Proceedings Article | 7 November 2007
KEYWORDS: Antennas, Interferometry, Point spread functions, Microlens array, Interferometers, Extremely high frequency, Ultraviolet radiation, Detector arrays, Fizeau interferometers, Electrooptic modulators
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.
