(This is almost verbatim the text of the conference presented in plenary session PLO6 at the E.C.O. 2 symposium in Paris on april 27, 1989.) I would like to devote this talk to a tutorial review of the domain of optical computing as seen from the viewpoint of optical interconnects. We shall start with a short critical assessment of the state of optical computing, showing that optical interconnects are an important part of it. We shall then review the technologies and the families of components available and whose continuing development appears necessary in this context. The question of interconnects in microelectronics will then be examined, clearly supporting the assertion that optics has a part to play there. We shall close with a review of the fundamental advantages that can reasonably be expected from the introduction of optical interconnects in electronic computers. The general ideas will be illustrated whenever possible by recent examples from the litterature or from projects presently under investigation in our Institute in Orsay. To a large extent, this presentation will elaborate on work inspired by the pionneering 1984 article by Goodman, Leonberger, Kung and Athale.
Gratings formed in dichromated gelatin due to the nonlinear recording characteristics of the material are investigated for a range of single and double exposures. We found that for a high enough single exposure not only the grating with the grating vector K but spatial harmonics at 2K and 3K are also present. Similarly, for double exposures (sequential exposures for gratings with grating vectors K1 and K2) additional harmonics with wave vectors K1 - K2, K1 + K2, 2K1 and 2K2 may be detected. We developed a simple theory to relate the modulation strength of each grating to a nonlinear recording characteristics of dichromated gelatin, and compare the theoretical values to those derived from experiments. Data is presented as plots of modulation against exposure. For most cases such procedure gives good agreement between theoretical and experimental results but it needs to be noted that for the transmission dip corresponding to the K1 - K2 case there may be significant contribution due to multiple diffraction at gratings with grating vectors K1 and K2. The application of this theory to a coupled wave solution of two sequentially recorded transmission gratings is discussed. We also apply the theory to the problem of varying the beam ratio of single reflection and transmission gratings. Experimental results are presented.
We review and analyze several different methods of fabricating periodic, Fourier-transform type computer-generated kinoform holograms with the aim of using them as fan out elements in digital optical computing/optical processing systems. It is concluded that none of the presently known recording techniques or materials are suitable for producing an on axis kinoform with the required accuracy in its phase relief structure. The fabrication of a certain modified, but essentially equivalent, type of kinoform is however feasible. This involves a microlithographic process yielding an eight-level surface relief profile.
Various applications of holographic optical elements (HOEs) have been widely studied in the litterature. They include array illuminators for input, output and offset beams on nonlinear optical arrays, connectors in optical cellular processors, head up displays, etc... Thick holograms show the potential of very high diffraction efficiencies and therefore attract particular attention. Photopolymers and dichromated gelatin (DCG) are among the best materials for a large index modulation extending over a thickness of several microns. Three practical and theoretical problems are associated with their use for thick HOEs: . chromatic aberration results from the wavelength change between recording -usually in the blue part of the spectrum- and reconstruction which often involves near infrared lines; . field aberrations in the case of reconstruction by more than one source point; . optimization of diffraction efficiency. In the present communication, we concentrate on diffraction efficiency; we elude the problem of aberrations by considering only plane waves holograms, i.e. holographic gratings. Firstly,we review previous approaches to diffraction modelling and reformulate them using an alternate rigourous analysis. Secondly, we discuss some issues of practical interest with reference to the various models and to our experimental results, such as Bragg peaks profile, optimal index modulation depth as a function of wavelength. Emphasis is put on the obtention of high efficiency in the near infrared. Thirdly, we apply our analysis to study the influence of polarisation and of index profile nonlinearities.
