I. Introduction
Semiconductor ring lasers (SRLs) are attractive as potential low cost integrated single mode laser sources compared with conventional FP and DFB lasers. The cost reduction would derive from several features—they do not require cleaved facet to form a resonant cavity, thus are particularly suitable for monolithic integration and can be tested on a wafer scale before dicing, and they can achieve single mode lasing without the use of gratings and therefore are simple to fabricate, because spatial hole burning, one of the main sources of multi-longitudinal behavior in FP lasers, can be eliminated when SRL operates in unidirectional mode.