Matteo and Yuhang

The suppression of filter cavity length noise provides stable detuning, which is vital for the production of frequency dependent squeezing. The CCFC control loop is designed to achieve this goal.

To understand better how CCFC control works, several characterization works have been done recently. They are listed as follows:

1. Figure 1 shows many length error signals and noise curves. The addition of CCFC error signal introduces length noise for GR loop at low frequency. This is validated by figure 3 and 4. The GR+IR error signal doesn't change because the filter cavity length change doesn't change.

2. Figure 2 shows correction signals. For the correction signals send to the main laser or end mirror, they are the same whether there is CCFC or not. This is consistent with the unchange of IR+GR error signal.

3. Figures 3 and 4 show FC GR TRA/REF DC spectrums. CCFC causes the GR length noise increase, which translates into intensity noise.