In the fabrication of holographic optical elements, it is important to have careful characterization of the recording medium. Hence, we have made measurements of the increment in the index of refraction vs. exposure for DMP-128 manufactured by Polaroid corporation. The results of measurements are presented for a series of holographic diffraction gratings. A novel spectrophotometer technique that provides a simultaneous measure of both thickness and refractive index is described. Data illustrating its use are presented for the holographic photopolymer. The feasibility of filling voids in the processed photopolymer for device applications was demonstrated by construction of a distributed feedback dye laser in a DMP-128 grating. At high spatial frequencies, diffraction efficiency is used to estimate refractive index variations. As an initial finding, we report a rather large modulation in the index of refraction of 0.2 at 1000 cycles/mm.
Kinoforms manufactured in photoresist with photolithographic techniques using a single, ten-level, grey-scale photomask, exposed in a specially designed laser exposure system, are described. Kinoforms designed for uniform as well as for partial Gaussian beam illumination are investigated. Measured diffraction efficiencies exceeded 50 percent.
Silver-halide sensitized gelatin has the sensitometric and spectral response of silver halide emulsions and the holographic characteristics of dichromated gelatin, and therefore when properly processed it has little scattering and absorption. Silver-halide sensitized gelatin is one of the most promising techniques today for the fabrication of transmission holographic optical elements. In this paper we will first review the mechanism of hologram formation in silver halide sensitized holograms. The most important steps of processing -the influences of the developer on the modulation transfer function and of bleaching on the difraction efficiency- will be analized. Holographic characteristics such as exposure sensitivity, resolution, thickness change and evironmental stability will be pointed out. Through optimized processing we have been able to fabricate holographic optical elements, including a holographic scanner and different off axis holographic lenses. These will be analyzed from the point of view of the recording material.
When recording thick holograms, Bragg's Law must be taken into account; this imposes additional limitations on the geometry of the recording sources if the reconstruction geometry is fixed. However, the results we have reported from previous papers do not enable the recording geometry required to give maximum diffraction efficiency of the hologram to be calculated for an arbitrary specified reconstruction geometry when there is a possible change in the thickness and refractive index of the recording medium. Bleached silver halide, dichromated gelatin and silver halide sensitized gelatin are well known photographic techniques for phase holographic formation in commercially available emulsion. These techniques are applicable to volume phase formation. This occurs when the refractive index and/or thickness of the exposed/non-exposed portion of the emulsion varies relative to the bulk unexposed portion. We present our theoretical expressions of third-order aberrations in off-axis holographic lenses of different f/No, taking into account the index and thickness variations of the recording materials.
The effect of the polarization self-action of resonant radiation on the holographic storage as well as the holographic reconstruction is investigated in alkali halides with reorienting impurities. The polarization preserving phase conjugation is observed under the degenerate four-wave mixing for KC1 containing FA(Li) centers.
A deep 3-D holograms with 102-103 mkm physical thickness were realized which accept postexposure enhancing and reversable a posteriori grating parameters variation. These properties are provided with a developed net of through capillaries in hologram volume. Capillary structure holograms are not destructed while dark stored and reconstructed, they have low thermal expansion coefficient and are practically shrinkproof.
Significant progress during the last several years has resulted in the successful development of read-only, write-once and erasable optical storage systems. Recently, several optical head desiu s have emerged in the literature that utilize holographic optical elements (HOEs) as replacements for convention. elements. The motivation is to reduce the numberof head components by capitalizing on the multifunctionality of an individual HOE, improve manufacturability, and reduce cost. There is also potential for improved drive performance by reducing the overall size and mass of the head. Following a brief review of 'conventional' optical head technology, this paper will describe design modifications utilizing HOEs, their impact on head performance, and the requirements that must be satisfied to permit wider usage. Emphasis will be placed on the issues associated with laser diode wavelength variability, high numerical aperture, high diffraction efficiency, and their use in magneto-optic systems where polarization considerations are important.
