KAGRA Logbook

MIF (ITF Control)

Hiroki Fujimoto - 0:40 Wednesday 13 May 2026 (36880)

SRM OPLEV QPD Centering

[Dan, Takano, Tanaka, Fujimoto]

Summary

From the previous SRC flash searches, it was found that a large yaw offset of the SRM is required to observe SRC flashes. Under such conditions, the SRM OPLEV beam went out of range of the QPD.
To address this issue, we performed centering of the OPLEV QPDs (TILT QPD and LEN QPD) for the aligned SRM.
Although the centering onto the QPDs were confirmed while we were in the mine, after returning to the control room we found that the beam was no longer incident on the TILT QPD. We plan to check the situation and recover the alignment tomorrow.

What we did

After obtaining a reasonable SRM alignment during the SRC flash search in the morning (klog #36876), we carried out the SRM OPLEV centering work in the mine in the afternoon.
Inspection around the QPD revealed that the beam was clipping on the holder of the first steering mirror. Therefore, we decided that rearrangement of all the optics, including the steering mirror, BS, TILT QPD, lens, folding mirror, and LEN QPD, was necessary.

First, we adjusted the position of the first steering mirror and aligned the beam onto the TILT QPD. After coarse alignment, fine adjustment was performed using the micrometer stage of the QPD while monitoring the QPD signals.

Next, alignment onto the LEN QPD was performed. For the LEN QPD, the distance relationships between the SRM and lens, and between the lens and LEN QPD, are important. Therefore, the downstream optics (BS, lens, folding mirror, and LEN QPD) were repositioned and aligned so that these distances matched the previous setup (klog #7952).

Fig. 1 shows a photograph of the final layout. The distances between optics are as follows:

TM to upper viewport: 990 mm (assumed to be the same as the previous setup)
Viewport to first steering mirror: 305 mm
First steering mirror to BS: 40 mm
BS to lens: 45 mm
Lens to folding mirror: 110 mm
Folding mirror to LEN QPD: 273 mm

After repositioning, final centering onto the QPD was performed using the micrometer stages of the QPD.

After the adjustment, the optical table was covered with aluminum foil, and we returned to the control room. At that point, we noticed that the beam was no longer incident on the TILT QPD. It is possible that the aluminum foil blocked the beam path or disturbed the alignment during covering. We plan to check the situation tomorrow and recover the alignment.

Images attached to this report

Comments to this report:

dan.chen - 16:44 Wednesday 13 May 2026 (36885)

[Tanaka, Fujimoto, Saito, Dan]

Summary

We checked the SRM OPLEV QPD signals after yesterday's centering work. The OPLEV beam was incident on both the TILT and LEN QPDs, but the environmental light and IR contribution were relatively large. We installed an IR filter also in front of the LEN QPD, centered both QPDs, and confirmed that the LEN OPLEV mainly responds to SRM longitudinal motion.

What we did

We first checked whether the OPLEV beam was incident on the QPDs. The environmental light level was large, so we reduced it as much as possible during the measurements.

For the TILT QPD, the sum signal was about 90 counts with the OPLEV beam and about 10 counts when the beam was blocked. Removing the IR filter increased the sum from about 90 to 230 counts, corresponding to an attenuation factor of about 2.6.

For the LEN QPD, no IR filter was installed at first. The sum signal was about 317 counts with the OPLEV beam and about 307 counts when the beam shutter was closed. After inserting an IR filter, the sum decreased to about 130 counts, corresponding to an attenuation factor of about 2.4. Opening the green shutter did not significantly change either QPD signal.

These checks showed that the OPLEV beam was already incident on both QPDs, but the IR contribution was non-negligible. Therefore, we decided to install/keep IR filters in front of both the TILT and LEN QPDs.

IR filter installation and QPD centering

We installed an IR filter in front of the LEN QPD using a right-angle clamp because of the limited space around the QPD.

After reducing the environmental light, with the OPLEV beam on, the IR shutter closed, (and the green shutter open,) the final sum values were:

TILT sum: 90.5 counts
LEN sum: 130 counts

We then centered both QPDs using the micrometer stages while monitoring the QPD signals.

LEN OPLEV response check

Finally, we excited the SRM in longitudinal and yaw at 10 Hz and checked the LEN OPLEV response. (Attached)

When the SRM was excited in the longitudinal direction, the response appeared in the LEN OPLEV signal. When the SRM was excited in yaw, the response clearly appeared in the TILT OPLEV signal. Therefore, we judged that the present oplev position is acceptable.

Files: `/users/VIS/TypeB/SRM/LogNotes/260513_oplev/`

Images attached to this comment

Hiroki Fujimoto - 3:34 Thursday 14 May 2026 (36892)

For additional information, I attached the photograph of the current SRM OPLEV QPD layout below.

Images attached to this comment