Monolithic Semiconductor Ring Lasers: Design, Experiments and Applications

Monolithic semiconductor ring lasers (SRLs) have the potential to simultaneously address two important issues of modern optoelectronic systems for high-speed optical communications: integrability and ultrafast all-optical signal processing. SRLs do not require chip cleaving or refractive index grating for the definition of the laser cavity, and hence they allow to integrate several laser devices on the same semiconductor chip. Since the early 90's, SRLs have been demonstrated as practical laser sources, showing interesting properties such as the directional bistability that occurs when only one of the two possible counter-propagating modes is active, while the other is highly suppressed. In this regime, the injection of an optical pulse can switch the direction of operation, hence realizing a new type of optically addressable digital memory unit (all-optical flip-flop). In this paper we review the technological development and experimental results achieved to date for SRLs, present a physical model for the description of device operation, and discuss device design rules for optimization of the performance and reduction of switching time