An optical apparatus comprises a first and second cladding layers and first and second core layers between the cladding layers. The second core has a set of diffractive elements. The first core and the claddings are arranged to form a slab waveguide supporting slab waveguide modes and confining in one transverse dimension optical signals propagating in two dimensions in the slab waveguide modes. The second core and the claddings are arranged to from a channel waveguide supporting one or more channel waveguide optical modes and confining in two transverse dimensions optical signals propagating in one dimension in the channel waveguide modes. The diffractive elements are arranged to couple at least one slab waveguide mode and at least one channel waveguide mode to enable transfer of an optical signal between the slab and channel waveguide optical modes thus coupled.