The Effect of Intracavity Field Variation on the Emission Properties of Quantum Cascade Lasers

Abstract

A common and powerful simplification in laser physics is to ignore the spatial dependence of the intracavity field intensity, and instead replace it with its average value. This approach can elucidate many aspects of laser behavior. In this work, however, we examine several problems, both theoretical and experimental, whose understanding requires that the intracavity intensity variation be properly taken into account. We first address theoretically the question of light reflecting from an amplifying slab, a simple problem to pose but one that reveals counterintuitive solutions of the Fresnel equations. These subtleties provide a deeper understanding of negative refraction in nonmagnetic media, amplified total internal reflection, and the perfect lens. Secondly, we fabricate multi-section sampled grating quantum cascade lasers (QCLs) and demonstrate single-mode operation and wide tunability by the Vernier effect. Thirdly, we theoretically investigate how the end mirror reflectivities of a laser affect the output power, and show that power output is reduced when the disparity of the two reflectivities increases. Finally, we demonstrate experimentally for the first time that the transition from single to multi-mode operation in QCLs begins with the appearance of sidebands on the primary lasing mode, separated by tens of free spectral ranges. We explain this state theoretically as the result of the parametric interaction between the primary lasing mode and the sidebands. The frequency separation of the sidebands and the temporal behavior of the emitted waveform are sensitive to the facet reflectivities. This discovery provides a new pathway toward mid-infrared frequency combs from quantum cascade lasers.