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Entry  Thu Aug 1 21:19:57 2013, tara, DailyProgress, BEAT, searching for beat 
    Reply  Fri Aug 2 12:18:16 2013, tara, DailyProgress, BEAT, searching for beat beat_2012_08_02.JPGnb_short_cav.png
       Reply  Fri Aug 16 22:05:27 2013, tara, DailyProgress, BEAT, noise hunting nb_short_cav.pngnb_short_cav.figbeat_psd.jpg
          Reply  Tue Aug 20 20:10:01 2013, tara, DailyProgress, BEAT, noise hunting beat_2013_08_20.png
       Reply  Thu Jun 26 17:10:09 2014, Evan, DailyProgress, BEAT, Beat 
Message ID: 1307     Entry time: Tue Aug 20 20:10:01 2013     In reply to: 1305
Author: tara 
Type: DailyProgress 
Category: BEAT 
Subject: noise hunting 

Noise hunting is in progress. Today I identified that scattered light from the window is one of the problem.

I spent sometime making sure that all the beams in the input optic and the beat areas were dumped properly. I also tightened all the screws on the optics and the mounts on the table.

I mentioned in the previous entry that for RCAV, the reflected beams from the cavity and the vacuum window overlapped a little bit. The window beam was much smaller and actually closer to the edge of the main beam, so I used an iris to remove the outer path, and let only the beam in the center area go through to the RFPD. With that I could increase the gain in RCAV loop to Common/Fast = 624/750, where they used to be ~ 600/600 before. The iris might introduce some extra scattered lights, since it clips a part of the beam.

The scattered noise around DC to 100 Hz is reduced a bit, see the below figure. However, not much improvement in the flat region (100Hz and above). Plus, some mechanical peaks around 1kHz appear with higher level than before.


I expected the scattered noise will be even lower if the cavities are tilted a bit to avoid the beams overlapping. At higher frequency, it might be the gain limit from RCAV loop where the modulation depth is very small.

Next thing to do is to increase more power in the modulation depth for RCAV.


I found out that the sidebands in the beat signal mentioned in the previous entry changed with the gain of the TTFSS (both ACAV and RCAV). With higher gain, the sidebands are suppressed more. It might have to do with the PZT resonant of the NPRO. 

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