Nonlinear dynamics and modulation properties of optically injected quantum cascade lasers

Quantum cascade (QC) lasers have been of great interest since its first demonstration at Bell labs in 1994 [1]. In contrast to interband semiconductor lasers, one of the fundamental features in QC lasers is that the stimulated emission relies on the intersubband transition, which is dominated by the ultra-fast electron-optical phonon scattering mechanism within several picoseconds. Flat modulation responses without relaxation-oscillation resonance and high 3-dB bandwidths of about tens of gigahertz have been demonstrated experimentally [2]. It is well known that optical injection can well enhance the dynamical properties of semiconductor lasers. To this end, Meng et al studied the modulation responses of injection-locking QC lasers employing a simple two-level rate equation model, where the non-existence of the unstable locking region in the locking regime was pointed out [3]. However, we remark that this conclusion needs further investigation using a more accurate rate equation model and an explicit approach based on the analysis of the local bifurcation scenario.