[Nagano, Yamamoto, Nakano]

The concave mirror was fine aligned so that the input beam to the reference cavity does not jit. The DC QPD was put about 2m after from AOM.
The procedure of alignment was as below

1. make reference.
To make sure that the laser reflect nomaly on concave mirror, the power after the double path AOM was measured and align the concave mirror to make power maximum.  Align the cavity with transmitted power. Then zeros the QPD output with steering mirror before QPD. We used the QPD and cavity as an alignment reference.

2.optimize the position
Inject the signal (2Vpp, 5Hz, 7.5V offset) and excited the beam jitter. With measuring the spectrum of yaw motion with diaggui, optimized the position of the concave mirror with a micrometer attached stage. The spectrum after optimization was attached. The beam jitter at 5 Hz was dumped and 10Hz was excited, that means the alignment is at around the best position.