## Summary of 2026.07.31 and 2026.08.04 works

(2026.07.31)

We redesigned ADSs for DRMI. First, we decided the dither frequency in order to solve the degeneracy betwen other DoFs. We prepared the frequency set per 2 Hz, which is above 1 Hz, typical suspension's resonant frequency, 4.125, 6.125, ...., 30.125, 32.125 Hz. This is because we want to see the sideband from alignment fluctuation until 0.5 Hz around one dither frequncy. So it is necessary to set the frequncy interval between each dither frequency which is used as PIT and YAW of one mirror.

Then, we measured the spectra of POP90 and AS34 with no exciations to look for the frequency region, which the only noise floor is in. There are some peak in the region from 14 Hz to 20 Hz in POP90. From this results, we decided the frequencies for ADSs as below. This time, we decided to use POP90 for PRM, BS, IMMT2 dithering and to use AS34 for SRM dithering.

• BS PIT: 22.125 Hz, YAW: 24.125 Hz
• PRM PIT: 4.125 Hz, YAW: 6.125 Hz
• SRM PIT: 26.125 Hz, YAW: 28.125 Hz
• IMMT2 PIT: 30.125 Hz, YAW: 32.125 Hz

We implemented the bandpasse filter at each corresponded frequency to K1:ADS-{PIT,YAW}_{IMMT2, PRM, SRM, BS}_DEMOD_SIG filter bank. and performed the phasing for each signal from alignment dither. This time, we decided the dither amplitude so that the sidelobe around the dither frquency could be seen largely enough.

After that, we implemented the 1.5 Hz cutoff lowpass filter, comb filter for corresponding frequency and its harmonics, and 2 Hz notch for the beat note between PIT and YAW in K1:ADS-{PIT,YAW}_{IMMT2, PRM, SRM, BS}_DEMOD_SIG_I, which is a filterbank for a demodulated signal.

Then, we tried to engage ADSs. PRM seems to work because POP90 increased and there seems to be harmonics of diter frequency, 8.25 Hz and 12.25 Hz in POP90. IMMT2 and SRM seems to work but their harmonics could not be seen in POP90 and AS34, respectively.

On the other hands, BS ADS seems not to work. ADS loop itself can be close but the POP90 and AS34 power decreased on engaging ADS.

During the work, we modified the ENGAGE_ADS_FOR_DRMI in order to engage current ADSs except for BS (I hope Hirose-san will report soon).

(2026.08.05 work)

In daytime, DRMI lock was too unstable to proceed the ADS. We suspected the seismic motion around 0.1 Hz disturbed the lock because the 0.1 Hz seismic motion got 1- order larger than the one in last Friday (klog37313) and BS and SRM IM YAW motion at 0.14 Hz, which is IM YAW resonant frequency, seems to become 1-order larger due to the large micro seismic. We implemented the resG filter for 1.4 Hz of FM5 in K1:VIS-BS_TM_OLDAMP_YAW, which was already implemented. It seems to reduce the Yaw motion at 0.14 Hz. However, DRMI lock was still unstable.

We observed two kinds of lock loss pattern in daytime. One is the oscillation at 20-30 Hz. Sometimes, DRMI LSC began oscillated at some frequency between 20-30 Hz. We measured the OLTFs. But the phase margins around their frequncies of DRMI LSC loop seem to be large enough even if the only overall gain was changed by some reasons. So something might change not only gain but also the phase of loop. Moreover, this oscilltion kicked the ADS, especially IMMT2 and SRM because their dither frequencies are close to oscillation frequencies and this kick changed the alignment them largely and often made the relock difficult.

Another pattern is the sudden lock loss. Sometimes, POP90 and AS34 dropped to 0 within less than 1 second. At that moment, any alignment seem to be not changed

In night time, DRMI lock seemed to become stable for some reason. Especially, DRMI was stable without ADS rather than with ADS. I attempted some trials to improve the stabilty, inclunding to review of ADSs.

• Decoupling the PRM motion from MICH error signal.
  • As reported in klog, I tried to decouple the PRM length motion from the MICH error signal with the same procedure of PRM-to-SRCL. I remeasured the MICH eror signal response when PRM length displacement was excited, including to the reprocebility of the check of PRM-to-SRCL decoupling factor.
  • I applied the obtained decouplig factor for PRCL-to-MICH but it seems not to work.
  • I also confirmed the decoupling factor of PRM-to-SRCL seemed to be almost the same. It seems to be good.
• Review of DRMI ADS
  • Since DRMI was stable without ADS rather than ADS, I reviewed the ADS
    • PRM
      • If PRM actuators is not optimized in terms of eular DoFs or if the beam spot on PRM is off from the center, the alignment excitation couples to length motion. On the other hands, if cavity length is tuned, this length motion should not be seen at the dither frequency. But if not so, the length fluction also contaminates in the alignment signal. Current DRMI lock is performed with some offsets in their error signals. But these offsets are not optimized. Therefore, I started to check whether the cavity length tunes or not by exciting the length.
      • I excited the PRM L displacement from K1:VIS-PRM_TM_ISCINF_L_EXC at the PRM PIT dither frequency, 4.125 Hz. Although PRCL control UGF is ~30 Hz, I could see the length excitation in POP90 and PRCL error signal. In this state, I changed the PRCL1 offset from -4.7 by 0.5 step. When the offset value was 0, the peak of PRM Length excitaiton disappeared from POP90 though it was in the PRCL error signal.
      • In this state, I switched the excitation from length to alignments and performed the phasing for PRM ADS again. Also, I found that the CLKGAIN = 30 cnts seems to be large enough to see the sidelobe. The phase seems to be almost the same. By the way, I tried to implement the resG filter for dither frequency in PRCL but the phasing is not changed.
    •  IMMT2
      • Also, as for IMMT2,  I performed the phasing. Before this, I found that there was the small peak at 30 Hz in POP90 without excitation. So I changed the PIT dither frequency from 30.125 Hz to 34.125 Hz. As for YAW, the domod phase was not changed so much (~several degrees)
    • I implemented ADSs of PRM and IMMT2 after re-phasing Then, POP90 are slightly improved (fig.1). The harmonics peaks of PRM dither frequencies (PIT -> 8.25 Hz, Yaw -> 12.25 Hz) also got slightly bigger (fig.2). 
    • SRM
      •  As Aritomi-san and Yokozawa-san already reported in klog37315, SRM phasing value seems to be not reproducibiltiy, the phase easily changed more than 90 degrees. 
      • I implemented the resG filter for SRM dither frequencies in the SRCL loop but the situation seems to be not changed.
      • Also, I tried to tune SRCL as the same procedure of PRCL but the AS34 fluctuation seems to be too large to adjust the offset.

During ADS work, we found that there was no anti-aliasing filter in AS34 and there was also no lowpass filter in normalization filter. we implemented them. (klog37314) 

### Next

• investigate SRM and BS ADS good configuration
• (give up ADS and move to WFS for DRMI?)

After the ITMY work, I investigated the SRM ADS phasing in some situations. This time, I locked DRMI with 1f and engaged PRM and IMMT2 ADSs. I found that the demod. phase of SRM dithering seems to be flipped with or without resG filters at corresponding frequecies of ADS in SRCL control loop. I summerized the phasing in each situation as below table. I'm not sure of the exact reason why the signe flip occured for now (A2L coupling? If A2L coupling is caused by the flip, the L conponent has the opposite sign from Angle conponent and the size is twice? SRCL control is reduced the L component to small enough?). However, the value itself seems to be reproducible if we measure it with good coherence, more than 0.9.