[Komori, Tanaka, Uchiyama, Ushiba]

Abstract:

We aligned POP FORWARD beam path and reduced the beam power on POP FORWARD QPDs for high power operation.
Now, POP FORWARD QPD SUM value is about 10 cnts with misaligned PRM, which is corresponding to 14000 cnts with 10W PRFPMI.

Detail:

Since the beam is almost clipped at FST1 (see JGW-T1909623-v12), we changed the mirror mount to lens mount.
Thanks to this modification, the beam from FST1 to FST2 seems to be enough large gap from the optical structures now.
One note here is that we set the HR side into lens folder backwards.

Initially we planed to reduce the QPD SUM value by a factor of 5 by changing the gain switch and by a factor of 2 by inserting 50-50 BS; however we noticed the gain switches has already been low gain, so we need to reduce laser power more than 10 times.
So, we inserted BSX11 between FLNS1 and FBS1 instead of BS, which have more than 95% reflectivity for s-pol beam, according to the spcification.
Reflected beam at BSX11 is dumped by LB1/M.

Then, we aligned the beam to QPD centers and checked if all beam dumps propery dump the ghost beam.
Figure 1 shows the QPD SUM values when PRFPMI is locked with 1W after the work.
QPD SUM values are less than 2000 cnts, so QPD SUMs won't be saturated even with 10W PRFPMI.

Note:

We found that the POP FORWARD beam has a ring around the main circular beam (fig2).
This ring is not seen just after the viewport but appeares after the beam flies.
Since there seems no clipping on the optical table and the ring appears between the upper periscope mirror and the lower periscope mirror when the beam passes in air, we thought it might come from inside the chamber, but we could not confirm it.