As we talked in the meeting, I did the measurement of beam reflection jitter by using QPD.

I did measurement first time just after the meeting, I measured the GR reflection beam jitter by QPD at yaw and pitch direction. I tried my best to center this beam and made the measurement. I also measured the beam jitter difference when the filter cavity is locked and unlocked. The result is attached in figure 1.

I did measurement again before leaving. This time I just measured when the filter cavity is unlocked. The comparison is in attached figure 2.

conclusion:

1. The lock of the filter cavity can reduce this beam jettering above ~10Hz. We know the correction signal we are sending to Main laser PZT is limited by laser noise above 10Hz. So I guess the lock of the filter cavity reduces the power coupling from higher-order modes to TEM00. And at low frequency, this is limited by the suspension. So if we engage the filter cavity length control, we may solve a bit this beam jittering problem. We should try to see this effect.

2. From the first plot, It seems pitch and yaw have different peaks. Yaw is worse at a higher frequency while the pitch is worse at a lower frequency.

3. From the second plot, it seems there is no obvious difference between 6 pm and 9.5 pm of beam jittering. I will check the situation in the morning and just after lunch.