KAGRA Logbook

MIF (ITF Control)

kentaro.komori - 21:28 Wednesday 26 November 2025 (35679)

Recovering the 'INCREASING LAS POWER' state by reducing the IMC loop gain

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

Abstract:

We resolved the lock-loss issue that occurred when increasing the laser power by tuning the IMC loop gain.

Details:

As reported in klog:35668, lock loss consistently occurred when the input power was increased.
Yokozawa-san found that the CARM open-loop gain exhibited an unexpected peak around 200 kHz (Fig. 1), which was a strong candidate for the cause of the lock loss.
Ushiba-san suggested that this could be due to an excessively high IMC open-loop gain.
I measured the IMC loop and confirmed that this was indeed the case: the UGF was around 200 kHz, and the phase margin was small—approximately 20 degrees or less (Fig. 2).

To address this, I reduced the IMC CMS in1gain by 3 dB, lowering the UGF to around 100 kHz and improving the phase margin to ~30 degrees (Fig. 3).
After this adjustment, we were able to increase the laser power without losing lock.

Tanaka-san updated the guardian setting for in1gain in the LOCK PREP state of the IMC guardian.
With this change, the system can now transition through the INCREASING LAS POWER state without lock loss.

Images attached to this report

Comments to this report:

kenta.tanaka - 22:04 Wednesday 26 November 2025 (35680)

This afternoon, we found that the lock loss was occured whenever the IN1GAIN value was changed from -23 dB to -22 dB after increasing the laser power itself. This process is performed after increasing the IMC output power.

At first, we measured the CARM OLTF just before changing the gain values related the CARM loop (K1:IMC-SERVO_{IN1,FAST}GAIN, K1:LSC-REFL_SERVO_SLOW_GAIN). In this state, the OLTF itself should be the same as the state after passing through the INCRASING_LAS_POWER state. Fig. 1 shows the CARM OLTF, dark curve is the one before the adjustment. The UGF at that time is ~60 kHz, and the phase margin seems to be roughly 30 degrees. The gain changing process is performed to keep this OLTF so we considered that the phase margin is not enough to perform the changing process. Therefore, we attempted the process in the state which the overall gain reduced -3 dB by decreasing K1:IMC-SERVO_FASTGAIN, K1:LSC-REFL_SERVO_SLOW_GAIN to -3dB in advance. That is, the process started when IN1GAIN was -24dB, FASTGAIN was 1 dB, and SLOW_GAIN was 0.32

After changing the process, we succeeded in transiting the INCREASING_LAS_POWER state manually. The bright curve is the one after the adjustment. Current CARM UGF is ~40 kHz, phase margin is 43 deg. I wonder that the gain above ~ 150 kHz seems to be not changed even though we adjusted the overall gain. We need more investigation.

Then, we modified the script in the INCREASING_LAS_POWER of the LSC_LOCK guardian. We tested the process by the guardian at twice, the first trial is failed on the way which FAST_GAIN decreased from 1 dB to -2 dB. The second trial was succeeded. So we would like to know the succeess rate of this process. We left the IFO with this state.

Images attached to this comment

35680_1764162269_MokuFrequencyResponseAnalyzerData_20251126_211941_Screenshot.png

takafumi.ushiba - 0:24 Thursday 27 November 2025 (35681)

As discussed in klog35456 and klog35472, high CARM gain might introduce the instability during increasing laser power.
So, 40kHz UGF would be good at least for achieving the stable switching of CARM input gains.

During the work reported in klog35472, I set CARM UGF as 40kHz, so the initially measured UGF seems unexpectedly too high.
I'm not so sure why the CARM UGF (and also IMC UGF) became so high, so it would be nice to investigate the reason.

takafumi.ushiba - 8:16 Thursday 27 November 2025 (35684)

>To address this, I reduced the IMC CMS in1gain by 3 dB

Did you reduce IN2 gain as well?
To keep the relative gain between IMC loop and CARM loop, we also need to change IN2 gain if IN1 gain is changed.

kentaro.komori - 20:29 Thursday 27 November 2025 (35698)

We already discussed this topic in the today's morning meeting, but I answer here just in case.
We did not change the in2 gain.
Instead of that, we changed the CARM CMS fast gain.

However, in terms of the noise, reducing the in2 gain in the CARM CMS will be better because it is still 7 dB but the CARM fast gain is already -23 dB.