Source: http://laserfaq.ru/sam/laserdps.htm
Timestamp: 2019-04-24 20:49:52+00:00

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Sam's Laser FAQ, Copyright © 1994-2009, Samuel M. Goldwasser, All Rights Reserved.
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Blinking Laser Pointer or Diode Laser Module?
Back to Laser Diode Power Supplies Sub-Table of Contents.
Absolute current limiting. This includes immunity to power line transients as well as those that may occur during power-on and power-off cycling. The parameters of many electronic components like ICs are rarely specified during periods of changing input power. Special laser diode drive chips are available which meet these requirements but a common op-amp may not be suitable without extreme care in circuit design - if at all.
Current regulation. Efficiency and optical power output of a laser diode goes up with decreasing temperature. This means that without optical feedback, a laser diode switched on and adjusted at room temperature will have reduced output once it warms up. Conversely, if the current is set up after the laser diode has warmed up, it will likely blow out the next time it is switched on at room temperature if there is no optical feedback based regulation.
Note that the damage from improper drive is not only due to thermal effects (though overheating is also possible) but due to exceeding the maximum optical power density (E/M field gradients?) at one of the end facets (mirrors) - and thus the nearly instantaneous nature of the risk.
The optical output of a laser diode also declines as it heats up. This is reversible as long as no actual thermal damage has taken place. However, facet damage due to exceeding the optical output specifications is permanent. The result may be an expensive LED or (possibly greatly) reduced laser emission.
Another one was blown by assuming that a particular driver circuit would work over a range of input voltages when in fact it was supposed to be powered from a regulated source. At first the degradation in brightness appeared to be reversible. However, what was probably happening was that damage to the laser diode was occurring as soon as the brightness appeared to level off. The natural tendency was then to back off and approach this same point again. Not quite as bright? Crank up the current. Finally, once it is much too late, the realization sets in that it will *never* be quite as bright as it was originally - ever again. This one still lases but at about 1/10th of its former brightness.
If you then try to power this damaged laser diode with a driver circuit using optical feedback, further instantaneous damage will occur as the driver attempts to maintain the normal optical output - which is now impossible to achieve and only succeeds in totally frying the device as it increases the current in a futile attempt to compensate.
And a comment about the expensive Nichia violet laser diodes (see the section: Availability of Green, Blue, and Violet Laser Diodes). Physically, they look like ordinary laser diodes and except for a higher voltage drop, the driving characteristics are basically similar. However, I've heard that they are even more sensitive to EVERYTHING than their visible and IR cousins and will degrade or die more easily. Since the wavelength of these diodes (in the 400 to 420 nm range) is basically useless for applications requiring visibility, aside from the "being the first kid on your block" factor, I'd stay away from them until the price comes down dramatically! I suspect that the newest 430 to 445 nm Nichia diodes are equally tempermental.
Also see the section: How Sensitive are Laser Diodes, Really?.
Back to Diode Laser Power Supplies Sub-Table of Contents.
Where what you really want is a working visible diode laser, a commercial laser pointer or diode laser module may be the best option. Both of these include the driver circuit and will run off of unregulated low voltage DC. While the cost may be somewhat higher than that of a bare laser diode, the much reduced risk of blowout and built-in optics may be well worth the added cost. It doesn't take too many fried laser diodes to make up this cost difference!
Believe me, it can get to be really frustrating very quickly blowing expensive laser diodes especially if you don't really know why they failed. This will be particularly true where the specifications of the laser diode and/or driver circuit are not entirely known - as is often the case. Helium-neon lasers are much more forgiving!
Buy one that accepts an unregulated input voltage. Otherwise, you can still have problems even if you run the device from a regulated power supply. All laser pointers and most (but not all) modules will be of this type. However, if you get a deal that is too good to be true, corners may have been cut. A proper drive circuit will be more than a resistor and a couple of capacitors!
To confirm that the driver is regulating, start with an input near the bottom of the claimed voltage range and increase it slowly. The brightness of your laser diode should be rock solid. If it continues to increase even within the supposedly acceptable range of input voltage, something is wrong with either the laser diode (it is incompatible with the driver or damaged) or driver (it actually requires a regulated input or is incorrectly set up for the laser diode you are using). Stop right here and rectify the situation before you blow (yet another) laser diode!
See the chapter: Laser and Parts Sources for a number of suppliers of both diode laser pointers and diode laser modules.
If you still aren't convinced that someone else should deal with laser diode drive design issues, the remainder of this chapter provides suggestions for integrated drive chips, sample circuits, and complete power supply schematics. But don't complain that you haven't been warned of the sensitive nature of laser diodes.
Series resistor: There is no active regulator. A resistor limits current to a safe value with a fresh set of batteries. The laser diode is driven like an LED. As the batteries are drained, current decreases proportional to the difference between the battery voltage and the diode drop (about 2 V) divided by the resistances. Since output power and thus brightness would also decline dramatically with battery use, this approach is only found in the cheapest of laser pointers. See the section: Laser Pointer with a Resistor for a Regulator.
Constant current: Laser diode current is set to a safe value between threshold and maximum. This takes care of battery voltage variations but still would have problems with changes in the laser diode output with temperature. This is rarely, if ever, found on red laser pointers but is used for green laser pointers since the high power pump diodes for the DPSS laser module do not have or need optical feedback for adequate regulation.
Optical feedback - unregulated reference: Some laser diode drivers use the monitor photodiode to control laser diode current but do not have constant voltage source like a zener diode circuit to use as a reference. This is fairly safe for the laser diode as long as the correct battery types are used. For these, output brightness will vary somewhat with battery voltage and will thus decline as the batteries are drained.
Optical feedback - regulated reference: The best designs (and all those using IC driver chips) will maintain nearly constant output power until the batteries are nearly exhausted.
I'd expect to only see (3) and (4) in modern red laser pointers with (4) predominating in more modern designs. Expect (2) in green DPSS laser pointers (but many or most of these will also be pulsed).
Visible laser diodes generally have very precise drive requirements. Too little current and they don't lase; too much current and they quickly turn into poor imitations of LEDs or die entirely. At least that's true of most of them. In order for a simple resistor to set the current precisely enough, it would have to be selected for each laser diode to limit the current to a safe value with fresh batteries over the expected temperature range. With only 5 to 10 percent between lasing threshold and maximum current for a typical visible laser diode, this could be impossible. Until recently, I had heard that this type of design (or lack thereof) has been used but had never seen such a simple circuit in a laser pointer. Apparently, visible laser diodes are now mass produced with a much larger range of current between threshold and operating limits - possibly engineered specifically for the ultra-cheap laser pointer market.
Well, I have in my hands a laser pointer that has only a resistor to limit the current instead of the transistorized circuits usually found. It have a 51 ohm SMD type resistor on the PCB in series with the power switch, the laser diode, and 3 LR44 batteries (1.5 V each).
In fact, the laser diode has no monitor photodiode at all - it have only 2 terminals. The metal case is open on the rear, so one can easily see the laser diode itself inside it. Interesting enough is that it is the only type of laser pointer that I can actually now find here (Brazil), but some years ago I bought some pointers having a complete regulator circuit.
He's has sent me a sample, all the way from Brazil! Heck, it arrived faster than some of the stuff I send next door. :) As advertised, it certainly appears not to have anything inside other than a laser diode chip on a heat sink, 51 ohm surface mount resistor, on-off switch, and battery.
I have measured the I-V curve for both the overall circuit and just the laser diode. It is consistent with a 51 ohm series resistor and 20 ohm diode resistance with about a 2 V drop at just above 0 mA (the knee of the diode I-V curve). The threshold is around 15 mA and the operating current is 35 mA at 4.5 V (the normal battery voltage) - a rather wide range for a visible edge emitting diode. My hypothesis is that these laser diodes are specifically designed to have a wide operating range - possibly by reducing the reflectance of the output facet and thus the gain, possibly by varying the doping, or something else. So, efficiency is lower but with the benefit of increased tolerance to power supply current variation (though 35 mA for a few mW of output power is a very respectable value).
Someone else sent me a similar pointer and while I haven't actually measured its I-V curve, I expect that it behaves basically the same. These are both bullet-style pointers of obviously really cheap construction that came with 5 screw-in pattern heads (1 clear and 4 HOEs). Another better quality bullet-style pointer I have uses the normal laser diode in a can package with a regulated driver.
I also bought a couple dozen as-is pointers in a single lot on eBay which are all of this type.
"My laser pointer requires those little button cells which are really expensive and hard to find. I was wondering if I can instead connect 2 wires and make a battery pack for it using 3 AA batteries. Do all pointers have power regulators?"
They all have some sort of regulation but it may not be adequate to deal with much of a change. You would have to check circuit to be sure or use batteries that are exactly the same maximum voltage. Even that isn't totally guaranteed as really dreadful designs could depend on the internal resistance of the batteries to limit current. So, replacing AAA Alkalines with D Alkalines could cause problems with some designs.
To be reasonably safe, you would have to measure the current using a fresh set of the recommended button cells and then add enough series resistance to make sure the current can never exceed this value even with brand new AAs (or whatever you are using).
Note that the much more complex and expensive green laser pointers should have decent regulation but they may still assume that nicely behaved batteries are used. Therefore, if adding an external power source to one of these, it is best to make sure it is well filtered, regulated, and has absolutely no overshoot during power cycling. Also see the next section.
Unlike high quality and expensive diode laser modules, laser pointers may have less than stellar internal regulation. Thus, you could easily destroy them instantly by attaching an external power supply, wall adapter, or even a higher capacity battery of the same voltage as the one used originally. Some pointers may even depend on the internal voltage drop inside the recommended (internal) batteries to provide some of the current regulation!
So, if you really want to run a pointer from an external source, the best thing to do would be to measure the voltage across a fresh set of batteries powering the pointer and build a highly filtered, well regulated power supply to match it. The power supply must have absolutely no overshoot or undershoot when power cycling.
