khanh.vu@LIGO.ORG - posted 14:32, Tuesday 04 August 2026 - last comment - 09:38, Wednesday 05 August 2026(91386)
IOT1 Table Work
Jennie Wright, Masayuki Nakano, Khanh Vu This morning we worked on several tasks on the IOT1 table, including installing the camera and shutter, profiling the beam, and calibrating the DC power. We identified a new location for the camera using the beam transmitted through JACR_M5. During this process, Masayuki noticed that the beam was being clipped by the shutter. We suspect that the beam may have been clipped for some time. We then installed the camera in its new location, and Masayuki aligned the shutter on the table. Next, we profiled the beam for the wavefront sensors. We found that the Gouy phase separation between the two WFSs is approximately 70 degrees. We decided to leave the current configuration as it is since the separation is good enough. Masayuki will make a plot and perform a more detailed calculation later. We also maximized the laser power in the REFL path by optimizing the waveplate angle. When JAC is unlocked, the measured power on the RFPD is 5.1 mW, and the trigger PD voltage is 0.28 V. When JAC is locked, the measured power decreases to 0.4 mW, and the trigger PD voltage is 0.02 V. Since the beam is split evenly, each WFS receives approximately 2.55 mW of optical power. Finally, we calibrated the DC readout of RFPD by converting counts to mW. Before performing the calibration, Masayuki checked the alignment and recentered the RFPD. He then recorded two sets of measurements, each averaged over 10 seconds: Measurement #1 * JAC_REFL_A_LF_INMON: 1072.3966186523437 counts * DC power: 4.4 mW Measurement #2 * JAC_REFL_A_LF_INMON: 1073.5228637 counts * DC power: 4.5 mW After the calibration, we updated filter number 10 with the new coefficients.
The beam profile between the beamsplitter and the JAC WFS was measured. Here I fit a Gaussian beam to those measured beam sizes and convert the WFS locations into Gouy phase.
Fit
The measured beam diameters were fit independently in x and y with the standard Gaussian beam model, w(z) = w0 * sqrt(1 + ((z - z0)/zR)^2) with zR = pi * w0^2 / lambda and lambda = 1064 nm:
The beam is slightly astigmatic, so x and y are treated separately throughout.
WFS positions and Gouy phase
The positions of WFS A and WFS B were measured with a ruler from the same reference as the profile scan: WFS A at z = 0.325 m, WFS B at z = 0.410 m. The corresponding Gouy phases, psi(z) = arctan((z - z0)/zR), are:
Assessment
The separation is 71 deg in x and 64 deg in y, not the optimal 90 deg. This is not optimal, but it is not terrible either: the two WFS remain well separated in Gouy phase and the sensing matrix would not be close to degenerate. Given the time available we did not optimize the layout.
If we want to optimize it later, the fix is straightforward: moving WFS A upstream (toward the BS) by about 5 cm in x / 7 cm in y, i.e. from z = 0.325 m to roughly z = 0.27 m, brings the separation to 90 deg. WFS B does not need to move.