The authors' work is reviewed on implementation of integrated-optic disc pickup devices using waveguides and holographic components. The integrated-optic disc pickup (IODPU) is constructed by integrating a focusing grating coupler (FGC), a grating beam splitter and photodiodes in a waveguide on Si, and is capable of detecting readout signal and tracking/ focusing error signals. The gratings are fabricated by computer-controlled electron-beam writing and reactive ion etching. Integration of detection optics for magneto-optical disc pickup is also presented. The device uses a trifocal-FGC, which serves as a polarization splitter based on waveguide mode dispersion, and performs differential detection of MO read-out signal. The design considerations, fabrication processes and experimental works are presented including the most recent results. Although the performances obtained so far are not sufficient for practical application, the elementary functions have been successfully demonstrated.
Holographic scanning has been used in bar code readers since 1980. The technology has evolved since that time to include industrial bar code scanners as well as the more familiar supermarket scanners. During this evolution, a number of innovations have allowed the capabilities of the technology to be more fully exploited. This paper will review the technical developments made at IBM over the last ten years, with emphasis on the application requirements that motivated the developments. The major developments to be discussed include: multiple focal-plane scanning, retro-collection, variable facet width, scan-angle multiplication, overlapping focal-zones, complex facet shapes, facet identification, and raster scanning. Some recent developments in holographic disk fabrication will also be discussed. These new developments have allowed the construction of complex facet shapes and offer the potential for significant reduction in disk manufacturing costs.
We have developed an all holographic line scanner for use in diode laser printers. The scanner consists of only a holographic disk and a holographic lens for laser scanning and focusing. No other optics such as a collimating lens and f- θ lens are necessary. The holographic disk performs high precision straight line scanning. The focusing function is achieved by the compound optical power of a holographic disk and lens. To make the disk more compact, while retaining a required scanning width and scanning beam, the increasing deviation from a straight line and scanning beam aberration need be overcome. In this paper, we devised the method to obtain the required phase transfer functions of the holograms and discussed the holographic recording method to fabricate the holograms.
A method for designing and recording holographic optical elements, using recursive techniques, has been developed. The elements are recorded with complex wavefronts that are derived recursively from interim holograms. With this design method the elements, can have high diffraction efficiencies and low aberrations. so they are useful for a variety of applications. We shall review the design method and illustrate it with examples of a Fourier transform holographic lens, a holographic imaging element for helmet displays and a focussing element for the near infra-red radiation.
A single diffractive lens with a separated aperture stop can be corrected for third-order coma, astigmatism, and Petzval curvature of field. This simple system provides a high quality imaging over a much wider field of view than conventional optically recorded holographic optical elements. This lens is compared with other lenses using modulation transfer function techniques. The correction of spherical aberration is achieved with a suitably designed Schmidt-camera-type aspheric corrector plate. The telecentric layout and distortion of the lens are shown to be suitable for use as a Fourier transform lens.
The interconnection problems in the use of optical fibers for signal transmission is a very active area of research involving several fields such as optical communication techniques and holography. Various authors have studied the efficiency of the transfer of energy from the first fiber to the second one by using a holocoupler as connecting medium. In an earlier workwe developed a mathematical model based upon the scattering by a dielectric optical waveguide, the scattered field acting as the object wave in the recording process of the holocoupler. In the present work we propose a new system where a GRIN medium is acting as the signal transmitter. We have studied the optimal conditions for the holocoupler/Grin medium system. For that we have considered two different taper shapes generating the GRIN medium and, we have made comparative studies for the results obtained in both cases. Some conclussions are withdrawn in relation to the parameters of the GRIN medium for the coupling optimization.
By holographic tandem arrays we mean optical components which consist of two holographic arrays in series. We present two examples: holographic telescope arrays, which consist of two lenslet arrays, and holographic perfect shuffle components, which consist of an element for channel permutation and a second element for the redirectioning of the channels into a common output direction. The fabrication of such tandem arrays can be facilitated with in situ-recording methods. In situ-recording means that the two holographic elements for one tandem array are recorded in rigid coupling, which avoids their adjustment lateron. Demonstration components have been fabricated in dichromated gelatin.
