Grating-based surface-emitting tapered unstable resonator laser simulations and experiments

We have fabricated and characterized tapered unstable resonator lasers with two slightly different resonator geometries, one with a linear taper and one with a nonlinear taper, to investigate the effect of resonator geometry on the performance characteristics. Special attention has been paid to the modal behavior and its dependence on drive current. We investigate the modal behavior by monitoring the focused spot produced by an integrated focusing grating coupler. To aid in the design of the lasers and the analysis of the measurement results, we implemented numerical simulations based on the beam-propagation method and obtained good agreement between measurements and simulations. We find that the threshold current, the efficiency and the optical field stability, including filamentation threshold, are significantly improved by adjusting the shape of the current injection region to the diverging field profile in the tapered section. Improvements in threshold current and efficiency are attributed to a more efficient use of the injected carriers, while improvement in optical field stability is attributed to reduced carrier induced phase front distortion. We conclude that a relatively small modification of the resonator geometry can lead to large improvements in laser performance.