Thank you for your comment.
And I’m sorry, this is a mistake.

I think I probably forgot to remove this value after trying the 1f decoupling.
I checked and found that this value was also present during the 3f diagonalization, so redoing the diagonalization is necessary.

Fortunately, POP17I, rather than POP17Q, was being sent to MICH, so it does not seem that MICH was being locked with the 1f signal.

[Hirose, Tanaka, Ushiba, Komori (remote), Fujimoto]

Abstract

We performed the MICH diagonalization for the DRMI 3f lock again, this time with the POP17I removed from the MICH signal.
The OLTFs were still clean by the MICH diagonalization, but the lock duration was only a few minutes, unlike the previous lock.

Further investigation is necessary to understand the cause of the short lock duration.
Possible causes are:

• Interferometer misalignment (reproducibility may be improved by implementing the ADS loops for SRM and BS)
• Mis-diagonalization of MICH

Details

As Yokozawa-san pointed out, in the previous MICH diagonalization work (klog #37352⁠), POP17I was mistakenly sent to MICH with a factor of 2.06500.
This time, we turned off this POP17I path and performed the MICH diagonalization using only the 3f signals.
Since the procedure was the same as before, please refer to the previous klog for the details.

Removal of PRCL coupling

We excited PRM in the length direction above the control bandwidth and removed the PRCL signal from MICH and SRCL.
Fig. 1 shows the peaks after reducing the coupling from PRCL.
The factors were changed as follows.

• REFL135I_to_MICH: -1.38 ⇒ -1.139
• REFL135I_to_SRCL: 0.705 ⇒ 0.65

The coupling to MICH was reduced by about a factor of 4, and the coupling to SRCL was reduced by about a factor of 5.
These values were implemented in the guardian.

Removal of SRCL coupling

In the same way, we excited SRM above the control bandwidth and tried to remove the SRCL signal from PRCL and MICH.
However, as in the previous measurement, the coupling from SRCL was already sufficiently small, as shown in Fig. 2.
Therefore, we did not perform any further coupling reduction for SRCL.

Diagonalization of the MICH actuator

Fig. 3 shows the spectra when we diagonalized the MICH actuator by combining BS, PRM, and SRM.
The reference traces show the spectra when only BS was excited.

The OUTPUT_MTRX values to BS, PRM, and SRM used in this adjustment were as follows.

• BS: 15
• PRM: -0.61
• SRM: 5.35

With this adjustment, the coupling to SRCL was reduced by about a factor of 10, and the coupling to PRCL was reduced by about a factor of 80.

Using the ratios obtained here, we changed the MICH actuator settings in the OUTPUT_MTRX of LSC_OVERVIEW as follows.

• MICH_to_PRM: 0 ⇒ -0.04066
• MICH_to_SRM: 0 ⇒ 0.3566
• MICH_to_BS: 1.0 ⇒ 1.0 (unchanged)

These values were also implemented in the guardian.

OLTFs

After the sensor and actuator diagonalization described above, we measured the OLTFs of MICH, PRCL, and SRCL, and adjusted the overall gains.

Figs. 4, 5, and 6 show the OLTFs of MICH, PRCL, and SRCL after the gain adjustment, respectively.

The current overall gains and filters are as follows.

• MICH1 gain: 2.1 ⇒ 9.0
• PRCL1 gain: 2.0 ⇒ 2.0 (unchanged)
• SRCL1 gain: 1.4 ⇒ 2.0
• MICH2 roll-off: ELP500 ⇒ ELP100

As in the previous measurement, the OLTF shapes themselves were clean, and the strange-shape problem seen without the diagonalization was again resolved.

In addition, since the MICH output tended to be close to saturation, we changed the roll-off filter from ELP500 to ELP100 to reduce the RMS of the output.

Lock duration

Although the OLTFs were as clean as those obtained in the previous measurement, where POP17I was mixed into MICH, the lock duration was only a few minutes.

The cause of the short lock duration is still unclear, and further investigation is necessary.
Possible causes include:

• Mis-diagonalization of the sensors and/or actuators
• Strong dependence of the lock duration on the interferometer alignment (Currently, the ADS loops for SRM and BS have not yet been implemented for DRMI, so the alignment reproducibility is not good.)

The first possibility is briefly discussed below.

Discussion

Possible mis-diagonalization of the sensors and actuators

In this measurement, the following OUTPUT matrix elements were obtained as a result of the MICH actuator diagonalization.

• MICH_to_PRM: -0.04066
• MICH_to_SRM: 0.3566
• MICH_to_BS: 1.0

On the other hand, according to the suspension model checked by Ushiba-san, the actuator efficiencies at 100 Hz are:

• PRM: -143 dB
• SRM: -157 dB
• BS: -168 dB

From these values, the theoretically expected absolute values of the MICH actuator factors, including the correction for the 45-degree angle of BS, are:

• MICH_to_PRM (model): 0.0398
• MICH_to_SRM (model): 0.200
• MICH_to_BS (model): 1.0

The measured MICH_to_PRM factor is roughly consistent with the model prediction.
However, the measured MICH_to_SRM factor is about 1.8 times larger than the model prediction.

One possible reason is that the demodulation phase of REFL51I/Q was not well tuned, and the MICH signal leaked into REFL51I.

Therefore, it may be useful to assume that the values from the suspension model correctly produce a pure MICH actuator, and then use this actuator to phase REFL51I/Q. 

Plan for tomorrow

One possible plan is to implement the ADS loops for SRM and BS in DRMI in order to improve the alignment reproducibility.
This will help us check whether the lock duration depends strongly on the alignment.

Also, as mentioned in the discussion above, it may be useful to construct a pure MICH actuator using the theoretical values from the suspension model, or using the length oplev signals as suggested by Ushiba-san.
Then, we can use this pure MICH actuator to determine the sensor diagonalization.

Fujimoto, Tanaka

We also performed the MICH actuator diagonalization by using 1f sensors with the same procedure as 3f sensors in klog37352, in order to check whether the diagonalization value by 1f sensors become the same as the value by 3f sensors. This time, we did not perform the 1f sensor diagonalization before the actuator diagonalization.

The values are summarized as below

• MICH to PRM: 0 -> -0.0425
• MICH to SRM: 0 -> 0.25
• MICH to BS: 1 (unchanged)

Also, we compared these ratios with the ratios of the actuator effeciecies measued by Length OPLEVs. According to suspensions' plant models in FM10 of {PRM,SRM,BS}_TM_OLDAMP, the actuator effieciencies @ 100 Hz are as below,

• BS @ 100 Hz: -171 dB (According to the model, the efficiency of BS is -168 dB. Since the BS is oriented at 45 degrees to the optical axis, its contribution to the optical path length is changed by a factor of 1/sqrt(2), corresponding to -3dB. Thus, the effective efficiency in terms of MICH is -171dB @ 100 Hz.)
• PRM @ 100 Hz: -143dB
• SRM @ 100 Hz: -157dB

From above the efficiecies, the ratio between BS and PRM efficiencies is -143dB - (-171dB) = 28 dB. The diagonalization ratio between BS and PRM is also 20*log10(1/0.0425) = 27dB. They seem to be almost consistent. Similarly, the ratio between BS and SRM efficiencies is -157dB - (-171dB) = 14 dB. The diagonalization ratio between BS and SRM is also 20*log10(1/0.25) = 12dB. They have 3dB discrepancy. 

We input these values in LSC OUTPUT MTRX and these value are implemented by the VERTEX guardian if the guardain are requested to DRMI_1F_LOCKED.