KAGRA Logbook

MIF (General)

matteo.leonardi - 18:37 Wednesday 16 March 2022 (20103)

Attempt to measure single bounce from ITMX at POP

[Matteo, Haoyu]

We tried to measure the beam shape of the single bounce reflection from ITMX at POP. The beam shape was ugly and most definetely not gaussian. Initially we suspected some clipping at the level of POP but we found none. We measured the beam before the periscope but it was still ugly. A location where clipping might happen is at the gate valve between ITMX and BS. We might need to check the situation after this gate valve is open (July?).

We also rotated the half waveplate which controls the splitting ratio before s and p pol cameras. The idea was to minimize the power on p pol camera. This is to ensure that the image collected at POP reflects the conditions at ITMX. After rotation (which indeed changed not only the total power but also the shape of the s and p pol images), we increased the exposure of p pol cam (K1:CAM-POP_P_EXP). New value is 34310.

Tomorrow we will try again to measure the beam shape in s and p to compare it with simulations.

Comments to this report:

Haoyu Wang - 21:42 Wednesday 16 March 2022 (20106)

The attached pictures show the position and shape of the beam measured before the POP periscope.

Images attached to this comment

Haoyu Wang - 22:53 Thursday 17 March 2022 (20140)

After yesterday's earthquake, IR beam is realigned. We remeasured the beam shape at POP. The beam is measured before the periscope, same as yesterday. The beam shape looks better than yesterday. But there seems to be still some clipping effect.

We then replaced the CCD for s-pol with the beam profiler and measured the shape of s-pol beam. The p-pol beam was also measured by rotating the HWP and minimizing the beam power. See attached pictures. The measured p-pol beam shape shows some overlap with our single bounce simulation.

Images attached to this comment

tomotada.akutsu - 23:43 Thursday 17 March 2022 (20142)

You may also measure some beam profiles around IMMT1 and IMMT2 instead of the POP-forward beam. Anyway, nice(?) similarity with the simulation and the actual measurements.

Haoyu Wang - 15:39 Tuesday 22 March 2022 (20183)

Hi Akutsu-san,

We think the input beam seems in a good shape: klog #20128. We measured the POP forward beam size beccause we want to order a lens for matching two QPDs at POP forward.

The measured p-pol shape from ITMX seems similar to the simulation by constructing birefringence maps with TWE maps. To confirm this, we would like to measure beam shapes when the spot is on different positions on ITMX, as well as measurements for ITMY.