An optical apparatus comprises a planar optical waveguide having at least two reflectors. The planar optical waveguide substantially confines in at least one transverse spatial dimension optical signals propagating therein, and the reflectors define an optical resonator that supports at least one resonant optical cavity mode. At least one of the reflectors comprises a set of diffractive elements arranged: so that an optical signal in one of the resonant optical cavity modes is successively incident on the diffractive elements; so as to exhibit a positional variation in amplitude, optical separation, or spatial phase; and so as to apply a transfer function to the optical signal successively incident on the diffractive elements. The transfer function is determined at least in part by said positional variation in amplitude, optical separation, or spatial phase exhibited by the diffractive elements.