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 power of >70 dB at 100 GHz and 100 ms integration time, and still approximately 30 dB at 1 THz. As an example for high-resolution terahertz spectroscopy, we characterize the transmission properties of a subwavelength metal grating.

A mode-hop-free tunable external-cavity Littrow diode laser with intracavity acousto-optic modulators (AOMs) has been built. The modes of the red laser diode without a special antireflection coating are shifted by varying the injection current. The external resonator modes and the grating selectivity are independently electrically alterable by two AOMs. Thus, a tuning of the external resonator over up to 1900 GHz is possible. A precise computer control of laser diode and AOMs allowed a single-mode tuning of the whole laser with a tuning range of 225 GHz in 250 s. Additionally, we demonstrated fast tuning over 90 GHz in 190 micros and a repetition rate of 2.5 kHz.

We report on a calibration procedure that enhances the precision of an interferometer based frequency stabilization by several orders of magnitude. For this purpose, the frequency deviations of the stabilization are measured precisely by means of a frequency comb. This allows us to implement several calibration steps that compensate different systematic errors. The resulting frequency deviation is shown to be less than 5.7 MHz (rms 1.6 MHz) in the whole wavelength interval 750-795 nm. Wide tuning of a stabilized laser at this exceptional precision is demonstrated.