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Entry  Sat Jun 23 02:29:18 2012, yuta, Update, Green Locking, X arm mode scan results XarmScan20120623.pngfine8FSRscanXarm.png
    Reply  Wed Jun 27 03:43:52 2012, yuta, Summary, IOO, how to improve mode matching to arms modematchMCtoARM_design.pngmodematchMCtoARM_MMT1MMT2longer.png
       Reply  Wed Jun 27 10:27:09 2012, rana, Summary, IOO, how to improve mode matching to arms 
Message ID: 6876     Entry time: Wed Jun 27 03:43:52 2012     In reply to: 6859     Reply to this: 6877
Author: yuta 
Type: Summary 
Category: IOO 
Subject: how to improve mode matching to arms 

From the mode scan measurements of the arms(elog #6859), ~6% of mode-mismatch comes from 2nd-order mode. That means we have longitudinal mismatch.

Suppose every mirrors are well positioned and well polished with designed RoC, except for the MMT1-MMT2 length. To get ~6% of mode-mismatch, MMT1-MMT2 length should be ~28cm longer (or ~26cm shorter) than designed value.
I don't know whether this is possible or not, but if they are actually longer(or shorter), we should fix it on the next vent.
I found some related elog on MMT (see #3088).

modematchMCtoARM_design.pngmodematchMCtoARM_MMT1MMT2longer.png


RoC and length parameters I used is below. They maybe wrong because I just guessed them. Please tell me the actual values.
Mirror thickness and effect of the incident angle is not considered yet.

== RoCs ==
MC2 19.965 m (???)
PRM 115.5 m (not used in calculation; just used to guess MC parameters)
ITM flat
ETM 57.37 m

== Lengths ==
MC round trip 27.084 m (???)
MC1 - MC3  0.18 m (???)
MC3 - MMT1 0.884+1.0442 m
MMT1 - MMT2 1.876 m
MMT2 - PRM 2.0079+0.4956 m
PRM - ITM 4.4433+2.2738 m
ITM - ETM 39 m

ELOG V3.1.3-