Modal cutoffs in single- and few-mode fiber couplers |
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Authors: | Love J. Ankiewicz A. |
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Affiliation: | Australian National University, Canberra, Australia; |
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Abstract: | Optical coupling, or crosstalk, between two parallel, identical single- or few-mode fibers is describable as a superposition of the modes of the composite waveguide. Like the modes of a single-fiber waveguide, the modes of the two-fiber waveguide are cut off. Exact and simple accurate asymptotic formulae are derived giving the cutoff value as a function of separation. For single-mode couplers, perturbation theory gives the fundamental and second modesPsi+andPsi-as the sum and difference of the fundamental modesPsi_{1}andPsi_{2}of each fiber. In realityPsi+is the fundamental mode of the composite waveguide, whereasPsi-is the second mode and has a finite cutoff. This limits the smallestV-value for which low-loss coupling is possible. The limitation is least severe for step-profile fibers. |
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