Also, is there any PD satulation? Actually, one story is as follows,

: If satulation happens in anywhere (circuit, PD, etc), the low frequency signals will be lost firstly, I think. Then, the loop gain for low rfrequencies will be lost, then ithe system starts drifting, and finally reaches lockloss.

I checked the signals while increasing the laser power in the IRX LOCKED state (Fig. 1).
The laser power increase finished well ahead of the lockloss, so the glitch due to the gain switching doesn't seem the trigger of the lockloss.
In addition, we can see the slow drift of CARM FAST and IMC FAST signals just before the lockloss, which might cause the lockloss.

Since high-pass filter was engaged before the FAST DAQ (2nd-order 5Hz for CARM and 1st-order 2kHz for IMC), I have no idea why this kind of slow drift can be seen in these channels.
One possibility is saturation of opamp inside the circuit, so I would like to check if the same phenomena can be seen even swapping IMC CMS to the old one tomorrow.

Shimode, Aso

We checked the noise of the E8863D signal generator used for the PLLlocal oscillator of ALS-X (https://klog.icrr.u-tokyo.ac.jp/osl/?r=35425).

We generated 40MHz/10dBm with the E8863D and measured the spectrum with an Agilent CXA N9000A.

The spectra with span of 20MHz and BW of 5.1kHz are the following.
Left: when RF OUT was ON. Right: when RF OUT was turned off (still the cable was connected). 

As you can see, the noise siderobe is higher than the measurement noise below 4MHz offset from the carrier.

Following is the noise spectrum when the same 40MHz/10dBm signal is generated from Moku:Lab.
Even Moku:Lab is slightly better than the broken E8863D.

The followings are the spectra measured with 200kHz span and 1kHz BW.
Left: RF ON,  Right: RF OFF.

The SSB phase noise is about -105dBc in within the 200kHz offset. (10dBm carrier and -65dBm noise makes -75dBc with 1kHz BW. Converting to 1Hz BW, it is -105dBc).
According to the data sheet of E8663D, the SSB phase noise with UNY option should be less than -160dBc. So the measured noise is way larger than the specification.

I will ask KEYSIGHT if they can repair this or not.

[Aso, Yokozawa, Ushiba]

Abstract:

To solve the issue that the waveform coming out from VCO circuit distorts, we added 3 dB attenuator between 80 MHz BPF and the last RF amplifier.
Then, we repaired VCO circuit and confirmed it works well.

Detail:

This morning, we measured the waveform of the RF amplifier output with an oscilloscope with the several configurations.
First, we injected 80 MHz sine signals directly into the last stage RF amplifier from Moku:Lab with an amplitude of 2 dBm, as indicated on the front panel.
The output waveform appeared clean, and the amplitude was approximately 26 dBm, also written on the front panel, so the RF amplifier itself seemed fine.

Next, we checked the waveform coming out of the VCO circuit to see if the distorted waveform was reproduced.
It was, and we found that the amplitude of the output signal was too high (33 dBm = 15 Vpeak, almost the same as the supply voltage of 15 V).

Next, we checked the input signals to the RF amplifier.
The amplitude was about 12 dBm, which seemed too high according to the value written on the front panel.

We discussed how to solve this issue and decided to put a 3 dB attenuator between the 80 MHz BPF and the RF amplifier.
With this configuration, the VCO circuit output is about 31.5 dBm (12 Vpeak), which is lower than the supply voltage (15 V).
Although the waveform is not perfect, we stopped further attenuation because it might reduce the GRX power too much.
The AOM is used in a double-path configuration; a slight reduction in RF power would cause significant power degradation for the GRX beam.

After making the above modification, we checked that the VCO circuit was working properly and found that the soldering for the VCO input was broken.
We soldered it again, and the VCO circuit recovered.

Operator: Dan Chen
Shift time: 9:00-17:00 JST

Check Items:
VAC: No issues were found.
CRY temp: No issues were found.
CRY cooling water: No issues were found.
Temp: FIELD_IYA changed by +0.5 deg in the morning and -0.4 deg in the afternoon
VIS GAS filter: No issues were found.

IFO state(JST):
9:00 : STANDBY
16:05 : LOCKING

Recovery of GRX lock and RF_LOCKED was achieved

Date: 2025/10/30

Members: Takaaki Yokozawa, Dan Chen

Summary

The repaired VCO circuit for GRX was reinstalled in the original configuration. After reinstallation, we tried locking the Xarm and successfully reached the ALS_LOCKED state. Then, we performed the initial alignment, and the interferometer recovered to the RF_LOCKED state.