Another not quite as robust alternative is to obtain a wall adapter with an adequate current rating and slightly higher voltage rating than the pointer's battery. Then, add series resistance until the voltage at the pointer is the same as when powered with its internal battery. This is risky, however, since unless the wall adapter is regulated (few are), ripple, line voltage fluctuations, and power surges will get through it - and any of these can fry a laser diode in next to zero time.
Also note that a fancy regulated power adapter may actually be deadly to a laser pointer. Power supplies that include active components (those using switchmode or linear regulators as opposed to simple wall adapters with only a transformer, rectifier, and filter capacitor) may produce sub-microsecond (or longer) overvoltage spikes when power cycled (at power-on or power-off). These will have no effect on most electronic equipment but may be fatal to laser diodes.
As far as connecting the power supply: If you don't mind drilling a hole in the case or end-cap, construct a dummy battery with contacts at each end which you wire to your external power supply. Drill a hole in the side of the case, or better yet in the cap (but off to one side so the cap will still make proper contact with the battery if you decide to use the pointer with a battery in the future) to allow the pair of wires to pass through after the cap is screwed on. There are all sorts of ways of doing this. The connections have to be made to the center spring contact on the circuit board at the bottom of the battery compartment and the case. Make sure you get the polarity correct!
Also see the section: Power Regulators in Laser Pointers.
With the wide availability of inexpensive laser pointers in particular, it would be nice if there were a way to make them do something more exciting than just project a steady red dot.
"Hi is there any way I can make my laser pointer blink at an adjustable rate, something that will turn on/of maybe with the control of a adjustable resistor? Are there any schematics or something to help me out?"
In principle, a simple circuit based on a 555 timer, for example, could be used to control power to the pointer or module - perhaps even just control a relay to act as the on/off button.
In practice, whether this will work or not depends on the design of your laser pointer or diode laser module. Some have significant filtering and delays circuits inside which will make blinking at a useful rate impossible. Others will work fine. Still others will fail due to the repeated stress of on/off cycles.
Going any deeper into the circuitry than the batteries/power supply or on/off switch is definitely not for the beginner - if possible at all. Unfortunately, however, that may be necessary to achieve a useful result. For more info, see the sections of this chapter on laser diode power requirements, modulation, and the sample laser diode driver schematics.
Yes! I've used a simple 555 timer circuit driving an emitter follower transistor buffer amp, to drive several laser pointers. I've had little trouble recovering a near square wave at the receiving end with a phototransistor driven amplifier, up to about 5 kHz. After that, the residual energy stored in the laser module's driver circuit starts to degrade the square wave, but this can usually be extended, at least through the remainder of the audio range, by using a push-pull or complementary-symmetry type buffer, instead of a simple emitter follower. If you need to go beyond 4 kHz though, it is better to attempt to modulate the intensity rather then try to accomplish complete shut down/turn on.
Many semiconductor manufacturers offer laser driver chips. Some of these support high bit rate modulation in addition to providing the constant current optically stabilized power supply. Other types of chips including linear and switching regulators can be easily adapted to laser diode applications in many cases. However, some of these chips are designed in such a way that they will work only at the high bit rates advertised maintaining a continuous carrier at all times or with a 50 percent average duty cycle or something equally annoying if all you want is a CW laser diode power supply or even one for low bit rate communications. You need to check the specs very carefully for non-standard (e.g., not covered in the datasheet or app-note) applications.
Note: Free samples of ICs like laser diode drivers may be available for the asking even if you won't be buying a million parts in the future. Manufacturers often provide some means of requesting free samples at their web sites. Just be honest about your needs - they consider it good PR and you might just tell a friend or colleague who WILL buy a million parts!
Analog Devices (http://www.analog.com/) has several laser diode drivers including the AD9660 which provides for full current control using the photodiode for feedback and permits high speed modulation between two power levels.
We are using the OPA 2662 (Burr-Brown) for this. It is an OTA with 370MHz BW, 59 mA/ns SR, and can source/sink 75mA of current per channel (two channels per chip which may be paralleled quite easily). The part provides the emitter of the current source to an external pin (programming side of an internal current mirror), so that a single resistor sets the voltage-current transfer characteristic. Watch out for the dependence of the harmonic distortion specs upon the supplied current and frequency though...if this will be a problem for your particular application that is (didn't matter much for mine).
Elantec (http://www.elantec.com) offerings include the EL6251C and EL6258C which provide laser diode driver and sense circuitry. They support high speed control of laser diode current with selectable levels for read and write, optical feedback regulation, and protection from low power supply or open input conditions. These parts are intended for CD, CD-R, CD-RW, and other optical data storage applications.
Another chip, the EL6270C, features an integrated high frequency modulator (HFM) oscillator to provide output current drive of up to 100 mA, an external resistor that controls the average laser diode output power, and a low power disable mode that powers down to 5 uA.
Complete datasheets are available at the Elantec Web site.
Check out the datasheets for several laser driver circuits available on the market for high speed fiber communications. See Maxim, HP, Sony, Philips, Fujitsu, Microcosm, etc. Also, there are many papers in Bell System Technical Journals that deal with other bias control schemes that don't involve optical feedback.
iC-Haus Corporation (http://www.ichauscorp.com/) offers several CW laser diode driver and controller chips. The complete datasheets are available on-line and include functional block diagrams and application information. These devices require only a few common external components and can be used for CW and modulation/pulsed operation up to several hundred kHz (depending on model). iC-Haus parts are available through electronics distributors.
Laser Diode Power Supply 3 (RE-LD3) uses a similar chip - the LT1054 DC-DC Converter, not for voltage stepup but to very effectively isolate the laser diode from input voltage spikes.
The MAX3261 (1.2 Gbps), MAX3667 and MAX3766 (622 Mbps), and MAX3263 (155 Mbps) are examples of their highly integrated laser driver chips.
The Maxim Engineering Journal (a monthly or so publication you will receive if you have requested their CDROM and possibly included in trade rags like EDN and Electronic Design) sometimes has laser diode related articles. For example, the Special Fiber Optic Edition (early 1999) is devoted to applications of Maxim's high speed (622 Mbps and up!) optical interface components including laser diode drivers and sensors. (The Maxim application note Driving a Laser diode at 622 Mbps From a Single +3.3V Power Supply may be one of those from this publication.) The next issue I received, Volume 33, included a circuit similar to the one described in Digitally Controlled Laser Diode Driver.
Both Sharp and Mitsubishi manufacture IC's for driving laser diodes. Most will maintain constant power. Some require two voltages, others just one. These circuits will drive the common cathode lasers, or the Sharp "P" or the Mitsubishi "R" configuration which has the laser's cathode connected the the anode of the photo diode. The Sharp IR3C07 is a good for CW or analog modulation, and the IR3C08 or IR3C09 will allow digital modulation to 10 MHz. These parts are quite inexpensive.
The bottom line is that these should be fine for CW laser lights and laser pointer type applications but NOT for modulation as may be claimed by the distributors of these modules.
I called a person I know who works for a major surplus house. He asked NOT to be identified. He did give me valuable information regarding the NS102 laser driver modules that are being sold for $3 each (in large quantities) on the internet.
Here's what I was told.
The NS102 is mass produced in Asia. The chip that the NS102 PCB is based on is unknown, and probably made in Taiwan too. There are no specs for it. Only DC parameters are given on the 'rough spec' sheet (advertising quality literature) the sellers give you.
They do work and they work well.
They use low power and they are stable-if the voltage in changes from 4v to 8v, the LD output remains fairly constant.
However, they are NOT suitable for modulation of laser diodes and should only be used as a laser pen power supply!
I have an email from a vendor here which sparked all this speculation regarding their suitability for our purposes. The email CLAIMS they can be driven to 12 Mhz output pulses while maintaining FULL APC (not average output monitoring as they do in fiber optic drivers). As far as I can tell, this is just plain a lie and no one should purchase these expecting to modulate a laser diode for communications purposes.
They are probably little more than the standard 2 transistor laser driver that can be used for a laser pointer because it is heavily bypassed with a heavy duty slow start ramp up circuit.
Some vendors are now selling these for $20 in small quantities - don't get taken in - it's a laser pointer driver and NOTHING MORE.
If anyone has better info or has tested one of these on the bench, please let me know. I'd really like to get info on the chip contained on the PCB too.
The following refers to chips available from NVG, Inc. and other resellers of their products. See the section: Mail Order - Lasers, Laser Parts, Optics, Accessories for more info on NVG.
The NVG laser driver circuit was originally designed for CW only. While I did not design the driver circuit, I was able to find a way to get it to modulate successfully up to 2 MHz. I have successfully built a free-space FM modulated data/voice transmission system using the NVG laser modules (diode, driver, collimator, enclosure) already set and burned in).
In addition I have helped a number of customers from around the world (Spain, Italy, Switzerland and the US) use the NVG modules in a modulated design.
While the NS102 type driver circuit does have a 0.1 uF capacitor to act as a 'soft on'/filter protection of the laser diode, by providing enough voltage to keep the module/laser just below the threshold, you can modulate the NVG modules (or any suitable diode attached to the NS102 driver) up to 2 MHz. At that point, it seems that the capacitor effectively filters the modulation and the circuit 'saturates' and only produces CW output.
As far as modulation is concerned, the Analog Devices driver is hard to beat for three bucks. Couple that with a 555 and a battle proven LM317 front end and cry 'BINGO'. Maxim used PECL inputs ... arrgh! I don't need to spit photon packets at 150 mhz! Linear Tech IR receiver looks good, although the $7.00 price tag + a handful of linear doesn't really appeal to me. Too bad you can't get inside the Epoxy covered die in the Sharp TV/VCR consumer IR receiver modules (apx $1.50/100 pcs). Not everyone in the world wants to decode bursts of 40 kHz back into data!
Oh, by the way - an Optek BP812 Optologic sensor performs quite well at at 760 nm. It's an active device available in either totem pole or open collector outputs. The applications guy at Optek says the device won't work at 760 but looking at the response curve, I disagree. It's response is only down about 10% in the reds! Most silicon photo stuff is down about 60-75% at 760ish nm. From what I have seen, the device is very usable at 760 nm. Useful part for red diodes and HeNe stuff.
