I measured CCFC error signal with 20% pick off. The CCFC calibration amplitude is 452mVpp. Fig 1 shows the CCFC error signal with different demodulation phases. The CCFC error signal agrees well with theory, but the CC detuning changed by 20 Hz from the previous measurement. This means that the filter cavity length changed. The CC PLL frequency can be written as follows:

CC PLL frequency = 14*FSR + CC detuning

From this formula, the CC detuning change of 20 Hz corresponds to the FSR change of 1.4 Hz and the filter cavity length change of delta L = delta FSR /FSR * L = 0.8 mm. We need to tune the CC PLL frequency.

Note that I fixed the mode mismatch between OPO/FC to 6% in the calculation.

Then I locked CCFC. The filter setting is gain of 10000 and LPF of 0.03Hz. The CCFC can lock only for a few minutes due to the CC1 saturation.

Fig 2 shows the IR locking accuracy with/without CCFC. Now the IR locking accuracy with CCFC is 1.2 Hz and the high frequency noise shape looks similar to the best locking accuracy we obtained on 20201211. I compared the IR locking accuracy on 20210517 with the one on 20210514. The difference of these is whether the laser is kept on for more than one day or not. It seems that keeping laser on makes the high frequency noise better.