Details

We installed the repaired VCO circuit for GRX and confirmed that Xarm could reach ALS_LOCKED.
Then, after completing the initial alignment, we requested RF_LOCKED from the center area and returned to Mozumi.
However, we found that PLLY could not be locked. So, during the PLL-Y locking process, we changed TEMPY_BIAS (K1:ALS-Y_LASER_TEMP_BIAS_OFFSET) from -0.50 to -0.44, and it successfully locked after this adjustment.
After that IFO could achieve RF_LOCKED state.

The laser power is still decreasing.

[miyoki, ushiba]

>8. Checked the actuator side of GRX

Photo.1 showed the power lines for four Wenzel parts in the VCO box for X. As you can see, 4 cables are tried to be clamped in one terminal segment for +15V and GND, individually. Actually, one of red cables seemed to be detached from the terminal when I opened the cover as Photo.2 .

Photo.3 shows detached each power line.

Fig.4 shows the improved connection of these powe lines. The power lines and GNDs for two Wenzael parts are solded with each other with a single red(for +15) and black(for GND) lines, individually, then new  two red and black lines are fixed in the terminals.   

Fig.5 shows the top view inside of the VCO box. The left bottom Wenzel part's poweline seemed to be loosened maybe during some activities yesterday, then completely detached in the last night.

After improvement, we checked VCO performance. Anyway, we can confirmed that the output frequency (~ 79.5MHz) can be changed according to the applied control DC voltage. So, the trouble reason seemed just to be the detached power line cable.

GRX/GRY lock issue investigation

Date: 2025/10/29

Members: Takafumi Ushiba, Shinji Miyoki, Dan Chen

Summary

We found that the GRX signal became strange around 20:00 last night, and we investigated both GRX and GRY. As a result, we discovered that the power line inside the VCO chassis for GRX was loosely fixed and easily detached. The circuit has been brought back to Mozumi and will be inspected, repaired, and reinstalled tomorrow. For GRY, we found that the suitable phase difference between modulation and demodulation had shifted by about 135(? need to be confirmed as a exact value) degrees. After adjusting the phase of the function generator (FG), Yarm could reach the ALS_LOCKED state.

Details

1. Checked GRY function generator (FG)
   The output settings were confirmed as ch1: 0 dBm and ch2: 10 dBm, consistent with the previous report.

2. Power-cycled the FG
   Tried to lock after restarting the FG, but still could not lock.

3. Tried swapping FGs
   Used the FG for GRX to drive GRY, but it did not lock and the signals were unchanged.

4. Checked FG output signals
   The FG output was normal. After restoring the original configuration (GRX with GRX FG, GRY with GRY FG), we noticed that GRX also could not be locked. The GRX signal had already looked strange since around 20:00 last night (see fig_001.jpg).

5. Investigated the Dual I&Q Demodulator
   Since both GRX and GRY could not be locked, we checked the common circuit, the Dual I&Q Demodulator (S1809168). Power-cycling it made no difference. Replaced it with another unit (S1809171), but there was still no improvement. (We continued using S1809171 after this.)

6. Power-cycled the RFPD interface
   No change; still could not lock.

7. Checked signals around the I&Q Demodulator using MokuLab

   • The RFPD signals for both X and Y were visible at appropriate frequencies, though the GRY signal was relatively smaller.
   • Disconnected the EOM to check possible coupling effects.
   • The LO signals from FG to the demodulator were large for both X and Y, though the X channel appeared slightly noisy.
   • Signals just after the RFPD looked normal.
   • When rechecking the I-Mon port, GRX showed a clear PDH signal, while GRY showed only a small one (see fig_002.jpg).

8. Checked the actuator side of GRX
   Since the PDH signal of GRX was clear, we suspected a problem on the actuator side for GRX. Using the GRY VCO circuit (S1809061) for GRX allowed us to lock. We then opened the original GRX VCO circuit (S1809060) and found that the power amp supply line inside the chassis was very loose and could be easily detached, and some solder joints were also poor (see fig_003.jpg). We concluded that this was the cause of the GRX issue. The circuit was brought back to Mozumi for repair.

