After the implementation of CC1 mirror mount, we used the 1/f integrator and locked this loop(with unity gain frequency of ~2.2kHz). We measured open loop transfer function and the noise spectrum of the error signal. The result is as follows.

But the measurement of the noise spectrum when the turbopump was on/off was not so reasonable.

We decide to characterize this loop again. Including the measurement of free-running noise, noise while locking, PD dark noise and also the optomechanical transfer function.