Optical Digital Processing Research Articles

Sparse matrix wave-front estimators for adaptive-optics systems for large ground-based telescopes

A closed-loop adaptive-optics servo system is an iterative optical–digital processor that can be characterized by a first-order difference equation. This identification leads to a new class of wave-front estimators, some of which are extremely sparse and permit real-time subaperture intensity weighting.

Non-linear optical materials and applications

A review is given of the present status of the non-linear optical materials and devices, and their applications for optical signal processing and computing. The primary motivation behind this review is to introduce the different areas involved, paying special attention to their interfaces. First, the optical non-linearities in semiconductor materials, semiconductor microstructures and photorefractive materials are introduced. For the semiconductors we discuss the third-order non-linear susceptibilities due to virtual transitions (e.g., bound electrons in intrinsic semiconductors and non-linear motion and energy relaxation of free carriers in doped semiconductors) and due to real transitions (e.g. valence-to-conduction-band transitions, free-carrier transitions, impurity transitions and transitions in excitons and excitonic complexes). Recent advances in engineering semiconductor microstructures are discussed and shown to enhance their third-order non-linear optical susceptibilities in comparison with the bulk semiconductors. The mechanism of the photorefractive effect is next introduced and analyzed in conjunction with several engineering approaches to enhance the performances of the photorefractive non-linearities (e.g., non-stationary recording, multiphoton excitation, combination of the photorefractive effect with the electrorefractive non-linearities near the band edge, etc.). Using the photorefractive materials as an example, we define a set of figures of merit based on the requirements from the optical devices for system applications. Using these figures of merit we evaluate and discuss the optimization of different photorefractive materials: ferroelectric oxides (e.g., LiNbO 3, BaTiO 3, KNbO 3, tungsten bronze family, etc.), compound semiconductors (e.g., GaAs, GaP, InP, CdS, CdSe, CdTe, etc.), silenites (e.g., Bi 12SiO 20, Bi 12GeO 20), and ceramics. Finally, the applications of non-linear optical materials are discussed using the photorefractive materials as an example. Two types of devices are distinguished: passive devices that are based on volume holographic storage of information and active devices that are based on continuous two-wave and four-wave mixing of optical information carrying waves. These devices are evaluated and their optimization is discussed in conjunction with applications to analog and digital optical signal processing and computing. Examples of analog (e.g., inverse filter and linear algebra processor) and digital (e.g., parallel access optical memories and reconfigurable interconnects) optical computing applications will be also discussed.

Ultrafast digital soliton logic gates

We demonstrate all-optical fibre switches, including soliton-dragging logic gates, soliton-interaction gates and soliton-trapping AND-gates, that have the potential of operating up to speeds of 0.2 Tbps. Solitons in fibres are attractive for ultrafast timedomain switching because they avoid pulse distortion during propagation and because they exhibit particle-like properties. Soliton-dragging logic gates satisfy all requirements for a digital optical processor and having switching energies approaching 1 pJ. In addition, soliton-dragging logic gates are one example of a more general timedomain chirp switch architecture in which a dispersive delay line acts as a ‘lever-arm’ to reduce the switching energy. Soliton-interaction gates are based on elastic collisions between solitons and illustrate that solitons can be used to implement conservative, billiard-ball logic operations. Soliton-trapping AND-gates are sensitive to the timing of the input pulses and display on/off contrast ratios greater than 20∶1. The soliton-trapping AND-gate can serve as the final stage in an all-optical system and as the interface to electronics. These ultrafast gates may prove advantageous in applications where the switch bandwidth limits the performance of the system

Design of digital–optical processors by using both intensity and polarization–encoding schemes

Here we propose two encoding schemes that utilize both light intensity and polarization to code input data in digital-optical computing. This approach results in a more efficient information carrying and processing technique when compared with the traditional intensity-only or polarization-only encoding schemes. Optical logic units that are based on the proposed encoding schemes are designed. A recursive parallel adder and a carry look-ahead adder that use these logic units as building blocks are also described.

Digital optical processing based on higher-order modified signed-digit symbolic substitution

An efficient higher-order modified signed-digit optical symbolic substitution technique for addition and subtraction has been derived. This two-step symbolic substitution technique, which is independent of the number of bits to be processed, saves a tremendous amount of computation time. Two separate approaches for optical implementation that use content-addressable memory and the joint transform correlator are also suggested.

