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Message ID: 5773     Entry time: Mon Oct 31 21:46:32 2011
Author: kiwamu 
Type: Update 
Category: LSC 
Subject: Dependence of Recycling gain on incident beam pointing 

I horizontally swept the translation of the incident beam in order to investigate a possible clipping in Power-recycled Michelson (PRMI).

The recycling gain of PRMI depended on the beam pointing but it did't improve the recycling gain.

I guess the amount of the entire shift I introduced was about +/- the beam diameter = +/- 5 mm or so.

Within the range of about +/- 5mm in the horizontal beam translation I didn't find any obvious sign of a clipping.



 This is the procedure which I did:
  (1) Some amount of offsets were introduced on MC2 in both PIT and YAW such that the PZT1 won't rail (#5762).
      => Every time when I introduced the offset I realigned the zig-zag mirrors on the PSL table to maintain the high transmissivity of MC.
  (2) Fine tuning of the MC offsets so that the PZT1_X EPICS value becomes almost zero when the beam is aligned down to the Y arm.
     => 0.523 in C1:ASC_PZT1_X became a point where the coupling of the beam into the Y arm was maximized.
     => Last time the direction which we investigated was the positive side from this zero point (#5709) in PZT1_X.
  (3) Aligned MICH by steering BS.
  (4) Locked PRMI with carrier resonating and aligned PRM to maximized the power recycling gain which was obtained from POYDC.
  (5) Translated the beam pointing
     => First I shifted PZT1_X by a wanted amount.
    => Then I locked the Y arm and realigned PZT2_X by maximizing the Y arm transmission.
          This procedure should give us a pure translation on the incident beam.
  (6) Repeated the same procedure (3) through (5) in each PZT1 position.


Here shows the measured recycling gain and the power reflectivity of PRMI as a function of the beam pointing.


 Upper plot : measured recycling gains (Red) observed maximum values (Black) measured values on average.
 Lower plot : measured power reflectivity of PRMI (Blue) observed minimum values (Black) measured values on average.
 As shown in the plots the recycling gain could go up to 8 at some points.
As the PZT went away from 0 it decreased and eventually became about 3 in each side.
The reflectivity showed the minimum value of 0.4 when the PZT1 was at -1 in EPICS value.
One hypothesis to explain this plot can be that : we are just seeing the effect of the incident beam misalignment.


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