We present an approach to receive-mode broadband beam forming and jammer nulling for large adaptive antenna arrays as well as its efficient and compact optical implementation. This broadband efficient adaptive method for true-time-delay array processing (BEAMTAP) algorithm decreases the number of tapped delay lines required for processing an N-element phased-array antenna from N to only 2, producing an enormous savings in delay-line hardware (especially for large broadband arrays) while still providing the full NM degrees of freedom of a conventional N-element time-delay-and-sum beam former that requires N tapped delay lines with M taps each. This allows the system to adapt fully and optimally to an arbitrarily complex spatiotemporal signal environment that can contain broadband signals of interest, as well as interference sources and narrow-band and broadband jammers--all of which can arrive from arbitrary angles onto an arbitrarily shaped array--thus enabling a variety of applications in radar, sonar, and communication. This algorithm is an excellent match with the capabilities of radio frequency (rf) photonic systems, as it uses a coherent optically modulated fiber-optic feed network, gratings in a photorefractive crystal as adaptive weights, a traveling-wave detector for generating time delay, and an acousto-optic device to control weight adaptation. Because the number of available adaptive coefficients in a photorefractive crystal is as large as 10(9), these photonic systems can adaptively control arbitrarily large one- or two-dimensional antenna arrays that are well beyond the capabilities of conventional rf and real-time digital signal processing techniques or alternative photonic techniques.

Optical BEAMTAP beam-forming and jammer-nulling system for broadband phased-array antennas.

The requirement for diverse applications such as digital video, interactive media etc. demands high storage densities, rapid data transfer rates, random access to data and removability of the storage media. The complete technology involves suitable recording medium and the processes for recording, post treatment and reading. The challenge in the development of holographic storage system is the availability of appropriate recording medium which shows maximum refractive index change on irradiation. In this article, attempt has been made to discuss the important disclosure of patents made during rapid development of the holographic recording medium. Silver halide emulsions to variety of photopolymers are developed to achieve minimum polymer film shrinkage, minimum post processing and multiple writing. All types of polymer based mediums are incorporated in this article. Few discussions are divided into subsections. It has generally been observed that acrylates, acrylamide, carbazolyl-, fluorenyl-, anthracenyl- containing monomers are common monomers and polymethyl metha- crylates, polyvinyl alcohols, epoxy resin, sol-gel to polyelectrolytes are the common matrix resin. A recent addition is the nanoparticles in the composition to improve the diffraction efficiency and hologram stability. A different class of photopolymer called photorefractive polymer is also reported. Liquid crystalline polymer is photopolymer which provides erasable holographic media. Claims chosen are mostly from US patents. Some other literatures are also included as demanded by the discussion. The important features of the medium used in patents are highlighted. The details about performance, as claimed in certain patents, are incorporated. Rapid developments are observed in the subject.

Holographic Recording Medium

We present a spatio-temporal operator formalism and beam propagation simulations that describe the broadband efficient adaptive method for a true-time-delay array processing (BEAMTAP) algorithm for an optical beamformer by use of a photorefractive crystal. The optical system consists of a tapped-delay line implemented with an acoustooptic Bragg cell, an accumulating scrolling time-delay detector achieved with a traveling-fringes detector, and a photorefractive crystal to store the adaptive spatio-temporal weights as volume holographic gratings. In this analysis, linear shift-invariant integral operators are used to describe the propagation, interference, grating accumulation, and volume holographic diffraction of the spatio-temporally modulated optical fields in the system to compute the adaptive array processing operation. In addition, it is shown that the random fluctuation in time and phase delays of the optically modulated and transmitted array signals produced by fiber perturbations (temperature fluctuations, vibrations, or bending) are dynamically compensated for through the process of holographic wavefront reconstruction as a byproduct of the adaptive beam-forming and jammer-excision operation. The complexity of the cascaded spatial-temporal integrals describing the holographic formation, and subsequent readout processes, is shown to collapse to a simple imaging condition through standard operator manipulation. We also present spatio-temporal beam propagation simulation results as an illustrative demonstration of our analysis and the operation of a BEAMTAP beamformer.

