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Message ID: 16128     Entry time: Mon May 10 10:57:54 2021
Author: Anchal, Paco 
Type: Summary 
Category: Calibration 
Subject: Using ALS beatnote for calibration, test 

Test details:

  • We locked both arms and opened the shutter for Yend green laser.
  • After toggling the shutter on.off, we got a TEM00 mode of green laser locked to YARM.
  • We then cleared the phase Y history by clicking "CLEAR PHASE Y HISTROY" on C1LSC_ALS.adl (opened from sitemap > ALS > ALS).
  • We sent excitation signal at ITMY_LSC_EXC using awggui at 43Hz, 77Hz and 57Hz.
  • We measured the power spectrum and coherence of C1:ALS-BEATY_FINE_PHASE_OUT_HZ_DQ and C1:SUS-ITMY_LSC_OUT_DQ.
  • The BEATY_FINE_PHASE_OUT_HZ is already calibrated in Hz. This we assume is done by multip[lying the VCO slope in Hz/cts to the error signal of the digital PLL loop that tracks the phase of beatnote.
  • We calibrated C1:SUS-ITMY_LSC_OUT_DQ by multiplying with
    \large 3 \times \frac{2.44 \, nm/cts}{f^2} \times \frac{c}{1064\,nm \times 37.79\, m} = \frac{54.77}{f^2} kHz/cts where f is in Hz.
    The 2.44/f2 nm/cts is taken from 13984.
  • We added the calibration as Poles/zeros option in diaggui using gain=54.577e3 and poles as "0, 0".
  • We found that ITMY_LSC_OUT_DQ calibration matches well at 57Hz but overshoots (80 vs 40) at 43 Hz and undershoots (50 vs 80) at 77Hz.


  • If we had DRFPMI locked, we could have used the beatnote spectrum as independent measurement of arm lengths to calibrate the interferometer output.
  • We can also use the beatnote to confirm or correct the ITM actuator calibrations. Maybe shape is not exactly 1/f2 unless we did something wrong here or the PLL bandwidth is too short.
Attachment 1: BeatY_ITMY_CalibrationAt57Hz.pdf  10 kB  Uploaded Mon May 10 12:13:41 2021  | Hide | Hide all
BeatY_ITMY_CalibrationAt57Hz.pdf BeatY_ITMY_CalibrationAt57Hz.pdf
Attachment 2: BeatY_ITMY_CalibrationAt43Hz.pdf  10 kB  Uploaded Mon May 10 12:13:50 2021  | Hide | Hide all
BeatY_ITMY_CalibrationAt43Hz.pdf BeatY_ITMY_CalibrationAt43Hz.pdf
Attachment 3: BeatY_ITMY_CalibrationAt77Hz.pdf  10 kB  Uploaded Mon May 10 12:14:01 2021  | Hide | Hide all
BeatY_ITMY_CalibrationAt77Hz.pdf BeatY_ITMY_CalibrationAt77Hz.pdf
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