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Entry  Mon Nov 26 23:45:52 2012, Ayaka, Update, IOO, Tuning MC_L POYerr_MCL.jpgMCL-MCFxover.jpgMCL-MCFxover2.jpg121126.tar.gz
    Reply  Tue Nov 27 00:45:20 2012, jamie, Update, IOO, MC_L and laser frequency noise spectra laser_noise.pdfMCL_noise.pdf
Message ID: 7748     Entry time: Mon Nov 26 23:45:52 2012     Reply to this: 7750
Author: Ayaka 
Type: Update 
Category: IOO 
Subject: Tuning MC_L 

[Rana, Jamie, Ayaka]

We could not lock the arms with MC_L loop on. Therefore we measured the change in YARM error signal when the MC_L is turned on.

POYerr_MCL.jpg

(data; POYerr_MCF.xml in zip file)

Green line; POY error signal when MCL loop was on and the YARM loop gain (0.5) was so high that the saturated control signal made funny peak around 250 Hz.
Blue line; POY error signal when MCL loop was off and the YARM loop gain was low (0.2).
Pink line; POY error signal when MCL loop was on (the gain was -300) and the YARM loop gain was low (0.2).
Red line; POY error signal when MCL loop was on, another low pass filter (2nd order, cut off frequency of 55Hz) was added to MCL loop and the YARM loop gain was low (0.2).

We also changed the filter trigger in order to lock YARM. The FM7 and 8 trigger was turned off. It means that spectrum above was took with FM2,3,4,5,6,9,10 on. Whitening filters were also on.

MCL control signal makes the arm spectrum bad because the MCL control signal moves MC2 mirror additionally and adds extra frequency noise.
Ideally, error signal should be the same at higher frequencies and go down at the lower frequencies when the MCL loop is on because MCL signal should suppress the seismic noise.

Before we added the LPF, MCF/MCL loop cross over (which was taken with the template /users/Templates/MC/MCL-MCF_xover-2012-8-23.xml) is below;

MCL-MCFxover.jpg

(MCL-MCF_xover.xml in zip file)

After the LPF is added, the cross over has been changed as below;
MCL-MCFxover2.jpg

(MCL-MCF_xover2.xml in zip file)

For now, I will just turn off the MCL loop for the acoustic noise experiments.

Attachment 4: 121126.tar.gz  358 kB
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