Just because it isn't hot doesn't mean you didn't already fry it.
Unlike most other things, running them at the "typical" data sheet values won't work. I'm not talking suboptimal here; I mean that it won't work, not even a little bit.
You must never, never, never exceed the full rated *optical* power output of the laser, not even for a fraction of a microsecond. If you do, your laser will be degraded or dead. This means LOTS of careful design to avoid nasty switch-on and switch-off transients, for example.
You can provide an adjustment anyway.
because your feedback circuit rings (or worse. oscillates) so that the drive current occasionally exceeds the maximum.
because of PSU on/off transients.
because you have used a socket for the laser, and the photodiode connection is flaky: if it comes disconnected, your feedback circuit will think there isn't enough drive to the laser and will crank up the current to destruction level.
because you are trying to modulate the laser brightness with some AC signal and either you overdo it, or the feedback circuit overshoots.
because you have a pot. somewhere in the circuit to adjust for full output, and its wiper is noisy.
Above all, remember that it is excessive light output that destroys lasers. The heating effect of the drive current is not a big problem except that it has the effect of pushing the threshold current down. Excessive light levels, on the other hand, can damage the tiny end mirrors of the lasing crystal.
Sharp (one of the big suppliers of laser diodes) also make some nifty 8-pin drive chips that are pretty good if you don't need to modulate the laser rapidly. For modulation, consider setting the light output close to 50% of full output using a really slooooowww-responding feedback circuit, and then impressing a fixed-amplitude modulating current on the laser. This is OK because the gradient of the light/current graph is reasonably predictable for any given laser type, so it's possible to calculate a suitable safe modulating current from the data sheet.
Good luck to all - and don't forget the eye safety regulations.
Laser diode structures are usually so small that damage thresholds are very low on every dimension. The general approach to protecting them is to series AND shunt filter (and/or clamp) supply voltages to limit the voltage compliance of current source driving circuits. Also, consider having some of the current limiting be by means of an actual resistor rather than just active circuitry. The parasitic capacitances in active driving circuitry can interact with dv/dt on supply lines to turn on the drive circuit (e.g., drain to gate capacitance with MOSFET drive), so the resistor limits current even when this happens. Using bypass capacitance local to the pulse current loop has the dual benefit of absorbing residual transients and avoiding any effects of upstream series filter components on speed.
(From: Mark W. Lund (lundm@physc2.edu).
You can blow out the laser in nanoseconds if there is enough voltage and/or power in the pulse. Two methods: electrostatic discharge type damage which punches holes in the cavity; brief high power which damages the front facet.
Make sure that the power supply to the modulation circuit is filtered to prevent surges, isolate the signal circuit to prevent surges on the input line from getting to the laser.
Well, that was embarrassing, but I hope it encourages others to save a few (laser diode) lives.
Semiconductor lasers are very sensitive to power spikes. The level of current that is a problem depends on the laser structure and how much of the current is converted into optical power vs. heat. In general, reverse current spikes are very damaging, no matter what level. Make sure that you are modulating the diode so that you go below laser threshold but not below 0V. In the forward direction, very short overshoots (<1microseconds) in current can be handled until you blow the facet off of the device (catastrophic optical damage - COD). Longer pulse overshoots aren't any better. The current level that damage occurs varies from device to device. I tend to recommend less than 10% overshoot in all cases. COD is very easy to note, just look at the laser (while it is not operating) under a microscope. The facet coating is damaged near the emission region, if there is a coating. Otherwise, you will see an enhanced region (darker area) when looking under Nomarski - maybe not so easy to see.
Another problem that you might be having is spiking during start-up or shut-down of the device. Current supplies that look lovely during operation sometimes have spikes in the output when you turn them on or off. You might want to short the device, making sure that there is no bounce during the shorting, before turning your supply on or off. There are several laser diode driver companies out there that make current generators with slow starts and minimal overshoots. Avtech, Melles Griot, ILX Lightwave, WAvelength Electronics, etc.
It would be nice if the monitor photodiodes associated with all laser diodes had the same sensitivity - or even were consistent for a given model. But, unfortunately, this is not the case.
"I am designing a driver circuit for a laser diode (NEC NDL3220S). The problem is that the spec sheet says the output of the monitor photodiode at rated power is max: 0.5 ma, typical: 0.3 ma, min 0.1 ma, at 5 V. This is a huge range! If I set for 0.3 ma and the actual output is 0.1 mA I will burn out the laser. I do not have equipment for calibrating the laser output directly."
Welcome to the wonderful world of laser diodes! You'll find that a 5:1 range in monitor current is typical, with even a full order of magnitude being common! This is one reason why most laser diode based applications have a provision for trimming/tuning the driver circuit to the particular laser.
Your safest bet is to design the feedback loop to operate with less than the minimum monitor current, and provide the ability to actively tune it to the appropriate operating point. Thankfully, the relationship between output power and monitor current will remain reasonably constant over the lifetime of each particular device. So, once it is properly set, you're done.
The laser diode start time is greatly increased if the LD starts from zero rather than an LED-level current flow. Wish I'd seen this two years ago!
I never tried biasing it down to BELOW laser threshold at the 'LED' level. Although this would be an improvement over cutting it off completely, I would think this would be slower than biasing to 1/2 laser power.
Also see the section: Digitally Controlled Laser Diode Driver which has a bit more on the circuit mentioned above.
Altering the amount of light hitting the monitor photodiode inside the laser diode package. This will change the power level setting if the APC (Automatic Power Control) circuit is being used (as it should be in most cases).
Destabilizing the lasing process due to reflected light entering the laser cavity. This effect actually may be more common with low power laser diodes than one would think. See the section: Causes of Laser Pointer Output Power Changing When Directed at a Mirror. However, where the behavior is repeatable and stable, I'd be more inclined to believe it is the simpler explanation, above.
Note that the losses in the optics are usually only a minor factor where the power decreases. Even uncoated surfaces reflect only about 4 percent so if you are getting a 30 percent decrease in power, this probably isn't the cause!
CAUTION: If you remove the optics from a diode laser module, the power may increase resulting in laser diode destruction, especially if the unit is being run near its maximum ratings.
The information sheet for a Power Techologies 35 mW module states in bold capital letters not to even ADJUST the collimation while the diode is running at full power!
Without Collimating Optics: 10.8 mW.
With Collimating Optics: 10.5 mW.
It is interesting to note that the second reading WITHOUT optics was 3.8 mW and the third reading 2.6 mW. The barrel was becoming very hot. I killed the power before I killed the diode (I'm learning!). So this particular diode (from NVG, Inc.) obviously was set up with the collimating optics in place NEEDS the feedback (reflection) for the photodiode to control the current.
There is no law that says the internal monitor photodiode must be used in the driver optical feedback circuit. For some applications, it is desirable to substitute an external one or use both together. This could be used to control beam power based on some mechanical condition like position or angle or to compensate for variations in the behavior of the external optics.
You can't modify a sealed diode laser module in this manner unless it already has a modulation input but if you are building something from components, it should be possible. Loop stability must take into account optical path delays if the distance between the laser diode and photodiode is significant but this shouldn't be a problem unless you are also trying to modulate the thing at a very high rate. Obviously, any such scheme must assure that the external photodiode always intercepts enough of the beam and/or that a hard limit is imposed by feedback from the *internal* monitor photodiode to assure that the laser diode specifications are not exceeded under any conditions. Otherwise, even an errant dust particle or house fly wondering into the portion of the beam path used for feedback could ruin your laser diode!
Laser diodes in the several hundred mW to multi-watt range which do not have internal monitor photodiodes have a different set of issues with respect to safe (for the laser diode, that is) drive circuits.
The dire warnings about instant destruction from overcurrent still apply but but the extreme non-linearity typical of low power laser diodes isn't usually present with higher power devices. There is still a lasing threshold but above this, the output power increases linearly with current and there is likely to be decent consistency from unit to unit. However, proper current control and temperature compensation (or adequate derating) is still essential.
When you get into the 1 amp diodes (or anything over 200 or 300 mw), the driver becomes less dependent on the laser power feedback PD and many of these higher powered diodes just don't have the power sensing PD on-board for this reason.
While the threshold current is still very dependent on the temperature of the diode, the DIFFERENCE between the max current and the smoke release current widens a lot - meaning that the larger diodes can be operated fairly safely without sampling the output and applying variable current based on the power sensing PD.
The 1 watt diodes that I was trying to buy several years ago had 2 sets of specs-one at ambient room temperature and the other set for diodes at actual operating temperatures-the inference being that the preferred driver needed TEMPERATURE feedback in order to ramp the diode up to operating temperature.
Note that these diodes were used to drive fiber optic cables where they operate as an FM transmitter (constant carrier/fixed duty cycle transmit), so they probably used a time delay circuit to ramp them up to temperature rather than an actual temperature sensor.
Where the diode (probably) isn't on constantly, it might be necessary to derate the diodes and operate them just above threshold in order to be safe.
For your high power diodes, you can use a simple constant current driver (assuming the diode doesn't require PD based power sensing feedback.
The Vishay Siliconix catalog has an ABSOLUTELY O-U-T-S-T-A-N-D-I-N-G technical description of MOSFET based constant current source design. You can request the hard copy of the catalog from their website, make sure you get the full catalog with the ap notes.
I ended up feeding half of the 33 ohm resistors from one 7805 voltage regulator and the other half from a second 7805. Even though one 7805 can handle one amp of current it began to show signs of thermal drift when running at this level. By splitting the resistor bank in half each regulator only needs to supply 1/2 amp.
A 808 nm 500 mW laser diodes are visible but barely. Do NOT be fooled into thinking it's not really putting out much power. Human eyes aren't that sensitive to 800 nm radiation BUT you can easily burn a hole clean through your retina with this much power. If you doubt this, try focusing your 808 nm 500 mw laser on the black plastic part of a VHS video cassette and see what it does. When I do this with mine I get instant smoke and liquid plastic. So, BE CAREFUL especially when focusing this diode down to a small spot.
