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Entry  Sun May 13 21:18:55 2018, awade, DailyProgress, BEAT, Reinstalling beam splitter and beat detector IMG_3368_DamageOnSouthBeatSteeringMirror.JPGIMG_CurrentBeatBoardLayout.JPG
    Reply  Mon May 14 09:25:44 2018, Craig, DailyProgress, BEAT, Reinstalling beam splitter and beat detector 
       Reply  Tue May 15 14:24:25 2018, awade, DailyProgress, BEAT, Reinstalling beam splitter and beat detector 
Message ID: 2184     Entry time: Mon May 14 09:25:44 2018     In reply to: 2183     Reply to this: 2185
Author: Craig 
Type: DailyProgress 
Category: BEAT 
Subject: Reinstalling beam splitter and beat detector 

If I recall correctly, I placed those beam dumps everywhere because the transmission table ghost beams had gotten out of control.  Particularly bad was out of the cavity, it seems you have left those beam dumps there but I would double check their status.  There was also reflection from the first lenses back into the cavity that I tried to have the post-cav beam dumps block, unclear how successful I was. 

Also bad was reflection from the ND filter we had on our pre-RFPD lens, it seems like you have removed this lens which is great.  Also, there was some backscatter from the third camera I had installed at the other end of the recombination BS that unites the North and South path light, seems like that is gone as well, you might dump that light if you aren't already. 

Finally, the scatter from the IR cameras was like a diffraction grating pattern that went everywhere, it might be prudent to place those black glass beam dumps with the holes in front of them.



I reinstalled the beam splitter, steering mirror and PD in the transmission path.  We have the beam profiling measurement and can reposition the detector more optimally in the near future.  But for now we need to get back to the scatter hunt: specifically the 500 Hz feature that haunts the area directly around the north path pre-mode cleaner (PMC).  

Some of the beam dumps were removed as they weren't bolted down or were in the way of the beam profiling (sorry Craig).

A few things to note:

  • I have now permanently removed the black cube steering mirror mount from the beat setup.  There are visable damage marks on the mirror that make it a dubious choice.  Also the mount just has too much close proximity surfaces on the reflection side that can be potential scatter points.  
  • I replaced the south path steering mirror (previously the black cube beam mount) with a Newport 10Q20HE.1 mirror.  The surface specs are ok 10-5 scratch dig and better than lambda/10.  Its optimized to be over 99% on both polarizations but not technically ultra good HR. We're out of the really good HR reflectors so I might be time to buy some more
  • After a bit of checking around I've concluded that the first beam splitters directly after the cavity are actually 50:50 splitters optimized for 45 deg on s-pol. The reflection is used to generate the beat and transmission of these splitters is used for ISS trans PDs and CCD cameras.  There is a 99% 45 s-pol splitter after the 50:50 splitters that picks most of the light off for the ISS PDs.  Its seems like we are wasting a lot of light going to the ISS here and should actually be using p-pol to get critical coupling into the InGaAs diodes.
  • We're still using two separate raised breadboards here in this optical setup.  I couldn't find any larger 3' by 1' boards in the Bridge West labs, might need to make a purchase to get this together all on one board.
  • I looks like John Martin will be using the ISS from the ATF lab in his SURF project.  I need to get onto getting more stuff fabricated so we can build the ISS better. Maybe a good task for Anchal would be to make a SolidWorks assembly of the new board to see how it will fit together.
  • There are still a few 1/4" adjustble hight mounts in the ISS part of the board.  These should be replaced with 1" mounts as we now have them in stock.


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