Laser Diode Do's (more like Maybe's) and Don'ts If you have a laser diode and want to try your luck getting it to work, then you might want to consider this wisdom from Sam Goldwasser, who frequently posts this and other hobbyist/electronics wisdom to various Usenet newsgroups: Begin frequently posted wisdom from Sam Goldwasser You cannot drive LDs like LEDs, they will either not work or burn out more quickly than instantly: How do I use a laserdiode? ------------------------- The quick answer - very carefully for two reasons: I am assuming this is a typical 5 mW visible laser diode. 1. You can easily destroy the typical laserdiode through instantaneous overcurrent, static discharge, probing them with a VOM, or just looking at them the wrong way. 2. Anytime you are working with laser light you need to be careful with respect to exposure of a beam to your eyes. This is particularly true if you collimate the beam as this will result in the lens of your eye bringing it to a sharp focus with poassible instantaneous retinal damage. The third lead is for an optical sensing photodiode used to regulate power output when used in a feedback circuit which controls your current. Typical currents are in the 30-100 mA range at 1.7-2.5 V. However, the power curve is extremely non-linear. There is a lasing threshold below which there will be no coherent output (though there may be LED type emission). For a diode rated at a typical current of 85 mA, the threshold current may be 75 mA. This is one reason why many applications of laser diodes include optical sensing (there is a built in photodiode in the same case as the laser emitter) to regulate beam power. You can easily destroy a laser diode by exceeding the safe current even for an instant. It is critical to the life of the laser diode that under no circumstances do you exceed the safe current limit even for a microsecond! Laser diodes are also extremely static sensitive, so use appropriate precautions. Also, do not try to test them with a VOM which could on the low ohms scale supply too much current. It is possible to drive laser diodes with a DC supply and resistor, but unless you know the precise value needed, you can easily exceed the ratings. You can identify the laser diode and the photodiode because the photodiode's forward voltage drop will be in the approximately .7 V range rather than 1.7-2.5 V for the laserdiode. So, for the test below if you get a forward voltage drop of under a volt, you are on the photodiode leads. If your voltage goes above 3 V, you have the polarity backwards. Warning: some laserdiodes have very low reverse voltage ratings and will be destroyed by modest reverse voltage. Check your spec sheet. One approach that works for testing is to use a 0-10 VDC supply with a, say 100 ohm resistor in series with the diode, and slowly bring the current up until you get a beam. However, you still have no idea of when you are at the safe current limit without an optical power meter. For an actual application, you should use the optical feedback to regulate beam power. You should also use a heatsink if you do not already have the laserdiode mounted on one. The raw beam from a typical laserdiode is wedge shaped - 10x30 degrees typical divergence. You will need a short focal length convex lens to produce anything approaching a collimated beam. The circuit below is of a constant current regulated power supply for a CW visible laser light but it may not apply directly to your configuration of laserdiode and photodiode (polarities may differ). Three terminal regulators can be used in constant current mode as long as you guarantee that there will be no overshoot, etc. These things really are finicky. CW Laser Light (reverse engineered from commercial unit). -------------------------------------------------------- This circuit was traced from a commercial CW laser light. Errors may have been made in the transcription. The type and specifications for the laserdiode assembly (LD and PD) are unknown. The available output power is unknown but the circuit should be suitable for the typical 3-5 mW visible laserdiode (assuming the same polarity of LD and PD or with suitable modifications for different polarity units.) If you do build this or any other circuit for driving a laserdiode, I suggest testing it first with LEDs and discrete photodiodes to verify current limited operation. Then with the laserdiode in place, start with a low voltage supply rather than 9V until you have determined optimal settings and work up gradually. Laserdiodes are very unforgiving. Note the heavy capacitive filtering. Change would be needed to enable this circuit to be modulated at any reasonable rate. D1 +9 >------|>|-------+------------+-----------------+-----+--------+ 1N4001 | | | | | Rev. Prot. | | Pwr Adj | _|_ __|__ | / R3 10K (2) | PD /_\ LD _\_/_ | R2 \ +----+ | | | | 560 / | | +-----|---||---+ | \ +---/\/\--+-------+ C4 | | | | | .1 uF | |+ | | +----||----+ | __|__ | | __|__ C2 (1)| / C1 ----- | | E / \ 100 pF| \ 10 uF | - +-----|------' Q1 '-------+ / R4 | | | BC328-25 (5) | \ 3.9 | | | (PNP) | | | | | | | | +---+ | | |/ Q2 | |_ _|_i | +---| BD139 (NPN) | VR1 _'/_\_ | +| |\ (5) | LM431 | | C3 __|__ E| | 2.5 V | | 10 uF ----- | | (3) | | -| | R1 3.9 | | | | | GND >----/\/\/\-----+------------+-----+--------------------+-----+ Notes: 1. Capacitor C4 value estimated. 2. Potentiometer R3 measured at 6K. 3. LM431 shunt regulator setup as 2.5 V zener. 4. Supply current measured at 150 mA (includes power on LED not shown). 5. Transistor types do not appear to be critical. End frequently posted wisdom from Sam Goldwasser. EXPERIMENTER BEWARE: Please use the above only with due caution, with respect to hazards to humans / parts thereof / other life forms / property including but not limited to laser diodes themselves. Use of the above or anything else should only be attempted with appropriate precautions along with the understanding that laser diodes are fussy and are easily ruined by exceeding any of their ratings even for a microsecond. The above or anything else you try to operate a laser diode should only be attempted by those who are skilled in constructing / testing / operating homebrew electronic circuits. Attempt to operate bare laser diodes only at your own risk. Sam Goldwasser and I will *NOT* be responsible for anything that goes wrong with any laser diodes or anything affected by any radiation they emit, nor will anyone who stores/transmits this information be responsible. Written by Don Klipstein except as noted. Back up to Don's home page.