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6 December 1989 Recursive Optical Notching Filter
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A Recursive Optical Notching Filter(RONF)7 was developed to eliminate narrow band interference (NBI) experienced by RF receiver systems such as those used in radar and communications. Electromagnetic interference(EMI) can drastically reduce the effective range of radars and cause intermodulation products in the receiver and subsequent demodulation. The RONF functions to identify the interferers, their numbers and locations, as well as adaptively construct a notch filter structure which 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 wave length, 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.8 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.
© (1989) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Robert W. Brandstetter and Philip G. Grieve "Recursive Optical Notching Filter", Proc. SPIE 1154, Real-Time Signal Processing XII, (6 December 1989);

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