Multibeam Beamforming Demonstration of a Hybrid Integrated Photonic Module for a Synthetic Aperture Radar Receiver

This article presents an experimental characterization of the first photonic-assisted receiver module developed for a Synthetic Aperture Radar (SAR) designed for Earth observation from space as part of the SPACEBEAM project. The receiver module aims to achieve high-resolution wide-swath SAR imaging using photonic beamforming techniques implemented in a hybrid photonic integrated circuit (PIC). It leverages the Scan-on-Receive (SCORE) method to perform precise and continuous beamforming-on-receive of wideband signals from a 12-element antenna array, synthesizing up to three simultaneous receiving beams. The PIC also employs frequency-agnostic photonic down-conversion to enable direct digitization of the synthesized beams. The design and fabrication of the hybrid PIC are discussed, highlighting the use of indium phosphide (InP) and silicon nitride (Si₃N₄) platforms for active and passive components, respectively. The description of the receiver module is detailed, including functional architecture, PIC layout, chip fabrication, and hybrid assembly with custom-designed PCB and control electronics. Experimental results demonstrate successful frequency down-conversion from X-band at 9.65 GHz to a low intermediate frequency at 1.35 GHz with a measured conversion loss of 50 dB. Furthermore, both single- and multi-beam beamforming operations of the PIC receiver module are demonstrated through radiation diagrams showing beam steering at different pointing angles. To the best of our knowledge, this outcome marks the first-ever multibeam demonstration of a fully integrated optical beamforming network (OBFN). It underscores the effectiveness of photonic technologies in implementing OBFNs with high scalability, flexibility, and suitability for small satellite platforms.