masayuki.nakano@LIGO.ORG - posted 16:50, Sunday 16 August 2026 (91558)
JAC pico motor calibration by non-locked sweeps.
All four JAC pico axes (heads A and B, X and Y motors) have been calibrated in counts to beam displacement on the WFS, by non-locked sweeps read through the WFS DC signals. Efficiencies come out at 0.6–1.2e-3 beam radii per count (0.09–0.19 μm, 0.8–1.6 μrad of beam pointing), with a direction dependence of 9–27 %.
Setup
- 2 W input. Cavity intentionally unlocked before the measurement; each pico was first moved to bring its WFS DC readings near zero (beam near the QPD center).
- Each pico axis is swept at 1 count/s, 200 counts per leg, out and back, so the calibration includes the direction dependence (hysteresis) of the motor.
- Signal: SUM-normalized WFS DC. With the cavity unlocked the beam is a clean Gaussian, so the normalized quadrant signal gives the beam displacement in units of the beam radius directly. The efficiency is fitted per leg.
Conversion
The fit gives the efficiency in beam radii (d/w) per count. Beam radii at the QPDs from the JAC reflected-path optical model:
| plane | wx [μm] | wy [μm] |
|---|---|---|
| WFS A | 199.4 | 181.0 |
| WFS B | 150.0 | 155.9 |
Results
Denominator: SYS-MOTION_C_PICO_D_CURRENT_X/Y_POSITION [counts] of the swept motor. "up" / "down" refer to the sign of the count sweep.
| pico / motor | QPD axis | direction | d/w per count | μm per count | μrad per count (beam) |
|---|---|---|---|---|---|
| A / X | A YAW (x) | up | -9.41e-4 | 0.188 | 1.64 |
| A / X | A YAW (x) | down | -7.82e-4 | 0.156 | 1.36 |
| A / Y | A PIT (y) | up | -7.33e-4 | 0.133 | 1.16 |
| A / Y | A PIT (y) | down | -8.05e-4 | 0.146 | 1.27 |
| B / X | B YAW (x) | up | -1.22e-3 | 0.183 | 1.60 |
| B / X | B YAW (x) | down | -9.32e-4 | 0.140 | 1.22 |
| B / Y | B PIT (y) | up | -5.72e-4 | 0.089 | 0.78 |
| B / Y | B PIT (y) | down | -7.00e-4 | 0.109 | 0.95 |
Attachments: per-axis sweep figures (normalized signal against pico counts).
Images attached to this report
