For an example of one such system using a tiny Nd:YAG laser rod pumped by the electronic flash unit from a disposable (single use) 35 mm pocket camera, see the paper: Micro-Laser Range Finder Development: Using the Monolithic Approach.
(difference frequency) * c Distance = ---------------------------- 2 * (chirp rate)Where c is the velocity of light.
Dynamic implementation in the form of a laser scanner can actually be used to implement a 3-D profile measurement system. If a laser beam is scanned across a 3-D object, and the spot is viewed (by optical sensors) from two different locations, it is possible to determine the instantaneous distance to the spot (on the object). This can be down digitally (using a pair of CCD cameras - slow) or analog using a pair of 4-quadrant photodiodes. With a more constrained system (see below), only a single sensor is needed. This isn't a simple project either but at least doesn't depend on precision on the order of the wavelength of light! Such scanners exist and are used in conjunction with robotics (and other research), in industrial CAD/CAM for construction of computer models from real-world objects, and many other applications.
(From: Steve Roberts (osteven@akrobiz.com).)
One approach is to use a frame grabber, a translation stage, and a simple laser with a simple line generating optic. You put the piece to be scanned on the translation stage, shoot the line onto it from above and look at it with the camera. The line creates a cross-section of one small part of the object and the camera records it. Then you process out the laser light from the background, advance the translation stage one more linewidth, and take the next slice and so on - sort of a crude from of computed tomography.
(From: Paul Mathews (optoeng@whidbey.com).)
You might want to look at some modules designed for this purpose. The Sharp's Distance Measuring Sensors are compact and sensitive. They include the emitter LED, detector photodiodes, and signal processing circuitry in a compact integrated module.
There are also some nice application notes available from Hamamatsu for use with their Position Sensing Diodes and related ICs.
Manufactures/suppliers of devices used in laser rangefinders include: E-O Devices and Analog Modules.
To distant scene. ^ ^ | | | C/------/D |A | \--------\ (B is partially silvered or a half mirror to adjust B| permit viewing of both sides from the scene.) angle ^ view here | | |<- baseline -->|The further apart the mirrors are (size of baseline), the greater the useful range. Adjust the angle of mirror A or D until the images are superimposed. Calibrate the angular setting to distance.
The distance from A to the scene is then: tan(angle A) * baseline.
For long distances, C and D can be eliminated - they compensate for the difference in path lengths of the two views - else the sizes would not be the same. (Even this doesn't work perfectly in any case. Can you figure out why?)
You can add telescopes and other optics if you like - this is just the basics.
Look Ma, no electronics. :-)
Note that SLR cameras do NOT use this approach as they are entirely optical (meaning that adjusting the focus only controls the lens - nothing else!). With SLRs, a pair of shallow prisms oriented in opposite directions (or many in the case of a 'microscreen' type) are cemented onto a clear area of the ground glass. When the image is precisely focused onto the ground glass, the prisms have no effect. However, when the image is in front or behind, they divert the rays such that the two halves of the image move apart (or the image breaks up in the case of the 'microscreen').
There were some "Amateur Scientist" articles in Scientific American a few decades ago on constructing several types of optical range finders. These were included in the book, "Light and Its Uses". See the section: A HREF="laserclt.htm#cltsi">Scientific American Articles on Lasers and Related Topics.
My students construct a simple laser rangefinder using a few basic parts:
Equipment:
Basic procedure:
Rough diagram of rangefinder setup:
To wall To wall ^ ^ | \ distance | first reflected beam \ second reflected beam | \ | angle \ Laser --3"---/------------------------------------/ Beam splitter Rotary table with mirror |<------------- 6 feet ------------->|Of course, you can make the non-laser version of this type of rangefinder (but this is a laser FAQ! --- sam). My students also make that one as well. Both are pretty neat and demonstrate the power of trig to determine distances!
I am just finishing the development of a range finder based on the TOF (pulse-Time-Of-Flight) measurement method. There are also different methods like phase-shift method which compares the phase shift between outgoing modulated beam and reflected light.
The Pulse TOF method has some advantages which make it very useful: you can use relatively high pulse power and still be in the Class I safety range.
While building such a range finder there are two crucial components which have influence on its accuracy: the time measurement circuits and the receiver. Our aim was to build a laser scanner with the resolution of 1 cm which means that you have to be able to measure the time with the resolution of 67 ps. The range of the scanner should be approx. 30m. We are not ready yet but there are some results.
For the first prototype we used a 1.25 GHz oscillator and special microstrip design to get the resolution of 70 ps. In the current prototype we use a special prototype IC which should deliver 50 ps resolution.
The problems are on the receiver side, a relatively large jitter (which I'm fighting now) destroys my high time measurement precision. The jitter on the input results in the distance differences of approximately 10 cm). This can be filtered out by averaging of a number of measurements and that is what we are doing now. Our measurement frequency is at present 100 kHz, but we will probably perform the averaging over 10 measurements so that effective measurement rate will be 10 kHz.
(From: jfd (jezebel@snet.net).)
The problem is getting simultaneous long standoff range and extremely accurate range. You can phase detect with accuracies in the sub-inch range using direct detected RF modulated LIDARS or you can use an interferometric technique with a reference to get sub-micron distances.
(From: Robert (romapa@earthlink.net).)
For much better resolution than would be possible with simple sampling while still maintaining low cost, digital TOF rangefinders can combine a precision analog temporal interpolator with say a CMOS system running at 100 MHz. The analog circuitry to accomplish this is in many production units (for different applications) - but 5 ps resolution has been achieved with low-cost components and in production for 15 years from at least one manufacturer. The idea is interpolate between the digital count periods with a precision time-to-voltage converter which is then sampled by microcontroller and combined with the digital counter results.
(From: Bill Sloman (bill_sloman@my-deja.com).)
You may be able to achieve this at low unit cost, but getting a precision analog temporal interpolator to work well next to CMOS running at 100 MHz isn't something I'd describe as easy.
We developed a system of this sort at Cambridge Instruments between 1988 and 1991 using a mixture of 100K ECL and GigaBit Logic's GaAs for the digital logic. Any digital signal going to or from the analog temporal interpolator was routed as a balanced pair on adjacent tracks, and we were very careful about the layout, but we still had to work at getting the noise on the interpolator output down to the 60 picosecond jitter on our 800 MHz master clock (getting a better master clock was the next priority).
Current-steering logic (like ECL and GaAs) is a lot quieter than voltage-steering logic (like TTL and CMOS), which is why very fast DACs and ADCs use ECL interfaces. Precision analog interpolators are no less sensitive.
Do you know who has actually achieved that 5 ps resolution and for what application? Tektronix and time domain reflectometers come to mind, though Tektronix isn't exactly cheap. IIRR Triquint was originally their in-house analog foundry and I think Tektronix has been using GaAs ASICs in their faster gear for quite some time now.
The hybrid approach certainly isn't new, but getting it to work is a fair test of one's analog skills.
Of course, using phase-shift not only makes for easier circuit design, but also lets you run your LED at a 50% duty cycle, giving you a lot more reflected photons to work with than the 0.01% you get with TOF.
(From: Lou Boyd (boyd@fairborn.dakotacom.net).)
The Texas Instruments book "Optoelectronics: Theory and Practice" published by McGraw-Hill had a chapter (23) on the design of an LED/Si Diode rangefinder with schematics of the transmitter, receiver, and timing section. This was a phase modulated design but obsolete by todays standards. Low cost modern rangefinders like those by Leica or even Bushnell are far more advanced in the detection circuit than that in the TI book. Most eye-safe commercial rangefinders use phase modulated techniques. This gives good accuracy but limited range, usually less than 1 kilometer with measurement times typically 1/10 second.
Most military rangefinders use a much higher power transmitter with a time of flight method. A time of flight rangefinder just sends a single pulse and receives it. Some use multiple pulses for improved resolution and range but that typically isn't necessary. A counter is started on the rising edge of the transmitted pulse and stopped when the rising edge of the receive pulse is detected. If the counter is measuring a 150 mhz (approx) clock the range will be displayed in meters. Unfortunately that fast of counter requires at least a few high speed chips beyond the capability of standard CMOS or TTL logic. Since the round trip takes only 6.667 microseconds per kilometer you don't even need blanking on the displays. They can be attached directly to the counters or just read by a computer. A four or five digit counter suffices for most purposes. There is a little added complexity on sophisticated units for making the sensitivity of the receiver increase with time after the pulse is transmitted. This is sometimes done by charging a capacitor attached to a gain control which increases the gain with the square of time out to the maximum the unit is capable of. These rangefinders tend to be expensive because of the technology but the electronics is simple in concept. Ranges are limited only by the transmit power which can be extremely high using solid state Q switched lasers.
Surplus lasers and the associated electronics from military rangefinders have been showing up on the surplus market in the $300 range. Unfortunately the receivers have not.
For some insight on the level of complexity involved look at the boards sold by E-O Devices These are time of flight pulsed laser rangefinder components designed for use primarily with LED's or diode lasers. Also check Analog Modules for examples of state of the art variable gain rangefinder receivers. If you want one of their modules plan on spending between $1,000 and $2,000. :-(
A Q-switched solid state laser will give you short pulses with minimal fuss. A unit like the small surplus Nd:YAG laser (SSY1) described in chapter: Solid State Lasers was originally part of the M-1 tank rangefinders and thus should be ideal. It is quite trivial to build a suitable power supply these laser heads since a passive Q-switch is used and this doesn't require any electrical control.
A few mJ should be sufficient. (SSY1 is probably in the 10 to 30 mJ range using the recommended pulse forming network.) With a Q-switched laser, the required short pulse if created automagically eliminating much of the complexity of the laser itself.
Diode laser assemblies from the Chieftain tank rangefinder are also available on the surplus market but you probably would have to build a pulsed driver for them which would be more work.
For the detector, a PIN photodiode or avalanche photodiode (APD) would be suitable. The preamp is the critical component to get the required ns response time. You need to sample both the pulse going out and the return since the delay from firing the flashlamp (if you are using a solid state laser) to its output pulse is not known or constant.
15 cm resolution requires a time resolution of about 1 ns (twice what you might think because the pulse goes out and back). GHz class counters are no big deal these days.
However, approaches that are partially analog (ramp and A/D) which don't require such high speed counters are also possible. In fact, if your digital design skills aren't so great, this is probably the easiest way to get decent resolution, if possibly not the greatest accuracy/consistency. All you need is a constant current source and an A/D (Analog to Digital converter). This can be as simple as a FF driving a transistor buffer to turn the voltage to charge the capacitor on and off with a transistor set up with emitter feedback for as a constant current source. Or, it can just be an exponential charge with non-linear correction done in software. The A/D doesn't need to be fast as long as its output word has enough bits for your desired resolution. For a typical exponential charging waveform, add 1 bit to the required A/D word size. For example, determining distance over 100 meters to to 5 cm resolution would require that the full voltage ramp be about 700 ns in duration (a bit over maximum round trip time, cut off sooner if there is a return pulse) and then sampled with a 12 bit A/D.
Another even simpler way of doing this is to charge the capacitor as above but then discharge it with a much longer time constant and determine how long it takes to reach a fixed voltage. By making the discharge time constant sufficiently large, any vanilla flavored microprocessor could be used for control and timing.
All in all, these are non-trivial but doable projects.
See the previous sections on laser rangefinders for more info.
If the surface is smooth and flat over a scale of 5 to 10 um, this could work as a way of determining distance to the pickup. In other words, the dominant return from the surface has to be a specular reflection back to the source in order for the focus servo to lock properly. (The width and depth of the pits/lands of the CD or DVD disc is small compared to the beam so they are mostly ignored by the focus servo.) I don't know how much angular deviation could be tolerated.
The output would be an analog voltage roughly proportional to focus error which could be mapped to lens height (assuming the device is in a fixed orientation with respect to gravity - more complex if you want to do this while on a roller coaster or in microgravity!). The total range would be 1 to 2 mm with an accuracy of a few um.
Also see the section: Can I Use the Pickup from a CD/DVD Player or CD/DVDROM Drive for Interferometry?, which would be even more precise but more complex. The practical issues of using the guts of these devices are also discussed there.
Since the 'stylus' of a CD player has an effective size of around 1 um (DVD would be even less), it could in principle be used to implement a very high resolution optical encoder for use in linear, rotary, or other sensing application. The stand-off distance (from objective lens to focal point) can be a couple of mm which may be an advantage as well. While this is probably somewhat less difficult than turning a CD player into an interferometer (see below), it still is far from trivial. You will have to create an encoder disc or strip with a suitable reflective pattern with microscopic dimensions. Without access to something like a CD/DVD mastering unit or semiconductor wafer fab, this may be next to impossible. Your servo systems will need to maintain focus (at least, possibly some sort of tracking as well) to the precision of the pattern's feature size. To obtain direction information, the 'track' would need to have a gray code pattern similar to that of a normal optical encoder - but laid down with um accuracy in such a way that the photodiode array output would pick it up. (Implementing an absolute encoding scheme would probably require so many changes to the pickup as to make it extremely unlikely to be worth the effort.) Of course, you also need laser diode driver circuitry and the front-end electronics to extract the data signal. Not to mention the need for a suitable enclosure to prevent contamination (like lathe turnings) from gumming up the works. And, with your device in operation, any sort of vibration or mechanical shock could cause a momentarily or longer term loss of focus and thus loss of your position or angle reference.
If you are still interested, see the section: Can I Use the Pickup from a CD/DVD Player or CD/DVDROM Drive for Interferometry? since some of the practical issues of using the guts of these devices are discussed there.
For example, if the outgoing laser beam is modulated at 1 GHz and the reflected beam is combined with this same reference 1 GHz in the sensor photodiode or a mixer, for relative speeds small compared to c (the velocity of light), the difference frequency will be approximately 1 Hz per 0.5 foot/second.
"INTERFEROMETER: An instrument designed to produce optical interference fringes for measuring wavelengths, testing flat surfaces, measuring small distances, etc."As an example of an interferometer for making precise physical measurements, split a beam of monochromatic coherent light from a laser into two parts, bounce the beams around a bit and then recombine them at a screen, optical viewer, or sensor array. The beams will constructively or destructively interfere with each-other on a point-by-point basis depending on the net path-length difference between them. This will result in a pattern of light and dark fringes. If one of the beams is reflected from a mirror or corner reflector mounted on something whose position you need to monitor extremely precisely (like a multi-axis machine tool), then as it moves, the pattern will change. Counting the passage of the fringes can provide measurements accurate to a few nanometers!
