Modal cutoffs in single- and few-mode fiber couplers
- 1 February 1985
- journal article
- Published by Institute of Electrical and Electronics Engineers (IEEE) in Journal of Lightwave Technology
- Vol. 3 (1) , 100-110
- https://doi.org/10.1109/jlt.1985.1074138
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 modes\Psi+and\Psi-as the sum and difference of the fundamental modes\Psi_{1}and\Psi_{2}of each fiber. In reality\Psi+is the fundamental mode of the composite waveguide, whereas\Psi-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.Keywords
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