AN IMAGING SCATTEROMETER FOR CHARACTERIZING SURFACE AND BULK SCATTER
Joshua Hacker, Joshua Smith.
California State University Fullerton, Fullerton, CA.
The Advanced Laser Interferometer Gravitational-wave Observatory (aLIGO) requires optics with specific scatter characteristics in order to detect gravitational waves. With this in mind, an angle-resolved imaging scatterometer has been developed to characterize scatter from diverse optics in order to determine their suitability for use in aLIGO. The scatterometer laser operates at 1,064 nm, to match the lasers used in aLIGO, with a fixed angle of incidence onto a sample optic. The laser and optic are placed on a rotating stage and the scatter is recorded by a ccd camera over a discrete range of viewing angles. Like photodiode-based angle-resolved scatterometers, this setup is used to measure a sample's bidirectional reflectance distribution function (BRDF) and estimate the total integrated scatter (TIS). The primary advantage added by the ccd imaging is the ability to distinguish the character and spatial distribution of the scatter, allowing discrimination of point defects from micro-roughness, bulk scatter from surface scatter, and identify scatter associated with the setup, not the sample. In this article, we discuss the design, advantages, and limitations of the imaging scatterometer, giving examples of how it is used to characterize aLIGO optics and how it might be used for other applications.