want to play with VCSELs, bare chips, packaged chips, and even VCSEL arrays are available from various laser suppliers and prices aren't totally rediculous. For example, see Roithner Lasertechnik's VCSEL Page. Available wavelengths are currently 785 and 850 nm.

    Optically Pumped Semiconductor Laser (OPSL)

    Nearly all semiconductor lasers are powered by electrical current through the gain medium. However, for certain materials, it's also possible to use another laser to optically pump it. This has some significant advantages in terms of controlling transverse and longitudinal modes and beam shape.

    The first (that I know of) commercial OPSL is the Coherent, Inc. "Sapphire", a replacement for low power argon ion lasers at 488 nm. (I think the use of Sapphire is unfortunate as this has absolutely nothing to do with the Ti:Sapphire laser with which it may be confused.) The Sapphire is an Vertical External Cavity Surface Emitting Laser (VECSEL). The resonator is in many ways similar to that of a frequency doubled Diode Pumped Solid State (DPSS) laser but with an InGaAs quantum-well semiconductor instead of a laser crystal as the gain medium. It is pumped by a high power 808 nm laser diode and lasing at the fundamental IR wavelength of 946 nm. This is intracavity doubled to 488 nm.

    See the Coherent Sapphire Page for more information.

    On-line Introductions to Diode Lasers

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

    Additional Laser Diode Information

    Here are some Web sites that may be of interest:

    Some very good basic information about laser diodes is provided in of all places, manufacturer's catalogs! :) Try companies like Mitsubishi, Fujitsu, Hitachi, Sharp, Sony, NEC, etc. They have introductory sections at the front or the back of their laser diode catalogs. Just call the and ask for a laser diode catalog. Much of this is now on-line.



  • Back to Diode Lasers Sub-Table of Contents.

    Beam Characteristics, Correction, Comparison with Other Lasers

    Beam Characteristics of Laser Diodes

    Unlike the helium-neon and other common gas lasers (as well as most other types of lasers), the raw output beam from an edge emitting (also called Fabry Perot or FP) laser diode (the most common and until recently, only commercially available types) is highly divergent and suffers from two asymmetries: astigmatism and an elliptical beam profile. The beam is also inherently linearly polarized. These all follow directly from the shape of the emitting aperture of the edge emitting laser diode end facet which is highly elongated rather than circular.

    For more information (and some medium weight math) on the beam characteristics of common laser diodes, check the Power Technology, Inc. Technical Library.

    There are ways of correcting for all of these artifacts with a single special lens close to the laser diode itself. For example, Blue Sky Research offers combined laser diodes and microlenses which they claim perform as well as larger more expensive diode laser modules using various discrete lenses and prisms to implement the beam correction.

    Note that VCSEL (Vertical Cavity Surface Emitting Laser diodes) need not suffer from astigmatism and/or an elliptical beam profile since their emitting aperture can be made to be perfectly symmetrical. I would also expect them not to need to be polarized for this reason as well. See the section: Vertical Cavity Surface Emitting Laser Diodes (VCSELs).

    Measuring Laser Diode Beam Characteristics

    (From: Gregory J. Whaley (gwhaley@tiny.net).)

    At Philips we used three difference techniques to measure astigmatism in laser diodes:

    Not surprisingly, each technique would give slightly different numbers! :-)

    Correcting for Unwanted Laser Diode Beam Characteristics

    Without any type of correction, the output of a bare laser diode is more like that from a mediocre flashlight than what is normally thought of as a laser source. Some optics are needed to produce a reasonably well collimated beam (like the one from a cheap laser pointer) and more sophisticated optics are needed to provide optimal beam quality (which can be very good indeed). Of course, depending on the particular application, one or more of these so called 'defects' may actually be considered desirable. Still another approach which will correct for the elliptical beam profile and astigmatism all at once is to couple the beam into a single mode optical fiber using two short focal length lenses. With a sufficiently long fiber (well, relative to the wavelength), the output beam characteristics will be entirely determined by the quality of the output face of the fiber. Then, a simple collimating lens can be used.

    Whatever type of external optics are added, take care that significant power isn't reflected back into the laser diode itself. This can destabilize the lasing process as well as fooling the built-in photodiode into thinking the output power is higher than it really is causing the optical feedback circuit to reduce it.

    Some additional comments are provided below:

    (Portions from: Mark W. Lund (lundm@physc1.byu.edu).)

    A simple short focal length lens will collimate the beam. However, laser diodes tend to be astigmatic which means that you will have one axis collimated at a different focus than the other. A typical value for this astigmatism is 40 microns. A cylindrical lens in addition to the spherical collimating lens or a special lens designed for this purpose can correct this but may not be needed for non-critical applications.

    Any camera lens will be able to produce a reasonably well collimated beam (subject to the astigmatism mentioned above). Put the laser diode at the focal point of the lens. If you want the type of narrow beam produced by a HeNe laser, you need a short focal length lens, such as a microscope objective. A good compromise between cheap and short focal length would be an old disk camera lens. These cameras can be found at thrift shops, garage or yard sales, and flea markets for a couple dollars or less.

    The longer the focal length the larger your beam will be, but the less effect the astigmatism will have. The diameter of the beam will be the size of the aperture of the lens (in which case you are throwing away light) or the size of the beam at the distance of one focal length, whichever is less.

    (From: Steve Nosko (q10706@email.mot.com).)

    One thing to note is that the laser diode actually has two apparent point sources. One for the wide axis of the beam and another for the narrow axis. This means that the lens must be more like two crossed cylindrical lenses with different focal lengths. There are different types of laser diodes with varying degrees of this so that some are easier to to design lenses for. There probably are types, by now, where there aren't two.

    I think of it like this (right or wrong). The astigmatism has two components. One is the difference in divergence between the two axes. I think this can be even if there is ONLY one apparent point source. It is just a point source with an oval aperture letting light through. The other is the different apparent point sources for the two axes.

    Laser Diodes with Built-In Beam Correction

    Laser diodes are also available with the corrections built in. Check out the Circulaser(tm) from Blue Sky Research. These look like any other 3 pin bare laser diode in a standard 5.6 or 9 mm package but produce a nearly circular diffraction limited beam requiring no additional beam aberration correction for many applications. Their divergence is also much less than that of a typical normal laser diode (8 degrees typical) easing the requirements of additional collimating or fiber coupling optics and capturing more of the available optical output power. Complete specifications and a photo of a typical device can be found at their Web site.

    I have tested a PS106 which is a 650 nm, 7 mW Circulaser(tm). The beam is indeed nearly perfectly circular with a divergence of about 8 degrees FWHM - less than that of the lower divergence (slow) axis of the typical bare laser diode.

    Aside from the convenience of not having to worry about their funny beam shape, putting a microlens next to the laser diode itself results in much more of the light being used compared with what gets through inexpensive external optics. With the typical collimating lens used in laser pointers and diode laser modules, as much as 40% or more of the light from the diode may be wasted largely due to its high divergence in the fast axis (30 or 40 degrees total at the half power point, perhaps twice this at the 10% point) - a very significant fraction gets blocked by the small aperture of the collimating lens.

    Coherence Length of Laser Diodes

    The party line has generally been that internal cavity Fabry-Perot (a fancy name for the usual side-emitting type) laser diodes have coherence lengths on the order of millimeters. Such claims are based at least partially on a comparison with other much larger lasers where the coherence length is usually on the order of the physical length of the cavity. Laser diode chips are a fraction of a mm on a side. Thus, very short coherence lengths were expected.

    However, this general rule appears not to apply for all laser diodes including those in many common diode laser modules and even cheap ($9.95) laser pointers. These are now being used routinely for experiments in interferometry and even holography. While their stability over time (e.g., wavelength drift and susceptibility to mode hopping) - is probably less than stellar, over the short term, coherence lengths of 20 cm or more are not unusual. This is similar to that of a typical helium-neon laser.

    For more on applications that may benefit from long coherence length diode lasers, see the sections: Interferometers Using Inexpensive Laser DiodesCan I Use the Pickup from a CD Player or CDROM Drive for Interferometry?. Also see the section: Holography Using Cheap Diode Lasers.

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

    The 1970's grade pulsed laser diodes have coherence lengths of 500 microns or so. Modern CW single mode diodes have coherence lengths of meters. I once asked Don Scifries why they had such long coherence lengths compared to gas lasers with much larger cavities and he referred me some papers. The impression that remains after 13 years is not that laser diodes are so good, but that HeNe Lasers are so bad. Line width of a typical 780 nm CD laser can be 10s of kHz.

    (From: Prof Harvey Rutt (h.rutt@ecs.soton.ac.uk).)

    **Crudely**, a CW laser will go SLM (Single Longitudinal Mode) spontaneously if the mode separation exceeds the *inhomogeneous* linewidth. The homogeneous linewidth can exceed the mode separation because inter mode competition suppresses the other modes CW. But if mode than mode falls within an inhomogeneous width, and is above threshold, all may oscillate as they do not compete.

    The coherence length of a HeNe laser is a simple matter: inhomogeneous linewidth set by Doppler broadening, mode separation set by length, usually a few modes run (or it would power cycle badly) so coherence length is approximately the cavity length divided by number of modes. When it goes single mode (but, unless stabilized, very unstable power output) the coherence length is typically huge. *AND* the absolute frequency is then pretty stable, within half a mode spacing of the atomic line. Simple HeNes are so 'bad' to get reasonable power stability as the cavity length drifts; less than 3 modes->poor.

