True time delay photonic beamforming: A review

Wideband, large phased array antennas, using analog control for angle scanning, require true time delay (TTD), rather than phase shifters, to avoid squint and other impairments in both the temporal and spatial domains. While digital beamforming may eventually become a viable solution for such high-end systems, photonic technology can currently provide the required TTD capabilities. Common to most, if not all photonic solutions is the conversion of the RF signal to light, which after proper photonic processing is detected to produce a delayed RF replica of the input RF signal. This double conversion is quite lossy and often requires optical amplification to achieve the required dynamic range. Many architectures have been proposed for the implementation of TTDs. The talk will discuss some of these proposed architectures, including the use of dispersive components, switched fabrics, chirped Bragg gratings, wavelength division multiplexed approaches, as well as solutions based on nonlinear phenomena such as slow light and stimulated Brillouin scattering. In analyzing these various architectures, the following are among the important characteristics should be examined: (i) Flatness of their RF response; (ii) uniformity among the many TTDs populating the array; (iii) Dynamic range; (iv) Nonlinearities and their implications, as well as complexity, volume, weight and cost. In conclusion, it appears that when properly designed, photonic beamformers have matured to the point, where they can replace classical electronic beamformers, offering extremely wide bandwidth and large scanning angles.