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AOS (Beam Reducing Telescopes)
tomotada.akutsu - 20:53 Wednesday 28 March 2018 (4577) Print this report
Installation of additional optics to TMSY

K. Nagano, Ko. Yamamoto, Akutsu

Details here.


  • A collimated green beam is now coming out of the EYT chamber.
  • A periscope for the IR beam was constructed.
  • Memo: several parts should be replaced later (before being vacuum)

IR perisope

In the morning (Fig 1 and 2), we installed two steering mirrors, which can relfect green/IR lights, downstream of the 2nd BRT lens so that the green beam got out of the EYT chamber. We used the green beam to align a periscope for IR beam on the optical bench (outside of the EYT chamber); note that IR is too faint to use when we don't have Y arm cavity in bKAGRA phase 1, so we just used the green one as a reference.

Green beam collimation

In the afternoon, we changed our mind/plan from the one described here after discussion with Michimura-kun remotely. We determined to set the optics downstream of the 2nd BRT lens according to Morozumi-kun's version as much as possible (briefly described here), so changed the setup (only those setups installed this morning; the two steering mirrors) in EYT. After the 2nd BRT (Fig 3),

  1. a steering mirror: Sigma-koki's TFVMQ-50.8C10-20-W1D-532/1064-ARS-30/40D (from PSL room); it can reflect both 532/1064nm light.
  2. a harmonic BS: Thorlab's HBSY21 (from NAOJ) -> IR will just go outside of the chamber by this mirror, while most of green will transmit it.
  3. (roughly putting, only the green beam can reach here) a steering mirror as the same as (1); later we found the mirror here dose not need to be dichroic.That will be replaced later as the dichroic mirror is a precious.
  4. a lens (f=-200mm): Sigma-koki's SLB-50.8-200NM (from NAOJ)

Then the green beam was collimated by the (4) lens (Fig 4, 5, and 6). To achieve the setup, some parts were used from the ones for the other purpose, so need to be replaced later.


One question is the position of the lens. According to the calculation by Morozumi-kun, the (4) lens should be more closer to the 2nd BRT lens, but today we found more far away is better to collimate the green beam. Fortunately or unfortunately, if we need to put the lens in the position where he pointed, it was impossible because of the mechanical interfer with several parts like 1st BRT mirror (the large one). Note that we did not tune the positions of the lenses on the input table near the PR2 chamber, as we don't have any references. The arm cavity itself should be the reference, but we don't have it yet. The best location of the  (4) lens could be changed according to positions of the input lenses.

Other topics

We were suffering from sudden change of the green beam mode from yesterday (or more before?? I don't have clear memory). We firstly thought this would be due to some position changes of PR3 or 2 or something, but no idea. When the mode changed, a green beam spot (or pond?) appeared on the viewport window on the gate valve (actually pendulum valve) between EYT and EYC. Today we found the viewport window seemed get bedewing(!?); I'm not sure the bedewing was whether air side or vacuum side, but looked like so. The bedewing pattern was shrinking slowly, and synchronously the green beam pattern, which was made by the scattered light by the bedewing pattern, are shrinking to disappear.

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