An optical wavelength division multiplexer/demultiplexer device (2) is described that comprises a substrate (6) having a plurality of wavelength selecting filters (26, 28, 30, 32, 34, 36). The filters (26, 28, 30, 32 ,34 ,36) are arranged to provide conversion between a combined beam comprising a plurality of wavelength channels and a plurality of separate beams each comprising a subset of said plurality of wavelength channels. Hollow core waveguides (4) are formed in said substrate to guide light between the wavelength selecting filters. An add/drop multiplexer (100) is also described.