Sparse Aperture Detection and Imaging of Millimeter Sources via Optical Image-Plane Interferometry

摘要

We attempt to perform real time detection and direct high resolution imaging of millimeter blackbody sources using sparse aperture interferometry. We reject heterodyne technology for a multitude of factors including bulky equipment, cryogenic cooling, long integration times, and indirect imaging. An alternative method is to convert the incoming millimeter waves into optical and perform optical image-plane interferometry in real time. This method is suitable for snapshot-imaging of short-lived phenomena, often encountered in defense and security applications. The approach presented in this work utilizes a millimeter wave antenna array coupled to an optical interferometer which images directly on a detector array for image read-out, processing, and storage. To minimize the maximum sidelobes of the point spread function, we choose an antenna array composed of two concentric hexagonal rings, such that the outer ring is ~3 times the inner ring. This design ensures more or less uniform and isotropic spatial frequency coverage, eliminating difficulties associated with resolving out structures whose spatial frequencies are in between that of the single aperture diameter and those of the baselines. The Fourier coverage of this array is the sum of the Fourier coverage of the outer ring plus that of the inner ring added to that of the baselines between the inner and outer rings. The need for delay lines is done away with by mounting all the apertures on the same plane. The incoming millimeter signals are fed through electro-optical modulators for upconversion onto an optical carrier, which can be readily captured, routed, and processed using optical techniques. The optical waves are fed via a fiber optic array onto a microlens array which is a scaled down version of the antenna array configuration. Then homodyne interferometry is performed. We reject pupil-plane (Michelson) interferometry based on a multitude of factors. The main drawback is that pupil-plane interferometers don't produce images but rather gives the information about the autocorrelation of the object. We instead use a classical image-plane interferometer (Fizeau) setup and direct detection is performed on a detector array. Image-plane interferometry has its advantages. Unlike its pupil-plane cousin, a Fizeau interferometer is a true imaging device, where each beam is used to make an image of the object and are superimposed. Because Fizeau beam combiners work in the image plane, they don't suffer from ambiguities associated with the interpretation of visibility measurements. Also since the beams traverse the same paths and superpose, unmeasured phase changes do not creep in. In the design of the Fizeau interferometer, we preserve homothetic mapping, i.e., the entrance and exit pupils are replicas of one another, scaled only by a constant factor. This ensures direct imaging over a wide bandwidth with high angular resolution, high sensitivity, and a wide field of view. Since the Fizeau setup allows access to large fields, mosaicing wide fields is possible.