Electronic – Why did the laser die

Can it work this way: We start from, let's say, a 1.5 V supply, use a FET to connect diode to the supply for 50 ns and sense the peak current. Then we slowly increase the supply until we reach the required peak current. Will that work?

Yes it will work, but if it gets too much current you may burn it out, to little current and it will not lase. Max current is usually 10 to 15 percent above its min lase current.(depends on laser)

some good info here and an interesting chapter here and full table of contentsthey also have some driver references and schematics

I learned a lot from that site about how to drive some re-purposed red laser diodes that I had laying around.

I personally would start off with the lowest current possible, move it up till it lases and then calculate 10% above lasing current and make that your safe constant current, pulsed current can be higher.

In general, the laser hazard depends on the laser power, the output beam diameter, and the laser wavelength. For a class IIIa or 3R laser (the "IIIa" designation is basically obsolete, although it remains in use for products certified before the new classes were defined), you're at low risk if you don't force yourself to stare into the beam. If the beam just happens to stray into your eye, you'll generally have a reflex response to look away from the painfully bright light. (Don't fight this reflex -- keep yourself safe). Note that the high end of the class 3R power limits is defined by the power where 50% of people will have an "aversion response" sufficient to avoid injury --- and the other 50% won't.

Reducing the current to a diode laser will reduce the output power, and thus make the output safer. However, most laser accidents don't happen when the laser is operated as intended or as planned. You also need to consider all the possible "fault conditions" or ways that things can go wrong.

Say you design a control circuit that regulates the laser output to always be less than 1 mW (in most cases, a "safe" level) using a feedback photodiode. For real safety, you should also consider things like

• What if the optical feedback path to the photodiode is blocked (by dirt getting in your box)?
• What if the electrical path from the feedback diode to its amplifier is broken?
• What if there is a spike or drift in the power supply to the laser or laser drive circuit?
• What if some bit of metal junk gets in your box and short-circuits the laser to the power supply, bypassing the power controller?
• How are you calibrating the limit value for the photodiode current? If you're setting it with a pot, could the pot drift or be mis-adjusted after calibration? If you're using a digital circuit, could the EEPROM with the cal data be erased or damaged?
• etc.

These are the kind of conditions that marketable laser products need to consider before they can pass regulatory requirements. Before you risk your eyesight with your laser system, you should at least consider doing this kind of analysis for yourself.

If you used a laser with less power capability, you would know that before any of these kind of hazards could lead to the laser producing a dangerous beam, the laser itself would burn itself up. If you use a laser capable of producing 5 mW before burning out, you'd be wise to treat it with a proportional level of respect.