Self-mixing as a means of spectral characterisation of a Terahertz QCL

Fast, sensitive and compact coherent systems for imaging and interferometry can be developed through the use of a single terahertz (THz) quantum cascade laser (QCL) device as both emitter and detector in a self-mixing (SM) scheme [1]. Here, radiation re-injected to the laser cavity interferes (‘mixes’) with the intra-cavity electric field, causing small variations in the fundamental laser parameters [2]. Of particular importance is the voltage perturbation induced by optical feedback. This can be used as a method of measuring the self-mixing effect through monitoring the terminal voltage of the device, and is dependent on the external cavity length and lasing frequency. As such, interferometric fringes can be acquired in a SM system by simply changing the external cavity length. In this work we demonstrate the use of SM interferometry for performing spectral characterisation of a multi-mode THz QCL in a scheme that offers much reduced experimental complexity, compared with typical Fourier Transform infrared spectroscopy (FTIR) systems.