Some additional analysis.

Fig. 1 shows

• No features in the SUM of the IMMT1T QPD; the bump at 300 Hz would be due to one of harmonics of 60 Hz power supply.
• Somthing slightly seen in YAW but small.

I also tried to see POP forward QPDs but they are recoreded at 256 Hz, so the only frequency region less than 116 Hz can be seen in DQ. See Fig. 2. No relavant features are found in the POP forward QPDs (neither in pitch, yaw, or sum). Not sure if the would be under dark or ADC noise floor.

Ushiba, Miyakawa, Aso

We took spectra of various channels around the 220Hz peaks.

IMMT1 trans, MC REFL and MC trans

We only locked the MC and the IFO was misaligned (PRM, ITMX, ITMY are misaligned).
ISS is turned ON.
In this state, we measured the spectra of the IMMT1 trans QPD, MC REFL QPD (DC) and MC trans QPD.

Observations are:

1. The peak structure around 220Hz cannot be seen in any other channels.
2. The fact that IMMT1_TRANS_SUM does not see the peaks mean they are beam jitters, not intensity noise at IMMT1 trans.
3. No peaks found in MC REFL and MC trans suggests that the beam jitters are introduced after IMC.

OMC QPDs

We then moved on to the investigation using the OMC QPDs.

X-arm single bounce

PRM, ITMY, ETMX: misaligned
ITMX: aligned

We took spectra of the OMC QPD1 signals with the X-arm single bounce condition.
Observations are:

1. The 220Hz peaks can be seen both in PIT and YAW
2. SUM also sees the peaks but smaller.
Note: PIT and YAW are normalized by the SUM, which was about 4500. So, the PIT and YAW signals need to be multiplied by 4500 to compare the magnitude with the SUM signal. By doing so, one can see that the PIT/YAW peaks are much larger than the SUM peaks.
3. The above observation suggests that the peaks are mainly created by the beam jitter, not by the intensity fluctuation.
4. The small peaks in SUM may be from the coupling of the jitter into the SUM through the cross hair lines of the QPD.
5. It does not completely preclude the possibility of intensity fluctuation created by a beam clipping.

Y-arm single bounce and lock

PRM, ITMX, ETMX: misaligned
ITMY, ETMY: aligned

We then changed the IFO configuration to Y-arm single bounce.
We also took spectra with the Y-arm locked. When the arm was locked initially, the alignement was bad.
We engaged ADS to improve the alignment.
Spectra with these condisions are compared.
Observations are:

1. The 220Hz peaks in the single bounce spectra of SUM are a factor of few higher than the ones for X-arm.
Does this mean there is an extra beam clipping in the Y-arm path?

2. Both the beam jitter and intensity fluctuation get increased when the Y-arm is locked in a bad alignment.
3. The above noise level comes back to the same level as the single bounce case when the alignment is good.

Take away message for today

1. The 220Hz peaks are most likely coming from beam jitters generated somewhere between the IMC and IMMT1.
2. There are only two reflective optical elements there, STM1 and STM2.
3. In fact, the tapping test done today (klog will be posted elsewhere) showed that tapping the IFI chamber can easily excite the 220Hz peaks.
4. We still do not fully know the mechanism of converting the 220Hz beam jitters into the DARM noise.

I'm trying to subtract the beam jitter noise (witness: IMMT1 PIT) from DARM, by the ICA technic.
This is the first trial.

Fig. 1 & 2: Checking the spectrum and coherence (include other channels)
Fig. 3: Estimation for the transfer function derived from the cross-spectrum density (FFT length = 512s, smooshed by 64 points moving average)
Fig. 4: ASDs for the original DARM signal, the estimated noise, and the cleaned DARM signal
Fig. 5: Checking the time series with bandpass and zooming up.

I found the moving average for the TF was only for the real part.
After fixing this bug, the result is not changed so much.