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Entry  Fri May 6 14:20:43 2016, Gabriele, Electronics, Design, Design of optical lever electronics  qpd.PDFBreakout.PDF
    Reply  Sun May 8 13:35:04 2016, rana, Electronics, Design, Design of optical lever electronics  
       Reply  Tue May 10 13:32:17 2016, Gabriele, Electronics, Design, Design of optical lever electronics  
Message ID: 13     Entry time: Tue May 10 13:32:17 2016     In reply to: 12
Author: Gabriele 
Type: Electronics 
Category: Design 
Subject: Design of optical lever electronics  

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.

Quote:

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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