Optically controlled time delays for a phased-array antenna using a self-aligned dual-frequency beam

Abstract

The inherent parallelism of optics is attractive for true time delay beamforming in phased-array antennas performing wide instantaneous bandwidth over wide scan angles. Following the concepts presented in ref. (1), the microwave signal originates from the coherent detection by a high speed photodiode, of a dual frequency beam, obtained from an acousto optic Bragg cell. We present the implementation of an optical architecture based on polarization switching by N spatial light modulators of p × p pixels. This original arrangement of these modulators provides 2N optically controlled time delays. Furthermore we also propose and experimentally demonstrate a new technique that provides a dynamic alignment of two beams at different frequencies over a wide bandwidth. It involves an original application of the interaction between the beams in a photorefractive BaTiO3 crystal. A single frequency laser beam (ω0/2π) is focused through a Bragg cell excited by a continuous microwave signal (fo). The emerging beams at (ω0/2π) and (ω0/2π+f0) intersect in the BaTiO3 crystal. They undergo self fanning processes and are coupled, via noise figures in the direction of the (ω0/2π) beam. When fo is modified, the direction of the (ω0/2π + f0) beam changes but these two beams remain coupled in the fixed direction of (ω0/2π), allowing the use of a large bandwidth microwave signal. The optical amplification of a microwave signal in a photorefractive crystal (2) is also discussed.