Wavelength-multiplexing diffractive phase elements: design, fabrication, and performance evaluation.

We report on the wavelength-multiplexing diffractive phase element (WMDPE) capable of generating independent spot patterns for different wavelengths. The iterative method proposed by Bengtsson [Appl. Opt. 37, 1998] for designing a kinoform that produces different patterns for two wavelengths is extended to the WMDPE for multiple wavelengths (more than two wavelengths). Effectiveness of the design algorithm is verified by design and computer simulations on the WMDPE's for four and nine wavelengths. The WMDPE for three wavelengths (441.6, 543.5, and 633 nm) is designed with five phase levels and is fabricated by electron-beam lithography. We observed that the individual spot patterns are reconstructed for the design wavelengths correctly. Performance of the WMDPE is evaluated by computer simulations on the uniformity error, the light efficiency, and the contrast. On the basis of the results, the characteristics of the WMDPE's are discussed in terms of various conditions of fabrication and usage.

Optoelectronic parallel-matching architecture: architecture description, performance estimation, and prototype demonstration.

We propose an optoelectronic parallel-matching architecture (PMA) that provides powerful processing capabilities in global processing compared with conventional parallel-computing architectures. The PMA is composed of a global processor called a parallel-matching (PM) module and multiple processing elements (PE's). The PM module is implemented by a large-fan-out free-space optical interconnection and a PM smart-pixel array (PM-SPA). In the proposed architecture, by means of the PM module each PE can monitor the other PE's by use of several kinds of global data matching as well as interprocessor communication. Theoretical evaluation of the performance shows that the proposed PMA provides tremendous improvement in global processing. A prototype demonstrator of the PM module is constructed on the basis of state-of-the-art optoelectronic devices and a diffractive optical element. The prototype is assumed for use in a multiple-processor system composed of 4 x 4 PE's that are completely connected through bit-serial optical communication channels. The PM-SPA is emulated by a complex programmable device and a complementary metal-oxide semiconductor photodetector array. On the prototype demonstrator the fundamental operations of the PM module were verified at 15 MHz.

Thin Observation Module by Bound Optics (TOMBO): Concept and Experimental Verification.

A compact image-capturing system called TOMBO (an acronym for thin observation module by bound optics) is presented in which the compound-eye imaging system is utilized to achieve a thin optical configuration. The captured multiple images are processed to retrieve the image of the target object. For image retrieval, two kinds of processing method are considered: image sampling and backprojection. Computer simulations and preliminary experiments were executed on an evaluation system to verify the principles of the system and to clarify the issues related to its implementation.

Two-Dimensional Image Transmission Based on the Ultrafast Optical Data Format Conversion between a Temporal Signal and a Two-Dimensional Spatial Signal.

We have experimentally demonstrated a two-dimensional (2-D) image transmission based on the ultrafast optical data format conversion between a temporal signal and a spatial signal with an ultrashort optical pulse. In the proposed system we adopt a spectral holography technique to transmit a one-dimensional (1-D) spatial signal and use a spatial-domain time-frequency transform to realize a transform between 1-D and 2-D spatial signals. By use of these techniques, a low-optical-loss transmission system can be constructed. To demonstrate a 2-D image transmission with this technique, we achieved experimentally transmission of the alphabet letter T as a 3 x 3 pixel 2-D spatial image.

Fundamental functions for ultrafast optical routing by temporal frequency-to-space conversion.

We demonstrate fundamental functions for ultrafast optical routing by using an elemental part of an ultrafast optical technique for conversion of time to two-dimensional space. A preliminary experimental result shows that recognition of header signals can be achieved at a rate of more than 600 Gbytes/s.

Stream cipher based on pseudorandom number generation with optical affine transformation.

