KAGRA Logbook

ISC (General)

Hiroki Fujimoto - 23:05 Tuesday 13 October 2026 (37659)

Lock recovery for XARM and YARM

[Yokozawa, Ushiba, Fujimoto]

Abstract

We worked on lock recovery and initial alignment for XARM and YARM.
For XARM, we completed the initial alignment, including ADS, after recovering the error signal (klog 37656).
For YARM, I recovered the IR and GR resonances and successfully locked IRY. However, GRY was locked at a point away from resonance, and I could not lock it to the TEM00 mode.

Details

XARM
As described in klog 37656, we successfully recovered the IRX error signal by correcting the LO settings.
Thanks to the XARM alignment performed by Yokozawa-san in the morning, we were able to lock XARM with only minor alignment adjustments and complete the initial alignment procedure, including ADS.
However, as pointed out in klog 37656, some issues still need to be investigated, such as the change in the OLTF gain.

YARM
When I started the initial alignment of YARM, no flashes were observed for either IR or GR.
By adjusting the alignment of SR3, BS, IY, and EY while monitoring the camera images and PD signals, I improved the transmission to approximately 0.8 for both IR and GR and successfully locked IRY.
Fig. 1 shows a screenshot of the setup at that time.

However, when I attempted to lock GRY, it locked at a point away from resonance, as shown in Fig. 2.
As a result, the lock check repeatedly failed, causing the system to attempt to relock GRY.

If anyone has any ideas about what might be causing this issue, I would appreciate your comments at the next morning briefing.

Images attached to this report

37659_1791896513_Screenshot from 2026-10-13 21-59-55.png

37659_1791896589_Screenshot from 2026-10-13 22-02-53.png

Comments to this report:

takaaki.yokozawa - 6:35 Wednesday 14 October 2026 (37660)

I changed the target IFO_REFL from 125 to 140 for PREP_FOR_YARM_ALIGN in initial alignment guardian.
Same situation happened in this morning, GRY cannot be locked, even
K1:ALS-Y_PDH_COMCOMP
K1:ALS-Y_PDH_SLOWBOOST
were turned on (In case, GRX can be locked), so some trouble in PDH Y circuit?

I performed the initial alignment only for the IRY (ITMY and ETMY and BS), after then, manual alignment for SR3 to be large GRY flash.

Hiroki Fujimoto - 12:34 Wednesday 14 October 2026 (37666)

[Hirose, Yokozawa, Fujimoto]

Abstract

We investigated the GRY locking issue observed yesterday and found that both the phase modulation and demodulation LO outputs for Gr PDHY were turned off.
After turning on both LO outputs, we successfully recovered the PDH error signal and locked GRY.
We also found that the OLTF gain was more than 10 dB lower than previously measured due to an incorrect LO phase.
By optimizing the LO phase, we almos recovered the OLTF gain and achieved a UGF of approximately 5 kHz.

Details

Since we had confirmed this morning that the GRY error signal was missing, we investigated the analog electronics in the mine and resolved the issue as described below.

Turning on the LO outputs

We found that both the phase modulation and demodulation LO outputs for Gr PDHY were turned off (Figs. 1 and 2).
After turning on both outputs, we successfully recovered the Gr PDHY error signal and locked GRY using this signal.

LO phase adjustment

We first set the LO phase to −135 deg, which was the value recorded just before the planned power outage in March 2026 (Notion page).
However, the measured GRY OLTF gain was approximately 10 dB lower than that measured in September 2025 (klog 35124).

We therefore optimized the LO phase and found that −70 deg gave the best result.
As shown in Fig. 3, after performing ADS, the OLTF gain almost recovered to within approximately 1 dB of the previous measurement, with a UGF of approximately 5 kHz.

Images attached to this comment