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.