After the optimization of locking servo parameter, we locked PLL_CC. We are interested in this PLL because this phase noise is coupled into squeezing measurement directly. However, the other PLL phase noise changes the parametric amplification and de-amplification factor. How much can they affect squeezing needs to be considered further.

Anyway, the measurement result is attached. There is two main difference:

• The high-frequency PLL noise when the filter cavity is locked is lower
• The low-frequency PLL noise when the filter cavity is locked is higher

I think this is because we are making the main laser to follow filter cavity. Filter cavity suspension makes low frequency noisier while high frequency quieter.