Aritomi, Yaochin and Yuhang

To see the effect of better isolation on phase noise, we measured the CC loop phase noise just before and after the replacement of the Faraday isolator.

The result is shown in the attached figure 1. From this measurement, we could see the CC2 loop phase noise is reduced by more than a factor of 2.

In entry 1432, we measured once the phase noise and PD dark noise. From that entry, we could that dark noise is very close to the phase noise level in the kHz region. We want to confirm the situation, so we did the measurement of dark noise and shot noise again. The dark noise is measured when there is no light arriving at PD, all the others are the same(including demodulator, RF amplifier). The shot noise is measured when there is light arriving at PD, but we made sure OPO is locked at the same time. Because if we don't lock OPO, the light arriving CC1 PD has some frequency component. Also, all the others are the same setting for the measurement of shot noise(including demodulator and RF amplifier).

The result of CC1 phase noise and dark/shot noise comparison is attached in the second figure. We could see that this time, phase noise is at least 5 times higher than dark/shot noise above 300Hz. I checked the code I wrote for entry 1432's measurement, and I found the measurement of dark noise is consistent with the nowadays measurement. This proves that the CC1 phase noise is larger than before. Why the phase noise is higher now is still unknown.