Ushiba, Dan Chen, Tanaka, Uchiyama, Aso

Summary

Today, we tried to measure the transmissivity of the OMC. It was close to 1, but the precise measurement was not possible due to the large accoustic distrubances. We will perform more precise measurements once the OMC is in vacuum.

What we did

• Locked the X-arm.
• Adjusted the steering mirrors to center the beam on AS-PDA1.
• Measured the beam power in the chamber:
  • Before OSTM: 28.6mW
  • OSTM trans.: 0.256mW
  • Calculated input power to OMC: 28.6-0.256 = 28.3mW
  • Recorded the average values of the following channels before and after measuring the power:
    • K1:LAS-POW_IMC_OUT16: 1.207 cnt
    • K1:LSC-REFL_PDA1_DC_OUT16: 0.09027 cnt
    • K1:LSC-AS_PDA1_DC_OUT16: 0.0453 cnt
    • K1:LSC-AS_PDA2_DC_OUT16: 0.0208 cnt
    • K1:OMC-QPDV1_SUM_OUT16: 4968 cnt
    • K1:OMC-QPDV2_SUM_OUT16: 4525 cnt
  • Calculated the conversion factors from the readings of the above channels to the input power to the OMC:
    • K1:LAS-POW_IMC_OUT16: 23.45 mW/cnt
    • K1:LSC-REFL_PDA1_DC_OUT16: 313.5 mW/cnt
    • K1:LSC-AS_PDA1_DC_OUT16: 624.7 mW/cnt
    • K1:LSC-AS_PDA2_DC_OUT16: 1360 mW/cnt
    • K1:OMC-QPDV1_SUM_OUT16: 5.696e-3 mW/cnt
    • K1:OMC-QPDV2_SUM_OUT16: 6.254e-3 mW/cnt
• To measure the OMC transmission with reduced environmental noise:
  • Closed the side hatch of the OMC chamber.
  • Turned off the FFUs.
• Activated the OMC QPD centering servo.
• Locked the OMC to TEM00 mode using the OMC LSC Guardian:
  • Despite turning off the FFU, acoustic disturbances were still significant, causing instability in the lock.
  • The OMC DCPD SUM showed large fluctuations, likely due to the cavity length changes caused by acoustic noise.
  • From the time series, the average OMC DCPD SUM is estimated to be around 27mW.
  • From the calibration factors above, the OMC input power was 28.5mW
  • Then the transmissivity of the OMC is calculated to be ~95%

Discussion

• The peak transmitted power exceeded the input power to the OMC. This may be caused by the in-proper whitening/dewhitening for high-frequency large amplitude signals from the DCPDs.
• Since the OMC lock was not stable, an accurate transmission measurement cannot be performed now.
• Still, the transmission does not appear to be significantly poor.
• Thus, we decided to postpone the OMC scan until after the vacuum pumping and concluded today's verification.

To Do

• Measure the OMC transmission again after the OMC chamber has been evacuated.