  The two 4 km long gravitational wave detectors operated by the Laser
Interferometer Gravitational-wave Observatory (LIGO) were modified in 2008 to
read out the gravitational wave channel using the DC readout form of homodyne
detection and to include an optical filter cavity at the output of the
detector. As part of the upgrade to Enhanced LIGO, these modifications replaced
the radio-frequency (RF) heterodyne system used previously. We describe the
motivations for and the implementation of DC readout and the output mode
cleaner in Enhanced LIGO. We present characterizations of the system, including
measurements and models of the couplings of the noises from the laser source to
the gravitational wave readout channel. We show that noise couplings using DC
readout are improved over those for RF readout, and we find that the achieved
shot-noise-limited sensitivity is consistent with modeled results.
