Cutoff in single-mode optical fibre couplers

Optical coupling, or crosstalk, between two parallel, identical, single-mode fibres is describable as a superposition or the ¿+ and ¿¿ modes of the composite waveguide. Perturbation theory gives ¿+ and ¿¿ as the sum and difference, respectively, of the fundamental modes ¿1 and ¿2 of each fibre. In reality ¿+ is the fundamental mode of the composite waveguide, whereas ¿¿ is the second mode and has a finite cutoff. This limits the smallest V-value for which low-loss coupling is possible. Exact values and approximate analytical expressions for cutoff are presented and compared with experiment. The limitation is least severe for step-profile fibres.