In the beginning, we measured the optomechanical transfer function of the CC loop with Stanford Research 560 at the beginning. However, since this lock is not so stable, the measurement was very noisy.

Then Pierre put an integrator before 10kHz. We could lock loop with high gain. However, this is a small stable region. If we increase or decrease gain, we can both have oscillation. Anyway, we can lock in that small region. Then we measured both open loop transfer function and optomechanical transfer function.

The result is shown in the attached figure. There are several peaks in the low-frequency region.