[Oshima, Yamaguchi, Tanaka, Komori]

Abstract:

We optimized the optical alignment for GRY PNC to stabilize the PNC control.
As a result, the visibility improved by approximately a factor of two.
The current alignment is now more tolerant to displacement and misalignment under large seismic motion.

Details:

To address the frequent unlocking of GRY PNC, we realigned mirrors located before the BS to optimize the visibility between the reference light and the GRY reflection at the POS table.
We found that the two beam spots after the BS, where the two beams recombine, were misaligned, as observed with a sensor card.
After fine-tuning the GRY alignment to the Y-arm following the suggestion in klog:30234, we further improved the visibility by adjusting the mirror alignment just before the recombining BS, achieving a two-fold enhancement.

Due to today's significant seismic motion, visibility fluctuations around 0.4 Hz caused by displacement or misalignment of the Y-arm mirrors or the pick-off mirror for POS were considerably high.
To mitigate this, we adjusted the mirror alignment to ensure that visibility changes follow a quadratic dependence.
Since the visibility doubled, leading to an increased open-loop gain, we disabled the "-7 dB" setting introduced in klog:30303 and confirmed that the open-loop transfer function remained consistent with previous measurements (Fig.1).
This alignment improvement appears to have increased the probability of successful IR handover.