Mainly two mechanisms cause zero baseline drift (ZBD): the birefringence of electro-optic crystal and the etalon effect formed by two parallel end facets of the EOM crystal [Z. Li, et al., Optics Letters, 41, 14, 2016]. The reduction of birefringence effect can be done by controlling modulation voltage or the crystal's temperature [K. Kokeyama, et al., J. Opt. Soc. Am. A 31, 81 (2014)].

To monitor ZBD, we can lock cavity on anti-resonance. In this situation, the PDH signal is very much insensitive to phase change. Therefore, the signal magnitude change comes mainly from sideband amplitude change, usually addressed as residual amplitude modulation (RAM). This amount of RAM exists all the time, including the situation when cavity is on resonance. The control loop makes error signal to be zero with such RAM present, which makes the cavity not exactly on resonance. Or we can say RAM introducing detuning for cavity. This is the effect we are interested in. Notably, this effect may appear also for the CCFC control loop, which should be investigated.

We used the method described in the last paragraph to monitor ZBD. Since this effect exists when cavity is on resonance, we use the slope of PDH signal at resonance to calibrate ZBD. In this way, we have the detuning influence cause by ZBD for detuning change. The attached figure (is not yet calibrated), after calibration, shows a ~4Hz detuning change around zero.