An apparatus comprises an optical transmission element, a diffractive element set formed in or on the transmission element, and an optical component. The diffractive element set is positioned to enable spatial overlap of diffractive elements and an evanescent optical signal propagating in a suitably positioned optical waveguide. The diffractive elements are arranged to establish optical coupling between respective optical signals propagating within the transmission element and the optical waveguide. The optical component is arranged to launch or receive the optical signal propagating within the transmission element. The diffractive element set is arranged so that the optical signal in waveguide is successively incident on the diffractive elements. The optical apparatus can further include the optical waveguide, with the optical waveguide and the transmission element comprising discrete, assembled subunits.