[Fujimoto, Takano]

Summary

We noticed that the previous locking point of SRMI was not as intended, somewhere very weird. Finally, we found the good locking point, where MICH is at the dark fringe and the f1 sidebands are on resonance in the SRC.

Detail

Once we thought that we could lock SRMI (see here), but after further discussion, we concluded that this locking point was not as expected, because the AS34 buildup was -0.04, whereas we observed it can reach ~ 0.9. After several trials and errors, we found out the optimal conditions for locking it at the point we wanted: MICH is at the dark fringe and the f1 sidebands are on resonance in SRC (Fig. 3). We implemented the appropriate servo settings in the VERTEX guardian. We could lock SRMI by requesting it. The measured open loop transfer functions are as attached (Fig. 1 and Fig. 2).

During the trial, we noticed that PR3 often moved quite largely, sometimes more than 10 urad. It looked like big glitches pushed it. These glitches sometimes unlocked the interferometers during the initial alignment (Fig. 4).