Laser pointer (Red) - expected diode lifetime?

[0-8-15 User]

I disassembled my Hama LP-6 mini laser pointer to get to the laser diode and the driver.

• How high can I go with the input voltage? (The driver starts working at 2.3 V and draws about 14.5 mA. Increasing the voltage to 3.1 V increases the current consumption, to a little over 15 mA, and the output power to a small extent.)
• What lifetime can I expect from the diode (I have read everything from 2000 - 50.000 hours)?
• Can I pulse the output or will it hurt the lifetime?

[magic]

How high can I go with the input voltage? (The driver starts working at 2.3 V and draws about 14.5 mA. Increasing the voltage to 3.1 V increases the current consumption, to a little over 15 mA, and the output power to a small extent.)

Up to the breakdown voltage of the regulator.

I have seen one which should withstand tens of volts in theory (not tested) but other circuits may exist and you provided no means of identification

[james_s]

Driven within the specified limits a laser diode can last many tens of thousands of hours. Driven beyond these limits it may not last tens of nanoseconds. There are different types of damage that can occur, the one you have to be most careful about is catastrophic optical damage, even a very brief excursion into this area will cause immediate damage to the mirrored surfaces of the die due to heating. Old laser diodes had a very narrow window between where lasing starts and damage occurs, these had an integrated photodiode for closed loop regulation. Many modern laser diodes have a much wider operating range and many are driven open loop. You can pulse a laser diode but you need to avoid overshoot.

[0-8-15 User]

I have added pictures of the driver board. Perhaps you can help me understand how it works and what implications this has on my original questions.

The laser diode itself has three pins, does that mean that it is one of the old ones that has an integrated photodiode?

[james_s]

That looks like it's a closed loop diode with a photodiode but it's hard to tell. Some laser diodes have 3 pins with only two of them used.

[janoc]

You likely want to read the relevant sections of this:

http://www.repairfaq.org/sam/laserfaq.htm

[magic]

Definitely looks like optical regulation. My bets:

The switch disconnects the negative supply.
The laser pin near Q1 is the positive.
The laser pin near R2 is the feedback photodiode. When the laser is bright enough that photodiode current reaches some 0.1mA the transistor Q1 turns on (it has to be NPN) and steals current coming from R1 to the negative.
The reminder of R1 current somehow gets to the other side of the board and somehow ends up driving another NPN which sinks current through the third pin of the laser, which actually powers the light.

It will probably be good for a few volts. The transistors are likely some type that would withstand more than 20V, but at some point you may run into power dissipation limits.

If you want more details or higher confidence trace the exact connections and post the schematic.

[0-8-15 User]

You can pulse a laser diode but you need to avoid overshoot.

I'm using a 2.5V LDO Regulator (MCP1702-2502E) driven directly off an AVR microcontroller 5V pin, with a 1 µF capacitor on the output side.

It seems to work nicely for what I intend to use it for (the monitoring of an analog water meter). I currently run the laser at a frequency of 100 Hz and measure the intensity of the reflection with a BPW 34 photodiode attached to a very basic transimpedance amplifier. The pulsing is to compensate for fluctuations in ambient light to a certain extent.

Update:

• I just noticed that the start-up delay is caused by the MCP1702 and not by the laser diode driver. Replacing it with a NPN transistor (BC548B) powered by a 3.3V rail has fixed it.
• The two screenshots (1st MCP1702, 2nd BC548B) each show the power rail of the laser driver (pink line) and the ADC measurement activity (green line).
• I increased the frequency to 141 Hz. The third screenshot now also shows the output of the transimpedance amplifier (blue line).

[0-8-15 User]

I now run the laser with a pulse frequency of 1 kHz and am absolutely delighted with how well it works.

[SilverSolder]

I now run the laser with a pulse frequency of 1 kHz and am absolutely delighted with how well it works.

This sounds like a cool project.  What does the analog device you are "shooting" at look like?

[0-8-15 User]

I have attached a photo of it. The laser is aimed at the little black wheel.

[SilverSolder]

I have attached a photo of it. The laser is aimed at the little black wheel.

Ah, very cool, almost like it was designed for the purpose! 

[0-8-15 User]

I can only hope that the laser will live longer than a few thousand hours. All in all, I spent more than a whole week making this project happen. If the laser module breaks down I will definitely have to buy a new one to make the enormous effort worthwhile. Maybe one of these: http://www.roithner-laser.com/pricelist.pdf#page=33

[jmelson]

• What lifetime can I expect from the diode (I have read everything from 2000 - 50.000 hours)?

These lasers ought to be similar to what is used in CD players.  I have worn out the motors in several CD players, the lasers just keep on working.

• Can I pulse the output or will it hurt the lifetime?

Not likely, as long as the turn-on/turn-off transient remains well-regulated.  I have a laser photoplotter with a 5 mW 670 nm laser diode, and it is pulsed up to
200 KHz writing the raster image onto film.  I've been using the same laser for over 20 years in the unit with no issues.  (But, I only use it a few times a year.)

Jon