Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
2001-11-13
2004-03-09
Le, Que T. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C250S203100
Reexamination Certificate
active
06703596
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to apparatus for imaging from the reception of radio frequency electromagnetic signals.
2. Related Art
Array antennas for transmission and reception of radio frequency electromagnetic signals are well known in the art. One important use of such antenna arrays is the acquisition of information about a target object. It is well known to acquire information such as range, direction, and altitude of a target object. However, it is also useful to acquire additional information about a target object. An example of such information is information that would enable the target object to be identified. It is well known to use altitude and speed, which can be calculated from repeated range, direction, and angular position measurements, to infer the identity of a target object. However, a target object could be much more efficiently and reliably identified if it could be imaged.
Various techniques exist that can produce such an image, with a given resolution, such as synthetic aperture radar (SAR). The obtained resolution is dependent on various factors, including, but not limited to: antenna aperture size, frequency of operation, and bandwidth. In addition, factors that effect the practicality of implementing such systems include physical size of the antenna aperture, processing of the data generated from such a system and overall cost of implementing such a system. To produce images with increasing detail or image increasingly smaller targets, narrower antenna beamwidths, and/or higher frequencies are typically required. To achieve a narrower beamwidth, the physical antenna aperture size may be increased, which increased the complexity and cost of the system by increasing the number of antenna elements and associated electronics and support structure, or the antenna aperture may be increased synthetically, which increases the data processing requirements and also increases cost.
These increases in cost and complexity can become very significant, especially in spaceborne applications, in which computer processing power and storage is at a premium. For example, data generated from a spaceborne system may be transmitted to a ground station for further processing, in an attempt to avoid increasing cost and complexity of the spacecraft. However, practical link data rates will limit how much data can be transmitted at a given time, thus limiting how much data can be generated and stored before transmission can occur.
One approach to reduce the complexity of such systems makes use of optical components and processing techniques by translating received RF information onto light, via an electro-optic modulator (EOM). These signals are then optically combined for further processing via various lenses, and the resulting signal is projected onto a traditional electronic imaging array. The electronic imaging array, such as a charge coupled device (CCD) produces an image based on the projected signal. The image may be displayed for viewing or digitized for transmission to another location for viewing or processing.
In the prior art, antenna elements were typically connected directly to individual EOMs through transmission lines. In the case of imaging at millimeter wavelengths, the received RF signal may first be down converted using well known RF techniques to bring the signal within the operating range of the EOM and to avoid the high loss of millimeter-wave transmission lines. The down converted signal is then modulated directly onto an optical carrier, for processing, via the EOM. While this approach offers a reduction in the overall complexity of such a system, it still suffers from the performance penalties incurred by connecting discrete components together in terms of size, weight, and RF performance. For example, antenna elements must be connected to discrete EOMs via transmission lines, such as coaxial cables, which have RF loss and may vary in signal phase relative to adjacent elements, further complicating the eventual combination of signals. If the array is to operate at microwave or higher frequencies, the antenna array element spacing tends to restrict the space available for an EOM, as well as the orientation of the EOM behind each element. In the prior art, attempts have been made to directly integrate a traditional antenna element with an EOM in an effort to realize the size and weight advantages offered by the use of fiber optic components. This approach suffered from the fact that traditional antenna structures do not easily lend themselves to integration onto lithium niobate or gallium arsenide substrates, upon which commercially available EOMs are based.
What is needed is an antenna array structure that allows for direct integration of an EOM into its structure, but avoids the size, weight, and performance penalties incurred in present approaches, thereby making such an approach practical for use as an imaging sensor.
SUMMARY OF THE INVENTION
The present invention is an apparatus and system for imaging radio frequency electromagnetic signals. The present invention is useful for imaging a target object in order to identify an unknown object or to determine the condition or configuration of a known object.
In one embodiment, the present invention is an apparatus for imaging radio frequency electromagnetic signals comprising: an optical source operable to output an unmodulated optical signal; an image sensor operable to receive the unmodulated optical signal and an incident radio frequency electromagnetic signal and to modulate the unmodulated optical signal with the received radio frequency electromagnetic signal so as to form a modulated optical signal; a lens operable to receive the modulated optical signal and to focus the modulated optical signal; and a photodetector operable to receive the focused modulated optical signal and output an electrical signal representing the focused modulated optical signal. The optical source may be a laser.
One aspect of the present invention is the image sensor, which comprises: in a plurality of cells, each cell comprising: a first electro-optically active optical waveguide; a first planar electrode substantially parallel to the first waveguide; a second electro-optically active optical waveguide; a second planar electrode substantially parallel to the second waveguide, the first and second planar electrodes being substantially adjacent and coplanar; and a third planar electrode substantially parallel to the first and second planar electrodes and disposed such that the first waveguide lies between the first and third planar electrodes, and the second waveguide lies between the second and third planar electrodes.
The optical source may be coupled to a first end of each of the waveguides. The apparatus may further comprise an output optical waveguide coupled to the second end of each of the first and second waveguides. The apparatus may further comprise a coupler electrically connecting the first and third planar electrodes, whereby the first and third planar electrodes are kept at substantially the same electrical potential. The apparatus may further comprise a polymer layer in which the waveguides are formed and to which the planar electrodes are attached. The first planar electrode may be arranged so that an incident radio frequency electromagnetic signal will impinge upon the first planar electrode.
In one embodiment, the third planar electrode comprises a first portion and a second portion and is disposed such that the first waveguide lies between the first planar electrode and the first portion of the third planar electrode, and the second waveguide lies between the second planar electrode and the second portion of the third planar electrode.
The optical source may be coupled to a first end of each of the waveguides. The apparatus may further comprise an output optical waveguide coupled to the second end of each of the first and second waveguides. The apparatus may further comprise a coupler electrically connecting the first planar electrode and the first
Le Que T.
Lockheed Martin Corporation
Townsend and Townsend / and Crew LLP
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