Effects of current modulation on the transverse-mode dynamics of vertical-cavity surface-emitting lasers with weak optical feedback

A numerical study of the current modulation influence on the different feedback-induced transverse-mode regimes is presented. Simulations are based on a model that includes spatial profiles for the transverse optical modes and for the carrier density in the quantum-well active region of the VCSEL, and applies to weakly index-guided VCSELs, where the transverse modal profiles and modal frequencies are determined by the built-in refraction index distribution. Optical feedback is included as in the Lang-Kobayashi approach, taking into account a single reflection in the external cavity. Transport effects are also included by considering two carrier densities, one for the carriers in the quantum-well region, and one for the carriers in the barrier region. Results show that weak modulation preserves the in-phase and anti-phase regimes arising without modulation. As the modulation amplitude increases there is a transition to a dynamics governed by the current modulation, where the total output follows the injection current and there is either single-mode or in-phase multi-mode behavior. However, the effect of the modulation depends strongly on the modulation frequency. In the case of high frequency modulation the laser cannot follow the modulation and even under strong modulation the feedback-induced effects are dominant. On the other hand, if the modulation frequency is close to the internal relaxation oscillation frequency, the modulation strongly affects the transverse-mode behavior: even weak modulation induces a resonant pulsing with single-mode or in-phase multi-mode behavior.