Michael, Akutsu, Aso

This is a report on the work done on Nov. 11th (Fri).

We recovered the beam path for the OMC transmission camera.
In principle, this should have been a simple work. However, it turned out to be a tricky one.

The problem is that the OMC lock is unstable in the air. This is because the 800Hz mechanical resonance saturates the DCPD transimpedance circuit, when the transmitted beam power is large.
Therefore, a stable lock is achieved only when we locked to a higher order mode with much lower transmission power.
This made it difficult to track the subtle transmitted beam with a sensor card.

Here are several tricks we used or to be used in the future.

1. Engage the QPD centering servo (OMC ASC) only for DOF1 (both P and Y). This is because DOF1 is fed back to the OMMT2, which is in a separate chamber from the OMC. We did not want to engage the loop for OSTM because we are working in the chamber that contains the OSTM suspension. Engaging only DOF1 still stabilizes the alignment into the OMC reasonably well.

2. Lock the OMC to a higher order mode with transmission power of around 4mW. Alternatively, you can reduce the beam power at AS to be smaller, like 10mW and lock to the TEM00 mode.

3. Use a high sensitivity sensor card. We had an F-IRC2 from Newport at hand. However, this one has a lower sensitivity than the F-IRC1 sensor card at 1um.
F-IRC2 has a wider wavelength band (800 - 1700nm), but we don't need it, because we only use it at 1064nm. In KAGRA it must be prohibited to buy F-IRC2 in the future.

4. Turn off the lights in the tunnel. Turning off the LED lights in the clean booth was not enough.