about laser diode

Discussion in 'General Electronics Chat' started by CantataAndAria, Dec 4, 2014.

  1. CantataAndAria

    Thread Starter New Member

    Nov 14, 2014
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    on the datasheet, it says the Operating Current is 20-29mA, and Forward Voltage is 1.2-1.5V.
    But when I test on the circuit, when the Operating Current is between that range, the Forward Voltage is below 1.2V. When the Forward Voltage is between the range, the Operating Current is too high (~40-50mA).

    This is the Laser Driver circuit I designed:
    upload_2014-12-4_16-25-9.png
    during the test, there is some voltage drop on the BJT.
    The question is, I'm afraid the current through diode is too high and will damage the diode. But when the current is between the normal range, the laser diode spectrum is not perfect (linewidth is not narrow enough). Can anyone tell me how to solve this? Thanks!
     
  2. #12

    Expert

    Nov 30, 2010
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    With that circuit, the diode can never have 1.2 volts because the transistor is using up some of the 1.2 volts you supplied. And, that's not how you operate an LED. You just give it 20ma to 29ma with a constant current circuit and let it find its natural voltage. If its natural voltage is not in range, or its output is not up to the right quality, then you have a cheap or damaged laser diode.
     
  3. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    How stable is your supply voltage? Is it battery, or a regulated supply?
    If battery, can you buy a Zener, or a Voltage Reference IC?
     
  4. crutschow

    Expert

    Mar 14, 2008
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    As #12 stated, you need a constant current drive such as this, set to 20 to 29ma, for the diode, not constant voltage.
     
  5. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Also, the mosfet in your current mirror isn't really doing anything. A mosfet won't turn with a gate voltage only at 0.6 v above ground (clamped to that voltage by the emitter-base voltage on the NPN transistor.
     
  6. CantataAndAria

    Thread Starter New Member

    Nov 14, 2014
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    Hi everyone, I know there may be some problems of the driver circuit design, but the output current is in the range of Operating Current, so I just left it there.
    but here comes another problem. When I tested the circuit with the actual laser, the laser spectrum (multichannel) is not very perfect. I guess there may be some optical feedback in the system, and cause some noise.
    So my question is, is this because of the poor quality of laser, or is it because of the driver circuit design? I think the driver current just acts as a current source, it shouldn't influence the laser beam.....I don't know.....
    photo 2.JPG
     
  7. #12

    Expert

    Nov 30, 2010
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    My first instinct is that the supply has nothing to do with the center frequency or the bandwidth.

    However...if you would build a proper supply with capacitors across the laser diode to isolate it from stray inductance I would be sure. Local wisdom says to place a 10 uf capacitor of any kind in parallel with a 0.1 uf ceramic capacitor, place those physically close to the diode and electrically in parallel with it. A machine is only as good as the power it is receiving?
     
  8. ian field

    Distinguished Member

    Oct 27, 2012
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    The MOSFET part of the 'current mirror' won't conduct until VGS is a volt or few - the bipolar transistor only needs 0.7V to fully saturate - any light emitted is more likely to be incandescence than lasing.
     
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