The results of development of optical system with holographic optical elements (HOE) and the main possibilities of program complex DELOS for modelling and optimization of optical systems with HOE have been considered. Development methods of new classes of optical systems based on phase three-dimensional wide-angle HOE with a considerable remoteness of pupil and apochromats-anastigmats based on kinoform elements without use of special glasses or crystals are discussed. Analysis of aberration and correction possibilities of different types of HOE is given. Characteristic properties of HOE computation and modelling for control of aspherical mirrors have been considered.
The use of a laser diode source in the near-infrared region for making reflection holograms reconstructed in visible light is described. The source coherence function is investigated, and techniques for shrinking the holographic emulsion of silver halide recording materials are reported.
The combination of holographic metrology with endoscopic imaging allows the development of a new class of minute instruments for high-resolving, non-contactive, non-destructive intra-cavity measurements. New developments in micro-optics give main impulses to actual progress in holographic endoscopy: - Assembled gradient-index rod lenses are used for small submillimeter needle diameter rigid endoscopic instruments. - The use of (single-mode) optical fibers offers easy handling and flexibility. Especially, new single-mode imaging fiber bundles show improved performance data (high lateral resolution) at very small diameters. - The use of electro-optical crystals opens the possibility of developing a minute holographic storage device for an easy-to-handle, small holographic endoscopic camera system for in-situ recording, reconstruction and erasure with fast repetition rate.
Holographic combiner mirrors have been produced for an automotive head-up display prototype in the frame of a joint research program with VEGLIA BORLETTI, a subsidiary of FIAT. Up to this moment, plane, conformal fringe mirrors (150x200)mm2 in size have been recorded using Dichromated Gelatin (DCG) as the sensitive material. Peak reflectivity of more than 95% is obtained at 540 nm for a spectral band 20 nm wide. Future activity will be toward the addition of optical power to the holographic mirror to allow accomodation free operation and aberration control capability. Our complete procedure for producing DCG mirrors is reported in the paper.
As mechanical structures, robot manipulators deform when they are loaded, and any deformation causes the gripper to deviate from the desired position. This effect is one of the errors that manipulators exibit. The teach-in mode, used for programming today's robot manipulators, disguises deformation errors, and so deformation errors do not crucially affect the performance of the manupulator. However, when manipulators come to be programmed in off-line mode in the near future, deformation errors can be decisive for manipulator performance. In this paper, the structural response of a robot manupulator to load is investigated. Since results could provide a basis for the further analysis of structural components, holographic interferometry has been chosen as the method of investigation. A ruby laser with a 20 nsec pulse duration proved to be suitable for the application and hundreds of sandwich hologram pairs have been made. The results have disclosed the following facts: - Joints are the weakest parts of the structure. - There was a high degree of deformation of some components. The first result enabled the development of a mathematical expression for describing the deformation in relation to mechanical compliance. This expression is suitable for use in compensational measures. As regards to the second result, one of the highly deformed components was reinvestigated by means of finite element computations and, with minor mechanical modifications, a better design was obtained.
The invention of holography has sparked hopes for a three-dimensional electronic imaging systems analogous to television. Unfortunately, the extraordinary spatial detail of ordinary holographic recordings requires unattainable bandwidth and display resolution for three-dimensional moving imagery, effectively preventing their commercial development. However, the essential bandwidth of holographic images can be reduced enough to permit their transmission through fiber optic or coaxial cable, and the required resolution or space-bandwidth product of the display can be obtained by raster scanning the image of a commercially available acousto-optic modulator. No film recording or other photographic intermediate step is necessary as the projected modulator image is viewed directly. The design and construction of a working demonstration of the principles involved is also presented along with a discussion of engineering considerations in the system design. Finally, the theoretical and practical limitations of the system are addressed in the context of extending the system to real-time transmission of moving holograms synthesized from views of real and computer-generated three-dimensional scenes.
