<title>Photorefractive phased-array-radar processor dynamics</title>

Abstract

ABSTRACTWe derive, and experimentally verify the dynamic and steady state behavior of a high-bandwidth, large degree-of-freedomadaptive phased-array-radar optical processor. The large number of adaptive weights necessary for processing in a complexradar signal environment with large arrays are computed in the form of dynamic three-dimensional volume holograms in aphotorefractive crystal. The processor computes the angles-of-arrival of multiple interfering narrowband radarjammers andadaptively steers nulls in the antenna pattern in order to extinguish the jammers. The theoretical model developed providesanalytical expressions relating system parameters such as feedback gain and phase to suppression depth and convergence ratesfor multiple narrowband jammers of arbitrary spatial profile. We have obtained experimental verification of the systembehavior showing excellent agreement with the theoretical model and experimental jammer suppression as high as -35 dB.1, INTRODUCTIONLarge adaptive phased-array radar antennas can consist of thousands of antenna elements, have GHz bandwidths, and must beable to steer and adapt the antenna beam rapidly in a dynamic signal environment. This represents an extremely demandingsignal processing task. For example, broad band processing of a 10,000 element array, with 100 time delay samples each, atGHz rates can require 1015 multiplies per second. The calculation and updating of the adaptive, complex weights is wellsuited to three-dimensional optical processing techniques where two of these dimensions are used to represent the topology ofthe antenna array, and the temporal variation of the signals constitutes the third dimension. We are developing optical phased-array radar processors which use three-dimensional volume holograms in photorefractive crystals to time-integrate the adaptiveweights to perform beam-steering and jammer-cancellation signal processing tasks1'2'3. These processors use relativelysimple components requiring only a single photorefractive crystal, two single-channel high-speed detectors, and one or twosingle channel acoustooptic Bragg cells. The bandwidth capabilities of these components approach a GHz, allowing theprocessing of wide-band phased-array signals. The required number of processor components is independent of the number ofelements in the phased-array in contrast to traditional electronic or acousto-optic approaches4'5, where the hardwarecomplexity of the processor scales in proportion to the size of the array.We present experimental results from the first of such processors, an adaptive null-steering processor which computes theangle-of-arrival (AOA) of incoming narrowband jammers and extinguishes them. We begin with an overview of the processor