Optical processor design is the art of architecting a computing machine with components that are either currently available or which can be developed with a reasonable extension of current technology. There is no difference between this and the design of a processor using any other technology except that in optics the component selection is more limited and the cost of implementing certain classes of operations is far less. This paper outlines the characteristics of many current components and the impact of these characteristics on optical architectures. At the present time the most critical components in optical processing are at the interface with other processing technologies. These critical components are spatial light modulators and photosensors. Advances in semiconductor technology and opto-electronics for optical communications may greatly reduce the current component limitations. In this paper suggestions are provided for new component development and the impact of such developments on optical processing technology is discussed.
The Programmable Opto-Electronic Multiprocessor (POEM) combines free-space optical interconnects, optoelectronic devices, and electronic processors to perform computations. This paper investigates a specific POEM architecture for a multistage interconnection network application. For the chosen system, there is an optimum combination of optics and electronics. The effect of varying optoelectronic device parameters on the system performance is also examined.
A photonic parallel memory (PPM), an array of optoelectronic bistable switches, was proposed for application to optical parallel processing. Each of the switch consists of a heterojunction phototransistor (HPT) and a light-emitting diode (LED). In addition to the electrically erased PPM having only switches, an optically erasable PPM (OE-PPM) with functions of optical write-in, read-out and erasing was fabricated. The optical reset function was attained by an additional HPT connected to the switch electrically in parallel. The PPM can be used not only as a memory but as a logic gate. An optical parallel logic gate executing exclusive OR (EOR) was proposed as an application of the OE-PPM. The EOR operation with optical input and output was implemented.
This paper describes several photorefractive devices for optical information processing that have been developed in recent years. These include photorefractive spatial light modulators; numerical processors, such as parallel integrator and matrix-matrix multiplier; devices for optical neural network applications, such as parallel thresholding, Max operation and perceptron learning.
In this paper we present a new approach to incoherent-to-coherent optical conversion based on a real-time five-wave mixing technique in photoanisotropic organic film. A uniform grating is holographically written in the sample, and then locally erased by an incident white light image. Subsequent coherent diffraction of the spatially modulated grating imposes the inverse of the incoherent image onto the reading laser beam, allowing subsequent coherent optical processing. A theoretical analysis of the holographic recording and erasing mechanism in these photoanisotropic materials is presented, and the saturation is shown to be responsible for the grating intermodulation that produce the incoherent-to-coherent conversion. Experimental results of white light images converted to inverted coherent images in real-time are presented, and the resolution is shown to exceed 28 lp/mm.
In this paper the most recent advances on noncollinear guided-wave magnetooptic (MO) Bragg interactions in which the light waves propagate in directions nearly orthogonal to that of the magnetostatic waves, the resulting MO Bragg cells, and two applications are given. First, the basic interaction geometry and underlying physical mechanisms, and the realization of wideband MO Bragg cells in pure and Bismuth-doped YIG-GGG waveguides for the carrier frequencies ranging from 2 to 12 GHz are presented. Applications of the wideband MO Bragg cells to light beam scanning and real-time spectral analysis of microwave signals are then described. Finally, comparative advantages and disadvantages of guided-wave MO Bragg cells versus their acoustooptic (AO) counterparts are discussed.
We describe an acousto-optic interaction geometry in a uniaxial crystal which uses acoustic beam steering to couple incident and diffracted optical beams with parallel group velocities. The device uses tangential acoustic beam steering to provide large RF bandwidths at high efficiency. As with the acoustooptic tunable filter, the choice of parallel optic group velocities provides for a large input angular aperture, useful in multiplicative optical processing architectures. The parallel optical group velocities and beam steering geometry allow use of the Bragg cell with optical beams propagating in close proximity to the transducer, eliminating the dead time which is crucial in correlation cancelation feedback optical processors. The wide angular aperture allows this same interaction geometry to be used for high-efficiency wide- band acoustooptic modulators as well.
We describe tradeoffs which are required in obtaining desired characteristics from a-Si:H/FLC optically addressed spatial light modulators. The tradeoffs involve material selection as well as operating device parameters.
