(Sorry for the very late report)
Hirose, Tanaka, (supported by Yuzurihara-san and AGU members about TCam)
### Abstract
We mapped the beam spots on ITM and measure the offset in REFL WFS signals on Aug. 28th. We confirmed that there are some offsets in WFS signals and the offset amounts vary where the beam is on ITM (fig.2 and 3). However, the power of transmitted light and infected QPD decreased simultaneously. The beam entering the X-arm may be clipped on the way, and it is difficult to determine if this offset is due to birefringence. Also, since the error in identifying the beam position by T-cam is large, it may be necessary to measure the map more precisely.
### What we did
- We moved the beam spot on the ITM with 1 mm spacing, 20 mm × 20 mm vertically and horizontally, using the soft mode of the cavity axis late at night on August 28. Fig. 1 is the schematic view of the beam shift. We also engaged ADS to IMMT2 and PR2 so that the incident beam axis was aligned with the cavity axis.
- Figure 2 and 3 show the REFL WFS signals (K1:ASC-REFL_QPDA{1,2}_RF{17, 45}_I_PIT_OUT_DQ) plotted at the beam position obtained using the T-cam. The horizontal and vertical axis shows the beam position on ITM in horizontal and vertical direction respectively, and the color bar shows the WFS outputs.
- As you can see, WFS output has some offsets, and the offset amounts vary as the beam position on ITM.
- On the other hand, Fig. 4 shows a time trend during the measurement. The orange line shows the transmitted power of the X-arm, and the Green and Red lines show the SUM value of REFL RFQPD1 and RFQPD2. other lines show TM angles of ITMX and ETMX in PIT and YAW directions. When PIT angles became larger than the original angles where each beam spot on ITMX and ETMX is each center, the transmitted power and QPD sum values seem to be lower than the original values. Therefore, when the vertical beam position is lower than the center of ITM, the beam injected X-arm may be clipped. So, the WFS offset We cannot determine at this time whether the offset originates from clipping or birefringence. We need more investigation.
### Note
- The script for this measurement is in "/users/Commissioning/scripts/asc/Xarm/measure_WFS_offsetmap.py"
- These plots are in "/users/Commissioning/data/ASC/WFS/birefringence/archives/20220828/2309"
