Mode-locking in a semiconductor photonic bandgap laser

Multimode lasers have a very complex dynamics. Order emerges when the modes lock together, leading to a periodic train of pulses or a nearly constant power output with a linearly chirped frequency. The first is promoted by a saturable absorber, while the latter is connected to more subtle conditions, such as the fast dynamics of the gain. When none of these conditions is satisfied, we show that locking is still possible inside a photonic band-gap laser. We show, first in theory, that modes lock together to generate a variety of waveforms which are not trains of pulses nor chirped continuous power waves. Mode-locking (ML) is observed in experiments on a III-V/Silicon hybrid laser with the cavity made of a suitably tapered grating. The mode-locking beatnote is strongly dependent on the injected current and can be changed dynamically by more than 1 GHz in microseconds. The behaviour of the laser is critically determined by the dispersion, which can be controlled by the photonic crystal structure. By scaling up the number of interacting modes, this laser source may offer an effective and flexible way for the generation of custom waveforms. The typical platforms to generate multimode lasers rely on mode locking, achieved via a saturable absorber and fast dynamics of the gain. The authors demonstrate that mode locking in semiconductor photonic bandgap lasers is possible in the absence of a fast nonlinear mechanism, proposing an effective way of generating waveforms à la carte.