A simple version of a Michelson interferometer is shown below:
_____ Mirror 1 (Moving) ^ | | Beam | Splitter +-------+ | / | | Laser |=========>/<---------->| Mirror 2 (Fixed) +-------+ / | | | | | v Screen (or optical viewer, ------- magnifier, sensor, etc.)
In a perfectly symmetric Michelson interferometer, the fringe pattern should uniformly vary between bright and dark (rather than stripes or concentric circles of light) depending on the phase difference between the two beams that return from the two arms. A circular pattern is expected if the two curvatures of the wavefront are not identical due to a difference in arm-lengths or differently curved optics. Stripes (straight or curved) in any direction) would be an indication of a misalignment of some part of the interferometer (i.e. the beams do not perfectly overlap or one is tilted with respect to the other).
(Yes, about 50 percent of the light gets reflected back toward the laser and is wasted with this particular configuration. This light may also destabilize laser action if it enters the resonator. Both of these problems can be easily dealt with using slightly different optics than what are shown.)
A long coherence length laser producing a TEM00 beam is generally used for this application. HeNe lasers have excellent beam characteristics especially when frequency stabilized to operate in a single longitudinal mode. However, some types of diode lasers (which are normally not thought of as having respectable coherence lengths or stability) may also work. See the section: Interferometers Using Inexpensive Laser Diodes. Even conventional light sources (e.g., gas discharge lamps producing distinct emission lines with narrow band optical filters) have acceptable performance for some types of interferometry.
Such a setup is exceedingly sensitive to EVERYTHING since positional shifts of a small fraction of a wavelength of the laser light (10s of nm - that's nanometers!) will result in a noticeable change in the fringe pattern. This can be used to advantage in making extremely precise position or speed measurements. However, it also means that setting up such an instrument in a stable manner requires great care and isolated mountings. Walking across the room or a bus going by down the street will show up as a fringe shift!
Interferometry techniques can be used to measure vibrational modes of solid bodies, the quality (shape, flattness, etc.) of optical surfaces, shifts in ground position or tilt which may signal the precursor to an earthquake, long term continental drift, shift in position of large suspended masses in the search for gravitational waves, and much much more. Very long base-line interferometry can even be applied at cosmic distances (with radio telescopes a continent or even an earth orbit diameter apart, and using radio emitting stars or galaxies instead of lasers). And, holography is just a variation on this technique where the interference pattern (the hologram) stores complex 3-D information.
This isn't something that can be explained in a couple of paragraphs. You need to find a good book on optics or lasers. Gordon McComb's: "The Laser Cookbook [1} and the Scientific American collection: "Light and its Uses [5]" include various type of interferometers which can be built with (relatively) readily available parts. Hewlett Packard (among others) manufacture 'Laser Interferometry Measurement Systems' based on these techniques. Information and application notes are available by searching for the key words: "Laser" or "Dimensional Measurement" at the HP Test & Measurement Web Site Search Page.
Also see the Amateur Interferometry Group (AIG) Web site. The AIG is an informal gathering of people interested in designing, building, and operating various types of laser interferometers. Much of the information relates directly to the testing of optical components for astronomical telescopes but there should be much of general interest as well.
Your initial response might be: "Well, no system is ideal and the beams won't really be perfectly planar so, perhaps the energy will appear around the edges or this situation simply cannot exist - period". Sorry, this would be incorrect. The behavior will still be true for the ideal case of perfect non-diverging plane wave beams with perfect optics.
Perhaps, it is easier to think of this in terms of an RF or microwave, acoustic, or other source:
OK, I know the anticipation is unbearable at this point. The answer is that the light is reflected back to the source (the laser) and the entire optical path of the interferometer acts like a high-Q resonator in which the energy can build up as a standing wave. Light energy is being pumped into the resonator and has nowhere to go. In practice, unavoidable imperfections of the entire system aside, the reflected light can result in laser instability and possibly even damage to the laser itself. So, there is at least a chance that such an experiment could lead to smoke!
(From: Art Kotz (alkotz@mmm.com).)
We don't have to to think all that hard to figure out where all the energy is dissipated in a Michelson interferometer. Nor do we have to refer to imperfect components either. The thought experiment of perfect non-absorbing components still renders a physically correct solution.
To summarize a (correct) previous statement, in a Michelson interferometer with flat surfaces, you can get a uniform dark transmissive exit beam. The power is not dissipated as heat. There is an alternate path that light can follow, and in this case, it exits the way it came in (reflected back out to the light source).
In fact, with a good flat Fabry Perot interferometer, you can actually observe this (transmission and reflection from the interferometer alternate as you scan mirror spacing).
In the electrical case, imagine a transmitter with the antenna improperly sized so that most of the energy is not emitted. It is reflected back to the output stage of the transmitter. If the transmitter can't handle dissipating all that energy, then it will go up in smoke. Any Ham radio operators out there should be familiar with this.
(From: Don Stauffer (stauffer@htc.honeywell.com).)
Many of the devices mentioned have been at least in part optical resonators. It may be instructive to look at what happens in an acoustic resonator like an organ pipe or a Helmholtz resonator.
Let's start with a source of sound inside a perfect, infinite Q resonator. The energy density begins to build up with a value directly proportional to time. So we can store, theoretically, an infinite amount of acoustic energy within the resonator.
Of course, it is impossible to build an infinite Q resonator, but bear with me a little longer. It is hard to get an audio sound source inside the resonator without hurting the Q of the resonator. So lets cut a little hole in the resonator so we can beam acoustic energy in. Guess what, even theoretically, this hole prevents the resonator from being perfect. It WILL resonate.
No optical resonator can be perfect. Just like in nature there IS no perfectly reflecting surface (FTIR is about the closest thing we have). Every time an EM wave impinges on any real surface, energy is lost to heat. With any source of light beamed at any surface, light will be turned into heat. In fact, MOST of the energy is immediately turned to heat. By the laws of thermodynamics, even that that is not converted instantaneously into heat, but goes into some other form of energy, will eventually turn up as heat. You pay now, or you pay later, but you always pay the entropy tax.
(From: Bill Vareka (billv@srsys.com).)
And, something else to ponder:
If you combine light in a beam splitter there is a unavoidable phase relation between the light leaving one port and the light leaving the other.
So, if you have a perfect Mach-Zender interferometer like the following
+-------+ BS M | Laser |=====>[\]---------\ +-------+ | | M = Mirror | | BS = Beam Splitter | BS | M \---------[\]---->A | | V BIf you set it up so that there is total cancellation out of, say, port A, then Port B will have constructive interference and the intensity coming out port B will equal the combined intensity coming in the two input ports of that final beam splitter. This is due to the phase relation between the light which is reflected at the beam splitter. That which is reflected and goes out port A will be 180 degrees out of phase with that which is reflected and goes out port B. The transmitted part of port A and port B are the same. Hence the strict phase relationship between the light from the two output ports. This is an unavoidable result of the time-reversal symmetry of the propagation of light.
(From: A. Nowatzyk (agn@acm.org).)
A beam-splitter (say a half silvered mirror) is fundamentally a 4 port device. Say you direct the laser at a 45 degree angle at an ideal, 50% transparent mirror. Half of the light passes through straight, the rest is reflected at a 90 degree angle. However, the same would happen if you beam the light from the other side, which is the other input port here. If you reverse the direction of light (as long as you stay within the bounds of linear optics, the direction of light can always be reversed), you will see that light entering either output branch will come out 50/50 on the two input ports. An optical beam-splitter is the same as a directional coupler in the RF or microwave realm. Upon close inspection, you will find that the two beams of a beam-splitter are actually 90deg. out of phase, just like in an 1:1 directional RF coupler.
In an experiment where you split a laser beam in two with one splitter and then combine the two beams with another splitter, all light will either come out from one of the two ports of the second splitter, depending on the phase. It is called a Mach-Zehnder interferometer.
Ideal beam-splitters do not absorb any energy, whatever light enters will come out one of the two output ports.
There will be interference but you won't see any visible patterns unless the two sources are phase locked to each-other since even the tiny differences in wavelength between supposedly identical lasers (HeNe, for example) translate into beat frequencies of MHz or GHz!
(From: Charles Bloom (cbloom@caltech.edu).)
The short answer is yes.
Let's just do the math. For a wave-number k (2pi over wavelength), ordinary interference from two point-like apertures goes like:
Psi = (e^(ik(L+a).) + e^(ik(L-a).))/2 = e^(ikL) * cos(ka) I = Psi^* Psi = cos^2(ka)(a is actually like (x-d)^2/L where 2d is the slit separation, and x is the position along the screen; L is the distance from the center of the slits to our point on the screen).
Now for different wavenumbers:
Psi = ( e^(ik(L+a).)+ e^(iK(L-a).))/2 I = Psi^* Psi = 1/2 [ 1 + Re{ e^(i ( k(L+a) - K(L-a) ).)} ] = 1/2 [ 1 + cos( L(k-K) + a(k+K) ) ] = cos^2[ 1/2( L(k-K) + a(k+K) ) ]This is almost a nice interference pattern as we vary 'a', but we've got some nasty L dependence, and in the regime L >> a where our approximations are valid, the L dependence will dominate the a dependence (unless (k-K) is very small; in particular, we'll get interference roughly when a(k+K) ~ 10 and L(k-K) ~ 1 , and L >> a , which implies |k-K| << |k+K| , nearly equal wavelengths.)
The L dependence is the usual phenomenon of "beats" which is also a type of interference, but not the nice "fringes" we get with equal wavelengths (the L dependence is like a Michelson-Morely experiment to compare wavelengths of light, by varying L (the distance between the screen and the sources) I can count the frequency of light and dark flashes to determine k-K.
So you would like to add a precision measurement system to that CNC machining center you picked up at a garage sale or rewrite the servo tracks on all those dead hard drives. :) If you have looked at HP's products - megabucks (well 10s of K dollars at least), it isn't surprising that doing this may be a bit of a challenge. As noted in the section: Basics of Interferometry and Interferometers, a high quality (and expensive) frequency stabilized single mode HeNe laser is often used. For home use without one of these, a short HeNe laser with a short random polarized tube (e.g., 5 or 6 inches) will probably be better than a high power long one because it's possible only 2 longitudinal modes will be active and they will be orthogonally polarized with stable orientation fixed by the slight birefringence in the mirror coatings. As the tube heats up, the polarization will go back and forth between the two orientations but should remain constant for a fair amount of time after the tube warms up and stabilizes. Also see the section: Inexpensive Home-Built Frequency or Intensity Stabilized HeNe Laser.
The problem with cheap laser diodes is that most have a coherence length that is in the few mm range - not the several cm or meters needed for many applications (but see the section: Can I Use the Pickup from a CD Player or CDROM Drive for Interferometry?). There may be exceptions (see the section: Interferometers Using Inexpensive Laser Diodes) and apparently the newer shorter wavelength (e.g., 640 to 650 nm) laser pointers are much better than the older ones but I don't know that you can count on finding inexpensive long coherence length laser diodes. Even if you find that a common laser diode has adequate beam quality when you test it, the required stability with changes in temperature and use isn't likely to be there.
The detectors, front-end electronics, and processing, needed for an interferometer based measurement system are non-trivial but aren't likely to be the major stumbling block both technically and with respect to cost. But the laser, optics, and mounts could easily drive your cost way up. And, while it may be possible to use that $10 HeNe laser tube, by the time you get done stabilizing it, the effort and expense may be considerable.
Note that bits and pieces of commercial interferometric measurings systems like those from HP do show up on eBay and other auction sites from time to time as well as from laser surplus dealers. The average selling prices are far below original list but complete guaranteed functional systems or rare.
(From: Randy Johnson (randyj@nwlink.com).)
I'm an amateur telescope maker and optician and interferometry is a technique and method that can be used to quantify error in the quality of a wavefront. The methods used vary but essentially the task becomes one of reflecting a monochromatic light source, (one that is supplied from narrow spectral band source i.e., laser light) off of, or transmitting the light through a reference element, having the reference wavefront meet the wavefront from the test element and then observing the interference pattern (fringes) that are formed. Nice straight, unwavering fringe patterns indicate a matched surface quality, curved patterns indicate a variation from the reference element. By plotting the variation and feeding the plot into wavefront analysis software (i.e., E-Z Fringe by Peter Ceravolo and Doug George), one can assign a wavefront rating to the optic under test.
The simplest interference test would involve two similar optical surfaces in contact with each other, shining a monocromatic light source off the two and observing the faint fringe pattern that forms. This is known as a Newton contact interferometer and the fringe pattern that forms is known as Newton's rings or Newton's fringes, named for its discoverer, you guessed it, Sir Issac Newton. If you would like to demonstrate the principle for yourself, try a couple of pieces of ordinary plate glass in contact with each other, placed under a fluorescent light. Though not perfectly monochromatic, if you observe carefully you should be able to observe a fringe pattern.
Non-contact interferometry is much tougher as it involves the need to get a concentrated amount of monochromatic light through or reflected off of the reference, positioning it so it can be reflected off of the test piece, and then positioning the eye or imaging device so that the fringe pattern can be observed, all this while remaining perfectly still, for the slightest vibration will render the fringe pattern useless.
(From: Bill Sloman (sloman@sci.kun.nl).)
An interferometer is a high precision and expensive beast ($50,000?). You use a carefully stabilized mono-mode laser to launch a beam of light into a cavity defined by a fixed beam splitter and a moving mirror. As the length of the cavity changes, the round-trip length changes from an integral number of wavelengths of light - giving you constructive interference and plenty of light - to a half integral number of wavelengths - giving you destructive interference and no light.
This fluctuation in your light output is the measured signal. Practical systems produce two frequency-modulated outputs in quadrature, and let you resolve the length of a cavity to about 10 nm while the length is changing at a couple of meters per second. The precision is high enough that you have to correct for the changes in speed of light in air caused by the changes temperature and pressure in an air-conditioned laboratory.
Hewlett-Packard invented the modern interferometer. When I was last involved with interferometers, Zygo was busy trying to grab a chunk of the market from them with what looked liked a technically superior product. Both manufacturers offered good applications literature.
(From: Mark Kinsler (kinsler@froggy.frognet.net).)
You can get interferometer kits from several scientific supply houses. They are not theoretically difficult to build since they consist mostly of about five mirrors and a lens or two. But it's not so easy to get them to work right since they measure distances in terms of wavelengths of light, and that's *real* sensitive. You can't just build one on a table and have it work right. One possible source is: Central Scientific Company.
(From: Bill Wainwright (billmw@isomedia.com).)
