Participant: Marco, Yuhang and Eleonora

During the last few days, we installed the RF photodiode for mode cleaner reflection. We achieved the lock and measured the mechanical TF.

For the power supply of RF PD, we used a T to seperate the power from CMS 100-05.

For coupling the beam into the PD, we put a 75.6mm lens before PD. In order not to saturate the PD we put a OD 2 to attenuated power. After the OD, we got power around 200uW. Now, we can get the sideband information(78MHz) and create the error signal.

The demodulation frequency(78MHz) of mode cleaner is the same with the demodulation of filter cavity. The demodulation signal(LO) is provided by DDS2 board. We also maximize the error signal by select the best phase (175 degree).

We use SR560 for a temporarily lock. We set a first order pole at 3Hz with a gain of 10. Then we measured the mechanical transfer function of green mode cleaner (cavity + piezo). It is attached as Fig 1. As expected, it is flat at low frequecy. The structures due to the piezo resonances start from about 7 kHz.