Aritomi, Hirose, Tanaka
we decoupled the PRCL motion from SRCL 1f error signal to expand the lock duration. The coupling seems to be decreased by this decoupling. Also, we succeeded in transitting to 3f signals with decoupling factors. At least, the duration of 3f lock seems to be increased to 1 hour.
## what we did
The 1f lock duration seems to be too short to implement ADS. So we tried to improve the duration.
First, we found that PRCL-to-SRCL coupling in 1f signals seems to be large. Fig. 1 shows the spectra of DRMI 1f error signals (top) and coherences between them (bottom) in the DRMI 1F LOCKED state with ADSs except for BS. Coherence between PRCL and SRCL in the 30-70 Hz region seems to be large. We attempted to decouple PRCL and SRCL so that SRCL error signal (POP17I) could not see the PRM motion.
We excited PRM and SRM at 150.125 Hz, respectively and measured the ratio between the PRM motion and the error signal response at 150.125 Hz. Fig.2 and Fig.3 shows the results of PRM and SRM, respectively. The signals in K1:LSC-POP_PDA1_{RF45_I, RF17_I, RF17_Q}_NORM channels are calibrated to nano-meter unit. From the fig.3 result, when SRM was excited, the PRCL error signal(POP_RF45_I) did not see the SRM motion. This results seem to be reasonable because RF45 does not enter SRC thanks to the schnupp asymmetry. Also, the MICH error signal, POP_RF17_Q, did not see the SRM motion thanks to the phasing in klog36990. Therefore, it seems not to be necessary to subract SRM motion from PRCL or MICH error signals.
On the other hand, when PRM was excited as fig.2, the SRCL errof signal, POP_RF17_I see the PRM motion rather than PRCL error signals, This results are also expected because RF17 resonates in the coupled cavity consist of PRC-SRC due to the schnupp asymmetry. So we tried to decouple PRCL-to-SRCL by subtracting the PRM motion. From the top pane of the Fig.2 right panel the ratio of (calibrated POP17I)/(PRCL error) is +2.58. So we applied the value, -2.58*10(=POP_RF17Q calibration factor) = -25.8 in LSC_INPUTMTRX to subtract the PRM motion from SRCL error signal, and adjusted the K1:LSC-SRCL1_OFFSET value (SRCL offset:-3.2, PRCL offset: -4.7, then, -3.2 + (-4.7/10*(-25.8))) = 8.9 so that SRCL control keeps the resonance point of SRCL.
We tried to implemented this value into the matrix during the lock but the lock was down. So we input the 1/10 of the value at first, and increased the value to the nominal one, step by step. Fig. 4 shows the spectra and coherences of error signals when the matrix value was changed. Fig. 5 shows SRCL error signal and coherence PRCL and SRCL. I wrote down each measurement setting of each color line in the legend of top panel of fig.5. As you can see, the spectra and coherence around 20-90 Hz decreased as the MTRX value became close to nominal value= -25.8. Also, we found that BS dither can be seen in the PRCL-SRCL coherence at their frequency.
Moreover, there seems to be gain peaking around 140 Hz. So we measured the OLTF of SRCL. Fig.6 shows the result. The OLTF gain when MTRX value was -15 seems to increased. So we decreased the overall gain to -6dB. Then, the gain peaking around 140 Hz seems to be disappeard. Furthermore, in low frequency region, the OLTF gain shape seems to be improved.
After that, we tried to transit to 3f signals with decouplinf factor of PRCL-to-SRCL. We succeeded in transitting to 3f signals. Fig.7 shows the spectra of error signals and coherences before (without REF in label, measured in 2026/07/30 in klog37297) and after (with REF in label, measured in 2026/08/03) decoupling. As for 3f signal seems not to be changed so much.
By the way, Current 3f lock duration is at most 1 hour (fig.8)
