Quasi-Ray Analysis of Crosstalk Between Multimode Optical Fibers

Crosstalk between communications circuits has long been a problem concerning engineers and designers in the telecommunications industry. In a digital communications system using optical fibers as a transmission medium, at least a 30-dB signal-to-crosstalk ratio will be needed. In those applications where a lossy jacket around the fiber is undesirable, proper design of optical fiber cables and circuits requires an understanding of the parameters that control crosstalk between the fibers. A number of models can be found in the literature to describe crosstalk between optical fibers.1-8 Maxwell's equations are the usual starting point for these models, and a field theory approach describing the coupling coefficients between individual modes results. Each of these models provides insight into a possible mechanism for describing crosstalk between optical fibers, and each is operationally useful for calculating crosstalk between single-mode fibers or guides with a small number of propagating modes. A very interesting model describing crosstalk resulting from scattering from a rough core-cladding interface is also described in the literature.9 In this paper, a meridional quasi-ray tracing procedure is used to describe crosstalk between highly multimoded optical fibers. This approach is an extension of work developed by H. P. Yuen10 and N. S. Kapany.11-14 The mechanism for crosstalk coupling of energy between fibers is that of frustrated total internal reflection of waves in a multi17