Gain and Index Dynamics in Semiconductor Lasers

Semiconductor optical amplifiers (SOAs) provide ultrafast, i.e. broadband components for optical communication systems. They enter not only as signal generators and amplifiers, but also as nonlinear elements for ultrafast signal processing such as wavelength conversion, switching, and regeneration. The increasing demand for communication bandwidth has made the study of ultrafast processes in SOAs an important one. The ultimate limits to the speed of nonlinear optical signal processing can only be reached by proper materials design and engineering. We have seen that the active region in SOAs has gradually changed character from bulk semiconductor to quantum wells and most recently to quantum dots. By quantum confinement of the carriers, the light-matter interactions can be significantly modified and the optical properties, including dynamics, can be engineered to match the required functionalities and specifications. We have measured ultrafast gain and index dynamics of SOAs in pump-and-probe experiments applying 100 fs pulses and a heterodyne detection scheme, where both amplitude and phase of the probe pulses are determined. The gain depletion, and associated index change, and the subsequent recovery afte the passage of an intense short pump pulse yields detailed information about the population dynamics of the electronic states in the SOA active region. Recent results on long-wavelength quantum-dot SOAs with different degrees of electronic confinement will be discussed and compared to bult and quantum-well devices.