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Entry  Mon Mar 29 21:43:27 2010, Alberto, Configuration, PSL, Reference Cavity PD Noise Spectrum 2010-03-29_FSS_PD_shotnoise_and_darknoise.png2010-03-29_FSS_PD_shotnoise_and_darknoise.png
    Reply  Tue Mar 30 06:37:32 2010, rana, Configuration, PSL, Reference Cavity PD Noise Spectrum rc.png
       Reply  Wed Mar 31 15:31:53 2010, steve, Update, PSL, Reference Cavity RF PD base upgraded brspdbs.JPG
    Reply  Sat Apr 3 11:35:47 2010, rana, Configuration, PSL, Reference Cavity PD Noise Spectrum 
       Reply  Sat Apr 3 16:07:40 2010, Alberto, Configuration, PSL, Reference Cavity PD Noise Spectrum 2010-03-29_FSS_PD_shotnoise_and_darknoise.png
          Reply  Tue Apr 6 11:27:11 2010, Alberto, Update, Computer Scripts / Programs, Data formats in the Agilent AG4395a Spectrum Analyzer 
             Reply  Tue Apr 6 16:55:28 2010, Alberto, Update, Computer Scripts / Programs, Data formats in the Agilent AG4395a Spectrum Analyzer 
Message ID: 2742     Entry time: Wed Mar 31 15:31:53 2010     In reply to: 2733
Author: steve 
Type: Update 
Category: PSL 
Subject: Reference Cavity RF PD base upgraded 

Quote:

Some more words about the RFAM: I noticed that there was an excess RFAM by unlocking the RC and just looking at the RF out with the 50 Ohm input of the scope. It was ~100 mVp-p! In the end our method to minimize the AM was not so sensible - we aligned the waveplate before the EOM so as to minimize the p-pol light transmitted by the PBS cube just ahead of the AOM. At first, this did not minimize the RFAM. But after I got angry at the bad plastic mounting of the EOM and re-aligned it, the AM seemed to be small with the polarization aligned to the cube. It was too small to measure on the scope and on the spectrum analyzer, the peak was hopping around by ~10-20 dB on a few second timescale. Further reduction would require some kind of active temperature stabilization of the EOM housing (maybe a good SURF project!).

For the EOM mount we (meaning Steve) should replace the lame 2-post system that's in there with one of the mounts of the type that is used in the Mach-Zucker EOMs. I think we have spare in the cabinet next to one of the arms.

After the RFAM monkeying, I aligned the beam to the RC using the standard, 2-mirror, beam-walking approach. You can see from the attached plot that the transmission went up by ~20% ! And the reflection went down by ~30%. I doubt that I have developed any new alignment technique beyond what Yoichi and I already did last time. Most likely there was some beam shape corruption in the EOM, or the RFAM was causing us to lock far off the fringe. Now the reflected beam from the reference cavity is a nice donut shape and we could even make it better by doing some mode matching! This finally solves the eternal mystery of the bad REFL beam (or at least sweeps it under the rug).

At the end, I also fixed the alignment of the RFPD. It should be set so the incident angle of the beam is ~20-40 deg, but it was instead set to be near normal incidence ?! Its also on flimsy plastic legs. Steve, can you please replace this with the new brass ones?

 Teflon feet removed and heavy brass-delrin pd base installed. Ref-cavity reflected light remains to be beautiful doughnut shape on camera.

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