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

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.