Theoretical investigation for coupling between a laser diode and an optical waveguide via a gap

To develop a high‐grade optical communication system, numerous studies have been performed on the single longitudinal mode operation of a laser diode (LD). The optical feedback generated at the coupling junction of the LD and the optical waveguide is a serious problem since it is a cause of the deterioration of the stability of the operation of the single mode laser. This paper describes a theoretical study of the reflected and transmitted light at the open end of the LD and at the junction between the LD and the optical waveguide via a gap by means of the boundary element method and the eigenmode expansion method. It is confirmed from the viewpoint of electromagnetic theory that among various configurations for elimination of the feedback, the one with a double‐layered antireflection coating is the most effective. The analysis method used in this paper, unlike many other methods, has no restriction on the difference in the refractive index between the core and cladding. It can be extended easily to the case where the gap region consists of multimedia.