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Message ID: 10887     Entry time: Tue Jan 13 00:42:15 2015
Author: Jenne 
Type: Update 
Category: LSC 
Subject: Error signals for MICH with variable finesse technique 

In order to know where we should try to make the transition from REFL##Q to ASDC for MICH, I did a quick Optickle simulation to see what the error signals will look like.

The idea is to try to lock the PRMI on a single REFL diode (ex. REFL33 I&Q) with some MICH offset, and then transition over to ASDC.  As soon as we have completed the transition, we can engage the normalization matrix to normalize ASDC by POPDC, and also increase the MICH offset if we want.  Unfortunately, we do not as yet have the ability in our model to independently normalize different error signals, and then blend them, so we have to turn on the normalization after we've transitioned.

Here is the situation for PRMI-only:

You can see that REFL33Q has a slightly wider range than REFL165Q.  It seems like we can perhaps try to make the transition around -15nm or so.  Note that the error signals are not quite symmetric about 0nm, so we can use that to help determine what + and - mean.  We expect that we need to add about 1nm offset to REFL33Q to get a true minimum in ASDC, so the sign of the digital offset that we need will tell us if there is a sign flip or not between the digital offset and this x-axis. 

After we get this to work (hopefully in the next hour or so....), we will want to try the same thing with the arms held off resonance. 

Usually we lock the PRMI at an offset of about 3nm:

However we could do it lower, perhaps around 1nm (which is where we currently are doing our CARM/DARM ALS->IR signals transitions):

At some point, we will arrive at 0nm CARM offset, when we'll want to transition back to RF signals (although probably we could jump straight to a 1f signal, not plotted):

The moral of the story here is that I'm not sure how we were ever successfully locking MICH on REFL165Q, unless my phase-setting in Optickle is way off.  Certainly it looks like we should be sticking with REFL33 for PRFPMI.  Also, since we have an offset in REFL33Q anyway (which we have seen and have commented on before), at 3nm CARM offset it looks like we could try to just do the jump without any extra digital offset.  Here's a zoom of the 3nm situation:

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