Optical processor designs in the past have typically been analog in nature. Recently design concepts for a numerical optical processor have evolved in which we see digital techniques implemented with optical devices. This paper describes design concepts for a numerical optical processor which is based on the residue number system.
In the 1950s, digital designers noted that residue number systems offer the attractive feature of carry-free addition, subtraction, and multiplication. This interest waned, in part because residue number systems possess a multi-state character; flip flops and other binary state devices lend themselves more naturally to binary arithmetic than to multi-state arithmetic. Optics enjoys the requisite multi-state capability: lenses can resolve many discrete positions, gratings can resolve many discrete frequencies, and so forth. This fact has motivated efforts to create a residue based optical processor, which would combine the parallel, speed of light throughput of optics with processing accuracies possible only to digital systems. Our program has included the identification of appropriate roles for residue processors, the investigation of I/O techniques, such as analog to residue and residue to radix conversion, and the study of optical implementations of residue arithmetic.
An inner product processor is presented which is capable of performing 3.3 million inner products a second, where each vector consists of 100 elements each 20 bits wide. This is equivalent to more than 660 million 40 bit arithmetic operations a second. The latency of a particular calculation is 12.3 microseconds. The processor can be constructed entirely from 1024 by 6 bit ROMs with 300 ns cycle times and latchable inputs or outputs. Modular arithmetic is used internally; the input and output are binary. The specifications of the architecture are compatible with the stricter structural requirements needed by an optical implementation.
This paper describes work on generating arrays of stable states using a liquid crystal light valve (LCLV) and optical spatial feedback. Among the topics to be discussed will be feedback containing one and two LCLV's, giving both oscillatory and exponential decay to equilibrium, the possibility of obtaining more than two stable states in a given image element.
Two numerical optical processor configurations are presented. Both systems are based on holographic recording (to form the processing element) and holographic reconstruction (to perform the numerical processing). In these systems, pairs or arrays of pairs of binary words are presented as the input. The output may be the addition, subtraction, multiplication, division, or other operation on the pairs of words. There is one binary word answer for each pair of binary word inputs. Both numerical optical processor configurations use holographic wavefront addition and subtraction to construct Boolean logic operations. However, as contrasted with constructing arithmetic operations through multiple steps of combinational logic, a single-step truth-table look-up processor is devised. Numerical results are obtained with a single optical pass through the system. Both systems require only a limited part of the complete truth table to be recorded. The first system is based on the EXCLUSIVE OR operation performed holographically on binary arrays. The second system, more complex but more powerful, performs an operation that is equivalent to two levels of logic, a NAND and OR followed by another OR operation. For both the EXCLUSIVE OR numerical optical processor and the NAND-OR-OR numerical optical processor, the array sizes during recording and readout and the number of superposed recordings needed are presented and discussed.
The Air Force is actively supporting scientific research in optical processing through the award of contracts and grants to university and industrial research laboratories. The overall objective of this Air Force research program is to increase the flexibility of optical processors to the point of being able to perform any operation that is suitable for parallel processing. This paper will describe the currently active efforts in this program with an emphasis on those efforts dealing with integrated optics, fiber optics, and surface acoustic waves.
The Army is currently investigating the potential of optical and/or hybrid optical/digital techniques to solve many of its signal processing, target identification and communications problems. It is expected that optical or hybrid optical/digital techniques will find application where large quantities of data must be processed in real time or near real time. This paper will discuss the present Army activities in acousto-, electro-, integrated and fiber optics. A few representative current applications are image and signal correlation, spectral analysis and multi-mode fiber optic systems.
Significant advances have been made in the development of high-performance diode laser sources and photodiode detectors that will be essential for future optical signal pro-cessing systems. AlGaAs/GaAs diode lasers for 0.8-0.9 µm wavelengths are now commer-cially available and GaInAsP/InP for 0.9-1.7 µm should soon become available. Recent achievements include long lifetime lasers (>10,000 hours) in both materials systems, single longitudinal and transverse mode operation, high speed pulse modulation (1 Gbit/sec), and mode-locked operation with 20 psec pulses. High performance avalanche photodiodes have been developed in Si for wavelengths up to 0.9 µm and modest performance diodes in Ge up to 1.5 µm. For wavelengths beyond 1 4m, III-V compounds, especially the GaInAsP/InP system, show promise for fast, highly sensitive diodes that could be integrated into optical waveguide circuits along with diode lasers.
In recent years, considerable progress has been made in the development of components for wideband optical communication, using glass fibers as the transmission medium. (I") The most dramatic advance has been in the loss in the fibers themselAes, which has dropped from a minimum of about 1000 dB/km prior to 1970 to 0.2 dB/km today.(') Similarly, research in the field of integrated optics has led to the development of miniaturized, single-mode com-ponents for use with laser sources. Generation, modulation, directional coupling, switching, and detection of light are functions which have already been demonstrated with integrated optics devices.('-') Improvements in bandwidth, switching and multiplexing capability, size, and reliability for fiber communications systems could result from these efforts.
The scope of integrated optics is to perform signal processing at optical frequencies by using a miniature planar configuration consisting of thin films deposited on a suitable substrate. By utilizing guided optical waves rather than electric currents, such a planar configuration may perform functions analogous to those presently achieved by integrated electronics components. In this presentation, we shall review the forms and basic operation of passive components of current interest in integrated optics. Devices involving active components are being described in other presentations.
It has long been recognized that there exists a formal analogy between optical Fourier transforms performed by lenses and the chirp Fourier transforms implemented with dispersive delay lines. Progress in the technology for optical signal processing and for surface-acoustic-wave dispersive delay lines has advanced many signal-processing techniques from the theoretical to the practical realm. Analogous and competitive surface-acoustic-wave and optical techniques for spectral analysis, matched filtering, and convolution in one and two dimensions are compared and their limitations are discussed.