    Most diodes have a pretty broad spontaneous linewidth and how much it is homogeneous or inhomogeneous I'm not clear; possibly as manufacturing has improved the inhomogeneous component has tended to reduce to below the mode spacing? Cavity length is way sub-mm, so as soon as it does twin mode the coherence length is awful.

    I have *directly* measured the output spectrum of many near IR diodes, and all bar one set were severely multimode. One set (normal FP lasers) were all single, which surprised me. I think I've only looked at one visible (a while back) and it was heavily multi mode.

    When a simple diode does go SLM, surely one might expect it still to have pretty severe wavelength drift with chip temperature? This can certainly wreck holography.

    Obviously people have found pragmatically you can get away without an expensive DFB laser; that crude diodes can be SLM; it opens up the interesting qn of just why it seems modern diodes are tending to go SLM spontaneously, & how stable the output wavelength is when they do go SLM (order nm/degree from memory?)

    (From: Bret Cannon (bdcannon@owt.com).)

    There are two temperature tuning rates for a diode laser, one is the tuning of a given longitudinal mode with temperature and the other is the tuning over larger temperature changes where the lasing mode hope from longitudinal mode to longitudinal mode to be close to the peak of the gain curve. The average tuning rate for this later rate is typically 0.3 nm/°C while for small enough temperature changes the tuning of longitudinal mode is much smaller. For a temperature stability of 1 mK a diode laser frequency is stable to better than 0.001 cm-1, perhaps even a good as 0.0001 cm-1 as determined by tuning onto a Doppler-free atomic transition. Thus at 780 nm the temperature tuning of a longitudinal mode is less than 0.06 nm/°C. With a temperature tuning of less than 1 cm-1/C, a temperature stability of 0.1 °C during an exposure would give a coherence length longer than 10 cm.

    Unless there is external optical feedback or a very sophisticated electronic feedback there is no way that a 780 nm CD laser would have a linewidth of 10s of kHz. With a sufficiently low noise current supply (less than 1 microamp RMS in a 1 MHz bandwidth) and temperature stabilization to about 1 mK, the intrinsic linewidth of diode lasers can be measured and they are proportional to the inverse of the output power. Linewidths of about 50 MHz for a 3 mW laser and 5 MHz for a 30 mW laser are typical. These linewidths are 5 to 50 times the Shawlow-Townes linewidth for these lasers and results from the coupling of the refractive index and the population inversion. Moradian (sp?) who was at MIT at the time published experimental measurements in the late 1970s and early 1980s. Henry published an analysis of this line broadening mechanism but I don't remember exactly when.

    The linewidth decreases with the square of the cavity length and with external cavities a few cm long people have achieved linewidths of less than 1 kHz. An example of this is work by Leo Holberg and colleagues at NIST in Boulder for an optical clock based on an inter-combination line in optically cooled and trapped atomic calcium.

    Coherence Time of Laser Diodes

    (From: Bret Cannon (bret.cannon@pnl.gov).)

    It depends on the laser diode, the power supply that is used, and the external optical feedback into the diode laser. With a single longitudinal mode diode, without external optical feedback, and a current noise of less than 1 uA RMS in a 1 MHz bandwidth, you can get linewidths of 10 MHz for a coherence time of nanoseconds. With optical feedback the linewidth can collapse to a few Hz or explode to several terahertz, depending on its intensity and the delay time between the light leaving the diode and returning to it.

    Temperature Dependence of Laser Diodes

    In addition to impact on expected lifetime (power degradation and MTBF) (See the section: Laser Diode Life), temperature effects the wavelength of an unstabilized (internal cavity) laser diode due to changes in physical dimensions:

    The wavelength shift for 808 nm diodes is generally around 2.5 nm (+/- 0.2 or 0.3 nm) per 10 °C (or just say 0.3 nm/°C)(, with the wavelength shift to the red (longer) with increasing temperature.

    For the violet/blue Nichia laser diodes, it's typically 0.04 nm per °C.

    Note that diode current also affect wavelength, partially due to temperature. So, as a diode ages and requires more current for the same output, its wavelength will also change.

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

    It really depends on the laser (i.e., manufacturer) and temperature range you are talking about. A good rule of thumb is 0.3 nm per °C over the operating temperature range of the device (About 30 GHz per °C). That's the average slope of the curve though - it includes mode hops. If you're operating at a mode hop, you can get a lot more change than 30 GHz with a 1 °C temperature change. If you are between mode hops, it can be much less.

    Mode hops can be a moving target too. Optical feedback can cause them (even minute amounts). Or, you can operate at a specific temperature where there are no mode hops today, but next week it might mode hop at that temperature.

    Note that you can only go so far if you want to use temperature to reduce the wavelength. Even if you got the electronics to work under frigid conditions, there is a minimum laser wavelength you can get from a particular diode laser chip. I'm not a physicist, but it has to do with the bandgap of the materials used. What you would get, as you cooled the thing, is lower and lower threshold current, lower operating current, and longer lifetime.

    (From: Richard Alexander (pooua@aol.com).)

    Back in the old days, about 15 years ago, the only way to get visible light from a laser diode was by using cryogenic cooling. My textbooks from my laser degree program only knows of this type of visible laser diode (they were written in the early '80s). The first room temperature visible laser diode was invented about 1991; I still have a "Radio-Electronics" issue mentioning it.

    (From: Flavio Spedalieri (flavios@ihug.com.au).)

    All laser diodes have a tolerance when it comes to wavelength, these tolerances can be as high as +/- 10 nm.

    The wavelength tolerances are due to thermal effects, and current. As the diode heats up, the wavelength will change 0.3 nm/°C. and results in mode-hopping.



  • Back to Diode Lasers Sub-Table of Contents.

    Diode Laser Modules and Laser Pointers

    Alternatives to Using Raw Laser Diodes

    Where what you really want is a visible laser, a commercial diode laser module or some brands of laser pointers (those that include optical feedback based laser power regulation) may be the best option. Both of these include a driver circuit capable of operating reliably on unregulated low voltage DC input and a collimating lens matched to the laser diode. Many of the modules will permit fine adjustment of the lens position to optimize the collimation or permit focusing to a point at a particular distance. Line sources are also available or a point source can be turned into a line source with the addition of a cylindrical lens.

    However, neither of these devices is designed to be modulated at any more than a couple of Hz (if that) due to the heavy internal filtering to protect the laser diode from power spikes. Therefore, they are generally unsuitable for laser communications applications (though some laser pointers are so cheaply designed that such protection may be absent entirely). See the section: The Benefits of Cheap Laser Pointers for Modulation.

    Common visible laser diodes have a maximum optical output power of 3 to 5 mW. Due to the sensitivity curve of the human eye, a wavelength of 635 nm appears at least 4 times brighter than an equivalent power level at 670 nm. Thus, shorter wavelength laser diodes will be best where maximum visibility is important. However, these are currently much more expensive - but this will change as DVD technology takes off.

    Where the use of a diode laser module or laser pointer is suitable for your application, I would highly recommend this over attempting to cobble together something from a bare laser diode and homemade power supply - or even a commercial driver if it isn't explicitly designed for your particular laser diode. It really is all too easy to fry expensive laser diodes through improper drive or handling. Once blown, laser diodes don't even work very well as visible LEDs!

    See the chapter: Laser Parts Sources for a number of suppliers of both diode laser modules and laser pointers. In additiona, Don's Klipstein (don@misty.com) maintains a Web page with a List of Suppliers of Inexpensive Lasers. While not exhaustive, it does include some popular distributors and he does strive to keep it reasonably up to date. Some of these companies now sell laser pointers for under $6! Pretty soon, you will be able to find free laser pointers in cereal boxes. :)

    However, there is no way to know how reliable or robust an inexpensive laser pointer will be - or if the beam quality is acceptable before purchase. Diode laser modules are generally more expensive and of higher quality (though not always) so they may be a better bet for serious applications. Also consider a helium-neon laser since even the cheapest type is likely to generate a beam with better beam quality than the typical diode laser module or laser pointer. While any Tom, Dick, or Harry, can put together a laser pointer of questionable design from readily available parts and sell it on the Internet, only a handful of companies manufacturer HeNe tubes and their quality is all very high. With a HeNe laser, the tube alone determines most of its characteristics requiring at most a simple lens to collimate or focus the beam. See the chapter: Helium-Neon Lasers for more information.

    The Benefits of Cheap Laser Pointers for Modulation

    Ironically, many newer cheap laser pointers can be modulated at very high rates by simply controlling the current from the batteries/power supply. Why? Because they don't have any power regulation and the super cheap Far East imports have no filter capacitors at all. Of course, you risk blowing the laser diode if this isn't done carefully. See the additional comments below.

    On those that do have decent regulators, modulation frequency may be limited to a few Hz to a few hundred Hz depending on design and the actual output power may be more of a triangular wave shape due to the soft start (ramp up, ramp down) turn on, turn off behavior.

    (From: John, K3PGP (k3pgp@qsl.net).)

    The speed issue was true of many early (and pricey!) laser pointers which used a feedback power regulator. The capacitors and the feedback tended to reduce the speed at which the laser could be turned on and off.

    Now that the price has fallen everyone is competing to make them even cheaper. What this means is that most laser pointers today have NO power regulator at all. What I've been finding is a laser diode, resistor, switch, and two 1.5 volt batteries in series. Laser pointers like these can be modulated up into the hundreds of Mhz as there is nothing to interfere with the speed at which the laser can be turned on and off.

    Of course you stand the risk of easily damaging the diode in laser pointers like these with an overvoltage, spike, or static electricity if you don't use some common sense and are not careful when bringing wires out and hooking the laser pen to external circuitry.

