craig.cahillane@LIGO.ORG - posted 17:33, Friday 30 May 2025 - last comment - 18:13, Friday 30 May 2025(84676)
Small change in REFL A LF RIN during mod depth changes
Mr Todd, Craig Sheila reminded us about measuring mode matching using intensity noise: alog 70985. We were wondering about the assumption that the RF sidebands are small, to see if we can explain the noise lump around the arm pole at 45 Hz. We looked at the latest mod depth up down test at around GPS 1426014687. We compared the REFL A LF RIN noise at a nominal mod depth time, a 9 MHz 3dB low time, and a 9 MHz 3dB high time. We do tend to see a difference in the REFL A LF RIN noise between these times. The noise seems elevated when the 9 MHz is lower. The difference seems smaller between nominal and 9 MHz higher. I don't believe it's from a change in the raw power on the photodiode, which went from 7.25 mW to 7.16 mW, which is around a 1.5% change in power. Mr Todd will make a better plot of this change in noise.
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I'm attaching several figures here:
1. Transfer Function of Input RIN to REFL_A RIN
2. Transfer Function of Input RIN to REFL_A_9I_RIN
3. Transfer Function of Input RIN to POP_A_RIN
^These were calculated from the ISS injections I did in March
4. ASD of LSC_REFL_A_RIN during 9MHz mod-depth changes
5. ASD of LSC_REFL_A_RIN during 45MHz mod-depth changes
As Craig says, it seems the lowering mod-depth periods have a more dramatic effect on the RIN at REFL_A/B than the increased mod-depth. The increased mod-depth 45MHz period shows a slightly larger effect from nominal compared to the 9MHz increased mod-depth.
From our estimation of the DC power on the diodes, this is probably not a shot-noise limit thing, and is most likely something else.
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