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Holographic diffraction gratings are produced using a photolithographic approach. Unlike ruled gratings, which contain a reflective layer, holographic diffraction gratings use substrates coated with a photosensitive material (photoresist). The photoresist is specially positioned so that the coated blank lies between intersecting, monochromatic, coherent laser beams, usually from a 488nm Argon laser. The intersecting laser beams generate a sinusoidal intensity pattern of parallel, equally spaced interference fringes in the photoresist material. Since the solubility of the photoresist is dependent on its exposure to light, the gratings can be formed after the material is immersed into a specific solvent and etched accordingly. Holographic diffraction gratings are produced optically; hence, the groove spacing is extremely uniform. Most importantly, holographic gratings do not exhibit the ghosting effects seen in ruled gratings. They are designed to minimize aberrations and can enhance efficiency in a single plane of polarization.