Holographic endoscopy or endoholography combines the features of endoscopy and holography. It can be utilized in holographic imaging or interferometry inside natural cavities of the body. In imaging, the ability to record a three-dimensional, large focal depth, high resolution image of internal organs and tissue may greatly enhance the detection of disease and abnormality. With contact recording endoholography it is possible to obtain high resolution holographic recordings which can be examined microscopically. In this technique, a specially designed endoscope uses a single-mode optical fiber for illumination where Denisyuk holograms are recorded on film at the endoscope's distal end. The holograms are viewed under a stereomicroscope with high magnification, allowing for the observation of individual cells. Specific dyes are used to enhance the contrast of the tissue prior to the recording of the holographic images. In endoholographic interferometry, the use of conventional multi-fiber imaging structures is sufficient to obtain an adequate recording of interference patterns. The use of conventional fiberoptic imaging structures for holographic investigations of displacement and vibration is demonstrated. Image-plane holograms are formed at the proximal output end of an imaging multifiber. Pairs of double-pulsed holograms of dynamic events are recorded, one through the multifiber, the other in a conventional manner as a standard of reference. The fringes of both holograms are practically identical. The ability to perform measurements in holographic interferometry through optical fibers should lead to further developments in medical as well as industrial applications.
The holographic sampling extends the holographic measuring range and makes restoring of the running of displacement in time possible. The principle is based on the double-pulse holographic technique. Without discussing the solution of the technical details those basic conditions will be examined, which are needed taking into consideration a holographic sampling system established in practice.
Holographic lenses(HLs) have several advantages, compared with classical refractive lenses, for the fabrication of "Raman Laser Fibres Optics" (R.L.F.O.) optrodes, particularly since they are cheaper and easier to manufacture and it is possible to make "off-axis" lenses which can be used in some particular situation. A kind of HLs, which we called "petaloid optrode", together with its manufacture process and experimental results are presented in this paper.
Storage and display of 3-D objects are promising using computer-generated holograms. In the reconstruction step, the diffracted wave forms a virtual or real 3-D image. In the generation step, the complex amplitude of this wave is calculated in the hologram plane. To achieve realistic calculation times and storage requirements, two strategies are considered mainly: i) generalizations of standard calculation procedures. The introduction of extra degrees of freedom offers an adaptation to the 3-D object. The complexity of an individual calculation increases slightly, while the overall number of calculations is considerably reduced. ii) adaptations to the optical reconstruction setup. Limitations given by the detector allow to introduce simplifications in the hologram generation. The procedures of i and ii complement each other, because the resultant complex amplitudes are superposed coherently. Calculation time and storage requirements are reduced simultaneously, due to the enhanced mathematical descriptions of the optical systems. Storage and display of 3-D objects has become more realistic using computer-generated holograms.
Two different approaches to the production of computer generated diffractive optical elements have been studied. Computer generated diffractive optical elements have been written in binary form in chrome on glass using the electron beam facilities at the Rutherford Appleton Laboratory in the UK. These elements have been used as masters for subsequent copying into DCG to obtain higher diffraction efficiencies. Intermediate diffraction efficiency has been obtained by simple contact printing into photoresist. An alternative approach has been to write the elements directly into photoresist with careful control of the profile or blaze of each ruling. Using both these techniques conventional and novel elements have been produced and evaluated.
Different coding and quantization techniques in digital phase holography are considered. Characteristics of the methods are discussed with the help of a model of digital phase holography. The parameters diffraction efficiency and signal to noise ratio of the calculated hologram structures are investigated. The described methods are applied to calculate various examples of array illuminators.
We describe a method to transform a laser intensity gaussian distribution into a uniform distribution, using computer generated holograms; we explain the modelisation, the production and the realisation on DCG plates.
