Patent ID: 7260290

Claim:
An optical apparatus, comprising a planar optical waveguide having at least one set of diffractive elements, the planar optical waveguide arranged to substantially confine in at least one transverse spatial dimension optical signals propagating therein, wherein: each diffractive element set is arranged to route, 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 diffractive elements are arranged so that the input optical signal is successively incident thereon; the planar optical waveguide is arranged so that optical signals can propagate therein only in a fixed number of corresponding signal optical transverse modes substantially confined in at least one transverse dimension by the planar optical waveguide; the planar optical waveguide is arranged so that a modal index of the signal optical mode spatially varies along an optical signal propagation direction within the planar optical waveguide; said signal modal index variation yields a level of optical coupling between the signal optical mode and a loss optical mode at or below an operationally acceptable level; and the planar optical waveguide comprises a waveguide core, and spatial variation along the propagation direction of a bulk refractive index of core material provides said signal modal index variation.