Abstract:

We measured optimal demodulation phase of REFL PDA1/3 RF45, POP PDA1 RF17/45, and AS PDA1 RF17 by modulating laser frequency at 210 Hz to estimate the schnupp asymmetry.
The detail analysis will be done soon.

Detail:

To estimate the schnupp asymmetry with the same method written in klog14560, I modified the script and measured the optimal demodulation phase for each RF PD signals.
Measured data are stored at /users/Commissioning/scripts/schnupp/260213/.

The script to measure the optimal demodulation phase are updated and stored at /users/Commissioning/scripts/schnupp_asymmetry_measurement.py.
What done inside the script is:
1. Set input matrix to send the target PD's I and Q signals to XARM and YARM filter bank in k1lsc model.
2. Set lock-in frequency to 210Hz and amplitude to 100, which will be sent to MCE suspension.
3. Set filters for lock-in (210 Hz band-pass filter, 210 Hz comb notches, and low-pass filter after demodulation) at demod3 and demod4 filter bank.
4. Measure the lock-in output while changing demodulation phase step by step.
5. Fitting the result with sine signals to estimate the optimal demodulation phase.
6. Revert demodulation phase to the original values and set lock-in amplitude to 0.

Figure 1 and 2 shows examples for REFL PDA3 RF45 measurement with Y arm.
In addition, measured data and fitting summary are summarized in the txt file as shown in the attachments of the log.

The measured data is now stored at /users/Commissioning/scripts/schnupp/260213/.

Images attached to this report

Non-image files attached to this report

Comments to this report:

Carl Blair - 12:54 Friday 13 February 2026 (36347)

[Ushiba, Carl, Alex]

We processed the schnupp asymmetry measurements using a modified version of the script in klog14560. We also checked the relative phase of the I and Q demodulation channels. There are no significant changes since the last measurement. The script is in the measurement folder, it is well described in the original python notebook. The schnupp asymmetry measurements are:

Measured schnup asymmetry
-------------------------
average: 3.414 m
standard error:0.073 m

By PDA1REFL45: 3.318 m
By PDA3REFL45: 3.336 m
By AS17: 3.733 m
By POP17: 3.402 m
By POP45: 3.286 m

We additionally reameasured the Y arm PDA1 REFL 45 to see how much it varied shot to shot. It differed by 2 degrees which translates to 0.01m change in schnupp asymmetry measurement. This is consitent with the fitting uncertainty for this measurement that was +/- 2 deg 95% confidence interval. The PD with the lowest fit uncertainty is REFL PDA3 with X +/-0.14 and Y +/-0.3deg. I would trust this measurement most. I'm not sure its the same PD but in the previous measurements the REFL45 value was 3.321m.

The relative I/Q phasemeasurements below vary by as much as 2.5 degrees. However the statistical error from the worst fit has uncertainty +/-8 degrees (Y arm AS17 that has very low SNR), so I supect measured differences in I and Q demodulation phase are dominated by statistical error. To improve I/Q demodulation phase correction factor longer measurements should be taken to reduce the statistical uncertainty and the AS17 SNR should be improved.

X arm PDAREFL45: 89.89
Y arm PDAREFL45: 88.88
X arm PDBREFL45: 90.15 m
Y arm PDBREFL45: 89.93 m
X arm AS17: 90.23 m
Y arm AS17: 89.50 m
X arm POP17: 89.86 m
Y arm POP17: 90.52 m
X arm POP45: 92.49 m
Y arm POP45: 88.55 m

Carl Blair - 10:08 Friday 20 February 2026 (36405)

These are the numbers for each PD propogating the fitting uncertainty to the Schnupp assymetry measurement.
By PDA1REFL45: 3.32(5) m
By PDA3REFL45: 3.34(4) m
By AS17: 4(2) m
By POP17: 3.4(3)m
By POP45: 3.3(1) m