  We demonstrate a coherent imaging system based on a terahertz (THz) frequency
quantum cascade laser (QCL) phase-locked to a near-infrared fs-laser comb. The
phase locking enables coherent electro-optic sampling of the continuous-wave
radiation emitted by the QCL through the generation of a heterodyne beat-note
signal. We use this beat-note signal to demonstrate raster scan coherent
imaging using a QCL emitting at 2.5 THz. At this frequency the detection noise
floor of our system is of 3 pW/Hz and the long-term phase stability is <3
degrees/h, limited by the mechanical stability of the apparatus.
