A simplified model is presented that accurately predicts the coupling limitations of asymmetric fused multimode star (mxn) couplers fabricated from fibers with arbitrary core radii and numerical aperture values. The model assumes that the following conditions within the coupling region are met: a) the ratio of the cladding thickness between adjacent cores to the core radius (t/r) is much greater than 0.1, b) each fiber has an identical core index profile coefficient, c) the refractive index of each fiber's cladding is identical, and d) the fibers are substantially tapered. The model is shown to be equally applicable to symmetric couplers fused from identical fibers. In addition, experimental results supporting the model are presented.
J. D. Muhs, J. D. Muhs,
"Coupling Limitations Of Asymmetric Multimode Couplers", Proc. SPIE 0839, Components for Fiber Optic Applications II, (4 February 1988); doi: 10.1117/12.942543; https://doi.org/10.1117/12.942543