khanh.vu@LIGO.ORG - posted 14:32, Tuesday 04 August 2026 - last comment - 00:43, Tuesday 11 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.
Summary
Today we installed the iris to block the ghost beam on the JAC REFL path with an iris, and to re-measure the beam profile with it in place. The ghost beam was produced by the laser window which picks off the partial power of the JAC reflection beam (~0.4%). Since this laser window doesn't have the wedge on it, the AR reflection is not well separated. We observed this interference during the original beam profile measurement in this thread.
And now, the iris dumps the ghost beam, and we made a new the beam profile measurement. I made a good JAC REFL optical model which obtained by the fitting the beam profile measurement. We will use this model for the WFS signal calibration.
Iris installation
An iris was placed on the REFL path, between the first pick-off mirror and the first lens. To position it, the beam profiler was set just after the beam shutter, and the iris was closed while watching the profile, until the ghost was blocked and the main beam was left untouched. Actually, Since the ghost beam is very close, the main beam is partially blocked as shown in the attached pics. We will see if it would have any effect on our WFS signals.
Beam profile measurement after the change
1/e2 diameters along the REFL path, with
JACR_MB4as the origin:On-table distances were also measured:
JACR_L1toJACR_MB4= 24",JACR_MB4to WFS A = 12.5",JACR_MB4to WFS B = 15.5".New propagation model
I made a model of the beam propagation of the JAC REFL path from PSL to JAC and IOT1. This model was fitted to the new profile, with the PMC waist as origin.
Taken as known. The positions of lenses in PSL (
IO_MB_L1/L2/L3)and of the JAC waist are the design values, i.e. the PSL bench to HAM1 relative distance is trusted.Taken as unknown. The design placed the IOT1 table only loosely, and the periscope that matches the HAM1 beam height to the table height was estimated roughly. The distance from the JAC input to the IOT1 table is therefore the principal free parameter, allowed ±30 cm; it enters the calculation as the position of
JACR_L1measured from the PMC waist. The profile measurement was referenced toJACR_MB4, which carries its own error, so theJACR_MB4-JACR_L1distance is a second free parameter.The profile carries astigmatism, so a yaw tilt was allowed on each of the four lenses (three on the PSL bench, one on IOT1). This is deliberately over-parameterised: the individual tilts should not be read as physical alignment errors. However, the aim of this analysis is not to measure how each lens sits but a model accurate enough for the following calculation. So as long as the aquired prameters are physically reasonable, we can use these numbers as the following calculations.
One note:
The HAM1 periscope (
JAC_M1/JAC_M2) rotates the beam 90 degrees about its axis, so the transverse planes swap on the way to JAC: bench x (YAW) descends from the upstream sagittal channel and bench y (PIT) from the tangential one. A tilt therefore gives astigmatism of opposite sign depending on which side of the periscope the lens sits. This is the reason why the x/y beam size flipped at periscope in the attached plot.Result
The fitted path from the JAC input to the IOT1 table comes out about 21.5 cm shorter than design, which is the scale of looseness that was expected there.
Astigmatism at the WFS planes
Expressed as the difference in accumulated Gouy phase between the two transverse axes:
Sanity check: mode matching into JAC
Taking the cavity eigenmode as the reference, the fitted injection-lens tilts imply a mismatch of 0.48 % (tangential) and 0.43 % (sagittal), 0.91 % combined. Small enough not to conflict with the measured mode matching (~1%).
Model parameters
PMC eigen mode
The waist sits at z = 0 in both axes. Downstream of the periscope the tangential channel becomes bench y (PIT) and the sagittal channel bench x (YAW).
Elements
z is given from the PMC waist (the model's own origin) and from
JACR_MB4(the origin the bench profile was measured against).What was fitted, and what was not