In the afternoon we started placing components on HAM6 and HAM5 to measure the isolation ratio (green components in the attached).

The idea is to use the AUX laser we used for alignment, which will give us ~13mW through the SRM, and retroreflect the beam downstream of the OM1 so the return beam will be about 4mm in diameter at SRM (input beam is ~3mm in diameter, the IFO beam is ~4.4mm, so the return beam will be closer to the IFO beam in size).

Back-propagating return beam was located by inserting a HWP (not in the attached) between TFP and TGG, it's hitting the 50:50 cube and is going in the direction of point B in the first attachment. We blocked the forward propagating beam downstream and confirmed that the return beam went away.

The return beam is supposed to be colinear with the input beam but we haven't made any assessment of that. We will set up a temporary power meter holder at point B (haven't done that today). The HWP is still there in HAM5 but make sure to remove it when you measure the return beam power at point B.

The iris is roughly centered to the forward-propagating beam, but it's not very accurate. It's good enough to block the back-propagating rejected beam that is deflected away into -X direction.

50% of the input beam comes into the direction of point A, and we put a power meter holder there. You still have to adjust the height of the power meter but it's easy to see from the -X door using IR camera.

We misaligned SRM to see if we can locate the SRM reflection of back-propagating return beam in the direction of point A, but couldn't find any. We might try locating it again tomorrow.