Microwave photonic spatial-spectral imaging

Abstract

In this work, a method for millimeter-wave (mmW) imaging is expanded upon to produce optical Fourier transforms of the spatio-temporal aperture to identify angle-of-arrival and temporal frequency of received radio frequency (RF) wave fronts. In the foundational millimeter-wave imaging system, incident RF radiation is sampled with a phased antenna array and modulated onto a sideband upon an optical laser carrier. Leveraging a coherent photonic up-conversion process, both the amplitude and phase of the received complex field are preserved. In the case of mmW imaging, these up-converted signals propagate through optical fibers of equal length, which feed an optical phased array. The result is a passive steering of an optical beam based upon the incident angle of arrival of the received wavefront. 5203 An optical fiber true-time delay network, similar to existing arrayed waveguide gratings, is added to the mmW imaging system in a variant colloquially referred to as instant k-space imaging. Coupling these varied length optical fibers to a two-dimensional optical phased array allows for the passive steering of the optical beam based on not only the angle-of-arrival (AoA) but also on the frequency of the received RF signal. Effectively, this produces a two-dimensional Fourier transform of the spatial and temporal frequency upon a photodetector array prior to digital processing. With the benefit of added frequency discrimination, one of the angular dimensions of the array is consequently sacrificed. As a result, a novel capability is added to the millimeter-wave imaging system that allows the simultaneous channelization of radio signals based upon their angle-of-arrival and frequency. 5203 Subsequently, the two-dimensional spatial-temporal imaging technique is extended to full three-dimensional sensing by increasing the dimensionality of the antenna array. Introducing the additional spatial dimension disrupts the ability to directly produce an optical Fourier transform, however. Thus, tomography algorithms are applied to reconstruct this three-dimensional Fourier transform from the processed optical power distribution. 5203 Specific contributions of this work outline the theoretical sampling criteria for optimal reconstruction of both the 2D and 3D k-space imaging systems. Theoretical development of the receiving arrays is discussed prior to presenting the experimental validation of the proposed techniques.