Summary:

RIN of X arm green beam everywhere (input measured by QPD, transmission measured at corner, reflection measured by WFS) is much larger than Y arm for a broad frequency range, the descrepancy peaking at around 10Hz.

Looking at various things, the beam might be clipping somewhere between ISCTEX green Faraday and TMSX M3 (that's the HR/partial for IR/green to split the green main path and green QPD path).

This is not an ultra serious clipping, causing additional 1%-ish pk-pk RIN, but could be an issue for our initial alignment scheme. Needs more investigation.

Details:

First attachment, top panel shows various RIN when the arms are locked with reasonable alignment (ALS-TRX and TRY both more than 0.95).  Thick lines = X, thin ones =Y.

X arm RIN for input (QPD), transmission (ALS TR) and reflection (WFS DC) are all about 2 orders of magnitude larger than Y at 10Hz. The middle panel shows that the QPD signals for X arm shows similar structure, and it's mainly PIT.

On the bottom panel you see that the PIT to NSUM coherence is really high. This means that the structure is not due to some intensity noise from the laser, it's more likely that the PIT clipping is causing the RIN and increasing (or decreasing) PIT signal at the same time.

Perfect coherence between X QPDA and QPDB means that the clipping location is upstream of the QPD sled. High coherence between the QPD RIN and transmission RIN at 10Hz should mean that the clipping is upstream of TMSX M3 where transmission and QPD path are separated.

Back to the middle panel, the reflection RIN (WFS DC) is larger than input and transmission by a factor of 1.5 or so. This should mean that the clipping location cannot be upstream of the green Faraday on ISCTEX.

So the conclusion for now is that it's likely clipping between green Faraday and TMSX M3.

The noise is not stationary, it goes up and down, but it never comes down to the same level as Y arm.

The second plot shows that X arm has been like this for the past 6 months except for two brief periods.

(Note that I needed to set a factor of 0.685 calibration for X WFS DC NSUMs, as the DC value for these was about 1.45, not 1.00. I didn't have to do this to any other RINs.)