Yes, you can build one on a table top. I have done it. I was told it could not be done but tried it anyway. The info I read said you should have an isolation table to get rid of vibrations I did not, and even used modeling clay to hold the mirrors. The main problem I had was that the image was very dark and I think I will use a beam splitter in place of one of the mirrors next time. The setup I had was so sensitive that lightly placing your finger on the table top would make the fringes just fly. To be accurate you need to take into account barometric presure and humidity.
While I don't know how to select a laser diode to guarantee an adequate coherence length, it certainly must be a single spatial (transverse) mode type which is usually the case for lower power diodes but those above 50 to 100 mW are generally multimode. So, forget about trying to using a 1 W laser diode of any wavelength for interferometry or holography. However, single spatial mode doesn't guarantee that the diode operates with a single longitudinal mode or has the needed stability for these applications. And, any particular diode may operate with the desired mode structure only over a range of current/output power and/or when maintained within a particular temperature range.
(From: Steve Rogers (scrogers@pacbell.net).)
I have been involved with laser diodes for the last 15 years or so. My first was a pulsed (only ones available at that time) monster that peaked 35 watts at 2 kHz with 40 A pulses! It was a happy day when they could operate CW and visible to say the least. Anyway, in the course of my working travels, I have built numerous Twymann-Green double pass interferometers for the wave front distortion analysis of laser rods, i.e., Nd:Yag, Ruby, Alexandrite, etc. The standard reference light source for this instrument has always been the 632.8 nm HeNe laser. Good coherence length and relatively stable frequency was its strong suit.
When visible diode lasers came out I often wondered aloud about their suitability as a replacement for the HeNe. I despise HeNe lasers. They are bulky and I have been shocked too many times from their power supplies.
I assumed that since CD player laser diodes at 780 nm could have coherence lengths on the order of tens of centimeters or into the meters (!!, see, for example: Katherine Creath, "Interferometric Investigation of a Diode Laser Source", Applied Optics (24 1-May-1985) pp. 1291-1293), Visible Laser Diodes (VLDs) could make excellent replacements. As it turned out, VLDs tend to have coherence lengths which are considerably shorter according to the latest technical literature and I held off on experimenting with them. Last week, I went through my shop and found enough mirrors, beam splitter, assorted optics to throw together my own double-pass interferometer for home use. This coincided with my acquisition of a 635 nm 5 mw diode module - a good one from Laserex.
To make a longer story shorter, I assembled said equipment with the VLD and WOW! excellent fringe contrast (a test cavity of four inches using a .250" x 4.0" Nd:Yag rod as the test sample.) When a HeNe laser was substituted for the VLD, virtually no difference in the manual calculation of wave front distortion (WFD) and fringe curvature/fringe spacing. The only drawback with the VLD is that it produces a rectangular output beam. When collimated you have a LARGE rectangular beam rather than a nice round HeNe style beam. My interferometer now occupies a space of 10" x 10" and is fully self contained. It probably could even be made smaller. Not only that, but it runs on less than 3 V!!!
I am just as surprised as you are with the results that I achieved. This is one reason why it took me so long to attempt this experiment (something like 4 to 5 years). I have always assumed that a HeNe laser would be FAR superior in this configuration than a VLD would be. Perhaps others may know more about the physics than I do. One thing is certain, these are "single mode" index guided laser diodes and typically exhibit the classic gaussian intensity distribution which is not so evident with the "gain guided" diodes. This in turn implies a predominant lasing mode which in turn would imply a (somewhat) stable frequency output. Purists would note that this VLD has a nominal wavelength of 635 nm +/- 10 nm while the HeNe laser is pretty much fixed at 632.8 nm. This variable could account for extremely minor WFD differences.
(From: W. Letendre (wjlservo@my-dejanews.com).)
There's an outfit in Israel selling a diode based laser interferometer enough cheaper than Zeeman split HeNe units to suggest that they are using a laser diode in the 'CD player' class, or perhaps a little better. They are able to measure, 'single pass' (retro rather than plane mirror) over lengths of up to about 0.5 m, suggesting that as an upper limit for coherence length.
People sometimes ask about using the focused laser beam for for scanning or interferometry. This requires among other things convincing the logic in the CD/DVD player or CD/DVDROM drive to turn the laser on and leave it on despite the possible inability to focus, track, or read data. The alternative is to remove the optical pickup entirely and drive it externally.
If you keep the pickup installed in the CD player (or other equipment), what you want to do isn't going to be easy since the microcontroller will probably abort operation and turn off the laser based on a failure of the focus as well as inability to return valid data after some period of time.
However, you may be able to cheat:
CAUTION: Take care around the lens since the laser will be on even when there is no disc in place and its beam is essentially invisible. See the section: Diode Laser Safety before attempting to power a naked CD player or simlar device.
It may be easier to just remove the pickup entirely and drive it directly. Of course you need to provide a proper laser diode power supply to avoid damaging it. See the chapter: Diode Laser Power Supplies for details. You will then have to provide the focus and/or tracking servo front-end electronics (if you need to process their signals or drive their actuators) but these should not be that complex.
Some people have used intact CD player, CDROM, and other optical disc/k drive pickup assemblies to construct short range interferometers. While they have had some success, the 'instruments' constructed in this manner have proven to be noisy and finicky. I suspect this is due more to the construction of the optical block which doesn't usually take great care in suppressing stray and unwanted reflections (which may not matter that much for the original optical pickup application but can be very significant for interferometry) rather than a fundamental limitation with the coherence length or other properties of the diode laser light source itself as is generally assumed.
In any case, some of the components from the optical block of that dead CD/DVD player may be useful even if you will be substituting a nice HeNe laser for the original laser diode in your experiments. Although CD optics are optimized for the IR wavelength (generally 780 nm), parts like lenses, diffraction grating (if present and should you need it), and the photodiode array, will work fine for visible light. However, the mirrors and beam splitter (if present) may not be much better than pieces of clear glass! (DVDs lasers are 635 to 650 nm red, so the optics will be fine in any case.)
Unfortunately, everything in a modern pickup is quite small and may be a bit a challenge to extract from the optical block should this be required since they are usually glued in place.
If what you want is basic distance measurements, see the section: Using a CD or DVD Optical Pickup for Distance Measurements which discusses the use of the existing focusing mechanism for this purpose - which could be a considerably simpler approach.
Also see the section: Basics of Interferometry and Interferometers.
The ring laser gyroscope, in principle, can replace these with a fully solid state system using counter-rotating laser beams, photodetectors, and digital electronics with no moving parts larger than photons and electrons.
In practice, it isn't so easy.
In its simplest form, the ring laser gyro (RLG) consists of a triangular block of glass drilled out for 3 helium-neon laser bores with mirrors at the 120 degree points - the corners. Counter-rotating laser beams - one clockwise (CW) and the other counter-clockwise (CCW) coexist in this resonator. At some point, a photosensor monitors the beams where they intersect. They will constructively or destructively interfere with one-another depending on the precise phase of each beam.
What is actually be measured is the integral of angular velocity or angle turned since the counting began. The angular velocity will be the derivative of the beat frequency. A dual (quadrature) detector can be used to derive the direction of rotation (analogous to how computer mice work!).
A complete 3-axis inertial platform would require 3 RLGs mounted at 90 degrees to each-other. The entire affair can be fabricated inside a solid glass block!
However, there are problems with this simplistic implementation. To provide a suitable phase reference, both laser beams must come from the same source or be locked to it. However, the sort of design described above had problems with slow rotation as the two beams would tend to lock to each-other and there would be no output! Some approaches for solving this problem added noise (dither) in an attempt to force the beams to be more independent. Others have attempted to keep the beams separate as much as possible except where they intersect at the photosensors.
For the most part, these difficulties have been overcome and modern aircraft and perhaps spacecraft as well are now using inertial platforms based on RLGs in place of mechanical gyroscopes.
There is some interesting information on RLGs at the Canterbury Ring Laser Projects Page.
(From: Douglas P. McNutt (dmcnutt@macnauchtan.com).)
The mechanical precision is the hard part and that's what makes it virtually impossible for an amateur to construct a ring laser gyro. The two opposite traveling waves have to have extremely high spectral purity which translates to high quality, high reflectance flats at the corners. Not a home job.
It might be easier to build a fiber gyro in which the light passes many times around an effective ring through a wound fiber.
(From: Christopher R. Carlen (crobc@epix.net).)
The mechanical part is horrendous. We have an open cavity HeNe at my school's lab, and it is a challenge to keep lasing on a heavy damped breadboard with the mirrors mounted on a thick dovetail rail, bolted to the breadboard.
Then you complicate that by going from a straight, two-mirror cavity to a three or four mirror cavity ring configuration, and then spin it real fast. Can you say "centrifugal force?"
A fiber loop isn't quite the same as a ring laser, because the ring laser actually has the laser gain medium in the ring. As opposed to having the beam directed into a ring. The gain medium in the ring cavity ensures a standing wave is set up in the cavity, which would not be so for the fiber loop.
Of interest for the future of laser gyros are the new photorefractive polymer devices that exhibit the property of two-beam coupling. This device allows coherent transfer of energy from one beam to another, when the beams are intersected in the material. This can be used to assemble a ring resonant cavity, pumped from the outside by a laser. This can be done with a small diode laser resulting in an assembly much smaller and easier to keep still while spinning than a gas laser ring cavity.
Photorefractive oscillators using inorganic PR crystals have been studied for some time. The first announcement of a resonant cavity using a PR polymer has just occurred in the past few weeks (March, 1998).
(From: Douglas Dwyer (ddwyer@ddwyer.demon.co.uk).)
If you are trying to make a laser gyro as a home project you've got a lifetime project.
I think the ring laser is often carved out of a solid block for stability , a major problem with both ring lasers and fibre gyros is locking of the two phases - when rotated the phase relationship between the two paths sticks until a certain rotation rate is reached at which point the two paths unlock and it starts to work properly The solution to this could be to deliberately modulate the phase of the light with pseudo random noise and demodulate at the phase detector. Also as stated the fibre gyro is less attractive because of the inherent greater spectral width of the laser.
I wonder if one could bake a Mossbau gyro. I once saw turntable rotation detected by the relativistic effects on the gamma radiation and absorption. That could be easier.
Some applications for the Fourier transforms include:
The usual modern way of performing the Fourier transformer operation is to digitize the data and use a special optimized computer algorithm called the 'Fast Fourier Transformer' or FFT. However, even the most efficient variation of this approach is highly computationally intensive - especially when large multidimensional arrays like high resolution images are involved. To achieve adequate performance, digital signal processing accelerator cards, multiprocessors, or even supercomputers may be needed!
Enter Fourier optics.
It turns out that under certain conditions, a simple convex lens will perform the Fourier transform operation on a two dimensional (2D) image totally in *real time*. The theoretical implications of this statement are profound since real-time here means literally at the speed of light. In practice, it takes great effort and expense to make it work well. Many factors can degrade the contrast, resolution, and signal-to-noise ratio. Extremely high quality and expensive optics, precision positioning, and immaculate cleanliness are generally essential to produce a useful system. However, to demonstrate the basic principles of Fourier optics, all that is required is a common HeNe laser and some relatively simple low cost optics.
+-------+ Spatial Filter Input Fourier Transform Output | Laser |===>()===---:---===()::():::><:::():::><:::():::><:::():::><:::() +-------+ FL PH CL TR TL TP ITL OP |<-f1->|<-f2->| |<-- f -->|<-- f -->|<-- f -->|<-- f -->|A laser with a long coherence length is required. A diode laser will probably not work well. Therefore, this is likely to be a HeNe type. A medium power laser (i.e., 10 mW) will make for a brighter display but a 1 mW should work just fine. CAUTION: Take appropriate precautions especially with a higher power laser. However, once the beam has been collimated to a large diameter, the hazards are reduced.
Ideally, you have a nice optical bench to mount all these components. Otherwise, you will have to improvise. The first three items (the spatial filter components) really do need to be accurately and stably positioned. See the section: Laser Beam Cleanup - the Spatial Filter.
Laboratory quality lenses for Fourier optics research cost thousands of dollars each. However, you can demonstrate the basic principles and do some very interesting experiments with inexpensive optics.
The ratio of F1/F2 should be roughly the same as the ratio of the diameters of the useful aperture of CL (desired diameter of the field of view) to the HeNe beam.
For example, with a laser producing a 1 mm diameter beam and a useful field of view diameter of 1 inch, the following will work:
Hint: have a book with examples of Fourier Transform pairs handy.)
I just finished a class in this, using "Linear Systems, Fourier Transforms, and Optics", by Gaskill (Wiley).
A coherent source yields a Fourier transform of the electric field, including the phase factors. An incoherent source will perform essentially the same effects on the radiance, rather than the field. A coherent source is used to develop the concepts, and so most of the books show the experimental verifications of spatial imaging with coherent sources.
A negative lens will give a virtual image. If you want to perform spatial filtering, I think you're forced to use a positive lens. You also perform the inverse transform with another positive lens. You should therefore be able to confirm basic spatial filtering concepts with a hobbyists' telescope.
Gaskill talks about a few special configurations, but the easiest to get to is to locate a laser to one side of the lens, place the transparency at the front focal plane, and find the Fourier transform plane at the point where the point source (a laser) comes to focus. To make things really simple, put the laser twice the focal distance away from the lens, the image at the focal distance, and find the FT at twice the focal distance on the far side of the lens. An alternative is to take a laser, collimate the light to obtain plane wave illumination, place the image anywhere between the source and the lens, and find the FT plane at the focal distance on the other side of the lens. It is the focal point of the light source that determines the position of the FT plane.
Like I say, I just took the class, am still shell-shocked, and haven't had a chance to absorb or experiment with these techniques, so I could be misunderstanding the text. (From: Norman Axelrod (naxelrod@ix.netcom.com).) Yes, you need a laser. HeNe works, but not a diode (the laser needs to have good coherence). Focus the laser through a pinhole (focusing lens and pinhole combination is called a spatial filter). then re-collimate the light with a lens. Place the image or aperture 1 focal length from the collimating lens, then you can either use a bare screen placed at distance away, or a second collimating lens. This is necessary to get the far-field pattern.
(From: Brian Rich (science@west.net).)
A really cool book about this that I have a copy of but may be out of print is "Laser Art and Optical Transforms" by T. Kallard. Look for it at a good university library.
(From: Norman Axelrod (naxelrod@ix.netcom.com).)
There is another way to phrase what is happening that might make it more intuitive for folks with more of an optics background.
First, the light used should be parallel and coherent.
The light transmitted through the transparency (or light reflected from a 2-dimensional image) is diffracted by the transmission and phase changes provided by the image. As is done in elementary physics, a lens (here, a high quality lens) is used to take the light that is diffracted at different angles and focus them at a distance of one focal length from the lens (just like a burning lens, except you use parallel coherent light coming into the initial transparency and you have more than one beam at the burning distance).
