Jennie W, Jeff K, Keita K,

Summary: Scattering nosie from IFO couples into QPD A, QPDB segment four has larger dark noise b y factor of ~40 compared to other segments.

After we broke the ground loops on the TIA chassis inputs last week I redid the QPD dark noise measurements.

First I measured the average value of each quadrant input on both QPDs using diaggui to take a triggered time measurement withe the shutter open.

Then I closed the shutter and set the QPD_{A,B}_SEG{1,2,3,4}_OFFSET values to the average value for all four QPD A segments (15050 counts) and all four QPD B segments (16750 counts).

Then I measured the dark noise using the template in /ligo/home/jennifer.wright/git/2026/SPI/20260922_SPI_dark_noise_ASD.xml

The first measurement looked like it had some glitches on QPD A so Jeff got me to check the time series. At his suggestion we checked for coherence with ASC_AS_C (anti-symmetric port of H1) and it seems to be correlated. The AS port was flashing through fringes hence the fringing seen on the time series.

Zooming in this becomes more obvious.

I unlocked and mis-aligned the IMC and unlocked the JAC to stop scattered light through HAM 2 and 3 while I retook the measurement.

The QPDA dark noise now looks better but QPD B segment four is still significantly noisier than the other quadrants.

Looking at the time series this noise is obvious also.

Keita and I then checked the raw signal inputs before the anti-whitening filter on the QPD segments and these still show this noisier segment four. NB: this measurement was done with the IFO back to locking so scatter is again visible in the QPD A spectrum. The noise is flat, which suggests that the whitening stages in the TIA are not engaged so its not a whitening gain problem.

We then did a test with the SPI unshuttered and the dark offsets back to nominal. QPD B segment 4 now looks not that different to the others, which suggests that the four segments do not have wildly different electronic gains.

Try swapping chassis for TIA or disconnectiong input and measuring output ASD for TIA.

Next steps: 

- Measure TIA output spectra from each quadrant on QPD B with input unplugged.