I’m not sure about the AOM, but as for the VCO, I think (at least some) spare parts were available at UToyama.
Even if the VCO is broken, it might be enough to replace the active components, such as the seed VCO or amplifier.
Please find the related documents below.

https://gwdoc.icrr.u-tokyo.ac.jp/cgi-bin/private/DocDB/ShowDocument?docid=8968
https://gwdoc.icrr.u-tokyo.ac.jp/cgi-bin/private/DocDB/ShowDocument?docid=8433

We may want to check green frequency at the laser head with respect to IR frequency, to see if SHG in PROMRTHEUS is OK or not.

[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 ???

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

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.

[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).

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).

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