The key physical point is that the Fraunhofer diffraction pattern of an object is the Fourier transform of that object. This is true in the sense that the amplitude and the phase of the radiation at any point in the diffraction pattern are the amplitude and phase at the corresponding point in the Fourier transform.
For simple examples:
(From: Tom Sutherland (tom.sutherland@msfc.nasa.gov).)
Please allow me to recommend Professor Goodman's excellent and recently updated text "Fourier Optics". If I had my (last edition) copy in front of me I'd give you a better answer, however I do recall that the exact fourier transform of a pattern illuminated by a coherent plane wave is produced at the back focal plane of a lens if the pattern is located at the front focal plane of the lens. The intensity (but not the phase) of the fourier transform is produced if the pattern is located anywhere else in front of the lens (but of course there are some questions of scaling). (From: Robert Alcock (robert@fs4.ph.man.ac.uk).) Have a look at the book "Introduction to Fourier Optics" by J.W. Goodman. McGraw-Hill Book Company 1968. The first few chapters set the theoretical framework for the book by explaining 1D and 2D fourier transforms and scalar diffraction theory. I think that the chapters that you may find particularly interesting are:
(From: Herman de Jong (h.m.m.dejong@phys.tue.nl).)
Let me explain the optical Fourier Transform by lenses with an example: Suppose for simplification we essentially look at a two dimensional system: we use cylinder lenses and slit object.
When you use a broad laser beam and eliminate a slit (a pulse function), it will have a near field and a far-field pattern that is not exactly the same. The far-field pattern is a utopia but you get very close to the utopia the further away you put your screen. The intensity pattern is a squared sinc function (the sinc function is the FT of the pulse function) that scales with distance. We conclude the infinity pattern to be the squared of the FT of the slit and the associated E-field is actually the FT. If you use a cylindrical lens to image the slit on a screen you also get an FT provided you collect all relevant light from the slit onto your lens and the lens is perfect. It scales with the ratio of object an image distances It so happens that the FT of the FT the original but for a scaling factor and a minus sign in the inverse FT. I'm not sure how but in otical intensity FT's it makes no difference probably because of the squared of E-field that eliminates the minus sign.
It gets much more difficult to grasp with 3D and rotationally symmetrical optics, objects and images. You wouldn't want to know and I wouldn't be able to answer many questions.
(From: James A. Carter III (carter@photon-sys.com).)
It is possible to form the Fourier transform by placing the transparency in a convergent-cone optical field formed by a single laser. This technique is used when one wishes to scale the transform to be optimally sampled by a detector with fixed spatial sampling. Changing the location of the transparency with respect to the focus of the cone (i.e., changing the quadratic phase of the optical filed) will change the scale of the transforms as it maps spatial frequency (sometimes called the "plane wave spectrum") to spatial coordinates. Actually, no lens is required at all if you have a large enough lab and can invoke the "far field" condition. The "Fraunhofer" condition uses the quadratic phase of the lens to negate the second order term in the scalar diffraction integral using denoted as "Fresnel" diffraction. The far field condition puts the observation plane far enough away from the transparency plane to make it essentially a constant term in the integral and again you have a 2-D Fourier transform.
The lens can be thought of as a way to image the far field (ideally at infinity) to the back focal plane. If the transparency is not at the front focal plane, then the transform field (amplitude and phase) at the focal plane will have a quadratic phase term. The quadratic phase is irrelevant if the field is detected (with detector or film) because then all phase information is lost. If the field is recorded with a reference phase (i.e., a hologram), or is filtered for subsequently inversing the transform, then the quadratic phase should be corrected. The simplistic way to do this is to use a plane wave illumination (collimated source) and place the transparency at the front focal plane. Using your imagination and knowing the symmetry of the Fourier transform should justify this rational.
The field at the transform plane contains only the information that is collected and sampled by the lens. Thus, the ability to sample higher spatial frequencies depends on the collection angle (numerical aperture) of the lens. Some feel that the illumination beam must be spatially filtered to produce a uniform distribution. This is no more the case than saying that every Fast Fourier Transform should just be zero padded. Hamming, Hanning and other windowing algorithms are used to suppress the side-lobes produced by the finite sample extent. The Gaussian distribution of the laser can actually improve the fidelity of the transform and eliminate "ringing." The quality of the lens in terms of wavefront aberrations is important, but no more important than the wavefront quality of the beam. These phase aberrations may effect the point spread function of the system (seen when no transparency is present) and it is the point spread function that convolves with the transform and limits fidelity.
The text by Jack Gaskill and Joe Goodman are excellent for details. Another excellent source is the "(The New) Physical Optics Notebook: Tutorials in Fourier Optics" by Reynolds, DeVelis, Parrent, Thompson. This is available from Optical Engineering Press (SPIE). The "old" version of this was used in my training at the U. of Rochester when I took physical optics from one its early authors (Brian J. Thompson).
Many interesting things can be done with this simple engine. For more ideas, visit my (preliminary) Web site at http://www.photon-sys.com/
(From: Jeff Hunt (jhunt@ix.netcom.com).)
I'm a grad student at the Optical Sciences Center at the University of Arizona, and I think that Jack Gaskill's book on the subject is quite good. Just like Gaskill says, it covers what Goodman's text does, but it explains things in a way that is easier to understand (Goodman is the authority on the subject, from what I understand.)
(From: DeVon Griffin (devon@baggins.lerc.nasa.gov).)
Having done Gaskill ten years ago, I would say that the main drawback of the book is his notation. The m double-hat triple prime sort of thing makes trying to pick it back up after not having looked at it for awhile a daunting task.
Some would argue that the use of such technology in supermarkets at least, has dehumanized the buying experience and stacked the deck in favor of the merchant since prices tend to no longer be printed on each item and the checkout process is now so fast that it is virtually impossible to catch mistakes should they occur. Since the price-to-item relationship is stored in a computer somewhere, it is indeed possible for there to be errors - but in reality, these are generally rare.
Space and other factors prevent me from going into the details of the Universal Product Code itself but here are some Web sites that have info and many links to barcode manufacturers, barcode specifications, barcode generating software, and other information that may be useful:
The quick summary is that the pattern of black lines familiar on virtually all products nowadays - the UPC code - has been carefully designed to be easily decoded when scanned in either direction, at an arbitrary angle, and with variable speed. There are actually many other barcodes besides the UPC, used for inventory control, tracking, and other diverse applications. (If you should need to stay in a hospital, you will be given a barcode!)
The UPC consists of 12 total digits. The first digit is the type of product (0 is for groceries, 3 is for drugs, etc.), the next 5 digits on the left half are the manufacturer code, the first 5 digits of the right half are the product code, and the last one is a modulo check digit. Each digit as its name implies can have a value from 0 to 9, encoded as a set of 4 alternating bars and spaces, each of which may have a width of 1, 2, 3, or 4 units called "modules". The total width of each digit is defined to be 7 which allows for 20 unique codes - 10 used for the left 6 digits the other 10 for the right 6 digits. The left six digits are coded with odd parity; the right six digits with even parity. Additional details can be found at the first Web site, above.
The basic principle is to use a collimated laser beam, rotating multifaceted mirror, several stationary mirrors, and other optics, to generate a scan pattern above or beside the scanner which will intercept the UPC code printed on the item to be scanned in almost any orientation. While the scan may appear to consist of multiple lines or a continuous pattern, it is in reality a single rapidly moving spot.
Looking through the glass of the scanner, it may appear that all sorts of stuff is arranged at random. However, this is not the case. :) Refer to Optical Path of Typical Checkout Barcode Scanner as you read the description below (which also includes some comments on potentially useful parts that may be obtained from these units):
Some typical examples of HeNe tubes designed for barcode scanner applications are the Uniphase 098-1 HeNe Laser Tube and Siemens LGR-7641S HeNe Laser Tube. A typical small barcode scanner tube is shown in Uniphase HeNe Laser Tube with External Lens.
The HeNe laser power supply may be a self-contained 'brick' or built onto the mainboard. The former is of course much more desirable from the perspective of salvaging parts! In either case, to turn on the laser will probably require grounding or pulling up an enable signal since in most systems, the laser is automatically turned off after a period of inactivity.
The laser diode driver circuit will be in close proximity to the laser diode itself and may be on a separate board. However, it is most likely part of the mainboard. and difficult to determine correct use without a schematic.
These mirrors - particularly the dichroic type - are often of high enough quality to be used inside a laser resonator - even that of a low gain type like a HeNe laser.
The components of the this part can generally be separated to use individually using a combination of brute force and solvents. For example, to remove the lens and prism from the combo in the Orien 300, a pad of tissue paper is inserted in the hole followed by a wooden dowel that just fits. A couple of whacks to the dowel with a small hammer while holding the assembly should result in the prism/lens popping free. They can then be separated by soaking in acetone (nail polish remover). WARNING: Acetone and its vapors are flammable and toxic. CAUTION: Acetone will also damage many plastics including most likely, the large plastic lens, so don't let it contact that or other plastic optical components.
Unlike those in a laser printer, the mirror facets are large since they have to reflect the diffuse return beam as well as the tiny spot of the outgoing beam. They are fabricated as individual mirrors glued to a cast metal wheel type affair and are all set at slightly different angles so that each rotation of the mirror wheel results in scan lines at 3 to 6 slightly different locations depending on the number of facets.
These are usually decent quality aluminized first surface mirrors and could find all sorts of other uses. Although generally shaped as strange 4 sided polygons, they can be subdivided into more useful sizes using a glass cutter from the rear or a water-cooled diamond cutoff wheel.
For AC line powered units (no wall adapter), there will be some exposed 115 or 230 VAC points near the line cord and on the mainboard or power supply. For HeNe laser based systems with the high voltage power supply on the mainboard, there will be exposed pads with voltages up to 5 kV or more (during starting). Since these may not be clearly marked, it pays to identify them beforehand and take appropriate precautions. Those with 'brick' type HeNe power supplies are usually pretty well insulated.
Then, there is the rotating mirror which can catch long hair or jewelry.
Finally, since these scanners may have seen service under less than sterile conditions with all sorts of icky and disgusting stuff passing their way including meat and chicken parts dripping with blood, there can be all sorts of surprises in store for you from mummified mice to maggot colonies. Take appropriate precautions in your exploration and/or disassembly!
(From: Art Allen, KY1K (aballen@colby.edu).)
The unit I have which uses a power supply 100 percent identical to the schematic and PCB layout of IC-HI1 is a Metrologic Model MH290. It is labeled with a 1990 date of manufacture and says 12 VDC at 550 mA on the scanner unit itself. The wall wart that runs the system is rated at 12 VDC at 1 A.
The MH290 is a hand-held unit with a trigger, you pull the trigger when you are ready to scan and the laser starts scanning for 4 or 5 seconds and then shuts down. To attempt a second scan, you have to pull the trigger again. Inside the hand unit there is the receiver, a second PCB to support the receive electronics and the spinning mirrors (driven by a small 15 degree per step stepper motor). The MH290 is smart enough to know when the laser is on, and the error is produced if it doesn't come on OR if it stays on longer than it should.
The MH290 connects to another unit via a 9 pin RS232 type connector, the other unit has the EEPROM and related components for decoding and interfacing to the computer itself. The MH290 hand held scanner does not connect directly to the computer and all power sent to the MH290 comes from this other box.
Very old laser printers used helium-neon lasers but these are even rarer than HeNe laser based LaserDisc players. However, if you do find one, there will likely also be an Acousto-Optic Modulator (AOM) and driver since directly controlling HeNe lasers at high speed isn't feasible - don't neglect these very desirable components!
And, of course, those large graphic arts machines may have large HeNe lasers and even air-cooled argon ion lasers though newer ones will use Diode Pumped Solid State Frequency Doubled (DPSSFD) green lasers.
See the document: Notes on the Troubleshooting and Repair of Printers and Photocopiers for information on how the image exposure and fixing portions of this equipment works as well as warnings and precautions with respect to the hazards of toner dust. See the document: Sam's Gadget FAQ for more on salvaging parts from deceased equipment.
(Portions from: Erik Huber (erik.p.huber@uibk.ac.at).)
I worked in a big disco as LJ - Did a lot of raves and such stuff. I also DJ a little just for fun. The laser power you need depends on the room you have. If you want to scan pictures you need more power. If you just use rays, you won't need so much.
WARNING: Be aware that the maximum laser power level for the human eye is about (2.5 mW)/(cm2). Never look into the beam!
(From: Steve Roberts (osteven@akrobiz.com).)
If you wish to scan graphics on clouds, it takes from 10 to 20 watts of well collimated argon light to do so, and the problem is only people within about a 10 degree cone around the laser site from where it hits the cloud will see the graphics. Everybody else at best sees a faint flash from within the cloud, and in most places in the US the conditions for doing it will only be right a few days a year. It's also not a good surface for images, any thing more then a simple logo or spirograph pattern is unlikely to be recognizable. Scrolling text didn't work. How do I know, I was the one running the spirograph generator as a guest at the laser site.
In the USA, laser shows in clubs/bars/parks are regulated by the CDRH (Center for Devices and Radiological Health, a division of the FDA). Audience scanning is NOT permitted in the USA while it is common in the rest of the world. A large scanned effect spreads the laser power over a wide area and usually has some motion to it (such as the sine waves used to make rippling sheets of light). This means that the energy density and the exposure times are low.
If the laser beams are not scanning directly on the audience [dancers] then the effects are probably safe. If the system uses scanned beam effects, then it is probably following the rules of it's jurisdiction and is probably safe.
Having done a few of those shows overseas, it's not just moving fast, but that's part of it. In fact, moving too fast can in some cases brighten the beam to exceed the MPE (Maximum Permissable Exposure) because of the dynamic characteristics of the scanners. It's the dwell time on each point of the image as the scanners are tracing it out, it has to be carefully measured for each animation or effect with a scope, fast photodiode, and a laser power meter. Each image has to be carefully designed using the show software to avoid sharp corners and other hotspots. Just scanning it fast is not enough - you will note that only very large scans flow over the audience. There is what is referred to as the zero line, well above the audiences head. As the images dip below the zero line, they are reduced in brightness by the hardware and by the show programmer. A scan fail system is also usually in use that will cut off the system should a scanner fail, and this has to happen fast if the MPE is not to be exceeded, in a fraction of a millisecond, so very careful engineering has to go in this.
