Modal cutoffs in single- and few-mode fiber couplers

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 and as the sum and difference of the fundamental modes and of each fiber. In reality is the fundamental mode of the composite waveguide, whereas is the second mode and has a finite cutoff. This limits the smallest -value for which low-loss coupling is possible. The limitation is least severe for step-profile fibers.