This paper presents an overview of a cooperative research and development program between Ball Aerospace and Technologies Corp. and the Air Force Phillips Laboratory for laser communications. This effort employs hardware and equipment originally developed to support the crosslinking between geostationary Defense Support Program surveillance satellites. This joint activity modifies the existing hardware for ground based demonstrations and simulations focuses at risk reduction for future applications and technology insertion into operational architectures meeting future commercial, civil, and DOD communications requirements. The ultimate goal of the program is to produce hardware for a near term flight demonstration. A brief overview of the capabilities of the existing hardware will be presented followed by a status of the development efforts and future plans.
A high bandwidth, large degree-of-freedom photorefractive phased-array antenna beam-forming processor which uses 3D dynamic volume holograms in photorefractive crystals to time integrate the adaptive weights to perform beam steering and jammer-cancellation signal-processing tasks is described. The processor calculates the angle-of-arrival of a desired signal of interest and steers the antenna pattern in the direction of this desired signal by forming a dynamic holographic grating proportional to the correlation between the incoming signal of interest from the antenna array and the temporal waveform of the desired signal. Experimental results of main-beam formation and measured array-functions are presented in holographic index grating and the resulting processor output.
We present a novel and efficient approach to true-time-delay (TTD) beamforming for large adaptive phased arrays with N elements, for application in radar, sonar, and communication. This broadband and efficient adaptive method for time-delay array processing algorithm decreases the number of tapped delay lines required for N-element arrays form N to only 2, producing an enormous savings in optical hardware, especially for large arrays. This new adaptive system provides the full NM degrees of freedom of a conventional N element time delay beamformer with M taps, each, enabling it to fully and optimally adapt to an arbitrary complex spatio-temporal signal environment that can contain broadband signals, noise, and narrowband and broadband jammers, all of which can arrive from arbitrary angles onto an arbitrarily shaped array. The photonic implementation of this algorithm uses index gratings produce in the volume of photorefractive crystals as the adaptive weights in a TTD beamforming network, 1 or 2 acousto-optic devices for signal injection, and 1 or 2 time-delay-and- integrate detectors for signal extraction. This approach achieves significant reduction in hardware complexity when compared to systems employing discrete RF hardware for the weights or when compared to alternative optical systems that typically use N channel acousto-optic deflectors.
A photorefractive crystal can be used as a three-dimensionally parallel array of multipliers. In writing the photorefractive gratings, the crystal performs the operation of multiplying the inputs and integrating the resulting products. In readout the photorefractive crystal multiplies the inputs with the stored gratings. The three dimensional array of multipliers is only accessible from the two-dimensional faces. This restricts us to using the photorefractive crystal for outer- products in writing to the full three dimensions of parallelism and inner products in reading out the three dimensions, when we are using a single wavelength system. We have explored issues in using the full three dimensions of parallelism in the real time volume holograph for signal processing applications. These issues are illustrated with our successful implementation of a high-bandwidth large phased-array radar processing system. This example system leads to a new algorithm for processing phased-array-radar data, which has a great advantage in hardware complexity over the classic Widrow algorithm and leads to a significant hardware savings for true-time-delay phased-array-radar control systems.
The utilization of three dimensions of parallelism in photorefractive data processors is extended to parallel three- dimensional readout for the two radar scenarios of radar doppler and ranging processing, and 3D synthetic aperture radar. These are scenarios in which the data processing has full parallelism in all of three dimensions, making the volume holographic approach attractive. The result of this processing gives us a surface with the third dimension coded with the wavelength and the value represented by the intensity so that the three dimensions of data may be read out in parallel with the use of a three-color CCD.
We are developing a class of optical phased-array-radar processors which use the large number of degrees-of-freedom available in 3D photorefractive volume holograms to time integrate the adaptive weights to perform beam-steering and jammer-cancellation signal-processing tasks for very large phased-array antennas. We have experimentally demonstrated independently the two primary subsystems of the beam-steering and jammer-nulling phased-array radar processor, the beam-forming subsystem and the jammer-nulling subsystem, as well as simultaneous main beam formation and jammer suppression in the combined processor. The beam-steering subsystem calculates the angle of arrival of a desired signal of interest and steers the antenna pattern in the direction of this desired signal by forming a dynamic holographic grating proportional to the correlation between the incoming signal of interest from the antenna array and the temporal waveform of the desired signal. This grating is formed by repetitively applying the temporal waveform of the desired signal to a single acousto-optic Bragg cell and allowing the diffracted component from the Bragg cell to interfere with an optical mapping of the received phased-array antenna signal at a photorefractive crystal. The diffracted component from this grating is the antenna output modified by an array function pointed towards the desired signal of interest. This beam-steering task is performed with the only a priori information being that of the knowledge of a temporal waveform that correlates well with the desired signal and that the delay of the desired signal remains within the time aperture of the Bragg cell. The jammer-nulling subsystem computes the angles-of- arrival of multiple interfering narrowband radar jammers and adaptively steers nulls in the antenna pattern in order to extinguish the jammers by implementing a modified LMS algorithm in the optical domain. This task is performed in a second photorefractive crystal where holographic gratings are formed which are proportional to the correlation between the unprocessed antenna output and a delayed version of the formed main beam. The diffracted components from these gratings are subtracted from the formed main-beam signal producing a processor output with reduced jammer content.
We derive, and experimentally verify the dynamic and steady state behavior of a high- bandwidth, large degree-of-freedom adaptive phased-array-radar optical processor. The large number of adaptive weights necessary for processing in a complex radar signal environment with large arrays are computed in the form of dynamic three-dimensional volume holograms in a photorefractive crystal. The processor computes the angles-of-arrival of multiple interfering narrowband radar jammers and adaptively steers nulls in the antenna pattern in order to extinguish the jammers. The theoretical model developed provides analytical expressions relating system parameters such as feedback gain and phase to suppression depth and convergence rates for multiple narrowband jammers of arbitrary spatial profile. We have obtained experimental verification of the system behavior showing excellent agreement with the theoretical model and experimental jammer suppression as high as -35 dB.
We describe three real-time optical processing systems, for imaging radar, that form the array phase function and produce the radar image from the incoming signals. The first is an implementation of the Steinberg algorithm where we have modified the data format by frequency multiplexing the range information to allow for the data from all range bins to be present simultaneously. The second implementation uses this same input format, but provides hysteresis on the point scatterer used to calibrate the array. This allows for moving radar arrays where the point used to calibrate the array may be time varying. Finally we describe a self oscillating phase conjugate resonator, with a coherent optical to RF converter inside the cavity. This is a hybrid optical-microwave oscillator that adapts to the incoming signal.