We propose a new, to our knowledge, stream cipher technique for two-dimensional (2-D) image data that can be implemented by iterative optical transformation. The stream cipher uses a pseudorandom number generator (PRNG) to generate a pseudorandom bit sequence. The proposed method for the PRNG is composed of the iterative operation of 2-D affine transformation achieved by optical components and by modulo-n addition of the transformed images. We expect efficient execution of the method by optical parallel processing. We verify the performance of the proposed method in terms of security strength and clarify problems on optical implementation by the optical fractal synthesizer.

Direct control of fractal pattern generation on an optical fractal synthesizer.

We propose a method for direct control of position, rotation, and scaling of fractal patterns generated on an optical fractal synthesizer. In this method we introduce an iterated-function-system mother function to produce control parameters for arbitrary fractal patterns. We implemented the method experimentally and verified the effectiveness of the method.

Fabrication of multilevel phase computer-generated hologram elements based on effective medium theory.

A conventional method to synthesize diffractive optical elements and computer-generated holograms (CGH's) with high diffraction efficiency relies on an increase of phase levels. To fabricate such a device, one should perform electron-beam (e-beam) lithography with multiple-dose exposures or multiple-step photolithography. Here we describe a one-step method, which is based on the effective medium theory, for the fabrication of a multilevel phase CGH. The phase modulations required in cells of a CGH are constructed by means of dividing these cells into fine (subwavelength) structures. The surface features of these fine structures control their corresponding indices, and their values can be calculated according to the effective medium theory. By proper selection of the fine structures, based on the requirements of the phase modulation of the cells, a CGH with multilevel phases is synthesized when a binary structure is relieved on the dielectric material. Then the CGH can be fabricated by direct e-beam lithography or one-step photolithography through an amplitude mask followed by an ion-etching treatment. The experimental results showed that the reconstructed wave field is in good agreement with that simulated by a computer, indicating the effectiveness of the proposed method.

Discrete correlation processor as a building core of a digital optical computing system: architecture and optoelectronic embodiment.

In this paper we present a general-purpose discrete correlation processor (DCP) expected to be the building core block of a digital optical computing system. The DCP-1 is embodied by optoelectronic devices such as a VCSEL and a complementary metal-oxide silicon photodetector. The application targets of the DCP-1 are optical interconnection and various types of digital optical computing. It is expected that digital optical computing techniques coupled with the optoelectronic technology will provide large capability and flexibility in information processing. Introduction of a processing scheme of optical array logic enlarges the applicable field of the DCP-1 as well as its processing capability. With the experimental DCP-1 a bit error rate smaller than 10(-9) was obtained for A . B? operation under a 500-kHz clock rate.

Optical wavelet processor by holographic bipolar encoding and joint-transform correlation.

A novel optical wavelet processor based on the techniques of the joint-transform correlator and computer-generated holograms is proposed. A coding technique that is a simplified version of Lee's hologram [Appl. Opt. 9, 639 (1970)] is used to represent positive and negative values for the object signal and wavelet functions. We experimentally demonstrate that wavelet transforms of two different daughter wavelet functions can be simultaneously obtained by the appropriate arrangement of the daughter wavelet functions and the object signal on the input plane.

Sidelobeless multiple-object discriminant filters recorded as discrete-type computer-generated holograms.

The computer generation of sidelobeless multiple-object discriminant correlation filters has been stressed. We propose to synthesize the filter functions by use of the simulated-annealing algorithm. By this method the filters can be obtained as discrete-type computer-generated holograms. The filters can suppress the sidelobes and provide sharp correlation peaks. A computer simulation and an optical experiment were performed, and the expected correlation responses were obtained.

Array of photorefractive waveguides for massively parallel optical interconnections in lithium niobate.

An array of photorefractive (PR) waveguides is proposed for massively parallel optical interconnections. The array of PR waveguides can be fabricated in a lithium niobate crystal by illumination of an interference fringe pattern formed by plane waves. We numerically calculate the maximum density of the PR waveguides when the coupling between the PR waveguides is negligible. Experiments for the fabrication and guiding tests of the array of two-dimensional PR waveguides are demonstrated.

