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