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Entry  Thu Sep 5 10:52:27 2013, Evan, DailyProgress, ISS, Simple ISS with SR560 ctn_rin_clopen.pdfdata.zip
    Reply  Sat Sep 7 12:38:49 2013, Evan, DailyProgress, ISS, Simple ISS with SR560 - frequency noise coupling tf_fit.pdfrin_freq_est.pdf
Message ID: 1326     Entry time: Thu Sep 5 10:52:27 2013     Reply to this: 1328
Author: Evan 
Type: DailyProgress 
Category: ISS 
Subject: Simple ISS with SR560 

At Tara's suggestion, I implemented a simple ISS by feeding the output of the PDA10CS into an SR560 (ac-coupled) with some gain, and then the output of the SR560 into the EOAM.

I found that by turning on a 6 dB, 30 kHz low-pass filter on the SR560, I could put the gain at 500 without saturating the SR560 output. No inversion is necessary because positive voltage on the EOAM decreases the beam power (so there is already a minus sign in the loop).

I monitored the RIN by feeding the output of the PDA10CS into the SR785. The in-loop RIN is suppressed by a factor of 6 or so. Once Chas's board is here, the suppression should be much greater (since the gain will be 106 at low frequencies).

The shape of the RIN spectrum has changed compared to the previous RIN measurement. The 2 kHz peak is gone, and the shelf from 100 Hz to 1 kHz has dropped. Maybe it's because Tara has damped a lot of the mechanical resonances of the table optics with rubber stoppers. The low-frequency RIN remains at a few times 10−4/rtHz. According to Tara, this is probably induced by seismic coupling (not by fluctuations from the laser), and so the right way to make it go away is to float the table.

There is a minor mystery here. Based on the previous RIN measurement, I expect the dark noise of the PDA10CS to be at 7×10−7/rtHz or so abve 1 kHz. Why have I apparently been able to measure below this noise floor in the attached plot?

Attachment 1: ctn_rin_clopen.pdf  214 kB  | Hide | Hide all
Attachment 2: data.zip  38 kB
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