[Nakano, Kasuya, Enomoto]

Dirty error signal of RefCav was due to interference effect which was most likely caused by birefrengence of AOM crystal (we had injected circular polarized light into AOM, this AOM is used as a double path AOM).
We swapped the order of QWP and AOM so that we can inject linear polarized light into the AOM. Then we optimized the angle of QWP by minimizing the amplitude of the interference; at the optimal angle, the transmittance of PBS was maximum. 
Currently this problem was solved. Inject linear polarized light into AOM!!