A method comprises: formulating simulated design input and output optical signals propagating from/to respective designed optical input and output ports as optical beams substantially confined by a planar optical waveguide; computing an interference pattern between the simulated input and output signals; and computationally deriving an arrangement of diffractive elements of a diffractive element set from the computed interference pattern. When the diffractive element set is formed in the planar optical waveguide, each diffractive element routes, between corresponding input and output optical ports, a corresponding diffracted portion of an input optical signal propagating in the planar optical waveguide that is diffracted by the diffractive element set. The input optical signal is successively incident on the diffractive elements.