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Entry  Mon Mar 25 22:56:42 2013, Evan, DailyProgress, Electronics Equipment, Frequency readout noise of PDH loop res_and_pdh.pdferrsig_smooth_derivative.pdferrsig_voltage_noise.pdferrsig_freq_noise.pdf
    Reply  Tue Mar 26 10:00:19 2013, Evan, DailyProgress, Electronics Equipment, Frequency readout noise of PDH loop 
    Reply  Thu Mar 28 12:36:27 2013, Zach, DailyProgress, Electronics Equipment, Frequency readout noise of PDH loop 
       Reply  Sat Mar 30 01:42:16 2013, tara, DailyProgress, Electronics Equipment, Frequency readout noise of PDH loop 
Message ID: 1133     Entry time: Sat Mar 30 01:42:16 2013     In reply to: 1132
Author: tara 
Type: DailyProgress 
Category: Electronics Equipment 
Subject: Frequency readout noise of PDH loop 

It is indeed really high. We expect something in the order of ten nV/rtHz level (like what we had before, see PSL:781.). We are investigating it.

One of the possible causes is the back reflected beam. When I reduced the power input from 6mW to 1mW (with common/fast gain = 999/720, boost on), the error noise was down to ~ 10nV/rtHz up to 5 kHz, with bumps and peaks around 50kHz up to 100kHz. Our mode matching this time is not very good.

 I replaced the PBS for PDH locking to a bigger one, since the beam spot at that position was quite large. This got rid off some high frequency peaks and bumps. I still have to align the beam to minimize back reflection.

 

Quote:

Is this really the input noise of the PDH servo (i.e., output noise / servo TF)? If so, it seems pretty high. With the right components, you should to be able to do ~100x better.

Quote:

Tara was able to tame the cavity servo loop. The third attachment is the result of several SR785 measurements of the error signal power spectrum. I converted this to a frequency noise power spectrum (fourth attachment) by extracting the voltage/frequency calibration factor from the error signal as follows.

 

 

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