    Since we are dealing with a wide variety of styles and manufacturers, there will be some differences. For instance I've seen a few that have no power regulator, just a resistor to the 3 volt battery supply, BUT have an electrolytic capacitor across the diode. It was necessary to remove the capacitor to allow the laser to be switched at high speed.

    Laser Pointer Specifications

    Here are some of the things that manufacturers use to rate and promote both red and green laser pointers:

    By now, you're probably totally confused. My advice: Use the specs for guidance but if you really care about the quality of your laser pointer, try a few out which come with money back no-questions-asked warranties and keep the one you like. If, on the other hand, you just want to use the pointer for presentations (what a concept!) and not to stroke your ego, the cheapest red one will probably be just fine. :)

    Equivalent Brightness Ratings and Laser Pointer Visibility

    Some companies that sell laser pointers, rate them in terms of 'equivalent brightness' compared to a 670 nm device. The Mark-I eyeball is about 7 times more sensitive to light at 635 nm compared to 670 nm. (Green laser pointers at 532 nm will multiply this by another factor of 4 or 5.) (See the section: Relative Visibility of Light at Various Wavelengths.) For example, several of these companies offer laser pointers with a '30 mW equivalent' output. This just means they are comparing a 635 nm device optimistically to one of 670 nm. The actual output power is still less than 5 mW. I do not really consider this deceptive marketing as long as the meaning is understood. Here is a handy quick comparison chart for common and not so common laser pointer wavelengths:

       Wavelength    Relative   Factor    Color           Type
      ----------------------------------------------------------------
        543.5 nm      .974        30      Green      Green HeNe laser    
        532 nm        .885        28        "        Green DPSS laser
        632.8 nm      .237         8    Orange-red   Red HeNe laser
        635 nm        .217         7        "        Red diode laser
        640 nm        .175         5        "              "
        650 nm        .107         3       Red             "
        660 nm        .061         2        "              "
        670 nm        .032         1        "              "
    
    The term "Relative" refers to the visibility compared to the 555 nm peak of human vision; the "factor" compares the brightness to that of an older 670 nm pointer. Note that visual perception of brightness is not linear. Thus, a 1 mW 532 nm green laser pointer isn't actually going to appear 28 times brighter than a 1 mW 670 nm red model. What it means is that a 1 mW green pointer will appear similar in brightness to a 28 mW 670 nm red one (if such a thing existed).

    As far as I know, CDRH approval will not be granted for any device of this type over 5 mW actual beam power since their classification would then need to be IIIb. So, don't expect to find a laser diode with an actual output power of 30 mW in anything like a laser pointer! Frankly, I don't understand how laser pointers with an output above 1 mW gain approval in any case. The 670 nm pointers especially (since they APPEAR less bright) represent a definite hazard to vision at close range. Do not underestimate the stupidity of some people who totally ignore all the safety warnings - "Wow, look at these cool afterimages." - and then wonder why their vision never quite returns to normal (though I do not know of any confirmed cases of irreversible damage to vision even from this sort of abuse).

    Another popular 'specification' is how far away the laser pointer is visible. What the seller is probably actually referring to is the distance that their Marketing department *thinks* the beam should be visible so long as this value is greater than that of their competition. :-)

    Seriously, who knows? There is no standards organization overseeing these ratings. It could be the maximum distance to the screen that the beam is visible:

    1. to the person holding the pointer.
    2. to someone near the screen looking at the screen.
    3. to someone near the screen looking in the direction of the pointer.
    Another consideration, of course, is whether this requires a moonless night!

    Laser pointer marketers don't appear to have discovered (3) as yet (most likely due to liability issues) since the number would be extremely impressive - being in the many miles range! Apparently the Space Shuttle astronauts were able to see a 5 mW red (632.8 nm, similar to the best red laser pointers) HeNe laser from orbit, about 250 miles or 1.3 million feet. Claims could be even more impressive for a green (532 nm) DPSS laser pointer, being about 5 times brighter for the same output power. Any marketing types reading this? :)

    What's Inside a Laser Pointer?

    The description below applies to most red laser pointers sold today (pen or key-chain type). For info on green laser pointers, see the section: Green (or Other Color) Laser Pointers). For a quick introduction to both types, see: The LED Museum's Bit on Laser Pointers.

    A common red laser pointer contains the following components as shown in Typical Red Laser Pointer:

    Photos of the internal components of typical red laser pointers can be found in the Laser Equipment Gallery under "Assorted Diode Lasers". The actual laser diode is not visible in any of these being inside the brass cylinder next to the driver circuit board.

    Laser Pointers that Produce Multiple Patterns

    You've seen the Ads: "Laser Pointer with 42 Heads, $9.95.". These patterns may be in the form of arrows or stars or a company's logo. They are either built-in and selected by a thumb-wheel type arrangement or are in the form of interchangeable tips that slip over the end of the pointer (as in the 'Ad' above). There are 2 basic ways of accomplishing this: Constructing your own pattern generating heads is probably not a realistic option except perhaps for simple patterns using the template approach and even that would be quite a challenge given the small diameter of the beam as it leaves the pointer. Considering how cheap these things are now, it is also probably not worth the effort unless it's something very special.

    In my opinion, except possibly for an arrow, these things are really of little practical value.

    Orange, Yellow, and Green Light from Red Laser Pointer?

    While the lasing line from a diode laser or even a cheap laser pointer is quite narrow, there can be other wavelengths of incoherent light present in the beam. Since the effective aperture of the laser diode is nearly a point source (1x3 um typical), these spurious outputs will still collimate and/or focus nearly as well as the laser beam itself. However, it's highly unlikely that any of these are actual lasing lines except very near the main (design) wavelength. No, you can't convert a red laser pointer into a rainbow pointer with a simple modification performed on your kitchen table! :)

    I've seen the existence of faint non-lasing light from at least one cheap laser pointer - I only own two red pointers so 50 percent isn't to shoddy! To test for this (assuming you don't have an optical spectrum analyzer handy), if the pointer doesn't have an adjustable focusing lens, use a weak positive lens to focus the beam at a distance from the pointer of 0.5 to 1 meter - where the spot is still quite small, say less than 1 mm. Then, use a diffraction grating (almost any will do including a CD or DVD) to view one of these focused first order spots on a white card. Set things up so the spot is either blocked or misses the card entirely so all you see is the area towards the 0th order spot (undeflected beam). For my sample, there was a continuous tail amounting to a few dozen nm. I couldn't quite tell if it hit green but definitely was well into the yellow.

    Another approach is to pass the beam of the pointer through a series of mirrors that only transmit non-red wavelengths or reflect it from a series of mirrors that only reflect non-red wavelengths. Using a pair of HeNe laser resonator mirrors (an HR and OC in series) reduced the intensity of the red wavelengths by a factor of about 100,000 so only a hand full of red photons got through. :) This allowed me to clearly see the orange, yellow, and green output of the laser pointer mentioned above by looking into the beam through a diffraction grating. (Yes, this is safe once the red is filtered by the two mirrors. It's just a dim glow and barely visible when projected on a white screen in pitch blackness.) WARNING: Don't try the equivalent experiment (looking into the filtered beam) with a DPSS (green or blue) laser as there could be a significant amount of mostly invisible pump light at around 808 nm that gets through to fry your eyeballs.

    If you can power the pointer from an adjustable DC power supply (or have some weak batteries), there may be an even easier way to see the non-lasing colors - power the diode just below the lasing threshold. Under these conditions, output at the lasing wavelength won't drown out the broad-band LED emission and it will be easy to see its spectrum using any diffraction grating or prism (or even through the edge of lens in a strong pair of glasses!).

    The use of the human eye apparently works a lot better than a fancy Optical Spectrum Analyzer (OSA) because the intensity of the level for the non-lasing wavelengths is so low and spread over a substantial range. The only thing visible using an Ando OSA set to maximum sensitivity and averaging 10 times was a slow increase in amplitude starting at about 566 nm and continuing to the lasing wavelength of about 635 nm, but this wasn't even conclusively above the noise floor for the instrument.

    (From: Steve J. Quest (squest@att.net).)

    The keyword here is you have a CHEAP laser pointer. I'm going to presume the injection crystal lattice has contaminants in it, more likely if the manufacturer also builds LEDs in the same factory. What you are getting from your laser is a RED laser beam, and possibly green, orange, and yellow LED light (non-coherent) which is also coming from the same crystal. Fire it through a prism to see the various lines, I bet it's so polluted with foreign dopants, that it produces a bright red coherent line, and a few non-coherent red lines, an orange line, a yellow line, and a green line. That's all possible since the injection diode crystal is basically an LED crystal with perfectly cleaved ends, and a channeled electron injection pathway, axial to the beam.

    You can typically see this effect if you test the cheapest LEDs you can find with a prism. I've found that dirt cheap green LEDs usually produce both a green and a yellow line. Dirt cheap reds produce several lines of red. You can get many wavelengths out of a gallium arsenide crystal.

    Green (or Other Color) Laser Pointers

    Red laser pointers are by far the most common and now quite inexpensive. Pretty soon, they will be given away free in specially marked boxes of corn flakes. :) Seriously, prices under $5 aren't uncommon and dropping rapidly. However, except for various shades of red (depending on wavelength), all other colors are very expensive. In fact, there is really only one other color of any practical consequence - green. And this is a much different type of laser than the simple diode lasers used in red laser pointers.

    Currently, nearly all green laser pointers are based on Diode Pumped Solid State Frequency Doubled (DPSSFD) laser technology. They are not just red laser pointers with a different laser diode or green lens! (See the section: Diode Pumped Solid State Lasers.)

