Today we used the oscilloscope to monitor the homodyne SUB-DC channel. We can see clearly that the unlock of the IR phase part is related to the jump of the homodyne signal. This jump will rise up the whole spectrum level. The video can be checked from the following link. (blue line: IR coherent control error signal. yellow line: homodyne sub dc signal)

https://drive.google.com/open?id=1RpOabqKvQRVOS4uYonU8hqClGpDq8HDu

We can see from this video that every time homodyne signal jumps when the IR coherent control error signal (blue line) becomes thin.

The thickness of the thin line is comparable with the situation when there is no coherent control signal. So I checked the PLL and the lock of OPO. All of them are fine. The strange thing is this line doesn't drift away but get thinner.