When playing around with stuff like this you will notice that color has a LOT to do with how much energy is absorbed. Aiming the same laser at the while label on the same cassette resulted in nothing happening. There is a very important principal to be learned by this experiment. If the white label isn't absorbing much power from the laser beam then it has to be going some place else. The answer of course is it's being reflected (scattered) back from the white surface. Keep this in mind when playing around with this diode. If you hit something that's even remotely reflective you could end up with the beam coming right back at you and you might not even be aware of it since the human eye is not very sensitive to radiation in the 800 nm region.
For communications use you might want to consider expanding the beam. This will lower the power density and make it a LOT safer if you accidentally get in the beam. The beam exiting mine is approx. 4 inches in diameter. 500 mw spread across a 4 inch diameter circle is a LOT less dangerous than 500 mw focused down to 1 mm in diameter!!!
And remember that a 500 mW 808 nm laser diode needs a GOOD heatsink. If you notice the power dropping off shortly after you turn the laser on your heatsink is too small! If you are having problems with this and you don't have room for a bigger heatsink use a small 12 VDC fan. Try to direct the air across the heatsink and NOT across the optics!
You can monitor power output with a regular silicon solar cell hooked directly to a milliamp meter (not a voltmeter!!!). Do NOT use any series resistor between the solar cell and meter. Expect to see over 100 ma of current at this power level. I also suggest you expand the beam to make use of most of the surface of the solar cell. If you focus it down to a small diameter the power density goes up and you just might burn a hole in the solar cell! Plus a very narrow diameter beam could easily bounce off the shiny surface of the solar cell and hit you in the eyes with enough power density to do some real damage! Watch the angle between the solar cell and the laser and anticipate where the reflection might fall. You will get the same power reading no matter what the beam diameter is as long as all the energy hits the solar cell. You can substitute a white piece of paper to get some idea of beam diameter but be CAREFUL when doing this!
Treat this laser with respect. Anticipate reflections. Keep people, animals and airplanes out of it's path and above all THINK before you turn it on!
This is a basic power supply using a pair IC regulators to provide a variable voltage with adjustable current limit. Rather than combining these functions a brute force regulator pair is used - one for the voltage and the other for the current limit.
The idea is to be able to safely test laser diodes or complete drivers with the ability to limit current initially to a guaranteed safe value until circuit operation and/or laser diode behavior can be determined. This should substitute for an expensive lab supply for testing of lower power devices.
The circuit is shown in Sam's Laser Diode Test Supply 1 (SG-LT1). As drawn, it is suitable for laser diodes requiring between about 25 and 250 mA. With obvious changes to certain part values, the same circuit should be usable at up to an amp or more - but I won't be responsible for any destruction of expensive laser diodes that might result!
More modern lower dropout regulators like the LT1084 can be substituted for the LM317. For load currents above about 100 mA continuous, heat sinks will be required on the IC regulators.
With care, a very basic power supply can be used to safely drive low and medium power laser diodes.
The supply I have used to test diodes up to about 2 A is very basic consisting of a Variac, transformer, bridge rectifier, and filter capacitor with a current limiting resistor. For low power diodes, this is typically 50 to 250 ohms; for high power diodes, it is 8 ohms, 50 watt. A bleeder resistor assures that the filter capacitors discharge quickly once power is removed. A built in voltmeter shows the voltage into the current limiting resistor at all times. Using the equation: I=(V-2)/R (2 is the estimated voltage drop of the diode, R is the current limiting resistor) is often close enough. Adding a shorting relay which required a press of a button to re-enable when power is applied would further reduce the risk of accidentally overdriving the diode.
Since there is no active regulation, the output current has some 120 Hz ripple so the peak current may be slightly higher than the measured current. Installing a current meter (A or mA as appropriate) would be more precise but unless running near the maximum specifications of the diode, isn't really essential.
Batteries are in fact a relatively safe alternative to sophisticated power supplies if their characteristics are well understood. Since a properly connected battery can never put out more than its rated voltage when new or fully charged, and can't produce reverse polarity, all that is needed is current limiting via a high power resistor. I would still recommend a 0.1 uF capacitor, 1N4148 reverse protection diode, and 100 ohm resistor directly across the diode though.
A new or fully charged battery can have substantially more voltage than the nominal rating. For example, a new Alkaline is around 1.57 V, not 1.5 V. A NiCd may start out at 1.3 V or more when fully charged.
Don't get too greedy and use a battery voltage close to the diode voltage, include a reasonable size current limiting resistor and use a higher battery voltage. The internal resistance of NiCd and NiMH batteries is quite low and should never be depended upon for a significant part of the current limiting.
CAUTION: There must NOT be any filter capacitance in the power supply after the current limiting resistor. This is to minimize the chance that a bad connection to the diode will result in excessive current should such a capacitor charge to a much higher voltage and then discharge through the diode without current limiting.
It's fine to trickle charge a battery while it's being used since regardless of line voltage fluctuations and spikes, not much will happen to the battery voltage. However, due to the internal resistance of the battery, fast charging may not isolate the output enough. Better to implement a double buffering scheme where one battery is being charged while the other is in use, switching using a relay with an electrolytic capacitor to hold the voltage for the millisecond or so when the output is disconnected from either battery.
The voltage of Alkaline batteries drops steadily as they are used while that of NiCd and NiMH batteries is nearly constant until they are fully discharged. Without an active regulator, this must be taken into account.
To vary the current with no active components, a high power rheostat or selector switch must be used. Make sure it's wired so that intermittent contact can't result in current spikes.
Output power shown is approximate and depends on specific diode's threshold current and slope efficiency.
Double check polarity and take appropriate safety precautions!
Note that the sensitivity of this photodiode to the LED emission will vary considerably depending on its position and orientation. Tape the photodiode and one of the LEDs together (sort of like a homemade opto-isolator) to stabilize and maximize the response.
Where the laser diode current is below 20 or 30 mA, a suitable opto-coupler could also be used (see below).
Using this 'laser diode simulator', it will really only be possible to confirm that the laser driver current regulator is functional, not to actually set it up for your laser diode.
Once the circuit has been debugged, power down, and carefully install the laser diode. Double check all connections!
Set the power adjustment of the laser driver to minimum (usually maximum resistance).
If available, use a power supply with both voltage and current limit adjustments. Then, you can start with the voltage set to 0 and the current limit set just above the expected laser threshold current (plus the current drawn by the rest of the circuit - test with no laser diode in place). This can always be increased later.
Attach a voltmeter between the photodiode (PD) terminal and ground. This will effectively monitor relative optical power output.
If you have a (separate) current meter, put it in series with the power supply as well (or provide another means of measuring current).
CAUTION: Use clip leads. Leave the meters in place - do not attempt to change connections while the circuit is powered as this could result in a momentary current spike which may damage the laser diode.
Increase input voltage gradually. Once the laser diode starts lasing, the PD voltage should climb. The circuit should regulate when the PD voltage approaches the reference: 2.5 minus .7 V in circuits (1)-(3) or .5 Vcc for circuit (4). Then, the PD voltage and supply current should level off. If something doesn't behave as expected, shut down and determine why.
Once you are confident that the circuit is operating properly with the laser diode installed, the output power can be increased modestly. But, without a laser power meter, DO THIS AT YOUR OWN RISK!
For visible laser diodes, if you have a laser pointer or other visible diode laser module OF THE SAME WAVELENGTH, A-B brightness comparisons can be made if the beams are the same diameter. Otherwise, don't push your luck unless you have a bucketload of laser diodes you can afford to blow!
For IR laser diodes, visible light eyeballs won't work. The tiny red dot that may be visible from an IR laser diode cannot be used as an accurate indication of power output.
Laser diodes are generally NOT very forgiving. However, if you take your time and make sure you understand exactly what is happening at every step along the way, you and your laser diode will survive to light another day!
The Hewlett Packard HCPL4562 optocoupler appears excellent for incorporation into your laser diode simulator.
It is an LED optoisolator with a PD output stage. The PD is available by itself (without current amp transistor) or a moderate gain transistor is available (base/PD, emitter and collector)-so it's very flexible. The oveerall combination of LED, PD and output transistor has a 17 Mhz bandwidth rating.
My feeling was that the PD (standalone) should be used as we are trying to simulate a PD device itself that is normally inside the LD assy.
The goal is to be able to make the simulator have the same PD sensitivity as the actual LD/PD combination to be used. I think this is doable without adding a lot of complexity.
A quick and dirty audio monitor on the LD current would be neat too-you wouldn't have to depend on your eyes to tell you if the drive becomes unstable or drifts up/down.
The first five circuits are from published circuit diagrams or application notes, or were reverse engineered from actual devices. All use visible laser diodes though IR types would work with at most minor modifications to biasing points.
Laser drivers (1) to (3) were from CW laser lights used for positioning in medical applications. Laser driver (4) was from a UPC bar code scanner.
Errors may have been made in the transcription. The type and specifications for the laser diode assembly (LD and PD) are unknown.
The available output power of these devices was probably limited to about 1 mW but the circuits should be suitable for the typical 3 to 5 mW maximum power visible laser diode (assuming the same polarity of LD and PD or with suitable modifications for different polarity units).
Of the 5 designs presented below, I would probably recommend "Laser diode power supply 2" as a simple but solid circuit for general use. It doesn't require any special chips or other hard to obtain parts. However, I would add a reverse polarity protection diode (e.g., 1N4002) in series with the positive input of the power supply.
An enhanced version of this design including a printed circuit board (PCB) layout is presented in the section: Sam's Laser Diode Driver (SG-LD1).
A very basic and a high power laser diode drive circuit are also included (both open loop - no optical feedback) as well as one that can be programmed for 1024 levels of output intensity.
This circuit lacks some of the protective features of the circuits, below, but is clearly the same core design.
This is the circuit from a Scanditronix "Diolase 1" laser line generator, a unit designed for patient positioning in medical diagnostic and treatment applications like radiation therapy. No, it doesn't actually engrave the patient but just projects a red line to aid in placing the patient on the couch and adjusting couch position in relation to semi-indelible ink marks drawn on the skin surface.
It will run from a (wall adapter) power supply of about 6 to 9 VDC.
