Analysis of radiation coupling from high radiance IRED into an optical fiber

Abstract

A model is presented to describe coupling of radiation from high radiance IREDs into an optical fiber. This model is applied to judge the effects of junction current density distribution, thickness of the diode window layer, and roughness of the diode surface upon the coupling efficiency into a fiber. The transmission coefficient between the diode window media and the fiber core depends strongly on whether the emitting surface is rough or smooth. This necessitates two separate approaches for treating the radiation flux across the window layer. A rough surface randomizes incident radiation. Thus, a window layer flux treatment is generalized to evaluate the flux contributions to an annulus in the surface plane from all concentric annulii which lie in the junction plane. A smooth surface transmits within a small solid angle of incidence to the diode surface. Thus, the flux transfer is proportional to the emitted radiation from the corresponding position in the junction plane. Several combinations of the IRED source and fiber sink were treated with the above models. For a rough surface diode the flux transmission coefficient across the layer is the dominant factor and there is a strong dependence on the window layer thickness. For a smooth surface diode the transmission coefficient across the diode to fiber interface is dominate and there is only a weak dependence upon the layer thickness. As the window thickness is reduced the flux coefficient for either type of surface approaches unity, and the interface transmission coefficient dominates the total coupling efficiency. The models were tested against observed relative surface brightness distributions for Ga1-xAlxAS -- GaAs single heterojunction IRED. Changing the degree of current crowding by variations of the active layer thickness reveal the smooth surface model to more correctly predict the relative performance, of the diodes. This judgment is further confirmed by a lack of sensitivity in the optical power coupled into a fiber as the window layer is thinned. Using this treatment of the optical coupling characteristics the effects of parameters such as diode geometry, electrical properties of the structure and, surface charactreistics can be assessed. Examples in the performance evaluation of etched well or thick window planar high radiance diodes are presented.