We measured the optical gain (Dg) of reference cavity.

Sinusoidal signal at 35kHz was directly added to the laser's PZT, and then the 35kHz signal appeared in the error signal was observed.

Measured Dg was almost consistent with the value that was obtained by another method.

The amplitude of the signal added to PZT (Vin) and signal appeared in the error point were -38.681 dBVrms and -51.242 dBVrms, respectively.

The gain and phase of open-loop transfer function of the laser freq. stabilization were 1.83 and -149 deg. at 35 kHz.  =>  Consequently |1+Gop| = 1.108

Dg = |1+Gop| *(Vout / Vin) / Hpzt = 155 [mV/MHz], where Hpzt is voltage-to-frequency conversion efficiency of the laser's PZT and it was 1.68 [MHz/V].