Fig.1 shows the time series of the oplev signals in the optical path from IMMT2 to X-arm when PR3 was glitchy. Glitch size seems to be 1.9 urad in P, 0.97 urad in Y, respectively. These sizes are much larger than the usual RMS.

Even though the glitch was occuring, HANDOVER from ALS to IR seems to be fine (but not easy). However, the fluctuation of POP90 was terrible. Fig.2 show the timeseries including to POP90, PRC and MICH error/feedback. Between the cursors, there seems to be no glitch in PR3 oplev. in the out of this region, POP90 dropped to the less than half of the maxlmun.

[Kenta Tanaka, Dan Chen]

Follow-up Investigation of PR3 Glitches (2026/02/26)

Following the previous report on intermittent large motion of PR3, we performed an additional investigation today during a period when the interferometer was unavailable due to an earthquake.

Summary

• We investigated the PR3 glitches, but the root cause has not been identified.
• We suspected possible origins related to the oplev sensors and/or the oplev laser source; however, we found no evidence supporting these hypotheses.

Time-Series Observation

From the time-series data, the glitches appear to be correlated between the TM P and Y directions.

In the figure below, the red circles indicate periods where glitches are present, while the blue circles indicate quieter periods.

Oplev QPD Signals

We examined the QPD segment signals:

The glitches are visible in all four QPD segments and appear consistent across them. No abnormal behavior is observed in any single segment. Since the glitches do not appear in the SUM signal, they are unlikely to originate from the oplev laser source.

Spectral Comparison (PAY_FLOAT)

We compared the spectrum in PAY_FLOAT with data taken during a previously quiet period(2024/11/28):

• No significant spectral differences were observed compared to the earlier quiet dataset.
• Although the previous dataset appears to have had the loop manually closed, the high-frequency region should still be comparable.
• Note: during the present measurement the system was in PAY_FLOAT, so it is unclear whether glitches were actively occurring at that time.

Current Status

Despite these investigations, no clear cause of the PR3 glitches has been identified. The spectral shape is largely unchanged compared to the earlier quiet period, and the QPD behavior does not indicate a sensor- or laser-origin issue.