Tunable cw THz source with high-precision frequency control

We realized a tunable continuous-wave terahertz (cw THz) source with MHz frequency resolution. The system is based on optical heterodyning of two near-infrared Distributed Feedback (DFB) 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 @ 80 ms time scale. Using GaAs photomixers and log-periodic antennae, we achieve an SNR of the THz power of > 70 dB at 100 GHz and 100 ms integration time, and still ~ 30 dB @ 1 THz. As an example for high-resolution THz spectroscopy, we characterize the transmission properties of a sub-wavelength metal grating.