Baffle material characterization for Advanced LIGO

Abstract

The transition to Advanced LIGO introduces new sensitivity requirements for the LIGO interferometers. When light scatters away from the main laser beam, then scatters off the beam tube and returns to the main beam, noise is introduced into the phase of the laser. The Auxiliary Optics Support subsystem uses baffles and beam dumps to control this scatter, but the baffle material and shape contribute some scatter as well. Careful selection of baffle material for Advanced LIGO is necessary in order to minimize baffle backscatter. Characterization of potential materials will also inform the geometry and placement of baffles. To this end, I developed a scatterometer experiment designed to measure the Bidirectional Reflectance Distribution Function (BRDF) of a material. The arrangement was used to measure the BRDF for black welder's glass, the prime candidate material for baffles in Advanced LIGO. I found that black glass has a BRDF on the order of 10-, putting it within sensitivity requirements.