Contents I. Introduction
Since arrayed waveguide gratings (AWGs) were proposed [1], [2], a lot of applications have been presented due to their interesting routing capabilities for multiplexers and demultiplexers, add–drop multiplexers (ADMs) [3], or multiwavelength lasers [4]. AWGs have also been proposed to implement optical delay lines (ODLs) for different applications such as optical antenna beamformers [5], [6], optical code division multiple access [5], and analog-to-digital converters (ADCs) [7]. Optical beamforming allows us to overcome major drawbacks of traditional beamforming networks based on digital or intermediate frequency/radio frequency (IF/RF) processing when high carrier frequencies or high data-rate signals are considered. Optical beamforming architectures offer important advantages such as small size, low weight, no susceptibility to electro-magnetic interference, wide instantaneous bandwidth, and squint-free beam steering (owing to its true-time-delay feature) [8]–[10]. In [5], an antenna beamformer architecture using an AWG in a symmetric feedback configuration (loop-back configuration) was proposed. In such an architecture, the modulated optical carrier is driven by the AWG to a suitable delay line depending on its wavelength, so the generated microwave time delay is selected by switching the optical wavelength. The beamformer scheme proposed in [5]requires one AWG for each antenna array element, which is impractical and truly expensive for typical array antennas in terrestrial and space-based radio applications. Recently, an alternative architecture that reduces this limitation and therefore eases the commercial introduction of AWG-based ODL for antenna beamforming applications has been proposed [6]. Such architecture employs dispersion-based relative delays with multiple wavelengths (wavelength division mulipltexing (WDM) signals) instead of absolute propagation delays for a single wavelength. The spectral periodicity of the AWG response allows us to route multiple wavelengths through the same AWG output, obtaining simultaneously multiple progressive time delays with only one AWG. This architecture shows both true-time-delay (TTD) performance and multibeam capability as well as a drastic cost reduction, as a single AWG is able to drive several antenna array elements.
