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Entry  Tue Mar 31 02:51:39 2015, ericq, Update, LSC, Some locks 
    Reply  Tue Mar 31 03:02:44 2015, Koji, Update, LSC, Some locks 
       Reply  Tue Mar 31 18:27:58 2015, ericq, Update, LSC, Some locks qpdSwitch.pngqpdAlways.png
Message ID: 11185     Entry time: Tue Mar 31 18:27:58 2015     In reply to: 11183
Author: ericq 
Type: Update 
Category: LSC 
Subject: Some locks 

Can we plot the arm power trend for multiple locks to see if it is associated with any thermal phenomenan in the IFO?

I'm currently more inclined to believe that the arm power trends have more to do with the arm alignments. Here's a 10 minute lock from last night, where the QPD servos were switched on about halfway through. I couldn't get Den's new servos to turn on without blowing the lock, so I reverted to my previous design, but still only actuated on the ETMs, with their oplevs still on. 

The most obvious feature is the reduction in power that seems to correspond to a ~10urad pitch deflection of ITMX when the lock begins. Is this optical spring action?

Also, it looks like the Y arm Yaw loop was badly tuned, and injecting noise. Ooops.

As of Den's QPD tuning, the QPD servos just actuate on the ETM. This next lock effectively had the QPD servos on the entire time, and we can see a similar drift in ITMX, and how ETMX then follows it to keep the QPD spot stationary. (Here, I'm plotting the QPD servo control signals, unlike above, so we can see X pitch servo output drift with the ITMX deflection)

Again, ITMX is moving in pitch by ~10urad when the interferometer starts resonating. If this is an optical spring, why does this just happen to ITMX? If it is digital shenanigans, how does it correlate with the lock, since there is nothing actuating on ITMX but oplevs and OSEM damping? Is light scattering into the ITMX OSEMs?


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