THz Schottky diode harmonic mixers for QCL phase-locking

A fundamental 1.9–2.8 THz Schottky diode mixer has been modified to enable high harmonic mixing, thus allowing use of the mixer for phase locking of a quantum cascade laser (QCL) to an stable external microwave source. Phase locking to a reference will transfer the spectral line profile of the reference to the THz source; this will control the phase precisely. Stabilizing a QCL for a single frequency operation will help to reduce the drift due to temperature and other external biases. The mixer can be used for several applications, such as, heterodyne interferometer in far-infrared range and high-resolution heterodyne spectroscopy. The design and characterization of the harmonic mixer will be presented. A conversion loss of 63 dB for harmonic mixing at N=9 and 45 dB for N=3 were measured with a 2 THz input signal.