An optical apparatus (spectral filter, temporal encoder, or other) comprises a planar optical waveguide having at least one set of diffractive elements. Each diffractive element set routes by diffraction therefrom a portion of the optical signal propagating in the planar waveguide. The planar waveguide includes at least one material having thermo-optic properties chosen so as to yield a designed temperature dependence of spectral and/or temporal characteristics of the diffracted portion of the optical signal. Variations of material refractive indices, physical dimensions, and/or optical mode distributions with temperature may at least partly compensate one another to yield the designed temperature dependence. Optical materials with n/T of various magnitudes and signs may be variously incorporated into the waveguide core and/or cladding. The designed temperature dependence may be: i) substantial temperature independence; ii) a temperature derivative of zero at a selected temperature; or iii) a designed variation with respect to temperature.