[Hirose, Yokozawa, Ushiba, Fujimoto]

Abstract

We successfully recovered both the DRMI 1f and 3f locks.
For the DRMI 1f lock, we also recovered the ADS, and the lock duration reached approximately 20 minutes.
For the DRMI 3f lock, the lock duration was recovered to approximately 9 minutes, although the ADS has not yet been tested.

However, I observed strange lock transitions during the 3f lock, in which the AS DC signal suddenly increased and then returned to its original level.
The cause of this behavior is still unknown and requires further investigation.

Details: DRMI 1f Lock

Earlier today, the IFO LO amplitude was increased from 10 dB to 13 dB (klog 37678), which resolved the issue of the low POP90 signal during the PRMI lock (klog 37667).
We then proceeded with the recovery of the DRMI 1f lock.

Recovery of the 1f Lock

The following modifications were made to recover the DRMI 1f lock.

1. Restoration of filter settings
   There are several filters in MICH, PRCL, and SRCL that must remain enabled during the DRMI lock but are not automatically turned on or off by the VERTEX Guardian.
   We compared the current filter settings with those before the power outage and restored the necessary filters to their previous states.
    
2. Correction of the ELP filter settings
   We noticed that the MICH ELP100 filter was being enabled by the PREP_FM_FOR_DRMI of the VERTEX Guardian, although ELP300 must have been used for the 1f lock before.
   It is possible that ELP100 was mistakenly implemented in a way that also affected the 1f lock during the last DRMI 3f commissioning before the F2F meeting.
   We therefore modified the Guardian so that ELP300 is enabled in PREP_FM_FOR_DRMI, while ELP100 is enabled only in LOCKING_DRMI_3F.

After these modifications and the additional initial alignment, we successfully recovered the DRMI 1f lock.
The gains and offsets were left unchanged from the values already implemented in the Guardian.
Fig. 1 shows a screenshot of the setup during the DRMI 1f lock.
The maximum lock duration was approximately 20 minutes.

We also noticed that the POP90 and AS34 signals were approximately 1.44 and 1.35, respectively, which were significantly higher than their previous values of approximately 1.
This is likely because the IFO LO amplitude before the power outage was lower than the 13 dB setting used today, although we do not have an exact record of the previous amplitude.

OLTFs

Figs. 2, 3, and 4 show the OLTFs of MICH, PRCL, and SRCL measured during the DRMI 1f lock, respectively.
No significant abnormalities were observed.
A few comments on the comparison with the reference measurements from August 15:

• MICH: The measured OLTF gain was approximately 10 dB higher than the reference. This is because the MICH1 gain was increased during the implementation of the DRMI ADS after August 15.
• PRCL: The measured OLTF gain was approximately 4 dB lower than the reference. It is possible that some gain settings have been changed since August 15.
• SRCL: The measured OLTF was almost identical to the reference.

ADS

We tested the DRMI ADS that was implemented during the last DRMI commissioning before the F2F meeting (klog 37407).
The ADS worked properly for IMMT2, PRM, and BS. However, when the SRM ADS was enabled, the SRM alignment changed rapidly.
Upon investigation, we found that the signal demodulated at the SRM dither frequency contained not only the intended AS34 signal but also the POP DC signal.
The POP DC signal had been used for the SRM initial alignment, but it was not disabled when the ASC_LOCK Guardian went down.
As a result, the POP DC signal was unintentionally included in the DRMI ADS.

We modified the ASC_LOCK Guardian to resolve this issue, after which the SRM ADS also worked properly.
Fig. 5 shows a screenshot of the setup with the ADS enabled.

Details: DRMI 3f Lock

Recovery of the 3f Lock

The following modifications were made to recover the DRMI 3f lock.

1. Adjustment of the REFL HWP angle
   We noticed that the incident power on the REFL DC PD was approximately 30 mW.
   This was caused by the HWP rotary encoder losing its position information due to the power outage (klog 37518).
   We therefore changed the HWP setting from 146 deg to 166 deg to reduce the incident power on the REFL DC PD to 10 mW.
   We also updated the corresponding setting in the VERTEX Guardian.
    
2. Adjustment of the MICH1 gain
   The MICH1 gain had been set to 9 to increase the UGF during the 1f lock.
   However, with this gain setting, applying ELP100 to MICH resulted in insufficient phase margin and caused oscillation.
   I therefore modified the VERTEX Guardian so that the MICH1 gain is reduced to 2 for the 3f lock.
    
3. Adjustment of the MICH1 offset
   The MICH1 offset was originally set to 2 in the Guardian.
   However, I noticed that the AS DC signal exceeded 0.01 during the lock, whereas it should normally remain below 0.01 in the DRMI configuration.
   I therefore adjusted the MICH1 offset to minimize the AS DC signal.
   I found that changing the offset from 2 to 10 minimized the AS DC signal to below 0.01 and allowed the lock to remain stable.

With these adjustments, we successfully recovered the DRMI 3f lock.
Fig. 6 shows a screenshot of the setup during the DRMI 3f lock.
The maximum lock duration was approximately 9 minutes.
The ADS operation has not yet been tested for the 3f lock.
 

*Note: Strange Lock Transition

During the DRMI 3f lock, I repeatedly observed a strange behavior in which the AS DC signal suddenly increased to approximately 0.02–0.03 and returned to its original level a few seconds later.
Fig. 7 shows an example of this behavior.
Since the change occurred almost instantaneously, it appeared as if the interferometer had transitioned to a different lock point.
I also observed that the TEM00 mode image on the AS camera became brighter during this transition.
This behavior was not observed during the 1f lock, and its cause remains unknown.
Further investigation is needed to understand this phenomenon.