  New experiments that target the B-mode polarization signals in the Cosmic
Microwave Background require more sensitivity, more detectors, and thus
larger-aperture millimeter-wavelength telescopes, than previous experiments.
These larger apertures require ever larger vacuum windows to house cryogenic
optics. Scaling up conventional vacuum windows, such as those made of High
Density Polyethylene (HDPE), require a corresponding increase in the thickness
of the window material to handle the extra force from the atmospheric pressure.
Thicker windows cause more transmission loss at ambient temperatures,
increasing optical loading and decreasing sensitivity. We have developed the
use of woven High Modulus Polyethylene (HMPE), a material 100 times stronger
than HDPE, to manufacture stronger, thinner windows using a pressurized hot
lamination process. We discuss the development of a specialty autoclave for
generating thin laminate vacuum windows and the optical and mechanical
characterization of full scale science grade windows, with the goal of
developing a new window suitable for BICEP Array cryostats and for future CMB
applications.
