Matteo and Yuhang

As reported in elog2300, we optimized mode matching and mixer. We obtained a larger CCFC error signal after that. Then we used it to lock the filter cavity length for IR. Control loop information is summarized as follows:

1. Gain of CCFC loop: 50
2. Corner frequency of CCFC loop: 30Hz (one order low pass)
3. Error signal shape: figure 1
4. Open loop transfer function (only CCFC part): figure 2 (40mVpk-pk excitation used)
5. Error signal spectrum (loop on/off): figure 3.
6. Calibration for error signal: AOM speed (4000Hz/2.5s)(figure 4) divided by error signal slope around zero (75mV/43.7ms)(figure 1 and 5) divided by 2 (AOM scan green to IR) : 4000*43.7/2.5/75/2 = 466 Hz/V

We could see that the CCFC method stabilized length noise for IR below ~1kHz. The IR length noise reached 2.3Hz after closing the CCFC loop. Compared with the old case, we could see that the main difference in IR length noise is around 1kHz~10kHz. The reason for this difference is still unknown. But if the CCFC error signal can go back to the old case, the CCFC loop can reduce IR length noise to less than 1Hz.