Simulation study on semiconductor laser mode locking using nonlinear coupling in multimode-interference waveguide amplifiers

The feasibility of a passively mode-locked semiconductor laser with a multimode-interference (MMI) waveguide amplifier is numerically studied using a two-dimensional time-domain beam-propagation method. In an appropriately designed ring cavity, a pulse can be compressed from a few hundred picoseconds to several picoseconds, as a result of the interplay between linear coupling and gain saturation in the MMI waveguide amplifier. The asymptotically stable pulse peak position and pulsewidth imply the feasibility of passively mode-locking semiconductor lasers. The frequency chirping of the compressed pulse and the lateral distribution of the output beam are numerically analyzed in detail. Our simulations show that mode locking can be implemented within a fairly broad range of injection current and cavity alignment.