Characterization of an ultrafast uni-traveling-carrier absorber for monolithically integrated InGaAsP/InP mode-locked laser diodes

The monolithic integration of photonic circuits will open new perspectives for optical communication networks. It will enable higher transmission rates, new functionalities, higher functional densities, leading to all-optical networks and reduced cost for telecommunication. Mode-locked laser diodes (MLLDs) will play an important role for short pulse generation in Tb/s networks for the transmitters, as well as for clock recovery for the receivers and optical regenerators. To overcome the limitations of conventional monolithically integrated MLLDs, where the pulse width is limited by the relatively slow absorption recovery, we demonstrate an ultrafast semiconductor saturable absorber based on the uni-traveling-carrier (UTC) concept. The UTC absorber is designed to be monolithically integrated in InGaAsP/InP mode-locked laser diodes grown by MOVPE. The absorber shows a saturation energy of Esat,abs of 1pJ at 1.55μm and a voltage-dependent recovery time of 2ps at 2V reverse bias. The importance of an optimum absorption-bandgap to absorber-length ratio is demonstrated to keep the saturation energy low. The voltage-dependent absorption and absorption recovery time make this absorber ideal for hybrid mode-locking and synchronization to an external RF-source.