  The BICEP/Keck series of experiments target the Cosmic Microwave Background
at degree-scale resolution from the South Pole. Over the next few years, the
"Stage-3" BICEP Array (BA) telescope will improve the program's frequency
coverage and sensitivity to primordial B-mode polarization by an order of
magnitude. The first receiver in the array, BA1, began observing at 30/40 GHz
in early 2020. The next two receivers, BA2 and BA3, are currently being
assembled and will map the southern sky at frequencies ranging from 95 GHz to
150 GHz. Common to all BA receivers is a refractive, on-axis, cryogenic optical
design that focuses microwave radiation onto a focal plane populated with
antenna-coupled bolometers. High-performance antireflective coatings up to 760
mm in aperture are needed for each element in the optical chain, and must
withstand repeated thermal cycles down to 4 K. Here we present the design and
fabrication of the 30/40 GHz anti-reflection coatings for the recently deployed
BA1 receiver, then discuss laboratory measurements of their reflectance. We
review the lamination method for these single- and dual-layer plastic coatings
with indices matched to various polyethylene, nylon and alumina optics. We also
describe ongoing efforts to optimize coatings for the next BA cryostats, which
may inform technological choices for future Small-Aperture Telescopes of the
CMB "Stage 4" experiment.
