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Entry  Fri May 21 11:56:34 2010, Alberto, Update, 40m Upgrading, 40mUpgrade Field Power and RF Power Spectrum at the ports. 38m/38.55m arm length issue. 7x
    Reply  Fri May 21 16:24:11 2010, Koji, Update, LSC, 40mUpgrade Field Power and RF Power Spectrum at the ports. 38m/38.55m arm length issue. 
    Reply  Mon May 24 10:03:14 2010, rana, Update, LSC, 40mUpgrade Field Power and RF Power Spectrum at the ports. 38m/38.55m arm length issue. 
       Reply  Mon May 24 11:32:05 2010, rana, Update, LSC, 40mUpgrade Field Power and RF Power Spectrum at the ports. 38m/38.55m arm length issue. reflRFpowerVsArmLength.png
Message ID: 2966     Entry time: Fri May 21 11:56:34 2010     Reply to this: 2968   2973
Author: Alberto 
Type: Update 
Category: 40m Upgrading 
Subject: 40mUpgrade Field Power and RF Power Spectrum at the ports. 38m/38.55m arm length issue. 

I update my old 40mUpgrade Optickle model, by adding the latest updates in the optical layout (mirror distances, main optics transmissivities, folding mirror transmissivities, etc). I also cleaned it from a lot of useless, Advanced LIGO features.

I calculated the expected power in the fields present at the main ports of the interferometer.

I repeated the calculations for both the arms-locked/arms-unlocked configurations. I used a new set of functions that I wrote which let me evaluate the field power and RF power anywhere in the IFO. (all in my SVN directory)

As in Koji's optical layout, I set the arm length to 38m and I found that at the SP port there was much more power that I woud expect at 44Mhz and 110 MHz.

It's not straightforward to identify unequivocally what is causing it (I have about 100 frequencies going around in the IFO), but presumably the measured power at 44MHz was from the beat between f1 an f2 (55-11=44MHz), and that at 110MHz was from the f2 first sidebands.

Here's what i found:

RFPower_locked_38m.png

RFPower_unlocked_38m.png

FieldPower_locked_38m.png

FieldPower_unlocked_38m.png

 

I found that When I set the arm length to 38.55m (the old 40m average arm length), the power at 44 and 110 MHz went significantly down. See here:

RFPower_3855m.png

 FieldPower_3855m.png

I checked the distances between all the frequencies circulating in the IFO from the closest arm resonance to them.

I found that the f2 and 2*f2 are two of the closest frequencies to the arm resonance (~80KHz). With a arm cavity finesse of 450, that shouldn't be a problem, though.

40mUpgrade_distanceFromResonance_38m.png

 I'll keep using the numbers I got to nail down the culprit.

Anyways, now the question is: what is the design length of the arms? Because if it is really 38m rather than 38.55m, then maybe we should change it back to the old values.

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