An optical apparatus comprises a planar optical waveguide having at least two sets of diffractive elements. The planar optical waveguide substantially confines in at least one transverse spatial dimension optical signals propagating therein. The two diffractive element sets define an optical resonator that supports at least one resonant optical cavity mode. An optical signal in one of the resonant optical cavity modes is successively incident on the diffractive elements of each of the diffractive element sets.