40m QIL Cryo_Lab CTN SUS_Lab TCS_Lab OMC_Lab CRIME_Lab FEA ENG_Labs OptContFac Mariner WBEEShop
  40m Log  Not logged in ELOG logo
Entry  Mon Nov 4 17:08:23 2013, Jenne, Update, LSC, Thoughts on Transition to IR 
    Reply  Mon Nov 4 18:24:15 2013, Koji, Update, LSC, Thoughts on Transition to IR 
    Reply  Tue Nov 5 16:39:54 2013, Gabriele, Update, LSC, Thoughts on Transition to IR 
Message ID: 9339     Entry time: Mon Nov 4 17:08:23 2013     Reply to this: 9340   9344
Author: Jenne 
Type: Update 
Category: LSC 
Subject: Thoughts on Transition to IR 

Gabriele and I talked for a while on Wednesday afternoon about ideas for transitioning to IR control, from ALS. 

I think one of the baseline ideas was to use the sqrt(transmission) as an error signal.  Gabriele pointed out to me that to have a linear signal, really what we need is sqrt( [max transmission] - [current transmission] ), and this requires good knowledge of the maximum transmission that we expect.  However, we can't really measure this max transmission, since we aren't yet able to hold the arms that close to resonance.  If we get this number wrong, the error signal close to the resonance won't be very good.

Gabriele suggested maybe using just the raw transmission signal.  When we're near the half-resonance point, the transmission gives us an approximately linear signal, although it becomes totally non-linear as we get close to resonance.  Using this technique, however, requires lowering the finesse of PRCL by putting in a medium-large MICH offset, so that the PRC is lossy.  This lowering of the PRC finesse prevents the coupled-cavity linewidth of the arm to get too tiny.  Apparently this trick was very handy for Virgo when locking the PRFPMI, but it's not so clear that it will work for the DRFPMI, because the signal recycling cavity complicates things.

I need to look at, and meditate over, some Optickle simulations before I say much else about this stuff.

ELOG V3.1.3-