9. Checked the sensing side of GRY
   Since the GRY signal was small, we investigated further. Using the IQ demodulator (S1809171) previously used for GRX, GRY could be locked by inverting the sign in software. Observing I-Mon and Q-Mon with an oscilloscope revealed that the suitable demodulation phase was shifted. After adjusting the FG demodulation phase to maximize the I-Mon signal, Yarm could be locked successfully. When switching back to the original IQ demodulator (S1809168), the lock was still successful. Therefore, it seems that the phase somehow shifted around 17:00 yesterday, which made the lock fail.

Conclusion

• GRY: Successfully locked after adjusting the demodulation phase of the FG.
• GRX: Found that the VCO circuit (S1809060) had a loose power supply line inside the chassis (fig_003.jpg). The circuit was taken back to Mozumi for repair and will be reinstalled tomorrow.

Operator: Dan Chen
Shift time: 9:00-17:00 JST

Check Items:
VAC: No issues were found.
CRY temp: No issues were found.
CRY cooling water: No issues were found.
Temp: FIELD_IYA changed by -0.4 deg
VIS GAS filter: No issues were found.

IFO state(JST):
9:00 : STANDBY
16:45 : STANDBY

[Yuzurihara, Ishikawa]
As a continuous analysis of klog35268, we found scattered light noise appearing the strain channel(K1:CAL-CS_PROC_DARM_STRAIN_DBL_DQ) on 2025/7/15 and 16.
The scattered light noise appeared in the frequency range around 70 Hz.
Figure 1~3 show the Q-spectrogram of the strain signal. This segment bar indicates the approximate time of the inspiral range drop found in this analysis.

In this morning, we noticed GRX also cannot be locked now.
Around 20:47 JST yesterday, GRX signals became strange (fig1).
At that time, GRY already could not be locked, so te broken time seems different for GRX and GRY.

Figure 1 shows the filter bank I modified.
Old filter was moved from FM3 to FM2 and new filter was implemented at FM3, so no SDF change happens.

The EY IM temperature changed as Fig.1 in the last 30 days from 70K to 45K. 

We tried to keep IFO at PRFPMI_RF_LOCKED state but GRY cannot be locked now somehow.
So, we gave up locking IFO.

To solve the oscillation issue for ETMY, I tuned ETMY NBDAMP_L5.
Figure 1 shows the OLTF of NBDAMP_L5.
Due to the double peak around 1.7Hz, gain peaking at 1.73Hz is higher than before, so further tuning might be necessary.

After replacement of PLLX LO, we could lock ALS_DARM and LSC_LOCK guardian can reach PRFPMI_RF_LOCKED state.
However, ETMY and PR3 started to oscilate around 1.9Hz and 1.7Hz, respectively, which seems to cause lockloss (fig1).

[Michimura, Ushiba]

We investigated why lockloss happened during increasing laser power by checking following conditions:
1. Increasing laser power only locking IMC: OK
2. Increasing laser power with IRX locked: lockloss
3. Reducing IMC CMS IN1 gain manually: lockloss
4. Reducing IMC CMS IN1/IN2 gains at the same time: OK

We will investigate more tomorrow.

[Aso, Ushiba, Michimura]

The culprit was the signal generator for PLLX LO.

We have replaced it by repurposing the signal generator for IMC modulation (Keysight E8663D) and now the noise level for Gr X and Gr Y look the same.
The signal generator for IMC modulation is now Tektronix AFG31022 (JGW-S2314951).

