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A recursive optical notching filter (RONF) [U.S. Patent No. 4,522,466 (1985)] was developed to eliminate narrowband interference (NBI) expe-rienced by rf receiver systems such as those used in radar and commu-nications. Electromagnetic interference (EMI) can drastically reduce the effective range of radars and cause intermodulation products in the re-ceiver and subsequent demodulation. The RONF functions to identify the
interferers and their numbers and locations as well as to adaptively construct a notch filter structure that uniquely excises the NBI from the rf signal spectrum. Further, by the use of a novel recursive architecture, it is possible to develop notch depths approaching the signal dynamic range. The RONF operations are accomplished by impressing the unprocessed rf signal onto a laser carrier by means of an acousto-optic modulator (Bragg cell). The modulated laser beam is then optically Fourier transformed to produce a real-time frequency spectrum. At the carrier optical wavelength, the frequency-translated rf signal with NBI appears in spatial coordinates for parallel processing with spatial intercepts of the EMI provided by programmed amplitude, and phase "blockers" constructed by the programmable spatial filter (PSF). It is at the PSF that optical recursion is used to obtain superior notching depth [U.S. Patent No. 4,645,300 (1987)]. With the NBI removed, the optical signal is then inverse Fourier transformed and the original radio frequency signal is recovered by optical heterodyne conversion. Laboratory tests with radar systems as well as various related stimuli have been conducted under field and on-site conditions. The RONF test systems have demonstrated notch depths greater than 40 dB using the recursive architecture.
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