Design and Performance of a Distributed Aperture Millimeter-Wave Imaging System using Optical Upconversion

Passive imaging using millimeter waves (mmWs) has many advantages and applications in the defense and security markets. All terrestrial bodies emit mmW radiation and these wavelengths are able to penetrate smoke, blowing dust or sand, fog/clouds/marine layers, and even clothing. One primary obstacle to imaging in this spectrum is that longer wavelengths require larger apertures to achieve the resolutions typically desired in surveillance applications. As a result, lens-based focal plane systems tend to require large aperture optics, which severely limit the minimum achievable volume and weight of such systems. To overcome this limitation, a distributed aperture detection scheme is used in which the effective aperture size can be increased without the associated volumetric increase in imager size. However, such systems typically require high frequency (~ 30 – 300 GHz) signal routing and down conversion as well as large correlator banks. Herein, we describe an alternate approach to distributed aperture mmW imaging using optical upconversion of the mmW signal onto an optical carrier. This conversion serves, in essence, to scale the mmW sparse aperture array signals onto a complementary optical array. The optical side bands are subsequently stripped from the optical carrier and optically recombined to provide a real-time snapshot of the mmW signal. In this paper, the design tradeoffs of resolution, bandwidth, number of elements, and field of view inherent in this type of system will be discussed. We also will present the performance of a 30 element distributed aperture proof of concept imaging system operating at 35 GHz.