Note the heavy capacitive filtering in this circuit. Changes would be needed to enable this circuit to be modulated at any reasonable rate.
Potentiometer R3 measured at 6K.
LM431 shunt regulator set up as 2.5 V reference. A 2.5 V zener or even a visible LED could also be used.
Supply current measured at 150 mA (includes power on LED not shown).
Transistor types do not appear to be critical.
This is the circuit from a Scanditronix "Diolase 2" laser line generator, similar to the Diolase 1 described in the section: Laser Diode Power Supply 1 (RE-LD1) but containing a pair of diode laser modules, normally adjusted to produce a horizontal and vertical line. It appears to be an improved design including a soft-start (ramp-up) circuit and an inductor in series with the laser diode. Otherwise, it is virtually identical and will run from a 6 to 9 VDC source.
Since both units were from the same company, I assume that these refinements were added as a result of reliability problems with the previous design - in fact, I have recently discovered that the unit from which I traced that schematic is not as bright as it should be!
Q2 is the feedback transistor and compares the reference voltage on VR1 with the voltage developed across R3+R6 by the monitor photodiode current.
Q3 is the LD driver.
Capacitor C3 was marked n47 and very small, probably .47nF (470pF).
Capacitor C4 was marked 10n and very small, probably 10nF (.01uF).
Inductor marked Red-Black-Black-Silver, probably 20uH.
Potentiometer R6 setting not measured.
LM431 shunt regulator set up as 2.5V reference. A 2.5V zener or even a visible LED could also be used.
This design is virtually identical to the circuitry found in typical laser pointers like the Laser Diode Driver from Radio Shack 63-1040 Laser Pointer reverse engineered by Walter Gray.
This one runs off of a (wall adapter) power supply providing about 8 to 15 V.
Soft start circuit (slow voltage ramp up).
Optical power based current source.
The first part of the circuit consists of the input filter, soft start circuit, voltage regulator, and DC-DC voltage converter. Its output should be s super clean, filtered, despiked, regulated, smoothed, massaged, source of -5 V ;-).
It was not possible to determine the values of L1 and L2 other than to measure their DC resistance - 4.3 ohms. The LT1054 (Linear Technology) is a 'Switched Capacitor Voltage Converter with Regulator' running at a 25 kHz switching frequency. A full datasheet is available at the Web site, above.
The output of Q1 ramps up with a time constant of about 50 ms (R4 charging C9). This is then regulated by the 7805.
The LT1054 takes the regulated 5 V input and creates a regulated -5 V output. There is no obvious reason for using this part except the desire to isolate the laser diode as completely as possible from outside influences. Like the use of an Uninterruptible Power Source (UPS) to protect computer equipment from power surges, a DC-DC converter will similarly isolate the laser diode circuit from any noise or spikes on its input.
I suspect that there are additional components inside the laser diode assembly itself (like the hypothetical Rx, probably a few ohms) but could not identify anything since it is totally potted.
This more sophisticated (or at least more complicated) driver board uses a dual op-amp (LM358) chip instead of discrete parts to control a transistor current source. Due to the relative complexity of this design, and the fact that it is entirely constructed of itty-bitty surface mount parts, errors or omissions with respect to both transcription and interpretation are quite possible!
Get the schematic for LDDRIVE in PDF format: LDDRIVE-SCH.
The feedback loop consists of the photodiode (PD, part of D1), a non-inverting buffer (U2A), the inverting amp/low pass filter (U2B, R9, R11, C2, bandwidth of about 1 kHz), and emitter following current source (Q1, R13, R14, with a sensitivity of 36 mA/V) driving the laser diode (LD, part of D1).
Separate DC inputs are shown for the laser diode/photodiode itself (Vcc1) and the other circuitry (Vcc2). Vcc1 must be a regulated supply as there is no on-board voltage reference. It appears as though Vcc1 and Vcc2 should be set equal to one-another though there may have been (external) power sequencing in the original application. If Vcc1 is less than Vcc2 by more than a volt or so, the laser diode will be turned off. The input voltage range can be from 5 to 12 VDC though I would recommend running on 5 VDC if possible since this will minimize power consumption and heat dissipation in the current driver transistor and other circuitry. This is adequate for laser diodes with an operating current of up to about 80 mA. For laser diodes with an operating current greater than this, a slightly higher voltage will be required.
The set-point is at about 1/2 Vcc1 so that the laser diode optical output will be controlled to maintain photodiode current at: I(PD) = .5 Vcc1 / (R6||R7). Use this to determine the setting for R7 (SBT, Select By Test, Power Adjust) for the photodiode in your particular laser diode. Or replace R7 by a low noise variable resistor and use a laser power meter to set the operating current. (Hint: Start with the minimum current - maximum resistance).
For example, with Vcc1 = Vcc2 = 5 VDC, maximum laser diode current will be limited to about 90 mA. With R7 (SBT) equal to 5.9K, photodiode current will be .5 mA. For some laser diodes, this is approximately the value for 1 mW of optical beam power BUT YOURS MAY BE TOTALLY DIFFERENT!
If you then increase Vcc1 = Vcc2 to 10 V or halve the parallel combination of R6||R7, the output power will double or the laser diode will die in a futile attempt to achieve the impossible.
A cutoff circuit is provided to disable current to the laser diode as long as Vcc2 is more than about 1 V greater than Vcc1 or from an external input logic signal (ground J1-2 to disable). This consists of Q2, Q3, and their associated resistors. When Q2 is biased on, it turns on Q3 which shorts out the input to the main current driver, Q1.
The comparator (U1, LM311) would appear to output a signal based on photodiode current being above a threshold but its true purpose and function is not at all clear (or there is a mistake in the schematic).
As noted above, there is NO on-board voltage or current reference. Thus, Vcc1 must be a well regulated DC supply with low ripple and noise and NO power-on overshoot (especially if the laser diode is being run close to its optical power limit). However, this isn't quite as critical as driving the laser diode directly since optical output power (photodiode current) and not laser diode current is the controlled parameter. A power supply using an LM317 or 7805 type IC regulator with a large high quality filter capacitor on its output (e.g., 100uF, 16V, tantalum, in parallel with a .01uF ceramic) should be adequate.
Although the original version of this board uses surface mount devices, common through-hole equivalents are available for all parts and these are labeled on the schematic. Note: A heat sink is essential for (Q1) where Vcc1 is greater than 5 VDC - this part gets warm.
SG-LD1 is an enhanced version of the design described in the section: Laser Diode Power Supply 2 (RE-LD2) with the addition of bilevel (digital) modulation as described in the section: "Laser diode modulation". It should be capable of driving most typical small laser diodes including those found in CD players and CDROM and other optical drives, and visible laser diodes similar to those found in laser pointers, bar code scanners, medical positioning laser lights, and other similar devices.
This design assumes a laser diode assembly where the laser diode anode and photodiode cathode are common (this seems to be the arrangement used most). If the opposite is true with your device (laser diode cathode and photodiode anode are common), reversing the direction of polarized components and power supply input, and changing NPN transistors to PNPs and vice-versa will permit the same PCB layout to be used. However, if your laser diode assembly has both anodes or cathodes in common, this circuit is not suitable unless an external photodiode is used for the optical feedback.
Get the schematic for SG-LD1 in PDF format: SG-LD1-SCH.
In some cases, the part values listed should be considered as suggestions as many modifications are possible depending on your particular laser diode specifications and application needs. Transistors with heat sinks for Q2 and Q4 are advised if operating continuously near the upper end of the input voltage range (say above 10 V) and/or at laser diode currents of 100 mA or higher.
Input power (Vcc) can be anything in the range of about 10 to 15 VDC. It's not critical and will have no effect on the output power. A regulated supply isn't required.
Ebl should normally be left open. A switch closure (or open collector NPN transistor or open drain MOSFET) to Gnd shuts off the driver. Do NOT apply any active high signal to this input.
The monitor photodiode current at rated power will be in the specifications for the laser diode, usually with a rather wide range of sensitivity (10:1 or more). To start out, assume it's the minimum value and then if that doesn't result in enough output power (or any lasing at all with proper circuit operation confirmed), reduce the resistor values to obtain the desired output power. The reference point is a voltage of about 3.2 V on the base of Q1. For example, if the monitor photodiode current at full power is 0.5 mA, the total resistance would need to be about 6.4K ohms minimum. However, since the monitor photodiode sensitivity can vary widely, start with a high enough total resistance so that even worst case, the laser diode will be safe. Then, reduce the resistance once the behavior has been determined.
A positive voltage (3 to 15 V) applied to Mod turns on Q3 which shorts out R7 and increases the output power by an amount determined by the values of R4, R7, and the setting of R5. The specific resistance values must be selected based on the desired output power, modulation index, and monitor photodiode sensitivity.
CAUTION: As with all low power laser diodes, it is essential to use a laser power meter to determine the setting for maximum power.
A printed circuit board layout is also available. The entire single sided circuit board is 1.7" x 1.15" and includes modulation and enable inputs. It will run on an unregulated power supply of around 6 to 12 VDC.
The layout may be viewed as a GIF file (draft quality) as: sgld1pcb.gif.
A complete PCB artwork package for SG-LD1 may be downloaded in standard (full resolution 1:1) Gerber PCB format (zipped) as: sgld1grb.zip.
The Gerber files include the solder side copper, soldermask, top silkscreen, optional component side pads, and drill control artwork. The original printed circuit board CAD files and netlist (in Tango PCB format) are provided so that the circuit layout can be modified or imported to another system if desired. The text file 'sgld1.doc' (in sgld1grb.zip) describes the file contents in more detail.
I have a few bare (unpopulated) PCBs fabbed from this artwork available, as yet untested.
While most laser diode packages have the configuration assumed by all the previous driver circuits, there are some that don't fit the mold. This section deals with one variation in particular - those with a common cathode connection.
A simple modification to the basic SG-LD1 circuit (or any of the others that are similar) should permit these types of laser diodes to be safety driven.
