<title>Adaptive signal processing with photorefractive systems</title>

Abstract

Radio frequency (RF) signal processing systems that use photorefractive crystals to provide key functions are described. Some new experimental results obtained with a time integrating correlator developed to support the acoustooptic null steering processor effort [111 are presented which demonstrate the effectiveness of the photorefractive time integrating approach. We also describe a new adaptive notch filtering system which provides effective and flexible excision of narrowband interference. The system architecture makes optimal use of the unique properties of the photorefractive effect to provide a simple yet highly effective solution for the adaptive signal processing problem.The processing of radio frequency (RF) signals has been realized in many forms with the use of acoustooptic deflectors (AOD) to impress the information both spatially and temporally onto optical carriers. For the particular application of signal correlation, time integration techniques using detectors with integrative temporal response have been successful where long integration times are necessary. The conventional approach uses charge-coupled device (CCD) detector arrays where photogenerated charge is accumulated in potential wells formed for each detection element to perform the necessary integration. One of its drawbacks is the undesirable background bias that builds up in each detection element which limits both the integration time and the dynamic range of the system. These problems can be circumvented by the use of a photorefractive crystal as a holographic, time-integrating detector as has been reported [1-3J. A schematic diagram of that technique is shown in Figure 1 ; the system consists of two AODs (one for the input signal and one for the reference signal), imaging lenses, a photorefractive crystal and a linear detector array. The diffracted light from the two AODs is imaged onto the photorefractive crystal, each incident at a different angle so that a holographic grating is formed within the crystal. Timeintegration is achieved since the hologram forms with a characteristic time constant which is proportional to the incident intensity. Mathematically, the amplitude of the index grating that is formed is given bywhere s (t) and s2(t) are the electrical signals driving the two AODs, V is the velocity of acoustic waves traveling in the xdirection inside the AODs, 'r is the photorefractive material response time, ? is the wavelength of light and 2 is the full angle between the two writing beams incident on the crystal. A weak readout beam is Bragg-tuned to read out the hologram, and the diffracted output that carries the correlation information is imaged onto a linear detector array. Since the readout beam is coherent, further optical processing of the output can be performed directly if so desired. In Martin Marietta's acoustooptic null steering processor (AONSP) system, such a photorefractive correlator serves as an important link in the adaptive approach to sidelobe cancellation for a phased array antenna. The details of the ...