[Pierre, Matteo L., Yuhang, Eleonora]

In the past days we have tried to lock the cavity using the new servo, with an increased bandwidth. The differences between the old and the new servo are reported in entry #736.

Unfortunatley we didn't succeed in improving the lock perfomances.

We menaged to lock the cavity but the error signal shows a strong oscillation and the transmitted power is much lower than the peak observed during a scan of the cavity:

- If we increase the gain the situation doesn't change (see video here) and the UFG which has been found at about 4 kHz doesn't increase.

- If we reduce the gain, the trasmitted power increases slightly but at some point the lock becomes less stable. (see video here)

We suspected the the loop instability can be caused by the piezo resonances, which according to Matteo L's experience (entry #737) cannot be modeled as a simple pole and could have a stonger impact on the loop stability after the servo modification.  

In order to better characterize such resonances we tried to measure the openloop TF (with the old servo) at higher frequency.

We found in TAMA a network analyzer which goes from 10 kHz to 150 MHz (see second pdf attached) and performed the measurement up to 200 kHz with a swept sine.

The results are plotted in the attached picture (top plot). Since we were not able to extract the data from the instrument, we had no other option than to extract them from the picture (bottom plot).

These data and the matlab script to plot them can be downloaded from this link.

https://drive.google.com/drive/folders/1HrR6Bq3lwTRyxw1Q6pXoPU0TxDqfFtRs?usp=sharing

We will try to fit them and see if it is possible to extract usefull information about the piezo resonances.