    The exceptions are older models using green helium-neon (HeNe) lasers. I bet you didn't know HeNe lasers came in green, huh? :) These had power outputs of less than 1 mW and were much bulkier than modern laser pointers. And they are now about as common as raw dinosaur eggs. (See the section: HeNe Tubes of a Different Color if you are curious.)

    The wavelength of the DPSSFD lasers is 532 nm based on the intracavity frequency doubling a Nd:YVO4 (vanadate) chip using a KTP crystal inside the laser cavity. Their output may either be CW, quasi-CW, or pulsed. CW means "continuous wave" which results in a constant intensity spot. Quasi-CW and pulsed both result in a spot that varies in intensity (so they are really both pulsed output) but the pulses for the quasi-CW variety may be at a much higher frequency (e.g., 5 kHz versus 300 Hz). You can tell which you have by moving the spot rapidly across a screen - the trace from the quasi-CW and pulsed types will break into discrete spots. However, the spot spacing for the quasi-CW pointers may be so small for normal use that for all intents and purposes, they will appear continuous. However, a quasi-CW pointer would not be a good choice to use in a laser show application. (Note that there is no standard for calling a particular pointer quasi-CW or pulsed so your advertising blurb mileage may vary!)

    Visibility of these green pointers is 4 to 5 times that of 635 nm diode lasers or 632.8 nm red HeNe lasers, which in turn appear 6 or 7 times brighter than the older 670 nm laser diode based laser pointers for the same power output. The maximum legal green laser pointer power is still only 5 mW but this would be equivalent in brightness to something like a 150 mW, 670 nm device! And, the sellers of these things don't let you forget it! :)

    Battery life of any green pointer is likely to be much worse than that of the simpler red variety though for actual uses as a *pointer* (what a concept!), it probably doesn't matter all that much. The quasi-CW and pulsed variety should be somewhat better in this regard. (The "spec" sheet that comes with the Edmund Scientific L54-101 green laser pointer claims a 3 to 4 hour battery life from a CR2 lithium cell though I'm not sure I believe it.) 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 (frequency in the kHz range) 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. Pulsed pointers (frequency in the hundreds of Hz range or less) use a pulsed pulsed diode.

    Note that since there is no real control of temperature, power output may change significantly (up or down or both) if the pointer is kept on for an extended period of time. Usually, since pointers are really intended to be used for brief periods of time for pointing at something, if any optimization was done, the manufacturer would attempt to select the laser diode wavelength to match the vanadate's absorption band when the components are cool. As the laser diode heats up, its wavelength increases (about 0.3 nm/°C) and drifts away from the optimal value. (Even though the absoprtion band is quite broad, there may still be some noticeable effect.) However, if the wavelength was low to begin with, the power would increase as the wavelength moved toward the peak absorption for the crystal and then decrease if it went far enough. From my experience with these as well as other basic green DPSS lasers, unlike red laser pointers whose output is either constant or gradually dropping in intensity until the batteries poop out, expect a modest amount of slow cyclical and even possibly some sudden power fluctuations as the temperature of key components increase and lasing characteristics change. So, a typical green pointer may actually dip to less than 2/3rds of its rated power at times, hitting the rated power only occasionally. None are rock steady at their rated power. For that, you pay really big bucks. :) Maybe the next wave of green pointer technology is a light sensor to maintain constant output power! And don't forget that just because the CDRH safety sticker may say 5 mW max, your actual model may not come anywhere near that - ever. The actual power rating would be listed elsewhere. But providing it at all is rare, partially due to the fluctuation problem, but mostly because the manufacturers figure you're better off not knowing how mediocre the pointer realy is!

    With the much higher prices for green pointers, make sure you get a decent written warranty. Prices are currently averaging about $300 (in 2001) though I've seen some 3 mW models advertized on the Web for as little as $180, lower on eBay). And supposedly, though I haven't tried to buy one, there is at least one company (Leadlight Technology, Inc., Taiwan) who will sell 1 to 3 mW green pointers for as low as $88, quantity 1. Although some may consider it unethical, ordering several pointers and only keeping the best may be the only way to assure satisfactory performance as they are quite variable in output and stability. The additional complexity and more delicate nature of the individual components means that reliability and robustness may not be as good as for their red cousins (to the extent that these are reliable and robust!). This means that while those fancy polished wood cases look impressive, transporting the pointer in a well padded case is probably a better idea. Comparing the detailed diagrams of a Typical Red Laser Pointer and the Edmund Scientific L54-101 Green DPSS Laser Pointer, or the single diagram Comparison of Red and Green Laser Pointer Complexity. (The L54-101 is a $395 model, but even so, it's amazing prices aren't a lot higher as it has all the sophistication of a much more expensive DPSS laser.) Even a failed switch just out of warranty (assuming there is a warranty that will be honored in the first place!), can render a $300 pointer useless since there is often no non-destructive way of getting inside to repair it. (And, I've heard that the switches they use on these things are often not adequately rated for the much higher current green laser pointers use compared to red ones.)

    For more information on DPSS lasers and green laser pointers including details of the L54-101, see the sections starting with: Diode Pumped Solid State Lasers.

    And, what about those other colors? As a practical matter, there isn't much need for anything beyond green since its wavelength (532 nm) is near the peak (555 nm) of the human eye's response curve. However, to impress those high flying corporate executives, blue might be cool - but expect to spend a $2,000 for one using DPSSFD technology that isn't as bright as a $5 red pointer. I think yellow would look nice on dark color slides, but the only way to do this now would be to use a yellow HeNe laser (yep, they come in yellow also!) as there are no yellow laser diodes or practical DPSSFD alternatives. Same for orange. At the other end of the spectrum, violet (which would be really hard to see) laser pointers using the Nichia violet (400 to 415 nm) laser diodes could be built inexpensively like red ones since the circuitry is about as simple - except for one minor detail: the cost of these violet laser diodes is presently (February, 2001) still around $1,000 each! A violet pointer might impress the corporate big-wigs but due to the lack of visibility, would be quite useless for presentations unless the projection screen had a coating that glowed when hit by violet light. Hmmm, now that's an idea. :)

    There are inexpensive LED-based keychain pointers in bright blue and other colors but these are not true lasers and the divergence is typically 5 to 10 degrees instead of 1 or 2 milliradians (1 degree = 17 mR). But, if all you want to do is impress management types, that may be good enough. :)

    And, no, there is currently no technology capable of producing a variable color laser pointer.

    Additional Precautions with Respect to Green DPSS Laser Pointers

    Unfortunately, these usually don't come with any sort of useful user manual.

    Much of the following applies to any laser pointer but especially to the expensive green variety:

    Difference Between Diode Laser Modules and Laser Pointers

    A collimated diode laser module and pocket laser pointer both produce a spot of light. So why the typical huge difference in price?

    The simple answer is: It all depends. :) There can be variability in any type of product. While the desired output of a laser pointer and collimated diode laser module is similar, how fussy the end-user is and how one gets there may not be:

    In the end, it is probably the mass production that is the most significant factor in keeping costs down.

    There is also another difference between the two which relates to output power:

    Sources for Inexpensive Diode Laser Modules

    Unless you find a really good deal on excess inventory or the like, the guts of laser pointers are probably the cheapest source of decent quality diode laser modules for many applications. These are mass produced so cost can be quite low. There are many suppliers who will sell you just the laser diode in a brass mount with adjustable collimating lens and attached driver circuit on a tiny PCB for under $10 for a single unit, less in larger quantities.

    These aren't likely to be in the same league as the $300 diode laser modules from Edmund Scientific or even $100 units from other sources which will meet or exceed all specifications and have protection against all reasonable abuse, for the price, they can't be beat!

    With respect to specifications:

    See the suppliers listed in the chapter: Laser and Parts Sources.

    How to Determine if You Have a Diode Laser Module

    So you found a bag of cute little brass devices marked 'barcode lasers' at a garage sale. They have wires coming out of one end and a lens at the other. Are they bare laser diodes or do they have a built in driver circuit? Size alone is no real indication as the driver circuits can be quite tiny. Assuming that analyzing the circuit isn't possible or appealing and they are not clearly labeled (in which case you wouldn't be reading this anyhow), closely examine the wire leads:

    Brightest Laser Pointer for Outdoor Use?

    A laser pointer is a bright source of light but so is the Sun. :)

    The maximum legal limit for power output from any laser pointer in the U.S.A. is 5 mW - Class IIIa (there may also be more restrictive local regulations and it's lower in some other countries). The best color to use is green since the wavelength of modern green laser pointers based on Diode Pumped Solid State (DPSS) laser technology (532 nm) is very near the peak of human visual sensitivity (555 nm). Thus, a 5 mW green laser pointer produces nearly the brightest beam allowable by law (about 0.9 relative to 555 nm). (Although older green laser pointers based on green helium-neon lasers were a bit closer at 543.5 nm, one capable of 5 mW would be almost a meter long and weigh several kilograms with the required backpack mounted battery and high voltage power supply.) Whether the beam is pulsed or continuous doesn't make much difference. However, the spot from a low divergence beam may be somewhat more visible at a distance on a brightly illuminated surface (see below). The difference between a 4 or 5 mW pointer isn't really that significant (it's barely detectable even with two pointers side-by-side), and as a practical matter due to the technology, output may vary by as much as 30 percent (up, down, or in a cycle) as components heat during use.

