We measured the transfer function from the ISS out-of-loop PD to DARM displacement without ISS.
And we projected to the DARM displacement.
[miyoki post miyakawa-kun's mail via gokagra ]
As Kuromiya-kun reported in and Nakano-kun reported in free run intensity noise increases with laser power drastically. Comparing both results above, Kuromiya-kun’s results seems to correspond to 20A and 30A on Nakano-kun’s results. Did you try integrator on ISS? It reduce the intensity noise below 4kHz. You said boost was tried, so probably it means the integrator. UGH of ISS was set to 46kHz or something. It can be increased a bit, probably 1.5times. Another idea is to reduce the laser source power, for example to 30A (22W), and maximize output of PSL.
I measured the common mode rejection ratio of the interferometer to intensity noise.
CMRR is roughly 1/17 @ 100 Hz.
Method:
1. Measured the transfer function from K1:PSL-ISS_FIRST_SERVO_PDA_INF_OUT_DQ to K1:OMC-TRANS_DC_SUM_OUT_DQ while ISS is not on (the same timing as Yokozawa-san's measurement in klog #12983; 15-Feb-2020 at 18:30 UTC).
2. Dividing the transfer function by 8 (8 mW offset in TRANS_DC_SUM) will give you the transfer function from RIN of IFO injected beam to RIN of OMC DC PD. (Actually, K1:PSL-ISS_FIRST_SERVO_PDA_RIN_OUT_DQ is the RIN channel, but INF channel is almost the same since normalization is done by 0.995 or so, and RIN channel was not DQ-ed at this time).
Result:
Attached. Measurement below ~100 Hz is not very reliable due to low coherence (shaded region shows the error).
Discussion:
Although there's a finesse asymmetry between two arms, more common mode rejection is expected at high frequencies considering CARM cavity pole.
Flat RIN coupling at high frequencies suggests that intensity noise coupling is mostly from MICH contrast defect.
Next:
- Measure CMRR for frequency noise (CARM coupling is already measured (klog #12998), so what we need is a calibration of CARM error signal to Hz).
- Compare them with the simulation in JGW-T1910352
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.