Please folks, just because you saw a beam scanned over the audience in a club and you have a laser, don't try it at home without getting the equipment to make the measurements and calculate the MPEs. It is not possible to determine if a effect is eye-safe by eyeball alone. The European clubs pay between $50,000 and $100,000 for these systems so a lot of time and money is spent on doing the safety analysis when programming a show. There are permits and licenses involved as well. Each frame of the show - and there is usually 6 to 15 frames per second - must be checked and carefully designed when doing this. The show must be checked for each facility it is ran in as well. You really need to take a class in how to do it safely. Such classes are offered at ILDA and ELA meetings and by safety inspectors/laser providers in Europe.
Please note that this is not normally legal in the US as we have lower MPEs that make it ineffective when done anyways. It was suspended in much of Europe recently for a review of the power levels in use, new standards were implemented with tighter controls and it is again legal in parts of Europe. It is also legal in Canada, but again, measurements have to be made.
If you're gonna do a show and you don't know what your doing, the basic guidelines for where the beam may go are a minimum of 3 meters up from the highest point in the audience and a two meter horizontal separation from the audience to any beam. In the USA, a CDRH Variance is required for any public show above 4.95 mW, and the penalties are draconian for failure to obtain them. The MPE in the US is about 2.3 mW per square centimeter per second for visible lasers.
In my opinion, I would rather have a single mixed-gas 'white light' laser to avoid the hassles of beam collimation of two independent lasers. This is especially true if you do shows on the road where everything is jostled around. You may get better life with a red-only krypton tube, but you are almost always fiddling with near- and far-field collimation to keep your PCAOM output efficient across the entire spectrum.
The color balance in a single mixed-gas laser will slowly change over time, but it is easy to make software color palette corrections on the white-light balance in a few minutes. (At least until the red output drops too much.)
As for tube lifetime, I think it is function of art, science, tube current, luck, and the phase of the moon when the tube was installed. I know one laserist who only got 600 hours on a tube. I know another that has lasted for many years.
(From: Patrick Murphy (pmurph5@attglobal.net).)
The Schneider solid-state RGB laser does exist and is in use for laser shows, including the Hershey Park outdoor show in the U.S. There are two main versions of the laser. One is just the laser for light-show type applications. The other is the laser plus a video projection head (scanning mirror type) to create infinite-focus, wide color-gamut video. I saw both versions doing a combined show (video + laser graphics + laser beams), a few weeks ago at the Schneider factory in Germany.
The following information relates just to the light-show model,
imaginatively called "Showlaser"
The original $160,000 price mentioned elsewhere was an estimate; the
actual U.S. price will be somewhat lower than this ($120K? $140K?). This is
still a lot, but not quite as much as the estimate. Schneider realizes the
price is high for the laser light show market and will be seeing if it is
possible to lower it.
The useful output power is 13 watts of modulated white-light from the end of
a fiber (e.g., into your scanners). The colors are nicely spread -- red at
628 nm, green at 532 nm, blue at 446 nm -- so you get very dramatic violet
and purple. (In video applications, there is no speckle, skin colors are
normal, and saturated colors are quite striking.)
The input power is 220 VAC at 3,000 W (e.g., about the same as two hair
dryers). It has its own internal chiller, which you fill every few months
with a gallon of distilled water. So in this sense it is "air-cooled", as
you don't have to hook up an external chiller.
Because everything -- laser head, modulators, chiller, power supply -- is
built into one unit, the Showlaser weighs 660 pounds. This is roughly the
same as all the parts of a medium- or large-frame ion laser together. The
unit is compact and is on casters so the weight is not quite as bad as it
could be.
The working part of the laser is manufactured by Jenoptik (it says so it in
a big decal on the Showlaser's side). The working principle is described in
this paper:
RGB
Lasers for Laser Projection Displays. Here is the abstract:
The working part contains numerous optical components on a breadboard.
Although it looks like a nightmare to align, everything is actually
controlled by a computer. Once it is factory-set, in theory you never need
concern yourself with what is inside. Schneider says the laser will last
10,000 hours before the diodes need replacing.
"AVI-Imagineering With Lasers" is the U.S. distributor. They've received the
one for Hershey Park, with more on order. So far, the Hershey Park laser has
traveled well for AVI. It was trucked five times and four times there were
no problems at all when the laser was turned on. The fifth time there was a
power loss which may or may not have been due to traveling. (The cause is
still being studied.) Since the solid-state laser is much newer than
decades-old ion technology, I think people should expect a few "teething
pains" to be worked out.
Schneider also makes high-end TVs sold in Europe. I have been through the
factory (same place as the laser division) and it is an amazing place, with
raw materials such as plastics and electronic components coming in one end,
and consumer boxed TVs coming out the other. Schneider also recently bought
a majority interest in "tarm", the well-known German laser show company. So
Schneider does things on a big scale, they know what they are doing in
laser, and they want to do it at a consumer level.
Obviously, it's pretty amazing for an RGB laser to get 13 watts of modulated
light from a standard 220 VAC dryer-type outlet, with only occasional water
top-offs, and a 10,000 hour claimed life. On the downside is the weight and
the natural bugs that come with development of any new technology. The price
is the biggest obstacle at this moment. With luck that may be coming down to
a more affordable level, as volume, development, technology etc. improve.
(From: John R (scifind@indy.net).)
White-light color control with a red HeNe and multiline argon ion laser
and be done without a PCAOM, but you may not like the answer. It is much
cheaper than the PCAOM method, but still involves lots of work and moderate
costs. Of course, if you are a laser hobbyist, nothing is cheap, especially
if you want laser beams other than 632.8 nm red!
For a minimum white light color control system:
I once built one of these "RGB color boxes" using an argon and HeNe laser. It
worked quite well, but there was the major hassle of alignment of multiple
dichros, other mirrors, and three AOMS. A significant portion of the Argon
power may be lost because it has to pass through three dichros.
As for costs, if you can get surplus AOMs, dichros, and make your own mirror
mounts, maybe $200 to $400 - if you're lucky!
Unfortunately, there is no simple or cheap way of doing it.
And, if you are thinking about mixing yellow and orange HeNe's with argons and
red HeNe's, I seriously doubt you will achieve the performance (and ultimate
cost) of even a used PCAOM.
Why?
You may also run into problems as each independent laser has its one beam
diameter, divergence, and spatial TEM characteristics. So if you could
collimate them, the resultant "white light" beam will have lots of color
fringes.
Of course, it is your time, money, and effort, therefore, I wish you good
success. But using a higher power red HeNe and then blending it with the
multiline argon is still the better approach.
For more information, try Laser FX.
Their Website author also has an excellent handbook on lasers and laser shows.
There are a couple of chapters devoted to RGB color control in lasers,
including HeNe/Argon methods. If you are serious about making white light
beams (and learning about lasers and shows), this is the book to have!
Also, other ideas. Neos Technology has a 4-channel PCAOM crystal for $680 and
driver for $600. If you are a hobbyist, this is not cheap. However, if you
can get a PCAOM system, it is vastly superior to the RGB/dichro color method.
(From: L. Michael Roberts (NewsMail@LaserFX.com).)
To combine the two lasers your best and lowest cost solution would be a
dichotic. Firstly you need to have a set of two FS mirrors on optics mounts
[E.G. Newport MMI or RMSM OM3/4] to level and steer the beam. Purchase a cyan
or red dichro [from Edmund or PPS]; mount it on another optics mount. With a
cyan dichro, you shine the argon through the dichro [which transmits
green/blue wavelengths]. Set the dichro in the beam at 45 degrees at the
point where the ar and HeNe beams are made to cross at a right angle.
Careful adjustment of the steering mirror pair on each laser will allow you to
produce two beams that are level relative to each other [and the baseplate of
your projector] and cross at right angles. Set the dichro in the position
where the beams cross at a 45 degree angle relative to the Ar beam [with the
45 degree angle such that the HeNe beam is reflected away from the Ar source].
Adjust the beams until the HeNe and argon beams overlay each other on the
dichro [near field adjustment]. Now look at the resultant beam at some
distance or on the projection surface. Adjust the dichro so that the two
spots overlap [far field adjustment].
Adjusting the dichro will cause some change in the position of the Ar and HeNe
beams so you then re-adjust the near field [laser steering mirrors to overlap
the beams on the dichro]; then the far field [dichro to overlap beams on the
screen]. 2-4 adjustments going back and forth form near to far field may be
required, but in the end you will have the two beams exactly overlaid on each
other. To the eye, the beam will appear a pinkish white - colour balance can
be adjusted by varying the brightness of the Ar laser.
A cyan dichro is recommended as it reflects red and you want to conserve red
photons. You will note that some of the argon beam is deflected in the
direction the HeNe would have been going if not reflected. This is due to
beam splitting at the surface of the dichro. If you use a red dichro, those
would be red photons you would be throwing away.
You can now place a PCAOM [from NEOS or MVM] in the combined beam. Make sure
the polarization of the HeNe is vertical [check the ar while you are at it -
they are usually polarized vertically but poor alignment could have you a bit
off] and that the PCAOM cell is correctly oriented. Varying the control
voltages to the PCAOM will allow you to have additive [RGB] colour control.
You can get 16.7 million colours or more depending on the PCAOM and the system
used to control it.
(From: Steve Roberts" (osteven@akrobiz.com).)
There are 3 quality sources of laser show dichros that I have used:
Prisms are generally only useful for separating one line, and for laser
display purposes, you need all the power you can get, so you want all the blue
or all the green lines, etc. They are also a pain in the neck as dispersion
versus angle is constant, and a dichro can be tilted off axis quite a bit and
still have throughput. Many traditional laser projectors for planetariums did
just that, have a prism and a color selection galvo, but this takes up several
feet of space to do and is difficult to support from a control systems point
of view and to align. With a prism, you're wasting from 60 to 85% of your
light at any one time, as you're only using one line.
Also beware that Edmund Scientific's dichros are more or less coated for
TV/spotlight applications and thus leak some blue or green that a laser show
dichro wouldn't. This spoils the effect of clean contrasting colors, so you
need a dichro designed for laser display. Edmund's dichros are great with a
tungsten source however.
When you order, ask for backside AR coats on your dichros if available.
Otherwise you'd have 8 to 10% Losses from the Fresnel losses.
(From: L. Michael Roberts (newsmail@LaserFX.com).)
To create visible beams in *total* darkness you can get away with as little as
100 mW. For beam effects in a club or other venue with some ambient lighting,
1 watt is about the minimum you need to make visible beam effects. Outdoors
you will need 5-6 watts to make visible beams [again depending n ambient
lighting conditions].
In all cases, a scattering medium (smoke or dust) is required to deflect the
light towards the observer's eyes. In clean, clear air in winter, I have seen
the beams from a 20 watt argon look lamer than the beams from a 1 watt indoors
with a good haze.
(From: Steve Roberts (osteven@akrobiz.com).)
In a dark room with average dust levels and high humidity you can start to see
the forward scattering of an HeNe beam at about 1 mW! 30 to 40 mW of argon
makes an OK side view beam in a dim room, but its not exactly a Star Trek
photon torpedo kind of glow. It helps if the argon is configured multiline and
is doing more green then blue, as the eye peaks in the green. To see the beam
in a well lit room requires smoke of some form.
Most laser light show types don't like the common aquafog, it irritates your
lungs after constant exposure, so we use hazers indoors. A hazer works by
making very tiny particles of medical grade oil. These are small enough to be
flushed out of your lungs by normal breathing and if properly set up, are
odorless and OSHA approved. Fog machines for the most part are crackers, they
work by incomplete combustion of glycols (aquafog) or burning of oil in air.
Hazers fragment the oil in CO2 and thus are almost odorless. Plans for a
homemade hazer of sorts that uses air are at LaserFX on the
"Backstage" pages. It has a slight odor but is not that bad to be around, and
mind you I have asthma! I have done indoor shows for 1,200 people using 60 mW
and a cracker. I have also done shows indoors for 100 people with a 5 mW hene,
it depends on ambient lighting and air circulation/humidity.
It is a minimum of about 5 watts of argon light for a decent outdoor smokeless
beam show, with 20 watts being more typical.
(From: Steve Quest (Squest@cris.com).)
Visible wavelength lasers are more visible in 'plain air' if the angle of
incidence is low (you're close to the same angle of the beam) and if the power
is greater than about 5 watts. I perform an outdoor laser show using a 30 to
57 (max) watt YAG (frequency doubled to 532 nm) which is plainly visible in
mostly clear air (no need to smoke, or fog the air). When I want to do beam
effects with a 5 watt argon/krypton white-light laser, I have to fog the air
up.
Plain outdoor air has enough particulate matter to scatter a laser beam so
long as it is above 25 or so watts, thus making the beam visible. Of course,
the more power, the brighter the beam looks, but CDRH has limits, and that
limit is .9725 mw/cm2 at 750 feet, so the days of power beam shows
going all the way to outer space and beyond is over :-(.
I use a Laserscope laser, which is FDA (Food and Drug Administration)
approved, and am following CDRH (Center for Devices and Radiological Health)
guidelines, receive FAA (Federal Aviation Administration) approval and air
clearance before every show, and make sure that NOTAM (NOtice To AirMen) are
issued to pilots flying in the area of my shows, giving exact details as to
what is going on. Pilots love the shows, and air traffic routes planes WAY
out of their flightpaths to fly near the beam shows to get the best seats in
the house. :) However, I have to beam-off when they get too close, then they
return to their flightpath, and I can resume the show.
I used to be able to sparkle off the new moon with my YAG at full power and
full convergence. It takes some doing but you can see the sparkle from the
Sea of Tranquillity with the naked eye off the corner cube reflector, aka:
retroreflector left there in 1969 by the astronauts.
(From: Sam.)
WARNING: Shooting a laser into the sky is irresponsible and highly illegal
without prior approval from the proper agencies. Airline pilots do not
appreciate being blinded!
Here are some additional comments on the effects of viewing direction on
apparent brightness:
(From: Johannes Swartling (Johannes.Swartling@fysik.lth.se).)
What you see is light that has been scattered by the small particles in
the fog or smoke. This kind of scattering is called Mie scattering, and
occurs when the size of the particles is comparable to or a little
smaller than the wavelength of the light. In Mie theory, there is
something called a scattering profile - i.e., the probability that the
light will scatter in a certain direction.
Now, in the case of very small particles, such as molecules, this
scattering profile is isotropic. That means that the light will scatter
in all directions with equal probability. This special case is called
Rayleigh scattering, and can be seen from pure air if you have a strong
enough laser, such as an Ar-ion laser. When the particles get larger,
however, the light will tend to scatter more and more in the forward
direction. That is what you see from the smoke. When you look along the
beam in the direction where it comes from, you see a lot of light that
has been scattered just a little bit off the direction of the beam. When
you look along the beam away from the laser, there's a lot less light
that has been scattered backwards.
