Detail:

We saw IMC transmission power was reduced during PRFPMI lock acquisition.
It was happened when ALS_CARM was engaged (fig1: 110 of K1:LGRD-LSC_LOCK_STATE_N if ENGAGE_ALS_CARM state), more strictly speaking, when IMC CMS IN2 was closed.
After several investigation, we found that IMC ASC signals obtained some offsets when closing IMC CMS IN2 (fig2), especially in yaw signals.
First, we thought ALS_CARM had some trobles but it seemed not true because even though we opened the switch of CARM CMS FAST switch, the same thing happened.

We also checked the QPD signals (K1:PSL-ALIGNMON_OUTPUT_QPDA1_DC_{PIT,YAW}_OUTPUT) for checking PSL output axis but seems no drastic changes (the value is calibrated to the beam spot change on QPDs in the unit of mm).
We will investigate more today.

- Checked the IMC PDH error signal. It has 100mVpp for TEM00 at K1:IMC-SERVO_OUT2_OUT with 10dB In1 gain.

- Checked the IMC WFS demod phase. 

1. LOCK the IMC and excited the K1:LSC-DGCARM_EXC with the frequency of 10Hz and amp of 100cnts.
2. Check the path through IMC IN2 
   SUMMING NODE: IN1-disable IN2-disable 
   CARM: IN1-disable, IN2 enable, IN2GAIN 0dB, FASTEN-enable
   IMC SERVO: IN2-enable, IN2GAIN:0dB
3. Adjust the demod phase to minimize the I signals (e.g. K1:IMC-REFL_QPDA1_RF14_I1_ERR) 

  The screenshot of the demod setting and the spectra before and after the adjustment are shown in the next post. We changed the demod phase for all channels with ~45deg.

- Tweak the IMC alignment with the IP2 PZT in the PSL. We can't optimize the alignment due to the range of the PZT.

All of the attempt doesn't solve the situation.

The same IMC out drop happened on 31st August when I was trying the PRFPMI_RF_LOCKED recovery as the attached file.

In this timing after ~18:50, PRFPMI RF LOCKED was almost recovered even if this phenomena happened. (Fig1,2)

Fig.3 shows the data just before 18:50 where lock loss happened many times..

I also checked the O4a early-stage lock loss on 24th May 2023 [ID:2 in the O4a run summary] and recovery timing. This drop existed as Fig.1

However, the drop ratio was about ~0.01(including 2 step downs)/1.2, while it became 0.1/1.3 ~ 0.2/1.5 at present. Actually, it seemed to become larger.

Precisely speaking, 24th May case (and 20th June case which corresponds to ID:116 lock loss and recovery at the last stage of O4a) shows 2 steps down. The first is that we are now discussing. While there is the other drop just after this action. What is this (not CARM but DARM)?

---

Fig.2 shows the drop on 11th August using FB. The ratio ~ 0.046/1.3. Fig.3 shows the drop on 21st August using HPL. The ratio is ~0.05/1.3. Almost no change between FB and HPL. Both ratios are around the mid of 0.01/1.2 and 0.1/1.3.

IMC WFS (QPD1) demode phase was off by ~45deg for all quadrants.

What was done: Injected a 10Hz signal into DGCARM_EXC (which is connected to EXCB of Summing node?) which is routed to CARM servo board which is routed to MC servo board In2. We're basically making frequency modulation on laser which should be detected in I-phase for IMC WFS. Demod phase was changed to roughly minimize Q phase. Before the measurement, IMC ASC was turned off, IMC servo IN2 gain was set to 0dB (nominally -10) and the awg amplitude was set to 100. After the measurement IN2 gain was set back to -10dB.

1st attachment -> before the demod phase adjustment of IMC QPD1. All quadrants were off by ~45deg.

2nd attachment -> after the demod phase adjustment.