KAGRA Logbook

Nakano, Akutsu

While locking the green laser to the X arm cavity, we measured the separation angle of a ghost beam with respect to the main optical path in the EXT chamber. The green ghost beam should be created at the wedge of the ETMX AR. The method is simple and the same as the one described 4520, 4523, 4525 already done at Yend; measure the separation of the spot by the ghos beamt with respect to that by main beam, and do it again at the different postion, and calculate the separation yaw angle.

At the back of BRT2 lens, the ghost beam was shifed with respect to the main beam by about 9.5 mm in the -Y direction (the coordinates is KAGRA global). See Figs 1, 2, and 3. Qualitatively, the displacement in the -Y direction is consistent with the direction of the thicker side of the wedge; see here.
At the front of the steering mirror (STM3), the shift was about 13 mm. See Figs 4, 5, 6, and 7.
The two locations separated by 5x25=125mm, while the ghost beam was shifted 13-9.5 = 3.5 mm. So, the separation angle is 3.5/125 = 0.028 rad.
The BRT is a telescope with magnification of about 10 times, so the original ghost beam entering the BRT would be about 0.028/10 = 0.0028 rad separated with respect to the main beam.
In the same manner shown in 4520; generally speaking, the separation angle of a ghost beam, which is created by a wedged back surface with a wedge angle of w and goes through a medium with the index of refractance n, is 2nw. Substituting Sapphire's n=1.76 for 532nm, and considering the 2nw = 0.0028, the corresponding w is 0.0008 rad or 0.0456 deg. Also note that the measurement error for each number would be +/- 1mm order.
According to Hirose-san (on Oct 12, 2018), the measured wedge angle of the ETMX is 0.047 deg, so the number above would be well consistent, and the ghost beam should be due to the wedge.

At the same time, it seems the beam height just after BRT2 lens is around 165 mm (see Fig 2 and 6) , while the nominal design is 166mm.

By the way, I found that the green spots on most of the optics were about on center today! See those Figs 8-12, and compare the situation in the past reported in 6921... Does anybody have any idea regarding this?