  We investigate the reflectivity and optical scattering characteristics at
1064\,nm of an antireflection coated fused silica window of the type being used
in the Advanced LIGO gravitational-wave detectors. Reflectivity is measured in
the ultra-low range of 5-10\,ppm (by vendor) and 14-30\,ppm (by us). Using an
angle-resolved scatterometer we measure the sample's Bidirectional Scattering
Distribution Function (BSDF) and use this to estimate its transmitted and
reflected scatter at roughly 20-40\,ppm and 1\,ppm, respectively, over the
range of angles measured. We further inspect the sample's low backscatter using
an imaging scatterometer, measuring an angle resolved BSDF below $10^{-6}$
sr$^{-1}$ for large angles (10$^\circ$--80$^\circ$ from incidence in the plane
of the beam). We use the associated images to (partially) isolate scatter from
different regions of the sample and find that scattering from the bulk fused
silica is on par with backscatter from the antireflection coated optical
surfaces. To confirm that the bulk scattering is caused by Rayleigh scattering,
we perform a separate experiment, measuring the scattering intensity versus
input polarization angle. We estimate that 0.9--1.3\,ppm of the backscatter can
be accounted for by Rayleigh scattering of the bulk fused silica. These results
indicate that modern antireflection coatings have low enough scatter to not
limit the total backscattering of thick fused silica optics.
