Contents I. Introduction
Multi-beamforming networks are essential technologies for next-generation phased array systems, enabling capabilities such as multi-target tracking, multi-functionality, and various operational modes [1]. Traditional electronic phased array systems typically rely on phase shifters to construct beamforming networks. However, the instantaneous bandwidth and scanning angle of these signals are constrained by aperture traverse time and aperture effects, leading to beam squint, which significantly limits the operational bandwidth of phased array systems. Furthermore, as the scale of phased arrays and the number of beams increase, additional challenges such as large system size, degraded electromagnetic interference, increased complexity, and decreased reliability become prominent. To overcome these limitations, microwave photonic beamforming technologies have been proposed as promising alternatives for processing microwave signals in the optical domain. Since the frequencies of optical carriers are more than four orders higher than the microwave, it is much easier to process broadband microwave signal, enabling high-performance multi-beamforming. Efforts in this direction include the development of fiber-based optical true time delay lines (OTTDLs) [2], Bragg gratings [3], Fourier transform lenses [4], and spatial light modulators [5]. While these innovations have been instrumental in demonstrating the potential of optical beamformers for multi-beam operations, they often rely on bulky implementations and face inherent challenges in scaling to accommodate a large number of wireless beams and antenna elements (AEs).
