Participaint: Eleonora and Yuhang
Following the procedure of Marco, we measured the phase noise again. The difference is now rampeauto doesn't have ramp-in port and one of the resistors is changed.
Before, we found the error noise spectrum is limited by the laser frequency noise after change.
The measurement of phase is performed in three different cases: rampeauto off, rampeauto on and lock on.
The result is shown in attached figure. However, we found the phase noise level is comparable with before while locking.
I'm wondering if the increase of the phase noise at high frequency when the cavity is locked is due to the fact that, when the cavity is locked, the frequency changes of the master laser are very large ~ MHz.
Possible tests to check this hypothesis are to damp the mirrors (sending the PZT correction signal to the mirrors, upon filtering) or to excite the mirror oscillations, to artificially increase the laser frequency changes.
A related question: when we compute the residual RMS phase noise between the main laser and the auxiliary laser we integrate down to 100 Hz. Maybe the 1 Hz region is dominant with respect to the high frequency region, and thus we should solve in any case this problem of the main laser frequency changes in the Hz region.