Injection locking of a terahertz quantum cascade laser to a telecommunications wavelength frequency comb

High resolution spectroscopy can not only identify atoms and molecules, but can also provide detailed information on their chemical and physical environment, and relative motion. In the terahertz frequency region of the electromagnetic spectrum, where many molecules have fundamental vibrational modes, there is a lack of powerful sources with narrow linewidths that can be used for absorption measurements or as local oscillators in heterodyne detectors. The most promising solid-state source is the THz frequency quantum cascade laser (QCL), however, the linewidth of this compact semiconductor laser is typically too broad for many applications and its frequency is not directly referenced to primary frequency standards. In this work we injection lock a QCL operating at 2 THz to a compact fibre-based telecommunications wavelength frequency comb, where the comb line spacing is referenced to a microwave frequency reference. This results in the QCL frequency locking to an integer harmonic of the microwave reference, and the QCL linewidth reducing to the multiplied linewidth of the microwave reference, < 100 Hz. Furthermore, we perform phase-resolved detection of the locked QCL and measure the phase noise of the locked system to be –75 dBc/Hz at 10 kHz offset from 2 THz carrier.