1. Stefan Hild For the GEO-team March 2008, ILIAS WG1, Pisa Projection of small angle scattering

2. Stefan HildILIAS WG1 meeting, March 2008, PisaSlide 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

3. Stefan HildILIAS WG1 meeting, March 2008, PisaSlide 3 How much scattered light is necessary to limit the sensitivity of GEO600 ? 1.5e-19 m/sqrt(Hz) Achieved displacement sensitivity Accurary of the phase readout Frequency [Hz] sensitivity [m/sqrt(Hz)]

4. Stefan HildILIAS WG1 meeting, March 2008, PisaSlide 4 How much scattered light is necessary to limit the sensitivity of GEO600 ? ECEC Assuming angle between E C and  to be 90 deg. With E C = sqrt(2.7kW) we get tiny amount of light

5. Stefan HildILIAS WG1 meeting, March 2008, PisaSlide 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 amplitude Sandblasted metal works Super polished mirror

6. Stefan HildILIAS WG1 meeting, March 2008, PisaSlide 6 Photographs of the north end mirror

7. Stefan HildILIAS WG1 meeting, March 2008, PisaSlide 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:

8. Stefan HildILIAS WG1 meeting, March 2008, PisaSlide 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.

9. Stefan HildILIAS WG1 meeting, March 2008, PisaSlide 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.

10. Stefan HildILIAS WG1 meeting, March 2008, PisaSlide 10 12W Laser modecleaner interferometer with „dual recycling“ detector Slide stolen from Haralds talk at the LSC meeting Oct 2008

11. Stefan HildILIAS WG1 meeting, March 2008, PisaSlide 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.

12. Stefan HildILIAS WG1 meeting, March 2008, PisaSlide 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

13. Stefan HildILIAS WG1 meeting, March 2008, PisaSlide 13 Projection: Step 1 Light power at catcher (90mW) Effective mode diameter (from W. Winkler PhD thesis)

14. Stefan HildILIAS WG1 meeting, March 2008, PisaSlide 14 Projection: Step 2 ECEC Transforming light power into effective scattering amplitude:

15. Stefan HildILIAS WG1 meeting, March 2008, PisaSlide 15 Projection: Step 3 ASD of catcher movement ASD of displacement caused by scattered light from MFn catcher

16. Stefan HildILIAS WG1 meeting, March 2008, PisaSlide 16 Projection: Step 3

17. Stefan HildILIAS WG1 meeting, March 2008, PisaSlide 17 More Details can be found in:

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