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A new passband flattening technique for waveguide grating devices is proposed and experimentally demonstrated. It uses phase dithering of grating elements to obtain a flat passband with sharp transitions while maintaining single-mode input and output waveguides. The chromatic dispersion characteristics of the device can be improved simultaneously over the passband. The theory and design method based on an iterative Fourier transform algorithm are presented in detail with simulation and experimental results. The technique can be used in both arrayed waveguide grating and echelle grating-based devices.