After a very useful discussion with Rich this morning, I think the circuit based on aLIGO optical levers design should be good for our applications.
It uses a LT1125 as input stage, which has
- (maximum) current noise of 1e-12 A/rHz @ 100 Hz and 0.4e-12 A/rHz @ 1 kHz
- (maximum) voltage noise of 4e-9 V/rHz @ >100 Hz
We expect to send about 5 mW into the disk, getting back a 4% reflection, which would correspond to 200 uW on the QPD. Let's say we lose half of this power in reflections through viewports and such, so we have a total of 100 uW on the QPD, or 25 uW on each quadrant. From the QPD datasheet the repsonse is about 0.4 A/W, so we have a photocurrent of 10 uA. The corresponding shot noise limit is about 1.8e-12 A/rHz.
Using a transimpendace of 200k, the noise at the output of the transimpendance is
- shot noise = 3.6e-7 V/rHz
- voltage noise = 4e-9 V/rHz
- current noise = 2e-7 V/rHz @ 100 Hz, 0.8e-7 V/rHz @ 1 kHz
So in the worst case the current noise will be about half of the shot noise. This seems good enough.
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Koji, Rich, and I recently came up with a new QPD design which is better for general lab use than the aLIGO ones (which have a high-noise preamp copied from iLIGO).
https://dcc.ligo.org/LIGO-D1500467
This page has the mechanical drawing only, but perhaps Rich can tell us if he's ready to make the first version for you or not. I think you can get by with the old design, but this new one should be lower noise for low light levels.
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