John N. Lee

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.

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.

Radar And Communication Band Signal Processing Using Time-Integration Processors

A new architecture for performing time-integration correlation is described. The correlator 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 time integration is performed by a photodiode array which detects the diffracted light. Time-bandwidth products exceeding 106 (50 MHz times 30 ms) have been achieved. This two-beam SAW acousto-optic time-integrating correlator has been used to detect a number of wideband spread-spectrum signals. It has several attributes which make it particularly well suited for use as a spread-spectrum signal processor. These include linearity of operation, large time aperture over which the correlation can be observed, and the ability to determine the center frequency and bandwidth of the signals. The suitability of this correlator for use as a signal processor in spread-spectrum systems is considered. In addition, a two-dimensional realization of this correlator is proposed for frequency scanning correlation. The use of this frequency scanning correlator as an LPI radar signal processor is discussed.

IR-induced acousto-photorefractive memory effect in LiNbO3

Abstract An acousto-photorefractive effect in Y-Z LiNbO3 has been obtained using short-duration, high-intensity, 1060-nm laser pulses. Storage amplitude versus laser energy density has been measured for 10- and 30-MHz acoustic signals, for pure and iron-doped LiNb03, and as a function of the number of laser pulses. A mechanism based on surface storage is proposed that is consistent with results from photoconductivity and photovoltage measurements and from experimental results obtained using scanning electron microscopy, electrostatic probes, and various surface treatments on a variety of materials.

An acousto‐optic multichannel signal processor

The feasibility of using acousto‐optic frequency multiplexing to process several different signals simultaneously has been demonstrated. A four‐channel acousto‐optical system for correlating signals was operated successfully; each channel had a 45‐MHz bandwidth and a minimum of 45‐dB channel isolation.