Re: Screwy laser diode with monitor photodiode

redbelly wrote:
(snip)
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


I haven't used vcsel's, just edge emitters. But I have never trusted
the internal photodiode on those. Any reflections that feed back to
the diode will also affect the internal photodiode.

If your setup is such that there are no reflections coming back to the
diode, then I don't know what else would cause this. I'm not sure if
vcsel's would behave any better or worse than edge emitters.

Almost all VCSELs have multiple transverse modes. I saw one that ran in an almost pure 7th order mode--12 off-axis peaks in the angular spectrum, and a nice null in the centre. Multiple modes will give you funny speckly patterns that are extremely noisy--often as much as 20 dB noisier than the total integrated output, due to competition between modes.

Getting the monitor PD to track the output accurately will be very difficult with a multimode device.

Cheers,

Phil Hobbs
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