Scattered Light Control in Advanced LIGO

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Abstract:: The phase shift of the light field scattered from baffles, beam dumps, optical elements, or from the vacuum enclosure walls that have horizontal motion, which is injected into the interferometer (IFO) optical mode, will cause an apparent differential arm signal. The magnitude of the scattered light noise depends upon the particular point in the IFO into which the light is injected; and for small phase angles, the phase noise is proportional to the rms amplitude of the horizontal displacement of the scattering surface and to the electric field amplitude of the scattered light injected into the IFO. Fringe wrapping will occur for large phase angles and will generate noise at odd harmonics of the fundamental motion frequency of the scattering surface. Transfer functions for the scattered light phase noise were used to calculate the apparent displacement noise cause by scattered light. For Advanced LIGO, the total scattered light phase noise, expressed in equivalent displacement noise, must be 1/10 of the quadrature sum of the suspension thermal noise and the test mass thermal noise.

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LIGO Document P1000002-v1