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Entry  Fri Aug 28 23:33:38 2020, gautam, Update, BHD, Some more hardware changes 
    Reply  Sat Aug 29 22:46:29 2020, gautam, Update, BHD, New homodyne-phase control electronics zetaDrive.pdftrekTFs.pdf
       Reply  Wed Sep 2 00:49:47 2020, gautam, Update, BHD, Some notes about homodyne phase heterodyneMICH.pdfunwrappedPhase.pdfunwrappedPhase_zoom.pdfphaseNoisePSD.pdf
          Reply  Thu Sep 3 15:55:04 2020, gautam, Update, BHD, Phase drift between LO and IFO after fiber replacement LOphaseDrift.pdfphaseDrift_tempCorr.png
Message ID: 15553     Entry time: Wed Sep 2 00:49:47 2020     In reply to: 15549     Reply to this: 15555
Author: gautam 
Type: Update 
Category: BHD 
Subject: Some notes about homodyne phase 

Summary:

Using a heterodyne measurement setup to track both quadratures, I estimated the relative phase fluctuation between the LO field and the interferometer output field. It may be that a single PZT to control the homodyne phase provides insufficient actuation range. I'll also need to think about a good sensing scheme for controlling the homodyne phase, given that it goes through ~3 fringes/sec - I didn't have any success with the double demodulation scheme in my (admittedly limited) trials.

For everything in this elog, the system under study was a simple Michelson (PRM, SRM and ETMs misaligned) locked on the dark fringe.

Details:

​This work was mainly motivated by my observation of rapid fringing on the BHD photodiodes with MICH locked on the dark fringe. The seismic-y appearance of these fringes reminded me that there are two tip-tilt suspensions (SR2, SR3), one SOS (SRM) + various steering optics on seismic stacks (6+ steering mirrors) between the dark port of the beamsplitter and the AS table, where the BHD readout resides. These suspensions modulate the phase of the output field of course. So even though the Michelson phase is tightly controlled by our LSC feedback loop, the field seen by the homodyne readout has additional phase noise due to these optics (this will be a problem for in vacuum BHD too, the question is whether we have sufficient actuator range to compensate).

To get a feel for how much relative phase noise there is between the LO field and the interferometer output field (this is the metric of interest), I decided to set up a heterodyne readout so that I can simultaneously monitor two orthogonal quadratures.

  • The idea is that with the Michelson locked, there is no DC carrier field from the interferometer.
  • The field incident on the DCPD from the interferometer should be dominated by the 55 MHz sideband transmitted to the dark port given the Schnupp asymmetry. 
  • The LO field is picked off before any RF sidebands are added to it (the PMC modulation sideband should be suppressed by the cavity transmission).
  • Therefore, the LO field should be dominantly at the carrier frequency.
  • By placing a broadband RFPD (PDA10CF) in place of one of the DCPDs, I can demodulate the optical beat between this 55 MHz sideband, which shares the same output path to the location of the DCPD as the audio-frequency sidebands on the carrier from the dark Michelson, to estimate the relative phase noise between the LO and IFO output fields.
  • The point is that with the heterodyne readout, I can track the fringe wrapping, which is not an option for the BHD readout with two DCPDs, and uncontrolled LO phase.

Attachment #1 shows the detailed measurement setup. I hijacked the ADC channels normally used by the DCPDs (along with the front-end whitening) to record these time-series.

Attachments #2, #3 shows the results in the time domain. The demodulated signal isn't very strong despite my pre-amplification of the PDA10CF output by a ZFL-500-HLN, but I think for the purposes of this measurement, there is sufficient SNR.

This would suggest that there are pretty huge (~200um) relative phase excursions between the LO and IFO fields. I suppose, over minutes, it is reasonable that the fiber length changes by 100um or so? If true, we'd need some actuator that has much more range to control the homodyne phasethan the single PZT we have available right now. Maybe some kind of thermal actuator on the fiber length? If there is some pre-packaged product available, that'd be best, making one from scratch may be a whole project in itself. Attachment #3 is just a zoomed-in version of the time series, showing the fringing more clearly.

Attachment #4 has the same information as Attachment #2, except it is in the frequency domain. The FFT length was 30 seconds. The features between ~1-3 Hz support my hypothesis that the SR2/SR3 suspensions are a dominant source of relative phase noise between LO and IFO fields at those frequencies. I guess we could infer something about the acoustic pickup in the fibers from the other peaks.

Attachment 1: heterodyneMICH.pdf  109 kB  Uploaded Wed Sep 2 02:07:13 2020  | Hide | Hide all
heterodyneMICH.pdf
Attachment 2: unwrappedPhase.pdf  167 kB  Uploaded Wed Sep 2 02:07:26 2020  | Hide | Hide all
unwrappedPhase.pdf
Attachment 3: unwrappedPhase_zoom.pdf  204 kB  Uploaded Wed Sep 2 02:08:21 2020  | Hide | Hide all
unwrappedPhase_zoom.pdf
Attachment 4: phaseNoisePSD.pdf  153 kB  Uploaded Wed Sep 2 02:08:49 2020  | Hide | Hide all
phaseNoisePSD.pdf
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