VERY THICK HOLOGRAPHIC NONSPATIAL FILTERING OF LASER-BEAMS

A novel device, the nonspatial filter, is described for laser beam cle anup. It is based on the Bragg selectivity of thick holograms. Unlike pinhole and fiber spatial filters, which employ lenses and apertures i n the transform plane, nonspatial filters operate directly on the lase r beam. This eliminates the need for laser beam focusing, which is the source of many of the material and alignment instabilities and laser power limitations of spatial filters. Standard holographic materials a re not suitable for this application because differential shrinkage du ring processing limits the maximum Bragg angle selectivity attainable, and because they are generally too thin. New technologies that elimin ate the problem of differential shrinkage are described. These technol ogies are based either on the use of a rigid porous substrate material , such as porous glass, filled with a light-sensitive material, such a s holographic photopolymers or dichromated gelatin, or on the use of a thick photopolymer with diffusion amplification (PDA), We report resu lts of holographic nonspatial filtering of a laser beam in one dimensi on, with an angular selectivity of better than 1 mrad. (C) 1997 Societ y of Photo-Optical Instrumentation Engineers.