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Entry  Tue Mar 21 15:13:44 2017, gautam, Update, IMC, IMC input beam mode matching IMG_7220.JPGIMC_ModeMatch.pdfsingleLensSensitivity.pdfsensitivity.pdfIMCmodeMatch.m
    Reply  Tue Mar 21 21:59:48 2017, gautam, Update, IMC, IMC input beam mode matching IMG_3654.JPGIMG_3655.JPG
       Reply  Wed Mar 22 00:33:00 2017, gautam, Update, IMC, IMC length offset nulling offsetInvestigation.pdfoffset_summing_amp.pdf
          Reply  Wed Mar 22 16:58:25 2017, gautam, Update, IMC, WFS sensing matrix measurements 
             Reply  Thu Mar 23 01:44:53 2017, gautam, Update, IMC, WFS sensing matrix measurements IMC_WFS_segment_TF.pdfIMC_WFS_channels_TF.pdfTFsummary.pdfIMC_WFS_170322.xlsx.zip
                Reply  Thu Mar 23 08:43:11 2017, rana, Update, IMC, WFS sensing matrix measurements 
                   Reply  Thu Mar 23 23:38:58 2017, gautam, Update, IMC, MC SUS damping gains stepped down 
                      Reply  Fri Mar 24 11:26:57 2017, gautam, Update, IMC, MC SUS damping gains restored 
                         Reply  Fri Mar 24 19:04:18 2017, gautam, Update, IMC, Seismic feedforward and WFS MCL_comparison.pdfseis_comparison.pdf
Message ID: 12896     Entry time: Tue Mar 21 15:13:44 2017     Reply to this: 12898
Author: gautam 
Type: Update 
Category: IMC 
Subject: IMC input beam mode matching 

[valera, gautam]

Last night, Valera and I looked into two aspects of the IMC:

  1. How can we accurately set the offset at the error point of the PDH servo such that we lock to the true center of the resonance?
  2. What's up with the large common mode offset on the WFS?

I will post a more detailed elog about last night's work, but Valera also thought it might be a good idea to try and improve the mode-matching into the IMC. I couldn't find anything on the wiki/elog about the mode matching situation on the PSL table, so I quickly went over yesterday to measure some lengths. From looking at the MCREFL DC levels when the mode cleaner is locked (~0.37V) and unlocked (~5.7V), the current mode matching efficiency seems to be about 88%, so there is definitely some headroom for improvement.

Here is my cartoon of the situation on the PSL table. All lengths are measured in mm, and I would say correct to +/- 5 mm, so there could be considerable error here...

  (L1 : f=+200mm. L2: f=-150mm. L3:  f=+400mm)

I extracted the lengths from the edge of the PSL table to IM1 and MC1 from (what I think are) the latest CAD drawings on the DCC. I then put all this into an a la mode script [Attachment #5] - I assumed a waist of 370um at the PMC output mirror, and a waist of 1.78mm at MC1. I neglected the passage through the in-vac Faraday, EOM and BS1 (on the sketch above) and the MC1 substrate. I was able to achieve a theoretical mode-matching efficiency of 1 by just moving the positions of L2 and L3. 

Given that there are probably errors of the order 0.5cm in the lengths on the PSL table, and also the in-vacuum distance to MC1, I figured it would be ideal to just move one lens and see if we can improve the efficiency. It looks like it may be more effective to move L2 than L3. The plot on the right shows that the sensitivity is approximately equal to the positioning of L2 and L3. Judging by this plot, looks like w.r.t. the coordinates in this plot, we are somewhere around (0.02,-0.02).

It looks like if we want to do this, moving L2 (f = -150mm) may be the best way to go.

Attachment 2: IMC_ModeMatch.pdf  69 kB  Uploaded Tue Mar 21 16:14:28 2017  | Hide | Hide all
Attachment 3: singleLensSensitivity.pdf  20 kB  Uploaded Tue Mar 21 16:24:59 2017  | Hide | Hide all
Attachment 4: sensitivity.pdf  42 kB  Uploaded Tue Mar 21 16:25:36 2017  | Hide | Hide all
Attachment 5: IMCmodeMatch.m  3 kB  Uploaded Tue Mar 21 23:03:35 2017  | Hide | Hide all
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%Create a beamPath object
InpPath = beamPath;
%Add components - for a first pass, ignore Faraday and HWPs, so only
%mirrors and lenses..
... 115 more lines ...
ELOG V3.1.3-