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MIF (Noise Budget)
yuta.michimura - 4:58 Wednesday 19 February 2020 (13028) Print this report
Comment to Intensity noise projection (12983)

We have remeasured the intensity noise coupling to DARM when PRFPMI is locked and ISS is turned off at 2020-02-19 18:10:50 UTC.
New coupling factor is incorporated in our sensitivity DTT (DARMsens.xml).
CMRR for the intensity noise during this measurement was about 1/55 at 100 Hz.

Method:
 1. Turned ISS off and measured the transfer function from K1:CAL-CS_PROC_DARM_DISPLACEMENT_DQ to K1:PSL-ISS_FIRST_SERVO_PDA_RIN_OUT_DQ (see this figure). The transfer function is calibrated to RIN to DARM coupling by inverting it and applying 5 zeros @ 10 Hz and 5 poles @ 1 Hz, gain of 1e-9 to account for the whitening filter and nm to m conversion (see this figure; error bar estimated from the coherence is shown with the shaded region). The measured coupling is eyeball fitted with gain of 1.1e-12 and zeros at 100 Hz, 2500 Hz. This zpk is used in the sensitivity DTT.

 2. Also measured the transfer function from K1:PSL-ISS_FIRST_SERVO_PDA_INF_OUT_DQ to K1:OMC-TRANS_DC_SUM_OUT_DQ to calculate CMRR to intensity noise with the same method described in klog #13008. The result is shown in this figure (error estimated from the coherence is shown with the shaded rigion).

Discussion:
 Compared with Yokozawa-san's measurement in klog #12983, the coupling is a factor of 2 smaller. A factor of 1.5 comes from a new DARM calibration factor (klog #12992). The rest is probably from better interferometer alignment. CMRR for intensity noise is also better than that measured in klog #13008.
 It seems like the intensity noise is not limiting the sensitivity below 1 kHz when the interferometer alignment is good.
 

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