Abstract
We cast a theoretical model based on effective semiconductor Maxwell-Bloch equations and study the dynamics of a multi-mode mid-infrared quantum cascade laser in a Fabry-Perot configuration with the aim to investigate the spontaneous generation of optical frequency combs. This model encompasses the key features of a semiconductor active medium, such as asymmetric, frequency-dependent gain and refractive index as well as the phase-amplitude coupling of the field dynamics provided by the linewidth enhancement factor, and some specific resonator features, such as spatial hole burning. Our numerical simulations are in excellent agreement with recent experimental results, showing broad ranges of comb formation in locked regimes, separated by chaotic dynamics when the field modes unlock. In the former case, we identify self-confined structures travelling along the cavity, while the instantaneous frequency is characterized by a linear chirp behaviour. In such regimes, we show that OFCs are characterized by concomitant and relevant amplitude and frequency modulation.
Highlights
Quantum Cascade Laser (QCLs) have attracted a remarkable interest as THz and Mid-IR sources capable of self-starting Optical Frequency Combs (OFCs) under DC current injection [1,2,3,4]
This additional information revealed the true nature of the self-generated OFC in QCLs: it occurs in presence of a Frequency Modulated (FM) laser emission, but its formation implies a significant Amplitude Modulation (AM), appearing as intra-cavity optical pulses which propagate on a quasi-homogeneous background field [11, 16]
In this paper we have presented results concerning spontaneous OFC obtained in an original model we developed to encompass critical features for the coherent multimode dynamics of a QCL such as 1) a FP resonator with counterpropagating fields, which allows to include Spatial Hole Burning (SHB) effect in the gain dynamics, 2) effective semiconductor medium dynamics which reproduce asymmetric gain and dispersion spectra
Summary
Quantum Cascade Laser (QCLs) have attracted a remarkable interest as THz and Mid-IR sources capable of self-starting Optical Frequency Combs (OFCs) under DC current injection [1,2,3,4]. Several theoretical efforts have been made in order to provide a physical understanding of the fascinating phenomenon of self-starting OFCs in QCLs, to the best of our knowledge there is still a lack of models able to reproduce the experimentally measured coexistence of optical pulses and linear frequency chirp, and the alternance between locked and unlocked regimes. We believe that such tools would be promising to predict possible strategies to extend the OFC dynamic range by employing externally controllable signals, by optimizing the device gain material or the laser cavity design. Where R is the reflectivity of each mirror of the FP cavity
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