To compare the IR locking accuracy (CCFC error signal) for different FC green gain, I compared the 1/f^4 and 1/f filters of FC green lock. The 1/f^4 filter has much larger gain than the 1/f filter at low frequency. The nominal filter is 1/f^4 and the nominal gain is 1.5. For the 1/f filter, I used the FC gain of 5 to have the UGF of 14kHz. Fig 1 shows FC OLTF for 1/f^4 and 1/f filters. Both filters have UGF of 14kHz.

Fig 2 shows the FC green error signal (EPS1, 1Vpp fixed range) with 1/f^4 and 1/f filters. The FC green error signal with 1/f^4 filter at low frequency is below the dark noise and limited by the spectrum analyzer noise. This means that increasing the green gain just reinjects the dark noise.

Note that BS pointing and AA were engaged during the measurement, but Z correction was not engaged due to the problem. I found that 1/f filter with gain 5 is stable even without Z correction, but 1/f^4 filter is unstable without Z correction.

Then I aligned BAB to FC. The optimal p pol PLL frequency without green was 275MHz and BAB power before FC was 466uW. The maximum IR transmission of FC was 490.

Then I checked the CCFC error signal. I optimized the p pol PLL frequency to maximize the CCFC error signal. The optimal p pol PLL frequency with 20mW green was 230MHz and the CCFC error signal was 122mVpp.

Fig 3 shows the CCFC error signal with 1/f^4 and 1/f filters. We can see that increasing the green gain improves the IR locking accuracy above 10Hz, but not below 10Hz. The IR locking accuracy is dominated by the FC length noise below 10Hz and the laser noise above 10 Hz. By increasing the green gain, the laser noise can be reduced because the laser noise is common for both IR and green, but the FC length noise cannot be reduced because the FC length noise is different for IR and green.