Ushiba, Miyo, Akutsu

Abstract

We revisited the POP table, and investigated the robustness (or immunity against mis-alignment of several parts) regarding the optical alignment in the setup of the GreenX input made until yesterday (17428). Seems ok. Added an iris diaphragm in front of GrRFPD.

Alignment of a steering mirror in front of GrRFPD

By mis-aligning ITMX so that the reflected green beam from ITMX no longer entered GrRFPD, only the reflection beam from ETMX entered it. Then we tweaked a steering mirror in front of GrRFPD to look for the best alignment (if any) of it while looking at the DC output of this PD (K1:ALS-X_ARM_REFL_OUT16), but no significant increase of the signal was found; this would mean the current alignment is not so bad.

By the way, a part of the motivation of this work is also related to the power budget mistery reported in 17430, 17432, 17435. So we measured the power of the ETMX reflection just in front of GrRFPD with a power meter (Thorlabs S121C), and was measured as ~ 12-14 uW, which was fluctuated due to maybe dancing of ITMX, ETMX, PR3 and so on. I can say this number is consistent with the value calibrated back from the read in the digital system (16 uW in 17430). Actually we were able to see the green beam with our eyeballs! (I'm not sure why I could not find it out in the past, but maybe it was too dancing...) See Fig. 1.

Sweeping ITMX

Then ITMX was re-aligned. Along with the alignment of the steering mirror in front of GrRFPD as reported above, we swept ITMX to expecting there might be nicer power input; no improved power, as a result. With the power meter, it was measured as ~2.5 mW just in front of GrRFPD. I can say this is consistent with value calibrated back from the read in the digital system (2.7mW in 17435).

Power budget and clipping check

To learnn more about the situation on the POP table, we measured power of the green beams at several locations (Ushiba-kun will report later in detail). A part of the motivation of this work is that we are worried whether any clippings would be at the Faraday isolator (FI; see 17407). According to my observation, the input part of the FI is obliqued, so it might not be strange the output beam (reflected from the main interferometer) was titled in pitch... maybe.

We also put the power meter just at the output port (for the reversing beam) of the FI, and applied tilt offset to ITMX in pitch or yaw to learn how much their mis-alignment will cause beam clipping anywhere. The detail will be reported by Ushiba-kun. According to the measurement result, the "well-aligned in our believe" ITMX alignment is not too bad (well within or near the middle of the non-clipping range).

Finally we put an iris diaphragm just in fron of GrRFPD so that some ghost beams from the FI input port can dump (Figs. 2 and 3).