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Entry  Wed Nov 12 18:08:35 2014, Jenne, Update, LSC, RIN in transmission a problem? RIN_TRX_TRY_MCtrans_12Nov2014_zip.xml.gzRIN_TRX_TRY_MCtrans_12Nov2014.pdfRINcontribution_12Nov2014_linearXscale.pngRINcontribution_12NOv2014_logXscale.png
    Reply  Thu Nov 13 01:00:37 2014, rana, Update, LSC, RIN in transmission a problem? 
    Reply  Thu Nov 13 04:28:28 2014, Jenne, Update, LSC, RIN in transmission a problem? zippyzip.zipRIN_TRX_TRY_IPPOS_NoLens_12Nov2014.pdf
       Reply  Thu Nov 13 23:42:01 2014, Jenne, Update, LSC, RIN vs. Seismic Seismic_TRXTRYandMC_13Nov2014.pdf
          Reply  Fri Nov 14 17:08:17 2014, Jenne, Update, LSC, RIN vs. Seismic 
       Reply  Fri Nov 14 20:31:13 2014, ericq, Update, LSC, RIN in transmission a problem? 
Message ID: 10703     Entry time: Wed Nov 12 18:08:35 2014     Reply to this: 10707   10709
Author: Jenne 
Type: Update 
Category: LSC 
Subject: RIN in transmission a problem? 

In my previous meditations about RIN, particularly elog 10258, I was only thinking about the RIN contribution at the offset that I was currently sitting at.  Also, In elog 10258 I was comparing to the ALS signals and just said that the trans signals are better which is true, although isn't super helpful when thinking of reduced CARM offsets. 

My summary today is that I think we want to reduce the RIN in arm transmissions by a factor of 3.

Rather than dig around, I just remeasured the RIN, for both the single arm transmissions and the MC transmission.  (Data attached as .xml file)

The RMS RIN for the Xarm is 1.3e-2.  The RMS RIN for the Yarm is 8.9e-3.  The RMS RIN for MCtrans is 4.0e-3.  For the simulations below, I will use 1e-2 as an average RIN for the arms.


As an estimate of the RIN's contribution to cavity fluctuations, I divide the RIN by the slope of the CARM transmission peak.  The slope (from optickle) gives me [ delta-W / delta-m ], and the inverse of that gives me [ delta-m / delta-W ].  I multiply this by RIN, which is [ delta-W / W ] to get [delta-m / W]. 

Then, since I'm using the DC transmission signals as my error signals, I use just TRX (normalized to be 1 for single arm resonance) as my Watts.

So, in total, the traces plotted are { TRX * RIN / slope }. 

The 2 plots are the same data, one with linear-x and the other with log-x.  They both include my estimate of the cavity length fluctuations due to RIN at the arm transmission, as well as an estimate of the cavity length fluctuations if the arm RIN was as good as the MC RIN.  I also show the DRFPMI CARM linewidth (23 pm for HWHM), and 1% of that linewidth.  The last trace is 1% of the half-width of the transmission peak, at the current CARM offset.  For example, 1000 pm away from full resonance the half-width is 1000 pm and 1% of that is 10 pm. 


What we want to see here is that the solid blue line is below one of the dotted lines.  I think that using the overall linewidth (purple dotted line) isn't really the right thing to look at.  Our goal is to prevent excursions that will get too close to the resonance peak, and cause a lockloss.  A one picometer excursion is a much bigger problem (relatively) below say 100 pm, as opposed to above 100 pm.  So, I think that we should be looking at the half-width of the resonance peak at whatever the current CARM offset is (orange dotted line).  Above 25 pm, the blue line is below the orange line for all offsets plotted.  If we made the arm RIN as good as the MC RIN, that would be true down to 12-ish pm. 

We should be able to safely transition to non-normalized RF signals at 10pm or below.  This implies that (since any RF signals normalized by this RIN-y trans signal will have the RIN), we want to improve the RIN of the transmission PDs by about a factor of 3. (This will lower the blue line such that it crosses the orange dotted line at 10 pm).


Attachment 1: RIN_TRX_TRY_MCtrans_12Nov2014_zip.xml.gz  303 kB
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