(From: Pissavin (pissavin@aol.com).)
One interesting phenomenon; Depending on whether dust or smoke is used, there
is an asymmetry: With smoke, if you put your head near the laser and look
down the beam, you see almost nothing. Now, look toward the laser (BUT NOT
DIRECTLY INTO THE BEAM!) and you see a clear beam. Then replace the smoke
with dust and the effect will be reversed.
(From: NeoLASE (neolase@lasers.org).)
Large particles like dust have more back scattering centers while small
particles like smoke and haze have more forward scattering centers.
Mie scattering effects, and all that stuff, I've heard/read of but I
haven't studied in detail. Used a lot in laser particle size analysis.
For the most power available, usually a krypton ion laser running red only and
an argon ion laer for the blue and green is combined. The krypton red
wavelength (647.1 nm) is not the best for color combination for true RGB
mixing but it is about all that is available with adequate power. Remember,
even if the argon were to produce 20 watts evenly split between green and blue,
and 10 watts of red from the krypton, a total wattage of only 30 watts is
available for the entire picture area. This really isn't that much for a
large scale presentation and is why Vegas uses RGB light bulb boards as well
and stadiums use Jumbotrons or Diamond visions, not lasers. The total light
available is 1,000's of times brighter, and even with coarse resolution, the
distance from the screen blends the image. Raster scanning with a laser is
very inefficient, but with vector scanning and raster some unique effects can
be created. Better yet use 10,000 watt lamps, one for each color via the
proper filtering and use light valves to control the each device for each
color. Like a projection TV except on a huge scale. And cost is always a
factor.
How well this works depends on the pulse rate and pulse width of your laser
and how fast you are scanning, and how much you like dots and dashes
in your image. It also depends on how you are shaping your image - i.e,,
some non-galvo imaging systems use pulsed YAGs for projection video.
However if you are talking about an AO Q-switched YAG at a high rep rate, you
can do, say, 10 to 12K galvo graphics. It just shimmers a lot and has faint
spots that wander through the image. The real killer is that the divergence of
pulsed YAG lasers of any significant power is extremely high and when the
divergence magnitude starts to catch up with the resolution of the points in
the image, you get a blob. When it catches up with the scan angle, you get a
bigger blob. This happens at say a couple of hundred feet from the laser.
I have witnessed this as a member of the crew on a show using a Q-switched
YAG for beam effects. The company owner wanted to try scanning images on a
building some distance away to see how his collimator worked. Up close it
wasn't bad. But, more then a hundred feet or so from the laser, it was "The
Green Blob".
(From: L. Michael Roberts (NewsMail@laserfx.com).)
The most common way of creating this illusion is to use a scrim or a water
screen. The scrim is a thin fabric screen, like mosquito netting, that is
often dyed black or dark grey. It is rolled/lowered/flown into place while
the audience is looking at something else, then used for laser graphics
projections. Using typical modern 30K PCAOM projectors, flicker free
images can be projected onto the scrim. While most of the laser beam goes
through the scrim, enough of the laser is intercepted and reflected by the
threads in the scrim to form an image.
The water screen is a similar concept except that it uses a thin film of
water droplets sprayed into the air as a projection surface. Both there
techniques allow one to create the look of an image suspended in mid-air -
especially if the audience is fixed in relation to the projection surface.
There is a beam interference technique in the early stages of development but
it isn't likely to ever result in a large scale display out in open air. It
was pioneered by Dr. Elizabeth Dowing. The image is generated inside a
specially doped glass cube using scanned IR lasers. At present. the display
s barely 2" on a side. For details see
Three-Colour, Solid-State,
Three-Dimensional Display.
(From: Gronk (gronk@concentric.net).)
I am fairly new to lasers (been studying and researching on internet for
about 1.5 years now, especially Sam's Laser FAQ) and decided a few months
ago to do my own laser show for our New Millennium eve party. We had about
30 or 35 people in attendance, and a musical show that lasted about 40
minutes. The equipment consisted of a home built Lissujous pattern
generator (not the spinning motor kind) with laser modulation driving a
GAL-2, a 1 watt stereo audio amp with raw audio from the show music
driving a GAL-2, and 2 lumia wheels with 3 lasers shining through them.
All this was projected on a silver screen (plastic tarp) suspended about
15 feet above the audience (no audience scanning done of course) . The
lasers were all laser pointer types with the batteries removed and wires
attached, and all connected to a home built laser power control station
which controlled power to each individual laser. Fog beam effects were
accomplished by spraying 'fog-in-a-can' at the beams. It turned out
great, with all who attended enjoying it very much (granted, most of
them had never seen a 'real' commercial laser show).
It was a really fun project and will be done again at years end this
year! I would encourage anyone who might be thinking of doing this to
go for it! It was not really expensive, and was worth every penny for
the all around experience. I also included my son (who was way better
than me at operating the pattern generator) in the show, so he got a
real kick out of it too. Highly recommended!
(From: John Craker (watts@dccnet.com).)
I built a basic laser show from a dead (semi dead?) LaserDisc player. When
hooked up to my home stereo, it displays lovely (and useless) Lissujous
patterns on my ceiling.
I basically robbed a section of the chassis that housed the HeNe laser and
another section that had two deflection mirrors. Pointed the output of the
laser into the mirrors. I hooked up the coil of each mirror to each channel
of my stereo. With the difference in the stereo signal, you have each mirror
oscillating at a slightly different rate, and since one mirror deflects in
the 'Y' axis, and the other in the 'X', you get this great ever changing
display. Size is pretty much adjusted via the volume. :)
(From: Sam.)
Based on a photo that John sent me, the deflector from this LaserDisc player
would appear to be virtually identical to the
Meredith Instruments
GAL-2. I wonder if that's where they got them! Along with a HeNe laser
or laser pointer, and low power audio amp, you're in the instant light show
business. Well, at least for those boring Lissujous patterns! :) The GAL-2
is sensitive enough to be driven by a personal stereo but the 4 ohm input
impedance may overload its output if it is designed for 32 ohm headphones.
Note that while the GAL-2 and the similar laserdisc deflector appear
superficially similar to a pair of loudspeaker voice coil/magnet assemblies,
the pole pieces of their magnets are on either side of each coil rather than
within and surrounding them as in a true loudspeaker. Thus, the coil, and
thus mirror, pivots from side-to-side as expected and desired rather than
moving in and out.
There are now companies marketing (or at least seriously demonstrating) laser
based TV displays. The most recent versions use a single multi-color diode
pumped solid state laser. One such unit has an optical output of about 13 W.
To put this in perspective: The visible output of a 250 W incandescent bulb
is about 13 W. So, that's a lot of light for a small screen but isn't going to
compete in a theater setting. And, you don't want to ask about the cost! :)
But, see the section: About the Schneider High
Power DPSS RGB Laser/Projector for info on one such unit.
Using laser diodes directly rather than solid state lasers has some fundamental
problems. The first has to do with color. You can have any color of laser
diode you want as long as it is red. :) While moderate power (perhaps up to
500 mW or 1 W) red laser diodes have been around for awhile, laser diodes with
an actual blue wavelength (430 to 445 nm as opposed to deep violet - around
400 nm) are just becoming available as costly engineering samples with all
sorts of strings attached and they have power outputs of only a few mW (see:
Availability of Green, Blue, and Violet Laser
Diodes). Of course, even 445 nm is more violet than blue, 460 would be
better, but it's a start. Green laser diodes aren't even on the horizon.
Unfortunately, even if high power RGB laser diodes could be purchased for $10,
due to the fact that they would operate with multiple spatial modes along one
axis, generating a tightly collimated beam suitable for scanning would be very
complex and expensive, if not outright impossible. Better go to plan B. :)
(From: James A. Carter III (jacarter3@earthlink.net).)
Just to let folks know where this Laser TV thing has been.
In the 1920's, a company in England, Scophony Labs (I think that's right)
patented a method for using Bragg diffraction on tanks of water (that's right
H20) to display TV signals using white light (thermal) sources. They had to
use BIG beams because they didn't have lasers. BIG beams mean low modulation
rates due to acoustic transit time. Their idea was to scan the spot so that
the acoustic pulse was stationary on the screen. I believe that they didn't
use galvonometric scanners for the horizontal scan, instead they put mirrors
on motor shafts (similar to what some cinemagraphic projectors used at the
time). The scan rate and magnification were selected so that the scan velocity
vector was equal and opposite to the image of the acoustic velocity
vector. This may have been an idea way ahead of its time.
Just ten years ago, I helped design the optics of a system that does display
not only NTSC images but scan to HDTV as well. This is not a cheap system and
is certainly is not suitable for avionics; although the Air Force (through
TRW) did buy many systems. It used an air bearing motor to drive a many
faceted polygonal mirror scanner for the horizontal scan and used a "galvo"
scanner for the vertical. The AO modulators had enough band-width (at least
500 times what you get from PCAOMs) to project NTSC images in a flying spot
mode. That is the scanner was going much to slow to give the Scophony
condition. When we ramped the system (it was a closed loop continuous
multiscan projector) to 1280 by 1024 sources, the scan was fast enough that we
achieved the Scophony condition and realized over 35 MHz of video bandwidth
per channel. This is somewhat inadequate for computer CAD graphics but was
quite acceptable at the time. The display was dazzling, to say the least. Per
laser color for each red, green and blue channel with red at a deep and rich
635 nm (dye laser pumped by the otherwise useless cyan lines), and the argon
lines for green and blue. We used a 10 watt argon from Spectra-Physics to be
the photon engine (SP was an investor here). One of these went to the NAB show
and displayed our beloved President Ron.
Unfortunately, the lasers were not reliable enough, to expensive to repair and
replace, and more light is always better. Further, the big guys (TRW and SP)
started to bicker and the company went under. The last time I saw one of
these systems was at SP Corporate in San Jose. I was there to install a 25
watt laser, but that's another story.
Current commercial work centers on dumping the high speed scanner and using an
AO cell to modulate the whole line at one time. Bragg cell technology can give
the Time-Bandwidth Product (TBP) required which is certainly over 1000 and
closer to 2000. Unfortunately, acoustic attenuation (Beer's law in time and
space) and the non-uniformity of the laser source (typically Gaussian) require
losses to make a nice uniform display. Even with HIGH power pulsed lasers
(repping at the horizontal line rate or at a multiple), the display can lack
luster.
As always, more photons... more photons...
(From: Tony Clynick (tony.clynick@btinternet.com).)
I am pleased to tell you that laser video projection is still very active
in the UK. Based on the original laser video projector (LVP) made by
Dwight-Cavendish in the early 1980's, the projector now made by the team at
LCI (Laser Creations International in London) has been installed at several
permanent sites in theme parks since 1994, mostly in East Asia, and has been
used for dozens of temporary shows world-wide since 1987. Most applications
are in exhibitions, outdoor shows and theme parks.
The LCI-LVP uses SP white-light lasers with special optics to provide good
flesh-tones so the need for dye lasers is eliminated. A polygon scanner (GEC
Marconi - thanks Alan) provides the line scanning, at rates of up to 36kHz.
AO modulation and Scophony balance provides video bandwidth up to 30MHz, so
HDTV (1250/50 and 1125/60), as well as PAL/NTSC/SECAM are available in the
LCI-LVP. Output on screen of a peak-white modulated raster of over 15 watts
has been achieved. The largest image projected so far was 50 metres wide.
The collimated scanned beam provides an infinite depth-of-field, which was put
to good use last year at the Singapore National Day on a giant 35m x 28m
high-gain screen laid over the slanted stadium seating. The difference in
projection distance between the top and bottom of the screen was nearly 100
metres, so the LVP was the only machine capable of a focussed image over the
whole screen. All LVP's supplied so far by LCI are also capable of vector
scanning using the waste AO beam.
(From: Chris Cebelenski).
I know of one experimental project that uses an array of galvo's to project a
raster image at 1/2 normal NTSC refresh rate (15 fps). The cost of this
endevour so far has been, well, let's just say it's been expensive. :-)
Currently it's configured like this:
There are several problems with this:
(From: Steve Roberts (osteven@en.com).)
Two years ago I was at a Laser-FX conference in Canada, we had the chance to
watch (I have it on tape) a Russian made scan system with no moving parts, all
acousto-optic and almost totally analog driven, that produced sharp clean
monochrome images without flicker the size of a billboard using a 6 watt 532
nm YAG . The marketing person explained that RGB existed in the lab and was
not far away. I believe the company name was Lasys Technologies. Scan head and
laser was about the size of a PC/AT case and sat on a tripod, and was easily
handled with low weight. Ran off 220 VAC three-phase, but I was told 220
single-phase would not be a problem. Further details can be obtained from:
L. Michael Roberts (lmichael@laser-fx.com) who was the organizer of the
conference.
(From: L. Michael Roberts" (NewsMail@laserfx.com).)
Some of the newer laser based video projectors (e.g. the Samsung unit) use
a white light laser [Ar/Kr] as the source - 3.5 to 10 watts depending on
the image size and brightness desired. The beam is split into it's prime
component colours, modulated, recombined and then scanned.
Many of the older units used a tandem laser pair - an Argon and a red-only
krypton]. Some units even use three lasers - an argon with blue optics,
and argon with green optics and a red-only krypton. This takes a LOT of
water and power to operate.
There is presently a lot of work being done on producing compact diode
pumped YAG based red and blue lasers. Laser Power showed prototypes of
these lasers at the ILDA meeting in Amsterdam last November. This would
allow people to build a fairly powerful [2 watts input approx.] laser based
video projector that is air-cooled and can run on 115 VAC.
(From: Sam.)
Here is a link to an article about a system that may be commercially viable
in the near future. It uses second and third harmonic generation to produce
green (532 nm, 13 W) and blue (447 nm, 7 W) output, respectively, from a pair
of Nd:YVO4 diode pumped solid state lasers along with a diode
pumped optical parametric oscillator to generate the red (628 nm, 10 W) beam.
And, here's a description with photos of a laser TV system built back in 1985
(along with some other related laser display gadgetry):
And some comments from Doug:
(From: Doug Dulmage (dulmage@visi.com).)
