KAGRA Logbook

AOS (Beam Reducing Telescopes)

tomotada.akutsu - 20:09 Monday 09 December 2019 (12019)

Investigation to resolve the pitch-yaw coupling at TMSX

Adam, Eleonora, Tomo

Continued from 11903. To put it shortly, no improvement.

Assuming the beam sizes on the QPDs were the matter (too small), we moved the 2nd lens of the GPT (Gouy phase telescope) or "RLNS2" named here so that we could increase the beam size.

First we measured the data points shown by filled circles in Fig. 1; the fitting paramters of the waist sizes and the waist locations are also shown in the figure.
We also did additional measurements and the data points are shown with empty circles; the fitting parameters are also shown in the same manner; actually, the estimated beam waist locations vary too much w. r. t. the data points used for the fitting...

Anyway we put QPDs at around "-600 mm" for QPD1, "+250mm" for QPD2. Then we shook the ITMX and/or ETMX to see the pitch-yaw coupling in the same manner as before, but the coupling was not resolved.

Then we tweaked the position of the QPD2 forward and backward, but what we observed was more complicated responses of them...

Well, looking at 10337 (locating QPDs at TMSY), the beam waists were 0.2xx mm and I maybe do some efforts to make the same beam waist sizes in X and Y (?).

Images attached to this report

Comments to this report:

shinji.miyoki - 2:39 Tuesday 10 December 2019 (12023)

If there are some optics that are sensitive to S/P polarization before the QPD such mirrors that is optimized for S? polarization, I am afraid that this coupling is a fake due to birefringence in ETMX.

Did you see the same thing in TMSY or not tried yet?

If the FP axis moves, of course the beam path position inside the ETMX moves. The effect of birefringence can vary according to positions. Even if the beam moves , for example horizaontally, the amount of S->P conversion in the upper and lower area in the beam can change. This can result in power change in the upper and lower part in the QPD after passing through mirrors that have different reflectivity for S/P. This effect can be recognized as pitch motion by QPD.

S->P loss was estimated around 5~9% in the case of ITMs. ETMs have worse value ?

If this is true, we should take two dimensional S->P conversion map (fake map), then fit this map with two dimensional curved sphere, then subtract this from the detected value by QPD with a proper normalization.

If the coupling rate is constant, this story is doubtful.

tomotada.akutsu - 10:07 Tuesday 10 December 2019 (12026)

Actually, this coupling is not observed for green at TMSX, even when the green and IR beams are monitored synchronously during shaking ITMX and/or ETMX. Moreover no couplings were observed at TMSY.

Anyway locking the X arm out of the center points of the mirrors might be the next plan (?)

shinji.miyoki - 10:33 Tuesday 10 December 2019 (12027)

Thank you for your explanation.

adam.mullavey - 15:19 Tuesday 10 December 2019 (12040)

I'm attaching the measurements of the QPD signals with excitations at 0.1Hz in ITMX pitch (1st attachment), ITMX yaw (2nd attachment), ETMX pitch (3rd), ETMX yaw (4th). As can be seen pitch and yaw are still very coupled on QPDA2. QPDA1 doesn't look as bad (I'm not sure it was bad before), and could be used for soft control, in theory we only need 1 QPD. I've also attached a photo of the new configuration.

Images attached to this comment

tomotada.akutsu - 16:19 Tuesday 10 December 2019 (12044)

Interestingly, the tendency of the pitch-yaw couplings are similar to the results reported on 11879.