Nonspatial filter for laser beams

A nonspatial filter was developed to perform the same task as a traditional pinhole or fibre spatial filter: the filtering of spatial frequencies in laser beams. However, the new filter operates directly in a laser beam without focusing it. This makes it possible to eliminate many of the alignment instabilities and laser power limitations of spatial filters. The new filter is based on the Bragg selectivity of thick holograms. Two-dimensional filtering requires insertion of two different holograms in the light path. The requirements which holograms must satisfy, as well as those imposed on a holographic material to reach a bandwidth of about 10^–3—10^–4 rad for the angular selectivity contour amounting, are considered. Standard holographic materials are unsuitable for this application because of differential shrinkage during processing, which limits the maximum attainable Bragg angular selectivity. A new ‘porous’ holographic material is developed which is heterogeneous: it consists of a porous silicate matrix impregnated with a photosensitive medium. Calculations and experiments show that it is an ideal material for our task and it satisfies the necessary requirements: its thickness is several millimetres or more, it does not shrink, it makes it possible to attain the necessary refractive index modulation, etc. Potential applications of such highly selective filters are wide: they can be used to ‘clean up’ conventional laboratory and industrial laser beams, they can be mounted inside laser cavities for filtering of spatial frequencies and mode selection, they are promising for spectroscopy and correction of corrupted wavefronts, etc.