Single-element holographic nonspatial filter

We have previously described a holographic laser apparatus that 'cleans up' a laser beam by discarding off-axis rays. The device uses two matched holograms carefully oriented with respect to one another so that the Bragg effect from the first hologram cleans up first one direction and then the second hologram cleans up the orthogonal direction. The device works well and reduces the beam divergence to 0.8 mrad with a 3.0 mm thick hologram. However, there are practical difficulties with this configuration including sensitivity to the mutual alignment of the holograms and a cleaned-up beam which is inconveniently not in the same horizontal plane as the input laser beam. We describe here a much simpler design using a single hologram that improves performance and avoids these difficulties.