One thing that is nice about TV using lasers is the use of a true red
"gun". I've built 3 or 4 different versions of laser video projectors using
argon and krypton lasers and the first thing you notice when you put a standard
color bar signal up is that it looks "different". The reason is that in normal
television there really is no such thing as a red phosphor. They are actually
closer to orange than red, but by color mixing and a little fooling of the
brain, you see red from the orange phosphor. So when you finally do see
a video display that comes from a fairly dark red line (like the 650 of the
krypton), things that normally look really bland like browns, violets, and
other colors that depend on red, look stunning. It makes normal television
look much more like film that video. Oddly enough, a couple of commercial
laser video companies went to great lengths to produce the orange line instead
of the red from a krypton by using argon pumped dye lasers to produce the
orange. I could never, ever figure out why go to such trouble except that they
were so anal about trying to follow NTSC standards for color that they
ignored the benefit of having a true red. I had a little secret method for
curing those situations where the client would complain about the color and I
could give them orange back without the use of the dye laser, but normally
once they saw real red, they wouldn't let you touch it. It makes sense, most
color CCD camera (at least with three CCD's) use color dividing prisms that
cutoff into the red more than orange.
Displays capable of providing information about the three-dimensional aspects
of a scene can be divided into two classes:
The advantages of these approaches are that they are well within the
capabilities of modern digital processors and display devices.
There are various technological hurdles to be overcome to make this sort of
display practical since with a 3-D volume, much more data needs to be
rendered and transferred to the actual display hardware. There are also
fundamental problems with implementing hidden surface removal.
Needless to say, just a bit of work needs to be done before one of these will
be as inexpensive as any TV set. However, see the section:
Holographic Video Displays.
Currently, there are technical issues to be resolved with respect to the
bandwidth of the channel to get the information into the display
(Gigabytes/second are required for adequate refresh rates). But more
fundamentally, these techniques are incapable by their design of rendering
solid shaded surface views. The volumetric display is one of 'look through'
or 'structured fog'. However, such a technique in a practical application
could be extremely useful.
With technologies as yet unavailable, one could conceive of a 'selective
activation' display where points in 3-space are rendered opaque or emissive
by intersecting Laser beams or something like that. There has been progress
in this area with emissive displays - intersecting laser beams resulting in
the production of colored points of light. However, all these technologies
suffer at present from serious resolution and bandwidth limitations - not
likely to be solved for decades at least. (See below.)
A true holographic display would be capable of an ***arbitrary*** viewing
mode including the display of solid surfaces with shading which would
be viewable with correct perspective and shading from a range of angles.
I do not know of any actual examples of such technology at present. An
emissive volumetric display like the one described below cannot implement
hidden surface removal - essential for life-like rendition. While wire-frames
and look-through displays have many uses, they aren't likely to be of much
value for a boob-tube replacement! :)
A brief description of some of the alternatives can be found at:
Pangolin's Laser Show Guide -
Making 3D, floating images. Additional details on one of these, the
spinning helix approach, can be found at:
Technical
Description of a 3D Volumetric Display System.
Also see the sections starting with: Introduction
to Holography
(From: L. Michael Roberts (NewsMail@laserfx.com).)
Already in the works! A "Three-Colour, Solid-State, Three-Dimensional Display
based on two-step, two-frequency upconversion in rare earth doped heavy metal
fluoride glass is described. The device employs infrared laser beams that
intersect inside a transparent volume of active optical material to address
red, green, and blue voxels via sequential two-step resonant absorption.
Three-dimensional wire-frame images, surface areas, and solids are drawn by
scanning the point of intersection of the lasers around inside the material.
The prototype device is driven with laser diodes, uses conventional focusing
optics and mechanical scanners, and is bright enough to be seen in ambient
room lighting conditions.
The full article is available on-line at
Three-Colour, Solid-State,
Three-Dimensional Display.
(From: Michiel Roos (roosmcd@dds.nl).)
That's a block of (expensive) glass with some lights in it? Last thing I
heard, they'd only got a low resolution. But a couple of years ago I was
at a Philips trade show. There was a true (?!?) 3D laserTV system. In a
room, a music video was shown. There were a number of layers displayed
in air (fog?) so you'd get a 3D view. Nice thing was that you could walk
right through the image and still see it. But I've never heard of it
again. Anybody knows if they're working on this now?
(From: Steve Roberts (osteven@akrobiz.com).)
Engraving inside a block of glass is a pretty easy thing to do if you
have a high power pulsed YAG laser. I've seen problems in labs with cheap
glass lenses developing spectacular defects in the middle of the glass, so a
variable focus lens, some galvanometer scanners for positioning, and a monster
pulsed YAG - plus some decent software and you should be able to carve in
flint or lead glass.
It's all too easy to create microcracks on the insides of the cheap lenses.
(From: David Toebaert (olx08152@online.be).)
The December 1999 issue of 'Laser und Optoelektronik' has a beautiful picture
on the cover of a piece of lead crystal with the Dresdner Frauenkirche inside,
3-D engraved using Nd:YLF (Q-switched AND mode locked) lasers. It was
developed by the Fraunhofer Institut fur
Werkstoff- und Strahltechnik.
However, in so far as the technology exists today, holography is NOT what is
often depicted in Sci-Fi and other movies and TV shows. Some of this
deficiency is due to fundamental principles of what holography is and how it
works while much of it is due to the inadequacy of present technology:
While holography is really still in it's infancy it already has many other
fascinating applications. Just a few of these include:
The best bet is to get a 5-10 mW HeNe surplus laser for about $200 to $300
dollars. This type of laser should have a coherence length of at least 6" or
so. You'll also need some holographic film (I used to use Kodak stuff many
years ago -- don't know if they still make it but it was relatively sensitive
and easy to use). Next, you'll need to build a stable table. In a pinch, a
heavy wooden plank, slab of marble, etc., laid on a few partially inflated
inner tubes will probably be enough. I strongly recommend against a sandbox as
it's more of a pain in the ass to keep things clean and to prevent optics from
constantly shifting as you move things in the sand. Set the table up on the
lowest floor, preferably on a concrete foundation, to minimize
vibrations. Then you'll need to get some redirection mirrors and expanding
lenses. Finally, you'll need the chemicals to develop the exposed film.
From complete scratch, you are looking at an investment of about $350 to make
a simple hologram.
Here are more detailed suggestions:
(From: Rick Poulin (rpoulin@rohcg.on.ca).)
I used to be a holographic experimenter and got my supplies from Agfa but
sadly they got out of the business and left many people scrambling for a new
cheap source. If you want to pay through the nose,
Edmund Scientific or
MWK Industries are
the high water marks for pricing.
If you want cheap film or glass plates there is a source in Russia called
Red Star. Go to the Royal
Holographic Art Gallery Film Page for the North American dealer in British
Columbia, Canada.
The following is from a posting to the USENET newsgroup
alt.lasers in early 1999. I have no direct
knowledge of the contents or quality of these kits or whether they are still
available.
(From: Steve McGrew (stevem@iea.com).)
I've just received and tested the first shipment of a new
holography kit for education. It includes a HeNe laser, an optical
breadboard, adjustable mounts, dielectric mirrors, and a detailed,
understandable manual in good English (I helped with the translation).
The manual details a series of experiments and explanations that will
lead a student through all the basics of optics up through 3D
holography. The kit and experiments are designed for a college-level
optics course, but would be suitable as well for science enrichment at
the high school level. The kits are made in China under the
supervision of a university optics professor. Each kit fits neatly
into an aluminum suitcase. If you were to buy all the parts for the
kit in the U.S., they would cost somewhere in the range of $1,500.
My cost is $525 plus shipping; I'll provide these kits to any
bona fide school for my cost plus 10%, and will provide advice as
needed to teachers and students. (Price subject to change, so please
ask for confirmation of current price.)
While I don't know how to select a laser diode to guarantee an adequate
coherence length, it certainly must be a single spatial (transverse) mode
type which is usually the case for lower power diodes but those above 50
to 100 mW are generally multimode. So, forget about trying to using a 1 W
laser diode of any wavelength for interferometry or holography. However,
single spatial mode doesn't guarantee that the diode operates with a single
longitudinal mode or has the needed stability for these applications. And,
any particular diode may operate with the desired mode structure only over
a range of current/output power and/or when maintained within a particular
temperature range.
For for information on laser pointer holography, see:
(From: Frank DeFreitas (director@holoworld.com).)
I had my fingers crossed tighter than ever for this one -- moving up to
35 mW of power for holography using a diode source. It worked!
The module used contained the Hitachi 35 mW, 658 nm diode, along with
AR-coated anamorphic prisms (optional) and high-grade collimating optics.
The measured optical output after collimating optics is 27 mW and total cost
for putting the whole thing together was about $50 to $60.
This little baby exceeds the performance of any HeNe in its power range,
including the $5,000 Spectra-Physics at 25 mW.
Those diodes are real little buggers once they're set up with an
interferometer. Very strange behavior (at least strange after working
with gas lasers for so many years) - and in a good way.
In any case, this baby is ROCK solid. The final test which put us over
the top was so incredible that I thought there was something wrong with
the set-up. I would tap on the table just to make sure. It's almost as if
a fringe-locker was in place. Even with the best HeNe that I've had here
(Spectra-Physics 124B Stabilite) there would ALWAYS be some "drift" or
what I call "float". (Float is the feeling that fringes are not entirely
still -- it's not something that shows up very clearly to the eye.
It's more of a "feeling" when testing). The fringes with the new diode
are locked so tight it's almost like watching a still photograph.
As far as the coherence length is concerned, I measured (using a Science
and Mechanics PhotoMeter placed in the fringes) out to 14 feet without any
change. As you may know, this amount of coherence would require a rather
expensive etalon on any lab laser. Up until this point, we were only capable
of recording a few inches using diode lasers.
This diode created two very bright test holograms that exhibited depth all
the way back with the object(s) (1. ocean coral, 2. angel statue with
wings). For a special twist, I used an initial set-up for a 30 x 40 cm
hologram and then just shot two 4 x 5s with the set-up as-is. Even though
the size of the holograms are 4 x 5, they will give you an indication of
what a 30 x 40 cm hologram would turn out like -- since your beam spread,
exposure, etc. are calibrated for that size.
For a complete report, along with photos of the module, the holograms, the
visible beam in my lab and a interesting size comparison to a Spectra-Physics
124B HeNe laser go to the
Our Own 25 mW Laser
Page. (There are also other reports preceeding this one which may be
accessed at the Holoworld site.)
D and S Lasers is a spinoff
of Holoworld offering plans, a kit, as well as an assembled 25+ mW diode laser
system with long coherence length suitable for holography.
Such a display is simple in principle:
That is still the case.
However, for stationary images (e.g., medical visualization where one wants to
view anatomy from various angles with proper perspective, etc.), the speed may
not matter as much as long as writing doesn't take more than a few seconds.
So let's see.... For a 10 cm x 10 cm SLM, resolution order of a wavelength
of visible light, that's only about 50 billion pixels. Not your ordinary
CRT electron gun - more like a scanning electron microscope. A few 10s of Giga
bytes per second (for a 1 second refresh rate) is the same order of magnitude
as the internal memory busses on some of the latest microprocessors, so no big
deal. :) Of course, then multiply that annoying frame rate thing. ;-)
A search of a patent database at using keywords like "Three Dimensional
Display" and "Holographic" should turn up a variety of interesting, though
probably for the most part unrealistic (as yet) approaches to this problem.
(From: Steve Roberts (osteven@akrobiz.com).)
The problem is twofold, resolution and bandwidth. Resolution, because a
hologram needs far more sensors per mm then available CCDs can provide, and
bandwidth because only a dedicated direct array of fiber optic lines could
handle the bandwidth. Your not going to see the scene shot with actual lasers.
A computer and two or more cameras will be used, to synthesize the data.
Experimental small scale displays have been made at low resolution, but the
Cray computer they used to do the calculations is not something I'd have room
for in my living room. Laser beams loose coherence after a short distance, so
the guys at Monday Night Football aren't going to go blind, as lasers will not
be used to gather the images. Maybe at the end of my lifetime in 30 years, but
not any time soon.
(From: James Hunter Heinlen (dracus@primenet.com).)
There have been a few made. Right now, the only applications that can afford
such tech is very high end medical, and government, mostly military, but I
believe the DoE has one in their nuclear power simulation program. At any
rate, they are fiendishly expensive, and the one I saw (when I was still
doing consulting in the explosives industry) used a couple of Cray YM/P-2E's
(when they were new) as signal processors, plus other computers to do the
modeling, run the simulation, and produce a real time data stream to use as a
signal to be processed. It was considered the low end of the tech, and
produced a dim (but beautifully detailed) 3D moving image of whatever you
wanted in real time. They were using it to display the progression of a
shock-wave through multiple layers of (non-ideal, realistic) rock in fine
detail. We had to turn off the lights to see the display. The 'monitor'
looked like a plexiglass fish tank. If you want more info, there was a
couple of good articles about the displays in Government Computer News when
they first started making this type of system.
(From: Ted (email address N/A).)
There have been a few attempts to display true interference pattern
holograms created by lasers on very high resolution LCD displays. I was at a
digital imaging conference and they had one there. The screen itself, I
think, had about 50,000 x 50,000 pixels. The actual holograms were scanned
by a drum scanner at 90K x 90K pixels each and displayed at 1:10 (or
something like that) on the screen, which was about 17"! The hologram was
very bright, more brilliant than most I've seen on film. The spokesman said
each hologram file took well over 100 MB.
Note our eye process signals at about 27 fps, so about 30 fps is needed. At
30 fps, a one-second holographic animation of such would be 3 GB! An hour
would be 180 GB+. Clearly, even true hologram motion, is still a long way.
Artificial interference holograms created by computers would require even
more storage and processing power. But, at the rate things are going in the
computer industry, it is highly feasible in 10-20 years this could become a
reality.
(From: Andre de Guerin (mandoline@gtonline.net).)
There is a new type of liquid crystal display that generates a hologram
directly by producing the interference patterns on the surface of the LCD then
illuminating it with visible light.
The display this produces is a moving 3-D hologram in real time.
One slight problem... The LCD density is something ridiculous like
3,800 x 3,800 pixels with a pixel size of 10 um x 10 um. There would be
major problems with mass producing this sort of display, given that standard
1280 x 1024 laptop screens 1/10th the size have problems with dead pixels.
(From: Sam.)
Actually, there are bit more than one slight problem, not the least of which
is that the resolution cited is at best marginal and feeding it with data must
be a real treat, bandwidth and processing-wise! :) However, dead pixels,
at least, would not be a major problem, just adding a bit to the background
noise since localized defects in a hologram do not appear localized in the
3-D reconstruction.
The Laser Reflector Web site provides archives of past discussions indexed by
date (year and month) and a large set of links to other laser and laser
communications sites.
Offers of inexpensive lasers, laser components, and other related items also
appear from time-to-time via this email discussion group.
Anyone with an interest in laser communications is welcome to join. You don't
need to be a ham radio operator. Just send email to majordomo@qth.net with
'subscribe laser' (without quotes) in the message body.
