  The Cosmology Large Angular Scale Surveyor (CLASS) will measure the
polarization of the Cosmic Microwave Background to search for and characterize
the polarized signature of inflation. CLASS will operate from the Atacama
Desert and observe $\sim$70% of the sky. A variable-delay polarization
modulator (VPM) modulates the polarization at $\sim$10 Hz to suppress the 1/f
noise of the atmosphere and enable the measurement of the large angular scale
polarization modes. The measurement of the inflationary signal across angular
scales that span both the recombination and reionization features allows a test
of the predicted shape of the polarized angular power spectra in addition to a
measurement of the energy scale of inflation.
  CLASS is an array of telescopes covering frequencies of 38, 93, 148, and 217
GHz. These frequencies straddle the foreground minimum and thus allow the
extraction of foregrounds from the primordial signal. Each focal plane contains
feedhorn-coupled transition-edge sensors that simultaneously detect two
orthogonal linear polarizations. The use of single-crystal silicon as the
dielectric for the on-chip transmission lines enables both high efficiency and
uniformity in fabrication. Integrated band definition has been implemented that
both controls the bandpass of the single mode transmission on the chip and
prevents stray light from coupling to the detectors.
