Robert T. Weverka

Photorefractive processing for large adaptive phased arrays

An adaptive null-steering phased-array optical processor that utilizes a photorefractive crystal to time integrate the adaptive weights and null out correlated jammers is described. This is a beam-steering processor in which the temporal waveform of the desired signal is known but the look direction is not. The processor computes the angle(s) of arrival of the desired signal and steers the array to look in that direction while rotating the nulls of the antenna pattern toward any narrow-band jammers that may be present. We have experimentally demonstrated a simplified version of this adaptive phased-array-radar processor that nulls out the narrow-band jammers by using feedback-correlation detection. In this processor it is assumed that we know a priori only that the signal is broadband and the jammers are narrow band. These are examples of a class of optical processors that use the angular selectivity of volume holograms to form the nulls and look directions in an adaptive phased-array-radar pattern and thereby to harness the computational abilities of three-dimensional parallelism in the volume of photorefractive crystals. The development of this processing in volume holographic system has led to a new algorithm for phased-array-radar processing that uses fewer tapped-delay lines than does the classic time-domain beam former. The optical implementation of the new algorithm has the further advantage of utilization of a single photorefractive crystal to implement as many as a million adaptive weights, allowing the radar system to scale to large size with no increase in processing hardware.

Efficient true-time-delay adaptive array processing

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.

Wide angular aperture holograms in photorefractive crystals by the use of orthogonally polarized write and read beams

We demonstrate a method of simultaneous holographic recording and readout in photorefractive crystals that provides high write-read beam isolation and wide angular bandwidth. The method uses orthogonally polarized read and write beams and parallel tangent diffraction geometry near the equal curvature condition to provide spatially separable, orthogonally polarized diffracted output beams with high isolation and wide Bragg-matched angular bandwidth. The available angular bandwidth of this read-write technique is analyzed, simulated, and experimentally investigated. The measured angular bandwidth internal to the crystal is approximately 18° × 6° for our 45°-cut BaTiO(3) crystal, yet the entire hologram still demonstrates high Bragg selectivity. In contrast, traditional nonparallel-tangent geometries yield angular apertures of the order of 1° × 4°.

Fully interconnected, two-dimensional neural arrays using wavelength-multiplexed volume holograms.

We present a compact method to provide independent weighted interconnections between every pixel in a two-dimensional input array and every pixel in a two-dimensional output array. The two input dimensions and two output dimensions are connected by a four-dimensional weight matrix consisting of wavelength-multiplexed volume holograms that use cryogenic spectral hole burning in a single holographic element.

Fourier treatment of nonlinear optics

As the spatial angular bandwidth and temporal frequency bandwidth of nonlinear optical interactions is increased it becomes necessary to fully account for the effects of diffraction, anisotropy, and dispersion upon propagation through the volume of the nonlinear optical media. This increased bandwidth is required in order to increase the peak intensities through focusing or the use of ultrashort pulses and thereby increase the strength and efficiency of nonlinear interactions, or in order to increase the information capacity of the optical fields. The increased bandwidth appears to significantly complicate the simple plane wave coupled mode analytic formulation, however in the paper we show that in the Born regime of weak scattering a simple Fourier decomposition mechanism can be employed that fully accounts for the spatial, temporal, and polarization structure of the nonlinearly generated fields. This momentum space approach allows simple visualizations of the process, allows optimizations of the efficiency, resolution, and bandwidth of the nonlinearity, and enables the design and interpretation of novel interaction geometries. The momentum space solution begins with a 3-D spatio-temporal Fourier transformation of the vector field applied to the planar boundary of the nonlinear optical medium.

Adaptive phased-array radar processing using photorefractive crystals

An unconstrained, adaptive, null-steering phased-array optical processor that utilizes a photorefractive crystal to time-integrate the adaptive weights, and null out correlated jammers is described. A passive processor is presented, where it is assumed that it is only known a priori that the signal is broad-band and the jammers are narrow-band. The passive processor computes the angle(s) of arrival of the jammers and extinguishes them. Also presented is an active processor in which the temporal waveform of the desired signal is known, but the look direction is not. The processor computes the angle(s) of arrival of the desired signal and steers the array to look in that direction while nulling any narrow-band jammers. These are two embodiments of a class of processors which use the angular selectivity of volume holograms to form the nulls and look directions in an adaptive phased array radar antenna pattern

Acousto-optic photonic crossbar switch. Part I: design.

We present the design of a 12 × 12 photonic crossbar interconnection network constructed using a single three-dimensional acousto-optic crystal. Previous crossbars based on bulk acousto-optic cells require multichannel deflectors with one deflector per optical input; in contrast the design presented here angularly multiplexes these independent deflectors into a single-transducer acousto-optic device. A Fourier-optics analysis of an acoustically lossy Bragg deflector is coupled to a momentum-space analysis that permits the derivation of complete design equations for the switch. As a concrete example, the complete design of a 12 × 12 crossbar is presented. Finally, a coupled-mode analysis of the first- and second-order diffractions in the angularly multiplexed Bragg cell reveals the fundamental efficiency bounds of the switching network.

Acousto-optic tunable filter using phased-array transducer with linearized RF to optical frequency mapping

We present an optimized design of an acousto-optic tunable filter (AOTF) using a phased-array transducer for a spectrally-multiplexed ultrafast pulse-shaping RF beamformer application. The momentum-space interaction geometry is used to optimize an AOTF using acoustic beam-steering techniques in combination with acoustic anisotropy in order to linearly map the applied RF frequency to the filtered output optical frequency. The appropriate crystal orientation and phased-array transducer design are determined to linearize the RF to optical frequency mapping even in the presence of optical dispersion of the birefringence. After optimizing the phased-array transducer, acoustic anisotropy, and optical anisotropic diffraction geometry, the designed AOTF will compensate for the birefringent dispersion of TeO2 to give a linear modulation of RF frequencies onto the corresponding optical frequencies. This linearized frequency mapped AOTF is required for a squint-compensated, wavelength-multiplexed, optically processed RF imager.

Wide-angular-aperture acousto-optic devices

A new class of acousto-optic device that simultaneously achieves a wide angular aperture, broad bandwidth, and high diffraction efficiency is presented. Parallel tangents and beam steering are used simultaneously, which enhances the product of acceptance angle, bandwidth, and diffraction efficiency to be larger than that of isotropic acousto-optic devices by more than one order of magnitude and to be larger than that of tangentially matched acousto-optic devices by more than four times. A wide-angular-aperture acousto-optic device with a center frequency of 150 MHz and operating at 514.5 nm was optimally designed, fabricated, and its performance measured. The consistency between experiment and theory is excellent. This device can also be used as a high speed and high efficiency modulator without any change in design. This was verified experimentally and risetime of 11.7 ns was obtained. Thus, this device can be optimally used as both a wide-angular-aperture Bragg cell/deflector and a high speed modulator.

Beam-steering and jammer-nulling photorefractive phased-array radar processor

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.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.