GR851762

An optical filtering system (50) in which a signal beam (54) of optical radiation is passed through the optical filtering means a multiplicity of times. The filtering system has an optical Fourier transformer means (14), a spatial filter (18), optical inverse Fourier transform means (20) and a pair of reflectors (38, 40) in the Fourier plane of the spatial filter. An input signal beam to be filtered is optically Fourier transformed by the transform means (14) and is passed through the spatial filter (18) where unwanted frequencies of the signal beam are attenuated. The filtered beam is reflected by one of the reflectors back through the filter and the second mirror reflects it back through the filter a third time. Subsequent reflections back through the filter with a consequent further filtering of the signal beam can be accomplished by appropriately positioning the reflectors. After multiple filtering passes, the filtered beam is extracted for utilization. By maintaining the recursions of the beam in the Fourier plane of the filter, which preferably is of a programmable type, the optical throw of the filter system can be significantly reduced and a flexible imaging system without the restraints of external recursion is realizable in a robust structure with fewer components. An embodiment of the system being used in a heterodyning RF optical filtering system is described.