Relaxation abnormalities in single cardiac myocytes from renovascular hypertensive rats.

In myocardial hypertrophy secondary to renovascular hypertension, the rate of intracellular Ca2+ concentration decline during relaxation in paced left ventricular (LV) myocytes isolated from hypertensive (Hyp) rats is much slower compared with that from normotensive (Sham) rats. By use of a novel liquid-crystal television-based optical-digital processor capable of performing on-line real-time Fourier transformation and the striated pattern (similar to 1-dimensional diffraction grating) of cardiac muscle cells, sarcomere shortening and relaxation velocities were measured in single Hyp and Sham myocytes 18 h after isolation. There were no differences in resting sarcomere length, percent of maximal shortening, time to peak shortening, and average sarcomere shortening velocity between Sham and Hyp cardiac cells. In contrast, average sarcomere relaxation velocity and half-relaxation time were significantly prolonged in Hyp myocytes. Contractile differences between Sham and Hyp myocytes detected by the optical-digital processor are confirmed by an independent method of video tracking of whole cell length changes during excitation-contraction. Despite the fact that freshly isolated myocytes contract more rigorously than 18-h-old myocytes, the relaxation abnormality was still observed in freshly isolated Hyp myocytes, suggesting impaired relaxation is an intrinsic property of Hyp myocytes rather than changes brought about by short-term culture. We postulate that reduced sarcomere relaxation velocity is a direct consequence of impaired Ca2+ sequestration-extrusion during relaxation in Hyp myocytes and may be responsible for diastolic dysfunction in hypertensive hypertrophic myocardium at the cellular level.

Bispectral magnitude and phase recovery using a wide bandwidth acousto-optic processor

A hybrid optical-digital processor has been developed that computes both the magnitude and phase of the bispectrum for wide bandwidth (10 MHz to 1 GHz) rf signals. The overall optical architecture is that of a modified Mach-Zehnder interferometer that contains three acousto-optic modulators and appropriate transforming lenses. The intensity distribution in the output plane of the interferometer contains an interference term that represents the real part of the bispectrum multiplied by a spatial carrier (the interference fringes). To isolate the bispectrum information, the output image is digitized and digitally filtered. The imaginary part of the bispectrum is obtained by Hilbert transforming the real part, and then computing the bispectrum magnitude and phase. The processor is tested with four different combinations of rf test signals. Each signal has a bandwidth of either 6 or 12 MHz. Test results that illustrate the performance of the processor in the recovery of magnitude and phase information for the bispectrum of quadratically related signals are presented.

Principles of construction of optical echo processors

An analysis is made of the possibility of utilization of the photon echo effect in optical digital processing of discrete images by the method of control operators, which makes it possible to solve a number of problems. Different regimes for the excitation of the photon echo are in practice usable in all component parts of a processor, which makes this effect potentially promising for the use in optical data processing.

Optical digital processor using arrays of symmetric self-electrooptic effect devices

Four arrays of thirty-two GaAs symmetric self-electrooptic effect devices were optically interconnected to form a looped-pipeline optical digital processor. Several circuits were demonstrated, including two shift registers and a decoder circuit. Clock frequencies of up to 1 MHz were attained. Possible extensions to and limitations of this system are described.

Effective hybrid processor to compute image moments for pattern recognition

A hybrid optical-digital processor is presented for computing the invariant moments of images in real time, which consists of a holographic mask, two lenses, a charge-coupled-device detector, and a microcomputer. The processor is tested by inputting some roman letters, and the produced results show that the invariant moments of a letter are approximately independent of shift and rotation and that the moments are distinct with different letters.

Optical computing and related microoptic devices

We investigate scaling with the cross-sectional area of energy and speed for optical devices and of optical design, speed, and thermal dissipation for device arrays. Theory and experiments clearly point to lower energy and faster speed for smaller devices and to simpler optical design, smaller propagation time delays, and higher thermal dissipation capability for smaller array sizes. We conclude that the development of high speed digital optical processors will depend on small devices interconnected by microoptic systems.

Optical digital implementation of the Wigner distribution function: use in space variant filtering of real images

An optical digital processor based on the Wigner distribution function (WDF) has been implemented, permitting space variant filtering of 2-D real images. The processor sequentially generates all space samples of the distribution; thus, each sample can be modified by a different filter. The filtered Wigner distribution function is optically inverted, and a digital image processor selects the information associated with each sample of the image. Compared to a digital implementation, the required computer time has been reduced to the point where future practical use of Wigner filtering can be expected.

