Some pictures for better understanding

The first picture shows the shape of the green beam when the cavity is locked at the right temperature.

For the second picture, I tried to scan the cavity with pretty small amplitude triangle wave, and then change the PZT driver by hand to get near to the resonance.

From the picture, there are two problems:

The first one is the what I described before, the slopes from two side of the peak is different, take the first peak as an example, the reduce process is faster than the increase part.

The second is that the two different peaks are actually the same mode but they did not look the same. I discussed with Manuel and we thought it maybe because of the hysteresis, there are some loss in the PZT.