29-05-2013, 03:20 PM
ULTRA WIDEBAND PHOTONIC CONTROL OF AN ADAPTIVE PHASED ARRAY ANTENNA
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ABSTRACT
This paper presents a new concept for a photonic implementation of a time reversed RF antenna array
beamforming system. The process does not require analog to digital conversion to implement and is
therefore particularly suited for high bandwidth applications. Significantly, propagation distortion due to
atmospheric effects, clutter, etc. is automatically accounted for with the time reversal process. The
approach utilizes the reflection of an initial interrogation signal from off an extended target to precisely
time match the radiating elements of the array so as to re-radiate signals precisely back to the target’s
location. The backscattered signal(s) from the desired location is captured by each antenna and used to
modulate a pulsed laser. An electrooptic switch acts as a time gate to eliminate any unwanted signals such
as those reflected from other targets whose range is different from that of the desired location resulting in
a spatial null at that location. A chromatic dispersion processor is used to extract the exact array
parameters of the received signal location. Hence, other than an approximate knowledge of the steering
direction needed only to approximately establish the time gating, no knowledge of the target position is
required, and hence no knowledge of the array element time delay is required. Target motion and/or array
element jitter is automatically accounted for. This paper presents the preliminary study of the photonic
processor, analytical justification, and simulated results. The technology has a broad range of
applications including aerospace and defense and in medical imaging.
INTRODUCTION
Photonic Processing for Microwave Phased Array Antennas It has been well established that
optical signal processing methods provide the antenna array designer with unique capabilities generally
not available using conventional microwave techniques. When compared with an all-microwave
approach, the utilization of optical components, especially the incorporation of low loss optical fiber, can
provide significant reduction in the size and weight of the system as well as providing a high degree of
immunity to electromagnetic interference (EMI) and electromagnetic pulse (EMP) effects Enabling.
THEORY OF OPERATION
Conventional Phased Arrays
Most optical beamforming systems extant fall into one of two categories; delay-and-sum beamforming and
Fourier-based beamforming. From a target tracking point of view, problems posed by these systems
include a-priori knowledge of the exact position of where one wishes to steer the array, precisely
specified stable antenna array locations, difficulty in specifying and generating antenna nulls to counteract
the effect of interference, and the inability to easily account for atmospheric effects.
Time Reversal
The time reversal process has long been of interest to the sonar community and applications similar to
those discussed in this paper have been explored in the literature under the title of “time lens” [8]. Its
applicability to the present discussion is based on the reciprocal nature of the wave equation. In essence,
a signal f ( t ) is passed through a system
Time Reve rsed Adaptive Array
Unlike with convention antenna array systems where the array elements are precisely arranged with
respect to one another, the N radiating elements which form the time reversed array processor are
conveniently placed in (fixed) locations or perhaps even mounted on a mobile platform. Variation in
element location is automatically accounted for in the time reversal process. As with any system of this
type, a priori knowledge of the existence of a target and a reasonably accurate initial estimate of its
location are required.
This information can be obtained from either an on-board or remote radar system, or from some other
intelligence gathering agent, and then communicated to the array.
Pulse Propagation in a Dispersive Medium
We briefly present here the well-known characteristics of Gaussian pulse propagation in a dispersive
transmission medium [10]. This forms the basis for the optical implementation of the time reversal
process and makes clear the design guidelines and system limitations. It is assumed that the pulse
originates from a source that is transform-limited and can be modeled as a Gaussian signal both spectrally
and temporally.
ANTICIPATED PERFORMANCE
Candidate Pulsed Laser Sources
Supercontinuum sources have been widely reported over the last decade [14-16] with wide spectral
breadth but typically without a flat enough power spectral density across the spectral range desirable for
the modulation of an RF signal, or too little spectral range for the application presented here.
Supercontinua from 1.0-km of Highly Nonlinear Fiber (HNLF) fiber pumped by a continuous wave
Raman fiber laser have been reported with >505-nm bandwidth centered at 1481-nm, and spectral
flatness of <2.4-dB [17], yet for time stretch applications, a pulsed supercontinuum source is required in
order to generate linear frequency chirping as the optical output. A supercontinuum source potentially
most useful for application in ultra -wideband photonic control of an adaptive phased array antenna has
been reported [18] whereby 1-psec pulses generated by a passively mode-locked Erbium doped fiber
amplifier were dispersed in three stages of single mode fiber, resulting in over 80-nm of linearly chirped
bandwidth centered at 1560-nm.