Device approaches are investigated for O-SLMs based on MBE engineered III-V materials and structures. Strong photo-optic effects can be achieved in periodically (delta) -doped multiple quantum well (MQW) structures. The doping-defined barriers serve to separate and delay recombination of the photo-generated electron-hole pairs. One can use this photo-effect to change the internal field across the MQWs giving rise to quantum-confined Stark shift. Alternately, the photo-generated electrons can be used to occupy the quantum wells, which in turn causes exciton quenching and a shift of the absorption edge. Recent work has shown that both of these predicted photo-optic effects can indeed be achieved in such MBE engineered structures. However, these enhanced effects are still insufficient for high contrast modulation with only single or double pass absorption through active layers of practical thickness. We use the asymmetric Fabry-Perot cavity approach which permits extinction of light due to interference of light reflected from the front and back surfaces of the cavity. Modulation of the absorption in the active cavity layers unbalances the cavity and 'turns on' the reflected output signal, thereby allowing large contrast ratios. This approach is realized with an all-MBE- grown structure consisting of a GaAs/AlAs quarter-wave stack reflector grown over the GaAs substrate as the high reflectance mirror (approximately equals 0.98) and the GaAs surface as the low reflectance mirror (approximately equals 0.3). We use for our active cavities InGaAs/GaAs MQWs separated by npn (delta) -doped GaAs barriers to achieve sensitive photo-optic effect due to exciton quenching. High contrast modulation (> 60:1) is achieved with the Fabry-Perot structures using low power (< 100 mW/cm2) InGaAs/GaAS quantum well lasers for a write signal.
The advanced development and characteristics of a new high resolution, high frame rate, reflected mode MOSLM is reported. This effort is aimed at the production of Miniature Ruggedized Optical Correlators. The device research and process development is being performed at Carnegie Mellon University NSF Data Storage System Center (formerly Magnetic Technology Center) under contract from Litton Data Systems. Pixel size is under one mil center to center, one third the dimension of present transmission mode evaluation devices. This development includes optimization of the optical and functional characteristics of the MOSLM for Mil Spec Systems.
Until now, the input/output response curve of a SLM has been regarded as a given. Often the curve we are given is not the curve we want, but we have no choice. We show that if it is possible to induce any sort of significant changes in the response curve, then we can simulate in parallel an almost arbitrary response curve using the available variations of response curves.
The development of a ferroelectric liquid-crystal-over-single-crystal-silicon spatial light modulator is described. The reflective SLM has an array of 176 X 176 pixels over a clear aperture of 5.28 mm X 5.28 mm. Prototype devices driven from a specially designed high speed frame store have been operated at frame rates of approximately equals 1 kHz.
The aimed requirements for optical processing device described here are the short term memory, high resolution and high frame rate, cascadability, high fan-out. One of the simpler candidate devices for such requirements are the bistable optically addressed liquid crystal spatial light modulators.
A class of integrated devices with high electronic circuit complexity and a multiplicity of optical inputs and outputs can appropriately be called optoelectronic integrated systems. Although free-space propagation implies that the complete system extend physically beyond the integrated device, the term is suitable if a primary system-level processing task is performed on the optoelectronic substrate itself. Examples of such devices are the VLSI-based spatial light modulator, the optical-in/optical-out silicon retina device, and monolithic, optically interconnected, digital processor arrays. These devices must match systems requirements; whereas conventional (VLSI) integrated systems must comply with constraints on speed, power, and area, optoelectronic integrated devices must satisfy additional requirements founded in optical physics and established by the imaging configuration in the overall system. The interplay of device constraints with characteristics of various technologies for electronic-optical transduction will be examined in different regimes of granularity, concurrency, and speed. The holistic consideration of optical devices as parts of systems leads to a number of non-obvious conclusions. For example, in optoelectronic systems that are optimal in a certain sense, light modulators need not always dissipate less power than one or even 100 transistors, nor need they always switch in less than 1 ns, or even 100 ns. Examination of the liberal, but nonetheless realistic, constraints arising from this analysis will reveal that a number of existing light modulator technologies can offer satisfactory performance in interesting and important applications.
The optical implementation of a neural network consists of two basic components: a 2-D array of neurons and interconnections. Each neuron is a nonlinear processing element that, in its simplest form, produces an output which is the thresholded version of the input. Liquid crystal spatial light modulators, optoelectronic integrated circuits (OEIC's), either hybrid, such as liquid crystal on silicon, Si-PLZT, and flip-chip devices, or monolithic integration in 111-V compounds, are examples of such a solution. In order for these devices to be used as neurons in a practical experiment, they must contain a large number of neurons (104/cm2 - 106/cm2) and exhibit high gain. This puts a stringent requirement on the electrical power dissipation. Thus, these devices have to be operated at low enough current levels so that the power dissipation on the chip does not exceed the heat-sinking capability, and yet the current levels need to be large enough to be able to produce high gain. This means sensitive input devices are a must. To achieve these goals, the speed requirement of the devices must be relaxed as the operation of neural network does not have to be too fast.
This paper explores our immediate efforts to exploit basic quantum-well physics and optics in an effort to improve the performance of electroabsorption modulators and Self Electro-optic Effect Devices (SEEDs) for applications in optical processing systems. SEEDs with thick-, thin-, and extremely-shallow-barrier multiple quantum wells are comprehensively and systematically studied and compared experimentally, and improvements noted. Room- temperature resonant tunneling in thin-barrier multiple-quantum-well structures and its effects on device performance will be reported. Enhancements and uniformity considerations using practical Fabry-Perot effects will also be discussed.
