Abstract: A spectrograph usable as a demultiplexer/detector in a wavelength division multiplexing optical system. The spectrograph comprises a planar waveguide and a detector array. The planar waveguide has a dispersive edge having an inwardly concave shape, an input edge, and a straight output edge. The dispersive edge has a reflective diffraction grating formed on it, the grating having a variable line spacing. An optical input signal comprising a plurality of different wavelength ranges enters the waveguide at the input edge, and travels through the waveguide and strikes the grating. The grating focuses the optical energy in each of the wavelength ranges at a focal spot at the output edge, the position of each focal spot being a function of wavelength. The detector array comprises a plurality of photodetectors positioned along a straight line, such that the photodetectors are positioned at the focal spots. Each photodetector therefore detects the optical energy in one of the input ranges.