Unification of nonlinear-optical fiber devices

Nonlinear coupling can be described in a universal form applicable to a general class of optical fiber devices involving the interaction of two modes. By knowing the solution to one type of device, we then have the solution to all other types. For example, the descriptions of birefringent fibers used as switches and amplifiers have their direct counterparts in directional (twin-core) couplers, found by merely redefining the meaning of subscripts in equations or on the phase-space portrait. The key to this unification follows, not by actually solving any one problem but rather by interrelating the coupling of normal modes with the coupling of submodes, e.g., the individual core modes of directional couplers.