    So, if even 5 mW of green isn't bright enough, the optimal solution would be to control the ambient illumination by putting a dimmer on the Sun. :) If this isn't an affordable option, the best that can be done is to use a screen or whatever that is a light color and has a diffuse surface, and orient it to avoid direct Sunlight. Unfortunately, if there is no way to control any of this as would be the case with use by an outdoor tour guide, there are no good solutions. Even the best laser pointers have a divergence no better than about 1 milliradian (1 part in 1,000) so the power density of a 5 mW green spot projected on a surface more than a few meters away drops well below that of the 0.5 to 1 mW per square millimeter of Sunlight. Even the pure green color of the laser pointer will be quickly overwhelmed by the ambient illumination.

    Can I Boost the Power Output of a Laser Pointer or Diode Laser Module?

    The quick answer is: Probably not, or at least, not by much.

    I know that in your fantasies, you have dreamed about the possibility of creating a burning laser or Star Wars style light saber from a laser pointer. Unfortunately, neither of these is even possible theoretically. The best you could ever hope for would be to obtain at most 5 mW from a device currently outputting 2 or 3 mW.

    While it might be feasible to increase the current to the laser diode, unless you know its specifications AND have an accurate laser power meter (mucho $$$), there is no way of knowing when to quit. Above their rated maximum optical power, laser diodes turn into DELDs (Dark Emitting Laser Diodes) or expensive LEDs. Exceed this rating for even a microsecond and your whimpy 3 mW output may be boosted to precisely 0.0 mW. This is called Catastrophic Optical Damage (COD) to the microscopic end-facets of the laser diode. There can be also be thermal runaway problems or a combination of both of these depending on design - or lack thereof. However, if you have a bag of these gadgets and are willing to blow a few, here are some guidelines:

    But, in any case, how will you know when to quit before the laser diode is irreversibly damaged? And, in addition to exceeding the maximum rated output power as you crank up the laser diode current, an electrostatic discharge, a voltage spike from an external power supply, a noisy power adjust pot, or the phase of the moon on an alternate Tuesday, may be enough to blow it! By the time you notice a problem, it will likely be far too late for the health of your poor little defenseless laser diode!

    This really IS like playing Russian Roulette and my serious recommendation would be to leave well enough alone. Save for a more powerful unit or even just a 635 nm laser pointer if your current model is 670 nm (which will appear at least 5 times brighter for the same output power).

    If you do insist on modifying the circuitry, use an antistatic wrist strap, grounded temperature controlled soldering iron, and the proper desoldering equipment (if needed). At least then, you'll know that it was more likely the changes to the circuit that blew out the laser diode, not your rework technique. :)

    Also see the section: Determining Characteristics and Testing of Laser Diodes and those starting with: Laser Diode Life, Damage Mechanisms, COD and ASE, Drive, Cooling.

    The same basic comments apply to boosting the output power of expensive green laser pointers (but of course there is much more to lose). The adjustment may vary current or for those that are pulsed (which are most of them), the duty cycle instead. With no thermal management, stability is likely to be significantly worse at higher power even if the laser diode survives. However, since 3 mW and 5 mW pointers may be physically identical inside and out, I don't know whether they are sorted on the basis of power output before labeling or is just a matter of the setting of the power adjust pot - it probably depends on manufacturer/model.

    Having said that, I've heard of this being successful and I've also heard of at least one sample of a green laser pointer producing 36 mW out of the box. :) The vanadate/KTP crystals should be capable of much more than 5 mW, at least for awhile. However, in the samples I've seen, the discrete vanadate is mounted by just two tiny dabs of adhesive which could easily come unglued if the crystal gets hot (which it would with higher pump power). Green pointers using composite (e.g., CASIX) crystals would eventually suffer from the dark spot problem in the glue used to hold them together. There are instances of very "lively" pointers where just tweaking the OC mirror could result in increased power if not optimally adjusted originally. I'd consider this the exception though. Most likely, boosting power would require higher current to the pump diode which will result in shorter life or no life at all!

    (From: HippyLaserTek (hippylasertek@aol.com).)

    Since the switch died in my green pointer, I said what the hell, and gave it a shot. (For crying out loud, why don't they replace the switch with a soft touch type like in a calculator and a saturation driven transistor! Hell at $200 to $300 a pop that's the LEAST they can do!)

    Well I didn't expect 50 mW out at reasonable currents but I DID get around 15 mW of green out just by carefully tweaking on the three setscrews which adjust the OC mirror position. The only sacrifice was a slight decrease in beam quality so it looks oval instead of round, but for a pointer module, who cares anyway.

    It was cool not only seeing that kind of power from the pointer, but the mode patterns as well were rather interesting too. Some of the patterns were very beautiful. By turning the current up from it's original 400 mA to 450 mA, it topped 25 mW, the max my low power laser meter reads! It's rated for HeNe light, so i don't think it responds the same for green. I think it gives a false low reading though, I KNOW it does for blue. (This is true for a typical silicon photodiode, possibly as much as 20 to 25 percent reduction at 532 nm compared to 632.8 nm. --- Sam.)

    Going the other way I got green threshold at a mere 140 mA and "rated power" of 4.8 mW at around 250 mA. I'd LOVE to install a 2 watter pump diode in place of the 0.5 W? (tested at 0.4 W at 400 mA on my Ophir power meter set on shg/dye/argon setting) pump diode in it. I am fairly certain with that diode pumping the DPSS laser guts it would EASILY give out 75 to 100 mW. (See cautions, above. --- Sam.)

    Other things of interest is the 1,064 nm IR was negligible in power, only about 0.03 mW and IS NOT BLOCKED BY THE LITTLE BLUE FILTER. When at 85 to 90 °F pump diode leakage was negligible also, but if it's cold, say 55 °F pump leakage was over 50 mW but this IS blocked by the filter. It is also blocked by the filter in my power meter too so I had to remove it to take a reading. (The power meter probably also reads load at 1,064 nm. --- Sam.)

    Despite the high power, this is not quite as much of a hazard as this was right at the output of the brass part, by the time it reaches the output lens it is reduced to only 7 mW or so and diverges very fast. The YAG beam is concentric with the green beam.

    The laser's life as a pointer is over, but it is turned into a nice module. I replaced the cheap lens in it with a nice 1/2" diameter lens assembly from a target designator. The assembly also gives it the badly needed heat sinking the module calls for. The best part is though the beam is now about 1/4" diameter it has SERIOUS range and can go 25 feet and still be about the same size!

    What About Using Rechargeable Batteries in a Laser Pointer?

    This probably only makes any sense for power hungry green laser pointers since the batteries in red ones should last a long time due to their lower current drain (about 1/5th to 1/10th that of greens).

    The problem with using NiCd or NiMH cells to replace Alkaline types is that since the voltage is lower (1.2 V/cell versus 1.5 V/cell when fresh), the output may not be as bright if the pointer doesn't include decent regulation or its compliance range is inadequate. Thus, it will be necessary to adjust or change whatever is used for current control in your pointer so it provides the proper current to the laser diode at the lower operating voltage of the rechargeable batteries. Note, however, that since the A-hr capacity of rechargeables is less than that of Alkalines, lasing time will be reduced if they are used. (This is somewhat compensated by the flatter discharge curve of NiCds and NiMH cells and your mileage may vary.) Of course, you risk blowing the circuitry and/or laser diode should you then install Alkalines, so you may not be able to easily go back to them. As with the other comments on modifications to laser pointers, this is quite risky both in terms of possible damage to the laser diode as well as being able to make any modifications to the teeny tiny circuit board if needed.

    I've have heard of people (apparently with money to burn), successfully doing this with a green ($$$) laser pointer. They changed the value of the resistor used to set the laser diode current and were able to get slightly more power at the same time (expected life unknown). (Interestingly, at the original power, the beam was TEM00; with increased power, it became multimode.)

    Can I Increase the Life of a Laser Pointer?

    While the typical 5 mW laser diode may have a specified life in excess of 100,000 hours (8 years, yeah, sure!), one often finds that the $6.95 variety of laser pointers last a whole lot less than 8 years. :) It isn't possible to provide a universal procedure that will get the most life from any laser pointer. However, knowing that excessive current and singular overcurrent events ruin laser diodes should provide a basis for some recommendations: You may be better off buying a better quality diode laser module as they will have the necessary current regulator using optical feedback and other laser diode protection circuitry. While diode laser modules are generally much more expensive than cheap laser pointers, there are some that are cheaper than fancy laser pointers (which still may be low quality inside). Got that? :-)

    Optical Modulation of a Laser Pointer or Diode Laser Module

    It's a cute idea: Introduce an external light source to 'fool' the internal optical feedback circuit into thinking that the laser power is higher than it should be. The driver will then cut back on current to compensate. If you shine certain laser pointers at a mirror, their output will drop dramatically. However, this effect may be due to the monitor photodiode sensing the added light and cutting back on laser diode current, or due to light getting inside the laser diode cavity and messing up lasing. Apparently, the latter effect as unlikely as it sounds, may be the one that is more likely, at least with certain models.

    One way to tell which effect is causing the change in output power is to measure the laser diode current: If it drops with the reflection, the cause is likely the simple optical feedback mechanism. If on the other hand it increases, then laser instability is likely. Also see the section: Causes of Laser Pointer Output Power Changing When Directed at a Mirror.

    Even if the photodiode sensitivity is the cause, several factors conspire against this being a viable technique in general (though it may work with specific devices):

    And, if it is actually a lasing interference effect, good luck succeeding in getting anything to be repeatable or stable unless you have a granite block or sand-box holography setup. :)

    If you still insist on experimenting, be aware that while this appears to be safe for the laser diode, there is no way of knowing for sure without tests. There could be funny resonances in the driver that will blow your laser diode at certain frequencies! And, if the effect is due to lasing instability, the regulator may attempt to boost the current to compensate resulting in possible overheating of the laser diode, driver, or both.

