Low-threshold RNGH instabilities in quantum cascade lasers

TL;DR

This theoretical study reveals that low-threshold RNGH instabilities in quantum cascade lasers are caused by carrier coherence gratings, enabling ultra-short pulse regimes, and explains their absence in laser diodes due to different gain media.

Contribution

It demonstrates that RNGH instabilities in QCLs originate from carrier coherence gratings, not saturable absorption, and explores their potential for ultrashort pulse generation.

Findings

RNGH instabilities in QCLs are due to carrier coherence gratings.

Short-cavity QCLs can exhibit regular RNGH self-pulsations.

The study explains the absence of RNGH pulsations in laser diodes.

Abstract

A theoretical study on low-threshold multimode instabilities in quantum cascade lasers (QCLs) and laser diodes (LDs) is presented. Previously, low threshold Risken-Nummedal-Graham-Haken (RNGH) instabilities were reported in several experimental studies of QCLs. They were attributed so far to the combined effect of the induced grating of carrier population and of a built-in saturable absorption feature that may be present in the monolithic single-section cavity of these Fabry-P\'erot lasers. Here we show that low-threshold RNGH instabilities in QCLs occur due to a carrier coherence grating induced in the gain medium and not due to intracavity saturable absorption. We find that QCLs with a few mm long cavity exhibit intermittent RNGH self-pulsations while regular self-pulsations are possible in short-cavity QCLs, with the cavity length of 100 um or smaller. We examine a transient behavior to RNGH self-pulsations in short-cavity QCLs and find features that resemble cooperative superradiance. Our findings open a practical way of achieving ultra-short pulse production regimes in the mid-infrared spectral range. Applying same approach to semiconductor laser diodes (LDs) we explain the absence of RNGH self-pulsation in LDs based on a quantum well gain media, while practically established method for reaching the ultrafast coherent emission regimes in LDs is to incorporate a separately contacted saturable absorber section in the LD cavity.