[miyoki, kokeyama, uchiyama]

Nakano- kun found again a strong scattering in REFL camera. So we checked in the REFL table. We douted the clipping at the HWP again because the beam size at the present HWP almost fits the size of the HWP aperture.

 

1) We found that a fixing ring for a half wave plate just after the periscope was "burnt" and melted as the first attached image0-1. Especially, the slight transparent area shape is same shape on the REFL camera in the control room. This ring is made of plastic. We guessed this burning ocuured by strong IR power. We replaced this HWP and the rotating optical part with newone.

In addition to this, we found a strong scattering on the surface of a beam splitter just after the HWP and PBS plate set.There seemed to be several pont scattering sources on the surface by using IR viewer. We wiped it by aceton and ethanol. Point like scattering souce vanished, however, we still saw smoky image where the IR hit. So we changed this BS itself with a new one. However, we saw the same smoky image. We concluded that this is due to quality of athis BS. However, we never see such smoky image on for example lens. So we should select better BS in the future.

In the same manner, the two mirrors had the possibility to be smoked by somke from HWP. So we also wiped their surface by using aceton and ethanol. We just believed they are cleaned now. Also we found a smoky coating on the optical table surface around the HWP. When I wiped them by ethanol, they were removed. So, this is also because of HWP burning. According to my memory, this smoky coating on the surface did exist in 2019. I and Miyakawa-kun discussed the possibility about this burning started from the high power injection test in FPMI configuration in 2019.

I forgot to check the surface of PBS plate near the HWP. Sorry, we should check its surface condition again.

Kokeyama-san, took the relation btw lasser power and counts in the digital system in the PDA1.

PDA2 DC output seemed to be broken. No signal. However, RF signals are alive and they used for IFO.

In principle, we should clean everything in the REFL table again in future.

2) Position change of HWP and PBS set

Because the size of the beam almost fit with the HWP aperture size, we decided to change the position of the HWP and PBS as the attached image 3 and 4. Image 4 showed the original position of HWP and PBS set. At present there are space, except for a large iris that was added. The image 3 showd the present position. We put an iris not to let the beam hit the plastic ring in the HWP in front of HWP. At this position, the beam size is about 1/3 of the HWP aperture. The PD1 position was also reset because of this HWP position change.

3) ASC optical path was also rearranged because of the change of HWP, PBS after the periscope by Kokeyama-san.