An Elastomer Storage Device (ESD) was constructed which has potential as an adaptive Spatial Filter in optical spectrum analysers. The ESD uses a PVK-TNF photoconductor and parallel plate charging in a low pressure Argon atmosphere. Resolution greater than 100 cycles/mm and a time-bandwidth product in excess of 1. 6 x 106 have been measured. A photometric sensitivity of 5µ J/cm2 at 514. 4nm was observed. The linear dynamic range in the Fourier plane was measured to be 40db. The device has a lifetime in excess of 107 cycles at a frame rate of 10 cycles/second.
A spatial light modulator is being developed that takes a time dependent electrical signal and formats it into a two dimensional array of phase modulators. This is accomplished with a charge coupled device to shift electrical charge into a two dimensional array, and metallized membranes which deform by an amount proportional to the charge. Preliminary results indicate adequate membrane sensitivity and frequency response to permit real time operation of large time-bandwidth product light modulators.
Thin films of V02 are capable of reversibly switching from a transparent to a metal state at high speeds and with high spatial resolution. The result is a dynamic reticle or mask in the IR with large on-off ratios for both reflection and transmission. Three applications for which V02 films are a key will be discussed: a high performance real time coherent optical data processor, a transmit/receive switch and a fast random access scan laser with a large field of view and high resolution.
We here discuss acousto-electric devices for electronic imaging of light. These devices are more versatile than line scan imaging devices in current use. They have the capability of presenting the image information in a variety of modes. The image can be read out in the conventional line scan mode. It can be read out in the form of the Fourier, Hadamard, or other transform. One can take the transform along one direction of the image and line scan in the other direction, or perform other combinations of image processing functions. This is accomplished by applying the appropriate electrical input signals to the device. Since the electrical output signal of these devices can be detected in a synchronous mode, substantial noise reduction is possible.
Signal processing functions, such as correlation and Fourier transformation have been performed for signals with 50 MHz bandwidth and 500 TB product. Correlation experiments using Barker and p-n coded signals show the expected sidelobe and compression ratios. To study the effect of the coherency requirement of the light source, both lasers and incoherent sources were used. Experimental results indicate that incoherent light is sufficient for some applications. This opens up the possibility of a completely packaged surface-acousto-optic signal processor with small dimensions. The performance of this processor will be compared with those of other ones, i.e. silicon-on-LiNb03 SAW convolver, with respect to dynamic range, TB product etc.
Optical power spectral (OPS) analysis has demonstrated significant advantages for rapidly classifying simple patterns from aerial transparencies. However, as the pattern sets become larger and more complicated, the required software is more complex, the equipment requirements more demanding, and the processing time increases. Additionally, potential ambiguity of the OPS data due to loss of phase information places some unknown upper limit on the information content in OPS. Integration of other pattern recognition data which supplements the OPS analysis leads to an increased pattern classification potential. The optical/ digital system discussed integrates a photo diode array detector in the image domain with the OPS system. Hierarchical strategies employ initial OPS sampling and preliminary classification of patterns of potential interest. Space domain sampling and classification is only initiated when cued from the OPS samples. This system leads to a wide range of exper-imental capabilities and operational potentials. The paper discusses system configuration, experimental results, and future plans.
Time integrating acousto-optic processors realize flexible, multi-purpose complex signal processing architectures based on correlation algorithms. One-and two-dimensional techniques are presented including examples of spectral analysis and ambiguity function processing. Noncoherent optical processor implementation using interferometric detection with electronic reference is described and experimental results are given.
A hybrid optical approach to coherent spectrum analysis realizes gains associated with time-integrating techniques while still retaining some of the advantages of space-integrating techniques. Measurement of large time-bandwidth spectra is performed with high sensitivity, efficiency, and range of detectable signal power levels, particularly in large bandwidth applications, where an optically transformed periodic chirp forms a distributed local oscillator as a reference for time integration.
A general optical system for processing temporal signal waveforms is described. In this system, a frequency-variant spectrum analyzer system is used in conjunction with an interferometric optical hetero-dyne processor. The capability of the frequency-variant analyzer to control the position and resolution of signal components allows for the general mixing of a signal and a local oscillator. This mixing results in a frequency dependent redistribution of the signal components useful for bandwidth compression and expansion or for more general spread-spectrum signaling.
Data on scenes obtained by multiple spatial filtering operations are combined linearly to form two signals (principal vectors) which drive x and y axes of a light deflector to scan a beam across an output plane. This approach permits us to take account of the statistics of variations of the input and to reduce to a minimum the number of filters used.
Fourier transforms of diatom specimens are studied for the purpose of making improved spatial filters for use in a real-time coherent optical recognition system. The system consists of directing a laser beam into an interference contrast microscope to produce a real, magnified coherent image of the specimen to be recognized. This technique provides a method of adjusting low frequency attenuation for image enhancement over a continuous range. Examples of results are shown.
This paper describes the performance of a wideband acousto-optic receiver whose function is targeted towards electronic warfare receiver applications. The receiver design employs a conventional acousto-optic spectrum analyzer which is followed by a linear fiber optic array to sense the emerging light. Each fiber in the array is connected to its own photo-detector, video amplifier and threshold circuitry thus providing channelized receiver operation. The array circuitry is interfaced with a digital processor which can analyze the array output in real time. In this paper a preliminary account of receiver performance will be discussed in terms of measured sensitivity, dynamic range and frequency resolution.