Digital optical processor based on symbolic substitution using holographic matched filtering

We propose a digital optical arithmetic processor design based on symbolic substitution using holographic matched and space-invariant filters. The proposed system performs Boolean logic, binary addition, and subtraction in a highly parallel manner; i.e., the processing time depends on word size but not array size. Algorithms for performing binary addition and subtraction in parallel are presented. A skew problem occurring when symbolic substitution is applied to binary addition and subtraction with space-invariant systems is addressed, and its solution is suggested. Crosstalk in symbolic substitution is described, and new symbols which can prevent the crosstalk are introduced. System analysis and fundamental limitations of the proposed system are also presented in terms of processing time, overall light efficiency, and the maximum array size of the input data plane. The performance of the proposed system with that of the current electronic supercomputers has been compared by combining information about the processing time and maximum array size.

Highly parallel arithmetic algorithms for a digital optical processor using symbolic substitution logic

Symbolic substitution logic is a powerful tool for realizing optical arithmetic in a digital optical processor, because it is matched to the parallelism of optics and to the properties of optical switching devices. Working with such processor enables the use of very efficient parallel algorithms. We have investigated the implemention of the basic arithmetic operations: addition, subtraction, multiplication, and division.

Design of epitaxial AlGaAs multilayer structures: interference filters and optically controllable reflection modulators

Dielectric interference filters incorporating an epitaxial anti-reflector structure and a concept for an all-optical reflection modulator are introduced. They are suitable for two-dimensional digital optical signal processing in an AlGaAs system employing molecular beam epitaxy. The reflection modulator concept uses dynamic band filling in multiquantum wells to obtain a bleachable absorption type optical non-linearity and promises more than 80% modulation depth. The numerical investigations are carried through with the help of two equivalent mathematical approaches which are explained in detail. Calculated results show satisfying agreement with experiments.

High accuracy optical processors: a new performance comparison

A hybrid time and space integrating processor is shown to provide more than one operation per analog to digital conversion. A preferred performance measure, operations per second per component, is used to compare bit-sliced digital and high accuracy optical processors using the digital multiplication-by-analogconvolution algorithm. Optical system performance is shown to increase nearly quadratically with the speed of its components, while digital systems increase only linearly.

Staircase telescope arrays for local beam compression in one dimension

A deflecting staircase can be used like a telescope array to produce local beam compression in l-D. Such a component may be useful for array illumination in digital optical information processing. This paper introduces the concept of staircase telescope arrays and discusses realizations with mirrors, prisms, and gratings. The addition of a macro deflecting element and the extension of the staircase concept to 2-D beam compression are further topics.

Mathematical morphology processor using ferroelectric liquid crystal light valves: principle

We describe the principle of an optical digital processor for mathematical morphology (MM). The proposed central processing unit is based on binary logic using states of polarization and spatial data shift using geometric optics. Two dimensional Boolean operations are performed by means of logical gates using ferroelectric liquid crystal light valves, which are shown to be able to perform iterative processes well suited to MM operation implementation. A complete optical architecture is proposed from which the programming of elementary MM transformations is demonstrated.

An optical system for full text search

In this paper we propose a full text search system based on optics. The storage and processing of the textual data are performed by an optical back-end system to an electronic computer. In this way we can take advantage of the speed and parallelism of digital optical processing. Using the proposed configuration we show how one might implement a set of text processing operations using lasers, spatial light modulators and photodetectors .

Image analysis through the Wigner distribution function

The Wigner distribution (WD) for discrete images is computed and applied for texture analysis. First, the WD for different test images with spatial frequency and gray level information content is computed, and the most important features of the WD are outlined. Likewise, the original image with 1-D variation is recovered from the discrete WD; local filtering operations have been performed on the distribution, and a filtered version of the original image is then obtained. Since the WD entails simultaneous representation of the intensity distribution and of the spatial interdependence, it seems specially adequate for processing of textured information. In this way, texture recognition is performed through the extraction of features from the WD, and the results are compared with other methods. Finally, to avoid the great computational effort that requires this kind of representation, a hybrid optical-digital processing system is proposed for the texture recognition process.