Norman J. Berg

A new acoustophotorefractive effect in lithium niobate

A new photorefractive effect resulting from the interaction of high‐intensity short‐duration laser pulses with propagating acoustic waves has been found to occur in LiNbO3. The index‐of‐refraction change (δn) is proportional to the rf signal and increases sublinearly with the number of laser pulses and as the 1.3 power of the incident laser energy density (J) per pulse. The decay time of δn varies from a few hours when only green illumination (530 nm) is used to several weeks when combined green and infrared illumination (1060 nm) are used. This acoustophotorefractive effect can be utilized to construct an acousto‐optic memory correlator.

Three and four product surface‐wave acousto‐optic time integrating correlators

A method and device for processing spread-spectrum and other wideband comications and radar signals to obtain three and four product correlated signals. A laser beam is split and shaped into first and second sheet beams. The first beam is directed to a first acousto-optic medium where it is doubly diffracted by first and second signals. The second beam is directed to a second acousto-optic medium which is spatially rotated 90° relative to the first acousto-optic medium where the second sheet beam is either singly diffracted by a third signal or doubly diffracted by a third signal and a fourth signal. The diffracted sheet beams are shaped into square beams, combined and directed to a photodiode area array.

Surface wave delay line acoustooptic devices for signal processing

Several acoustooptic devices have been developed for use as electronic signal processors at the Harry Diamond Laboratories. These devices use the Bragg interaction between a coherent light beam and surface acoustic waves propagating in a transparent crystalline delay line. Both real-time convolution and correlation of signals have been performed, and a real-time continuous Fourier transform has also been achieved. A programmable memory correlator has been demonstrated. This device uses a newly discovered photorefractive effect to store an image of a surface acoustic wave in a lithium niobate delay line. An acoustooptic implementation of the triple-product convolver is under active development. This device has been proposed for use in conjunction with charge-coupled-device chirp-Z-transform modules to perform very long discrete Fourier transforms and to do omega-k beam forming.

Wide-Band Signal Processing Using the Two-Beam Surface Acoustic Wave Acoustooptic Time Integrating Correlator

A new acoustooptic architecture for performing real-time correlation of high-frequency wide-band signals has been developed. It uses a surface-acoustic-wave (SAW) delay line, and features the optical interference of two coherent light beams which have been Bragg-diffracted by SAWs propagating in the line. The signal multiplication, and subsequent time integration of the product formed, is performed by a photodiode array which detects the diffracted light. This architecture has achieved time-bandwidths products exceeding 10/sup 6/ (34 MHz X 30 ms), and has several attributes which make it particularly well suited for use as a spread-spectrum signal processor. These include linearity of operation, large dynamic range, a large time aperture over which the correlation can be observed, and the ability to determine the center frequency and bandwidth of the signals. A correlator with this architecture has been used to detect a number of wide-band spread-spectrum signals. Its suitability for use as a signal processor in several spread-spectrum systems is considered.

A new surface‐wave acousto‐optic time integrating correlator

A new realization of the acousto‐optic time‐integrating correlator has been constructed. The new device uses a surface‐acoustic‐wave delay line and is configured so that no separate reference beam is required for coherent detection. Instantaneous bandwidths of 30 MHz and integration times of 30 ms have been achieved. Broadband signals, such as pseudonoise biphase modulated waveforms, with signal‐to‐noise ratios of −40 dB have been detected. In addition, center frequency, bandwidth, and relative time‐difference‐of‐arrival of these signals can be determined.

An acousto‐optic real‐time ’’two‐crystal’’ correlator

A real‐time correlator has been developed that utilizes successive acousto‐optic interactions in two adjacent piezoelectric crystals (lithium niobate, LiNbO3, and bismuth germanium oxide, BGO) having surface‐acoustic‐wave (SAW) propagation velocities that differ by almost a factor of 2. Complex waveforms, such as FM chirps and Barker codes, have been correlated successfully. A large time‐bandwidth product (∼3000) was obtained, and a 40‐dB dynamic range was shown to be feasible. The use of this two‐crystal device to obtain either pulse expansion or time inversion of a signal also was demonstrated.

Real‐time Fourier transformation via acousto‐optics

An implementation of the chirp transform algorithm for performing a real‐time Fourier transform is described. The implementation is based upon an acousto‐optic convolver with a large time‐bandwidth product An instantaneous bandwidth of about 75 MHz was achieved for the Fourier transformer with a dynamic range in excess of 60 dB for a cw time‐gated waveform. By using an almost‐uniform light‐beam intensity across the device, the measured sidelobe amplitudes were within 0.5 dB of the theoretical values. The corresponding measured phase errors were less than 5°. The adaptability of this acousto‐optic implementation is demonstrated by the ease with which one can vary both the sidelobe weighting function and the time rate of the transform.

Multichannel signal processing using acoustooptic techniques

The feasibility of acoustooptically processing several different signals simultaneously has been demonstrated using two distinct multiplexing techniques. A four-channel acoustooptical system using frequency multiplexing was successfully used for correlating signals;each channel had a 45-MHz bandwidth and about 45-dB interchannel isolation. A second technique for multiplexing, which uses the angular orientation of acoustic waveforms, was also demonstrated; a minimum of 20-dB channel isolation was measured, and parameters that may limit the number of channels and the channel isolation were identified and measured. Both multiplexing techniques can be used in a memory device for increased data-handling capability.

Acousto-optic diffraction and signal mixing device

An acousto-optic diffraction and signal mixing device. A laser light beam is expanded and shaped into a sheet beam which is directed across the surface of an acousto-optic medium. Four acoustic transducers are disposed on the acousto-optic medium, two at each end of the medium. Each acoustic transducer is supplied with a signal to be propagated on the surface of the acousto-optic medium. The first two signals diffract the sheet beam to produce a first product diffracted beam of light containing the product of the first two signals. The second two signals diffract the sheet beam to produce a second product diffracted beam of light containing the product of the second two signals.

Compact Architecture For Spectrum Analysis And Correlation

An acousto-optic architecture for simultaneously obtaining time integration correlation and high-speed power spectrum analysis has been constructed using commercially available Te02 modulators and photodiode detector-arrays. The correlator section of the processor uses coherent interferometry to attain maximum bandwidth and dynamic range while achieving a time-bandwidth product of 106. Two correlator outputs are achieved in this system configuration. One is optically filtered and magnified 2 : 1 to decrease the spatial frequency to a level where a 25-MHz bandwidth may be sampled by a 62-mm array with elements on 25-μm centers. The other output is magnified by a factor of 10 such that the center 4 μS of information is available for estimation of time-difference-of-arrival to within 10 ns. The Bragg cell spectrum-analyzer section, which also has two outputs, resolves a 25-MHz instantaneous bandwidth to 25 kHz and can determine discrete-frequency reception time to within 15 μs. A microprocessor combines spectrum analysis information with that obtained from the correlator.