Frequency stabilization, tuning, and spatial mode control of terahertz quantum cascade lasers for coherent transceiver applications

THz quantum cascade lasers (TQCLs) first demonstrated in 2002 are a promising source of THz coherent radiation for use as both transmitters and local oscillators in coherent heterodyne detection systems. However, present TQCLs have deficiencies which include lack of frequency tunability, as well as inadequate spatial and temporal coherence. In this thesis we have addressed these issues to demonstrate an improved 2.408 TQCL which operated as a transmitter in a coherent heterodyne detection imaging system. The 2.408 THz QCL used in this thesis was grown and fabricated by the Photonics Center (University of Massachusetts Lowell).;First, we showed that a short hollow Pyrex tube can act as a dielectric waveguide and transform the multimode, highly diverging TQCL beam into the lowest order dielectric waveguide hybrid mode, EH11, which then couples efficiently to the free-space Gaussian mode, TEMo0. This simple approach should enable TQCLs to be employed in applications where a spatially coherent beam is required.;Next, the tunability problem was addressed. A compact, tunable, narrowband terahertz source was demonstrated by mixing a single longitudinal mode, 2.408 THz, free running quantum cascade laser with a 2-20 GHz microwave sweeper in a corner-cube-mounted Schottky diode (SD). The sideband spectra were characterized with a Fourier transform spectrometer, and the radiation was tuned through several D2O rotational transitions to estimate the longer term (t ≥ several seconds) bandwidth of the source. A spectral resolution of 2 MHz in the CW mode was observed.;The temporal coherence of the TQCL was improved by assembling a simple analog locking circuit, which stabilizes the beat signal between the TQCL and a 2.40976 THz CO2 optically pumped molecular laser (OPL) line to 4 kHz full width at half maximum (FWHM). This is approximately a tenth of the observed long-term (t ∼ sec) linewidth of the OPL showing that the feedback loop corrects for much of the mechanical and acoustic-induced frequency jitter of the gas laser. The achieved stability is sufficient to enable the use of the TQCL as a transmitter in a coherent transceiver.;Finally, a coherent transceiver using the TQCL as a transmitter and an OPL as the local oscillator was used, with a pair of SD mixers in the receiver and reference channels, to acquire high-resolution inverse synthetic aperture radar (ISAR) images of fully illuminated targets, including scale models and concealed objects. Phase stability of the received signal was sufficient to allow coherent image processing of the rotating target (in azimuth and elevation). Several THz ISAR images with submillimeter resolution, obtained with the system, are presented. These images clearly illustrate the first use of a quantum cascade laser in a coherent detection system.