We focused on the effects of group-velocity dispersion (GVD) on the coherent pulse progression in mid-infrared (MIR) quantum-cascade lasers (QCLs). Comparison of GVD effects on the two kinds of typical QCL cavities, i.e., FP and ring cavities, brings insight into the interaction between the GVD and the spatial hole burning (SHB) effect which is only supported by FP cavities but not ring cavities. The theoretical model is built based on the Maxwell-Bloch formulism accounting for two-way propagations of electric field and polarization as well as the couplings among the electric field, the polarization, and the population inversion. The pulse evolution in time-spatial domains is simulated by the finite difference method with prior nondimensionalization, which is necessary for a convergent solution. Results predict that the SHB could broaden the QCL gain bandwidth and induce additional side modes closely around the central lasing mode with an intensity more pronounced than that of GVD associated side modes. Moreover, owing to the SHB, the lasing instability caused by GVD is weaker in a FP cavity than a ring cavity.
We focused on the effects of group-velocity dispersion (GVD) on the coherent pulse progression in midinfrared quantum-cascade lasers (QCLs). We carried out the study on Fabry–Perot (FP) cavities. Comparison of GVD effects on the two kinds of typical QCL cavities, i.e., FP and ring cavities, brings insight into the interaction between the GVD and spatial hole burning (SHB) effect, which is only supported by FP cavities but not ring cavities. The theoretical model is built based on the Maxwell–Bloch formulism accounting for two-way propagations of electric field and polarization as well as the couplings among the electric field, polarization, and population inversion. The pulse evolution in time–spatial domains is simulated by the finite-difference method with prior nondimensionalization, which is necessary for a convergent solution. Results predict that the SHB could broaden the QCL gain bandwidth and induce additional side modes closely around the central lasing mode with an intensity more pronounced than that of GVD associated side modes. Moreover, owing to the SHB, the lasing instability caused by GVD is weaker in an FP cavity than a ring cavity.
The effect of group-velocity dispersion (GVD) and self-phase modulation (SPM) on the coherent pulse progression in mid-infrared (MIR) quantum-cascade lasers (QCLs) is investigated. The background saturable absorber (SA) effect is included in the study. In this case, the lasing pulses can be built up from the instable continuous wave (CW) operation condition related to both SA effect and GVD under the influence of a small initial disturbance. The theoretical model is built based on the Maxwell-Bloch formulism accounting for the couplings among the electric field, the polarization, and the population inversion. The pulse evolution in time-spatial domains is simulated by the finite difference method with prior nondimensionalization, which is necessary for convergent solution. We first studied the QCLs with ring cavity to illustrate the interplay between GVD and SPM effects. It is found that the anomalous GVD, which receives less attention in the study of QCL dynamics, can significantly narrow the spectrum splitting between side modes. The SPM can broaden the linewidth of the spectral modes. Their combined effects can lead the possibility of forming solitons. We also extended our study to the same QCL medium with a Fabry-Perot (FP) cavity. The additional effect of spatial-hole burning (SHB) is identified in results obtained.
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