As previously noted, there are multiple beams coming back from the ETM surface onto the PDH PD on the end table. They are spread out in a vertical pattern. All the spots swing together (as the ETM moves?).
I moved the PDH Green PD on the end table so that it was further away from the Faraday and I added an iris in between the Faraday and the PD. Now only the principle reflection from the ETM is incident on the PD. See attached photos. In order to sneak past the neighbouring optics, I had to steer the beam down a bit, so the PD is now lower than it previously was.
Just FYI: the angle between the returning beams is about 5 or 6 mrad at the PD. Before the beams get to the PD they go through a telescope that de-magnifies the beam by about 5 or 6 times. This implies that the angle between adjacent returning beams from the ETM is around 1 mrad at the ETM.
This does make the position of the spot on the PD more susceptible to the alignment of the ETM - we should use a short focal length lens and image the ETM plane onto the PD.
First image - overview of table
Second image - the three returning beams immediately before the IRIS
Third image - a close up of the IRIS and PDH PD.