  Optical cavities to filter squeezed light for quantum noise reduction require
optics with very low scattering losses. We report on measured light scattering
from two super-polished fused silica optics before and after applying
highly-reflective ion-beam sputtered dielectric coatings. We used an imaging
scatterometer that illuminates the sample with a linearly polarized 1064 nm
wavelength laser at a fixed angle of incidence and records images of back
scatter for azimuthal angles in the plane of the laser beam. We extract from
these images the bidirectional reflectance distribution function (BRDF) of the
optics with and without coating and estimate their integrated scatter. We find
that application of these coatings led to a more than 50% increase of the
integrated wide-angle scatter, to 5.00+/-0.30 and 3.38+/-0.20 ppm for the two
coated samples. In addition, the BRDF function of the coated optics takes on a
pattern of maxima versus azimuthal angle. We compare with a scattering model to
show that this is qualitatively consistent with roughness scattering from the
coating layer interfaces. These results are part of a broader study to
understand and minimize optical loss in quantum noise filter cavities for
interferometric gravitational-wave detectors. The scattering measured for these
samples is acceptable for the 16 m long filter cavities envisioned for the
Laser Interferometer Gravitational-wave Observatory (LIGO), though reducing the
loss further would improve LIGO's quantum-noise limited performance.
