Bound solitons in a nonlinear optical coupler

It is predicted that a nonlinear optical coupler in the form of a twin-core fiber with a frequency mismatch between the cores, or two parallel planar light guides carrying nonparaxial beams, should produce a set of stable bound states of solitons with different separations between their centers. Unlike the recently considered bound states of far separated optical solitons in single-mode and bimodal fibers, with an exponentially small binding energy, in the present case the bound states are robust (their binding energy has no exponential smallness), and the corresponding separation can be comparable to the proper size of the solitons. The predicted bound states can be used in the design of soliton-based memory and data-processing devices.