Sam's Laser Diode Driver 2 shows the new circuit. The only changes are to the wiring of the laser diode package and the substitution of a zener diode (CR3) for R8. CR3 guarantees that the laser diode will not be driven should the voltage on the photodiode be insufficient for the feedback control to be active. At normal supply voltages, leaving R8 in as in SG-LD1 should work. The concern is that during power cycling or if run from a power supply voltage that is too low, the circuit could attempt to overdrive the laser diode thinking there is inadequate output power due to lack of bias on the photodiode and/or not enough voltage on the feedback components.
This one runs open loop (no optical feedback) but has been designed to permit safe modulation. It should be fine as long as you don't try to run too close to the laser diode's maximum current/power rating.
The circuit and an extensive description can be found at K3PGP's Experimenter's Corner under: Biasing and Modulating Laser Diodes - Safely!.
The circuit in Viacheslav's Laser Diode Driver (VS-LD1) is quite straightforward. I guess my main nit to pick would be that it uses more power than needed due to the constant current driver as opposed to a constant voltage source and a means of controlling the current via a pass transistor. But for a low power laser diode, this really isn't a major concern. There is enough filtering on the input that any transient conditions should not cause problems.
I started with a constant current source using a LM317L (DA1) and R1. The current then branches to laser diode (through R5 for fine adjustment of division ratio and R6 for monitoring) to KT3 (LD anode). Another branch on VT1 is made to sink the extra current, the more the feedback, the more current sinks through the transistor. R2 regulates the reverse bias of the photodiode (it actually doesn't need to be 20K, but I picked from what I had in local store).
KT3 is the LD anode, KT4 is the PD cathode.
This circuit looks pretty stable (I can only judge by eye and voltage meter). For tests I used 2 metal-cased LED's and some unknown photodiode. Green LEDs could not impress the photodiode so I just used a laser pointer to check that feedback works. After I was sure that everything was all right, I set current to about 50 ma and plugged in the laser diode (Mitsubishi ML1016R, I = 80 mA). Then it was easy to set the nominal current and test the feedback a little against circumstances (unattaching it from heatsink for a few seconds, for example).
Actually before this circuit I assembled one similar to SG-LD1, just altered it to adopt Mitsubishi's pinout. But while testing it I felt like I'm not 100% sure how it works and I was very paranoid about LD sensitivity to everything and knew very little practical stuff, so I decided to make my own circuit. Yes, it indeed draws 120 mA where only 90 mA are used for good, there's room for improvements.
This circuit was found in a 25 mW red laser diode module, model and manufacturerer unknown. It is almost an exact mirror image (with respect to polarities) of Toshiba Discrete Laser Diode Power Supply (TO-LD1). Note that the input voltage is negative.
Note the LED used in place of a zener. I confirmed that it actually does light up orange.
It is from a cheap laser pointer. Like the other discrete laser diode drivers, a single PNP transistor is used in the feedback loop to regulate laser diode current. However, although optical feedback of sorts is used, there appears to be no real reference. Thus, output power will depend on battery voltage, nominally 4.5 VDC (3 button cells, I assume) and the gain of Q2.
At first I thought some parts had been left out: At the very least, a zener or similar reference across C-E of Q2, and possibly some filter caps to keep the thing from oscillating. While was willing to believe that the design had the optical output depending on battery voltage, it seemed inconceivable for it to be directly affected by the gain of the driver transistor. However, I now believe that it is probably drawn correctly but the actual operating point is where the Q1 is almost in cutoff and its gain wouldn't be critical.
Ipd = Output Power(mW) * X (where X is the sensitivity of the monitor photodiode in uA/mW).
I1 is very nearly equal to lpd (minus Q1's base current).
V(R1) = I1 * R1.
V(R2) = V(Battery) - V(R1) - 0.7.
The operating point will depend slightly on the gain of both Q1 and Q2 but if the product ot their Hfes is high, for a given battery voltage, laser output power will be fairly constant.
You can crank the math for your favorite laser diode and transistor specs!
This is the circuit from another inexpensive laser pointer. Although very similar, it includes some capacitive filtering (and more optional filtering in C2, not installed), as well as a power adjust pot (VR1). However, like the previous circuit, this does not have any absolute reference so power output will be dependent on the battery voltage to some extent. People have successfully modulated this module at a reasonable frequency (upper limit not determined) by removing or greatly reducing the value of the filter capacitor, C1. However, do this at your own risk!
This unit was available from Oatley Electronics (AU) as the module LM-2 (January, 2000). Of course, they may have already switched to a different supplier or the manufacturer may have changed the design!
Vbatt = battery voltage under load.
Ipd = total photodiode current.
Vld = voltage across the laser diode.
Vbe1 = Base-emitter drop (.7 V) of Q1.
Since Ipd is proportional to optical power output, like LP-LD1 and LP-LD2 (above), brightness is dependent on battery voltage. In this case, it is a much more non-linear relationship as Vld and Vbe1 set a threshold of about 2 to 2.5 V below which there will be nothing and then output will increase based on Vbatt/(R1 + VR1). The circuit operates on 3 V but 4.5 V seems like the minimum to get any decent output.
This is the circuit from another inexpensive laser pointer. Well, actually it's from a diode laser module, but this was obviously just a pointer driver without the pushbutton (which I have added in the schematic). Battery voltage is 2.6 to 3.0 V. It's very similar to LP-LP1 and LP-LD2, above.
This is a very simple circuit from a 780 nm laser diode module sent to me by Shawo Hwa Industrial Co., Ltd., a Taiwanese manufacturer of laser pointers, laser modules, and other related laser devices. This unit is similar to the guts from a typical visible laser pointer. Connections are via wires though there is a battery contact spring hidden under heatshrink, but no switch or power adjust pot. The laser diode is in a 5.6 mm metal can though the window appears to be molded in place rather than glued from the inside.
The battery voltage is spec'd at 3 V. The only reference device is the B-E junction of Q1 so power output will vary with temperature and not very much with battery voltage. Both SMT transistors were labeled "RIP". R1 could be changed to a pot to provide a variable power adjustment. I assume that for this module, its value is selected for each laser diode. I'm not sure what the rated output power is for this module other than "<5mW" but it actually measured 2.3 mW.
Here is the schematic for the driver from a CW green DPSS laser pointer generously contributed to the cause by Laserpointers.co.uk. There is no model number on the case but it is manufactured by Lightvision Technologies Corp., Taiwan. The pointer was given to me because (1) it was broken and (2) Laserpointers.co.uk apparently doesn't deal with this supplier anymore so they couldn't send it back for repair.
Here is the schematic for the driver from the green DPSS laser pointer described in the section: The Edmund Scientific Model L54-101 Green Laser Pointer. Photos of the pointer are shown in Components of Edmund Scientific L54-101 Green DPSS Laser Pointer. This is a pulsed model operating at about 4.5 kHz with a 50% duty cycle. The driver board was designed by B&W Tek who are also the supplier of the pointer to Edmund Scientific.
The circuit in Green Laser Pointer Diode Driver 2 uses what appears to be a low voltage 33202 dual op-amp. Do a Google search for "MC33202".) It's configured as a squarewave oscillator feeding a constant current driver. Part values for the capacitors were all guessed because they wouldn't produce meaningful readings on either of my DMMs. This is still a mystery.
This driver is from a pointer that is externally identical to the one described in the section: Laser Diode Driver from Green Laser Pointer 1 (GLP-LD1) but the actual DPSS module and driver differ.
The circuit in Green Laser Pointer Diode Driver 3 is a basic current regulated driver using a single op-amp with a range of approximately 0.167 to .333 A. It was set to about 0.300 A.
Here is the schematic for the driver from a Z-Bolt BTMK-10 green DPSS laser pointer. This one is rated at 5 mW, though I assume the same design is used for some higher power versions. The ZBolt BTMK-10 is actually not a pointer in the usual sense since it doesn't have a momentary switch on the side and is aimed at (no pun...) targeting applications. But I'll call it a pointer here. :) The switch is on the rear end and is latching. This one differs from the 3 previous drivers in that it uses Automatic Power Control (APC) rather than Automatic Current Control (ACC, constant current). So, the feedback loop is closed by a photodiode that samples a portion of the output beam.
The circuit in Green Laser Pointer Diode Driver 4 uses what appears to be a low voltage ELM8548M1 dual op-amp. However, as can be seen in the schematic, there is no feedback resistor for the second op-amp so perhaps that has one built-in. The parts were all labeled, though I'm not positive about which labels went with which parts in a couple cases. There is also space for a tiny surface mount LED and its current limiting resistor.
While the APC circuit operation is quite straightforward, there would seem to be a potential issue should the circuit be incapable of obtaining the expected output power. Since there is no absolute current limit, it could drive the laser diode to destruction should someone power it in a cold environment where the diode wavelength doesn't match up with the vanadate absorption and it can't produce 5 mW at rated diode current. The current would then be limited only by circuit and battery resistance. However, if the designers were really clever, they might have set up the beam sampler to just enough pump light leaks through to the photodiode and limit the current even with insufficient green output. However, I rather doubt this to be the case since there is no way to adjust any current limit.
Note: Resistor value depends on your specific laser diode current requirements. Discussion below assumes a laser diode with a 72 to 100 mA drive range.
Power is 5.5 to 9 VDC. I use a 9 volt battery.
Watch the pin arrangement on the LM317. On the LM317L (the TO-92 plastic transistor type case) and the LM317T (TO-220 7805-type case), the pins are, left to right, Adjust-Output-Input.
For the resistor, I use a small carbon 10 ohm in series with a precision 10-turn 20 ohm adjustable. The combo was empirically set to about 17 ohms.
On initial power on, use three garden variety diodes stacked in series instead of the laser diode. Put a current meter in series with the diode stack and adjust the precision resistor for 50-60 mA. Disconnect power and replace the diode stack with the laser diode. Connect up power again, still watching on the current meter. The diode will probably initially glow dimly. I use a diode that lases at about 72 mA, and has a max rating of 100 mA. I use about 85 mA for normal ops.
Turn up the current, never exceeding your diode's max limit. The dim glow will increase in intensity, but at some point, a distinctive step in intensity will occur. Your diode is lasing. Remove the current meter as desired. Enjoy!
