We took a Thorlabs IO-5-1064-HP from a box of miscalleneous optics for KAGRA filter cavity testing (next to Mitsuhashi Roberts linkage experiment, purchased end of FY23).

We put it on a micro tilt adjustment stage and made sure it could be adjusted to the beam height from the Mephisto (72-73 mm). Actually the Faraday isolator that was already there was just put on a pedestal post instead of an adjustment stage, so it is likely that the vertical alignment through the FI was not good, generating a visible higher order mode and causing the improper beam profile.

Yesterday I prepared a new beam path to measure the beam profile directly from the laser. Today I measured the beam profile and at first it looks normal. So as we thought, the problem seems to be with the Faraday Isolator. I need to finish making the beam profile with all measurements to be sure of that but for now, it looks normal (at least with the W profile values).

Because the beam profile was normal, I put back the QWP and the HWP and verified if it was s-polarized yesterday. Today, I put the new Faraday Isolator and adjusted it. I measured the intensity without the FI at around 30mW and with at around 27mW. That gives a transmission about 90% and the manufacturer gives 91% of transmission. But because I had trouble adjusting it, I'll do a beam profile tomorrow to check the new FI and the setup.