Nic, Evan

Summary

The frequency noise in CARM is limited in part by the dark noise of REFLAIR9I between 30 Hz and 1 kHz.

Details

First, an explanation of the plot traces:

1. The first trace gives the out-of-loop CARM noise in full lock as sensed by REFL9I, taken on 2015-06-14 06:16:10.
2. The second trace is the dark noise of REFL9I, taken a few days later when the IMC was down.
3. The third trace shows the dark noise of the in-loop sensor (REFLAIR9I), taken today (out of lock, REFL shutter closed). Notes on this:
   • We had to temporarily increase the whitening gain by 30 dB in order to overcome the ADC noise.
   • We have also scaled this noise by a constant factor in order to give the equivalent amount of noise that would be seen on the out-of-loop sensor. [The total scale factor is 10^(-30/20) / 0.0525 ≈ 0.60­, where 0.0525 ct/ct was the previously measured conversion factor from REFL9I to REFLAIR9I in full lock on 2015-06-14.]
   • The detailed shape of the CARM loop is not taken into account (see explanation below about our assumptions for CARM). So above a few kilohertz, where the CARM gain is only a factor of a few, this in-loop dark noise is probably not impressed very strongly onto the OOL sensor.

In the case that the CARM loop is sensing-noise limited, the dark noise (and the in-lock shot noise) of REFLAIR9I should appear on REFL9I. The CARM ugf is about 15 to 20 kHz, and is >30 dB below 1 kHz. Additionally, the white(ish) noise in REFL9I from 30 Hz to 1 kHz strongly suggests that we are seeing some kind of sensing noise. So we believe that the sensing noise from REFLAIR9I is indeed being impressed onto the CARM loop, and is the dominant contributor to the OOL noise between 30 Hz and 1 kHz.

We would like to additionally measure the shot noise as seen in REFLAIR9I (with no lock), to see how large it is relative to the REFLAIR9I dark noise. Either way, it is probably advantageous to switch control of CARM to REFL9I.