A planar optical waveguide has a set of diffractive elements and confines propagating optical signals in at least one transverse spatial dimension. Each diffractive element set routes, between input and output 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. A desired level of birefringence for a chosen signal optical transverse mode is determined at least in part by i) selected areal density of diffractive elements, ii) selected diffractive element height and position along the confined transverse spatial dimension, iii) selected thicknesses and indices of materials comprising the diffractive elements, or iv) selected thicknesses and stress-optical coefficients of materials comprising the planar optical waveguide, and selected thermal expansion coefficient differentials among materials comprising the planar optical waveguide or comprising a waveguide substrate thereof.