Note: It is important to use a tantalum capacitor for C1.
Input power should be regulated 5 to 6 VDC. Since there is some interaction between diode voltage and current with this design, make sure to set up the current adjustment with a dummy (e.g., dead) laser diode, or make sure it is set low before applying power and increase it slowly to the operating point. Then, fine tweak the current once the temperature of the diode has stabilized.
This is a small printed circuit board (about 14x35 mm) which will drive laser diodes in constant current mode up to 800 mA without a heat sink and about 1.2 A with a heatsink (not included). It is suitable for driving laser diodes not requiring optical feedback such as DPSS laser pump diodes of up to about 0.5 W output.
The EU38 is available from Roithner Lasertechnik and formerly from B&W Tek. Thus, it is likely manufactured by someone else. Description and specs can be found on Roithner's Laser Diode Drivers Page.
The schematic I reverse engineered from the Roithner version can be found in EU38 Constant Current Laser Diode Driver. The circuit consists of an NPN power transistor controlled by a single op-amp. Feedback is taken from a 0.6 ohm series current sense resistor. One issue that I've found is that the reference is a zener diode (type unidentified) which probably doesn't have enough current going through it so while the feedback loop has enough gain and current regulation is quite good with respect to laser diode characteristics, the reference voltage changes slightly with input voltage. Thus, I recommend powering the unit from a regulated supply rather than a cheap wall adapter or batteries.
Not all components were labeled so it's quite possible there are errors. The zener voltage was determined by measurement with an input voltage to the board of about 4 VDC. I'm kind of guessing about the resistance of the Iadj pot (R4). It's more than 20K and less than 100K, so 50K is a nice standard intermediate value. The bias current or offset voltage or something :) of the mediocre op-amp (an LM358 clone) adds about 0.05 A to the output current.
I did find and fix two errors that were in my original schematic: (1) the value of R6 had been shown as 4.7K rather than 47K and (2) when I measured the voltage across the zener (ZD1), it was 1.05 V rather than the 1.5 V I had before. Although I was rather suspicious of that 1.05 V, a similar voltage has been confirmed by someone else. Perhaps the 1.5 V was wishful thinking when I originally traced the schematic.
The Roithner specs for the EU38 say that it can go to 1.2 A with a heatsink. As drawn, the maximum current is just about 1 A so there may still be errors in the schematic. If the resistance of the pot were much higher, the maximum current might almost get to 1.2 A. Or a user modification may be needed to go any higher. There are 6 through-pads on the PCB that I thought might have been intended for this purpose, but 4 are connected to ground, 1 is connected to power, and 1 is a no-connect.
I have used the EU38 to power the green demo laser described in the section: Even Simpler Instant Green DPSS Laser. The complete power supply is shown in Green Demo Laser Power Supply Using EU38. One complaint about the EU38 is that a jeweler's screwdriver must be used to adjust the current and the slot is in the metal wiper of the pot so it picks up 60 (or 50) Hz noise and modulates the diode current while touching it if the screwdriver handle isn't insulated!
The laser diode driver is an adjustable voltage regulator with a current limiting resistor. Added filtering and reverse polarity protection guarantee no overshoot or transients when power cycling. The cooling-only TEC driver is a MOSFET with a pot for the set-point. With only a MOSFET as the active component, this won't be very precise for temperature tuning but is adequate to keep the diode cool. I built it to power a Crystallaser 35 mW red diode laser. The numbers by LD1, TH1, and TEC1 refer to the 10 pin ribbon cable connector on the laser head. LED2 provides a rough indication of the voltage across the TEC, and thus the current through it.
Note that the voltage for the TEC is the same as the voltage for the laser diode based on the argument that there will be correlation between the LD power and the required TEC power. It could also come from the fixed 12 VDC input.
For this low power Crystallaser laser, the TEC is almost unnecessary as the maximum current to the laser diode is under 100 mA. But it was an excuse to implement this trivial scheme. In fact, acceptable cooling could be achieved even without using any active components by simply putting the laser diode in series with the TEC. But with the MOSFET, it was somewhat better.
A regualted 12 VDC power supply is recommended. Using a 7812 to provide this from a 15 to 20 VDC source would be ideal.
CAUTION: This is a more or less constant current driver without optical feedback. Therefore, it may not be suitable for laser diodes where the operating range of current is small.
The schematic in the section: Simple Laser Diode Power Supply is the standard circuit for making a constant current source from an LM317 or LM338 (e.g. see The Art of Electronics, fig 6.38). The problem with this circuit is that for large currents (the only currents for which it has good accuracy, and is a serious part saver) it's hard to make the current variable.
For example, for a 3.5 A current source, the resistor value is 0.357 ohms, and if you then want a 3.1 A current you've got to unsolder it and replace it with a 0.403 ohm resistor. Bummer.
One option would be to put a low value pot across the sense resistor and connect its tap to the voltage regulator common/adjust terminal. This will work reasonably well for a modest current range - perhaps up to 2:1 as shown below - but runs into difficulties where a wide range of control is desired.
The reason is that this arrangement can only *increase* the current from the nominal I = 1.25V/R. So, for example, to get a 10:1 range, the voltage across the sense resistor would be 12.5 V for the 10x current! In general this is not attractive for the high current condition because not only have you required a higher supply voltage, at the maximum current, but the power dissipation in the sense resistor is also quite high (more like HUGE --- sam).
Let me offer the following simple circuit, which I just created and haven't tried but 'oughta work' as a solution to this problem.
By contrast, this circuit can only *decrease* the current from the 1.25V/R value, but it easily handles a 10:1 range (or even much more) and the voltage across the sense resistor is never more than 1.25V, allowing low supply voltage (e.g. 5 V) and keeping the dissipation low.
The 1K pot selects a portion of the floating 1.23 V reference voltage, and tricks the LM317 or LM338 into correspondingly reducing the voltage across the 0.25 ohm current-sense resistor. The pot is conventional and may be panel mounted. It should be possible to nearly shut off the LM338 (a minimum quiescent current will still flow). The current sink, I, which powers the floating 1.23 V reference, is not critical and may be a simple current mirror (sorry to see the TL011 gone!), or even a resistor to ground or any available negative voltage, depending upon the desired current-source voltage-compliance range. That's it!
This isn't exactly an entire design but one that uses a common logic power supply in an unconventional way.
It may be possible to use a high current switchmode power supply as a variable current laser diode driver as long as it has remote sensing capability. The remote sensing feedback loop maintains a constant voltage (the spec'd supply voltage) between RS+ and RS-. Normally, this is used to compensate for the voltage drop in the wiring harness. By applying a variable control voltage between RS+ and V+, the power supply can be fooled into producing any output voltage from near 0 to its maximum rating as long as its minimum load requirement is satisfied. With a small resistor in series with the laser diode (or for those willing to take risks, the resistance of the laser diode), this results in a variable current to the laser diode. The only limit on output current is the maximum rating of the power supply. These types of power supplies, capable of 50 A, 100 A, or even higher current, are readily available on the surplus market. However, this scheme may only work with certain models, those which power their control circuitry separately from the main output and don't go into some sort of undervoltage shutdown if the output voltage goes too low. I don't know how to determine which models satisfy this requirement.
Vicor used to have application notes on doing this (among other things) with some of their Flatpac (among other) models. Search for "Programmable Current Source". The power supplies shown have an additional input called "Trim" which makes the modification particularly easy. But if there is nothing useful there anymore, I have an archived copy at Vicor - Flatpac Applications Circuits.
I have not yet attempted to close the loop and provide actual current control but have opted for voltage control for now at least. The unit I've been using for these tests is a Shindengen PS5V100A, a fully enclosed fan cooled switchmode power supply that's about 15 years old. This unit is also nice in that it regulates well with no load. All that was needed was to remove the shorting link between V+ and RS+ and install a 20 ohm, 2 W resistor in its place. Then applying 0 to +15 VDC current limited by a 47 ohm, 5 W resistor across RS+ (+) and V+ (-), the output voltage would vary from near 0 to 5 VDC.
Diode  o          o      Vout from 5 V to 0 V.
R3 can be constructed from a length of building wire. For example, 20 feet of #14 copper wire has a resistance of 0.05 ohms but water cooling would be needed if run near full current. I'm actually only using a head lamp load for testing and it works fine.
The same scheme using RS- did not have enough range, probably due to the internal circuit design. This is too bad because the op-amp circuitry to drive it might have been simpler, or at least more intuitive to design.
The challenge is to convert this to a user friendly form that is safe for the laser diode. I am designing a control panel which incorporates what I hope will be fail-safe circuits to minimize the chance of excessive current either from power cycling or by user error. It will use closed loop feedback so the actual current can be set (rather than voltage) and includes a multifunction panel meter (set current, actual current, diode voltage). It will enable diode current only if all power supplies are stable and correct, the 10 turn current adjust pot is at 0, and with the press of a green button.
However, initially, I'm using a 10 turn pot to control the current with a digital panel meter monitoring current via a 0.025 ohm sense resistor. Current is limited to between 50 A by a 0.06 ohm power resistor. Believe it or not, even 50 A is way below the limit for the diodes I need to test! See the section: Characteristics of Some Really High Power IR Diode Lasers.
The schematic in Sam's High Power Laser Diode Driver 1 includes the control panel, connections to the 100 A power supply, and laser diode wiring.
The basic control panel includes an Enable switch (eventually to be replaced with a keylock switch), Diode On and Off buttons, the 10 turn pot and DPM which reads 0 to 100 A. A differential amplifier converts the voltage across the current sense resistor into a DC voltage for the DPM. Without the differential amplifier, the control current was seriously affecting the readings as 1 A is only 2.5 mV. It's not possible (or at least not convenient) to separate the power and signal wiring to provide a proper single point ground.
Both the sense and current limiting resistors are simply lengths of #14 copper wire with forced air cooling. This works very well with the diode's output digging pits in my brick beam stop. :) However, for continuous operation, it may be necessary to replace the #14 with #8 because even the modest heating of the copper changes its resistance enough to noticeably affect current.