    My informal experiments have turned up both effects, one of each for a couple of laser pointers and quite noticeable photodiode based power suppression with an NVG D660-5 (just happened to be one I tried) on an optical feedback regulated driver - shining a laser pointer into the laser diode window resulted in almost total supression of lasing. I suspect that the pointer affected by interference inside the cavity went into overcurrent or thermal shutdown (as it refused to lase at all for several seconds after the test). And, a few days later, it was obvious that the output power had decreased and the beam pattern was messed up, a sure indication of facet damage, which probably happened immediately but I just didn't notice it.

    Causes of Laser Pointer Output Power Changing When Directed at a Mirror

    The following discussion resulted from the claim (mine and others) that reflecting the output of a laser pointer or diode laser module from a mirror might result in a decrease in output if it had optical feedback for power regulation. On one laser pointer I have, there is absolutely no effect. On another, output power drops by at least 50 percent. My assumption was that it was the light reflected back and falling on the monitor photodiode that caused the effect and not some weird interference to the lasing process. But given what is described below, I'll concede that in many cases, it may indeed be the latter.

    One way to tell which effect is causing the change in output power is to measure the laser diode current: If it drops with the reflection, the cause is likely the simple optical feedback mechanism. If on the other hand it increases, then laser instability is likely.

    It does seem that relatively low reflected power back to the laser diode can affect lasing. This has been used to advantage in narrowing the line width of common laser diodes with an external cavity. See, for example, U.S. Patent #4,907,237: Optical Feedback Locking of Semiconductor Lasers.

    CAUTION: While I've never heard of any damage resulting from these sorts of experiments with common laser pointers or diode laser modules, anything that affects the lasing stability and monitor photodiode feedback could result in excessive current and damage to the driver, laser diode, or both.

    (From: John, K3PGP (k3pgpalltel.net).)

    This is pretty much my findings here also.

    However, since laser pens seem to be built as cheaply as possible there are NO standards! What works with one may not work with another. This has caused me untold grief when trying to discuss most anything about laser pens!

    I have a few laser pens here that go nuts when you aim them at a mirror. With some pointers the mirror has to be precisely aligned much the same as the mirrors at the ends of the laser cavity itself. With others the alignment isn't as critical. These same pens seem to be unaffected by other light sources shining back into the laser including light from another laser pen with the same approximate wavelength.

    I think the important fact to those those units that were affected is whether or not the incoming radiation was precisely the same frequency as the oscillation in the laser cavity. When this experiment is set up with a pen that is sensitive to this effect, EVERYTHING affects the setup, even the slightest vibration which makes sense (to me anyway!). It kind of reminds me of the Michelson Interferometer or a holographic setup. I assume this interference effect is the same effect noticed with many HeNe lasers where no power sensing diode is involved.

    (From: Sam.)

    That would seem to confirm the hypothesis that interference with the lasing process is taking place, at least for those cases. I'm surprised they would be so sensitive.

    (From: John.)

    These pens seem to be somewhat rare though as most of the laser pens that I have don't seem to care what you shine back at them. Since laser pens differ so widely from one manufacturer to the next and even between identical model numbers from the same manufacturer I'm not sure if the differences are being caused by the use of different laser diodes or perhaps this effect is somewhat critical as to the amount of current passing through the laser diode or something else?

    (From: Sam.)

    Conceivably, the sensitive laser diodes are being operated on the verge of mode hopping or something like that but I'm more inclined to believe it is just a sample to sample variation or laser diode model dependent.

    (From: John.)

    When trying this experiment with several different HeNe lasers I've also noticed that some are effected to a much larger extent than others. I'm not sure why this is. Maybe it has something to due with the gas mixture, the pressure, the current passing through the tube, or what else?

    (From: Sam.)

    Also mirror reflectivity and curvature. The gas mixture, pressure, and current are probably less of an issue as long as it is running somewhere around the correct conditions.

    When you reflect a beam back into a HeNe laser, it's only .5 to 2 percent of the strength of the output beam and order of .01 percent of the strength of the circulating photon flux inside the tube unless the external mirror is very close to being parallel to the output mirror. Then, there will be multiple bounces and much of the light makes it back to the cavity... Hmmm. The distance also matters due to interference effects and the curvature of the mirrors affect the shape of the wavefront. Possibly HeNe lasers with close to planar mirrors are more sensitive to this. However, just the light bouncing back and forth and interfering with itself outside the cavity can confuse the observations. What a mess. :)

    Disassembling a Laser Pointer

    In the old days before the dinosaurs roamed the Earth and even before cell phones, laser pointers may have been constructed in a such a way that they could be taken apart and put back together again. Regrettably, that is no longer the case. Among your options are a hacksaw, lathe, hammer, and Dynamite - or something stronger. :) It can be done but don't expect to get the pieces together again, at least not in an aesthetic package.

    For red laser pointers, note that some/many/most of the newest and cheapest imports may not even use a packaged laser diode - the bare chip is attached directly to a metal header next to the lens. I wouldn't be too optimistic about repair or reuse of one of those.

    The deconstruction process for a typical green (DPSSFD) laser pointer - a much more complex device than the red variety - is shown in the Laser Equipment Gallery (Version 1.47 or higher) under "Dissection of Green Laser Pointer".

    Can a Fried Laser Pointer or Diode Laser Module be Repaired?

    Suppose someone offers you a diode laser module that has been damaged by applying incorrect power (the smoke all leaked out) for $5. Should you accept it? Is there any hope that the laser diode itself survived?

    The quick answer is a definite maybe IFF the module or pointer can be opened for examination or repair. If it is a potted block, forget it.

    The chances of success are much greater for a diode laser module since it is likely to have a proper laser diode driver with current regulation and optical feedback. These are typically so over-designed that while applying excessive voltage (well, within reason, not 120 VAC to a 5 VDC module!) or incorrect polarity may blow some components, chances are that the laser diode itself won't feel a thing and will survive unharmed.

    Assuming you can get inside, repair should be possible. And, even if you end up having to replace a 5 mW laser diode (for, perhaps $10), you have made out well. High quality diode laser modules go for anywhere from $50 to $300.

    However, depending on design, a laser pointer could be totally destroyed by even modest overvoltage (say 5 V instead of 3 V from 2 AAA batteries) or reverse polarity. Some of these don't have anything more than a resistor for current limiting. So the laser diode could very well have been damaged or turned into a DELD (Dark Emitting Laser Diode) or expensive LED. All you may end up with is a nice (or not so nice) case. :-( Of course that in itself may come in handy to package your own laser diode and driver - ignoring what was originally there. However, see the next section for more on this exciting topic. :)

    Repair of Diode Laser Pointers

    The following applies to laser pointers containing just a battery, driver, laser diode, and optics. For now, this is only the red variety though pointers using the Nichia violet laser diode, as useless and expensive as they may be, would also qualify. :) For green or other DPSS laser based laser pointers, there is the additional complexity of the DPSS laser module itself. See the section: Repair of DPSS Laser Pointers. And, for older style helium-neon laser based laser pointers, see the chapter: HeNe Laser Testing, Adjustment, Repair.

    With prices as low as $4.95, serious troubleshooting and repair of a cheap red laser pointer probably isn't worth the effort, time, and expense. But if you have one with 58 pattern generating heads or just want the educational experience, there may be a possibility of repair even though many of these things are not designed with user serviceable parts inside.

    Refer to Typical Red Laser Pointer for a general idea of what to expect. The detailed disassembly procedure will depend on the exact model. A combination of screw, press-fit, and glued construction is likely. Non-destructive disassembly may not be possible for some.

    Here are possible problem areas for a pointer that is weak or dead and hasn't been run over by a Sherman Tank:

    Cleaning Diode Laser Module Optics

    Note: There are additional considerations when cleaning the optical pickups found in CD and LD players, CDROM drives, and other optical storage devices. For more information, see the document: Notes on the Troubleshooting and Repair of Compact Disc Players and CDROM Drives.

    There are at least 3 surfaces that can collect dirt - the two sides of the lens (it is probably a single element) and the exterior of the laser diode window. However, in all likelihood, only the exposed surface of the lens will need cleaning.

    First, gently blow out any dust or dirt which may have collected inside the lens assembly. A photographic type of air bulb is fine but be extremely careful using any kind of compressed air source. Next, clean the lens itself. It may be made of plastic, so don't use strong solvents. There are special cleaners, but isopropyl alcohol usually is all that is needed. 91% medicinal should be fine, pure isopropyl is better. Avoid rubbing alcohol especially if it contains any additives.

    Lens tissue is best, Q-tips (cotton swabs) will work. They should be wet but not dripping. Be gentle - the plastic (probably) or glass and particularly the anti-reflection coating on lens is soft. Wipe in one direction only - do not rub. Also, do not dip the tissue or swab back into the bottle of alcohol after cleaning the optics as this may contaminate it.

    The alcohol should be all you need in most cases but some types of dirt (e.g., sugar) will respond better to just plain water.

    The inside surface of the lens, any other optics, and the window of the laser diode can be cleaned in a similar manner should this be necessary. Usually, it is not.

    Do NOT use strong solvents (which may attack plastic lenses) or anything with abrasives - you will destroy the optics surfaces.

    CAUTION: Lenses or other optical components may be bonded or mounted using adhesives that are soluble in alcohol or acetone (but probably not water). Don't make the mistake I made and use too much solvent. I still have not found the tiny collimating lens that popped out of a laser diode module and is now likely lost forever to the basement floor. Crunch :-(.

    Damage to Camera Sensor from Laser Pointer?

