Last night, we ran a quick TCS test where we attempted to minimize the intensity noise coupling to the DCPDs by changing the CO2 differential heating.
It seems that the following CO2 setting gives a much better intensity noise coupling when the PSL power is 25 W:
- CO2X = 460 mW
- CO2Y = 0 W
This did not improve the recycling gain so much. It seems to have increased by 2% only.
According to a past measurement with a lower power PSL of 2 W (alog 26264), a good differential CO2 power had been found to be P_{co2x} - P_{co2y} = 270 mW (or probably less than 270 mW because I did not explore the lower differential power).
This could be an indication that ITMY has a larger absorption for the 1064 nm light such that the differential self-heating linearly changes as a function of the PSL power. We should confirm this hypothesis using the HWS signals.
[The test]
No second or third loops engaged, DC readout, no SRC1 ASC loop.
- 3:34 UTC, full resonance with 2 W PSL
- 4:12 UTC, PSL --> 25 W
- 4:24 UTC, CO2Y 260 mW --> 0 mW
- 4:49 UTC, PSL --> 22 W for letting the 0.5 Hz instability go away
- 4:56 UTC, CO2X 480 mW --> 790 mW
- 5:12 UTC, lock loss
Drastic reduction of the intensity noise coupling was observed mostly between 4:24 and approximately 5:00, indicating that reducing the CO2Y power helped improved the coupling. After 5:00 UTC, we did not see a significant reduction. This may mean that we might have been already close to an optimum point where the coupling is minimized. The attached shows DARM spectra from various time during the test.
A broad peak at around 400 Hz is my intentional excitation to the first loop with band-passed gaussian noise in order to check the coupling to the DCPDs. As shown in the spectra, the reduction from the beginning to the end of the test is about a factor of 5. As reported in 27370, broad noise above 100 Hz up to several kHz is indeed intensity noise and therefore we see the noise floor in this frequency band decreasing too.
Because of the error in picomotor assignment, there's a good chance that the CO2Y laser is severly clipped and not properly aligned to the test mass. The result of this would be strong higher spatial order lensing (non-quadratic) on ITMY. We're certainly seeing an excess in lensing as measured with the HWS but the exact nature is unclear.
If there is signifiant higher order mode lensing then the best differential effect will be from having the CO2Y laser set to zero. However, this will not be the optimum lensing for the power recycling cavity.
So, poor CO2Y lensing is at least consistent with and a plausible explanation for requiring 0W on CO2Y to minimize intensity noise coupling while observing reduced PRC gain.
I looked back the intensity coupling of this particular day. See the attached.
The coupling at 100 Hz, even though the coherence is not high, seems to be too high by a factor of two or so comparing agaist the measurement from this February (25476). When dCO2 was sort of adjusted (red curve), the coupling at around 400 Hz and above seems comparable to what it was in this February.
