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Tue Jul 25 02:03:59 2017, gautam, Update, Optical Levers, Optical lever tuning thoughts
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Fri Jul 28 15:36:32 2017, gautam, Update, Optical Levers, Optical lever tuning thoughts  
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Tue Aug 1 16:05:01 2017, gautam, Update, Optical Levers, Optical lever tuning - cost function construction   
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Thu Nov 16 13:57:01 2017, gautam, Update, Optical Levers, Optical lever noise 
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Thu Nov 16 15:43:01 2017, rana, Update, Optical Levers, Optical lever noise
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Tue Nov 21 11:37:29 2017, gautam, Update, Optical Levers, BS OL calibration updated  
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Tue Nov 21 16:28:23 2017, gautam, Update, Optical Levers, BS OL calibration updated
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Tue Nov 21 23:04:12 2017, gautam, Update, Optical Levers, Oplev "noise budget"
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Wed Nov 22 05:41:32 2017, rana, Update, Optical Levers, Oplev "noise budget"
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Wed Nov 22 15:29:23 2017, gautam, Update, Optical Levers, Oplev "noise budget"
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Wed Nov 22 16:40:00 2017, Koji, Update, Optical Levers, Oplev "noise budget"
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Wed Nov 22 19:20:01 2017, rana, Update, Optical Levers, Oplev "noise budget"
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Wed Nov 22 23:56:14 2017, gautam, Update, Optical Levers, Oplev "noise budget"
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Thu Nov 23 18:03:52 2017, gautam, Update, Optical Levers, Oplev "noise budget" 
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Message ID: 13441
Entry time: Tue Nov 21 23:04:12 2017
In reply to: 13439
Reply to this: 13444
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Author: |
gautam |
Type: |
Update |
Category: |
Optical Levers |
Subject: |
Oplev "noise budget" |
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Per our discussions in the meetings over the last week, I've tried to put together a simple Oplev noise budget. The only two terms in this for now are the dark noise and a model for the seismic noise, and are plotted together with the measured open-loop error signal spectra.
- Dark noise
- Beam was taken off the OL QPD
- A small DC offset was added to all the oplev segment input filters to make the sum ~20-30 cts [call this testSum] (usually it varies from 4000-13000 for the BS/ITMs, call this nominalSum).
- I downloaded 20mins of dq-ed error signal data, and computed the ASD, dividing by a factor of nominalSum / testSum to account for the usual light intensity on the QPD.
- Seismic noise
- This is a very simplistic 1/f^2 pendulum TF with a pair of Q=2 poles at 1Hz.
- I adjusted the overall gain such that the 1Hz peak roughly line up in measurement and model.
- The stack isn't modelled at all.
Some remarks:
- The BS oplev doesn't have any whitening electronics, and so has a higher electronics noise floor compared to the ITMs. But it doesn't look like we are limited by this lower noise floor anywhere..
- I wonder what all those high frequency features seen in the ITM error signal spectra are - mechanical resonances of steering optics? It is definitely above the dark noise floor, so I am inclined to believe this is real beam motion on the QPD, but surely this can't be the test-mass motion? If it were, the measured A2L would be much higher than the level it is adjudged to be at now. Perhaps it's some resonances of steering mirrors?
- The seismic displacement @100Hz per the GWINC model is ~1e-19 m/rtHz. Assuming the model A2L = d_rms * theta(f) where d_rms is the rms offset of the beam spot from the optic center, and theta(f) is the angular control signal to the optic, for a 5mm rms offset of the spot from the center, theta(f) must be ~1e-17 urad @100Hz. This gives some requirement on the low pass required - I will look into adding this to the global optimization cost.
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