Optical loss from scattered light could limit the performance of
quantum-noise filter cavities being considered for an upgrade to the Advanced
LIGO gravitational-wave detectors. This paper describes imaging scatterometer
measurements of the large-angle scattered light from two high-quality sample
optics, a high reflector and a beam splitter. These optics are each
superpolished fused silica substrates with silica:tantala dielectric coatings.
They represent the current state-of-the art optical technology for use in
filter cavities. We present angle-resolved scatter values and integrate these
to estimate the total scatter over the measured angles. We find that the total
integrated light scattered into larger angles can be as small as 4 ppm.Comment: 11 pages, 9 figure