    Even a 1 mW laser beam can potentially produce permanent damage to the CCD or silicon sensor array insid e a video or still digital camera.

    If the camera is focused at infinity, a collimated laser beam will be focused to a tiny spot on the image sensor. Whether damage will occur depends on many factors including the type of image sensor, quality and focus of the optics, and how long the beam is held in one place. A 1 mW beam (much less than what some laser pointers produce) is roughly equivalent to the brightness of the noonday Sun at the equator on a clear day and when focused to a 10 um spot (the approximate size of one pixel on a typical video camera) it becomes 10,000 times more intense! Needless to say, pointing a camera at the Sun is generally not recommended.

    Anatomy of Fiber Coupled Laser Diodes

    These modules aren't the sort of thing you will find at your local K-Mart but may turn up surplus from communications, medical, or other applications requiring delivery of a high power laser beam over a fiber optic cable.

    WARNING: Class IV laser products - the output from the fiber will destroy vision and set things on fire!

    Fiber coupled laser diodes are much easier to use than bare laser diodes even though they still need an external high current driver. (Of course, they are also much more expensive.) Aside from the physical protection provided by the packaging, the output of the fiber is a nice circular beam with modest divergence (about 16 degrees full angle) which doesn't require correction for astigmatism or asymmetry. Thus, simple lenses can be used for collimation and focusing. I've used a good sample of the 808 nm version of the first laser described below to pump the guts from a green (DPSS) laser pointer just by holding the end of the fiber next to the Nd:YVO4 crystal. After adding a coupling with a GRIN lens for focusing, I can get a few mW of green light from it though I suspect the diameter of the pump beam is still larger than optimal. These will also easily pump the CASIX DPM0101 and DPM0102 Nd:YVO4/KTP composite crystals as well as other microchip lasers.

    (Note that Opto Power is now part of Spectra-Physics but these lasers predate the merger which may be one reason for the very different types of technology used in the construction of the first three lasers, below).

    Opto Power Corporation fiber coupled laser diode:

    The first unit I dissected is typical of 0.5 to 1.5 W fiber coupled diode lasers. Refer to Typical 1 Watt Fiber Coupled Diode Laser Showing Interior Construction and Closeup of 1 W Fiber Coupled Laser Diode Showing Cylindrical Microlens and Fiber Tip while reading the following description.

    WARNING: The output beam of high power laser diodes with an attached microlens (or other collimating optics) is much better collimated than we are used to for laser diodes - closer to that of a "real" laser. The divergence is typically 10x4 degrees as opposed to 10x40 degrees for a bare laser diode. What this means is that both the direct beam and any specular reflections are MUCH more dangerous to vision even several feet away from the source. Even the reflection from a shiny IR detector card can be dangerous. This is especially scary for people who have become complacent working with laser diodes being used to beams that spread out to safe levels in a few inches.

    The overall package is 1.5"(L) x 0.75"(W) x 0.5"(H) and is made of a block of gold plated brass with a milled cavity. There are red and black wires for power and a single-mode fiber with SMA 905 connector for beam delivery.

    After prying off the Epoxied lid, the following can be seen:

    Here are a couple of other lasers that yielded to my set of hex wrenches - no chisels or cutting torches required. They were mostly dead so no need to call out the SPCL (Society for the Prevention of Cruelty to Lasers:

    Spectra-Physics fiber coupled laser diode:

    This is an 803 nm unit with a power output of around 1 W model unidentified. It's application is also not known. Correction optics consist of a short focal length collimating lens glued to the rectangular diode "H" package to collimate one axis, a cylindrical lens to correct the other axis, and an adjustable (in X,Y,Z) focusing lens to get the light into the fiber core. The distance from the focusing lens to the fiber tip (Z) is quite critical but the position of the fiber in X and Y has a broad peak since the beam into it is quite well collimated and smaller than the lens.

    Opto Power Corporation high power fiber coupled laser diode:

    This one is strange. It has a rated output power of at least 6 W into a multimode fiber. Input power is the usual 2 V or so but up to 12 A!

    Spectra Diode Labs (SDL) fiber coupled laser diode:

    Here is a link to a Web site that shows details of the internal construction of an SDL 2300 series diode which is basically similar to the Opto Power laser, above:



  • Back to Diode Lasers Sub-Table of Contents.

    Low Power Visible and IR Laser Diodes

    Low Power Visible Laser Diodes

    These are the typical 3 to 5 mW (maximum power) visible laser diodes probably emitting at a wavelength in the 635 to 670 nm range. They are found in all modern red laser pointers, newer barcode scanners, laser light positioning devices, and now in DVD (Digital Versatile/Video Disc) players and DVDROM drives.
    1. You can easily destroy the typical laser diode through instantaneous overcurrent, static discharge, probing them with a VOM, or just looking at them the wrong way. :-)

    2. By far the easiest way to experiment with these devices is to obtain complete laser diode modules. Versions are available with both the drive circuitry and (adjustable) collimating optics. They are more expensive than raw laser diodes but are also virtually foolproof. Inexpensive laser pointers are one source for similar devices which may be adequate for your needs but modifying them could be more of a challenge. See the chapter: Laser and Parts Sources for suppliers of both raw laser diodes and laser diode modules.

    3. Any time you are working with laser light you need to be careful with respect to exposure of a beam to your eyes. This is particularly true if you collimate the beam as this will result in the lens of your eye bringing it to a sharp focus with possible instantaneous retinal damage.
    Typical currents are in the 30-100 mA range at 1.7 to 2.5 V. However, the power curve is extremely non-linear. There is a lasing threshold below which there will be no coherent output (though there may be LED type emission). For a diode rated at a typical current of 85 mA, the threshold current may be 75 mA. That 10 mA range is all you have to play with. Go to 86 mA (in this example) and your laser diode may be history in much less than the blink of an eye.

    This is one reason why most applications of laser diodes include optical sensing to regulate beam power. The third lead on the laser diode package is connected to an internal optical sensing photodiode used to regulate power output when used in a feedback circuit which controls your current. This is very important to achieve any sort of stable long term operation.

    You can easily destroy a laser diode by exceeding the safe current even for an instant. It is critical to the life of the laser diode that under no circumstances do you exceed the safe current limit even for a microsecond!

    In addition, as the temperature of the laser diode changes (heats while powered), the current requirement to produce a given optical output increases as well. Without optical feedback if you set the current to be correct once the temperature of the laser diode stabilizes, it will likely blow out instantly the next you turn it on from a cold start!

    Laser diodes are also extremely static sensitive, so take appropriate precautions when handling and soldering. Also, do not try to test them with an analog VOM which could on the low ohms scale supply too much current.

    It is possible to drive laser diodes with a DC supply and resistor, but unless you know the precise value needed or have a laser power meter at your disposal, you can easily exceed the ratings before you realize it.

    You might hear someone bragging "I have driven thousands of laser diodes by just connecting them to a battery and resistor and never have blown any". Sure, right. While it is quite possible that the susceptibility to instant damage due to overcurrent varies with the type of laser diode, unless you know the precise behavior, you must err on the side of caution. Some designers have gone to extremes, however. See the section: Laser Diode Power Supply 2 (RE-LD2) for a design with 5 levels of protection!

    For testing, see the section: Testing of Low Power Laser Diodes.

    For an actual application, you should use the optical feedback to regulate beam power. You should also use a heat sink if you do not already have the laser diode mounted on one. See the chapter: Laser Diode Power Supplies.

    The raw beam from a laser diode is generally wedge shaped - 10 x 30 degrees is a typical divergence. You will need a short focal length convex lens to produce anything approaching a collimated beam. The optics from a dead CD player (even though CD players and CDROM drives use infra-red laser diodes, the optics can likely still be used with visible laser diodes), a low to medium power microscope objective, or even an old disc camera can provide a lens that may be entirely suitable for your needs.

    CD player and Other Low Power IR Laser Diodes

    The major difference between these and the visible laser diodes discussed in the section: Low Power Visible Laser Diodes is that the output is near-IR - usually at 780 nm (wavelengths from 400 to 700 nm are generally considered the visible portion of the electromagnetic spectrum). Therefore, the emission is not readily visible and you must use an IR detector device to even confirm that the laser is operating properly. This also means that safety is even more of a consideration with these devices since what you cannot see CAN hurt you (or at least your vision).

    Thus, these devices make truly lousy laser pointers or laser light shows as the emission is just barely visible in subdued light. If you hoped for a Star Wars type laser beam, better go hunting for a 25 W argon laser. :-)

    However, for data or voice communications, various kinds of scanning or sensing, and electro-optic applications where visibility is not needed or not desirable, such low cost sources of coherent light are ideal.

    Similar types are found in CDROM drives and newer LD (LaserDisc) players. CD-R recorders, Minidisc equipment, magneto-optical, and other writable optical drives including WORM drives, use devices that are similar in appearance and drive requirements but may be capable of somewhat higher maximum power output - as much as 30 mW or more.

    Modern laser printers use laser diodes producing anywhere from 5 mW to 50 mW and beyond depending on their resolution and speed (pages per minute). High resolution laser imagers, typesetters, and plotters, may use laser diodes producing 150 mW or more. (However, equipment built before 1985 or so may use helium-neon or even argon lasers rather than diode lasers.)

    The laser diode in a laser printer is located inside the scanner unit which is probably a black plastic case about 6 or 8 inches on a side and a couple of inches thick with a motor protruding from the bottom. The laser diode is mounted (along with its driver board, collimating optics, and even possibly a Peltier solid state cooler on some) either near one corner or inside. There should be a laser safety sticker on it as well - but these fall off sometimes!