See the section: Laser (Email) Listservers
for more information about these private email discussion groups.
See the section: Amateur Laser Communications
Sites for additional Web sites related to this endeavor.
Bell Labs may have actually developed and produced some number of portable
demonstrators to promote the idea of optical communications. The typical unit
appears to have consisted of a HeNe laser tube, power supply, and modulator,
along with a separate receiver based on a solar cell, all packed in a handy
traveling salesman's type sample case. :) I say "may have" and "appears"
because I can't quite tell from the limited information and photos I have if
it actually had a working laser or just a cool-looking neon sign-type tube
for show - and actually did the communications with a separate conventional
modulated lamp (an arc lamp is mentioned in the description I have and its
presence doesn't make much sense otherwise). In any case, laser or not, this
unit was used in community relations and school programs to show how telephone
signals could travel over an optical beam. Some photos of one of these units
rescued from the dumpster can be found in the
Laser Equipment Gallery (Version 1.76
or higher) under "Assorted Helium-Neon Lasers" (giving it the benefit of the
doubt in actually containing a laser!).
(From: George Werner (glwerner@sprynet.com).)
Back in the middle 60's our group at Oak Ridge National Laboratory had
built a HeNe laser for the purpose of demonstrating to interested groups.
One time when I had brought it home in preparation to taking it "on the
road" I decided to test its long distance transmission. For distant
transmission we used a beam expander which was half of an 8x binocular with
a 30 mm objective. We also had built into our power supply a jack into
which we could plug in an audio modulation. I set up the laser on the
kitchen table near a window with a little pocket radio supplying a signal
to the modulator from the local radio station. With a mirror I directed
the beam out the window and across the valley to the parking lot I could
see where the city maintenance department has a number of vehicles parked.
It was about a mile away. Looking with another telescope I could see that
my beam was getting there when it retro-reflected from a car's tail light.
Then, taking with me a Fresnel lens and an audio amplifier attached to a
solar cell, I drove over there to see what it looked like up close. This
was at about 5:30 in the afternoon, still bright daylight, so the red spot
was not obvious, but I soon found it. About that time the night watchman,
as he should, came to see what it was about. I explained that I was
checking on this light that I was beaming down from halfway up the hill
across the Turnpike. He looked in that direction but didn't see anything.
Where he was standing, the beam was landing between his belt and his
shoulders. "You'll have to scootch down a little bit to see it," I said. He
found this hard to believe but he tried it and there was no mistaking there
was a light. I would compare it to the brightness of a locomotive
headlight about a half mile down the track at night (except that it was
red).
Then I put my 18 inch f/1 Fresnel lens in the beam and put the solar cell
at the focus (now bright enough to see the reflected light) and the radio
station came through loud and clear. With a Polaroid camera I photographed
the light coming from my house. Shot from that distance, all the houses
are very tiny, but magnification shows a white blob where my house should be.
P.S. I did not get arrested for trespassing. :)
(From: Sam.)
Although George was definitely not an amateur in the laser field of the day,
this could very well have been the earliest (or at least one of the earliest)
examples of amateur laser communications since it I bet it wasn't part of his
job description!
(From: Louis Boyd (boyd@apt0.sao.arizona.edu).)
In my experience a 5mw red laser does not do the job unless there's a
lot of dust or water droplets in the air. The problem is the dark
adapted human eye is very insensitive to red. Also backscatter from
small particles is reduced as wavelength increases. I can't give a
specific power level because it's so dependent on the particles
suspended in the air. Under the right conditions a 3mw green pointer
would be easily visible for a few people standing together but probably
won't be adequate in very clean air. Blinking the laser can make it
easier to detect and reduce power consumption. You also didn't state
the size of the group. The distance of the observer from the emitter
makes a difference.
The "vanishing point" for off axis viewer isn't at infinity and is
dependent on the power level and the hight of suspended particles. The
effect is that what you are pointing at may not be exactly where other's
perceive the end of the "beam" to be. You may actually be better off
with a larger beam diameter using a modified flashlight with a halogen
bulb.
One of the more powerful "MagLight" or "Surefire" flashlights with a an
extension of a couple of feet of ABS plastic with internal baffle rings
to prevent side scatter does a good job. This can put out around a watt
of light and it's a lot cheaper than an adequately powerful laser. If
this is for a large group get one of the "million candlepower" lamps and
make the baffle out of a "honeycomb" of tubes with black flocking blown
into them. Those have over 10 watts of light output. If you need to do
this for a large crowd like a stadium use a xenon short arc lamp
spotlight with hundreds of watts of output.
"JENOPTIK Laser, Optik, Systeme GmbH has developed the first industrial
all-solid-state Red-Green-Blue laser system for large image projection
systems. Compact in design (0.75 m 3 , 180 kg, 3 kW power
consumption), the system consists of a modelocked oscillator amplifier
subsystem with 7 ps pulse duration and 85 MHz pulse repetition
frequency, an optical parametric oscillator (OPO), and several
non-linear stages to generate radiation at 628 nm, 532 nm and 446 nm
with an average output power above 18 W. Each of the three colors is
modulated with the video signal in a contrast ratio of 1000:1 and
coupled into a common low order multi mode fiber. The system
architecture relies on efficiently manufacturable components. With the
help of FEM analysis, new engineering design principles and subsequent
climatic and mechanical tests, a length stability below 50 um and an
angle stability below 10 uR have been achieved. The design includes
efficient laser diodes with integrated thermo-electric cooler and a
life time above 10,000 hours. The stability of the output power is
better than +/- 2% in a temperature range from 5°C to 40°C. The
system operates reliably for more than 10,000 hours under field
conditions. The design is based (among others) on work by
Laser-Display-Technologie KG and the University of Kaiserslautern."
Inexpensive Combining of Argon Ion and HeNe Laser
Beams
Also see the section: Combining Light from
Multiple Lasers.
Thus, the final "white light" beam is made up resultant actions of three
dichros and three intensity controllers. If you have some type of analog
controller for each R/G/B color, you can blend them produce an incredible
amount of colors.
You will need some lots of custom dichros to combine the beams and numerous
beam leveling mirrors to achieve it. Lots of dichros and lots of mirrors
translates into "lots of losses" and a bitch to establish and maintain
collimation. Three dichro color systems are still lots of work. In this
case, you would have a FIVE-color dichro system.
Dichroic Mirrors for Separating Multiline Beams
Dichroic mirrors can be used to split a multiline laser beam into two or more
sets of separate lines. They enable the construction of simpler, smaller, and
more efficient systems compared to dispersive techniques like prisms or
gratings. But good quality dichros are not cheap.
For pricing, you're looking at $20 to $50 a square inch, depending on quality,
and whether a precut size is available. Some may charge a cutting fee or a
little more for the AR coated units. Keep in mind you need to know if you
want CMY or RGB and 0 or 45 degree incidence, as most folks will stock the
whole set of combinations. Be clear - specify that you want "transmit
blue reflect green at normal incidence" Or "pass blue/green combine red at 45
degrees". Most people don't think about it, but "pass deep blue and violet"
for a argon laser turns out to be a nice dichro to have.
Visibility of High Power Laser Beams
The following applies to the visibility of the beam itself (i.e., Star Wars
Light Saber style), not to its appearance then it strikes a surface.
Limitations of Lasers for Large Scale Shows
(From: Dean Glassburn (nitelite@concentric.net).)
Use of Pulsed Laser for Laser Shows?
(From: Steve Roberts (osteven@akrobiz.com).)
Holographic Laser Show Images?
Being able to project a 3-D image hundreds of feet into open space is pure
science fiction - there is no current technology and even basic theory that
would make this possible without some medium to act as a screen. However,
some pretty vivid illusions that may give the impression of such a display
do exist and you may experience one at your next large scale laser show:
Laser Show on a Shoe String
A low cost way of getting into laser shows is described at
LaserFX.com's Low
Budget Laser Graphics System which includes information on suggested
lasers, galvos, modifications to a sound card to pass DC, and the computer
system and software.
Galvo Type Deflectors for Laser Light Shows
May I suggest what I suggest to all beginners in Laser Shows?
Acceptable galvos for beginners:
Don't bother with galvos like CECs - they are designed for exposing beams in
small chart recorders using a ultraviolet arc source, they are referred to as
"pen" galvos, and thats what they are, about the size and shape of a ink pin,
with a small mirror about .5 mm across. They are thus too small to make a XY
mirror pair, especially since the external magnet needed is huge.
Laser Based Systems for 2-D and 3-D Display
Whatever Happened to Laser TV?
I am sure everyone has heard of the predictions that there would be mural (or
stadium) sized TV screens using lasers instead of the other silly technologies
like LCDs and light valves. This was 10, 20 years ago. Where are they? The
idea is simple: Replace the three electron guns in the color CRT with red,
green, and blue lasers and raster scan a TV picture onto your favorite screen,
barn, or mountain-side. :-)
Laser Based 3-D Displays
There have been a number of volumetric (not true holographic) displays
developed over the years using rotating mirrors, disks, LED arrays, disks
inside cathode ray tubes, etc. These are all scanned in such a way as to
cover a true volume of space at a rapid enough rate (at least that is the
objective) to produce the illusion of a solid 3-D volume floating in space.
The scanning source can be a laser, electron beam, or the projected output
of another 2-D display like a CRT or LCD panel.
3-D Laser Engraving Inside a Glass Block
Examples of art pieces made under computer control of a pulsed laser focused
inside a glass block can be found at
3D Laser Art Co.. They
have a basic explanation of the process but no specifics and no mention of
the type of laser that is used.
Introduction to Holography
What is Holography?
Holography represents a class of techniques which capture 3-D information about
a scene as an interference pattern on or in an extremely high
resolution 2-D film. When the film is developed and viewed under the right
conditions (some require a laser for viewing while others can use a suitable
white light source), the result is a recreation in every detail of the original
including the ability to move your viewpoint and look around objects, proper
hidden surface removal (solid objects appear solid), shadows and highlights,
and so forth. In principle, the hologram is optically indistinguishable from
the original. A normal photo of a hologram would look the same as a photo of
the scene itself.
(Portions from: Rick Poulin (rpoulin@rohcg.on.ca).)
Description of Holography Technique
While there are significant differences in the details of the process needed
to produce those little logos compared to large white light holograms used for
marketing or 3-D volume images for medical diagnosis, the basic techniques are
similar and can be summarized very briefly. The following is the sort of
holography setup that is within the capabilities of a determined amateur:
Basic Amateur Holography Setup
See the section: Holographic Information
Resources for alternatives - this is just one option.
(From: Brian Hogan (bhogan@bjgate.com)).
I haven't made holograms for a long time, but I started from the ground up. If
you've got $3K to play with, you can really start off very well. But if you
want to save money, you can build a complete setup for less than $1,000. It
may be far more advanced than what you may have intended, but you'll be able
to create pretty professional holograms.
Good luck and have fun.
Complete Holography Kits for Education
Several companies provide all the equipment and materials needed to get
started in holography. One example can be found at the
Arbor Scientific Holography
Page. Their prices may not be the best on individual pieces but the
convenience of one-stop shopping may outweigh the additional cost (except
probably for the laser especially if you opt to use a cheap laser pointer
for this!). Also check the various companies listed in the section:
New, Surplus, Walk-In, Mail Order, Kits/Plans
(Commercial).
Holography Using Cheap Diode Lasers
If you ask most laser 'experts' about the possibility of using a laser pointer
or inexpensive diode laser module for making holograms, the typical response
will be to forget it - the coherence length is only a few mm and therefore
inadequate. This apparently isn't the case. The coherence length for a
typical laser pointer or diode laser module may actually be more like 200 mm
(10 inches) - comparable to that of an HeNe laser and, with care, will remain
stable for long enough to make an exposure. While it may be unreasonable to
expect any old $8.95 laser pointer to produce the same quality results as
a $500 HeNe laser, surprisingly good holograms can be obtained on a budget.
And, it would appear, that in some cases, they can actually be superior.
Also see the section: Holographic Information
Resources.
Holographic Video Displays
To create a useful holographic display of a moving scene requires an almost
unbelievably large amount of data processing and throughput. Suppose you just
wanted to produce a holomovie of a 50 x 50 x 50 cm volume using a 50 x 50 cm
display device. Given that your typical holographic film must have a
resolution on the order of a wavelength of the light used to create/reconstruct
the hologram - 1000 line pairs/mm or better - this would mean that some sort
of spatial light modulator (e.g., LCD) would be needed with a similar
resolution to reproduce moving images. This means over 1.25x1012 or
1.25 Terapixels! An you thought SVGA resolution laptop screens were expensive!
To make things easier, we'll assume 1 bit per pixel for the interference
pattern, resulting in 100 Gbytes per frame! To provide smooth motion, one
needs a minimum of 24 to 30 fps so you are looking at 2.4 Terabytes/second.
Now, granted, various compression techniques (e.g., MPEG-26 by then) can be
used to reduce this by perhaps a factor of 10 to 100 or more (and no doubt
such processing will be much more advanced once this sort of folly
becomes at all practical) but that is still 24 Gbytes/second through the
communications channel. Hmmm, that doesn't look quite as impossible! This
doesn't take into account the need for color but at least the laser(s) will
probably be the least of your problems in bringing such technology to market!
I was actually discussing stuff like this (in a former life) in the early 1980s
realizing that either a dedicated special purpose computer or something as yet
non-existent would be needed to achieve any sort of througput.
Holographic Information Resources
See the chapter: Laser Information Resources,
specifically:
There is a weekly holography show on-line at
Holotalk which
has feature stories and special guests by hosted by
The Internet Webseum of Holography.
You may need special speech/video plugins for you browser to take advantage
of this Web page.
Laser Communications
Basic Description
The term 'laser communication' can mean many things but generally refers to
the transmission of information via a laser beam in free-space or a fiber-optic
cable. A laser communications system must then consist of:
Amateur Laser Communications
For more information and discussions on amateur laser communications, join the
Laser Reflector. It is
run by ham radio operators who do long distance free-space communications. One
is working on laser EME (Earth-Moon-Earth), and another is into
non-line-of-site weak signal operation using low baud rate long term
integration and advanced DSP techniques with coherent signals!
Early Laser Communications Experiment
Not surprising, the potential of optical communications was recognized by
researchers even long before the laser was invented. The following is just an
example of how easy it is to turn a laser that can be modulated and solar cell
into a line-of-site comm link. This was just an ad-hoc experiment but
>Miscellaneous
Use of Laser to Identify Stars in the Sky to a
Group
Of course you can't reach the stars but there may be enough scatter in the
air to show the direction. :)
I may be contacted via the
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Email Links Page.