Today, I tried to lock OPO but not successful. While checking OPO error signal, I found the error signal didn't seem to be reasonable. By adjusting only the phase of channel 3 (LO for OPO PDH demodulation), I found the optimal phase changed from 135deg to 205deg. This change of 70deg probably comes from the 90 phase change of DDS, combined with the original phase was not optimized.

Attached figure shows the PDH signal when new optimal phase is set.