[Shimasue, Takano, Michimura]
BS transmission at ~45 deg was measured to be:
50.8 +/- 0.9 % for s-pol
78.3 +/- 1.6 % for p-pol
They are consistent with the design (see, also, klog 29269 and JGW-T1503347)
What we did:
- Assembled a tripod setup for injecting the probe beam at IMC REFL area. We had a hard time collimating the beam for a few meters with the beam size smaller than the aperture of the power meter (Thorlabs).
- The injection bench consists from the laser source -> steering mirror -> Glan laser prism (rotated so that it transmits most of the beam, which was mostly s-pol) -> HWP -> f=-75 mm lens -> steering mirror -> Glan laser prism (rotated to define the polarization of the probe beam) -> f=300 mm lens (Attachment #1)
- Put the power meter head on a stick so that we can stick it inside the BS chamber (Attachment #2) to measure the power of transmitted beam. IR card was also put in front of the power meter head to find the beam.
- Brought these setup to the BS area, and put the tripod setup for injecting the probe beam between BS and PR2 chambers.
- The alignment of the incident beam, both in pitch and yaw was tuned by eye by centering the beam on BS.
- Measured the incident power at the injection bench, after the last lens (Attachment #3), and the transmitted power at the back of BS (Attachment #4) by holding the power meter on stick by hand. The stick was rested on a ground and/or some vacuum structures so that the head will be stable.
- The polarization of the probe beam was tuned by eye by rotating the second Glan laser prism (OptoSigma GLPB2-10-25.9SN-7/30). When the white line is vertical, it is s-pol and when horizontal, it is p-pol. The HWP was tuned to maximize the probe beam power.
Results:
- The table below summarizes the results
polarization | incident power | BS transmitted power |
s-pol | 44.98 +/- 0.75 mW | 22.87 +/- 0.18 mW |
p-pol | 42.44 +/- 0.54 mW | 33.21 +/- 0.52 mW |
- By calculating T = (BS transmitted power) / (incident power), BS transmission was estimated.
Discussions:
- The measured values are consistent with the coating design shown in JGW-T1503347.
- The incident angle was tuned within a few mrad (beam centering error of a few cm over injection bench to BS distance of 3.3 m).
- Mis-tuning of the incident angle by a few mrad can be converted into laser wavelength mis-tuning of a few 0.1 %.
- According to the coating design in JGW-T1503347, HR reflectivity and AR reflectivity does not depend very much on laser wavelength, and probably we will be still be limited by the uncertanties in the power measurement, even if we tune the incident angle more carefully.
Next steps:
- Measure reflectivity by sticking a power meter from the ICF203 flange between BS and ITMY.
- Re-do the measurements after aligning ITMs. Use ITM reflection to more carefully align the probe beam to the BS so that the incident angle will be the same with the main beam. (But this is probably uncesessary considering the uncertainties of the power measurement)
- Estimate systematic uncertainties from the polarization orientation of the incident beam, the incident angle etc.