  The search for inflationary primordial gravitational waves and the
measurement of the optical depth to reionization, both through their imprint on
the large angular scale correlations in the polarization of the cosmic
microwave background (CMB), has created the need for high sensitivity
measurements of polarization across large fractions of the sky at millimeter
wavelengths. These measurements are subject to instrumental and atmospheric
$1/f$ noise, which has motivated the development of polarization modulators to
facilitate the rejection of these large systematic effects.
  Variable-delay polarization modulators (VPMs) are used in the Cosmology Large
Angular Scale Surveyor (CLASS) telescopes as the first element in the optical
chain to rapidly modulate the incoming polarization. VPMs consist of a linearly
polarizing wire grid in front of a movable flat mirror. Varying the distance
between the grid and the mirror produces a changing phase shift between
polarization states parallel and perpendicular to the grid which modulates
Stokes U (linear polarization at $45^\circ$) and Stokes V (circular
polarization). The CLASS telescopes have VPMs as the first optical element from
the sky; this simultaneously allows a lock-in style polarization measurement
and the separation of sky polarization from any instrumental polarization
further along in the optical path. The Q-band CLASS VPM was the first VPM to
begin observing the CMB full time, starting in the Spring of 2016. The first
W-band CLASS VPM was installed in the Spring of 2018.