Spatio-temporal operator formalism for holographic recording and diffraction in a photorefractive-based true-time-delay phased-array processor

The finite impulse response neural network is described in detail. Different algorithms capable of temporal back-propagation are considered, including a novel modification to the conventional algorithm, called the delayed-feedback back-propagation algorithm. We present and analyze different optoelectronic processors making use of adaptive volume holograms and three-dimensional optical processing. Two single-layer architectures are presented: the input delay plane architecture and the output delay plane architecture. By combining them it is possible to implement both forward and backward propagation in two multi-layer architectures: the first making use of the conventional temporal back-propagation and the second making use of delayed-feedback back-propagation.

Optical finite impulse response neural networks

This thesis presents an approach to receive-mode, broadband beam forming and jammer nulling for large, adaptive antenna arrays as well its efficient opto-electronic implementation. The system is based upon a modified form of the least-mean-square (LMS) algorithm, known as the “Broadband Efficient Adaptive Method for True-time-delay Array Processing,” or BEAMTAP. The primary advantage of BEAMTAP is that it decreases the number of tapped delay lines that are required to process an N-element phased-array antenna from N to only 2, while still providing the full NM degrees of freedom of a conventional N-element time-delay-and-sum beam former requiring N tapped delay lines with M taps each. This produces an enormous savings in delay-line hardware and still allows the system to fully and optimally adapt to an arbitrarily complex spatio-temporal signal environment that can contain: broadband signals of interest, interference sources, and narrowband and broadband jammers. Because BEAMTAP allows for each of these signals to arrive from arbitrary angles onto an arbitrarily shaped array, the system enables a variety of applications in radar, sonar, and communication. 
The power of BEAMTAP lies in the fact that it represents a fundamental paradigm shift in the way that phased-array antennas should be processed. This is because the system efficiently uses output time delay, opposed to all other architectures that require input time delay to implement their array processing algorithms. In addition, BEAMTAP also provides an excellent match with the current capabilities of radio frequency (RF) photonic systems, as it uses a coherent, optically-modulated fiber-optic feed network, gratings in a photorefractive crystal as adaptive weights, a traveling-wave detector for generating time delay, and an acousto-optic deflector to control weight adaptation. Because the number of available adaptive coefficients in a photorefractive crystal is as large as 109, BEAMTAP can adaptively control arbitrarily large one- or two-dimensional antenna arrays that are well beyond the capabilities of conventional RF and real-time digital signal processing techniques, or alternative photonic approaches. 
This thesis, therefore, seeks to fully investigate the theoretical and experimental aspects of implementing the algorithm optically. A strong mathematical foundation is first provided in order to gain insight to the various advantages, complexities, and limitations of the processor. The remaining chapters are then focused upon the work leading up to, and including, an experimental demonstration of broadband, adaptive beam forming and jammer nulling using the BEAMTAP algorithm. The experimental results validate the robustness of the proposed architecture, as the system is capable of simultaneous beam forming and jammer nulling over a wide fractional bandwidth—even when the signal and jammer arrive from the same angle of arrival.

Coherent optical signal processing for broadband adaptive phased-array antennas using the beamtap algorithm

We demonstrate a method of simultaneous holographic recording and readout in photorefractive crystals that provides high write–read beam isolation and wide angular bandwidth. The method uses orthogonally polarized read and write beams and parallel tangent diffraction geometry near the equal curvature condition to provide spatially separable, orthogonally polarized diffracted output beams with high isolation and wide Bragg-matched angular bandwidth. The available angular bandwidth of this read–write technique is analyzed, simulated, and experimentally investigated. The measured angular bandwidth internal to the crystal is approximately 18° × 6° for our 45°-cut BaTiO3 crystal, yet the entire hologram still demonstrates high Bragg selectivity. In contrast, traditional nonparallel-tangent geometries yield angular apertures of the order of 1° × 4°.

Ernst K. Walge

Papers

Wide angular aperture holograms in photorefractive crystals by the use of orthogonally polarized write and read beams