Spectral Characterisation of a Terahertz QCL through Self-Mixing

The use of a single terahertz (THz) quantum cascade laser (QCL) device as both emitter and detector in a self-mixing (SM) scheme allows for the development of fast, sensitive and compact coherent systems for imaging and interferometry [1]. In this scheme, radiation re-injected to the laser cavity interferes (‘mixes’) with the intra-cavity electric field, causing small variations in the fundamental laser parameters, as described in the seminal paper by Lang and Kobashi (L–K) [2]. In particular, the voltage perturbation induced by optical feedback is described by a sinusoidal variation dependent on both the external cavity length LExt and the emission frequency under feedback ν, and is given byΔVsm ∝ cos(2 v/c). As such, interferometric fringes can be acquired in a SM system by simply changing the external cavity length and concurrently monitoring the terminal voltage of the device. 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 when compared with typical Fourier Transform infrared spectroscopy (FTIR) systems. In addition, we report the first direct measurements of the perturbation of the lasing frequency under feedback, and compare the results with predictions from the L–K model.