Phase Locking of a Quantum Cascade Laser Using a THz Schottky Diode Harmonic Mixer

Terahertz-frequency Quantum Cascade Lasers (THz QCLs) are compact electrically-driven sources of narrowband, coherent radiation in the 1–5 THz band. Although peak output powers in excess of 1 W have been demonstrated, most potential applications of THz QCLs as a local oscillator (LO) in THz astronomy and atmospheric spectroscopy require frequency stability better than 1 ppm and narrow linewidth, with low phase noise sidebands. However, temperature and current bias fluctuations in the gain media of the QCL can cause the refractive index to change with time, which affects the lasing frequency.

Phase locking improves the stability by using a negative-feedback system that combines a current-controlled QCL with a phase comparator and a stable reference so that the QCL maintains a constant phase angle relative to a reference signal. Since it is challenging to find compact and stable THz-frequency source to use as a reference, a THz mixer is needed to down-convert the signal to a lower frequency where frequency and phase comparisons are possible. A number of groups have accomplished QCL phase locking using a Hot Electron Bolometer (HEB) or Semiconductor Superlattice nonlinear device (SSL) as mixers. However, these mixers require an additional cryo-cooler, which increases the size and the complexity of the phase locking system. Furthermore, room temperature SSL devices exhibit conversion loss of 80 dB or more, which makes phase locking difficult. Therefore, a room-temperature, solid-state mixer with lower conversion loss is desirable to produce more compact phase-locked THz sources.

This thesis describes the phase locking of free running 2.518 THz and a 2.6906 THz QCLs, which have achieved a spectral resolution of approximately 1010. To phase lock THz QCLs, a room temperature Schottky diode based WM-86 (WR0.34) 1.8-3.2 THz harmonic mixer is developed. The mixer consists of quartz-based Local Oscillator (LO) and Intermediate-Frequency (IF) circuits and a GaAs based beam-lead THz circuit with an integrated diode. Measurements of the mixer are performed using a 2 THz solid state source and 2.6906 THz QCL, and a conversion loss of 27 dB for the 3rd harmonic mixing is achieved. This is the first time the development of a WM-86 (WR0.34) harmonic mixer with a beam-lead THz circuit for frequencies above 2.5 THz is demonstrated and the result represents the best Schottky-based harmonic mixer in this frequency range. Similarly, this thesis also prescribes the phase locking of QCLs at 2.518 THz and at 2.6906 THz using a room temperature Schottky diode for the first time after a 2.32 THz QCL phase locking was reported by a group at University of Massachusetts. A phase locked QCL can be used to build a heterodyne interferometer in the far-infrared range and high-resolution heterodyne tunable spectroscopy for different applications such as radio astronomy, molecular spectroscope, and plasma diagnostics.