    It is essential that additional precautions are taken if you have a higher power laser diode from equipment of this sort (or don't really know where yours spent its earlier life).

    There are now laser diodes (or possibly laser diode arrays) with optical output measured in 10s, even 100s of watts though these will not be what you would call tiny and will probably require buss bars for electrical power and plumbing for cooling!

    Example of Laser Printer Diode Laser Module

    (Portions from: EIO (ecsc@eio.com) and Chris Leubner).

    This Laser Printer Diode Laser Module is from an older unidentified laser printer, laser scanner/duplicator, or similar device. It shows an example of a typical assembly consisting of an IR laser diode, collimating optics, and electronics driver board.

    Laser diode and optics characteristics: Note that this is only the front-end. It does not include the beam scanner (motor driven multifaceted mirror), field correction and directing optics, or beam position sensor - which would be present in a complete laser printer. The output of this module is a collimated IR laser beam. The actual focal point will be at the image plane (photosensitive drum surface) after passing through the other optics.

    CD Player/CDROM Drive Laser Diode Characteristics

    Unless otherwise noted, the following discussion assumes the type of laser diode found in a CD player or CDROM drive. These are the most common devices you are likely to encounter. In fact, I bet you have at least one broken CD player or CDROM drive sitting in your junk box - or maybe you just retired your 16X CDROM drive because it was soooo slow and obsolete. :-)

    CD player laser diodes are infrared (IR) emitters, usually 780 nm, with a maximum power output of around 5 mW. Their emission will appear very slightly visible and deep red. This is the eye's response to the near-IR radiation but appearing about 10,000 times weaker than the actual beam would be it it's wavelength were centered in the visible part of the spectrum. Despite what the EM spectrum charts show, the eye's response does not drop off to zero at exactly 700 nm - there is decreasing sensitivity which may extend out beyond 820 nm depending on the individual (though some people can't even see the 780 nm). Just realize that the main beam is IR and almost totally invisible. Take care. A collimated 5 mW beam is potentially hazardous to your eyes. Don't be misled into thinking the laser is weak due to the dim appearance of the beam. It is not supposed to be visible at all!

    If you don't want to take even the minimal risk of looking into the lens at all, project the beam onto a piece of paper held close to the lens. In a dark room, it should be possible to detect a red spot on the paper when the laser is powered. For any laser more powerful than this or where the beam may be even approximately collimated, viewing the spot on a diffuse surface is the only safe method for checking the beam.

    Typical CD laser optics put out about 0.3 to 1 mW at the objective lens though the diodes themselves may be capable of up to 4 or 5 mW depending on type. If you saved the optical components, these may be useful in generating a collimated or focused beam. The aspheric objective lens will be optimized for producing a diffraction limited spot about 1 to 3 mm from its front surface when the optical system is used intact.

    The optics may include a collimating lens, diffraction grating (to produce the three beams in a three beam pickup), beam splitter prism or mirror, turning mirror (for horizontally mounted optics), and focusing (objective) lens. Older pickups tend to have larger and more complex sets of optics. Despite the fact that they are mass produced at low cost, these are all very high quality optical assemblies.

    However, depending on design, some of the parts may be missing or combined into one component. For example, many Sony pickups do not appear to use a collimating lens. For pickups with a collimating lens, if the objective lens is removed, you should get a more or less parallel main beam and two weaker side beams. Many newer designs have a combined laser diode/photodiode array rather than individual components. Mix and match parts for your needs (if you can get it apart non-destructively). Where there is no collimating lens, the objective lens may be used for this purpose if positioned closer to the laser diode.

    For examples of typical optical pickup/optical block designs, see:

    WARNING: A collimated 5 mW beam is hazardous especially since it is mostly invisible. By the time you realize you have a problem it will be too late.

    The coils around the pickup are used for servo control of focus and tracking by positioning the objective lens to within less than a um (1/25,400 of an inch) of optimal based on the return beam reflected from the CD. See the document: Notes on the Troubleshooting and Repair of Compact Disc Players and CDROM Drives for more information on optical pickup organization and operation.

    Typical drive currents are in the 30 to 100 mA range at 1.7 to 2.5 V. However, the power curve is quite non-linear (though perhaps not as extreme as the typical visible laser diode). There is a lasing threshold below which there will be no coherent output (just IR LED emission). For a diode rated at a nominal current of 50 mA (typical for Sony pickups, for example), the threshold current may be 30 mA. This is one reason why most applications of laser diodes include optical sensing (there is a built in photodiode in the same case as the laser emitter) to regulate beam power. You can easily destroy a laser diode by exceeding the safe current even for an instant. It is critical to the life of the laser diode that under no circumstances do you exceed the safe current limit even for a microsecond!

    Laser diodes are also supposed to be extremely static sensitive, so use appropriate precautions. Also, do not try to test them with an analog VOM which in particular could on the low ohms scale supply too much current.

    It is possible to drive laser diodes with a DC supply and resistor, but unless you know the precise value needed, you can easily exceed the ratings.

    For testing, see the section: Testing of Low Power Laser Diodes.

    For an actual application, you should use the optical feedback to regulate beam power. You should also use a heat sink if you do not already have the laser diode mounted on one. CD laser diodes are designed for continuous operation. See the chapter: Laser Diode Power Supplies.



  • Back to Diode Lasers Sub-Table of Contents.

    Determining Characteristics and Testing of Laser Diodes

    Working with Laser Diodes

    To minimize the chance of damage to your precious laser diodes (LDs) during assembly, rework, or removal from equipment, read and follow the guidelines below. Some of these apply only to those using optical feedback while others apply to all types.

    Again, double check all connections and circuitry before applying power after installation or rework. Especially check for solder bridges or damage to the circuit board. Make sure you read the pinout correctly! See the sections on testing to minimize the chances of blowing the laser diode when you power it up.

    Determining Characteristics of Laser Diodes Removed From Equipment

    The optical assemblies from CD players, laser printers, and other deceased or obsolete equipment represent a fabulous source of low cost laser diodes. It would be nice if something were known about their specifications! If none of these are viable, use the approaches described in the section: Testing of Low Power Laser Diodes understanding the risks involved.

    Testing of Low Power Laser Diodes

    If you have pinouts and specifications for your laser diode, these procedures can be greatly simplified. If you can at least identify the part number and manufacturer (look on the case if possible), check their Web site or locate an optoelectronics databook, or see K3PGP's Laser Diode Specifications maintained by K3PGP (Email: k3pgp@qsl.net).

    Note that if you have a device from a CD player, CDROM, or other optical drive with 8 or 10 pins, it is a combined laser diode and photodiode array in a single package. You will first have to identify the three connections to the laser diode itself. You should be able to determine this by tracing the wiring - there may even be markings on the circuit board. In many cases, the laser diode is driven by discrete components whereas everything else goes to a preamp IC. Once the pinout of the laser diode is determined, it can be treated in exactly the same way as the more common 3 pin type.

    Determining the Laser Diode Pinout

    The following assumes you know nothing about your device other than that it is a 3 to 5 mW laser diode.

    The first step is to identify which pair of terminals are the laser diode and photodiode. Your laser diode package will be configured like one of the following:

    
                LD                 LD                 LD                 LD
             +--|>|--o LDC      +--|>|--o LDC      +--|<|--o LDA      +--|<|--o LDA
             |                  |                  |                  |
      COM o--+           COM o--+           COM o--+           COM o--+
             |  PD              |  PD              |  PD              |  PD
             +--|>|--o PDC      +--|<|--o PDA      +--|>|--o PDC      +--|<|--o PDA
                (1)                (2)                (3)                (4)
    
    

    The most common polarities for low power laser diodes seems to be (2). The COM terminal will then be connected to a positive supply (+V) relative to LDC and PDA.

    If you are leaving the photodiode installed in the optical block, also see the section: Reasons to Leave the CD Laser Diode in the Optical Block for sample connections.

    Where you can see both the pins and the inside of the laser diode package, it is easy to identify which pins goes where:

    If you can confirm these 3 connections by inspection, only the LD and PD polarities will need to be determined experimentally.

    The following assumes you did not have this luxury:

    The photodiode's forward voltage drop will be in the approximately .7 V range compared to 1.7-2.5 V for the laser diode. So, for the test below if you get a forward voltage drop of under a volt, you are on the photodiode leads. If your voltage goes above 3 V, you have the polarity backwards.

    CAUTION: Some laser diodes have very low reverse voltage ratings (e.g., 2 V) and will be destroyed by modest reverse voltage. Check your spec sheet. However, the laser diodes found in CD players seem to be happy with 4 or 5 volts applied in reverse. Of course, a shorted or open reading could indicate a defective laser diode or photodiode.

    If the laser diode is still connected to its circuitry (probably a printed flex cable), it is likely that the laser diode will have a small capacitor directly across its terminals and the optical sensing photodiode will be connected to a resistor or potentiometer. In particular, this is true of Sony pickups and may help to identify the correct hookup.

    And finally, determining pinout without applying power to the laser diode package should be possible by taking advantage of the sensitivity of the laser diode (LD) and photodiode (PD) to external light. However, once the tests below have been performed, it's probably a good idea to confirm with an ohmmeter.

    (From: Nikos Aravantinos (aravantinos@ath.forthnet.gr).)

    After having played with several CD and CD/RW diodes, I believe that it is possible to determine the pinout to a high degree of confidence without applying any significant power to the laser diode.

    All that is needed is a voltmeter (rather a millivoltmeter) and an operating incandescent lamp (tungsten filament like a pocket flashlight). If you direct a light beam to the device under test and measure the voltage between common and each of the other two pins you will find two of the four following possibilities: