The measured RIN is measured and converted to frequency noise via photo thermal effect then compared to beat. The effect seems to be significant now since we lost the common mode rejection.
I measured RIN after ACAV (there is only one PD behind ACAV right now. we will add another one for RCAV soon). The magnitude is comparable from what we measured before but the peaks seem to change, see PSLPSL:1326, :PSL:1308, (8"cavity) PSL:742 .
The peaks around kHz are more clear. I'm not sure where they are from, but I think it is associated with vibration on mirror mounts that causes beam jitter. Because the peaks look like mechanical peaks, and this time the cavities are shorter, the beamsize is smaller from 8" cavities, the same beam misalignment will cause the power coupled into the cavities to change more compared to that of 8" cavity. We can check that by mis-aligning the input beam a bit, and see if RIN becomes larger or not.
The coupling from RIN to frequency noise is discussed in PSL:1328
I applied that to the measurement and here is the result. Note, only the effect from one cavity (ACAV) is taken into account.
The peaks seems to match up, especially around 20-30Hz and around 1kHz, see the zoomed in picture below. This makes me think that we might be limited by RIN noise now.
To Do next:
- Install ISS system on RCAV (PD behind the cavity / EOAM)
- Re-measure the coupling from RIN to frequency noise of the cavity
- Measure RIN and apply it to the noise budget.
- Find out what causes RIN to change