[Miyamoto, Akutsu]

We have changed the configuration of oplev for EYA from the original single arm one to the folded arm one. Decoulping of longitudinal DoF from angular DoFs is expected with the folded arm oplev.

The reason is as follows: the MI lost its lock because of tidal motion during the first test run of iKAGRA. Now the actuator range got improved, but if the mirror can follow the tidal motion, the amount of coupling from the length to the angular motions would be non-negligible. So the length coupling needed to be reduced.

First, the QPD assy was replaced by a mirror assy. The mirror is a cheap and usual one with aluminum deposited for visible wavelength; note that the light source of the oplevs have the wavelength of 670 nm, which is visible in red for human eyes. The the mirror was aligned so that the beam came back to the other side where the light source was. Then the QPD assy was put there. Before putting the QPD assy, we need to somehow clear and open a space fot the QPD on the optical table on the pillar. Then, using screws of the mirror holder for the stirring mirror, the beam was roughly brought to the QPD, and then more precise centering of the beam was done with micrometers on the QPD assy.

And then, as the arm length elongated, the spot size on the QPD increased. In order to reduce the spot size, we excahnged the collimator from LPC to LPF3500, and performed calibration again. Miyamoto-kun will report the detail later, hopefully.