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Vertical angular selectivity and grating degeneracy of volume holographic gratings

The vertical angular selectivity of volume holographic gratings in photorefractive crystals is investigated both theoretically and experimentally. The vertical selective angle and the shape of the grating degeneracy lines are derived by using a simplified geometrical model relating the k-vector sphere to the reference point plane. The study shows that volume gratings have finite selectivity in vertical direction, so that grating degeneracy does not occur along vertical lines.