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High-speed modulation dynamics of twin-stripe lasers

The high-speed modulation dynamics of semiconductor lasers is determined by a complex interplay of ultrafast carrier and light field dynamics. The characteristic time scales of the underlying physical processes determine the relaxation oscillations and set an upper limit to the modulation of a single-mode semiconductor laser. In spatially extended semiconductor lasers the longitudinal and transverse dimensions enable the coexistence of numerous longitudinal and transverse modes. A suitable design of the laser cavity and electronic contacts should consequently allow one to directly influence the lateral coupling and transverse mode dynamics. The twin-stripe semiconductor laser (realized with two parallel contacts on top of the active area) represents one of the simplest semiconductor lasers with coexisting transverse modes. Modulation of the current in the stripes with a beat frequency corresponding to the frequency separation of transverse modes may then lead to a significant increase of the high-frequency modulation response of the laser. In this paper, we present results of simulations on the modulation characteristics of twin-stripe semiconductor lasers on the basis of multi-mode Maxwell Bloch equations that include propagation effects and spatio-temporally varying mode competition. In particular, we analyze the dependence of light field dynamics and spectral properties on laser dimensions, carrier injection and modulation frequency. Our simulations reveal that it is both, the transverse and the longitudinal degree of freedom that influence the transverse mode dynamics as well as the laser response to high-frequency modulation.