Screwy laser diode with monitor photodiode

I am working on an instrument that uses a VCSEL light source for some
optical measurements. The current instrument has some problems with
the light output drifting with temperature. In order to correct this I
have been looking at using a VCSEL with a monitor photodiode to control
the light output. Presumably if I keep the signal at the monitor
photodiode constant, the output should be constant.

My problem is that as I change the current in the VCSEL the light
output as seen by an external photodiode does not track the light
output of the monitor photodiode packaged with the VCSEL. I have tried
several different devices, and they all have this problem, though
different devices have a different shaped relation between them, so
this is not a fixed transfer function that I can calibrate out. Since
the light out does not seem to track the monitor signal, if I control
the VCSEL current to keep the monitor current constant, the light
output is not nesicarily going to stay constant.

Does anybody have any suggestions as to what might be causing this? Is
there some mechanism for the monitor diode to pick up energy from the
VCSEL that is not from the light output?

My other thought is that it might be speckle on the two photodiodes
causing the inconsistancy. However this seems unlikely, since I think
the size of the photodiode is large relative to the size of a speck,
(what is signular for speckle?) that this would average out to
something insignificant.


Thanks for any suggestions.

Ethan Petersen

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Relevant Pages

  • Re: Screwy laser diode with monitor photodiode
    ... Presumably if I keep the signal at the monitor ... photodiode constant, ... My problem is that as I change the current in the VCSEL the light ... My other thought is that it might be speckle on the two photodiodes ...
    (sci.electronics.design)
  • Re: Screwy laser diode with monitor photodiode
    ... Presumably if I keep the signal at the monitor ... photodiode constant, ... My problem is that as I change the current in the VCSEL the light ... Ethan Petersen ...
    (sci.electronics.design)