This paper is a review of the development of Quantum Well SEED arrays for digital optics systems, which exploit the large space-bandwidth product of free-space optical interconnects to process or route digital information massively in parallel.
The use of acousto-optic processors in applications such as radar signal processing and spectrum analysis places high performance requirements on the optical detectors used to detect the processed signal. In many signal processing applications there is a need for high speed, high dynamic range detectors with commensurate crosstalk and image lag performance. A program supported by Harry Diamond Laboratories for improved performance of CCD detectors has been highly successful in reducing crosstalk and obtaining over 40 db of optical detection dynamic range in the zero image lag mode of operation. Two different types of epi- layer structures, P on P+ and P on N, have been fabricated for which the methods and design issues to achieve high dynamic range, low crosstalk, low image lag, and high speed are discussed. The design approach used to suppress blooming is also discussed and the results of recent devices testing are shown.
The inherent speed and parallelism offered by many optical computational devices represent a distinct advantage over their electronic counterparts. Spatial Light Modulators (SLM) and the Charge Coupled Detectors (CCD) form an important class of such optoelectronic devices. This paper presents device structures using SLM/CCD devices for computing vector dot products. Since the computation of vector dot products constitutes one of the most heavily used operations in digital signal processing, such devices lend themselves well for very high speed signal processing applications. The versatility of the SLM/CCD devices is demonstrated by presenting several architectures for a scalar multiplier. An analytical model characterizing the performance of a SLM/CCD pair is presented. Examples involving the use of the device for signal processing applications is also given.
New methods for wideband signal processing using fiber delay lines are described. The processors make use of fixed and programmable reflective taps, all-fiber switches, and ladder networks. A tapped delay line with over 10 GHz bandwidth has been demonstrated, as have variable-length delay lines with up to 16 different delay times.
Electro-optic sampling of wideband signals using short duration pulses from a diode laser may offer advantages over the electronic sampling performed in A/D converters. A hybrid optical/electronic A/D conversion technique is described which overcomes some of the difficulties encountered in previous efforts in this area. This hybrid technique uses electrooptic components to perform sampling and time-demultiplexing together with multiple electronic A/D converters. A system which uses 4 parallel electronic A/D converters and a pulsed diode laser is demonstrated with a sample rate of 2 GHz. For single-tone test signals in the range of 10 to 11 Ghz, the precision of this system is as high as 2.8 effective bits, limited principally by laser jitter.
Analog fiber-optic links using an integrated-optical intensity modulator have been demonstrated and analyzed. Experimental links operate at frequencies from 40 MHz to 22 GHz with electrical gain up to 11 dB, noise figure as low as 6 dB, and intermodulation-free dynamic range of up to 113 dB-Hz2/3. The gain, noise figure, and dynamic range are shown to have a simple dependence upon a few link design parameters. Other factors affecting link performance, such as stimulated Brillouin scattering, interferometric intensity noise, modulator linearization, and addition of an optical amplifier, are also discussed briefly.
Several advantages are afforded by the use of surface-acoustic-wave (SAW) acousto-optic (AO). Multiple transducer SAW AO delay lines allow several noninterfering wide-bandwidth signals to be introduced into the processing volume simultaneously. This allows overall processor size and complexity to be reduced. Rapid (often real-time) optical processing of signals can be performed at the interception point. One- and two-dimensional time-integrating AO correlation architectures using such SAW devices are shown, and their application in some systems are discussed. The same SAW AO device technology can be used with advantage in switching applications. This paper also discusses a new AO permutation network approach and device considerations for the implementation of this network. With the use of SAW AO device technology on lithium niobate, the permutation network can be integrated onto a single chip; several such optical chips can be interconnected to implement more complex switching networks for wide-bandwidth switching applications.
A simple and new optoelectronic sampling (OES) technique using III-V APD and PIN diodes is described for time-division-multiplexed (TDM) signal processing applications. An optoelectronic sampling receiver is designed for sampling 2 Mbit/s data from a 64 Mbit/s 1.3 micrometers optical data stream, with timing information provided by an optical in-band technique. Error free data extraction is demonstrated at -23 dBm optical power.
We have developed an electron-beam-addressed, lithium niobate spatial light modulator that converts RS-170 standard video signals into phase and/or amplitude modulation of coherent or incoherent radiation. This compact device has an 18 mm active area, greater than 1000:1 contrast ratio for monochromatic light, 16 gray levels, high optical efficiency, less than (lambda) /4 reflected wavefront distortion, greater than 5 line-pairs/mm resolution at 50% MTF and potential frame rates of up to 30 Hz. The device can be tailored for readout over a broad wavelength (0.4 to 4 micrometers ), has no pixel-fixed-pattern noise, and offers continuous gray scale in both the amplitude and phase operating modes. The device has been used as a general purpose programmable mask and as a computer generated image display. Its application areas also include optical computing, large screen projection displays, pattern recognition systems, optical neural networks, and reconfigurable interconnection system. This paper describes the device construction, its performance limitations, and its applications.
