High precision frequency characterization of THz quantum cascade lasers by heterodyne mixing

Terahertz quantum cascade lasers (QCLs) have been used together with a monolithic planar Schottky diode receiver to study the heterodyne mixing between dual internal modes of a QCL and between a single mode of a QCL and a known molecular line from a molecular gas laser. Dual mode mixing using a single QCL shows that the intrinsic linewidth of a free-running QCL is $<$ 30 kHz. Both standard and distributed feedback grating QCLs were mixed against known molecular gas laser lines. Resulting difference frequency spectra gave a high precision measurement of a QCL's absolute frequency against known references. Unusual slow transient turn-on behavior was also observed in a pulsed standard QCL. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.