Observation of optical feedback dynamics in single-mode terahertz quantum cascade lasers: Transient instabilities
Xiaoqiong Qi, Karl Bertling, Thomas Taimre, Gary Agnew, Yah Leng Lim, T. J. Gillespie, Ashley Robinson, Michael Brünig, Aleksandar Demić, Paul Dean, Lianhe Li, E. H. Linfield, A. G. Davies, D. Indjin, Aleksandar D. Rakić
Abstract
We provide an experimental evidence of transient instabilities (TIs) in a terahertz (THz) quantum cascade laser (QCL) under optical feedback, in contrast to the widely accepted claim that THz QCLs are ultrastable against feedback. The TIs appear as periodic oscillations in emitted power or terminal voltage of the laser with an increasing oscillation frequency as feedback increases. The absence of relaxation oscillations and low linewidth enhancement factor in THz QCLs makes them a platform uniquely suitable for exploring external-cavity-related dynamics in semiconductor lasers. This work opens a pathway to a THz sensing and imaging modality based on these TIs, which has much reduced complexity compared to existing approaches using laser feedback interferometry.