  The design and performance of a wide bandwidth linear polarization modulator
based on the Faraday effect is described. Faraday Rotation Modulators (FRMs)
are solid-state polarization switches that are capable of modulation up to ~10
kHz. Six FRMs were utilized during the 2006 observing season in the Background
Imaging of Cosmic Extragalactic Polarization (BICEP) experiment; three FRMs
were used at each of BICEP's 100 and 150 GHz frequency bands. The technology
was verified through high signal-to-noise detection of Galactic polarization
using two of the six FRMs during four observing runs in 2006. The features
exhibit strong agreement with BICEP's measurements of the Galaxy using non-FRM
pixels and with the Galactic polarization models. This marks the first
detection of high signal-to-noise mm-wave celestial polarization using fast,
active optical modulation. The performance of the FRMs during periods when they
were not modulated was also analyzed and compared to results from BICEP's 43
pixels without FRMs.
