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I have harvested several laser diodes out of old 16X to 22X DVD writers. I used a driver circuit based on LM 317 but I was not able to reach the lasing current of the diode. I do not have the specs sheet for any diode. I did experiment with various resistors to control the laser current but with no luck.

How can I experimentally determine threshold current of laser diode and the operating voltage of a unknown laser diode?

Laser driver circuit

This driver is the one that I used.

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  • \$\begingroup\$ please draw a proper schematic. The picture you've pasted is bright and colorful, but a schematic would actually have component names and would allow us to discuss your design problem. There's no substitute for a good schematic. Note: the question editor has a built-in schematic editor, so you can use that! \$\endgroup\$
    – Marcus Müller
    Jan 6, 2017 at 18:58
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    \$\begingroup\$ Your circuit produces up to 250 mA (limited by the 5 ohm minimum resistance between OUT and ADJ on the LM317). If this is not enough to turn on the laser, then for one thing you probably want to heat sink the laser well or it may not work for more than a couple milliseconds before cooking itself. You might also have damaged the laser when de-assembling it from where you got it --- these types of lasers are pretty fussy about ESD, etc. \$\endgroup\$
    – The Photon
    Jan 6, 2017 at 19:07
  • \$\begingroup\$ @ThePhoton Thank you, comment deleted. As constant current source, this circuit looks OK (except possibly for omitted input capacitor). Am unsure if laser diode is properly connected - these packages sometimes contain two diodes, one is a laser diode, while the other is a photodiode for feedback control. \$\endgroup\$
    – glen_geek
    Jan 6, 2017 at 19:08
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    \$\begingroup\$ anyone got any idea of why the 1N4001 is in there? From my perspective, that is always reverse-biased far below its breakthrough voltage, so it effectively has no effect (aside from a minimal junction of maybe 15pF or so), and the electrolytic cap certainly is not what I'd want in a constant-current supply, especially if I should be doing closed-loop current control based on the photodiode integrated in the package. Anyway, 20s of googling revealed that you'd expect a DVD laser diode to be in the range of 20-40 mA, not 250 mA (if @ThePhoton's calculations are correct). \$\endgroup\$
    – Marcus Müller
    Jan 6, 2017 at 19:17
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    \$\begingroup\$ Thank you guys. Please forget about the specific circuit and address the main question How to determine experimentally the threshold current of an unknown laser diode? \$\endgroup\$
    – Mohammad Al-Turkistany
    Jan 6, 2017 at 19:44

1 Answer 1

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Please forget about the specific circuit and address the main question How to determine experimentally the threshold current of an unknown laser diode?

  1. Get the datasheet. If that's not possible, get the datasheet of a "similar" part. This will at least give you a starting point for the device characteristics.

  2. Figure out which pins are the anode and cathode of the laser diode. This will be easier if you have completed step 1 successfully.

  3. Connect the laser to a variable current supply. Ramp up the current slowly until it turns on. Knowing what supply to use and what range to adjust it over will be easier if you have completed step 1 successfully.

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  • \$\begingroup\$ I can not afford variable current power supply. \$\endgroup\$
    – Mohammad Al-Turkistany
    Jan 6, 2017 at 19:50
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    \$\begingroup\$ The LM317 circuit you posted is a variable current supply, and probably costs $0.35. \$\endgroup\$
    – The Photon
    Jan 6, 2017 at 20:16
  • \$\begingroup\$ Can you suggest a better circuit using LM 317? The one in the post is not working for me? \$\endgroup\$
    – Mohammad Al-Turkistany
    Jan 6, 2017 at 20:22
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    \$\begingroup\$ If the circuit doesn't work, and Marcus is right about the required current, then probably you're connecting to the wrong pins of your laser, or you've blown up the laser already. \$\endgroup\$
    – The Photon
    Jan 6, 2017 at 21:21
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    \$\begingroup\$ @MohammadAl-Turkistany The LM317 circuit should work, however MarcusMuller has correctly pointed out that the 10 uF capacitor does not belong in the circuit. If you activate this current source before connecting the diode, it will dump a large current surge into the diode, easily causing damage. \$\endgroup\$
    – glen_geek
    Jan 6, 2017 at 22:11

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