A novel photonic Rotman-lens design for radar phased array antennas

Abstract

Future high performance large radar phased array antennas will require true time delay (TTD), rather than phase shifters, in order to support high bandwidth pulses over a range of scanning angles. Currently, RF Rotman lenses are used as TTD elements, offering a somewhat limited performance with respect to their RF characteristics. Recently, photonic architectures, comprising optical transmitters and receivers, together with some optical circuitry to optically process RF signals, have been proposed as candidates for inclusion within wideband RF systems, which need true time delay to cover an RF bandwidth as large as 18 GHz. In this paper a novel microwave photonic implementation of a Rotman-lens is proposed, offering improved functionality and performance. The proposed scanning unit is an optical module, where photo-detectors connected to the transmitting/receiving antennas are the interfaces, converting between the RF signals and their respective optical waves. The optical module is basically a photonic Rotman lens, quite similar in design to its RF counterpart. However, in addition to the improvement that is obtained by realizing the solution in a photonic module, the proposed photonic Rotman lens improved design is capable of realizing a linear phase profile with a varied slope, that is obtained at the output of the lens for any possible position at the input to the lens. This is in contrast to what is currently available with the conventional RF Rotman lens, where output phase front linearity is obtained for a small and discrete number of input positions. The improved performance is obtained by numerically optimizing the curves of the photonic input and output surfaces of the lens, having an off-centered elliptical profiles, rather than the classically used spherical curvatures. Performance characteristics will be reviewed.