Darkhan, Yutaro, Kiwamu,

Although we have been meaning to do some exciting measurements tonight, we ended up chasing an issue -- beam clipping.

Looking at the beam shape at the AS and POP ports, we started beliving that the main interferometer beam was clipped somewhere in chamber. Our initial assessment suggests that the clipping point may be in the reflection side of the BS. The reason is that the distortion of the beam shape occurs more strongly in the reflected light from ITMY than that from ITMX. See the pictures below for the evidence.

Pic.1 (ITMY misaligned. ITMX remained aligned. SRM and PRM are kept misaligned.) The beam is already distorted both at the POP and AS ports, but not as much as the other pictures shown below.

Pic.2 (ITMX misaligned, ITMY aligned, SRM and PRM remain misaligned) The distortion is more significant than the reflected light from ITMX. We think this is an indication that the clipping is happening in the reflection side of the BS.

Pic.3 (Same as Pic.2, but IMMT2 was steered in yaw by -90 urad) In this condition, our back-of-envelope calculation suggests that the beam must have moved by 2-3 cm, comparable to its beam size, on the BS towards the negative Y direction. We hoped that the effect of the clipping would reduce and indeed the beam shape seemed to be improved a bit.

Pic.4 (the same as Pic.2, but IMMT2 was steered in yaw by +90 urad) We expected that the distortion becomes more significant. Indeed the distortion seems more apparent.

Message:

It is likely that the interferometer beam is clipped somewhere in chamber. We believe that this is one of the reasons that make our DRMI locking harder by introducing unnecessary higher-order modes to the intracavity field.