[Aritomi, Yuhang]
Carrier and CC AOM frequency are as follows.
We changed a cable length for CCFC LO and checked the CCFC error signal. The result is as follows (Pic.1,2,3 are 0,2,4 m cable length).
| CCFC LO cable length | phase |
| 0 m | 0 deg (I phase) |
| 2 m | 90 deg (Q phase) |
| 4 m | 180 deg |
Then we added the CCFC I phase error signal to perturb of green FC servo. We used SR560 with lowpass filter 0.1 Hz and gain 200 before injecting the perturb. Then CCFC stably locked!
When SR560 gain is 1000, it oscillates with 88Hz (Pic. 4).
We measured CCFC phase noise. For CCFC calibration, we used Pic. 5. Since AOM scan speed is 1600 Hz/s, the calibration factor (Hz/V) is 1600 Hz/s*70 ms/40 mV = 2800 Hz/V. Measured CCFC phase noise is shown in Pic. 6. Note that free run CCFC error signal is out of linear range and the spectrum could be wrong. I also compared with IR locking accuracy we usually use (Pic. 7).
We measured the ratio of CCFC OLTF and green OLTF with SR560 (Pic. 8). The crossover frequency is 40 Hz.
When the CCFC demodulation phase is optimal, CCSB frequency separation can be obtained from the distance of two dips in Q phase signal since the two dips correspond to CCSB resonance (Pic. 9). From Pic. 2, time difference of the two dips is about 120 ms and therefore frequency separation of CCSB is 120 ms*1600 Hz/s = 192 Hz. This is a bit larger than optimal value 108Hz. Note that when the CCFC demodulation phase is not optimal, it will also change the distance of the two dips.
Just a reminder, sometimes we still suffer from the unlock of CC1 loop. This is mainly due to the saturation of CC1 correction signal. We need to investigate how to improve this.
I attached OLTF of CCFC and green lock. Note that I flipped the sign of measured data to match the measurement and theory. The measured phase is not consistent with theory.