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Entry  Fri Mar 29 19:56:02 2013, Gabriele, Metaphysics, LSC, Lock of PRMI on sidebands lock_prmi5.pdflock_prmi6.pdflock_prmi7.pdfoscillation.pdflock_prmi8.pdf
    Reply  Sun Mar 31 17:26:32 2013, rana, Update, LSC, Lock of PRMI on sidebands 
    Reply  Mon Apr 1 09:25:35 2013, Jenne, Metaphysics, LSC, Lock of PRMI on sidebands 
Message ID: 8376     Entry time: Fri Mar 29 19:56:02 2013     Reply to this: 8378   8380
Author: Gabriele 
Type: Metaphysics 
Category: LSC 
Subject: Lock of PRMI on sidebands 

I finally managed to get long stretches of PRMI lock, up to many minutes. The lock is not yest very stable, it seems to me that we are limited by some yaw oscillation that I could not trace down. The oscillation is very well visible on POP.

Presently, PRCL is controlled with REFL55_I, while MICH is controlled with AS55_Q. This configuration is maybe not optimal from the point of view of phase noise couplings, but at least it works quite well. I believe that the limit on the length of locks is given by the angular oscillation. I attach to this entry few plots showing some of the lock stretches. The alignment is not optimal, as visible from a quite large TEM01 mode at the dark port.

Here are the parameters I used:

MICH gain -10   PRCL gain -0.1

Normalization of both error signal on POP22_I with factor 0.004

Triggering on POP22: in at 100, out at 20 for both MICH and PRCL.

POP55 demodulation phase -9

MICH and PRCL control signal limits at 2000 counts

 

There is a high frequency (628 Hz) oscillation going on when locked (very annoying on the speakers...), but reducing the gain made the lock less stable. I could go down to MICH=-1.5 and PRCL=-0.02, still being able to acquire the lock. But the oscillation was still there. I suspect that it is not due to the loops, but maybe some resonance of the suspension or payload (violin mode?). There is still some room for fine tuning...

Lock is acquired without problems and maintained for minutes.

Have a nice week-end!

Attachment 1: lock_prmi5.pdf  63 kB  | Hide | Hide all | Show all
lock_prmi5.pdf
Attachment 2: lock_prmi6.pdf  24 kB  | Hide | Hide all | Show all
lock_prmi6.pdf
Attachment 3: lock_prmi7.pdf  69 kB  | Show | Hide all | Show all
Attachment 4: oscillation.pdf  12 kB  | Hide | Hide all | Show all
oscillation.pdf
Attachment 5: lock_prmi8.pdf  61 kB  | Hide | Hide all | Show all
lock_prmi8.pdf
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