With minor changes in part values for the current limiting resistors, and the set-point for the power supply output voltage, it should be possible to drive a pair of laser diodes in series as long as they can be isolated from the common point. (The positive connection to a high power laser diode is usually the mounting block of the diode but it may not be connected to the external case itself.) However, one risk with this setup is that if one of the laser diodes fails shorted, it will likely take the other one as well since the current will spike to a very high level.
The setup is shown in Photo of Sam's High Power Laser Diode Driver In Action. The water-cooled laser diode in the aluminum box is capable of 35 W output at around 55 to 60 A. The power supply is at the upper left with the control panel in front of it showing 40 A. Behind the power supply is the coil of white wire acting as a current limiting resistor next to its cooling fan. The current sense resistor is the 12 inches of so of red wire running from the power supply to the terminal strip. The blue-white glow is my digital camera's response to intense IR. The camera is really confused. :) When viewed through IR blocking laser goggles, a line on the brick starts glowing at a current of around 35 A and is white-hot at 45 A, where the current limit of the power supply is presently set (via the current limiting resistor and wiring resistance with the power supply adjusted for a maximum output of 5 VDC). The old darkroom enlarger timer in the upper right is used to turn the driver on for exactly the 20 seconds needed for my "meat thermometer" type power meter to take its reading, which would show about 23 W at 40 A for the diode in the photo. The reading at 45 A is about 27 W.
The circuit in Tim's High Power Laser Diode Driver is designed for high power laser diodes which include a monitor photodiode for optical feedback. Note that most common high power diodes are driven with a constant current but optical feedback enables more precise control of output power. Diodes like this are available from Roithner Lasertechnik at very reasonable prices.
The front-end is a current differential amplifier (very similar to the approach used in the LM2900 Norton op amp). I hand-picked the two transistors for the current mirror for close matching. They are mounted in a common heat sink to keep them at the same temperature.
The constant current sources are LM334s. These are cheap and work well. The one used on the non-inverting input of the current mirror is adjustable to about 2 mA. The one used as the common emitter amplifier load was set to about 1 mA.
There is a 100 uF, 16 V capacitor on board too as well as a reverse biased diode in parallel with an RC snubber directly across the laser leads (not shown).
This is a simple design good to at least 3 A that can easily be extended to even higher current. See Josh's Web Site. Go to "Physics", "Lasers", "Regulated Current Source for High Power Laser Diode".
What's still needed is protection to guarantee that the circuit is well behaved when power cycling. This circuit will probably evolve over time.
The circuit provides 1024 discrete output levels from a laser diode (with optical feedback) using a D/A converter with a 3 wire serial input. In essence, it is a basic laser diode driver with a programmable reference.
Also see the section: Laser Diode Drive Chips.
The following circuits would be suitable for driving the type of pulsed laser diodes found in the Chieftain tank rangefinder and currently available from OSRAM Opto Semiconductors and possibly other sources. These are very different than the sort of laser diodes with which we are generally familiar. A typical specification might be 8 W peak power at 850 or 900 nm (depending on model) with power requirements of 10 A at 0.1% maximum duty cycle. Thus, the average output power is actually in the mW range even though these laser diodes may be listed in some surplus suppliers' catalogs (like those of Bull Electronics) as multi-watt devices with the duty cycle restriction listed in fine print, if at all! Since the average power dissipation is also very low, they may come in plastic packages like LEDs with flat polished faces (and no possibility of adding a heatsink, which is one of the major limitations on average output power)! Other than time-of-flight laser rangefinders and related applications, I'm not sure what use these would be to a hobbyist. And, their output is totally invisible but very definitely not eye-safe.
A simple approach that should work is to use an SCR as the switch triggered by your favorite pulse generator, 555 timer based astable, or other oscillator circuit followed by a trigger device like a neon bulb, diac, or small SCR to guarantee fast turn-on of SCR1. The circuit below is similar to the one from Scientific American (see below) which describes the use of pulsed laser diodes back in March 1973 when no other types had been invented yet (or at least none were readily available). With the component values shown, the laser diode should have a peak current of about 10 A with a 100 ns time constant. Thus, it isn't a nice rectangular pulse but that's for the advanced course. :) R1 limits charging current, R2 limits discharge current, and D1 provides reverse polarity protection for the laser diode.
Scientific American had an article on driving a pulsed laser diode in "Infrared Diode Laser", March, 1973, pg. 114. This is also a part of the collection: "Light and its Uses".
There is a pulse drive circuit in Skip Campisi's "Laser Clinic" article in Poptronics, June 2001. It's based on an NPN transistor operating in avalanche mode to generate the required short high current pulses.
The RCA SG2002 laser diode is probably long obsolete but the ones found in Chieftain tank rangefinder should be similar (though the specific ratings may differ somewhat). OSRAM Opto Semiconductors currently manufactures similar devices.
The discrete totem pole buffer circuit designed to provide very fast turn-on and turn-off may be overkill depending on your requirements and it may be sufficient to just drive the power MOSFET directly from a pulse generator or other signal source.
Check out Directed Energy, Inc. for schematics, white papers, and specs using ultra fast power MOSFETS. You can also buy complete drivers for pulsed laser diodes with pulse widths down to at least 4 ns at 40 AMPs.
I just recently reverse engineered the IR laser driver out of an HP LaserJet IIP (Part number RG1-1594). I've drawn up the full schematic for the board and have got it working outside the printer with with a simple power supply using a 7808, 7805, and a couple of capacitors. See Hewlett Packard LaserJet IIP Laser Diode Driver (RG1-1594).
Also, I've seen these boards advertised as replacement parts on the net for $20, so they would make quite a nice unit for someone who doesn't have the time to build a driver board up.
The feedback loop seems to be 1:1 so pins 3 and 4 can be shorted together (Mine runs at about 43 mA under these conditions). Pin 5 was originally driven by a single gate from a 74LS08.
Please contact me via the email address, above, if anyone finds out what wavelength the laser is, or how many milliwatts. I'm presuming about 800 nm at about 5 mW.
Someone suggested it was 50 mW at 930 nm but the power seems high for a printer of this era. Though, perhaps the same driver has been used in newer higher performance ones.
The pinout of the LaserJet IIIP driver is the same as for the LaserJet IIP, above. I found that light emitted from the laser diode is 786.5 nm (measured with spectrometer) and average power is about 4.5 to 5 mW (measured with a laser power meter). The laser diode is enclosed in a TO-18 (5.6 mm) package with ground connected to the case.
Skip Campisi has a nice article entitled "Laser Clinic" Poptronics, June 2001. There are schematics with complete parts lists with component values selected for the Sharp LT022MC 780 nm LD, Mitsubishi ML720 1,300 nm LD, and the Hitachi HL6712G 670 nm LD (all 5 mW max) and a pulsed driver for the high power LASD59 (similar to the RCA 40861 and LDs from OSRAM and others).
CAUTION: While the author does provide some basic laser safety information, it would have been nice to have more on the the critical drive requirements of laser diodes. I'm afraid there may be some disappointment when more than a few laser diodes turn into DELDs. He notes the effects of ESD and reverse polarity but doesn't appear to deal with the very important maximum current ratings. The only way to set up these laser diodes for maximum safe (for the LD, that is) output is with a laser power meter since their characteristics vary from device to device.
Circuit Cellar Magazine has a design using a PLD that will drive a typical low power laser diode using optical feedback and includes modulation. See: Project 247: Laser Diode Controller. It can probably do a lot more than they have implemented without requiring additional parts. However, circuit to simply provide the features shown would only cost about $2 for discrete parts or a laser diode driver chip, no downloading of firmware needed! I'm also not convinced it handles power cycling or fault conditions reliably.
Laser Circuits at the Discovery Circuits Web Site has links to a few, mostly laser diode related, schematics.
SatSleuth Laser Schematic Collection has a variety of links - many back to Sam's Laser FAQ - but a few might be useful.
Raw laser diodes typically have an electrical->optical frequency response that extends to hundreds of MHz or beyond. However, most simple drivers designed for continuous wave (CW) operation (including all of the discrete circuits described elsewhere in this chapter) have such heavy filtering and isolation from power supply transients and noise that control beyond a few Hz is usually not possible.
The light versus current behavior is hideously non-linear below about 10% of full output, so you really need dynamic feedback control - but the photodiodes tend to be slow, so that's not on.
lasers don't turn on from fully-off anywhere near as fast as they can vary intensity around the 50% level.
It's pig-difficult to design the modulation circuit so it is guaranteed never to overshoot the current that gives 100% full light output (which is essential, because even very brief over-power transients dramatically shorten the laser's life).
But if the information from Honeywell and others is to be believed, the new vertical-cavity surface-emitting lasers (VCSELs) are much better behaved and can be modulated to extinction at quite high rates. They're also extremely cute bits of device technology.
A couple of simple such modulation circuits are shown below.
CAUTION: Since both the following affect the optical feedback, attempt at your own risk!
A bi-level modulation scheme could be easily implemented by connecting a general purpose NPN transistor across an additional resistor (at point XY). Then, full power will be achieved with the transistor turned on and reduced power with it turned off. Select a value for R2 that will still maintain the current above the lasing threshold - 1K is just a start.
|C    |     Typical transistors: 2N2222, 2N3904.
Here is another circuit which should achieve somewhat linear control of laser power since optical power output should be proportional to photodiode current. Resistor values shown are just a start - you will need to determine these for your specific laser diode and operating point.
Also see the section: Integrated Circuits for Driving Laser Diodes since most of these ICs are designed with speed modulation capability built in.
Derek Weston (Email: derekw@alphalink.com.au) has constructed an IrDA tranceiver based loosely on the driver in RE-LD1 and a Crystal Semiconductor CorporationCS8130 IR transceiver IC. A complete description of this project may be found at his: UPN Laser Transceiver Web Site on the Realtime Control Web site.
Peter Philips' Laser Link Communicator was originally published in "Electronics Australia", July 1997. This allows for the transmission of high quality audio up to distances of several hundred meters. Either a visible or IR laser diode may be used (the latter providing for greater security but increases the difficulty of initial alignment).

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