We realized a tunable continuous-wave terahertz source with megahertz frequency
resolution. The system is based on optical heterodyning of two near-infrared distributed
feedback diode lasers, each laser being stabilized by electronic feedback from a low-finesse
quadrature interferometer. The control loop permits precisely linear laser frequency scans
over> 1200 GHz⁠, and a beat signal linewidth of 1 MHz at 80 ms time scale. Using GaAs
photomixers and log-periodic antennae, we achieve a signal-to-noise ratio of the terahertz …