Aritomi, Yaochin and Yuhang

After we feedback CC2 correction to INPUT mirror, we calculated the phase noise. The result is shown in entry 1719. But when we compare this result to entry 1614, we found after feed it back to INPUT, the acuumulated RMS is even higher.

Actually, if we look into the detail, the main difference comes from basically high frequency region (kHz to 100kHz). Personally, I have the impression the phase noise is different sometimes. For example,  in entry 1522, we reported phase noise reduce after long time laser on. But for example, today(we kept laser on also for several days), the phase noise is a bit higher and we couldn't even lock CC2 loop(we could lock one month ago).

So it is important to compare the high frequency region of phase noise with/without feedback to INPUT mirror. We did this measurement with filter cavity unlock. Because we have some issue with the CC2 loop error signal(Aritomi-san may report later), so the comparison is not calibrated to the unit of [rad].

The comparison is only in high frequency region because we could lock CC2 only for very short time when there is only feedback to CC2 phase shifter PZT. But at least from this measurement, high frequency is not affected whether we feedback to INPUT mirror or not.