Multiport waveguide couplers with periodic energy exchange

In this Letter, a multiport directional optical coupler based on periodic energy exchange in a linear waveguide array is proposed. The periodic power transfer is achieved by choosing waveguide separations that render commensurate eigenvalues of the array coupling matrix. This is a general solution and offers a plethora of possibilities. Particularly interesting is an array that can be used to realize different couplers by simply choosing a different input waveguide. The proposed design principle is validated by full numerical simulations of realistic devices and the required fabrication precision is estimated. The proposed couplers are of interest for quantum optics, biosensing, and communications.