KAGRA Logbook

with takafumi.ushiba, takaaki.yokozawa, hirose.chiaki

Summary

We checked the effect of the SRCL resonant-gain (resG) filters at the ADS frequencies and measured the coupling from the LSC length error signals to AS RF34 and POP RF90. The SRCL resG filters suppressed the injected peaks in the RF signals. For MICH, no resG filter is currently used.

Measurement

The interferometer was brought to ALIGNING_DRMI. The IMMT2 and PRM ADS loops were kept on, while the BS and SRM ADS loops and the SRM BPC loop were off. The BS/SRM ADS CLK_GAINs were set to zero to avoid angular excitation from the ADS itself.

For the SRCL measurements, longitudinal excitation was applied to the SRM TM TEST L at 22.125 Hz and 24.125 Hz. For the MICH measurements, longitudinal excitation was applied to the BS TM TEST L at 14.125 Hz and 16.125 Hz.

Date directory: /users/Commissioning/data/DRMI/2026/0824

SRCL resG check

At both 22.125 Hz and 24.125 Hz, the excitation peaks seen in AS RF34 and POP RF90 were strongly suppressed when the SRCL resG filter was enabled.

Figures: SRCL_22p125.png, SRCL_24p125.png

Length-signal spectra: SRCL_peak_22p125.png, SRCL_peak_24p125.png

MICH measurement

MICH currently has no resG filter at these ADS frequencies, so the spectra were measured without resG. Clear excitation peaks were observed in the MICH length signal and in the RF signals.

Figures: MICH_14p125.png, MICH_16p125.png

Length-signal spectra: MICH_peak_14p125.png, MICH_peak_16p125.png

L-to-AS34/POP90 coupling

Using the measured LSC length-error amplitude and the corresponding RF peak amplitude, we estimated the conversion from the measured MICH/SRCL length error to AS RF34 and POP RF90.

Excitation	LSC error [nm/rtHz]	RF34 [cnt/rtHz]	RF34 / L [cnt/nm]	RF90 [cnt/rtHz]	RF90 / L [cnt/nm]
MICH 14.125 Hz	0.04416	0.001238	0.02803	0.0002600	0.005887
MICH 16.125 Hz	0.01430	0.0005095	0.03563	0.0001368	0.009564
SRCL 22.125 Hz	0.05841	0.0005105	0.008741	0.0005092	0.008719
SRCL 24.125 Hz	0.05326	0.0004240	0.007960	0.0004863	0.009131

These measurements will be used for evaluating how longitudinal motion excited by ADS can couple back into the angular control signals.

Detail:

To investigate that we need resonant gains in MICH loop quantitatively, I measured the spectrum of MICH, PRCL, and SRCL error signals with ADS EXCs were on while turning off ADS feedbacks (fig1).
Basic idea is that the peaks appeared in MICH error signals multiplied by the coupling function from MICH to POP90/AS34 should be lower than noise floor of POP90/AS34.

According to the coupling function measured in the original post (MICH to POP90: ~0.008 and MICH to AS34:~0.03), we can obtained the following table.

Frequency (Hz)

Acceptable MICH peak (nm/rtHz)

Measured MICH peak (nm/rtHz)

4.125 (PRM pitch)

0.25

0.044

6.125 (PRM yaw)

0.125

0.035

14.125 (BS yaw)

0.025

0.056

16.125 (BS pitch)

0.025

0.019

22.125 (SRM pitch)

0.0033

0.0051

24.125 (SRM yaw)

0.0033

0.0069

30.125 (IMMT2 pitch)

0.0038

0.049

32.125 IMMT2 yaw)

0.0038

0.014

According from the above tble, it is better to put resonant gains to reduce the offset of alignment signals at least for BS yaw, SRM pitch, SRM yaw, IMMT2 pitch, and IMMT2 yaw.
So, I tested the resonant gains.

Unfortunately, MICH UGF is close to 30 Hz, it is difficult to engage resonant gains at 30.125 Hz and 32.125 Hz.
On the other hands, resonant gains can engaged for the oher ADS frequencies
Figure 2 shows the filter bank of MICH1, and FM4 is the resonant gains I implemented.

For the other LSC loops, resonant gains are also implemented as shown in fig3 and 4 because the UGFs for these loops are higher than that of MICH, and we can engage the resonant gains without any problems.

After that, I tested the ADSs with DRMI 1F lock and DRMI seems stable with all ADSs are ON (fig5).
Also, when engaging the ADSs, POP90 and AS34 signals were increased, so ADSs seem to work well.