Improved holography for chirped gratings

Sources of high propagation loss in holographic gratings on planar waveguides have been identified by viewing the gratings as holograms of the optical exposure apparatus. In order to simplify the optical system and increase the large-area uniformity of the beams, collimating lenses were eliminated and the gratings were formed by interference of spherical wave fronts. In order to define the grating shape without causing mask diffraction patterns, the desired grating region was outlined by negative photoresist over the surface of the exposed and developed grating pattern.