Observation of optical feedback dynamics in single-mode terahertz quantum cascade lasers: Transient instabilities

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.

Automated summary

This study presents experimental evidence of transient instabilities in a terahertz quantum cascade laser under optical feedback, contradicting the widely accepted view of its ultra-stability. The absence of relaxation oscillations and low linewidth enhancement factor in THz QCLs make them uniquely suitable for exploring external-cavity dynamics in semiconductor lasers. These findings open up possibilities for a THz sensing and imaging modality based on these transient instabilities, offering a simpler alternative to existing approaches using laser feedback interferometry.