Optical implementation of visible gray-image morphology with the visual-area-coding technique.

We present a novel scheme of visible gray-image morphology with the visual-area-coding technique (VACT). The VACT is a technique of digitized analog-optical computing in which data are converted into visible coded patterns and processed with the visible form. Because the achievable operations in the VACT are identical to those of mathematical morphology, mathematical morphology is adapted to gray-image morphology with the VACT. Computer simulation and optical experiments of the several operations in mathematical morphology verify the correctness of the proposed technique. The processing capacity of the proposed method is estimated in terms of the space-bandwidth product.

High-accuracy optical computing based on interval arithmetic and the fixed-point theorem.

A method for high-accuracy analog optical computing based on interval arithmetic and the fixed-point theorem is considered. Two-variable simultaneous equations are studied to investigate the proposed method. An optical implementation is considered by the use of spatial coding of intervals, affine transformation, and image magnification. Computational simulation verifies the principle of the method.

Computer-generated multiple-object discriminant correlation filters: design by simulated annealing.

The computer generation of multiple-object discriminant correlation filters is studied. The quantization of filter functions influences the correlation response. This may cause misdetection or incorrect classification of patterns and is especially serious in the case of multiple-object discriminant filters. We propose synthesizing the matched-filter functions by the simulated-annealing algorithm. The recording of Lohmann-type computer-generated holograms is considered. By this method the filter functions can be encoded with a reduction in the quantization levels of amplitude and phase. Acomputer simulation is performed, and the expected correlation responses are obtained.

Incoherent-only joint-transform correlator.

An incoherent-only optical and electronic digital joint-transform correlator is proposed. A technique for the removal of extraneous signals inherent in the incoherent-only joint-transform correlator is also presented. A computer simulation and experimental results confirm the performance of the proposed incoherent-only joint-transform correlator.

Visual-area coding technique (VACT): optical parallel implementation of fuzzy logic and its visualization with the digital-halftoning process.

A novel technique, the visual-area coding technique (VACT), for the optical implementation of fuzzy logic with the capability of visualization of the results is presented. This technique is based on the microfont method and is considered to be an instance of digitized analog optical computing. Huge amounts of data an be processed in fuzzy logic with the VACT. In addition, real-time visualization of the processed result can be accomplished.

Optical characteristics of a wind-roughened water surface: a two-dimensional theory.

We present a two-dimensional theory of thermal emission and light scattering from an anisotropic wind-roughened water surface that is described by the Gaussian-Joint North Sea Wave Project model. The theory is developed through the use of the first-order geometrical-optics approxmation modified with shadowing effects, and it is valid when the average slopes of the surface are smaller than unity. The theory allows us to evaluate the effective emissivity and the effective bistatic reflectivity of a full-gravity-capillary wave surface at large viewing angles, for any direction relative to the average propagation direction of the surface wave. We also present an application of the theory to the recently proposed method for obtaining thermal imagery of a wind-roughened water surface from low altitudes, which is called statistically corrected ocean thermography. Corrected thermal images of the ocean surface, obtained by our field experiment, are shown.

Fabrication experiment of photorefractive three-dimensional waveguides in lithium niobate.

A technique for fabricating optical waveguides in a lithium niobate crystal is studied experimentally. We fabricate straight and curved structures of refractive index as the optical waveguides in a volume of the photorefractive crystal by scanning a focused laser beam. The experimental results show that the structure of refractive-index distribution is complex yet imply guiding of light in the part of high refractive index within the fabricated structure.

Reflective block optics for packaging of optical computing systems.

A new optical packaging technique, which we call reflective block optics, for optical computing systems is proposed and demonstrated experimentally. This technique is based on solid optics, which is advantageous with respect to stability, reliability, and alignability. Reflective lenses are used to attain high lens power, compactness, and a large space-bandwidth product. Glass blocks, cube beam splitters, and reflective optical elements are combined to form optical blocks. We can construct several optical systems by assembling the optical blocks.