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Stefan Hild For the GEO-team
March 2008, ILIAS WG1, Pisa
Projection of small angle scattering
Stefan Hild ILIAS WG1 meeting, March 2008, Pisa Slide 2
Overview
Intro: small angle scattering
Measurement of the small angle scattering function of a GEO600 core optic
Potential coupling paths of small angle scattering
Projection of the scattered light noise of one of the GEO600 catchers
Stefan Hild ILIAS WG1 meeting, March 2008, Pisa Slide 3
How much scattered light is necessary to limit the sensitivity of GEO600 ?
102
103
10-19
10-18
10-17
Frequency [Hz]
Displacement [m/sqrt(Hz)]
GEO600, May 06
102
103
10-19
10-18
10-17
Frequency [Hz]
Displacement [m/sqrt(Hz)]
GEO600, May 06
1.5e-19 m/sqrt(Hz)
Achieved displacement sensitivity
Accurary of the phase readout
Frequency [Hz]
sen
sitiv
ity [
m/s
qrt
(Hz)
]
Stefan Hild ILIAS WG1 meeting, March 2008, Pisa Slide 4
How much scattered light is necessary to limit the sensitivity of GEO600 ?
ECAssuming angle between EC and to be 90 deg.
With EC = sqrt(2.7kW) we get
tiny amount of light
Stefan Hild ILIAS WG1 meeting, March 2008, Pisa Slide 5
Small angle scattering in general The amount of scattered light varies with the mirror.
We are interested in very small angles, for instance = 0.1m/600m = 1.7mrad
Often the scattering funktion is assumed to be cosine-like
scattering angle
ampl
itude
Sandblasted metal works
Super polished mirror
Stefan Hild ILIAS WG1 meeting, March 2008, Pisa Slide 6
Photographs of the north end mirror
Stefan Hild ILIAS WG1 meeting, March 2008, Pisa Slide 7
Measurement of the small angle scattering of a GEO test mass
Can measure the light passes next to the mirror (red circle).
We get:
Since the modulation depth is similar to main beam:
Using:
We end up at:
Stefan Hild ILIAS WG1 meeting, March 2008, Pisa Slide 8
Two different scattering scenarios
A: Light on the mirror scattered directly into the incoming beam (instead of following the folded arm).
B: Scattered light hitting the metal works around the mirror is scattered back into the interferometer mode.
Stefan Hild ILIAS WG1 meeting, March 2008, Pisa Slide 9
Scenario A: Direct back scattering in the folded arms
Direct back scattering from MFn and MFe causes 1200m pathlength difference => increased frequency noise coupling (be covered by noise projection)
The position of MFn and MFe is only controlled with low bandwidth (0.1 Hz). MFe and MFn movement in combination with small angle scattering can cause noise in detection band.
Stefan Hild ILIAS WG1 meeting, March 2008, Pisa Slide 10
Slide stolen from Haralds talk at the LSC meeting Oct 2008
Stefan Hild ILIAS WG1 meeting, March 2008, Pisa Slide 11
Scenario B: Light scattered back from the catchers.
Catchers are not isolated from ground motion.
If we know: Scattering function of
the mirror and catcher The movement of the
catcher We can do a simple
projection.
Stefan Hild ILIAS WG1 meeting, March 2008, Pisa Slide 12
The scattering processes required
All together 3 scattering processes are required:
• light needs to be scattered onto the catcher.
• light at the catcher needs to be scattered back onto a mirror (MCn).
• at MCn the light needs to be scattered back into the interferometer mode
Stefan Hild ILIAS WG1 meeting, March 2008, Pisa Slide 13
Projection: Step 1
Light power at catcher (90mW)
Effective mode diameter (from W. Winkler PhD thesis)
Stefan Hild ILIAS WG1 meeting, March 2008, Pisa Slide 14
Projection: Step 2
EC
Transforming light power into effective scattering amplitude:
Stefan Hild ILIAS WG1 meeting, March 2008, Pisa Slide 15
Projection: Step 3
ASD of catcher movement
ASD of displacement causedby scattered light from MFn catcher
Stefan Hild ILIAS WG1 meeting, March 2008, Pisa Slide 16
Projection: Step 3
Stefan Hild ILIAS WG1 meeting, March 2008, Pisa Slide 17
More Details can be found in:
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