What we did:
 - We first checked if the intensity noise for green X is noisier than Y by comparing the spectra of K1;ALS-{X|Y}_ARM_INPUT_IN1 (see Fig. 1). RIN of X was slightly higher, but not enough to explain.
 - Next, we locked the arms in IR with a single arm configuration, and locked green PDH. Ideally, the feedback signals for green PDH loops should be zero, if PLL is working perfectly. Fig. 2 shows the spectra of the error signals K1:ALS-{X|Y}_PDH_MIXER_DAQ_OUT_DQ and the feedback signals K1:ALS-{X|Y}_PDH_SLOW_DAQ_OUT_DQ. Obviously, X was noisier and the feedback signal was almost flat.
 - We repeated the measurement with the phase noise cancellation loops on. No change observed (see green curves in Fig. 2). This means that the phase noise in green injection is not the cause.
 - Bypassed the phase shifter for LO of Gr X PDH demodulation and repeated the measurement (see cable diagram in JGW-T2112593). No change observed (see brown curves in Fig. 2), so we put the phase shifter back. The phase shifter was inserted only for X, and the amount of phase shift is K1:ALS-X_PDH_DEMOD_PHASE = 91.3 deg. To do this measurement, we connected Q phase instead of I phase to compensate for this phase difference.
 - Increased the amplitude of Gr X PDH modulation from 12.26 dBm to nominal 13 dBm (somehow this setpoint degrades over time. see this photo taken yesterday). No change observed (see magenta curves in Fig. 2). We decided to leave it 13 dBm.
 - Used output 2 of the signal generator used for Gr Y PDH (Keysight 33600A) instead of Agilent N5181B for Gr X PDH. No change observed (see cyan curves in Fig. 2). Put them back.
 - Used the IQ demodulator board used for Gr Y PDH to lock Gr X PDH. No change observed (see yellow curves in Fig. 2). Put them back.
 - We found that the signal generator used for LO for PLL X (Keysight E8663D JGW-S1706365) had "ERR" displayed (see Fig. 3, the signal generator on top). To clear this, we had to unplug the LAN cable to set it to local mode (see klog #23718 and klog #23713). The error message was "705, Sweep out of range; Phase Continuous Sweep to 40005000.000 Hz is impossible due to band-cross." Even if clearing the error, no change in the spectra observed.
 - We then used the signal generator used for LO for PLL Y (Keysight E8663D JGW-S1707148) for PLL X. The noise level for X matched with Y !!! See black curves in Fig. 2. So the signal generator used for LO for PLL X (Keysight E8663D JGW-S1706365) was the culprit!
 - We found that the signal generator used for IMC modulation was also Keysight E8663D JGW-S2516958 (was sitting between IOO and LSC racks). So, we repurposed this for PLL X, and used Tektronix AFG31022 JGW-S2314951 for IMC. Took the spectra again, confirming that the noise level didn't change with these changes (see right panels Fig. 2).
 - We have set them so that IP addresses for these signal generators for PLL X and IMC remain the same as previous signal generators. Fig. 4 and Fig. 5 are the photos of the signal generator situations after the work.

Discussion:
 - The reason why we didn't see the huge noise difference between X and Y in PLL error and feedback signals (klog 35419) is probably because these were dominated by the frequency noise of PROMETHEUS compared with the main laser. PLL LO noise contribution was smaller than the laser frequency noise contribution. PLL LO noise go into this loop as a sensor noise, so we could only see the noise with out-of-loop sensors. The PLL LO noise was flat in frequency, so the small difference could be seen at high frequencies in the PLL loop.

Next:
 - Check if IMC noise has not increased due to the signal generator change (unlikely, but just in case).

I checked the Mephisto 2000NE (SN 1228, 2002/08 bought by ICRR, Photo 1) that was used for ISS in Toyama University.

[Background]

Because of the air conditioner trouble in this summer in the clean room at Toyama University, the output power of this laser was reported to reduce at the level fo 0.9W (2A) from 1.6W.

[Check]

1)  FIrstly, I found a very dirty spot on the output AR window surface (no photo, sorry). Although I could not judge wheather this dirty spot was on the outside or inside of the AR window, anyway, I wiped it by a cotton bar with clean water (not pure water). Then, it can be completely removed at the level of eye balls judge.

2) I set up a power measuremnt setup as photo 2/3 in the clean room beside the draft chamber room in the KAGRA site. The used power meter is coherent Model 150-50, that can detect from 300mW to 150W. The expected 2W power is within this range.

3) The measured power was 1.75W at 2.0A as Photo 4/5. Photo 6/7 shows the wave length setting (1064nm), and mW level background without IR lasser input.

4) According to our (Miyoki, Uchiyama) memory, the max current was 2.1A for Mephisto 2000NE around 2004. Also, I found the record to use 2.3A in 2002 May to check other Mehisto 2000NE power. Anyway. 2.0A and 1.75W output might be enough for ISS experiment.

[Temporal conclusion and next step]

The main power reduction reason was the dirty spot on the AR window. I don't know wheather we can improve more when we will wipe the crystal or not.

Please check by using other power meter again to confirm the laser power, as the next step.

The laser power delivered from the main beam to the GR area was adjusted after increasing from 20W to 30W? If no such beam exists, this is my misunderstanding.

There seems to be a sterange unusual stray light hit on one optics in the green optics.

