[Aritomi, Hirose, Tanaka(remote)]
This work is continued from klog37320.

Summary

Following the previous investigation, we measured the optical power in front of the QPDs at the REFL, POP, and AS ports. We also investigated the cause of the QPD2 YAW PZT issue at the AS port and measured the optical power budget along the AS optical path.
We performed this measurement during the DRMI, but we haven't yet calibrated the power coming into the REFL, POP, and AS ports.

Result

REFL port

The optical power in front of each WFS was measured as follows:

Location	Power	note
In front of REFL QPD1	18.1mW
In front of REFL QPD2	30.5mW	Compared to case klog29836, the ratio of QPDA to QPDB is almost the same, so that's fine.
In front of REFL QPD3	23.0mW
In front of REFL QPD4	23.2mW	It's okay if the value is the same as QPDA4

A beam dump had been placed in front of WFS3. We removed the dump and instead installed a beam dump to absorb the reflection from the QPD.

By steering BS PZT1, we confirmed that the optical power toward the WFS1 and WFS2 directions was nearly identical.

POP port

The measured optical power was as follows:

Location	power	note
In front of POP QPDA1	5.8 μW (typically fluctuating between 5–6 μW)
In front of POP QPDA2	5–6 μW	It's okay if the value is the same as QPDA1

The dark level, measured by blocking the beam, was approximately 0.09 μW.

AS port

During the previous investigation, the QPD2 YAW PZT actuator did not respond.

The actuator configuration is

3-channel PZT driver outputs → SMA cables → REMO cables → mirror mounts for QPD1 PIT, QPD1 YAW, and QPD2 PIT
1-channel PZT driver output → SMA cable → REMO cable → mirror mount for QPD2 YAW

We performed a series of cable and driver swap tests to identify the faulty component.

The 1-channel PZT driver connected to the QPD2 YAW actuator showed only "3 V", indicating an abnormal condition.
Connecting the PIT cable to the 1-channel driver resulted in approximately "75 V", confirming that the 1-channel driver itself is functioning properly.
Connecting the YAW cable to the 3-channel driver still resulted in "3 V", suggesting that the problem is located downstream of the YAW cable.
Connecting the YAW cable to the mirror mount PIT caused the PZT driver output to change normally, indicating that the cable is not faulty.
Finally, connecting the PIT cable to the mirror mount YAW still resulted in "3 V", confirming that the mirror mount YAW actuator is faulty.

Therefore, the QPD2 YAW problem was identified as originating from the mirror mount YAW actuator rather than the PZT driver or cables.

The measured optical power at the AS port was

Location	Power
In front of AS QPD1	4–5 μW
In front of AS QPD2	~1 μW
Lens2 transmission	8–10 μW
BS3 transmission	3–4 μW
BS4 reflection	1–2 μW
Before PD1 (We set on Xarm configuration)	9.2–9.3 μW
Before PD2 (We set on Xarm configuration)	6.6–6.7 μW

A half-wave plate (HWP) is installed upstream of the AS QPD path. Considering the measured power at the QPDs, especially the low power at AS QPD2, the current HWP angle may be reducing the power sent to the QPD path too much. We should first recalculate the expected optical power at the AS port when the BS is misaligned. Based on the calculated power, the optimal HWP angle should be reconsidered to provide sufficient power to the QPDs while maintaining an appropriate power distribution to the other AS port photodetectors.

In addition, Tanaka-san measured the dark offset of the digital output for the AS port PD2 and updated the digital offset value accordingly. This calibration removes the DC offset of the PD2 signal and provides a more appropriate baseline for subsequent measurements and control.

Next step

Perform the calculation and comparison of the measurement result.
Insert the calibration(cnt->mW) on AS PDs.
Replace the faulty mirror mount YAW actuator for AS QPD2.
Perform the DC centring loop for REFL QPD3
Measure the sensing matrix for WFS on DRMI