Multimode RNGH instabilities of Fabry-Perot cavity QCLs: impact of diffusion

We analyze multimode instabilities in Fabry-Perot (FP) cavity MIR quantum cascade lasers (QCLs) caused by excitation of Rabi oscillations. Our analysis differs from previous treatments of this problem by inclusion of the grating of medium polarization (coherence grating) and its relaxation due to carrier diffusion. We show that low-threshold multimode Risken-Nummedal-Graham-Haken (RNGH) instabilities are possible in QCLs without a built-in saturable absorber, that is, in the configuration in which they have been experimentally observed. We also show that multimode RNGH instabilities are practically impossible in conventional VIS-NIR semiconductor quantum well laser diodes with a monolithic FP cavity. These explanations are supported by Lyapunov stability analysis and numerical simulations based on a travelling wave rate equation model.