W-Band Photonic Pulse Compression Radar with Dual Transmission Mode Beamforming

Millimeter wave (MMW) sensing systems have potential for enhanced range and angular resolution in comparison to lower radio frequencies. This article employs photonic-assisted radio-frequency arbitrary waveform generation to implement a high-bandwidth multi-target MMW radar. The presented sensing system operates in the W-band using chirped RF signals, with spectra between 80 and 95 GHz at the 10 dB points. We implement sum- and difference-mode beamforming using a two element transmitter array to perform azimuth angle sensing in addition to target ranging. Photonics-based waveform generation with programmable optical pulse shapers provides for incorporation of approximately uniform phase shifts across the entire bandwidth, which in turn enables control of the far-field interference patterns of the broadband waveforms. Our approach has analogies with monopulse tracking radar techniques, in which target angle information is revealed through sum- and difference-mode processing of the receiving antennae, but with the dual-mode processing implemented at the transmitter side via photonic radio-frequency arbitrary waveform generation. Experiments with a single target demonstrate unambiguous sensing of target angle with an estimated root-mean-square error of 0.18^\circ over a \pm 4^\circ angular range. We also demonstrate independent sensing of the angles of two targets that are resolved in range.