Ortwin Hess

Dynamics of High-Power Diode Lasers

This review gives an overview of the theory and discusses aspects of space-time modeling of high-power diode lasers. The dynamic interaction between the optical fields, the charge carriers, and the interband polarization are described on the basis of microscopic spatially resolved Maxwell—Bloch equations for spatially inhomogeneous semiconductor lasers. Thereby the influence of dynamic internal laser effects such as diffraction, self-focusing, scattering, carrier transport, and heating on the performance of broad-area or tapered amplifiers as well as the individual device properties (i.e. its epitaxial structure and geometry) are self-consistently considered.

High-frequency modulation of twin-stripe semiconductor lasers

Generally, the high-speed modulation dynamics ofsemiconductor lasers is determined by a complex interplay of ultrafast light-field and carrier dynamics with characteristic times-scales of inter- or intraband relaxation and scattering. Those determine the relaxation oscillations and set an upper limit to the modulation ofa single-mode semiconductor laser (cut-off frequency). In spatially extended semiconductor lasers, however, the longitudinal and transverse dimensions enable the coexistence ofnumerous longitudinal and transverse modes. With suitable resonator design allowing segmented contact carrier injection and modulation it should thus be possible to directly influence the lateral coupling and transverse mode dynamics ofa given laser structure and modulate the laser with a beat frequency associated with these modes. In this paper, we present results ofsimulations of high-frequency modulation characteristics oftwin-stripe semiconductor lasers. We show that the lateral segmentation of the contact may with proper asymmetric application of the injection current, indeed, lead to a more than five-fold increase of the modulation band-width. Our theory on the basis of multi-mode Maxwell Bloch equations includes propagation effects and spatiotemporally varying mode competition. Numerical simulations show that the increased high-speed modulation is closely associated with the coupled lateral and longitudinal multi-mode dynamics of the laser.

Spatio-temporal dynamics and fluctuations in quantum dot lasers: mesoscopic theory and modeling

We present a mesoscopic theory for the spatio-temporal carrier- and light field dynamics in quantum dot lasers based on spatially resolved semiconductor Bloch equations describing the dynamics of electrons and holes in each quantum dot. The Bloch equations are dynamically coupled to spatially resolved wave equations for the counterpropagating light fields and to a diffusion equation describing the carrier dynamics in the wetting layer of the quantum dot laser. These quantum dot Maxwell-Bloch equations (QD-MBEs) self-consistently consider the dynamic changes in the carrier distributions and the inter-level dipoles together with the spatially varying carrier-light field dynamics. Intradot scattering via emission and absorption of phonons, as well as the scattering with the carriers and phonons of the surrounding wetting layer are dynamically included on a mesoscopic level. Spatial fluctuations in size and energy levels of the quantum dots and irregularities in the spatial positioning of the quantum dots in the laser structure are simulated via statistical methods. Numerical simulations on the basis of the QD-MBEs reveal a complex carrier dynamics and a characteristic interplay of spontaneous and stimulated emission. For a specific set of QD-parameters the results of the modeling allow an analysis and interpretation of, e.g., saturation effects and dynamic pulse shaping in quantum dot lasers.

Propagating Anisotropic Solitons in Active Semiconductor Media

We predict and discuss the formation and propagation of anisotropic spatial optical solitons initiated by spatio-spectral mixing of two optical pulses in partially coherent active semiconductor media. The origin of the anisotropy is attributed to the combination of spatial effects (carrier transport, diffraction and self-focusing) and the microscopic dynamics of the nonlinear active semiconductor medium reflecting the spatio-spectral gain and refractive index dynamics.