Similar measurement with klog33306.
[Komori, Ushiba]
Abstract:
To measure the FSR of arm cavity, we conducted the cavity scan for X arm.
Analysis will be done tomorrow.
Detail:
To obtain the latest X arm and Y arm length, we tried to perform cavity scan of arms.
Today, we only finished X arm measurement but if the scan method is fine, we plan to perform Y arm scan with the same configuration tomorrow.
Followings are the procedure of the cavity scan.
1. Misaligned ITMY and PRM (request MISALIGNED state for ITMY guardian and MISALIGNED_BF state for PRM guardian).
2. Lock GRX by using XARM guardian (request ALS_LOCKED state).
3. Feedback ALS signals to IMC manually.
4. Increase laser power by rotating HWP at PSL.
5. Scan laser frequency by turning on/off FM1 of K1:ALS-SUM_OFS_SLOWOUT_CALI filter bank.
We modified the ramp time of FM1 of K1:ALS-SUM_OFS_SLOWOUT_CALI filter bank to make the scan speed about 1 kHz/s (100 seconds for ~2 FSR).
The start time of the scan is 8:20:04 UTC and 8:29:10 UTC for forward and backward scans, respectively.
Figure 1 and 2 shows the scan results when increasing and decreasing the offsets, respectively.
It is hard to see the sideband signals in transmission but PDH signals can be seen clearly, so probably this scan can be used for the analysis.
The analysis will be done tomorrow.
Note:
First, I tried to perform cavity scan for X and Y arm at the same time by closing laser shutter for GRY and did not misalign ITMY.
However, maybe due to the coupling between MICH fringes and photosensor signals, ITMX and ITMY were kicked a lot when increasing IMC output power.
So, we gave up scaning X and Y arm at the same time.
Followings are the detailed procedure to feedback ALS signals to IMC manually.
1. change K1:LSC-MCL_GAIN to 1.
2. Turn on FM4 of K1:LSC-MCL filter bank.
3. Close K1:IMC-SERVO_IN2EN switch.
4. Turn off FM2 of K1:LSC-MCL filter bank.
5. Increase K1:LSC-CARM_SERVO_IN2GAIN to 15.
6. Turn on 1st and 2nd K1:IMC-SERVO_COMBOOST.
7. Turn on FM5 of K1:LSC-MCL filter bank.
Diaggui files are stored at /users/Commissioning/data/XARM/2026/0209/.
