Instantaneous microwave-photonic spatial-spectral channelization via k-space imaging

The ability to both spatially and spectrally demultiplex wireless transmitters enables communication networks with higher spectral and energy efficiency. In practice, demultiplexing requires sub-millisecond latency to map the dynamics of the user space in real-time. Here, we present a system architecture, referred to as k-space imaging, which channelizes the radio frequency signals both spatially and spectrally through optical beamforming, where the latency is limited only by the speed of light traversing the optical components of the receiver. In this architecture, a phased antenna array samples radio signals, which are then coupled into electro-optic modulators (EOM) that coherently up-convert these signals to the optical domain, preserving their relative phases. The received signals, now optical sidebands, are transmitted in optical fibers of varying path lengths, which act as true time delays that yield frequency-dependent optical phases. The output facets of the optical fibers form a two-dimensional optical phased array in an arrangement preserving the phases generated by the angle of arrival (AoA) and the time-delay phases. Directing the beams emanating from the fibers through an optical lens produces a two-dimensional Fourier transform of the optical field at the fiber array. Accordingly, the optical beam formed at the back focal plane of the lens is steered based upon the phases, providing the angle of arrival and instantaneous frequency measurement (IFM), with latency determined by the speed of light over the optical path length. We present a numerical evaluation and experimental demonstration of this passive AoA- and frequency-detection capability.