Dynamics of distributed feedback lasers and semiconductor laser arrays.

Abstract

We study the dynamics of spatially modulated laser structures, which include distributed feedback (DFB) lasers with longitudinal modulation and semiconductor laser arrays with lateral modulation. A time domain model is developed to describe the dynamics of DFB lasers. Uniform grating, $\lambda$/4-shifted, chirped grating DFB (CG-DFB) semiconductor lasers, and CG-DFB fiber lasers are investigated. We found that a single section, asymmetric, linearly chirped DFB laser exhibits the dynamic behavior of high frequency, self-sustained pulsations. This high-frequency self-pulsating behavior, however, is absent in a uniform or symmetrically chirped DFB laser. The frequencies of these self-pulsations are multi-100 gigahertz in CG-DFB semiconductor lasers, and several gigahertz in CG-DFB fiber lasers. The physical mechanism is attributed to the mode beating between the two asymmetric, longitudinal modes with similar gain. The self-pulsation frequency depends on the coupling strength of the DFB laser. With sufficiently large coupling strengths, a terahertz self-pulsation frequency can be obtained. The effect of end mirror reflectivity on the self-pulsating behavior is also investigated in CG-DFB semiconductor lasers. For CG-DFB fiber lasers, we develop both a full model and a simplified model. The simplified model is based on the averaging over the inverted population distributions. Both models produce similar self-pulsations, while the simplified model needs much less computing time. For uniform grating DFB fiber lasers, we also study the effect of the intensity-dependent refractive index of the host material. This nonlinear index gives rise to a nonlinear phase shift, which could alter the dynamics in a high nonlinear index DFB fiber laser. We finally study the dynamics of semiconductor laser arrays by characterizing both the spatio-temporal and the spatio-spectral behaviors of the field in the lateral dimension. An eight-stripe channeled-substrate-plannar (CSP) semiconductor laser array is modeled and analyzed. The analyses give rise to the conclusion that a closely spaced CSP array with strong optical coupling between array elements can be characterized by a gain-guided, broad area laser. In summary, we conclude that multimode instabilities in spatially modulated laser structures can give rise to dynamic behaviors such as periodic self-pulsations, quasi-periodic self-pulsations and chaotic output.