One of possibilties why SRCL build up is lower than our expected is detuned from the resonance point. Actually, When DRMI was locked, RF51 I was not zero even though SRCL error signal by 1f was zero (fig.1). This offset is often caused by RFAM. So I checked current RFAM status of each demodulated signal from each REFL RFPD when the single bounced beam from PRM is on each PD.

Fig.2 shows that REFL demodulated signals and DC powers on REFL PDs. As you can see, RF56, RF45 seems to have some RFAMs. On the other hands, 3f PDs seems to be zero. Therefore, The offset on SRCL error signal cannot be explained by RFAM.