In this paper we present schemes of experimental setups for the radiation spectral brightness measurements in the range of 0.6 - 5 (mu) for N varying from 10-1 to 102 (if, for example, (lambda) equals 1 (mu) , this range of N corresponds to the brightness temperature range from 6 (DOT) 103 to 106 K), and for photomultipliers quantum efficiency measurements in the range of 0.4 - 5 (mu) with a dynamical range 10 - 1012 photon/sec and accuracy not worse than 1%. The new measurement methods are based on the utilization of the parametric light scattering phenomenon which is a spontaneous decay of laser pump photons in correlated photon pairs in crystals with quadratic nonlinear susceptibility. The first of two methods allows measurement of the radiation spectral brightness N in absolute units ('photons per mode') in visible and infrared range. The quantity N is related to the energetic brightness spectral density B through the equation B equals (hc2/(lambda) 5)N, where h is the Plank constant, c - the light velocity, (lambda) - the wavelength. The method is absolute and does not require any reference source or detector of radiation. Quantum noise of a parametric down-convertor, caused by the zero vacuum fluctuations with an effective brightness Nvac equals 1 photon per mode, is the reference in this case. The second method concerns the quantum efficiency of photodetectors determination, and it is based on the connection between the statistics of photocurrent and the radiation which causes it. The parametric scattering is a unique source of rather intensive and directed radiation flow consisting of photon pairs. Such a flow can be used to determine the absolute quantum efficiency of photodetectors.
We report batch-processed, totally planar, vertical-cavity top surface emitting GaAs/AlGaAs laser devices and arrays. Different size devices are studied experimentally. We measure continuous-wave threshold currents down to 1.7 mA and output powers > 3.7 mW at room temperature. We also discuss interesting characteristics such as differential quantum efficiencies exceeding unity and multi-transverse mode behavior. An array having 64 X 1 individually-accessed elements is characterized and shown to have uniform room-temperature continuous-wave operating characteristics in threshold current approximately equals 2.1 +/- 0.1 mA, wavelength approximately equals 849.4 +/- 0.8 nm, and output power approximately equals 0.5 +/- 0.1 mW.
In DFB and OBR lasers, phase of corrucation at the reflective end and possible change of reflectivity at the facets, may alter the conditions of oscillation. These result the variation on the output power amplitude and the wavelength. The purpose of tapering at the grating is to reduce these effects. In addition, with dissimilar tapering, we can increase mode selectivity of such lasers. In this paper, we consider reflection coefficient of tapered DBR as a function of end facet reflection coefficient, and deviation from the Bragg's condition. It is shown that tapering improves the operation of DFB and DBR lasers.
In this paper, the design and the simulation of a GaAs acousto-optic correlator for synthetic aperture radar (SAR) data processing are reported. The proposed integrated circuit is available for airborne applications and is able to operate with side-looking focused radar. The range compression is performed by the acousto-optic correlator, driven by the received signal, and the azimuth data compression is obtained by using a transmission mask/charge coupled device (CCD) system. Data for coherent compression are collected according to the pulse repetition frequency (PRF) of the reference chirp signal which modulates the laser beam. The proposed integrated circuit allows SAR signal processing on a substrate with very small sizes (37 X 17 mm2) and low power consumption. A range resolution of 126 cm and an azimuth resolution of 68 cm have been theoretically achieved.
The novel construction of a scanning, computer driven diffractometer is proposed. The Fourier diffractometry with directionally changed incident light wave, without sophisticated transforming lenses and with a single stable detector, was applied. The device and software were tested on geometrical object and complicated biomedical images.
Diffraction pattern representation of image (optical Fourier transform or Wiener spectrum) is very useful for computer image processing. An accurate analysis of spatial frequencies requires very well corrected lenses, and the diffraction pattern must by sampled by means of a moving detector or a stable multiple detector array strictly in the Fourier plane. The theoretical analysis of the diffraction pattern obtaining and sampling was provided. It was shown then the Fourier diffractometry with directionally changed incident light wave can be realized without above-mentioned limitations. In this method, a single stable detector is placed at the rear focus of a Fourier transform lens while the sampled diffraction pattern is moved over the Fourier plane. A suitable movement of the diffraction pattern can be obtained when the analyzed image is continuously illuminated with a parallel light beam whose inclination is changed with respect to the optical axis of the Fourier transform lens. This idea was applied for design a new generation of compact diffractometers with unsophisticated transforming lens.