We made a mistake by choosing the input power to the cavities to be 0.25 mW, so today I turned them back to 1mW and measure the beat.

Setup:

• input power: 1mW
• TTFSS gain (C/F): RCAV (760/980), ACAV (630/650)

Note about the measurement:

• The noise at error point from ACAV is pretty high (~up to 100 nV/rtHz around 6 kHz). Better characterization will be done later to see if the suppression is enough or not. I made sure that this measurement is good up to ~2kHz, (this was done by changing the gain level a bit and beat level did not change).
• Table was floated, but the air springs was not activated. I hope we will get better signal around 20-100 Hz once the air spring is re-installed.
•  Intensity noise->photothermal around a few hundred mHz, cause the PLL to drift away from the input range. This effect becomes worse when the input power is increased.
• RIN from ACAV is about a factor of 10 higher than that of RCAV.
• I measured the beat with ISS on/off on ACAV, nothing were significantly different. So maybe it is not a problem for now.

To do next:

• I'll compare this measurement with the one from 8" cavity to see if the results agree or not. They are different mirrors, but from the same coating run. I suspect that the loss in these mirrors are higher than what we estimate (Ta2O5=2.5e-4, SiO2 1e-4).
• Think about the error in the measurement (calibration, spotsize etc).