The output power of the NPRO laser has been gradually decreasing.
As a result, the PMC output has also been decreasing.
The decreasing rate is approximately 2% per 3 days.
I’m not sure whether this issue is already known, but I’d like to report it here.

[Ushiba, Michimura (and comments from Enomoto)]

To investigate the reason why Green X got suddenly noisier, we have done various swapping tests.
VCO, Summing Node, Gr PDH Common Mode Boards for both X and Y, arm length stability for both X and Y seem fine and not the cause.
Something real in green laser frequency ?? (PLL, AOM, ... ???)

Background:
 - ALS_DARM could not be engaged since around October 26 (Sun) 3-4 am JST (klog 35413).
 - Green X noise as measured at K1:ALS-{X,Y}_PDH_SLOW_DAQ_OUT_DQ is noisier than Y when Gr PDH loops are engaged, but not when arms are misaligned (klog 35416).

What we did:
 - Following klog 35416, we swapped VCO for green X and Y by swapping the input and output (see Fig. 2). No change in the spectra observed (see magenta in Fig. 1), suggesting that the noise is not from the VCO.
 - Measured the spectra of analog slow out for Gr PDH Common Mode Servo Board with doubt that the noise might be from SLOW_DAQ and analog slow out (which cannot be seen from the CDS). Fig. 3 shows the spectra when green shutters are closed (reference) and Gr PDH loops are engaged. 入力1 is X and 入力2 is Y. X clearly is noisier when the loops are engaged. X is also noisier when the shutters are closed, but this is from the gain difference in K1:ALS-{X|Y}_PDH_IN1GAIN (12 dB for X and -9 dB for Y). We confirmed this by increasing the gain for Y to 12 dB. These suggest that the noise is not from the analog slow out.
 - Swapped Gr PDH Common Mode Servo Board Chassis by swapping all the cables for X and Y, except for the power cables. No change in the spectra observed (see cyan in Fig. 1), suggesting that the noise is not from the Gr PDH CMB. When measuring the cyan spectra, we also engaged the fiber noise cancellation loops. There was a slight improvement for Y observed, but not for X, suggesting that the noise is not from the fiber noise nor the PZTs for the fiber noise cancellation.
 - Put them back, and measured the spectra when bypassing the summing node (see Fig. 4 for how we did), with doubt that something that follows the summing node is doing something bad. No change in the spectra observed (see yellow in Fig. 1), suggesting that this is not the case. We observed a factor of ~3 improvement above ~ 1 kHz in analog slow out as seen in Fig. 5, but not in X.
 - Finally, we put everything back and measured the spectra again (see black in Fig. 1).
 - To see if X green laser frequency is really noisier, we checked the error signal and the feedback signal for the PLL loops again (see Fig. 6). Actually, X is noisier above ~60 Hz by a factor of ~2. Actually, X was noisier than what we measured this morning (klog 35416). This PLL noise difference and time variation could be some hint, but not two orders of magnitude noisier as we see in K1:ALS-{X,Y}_PDH_SLOW_DAQ_OUT_DQ.
 - To check if arm lengh stability for X and Y are the same, we locked CARM with a sigle arm and measured the feedback signal at K1:LSC-CARM_SERVO_SLOW_DAQ_OUT_DQ. No difference in X and Y, suggesting that the arms have the same length noise level.

Other notes:
 - We also swapped X and Y in the Summing Node (X going from Input 1 to Out 2, Y going from Input 2 to Fast Out is the default but swapped), and measured the spectra, but no change was observed, suggesting that the noise is not from the Summing Node. Data was lost due to accidental logout from the machine (someone logged into our machine?).
 - We also checked if there are some secret RF amps or something between the VCO and the AOM by following the cables to AOM in the PSL room by our eyeballs. No secrets found.
 - Open loop transfer functions for Gr PDH loops seem normal, suggesting that something that changes the overall gain dramatically are unlikely (EOM modulation depth, AOM died, RF PD died, optical mis-alignment etc.).

Next:
 - There is some difference in the PLL noise. Investigate further. Maybe first by blocking the beams into the beat PDs and compare the spectra. 
 - See if RF PDs and demods for Gr PDH are working fine when some RF signal are present.
 - See if Gr PDH signal is noisy by locking the arm with IR, bring green on resonance with a fixed offset to VCO, and see Gr PDH signal
 - See if SHG in PROMETHEUS is OK or not (EDITED to take note of klog 35420)
 - Any other ideas ???