Digital phase holograms offer the possibility to generate special light distributions with high diffraction efficiency. We call this distribution signal. Because phase holograms only influence the phase of the illuminating wave a coding of the spectrum of the signal is required. In practice that means: the discrete spectrum WiJ has to be modified by a nonlinearity operator to fulfil the phase hologram constraint |Wij|=1. In addition often a quantization of the phase values of Wij is desired. Due to the coding and quantization constraints noise may diminish the quality of the signal reconstruction. Several iterative coding techniques have been developed to overcome this shortcoming. We want to present a one step procedure for coding and quantization of digital phase holograms. It is related to the error correction and diffusion method suggested in electronic half toning (binarization of greytone images), and pulse density coding of amplitude holograms. Its basic idea is to diffuse the local error due to the coding or quantization at one pixel to the next to be processed. Flexibility exists in the choice of the diffusion weights which control the distribution of the errors and influence the shape of the noise in the Fourier plane of the manipulated distribution. We want to utilize this flexibility to spatially separate the noise from the signal.
In addition to the compact size and light weight, a single HOE can often replace several conventional optics. We therefore can expect performance improvement and cost reduction for optical systems using HOE's. Examples are holographic optical heads for R/W optical disks and holographic scanners for laser printers. For many of these applications, laser diodes in ir wavelengths are used as the light source. Since the required recording media or a suitable laser source may not be readily available, we may have to use a computer to generate the HOE's. Ideally these computer generated HOE's are to be written directly by an e-beam machine. Often a moderate size (say 10 mm's) and high resolution (large than 1,000 1/mm) HOE is necessary. Such a HOE is still difficult to make using commercially available e-beam machines. We describe here an alternative method. This approach allows us to make a relative large and high resolution HOE without an e-beam machine. Another advantage is that the method can be extended to make Bragg (or volume) ir hologram using visible light. Basically we make a pre-distorted CGH with which the final HOE can be made using visible light. The HOE can then be reconstruct at the ir wavelength without aberrations. Since we need to generate the pre-distorted CGH from another hologram, we describe two methods, the Grating Period and Tangential Plane, to determine the additional phase change introduced by the grating structure. Both methods produced the correct CGH. Experiments with simple HOE designs and also with a more complicated holographic optical head design are shown.
Coding and quantization methods applicable in digital Fresnel holography are described. The introduction of an iterative Fresnel transform algorithm offers the possibility to reduce the space bandwidth product in digital Fresnel holography and to obtain binary amplitude holograms with high diffraction efficiency.
Cylindrical shaped computer-generated holograms are considered. The complex amplitude of an arbitrary 2-D transparency (object) situated in a plane is determined in the hologram surface. The calculation of the hologram transmittance bases mainly on a fast Fourier transform algorithm, a non-linear coordinate transformation in the hologram surface and the use of spherical phases. The feasibility and the limits of the procedure are demonstrated by optical reconstructions.
We compare the performance of different types of periodic phase-only computer-generated Fourier transform holograms (Lohmann and Dammann -type binary holograms and kinoforms) as space-invariant interconnects in digital optical computing. Optical interconnects must exhibit high diffraction efficiency PE and low reconstruction error ΔR. Optimization of the hologram structure by simulated annealing and a greedy algorithm yields (in reconstruction of 2D fully arbitrary interconnection patterns) PE = 25-32% and ΔR 5% for binary holograms, and PE = 70-93% and ΔR < 1% for kinoforms. Hence the kinoform appears to be the optimal type of computer-generated holographic interconnect.
In the animal brain there appear to be a variety of two-dimensional maps representing, in some cases, stimuli and, in other cases, responses. Links between these are performed by neurons which are trained during the animal's experience. This may offer a useful paradigm for complex optically controlled systems. We discuss here - mapping theory from N dimensions into 2, - map transformations by holography, and - map reconciliation by coherent optics.
We report on the design and fabrication of holographic optical elements in dichromated gelatin (DCG) for use in all-optical digital computing. Lenslet arrays as large as 56 x 56, optimised to replay in the near infrared region of the spectrum, and with diffraction efficiencies > 90% and a uniformity of ± 2% have been constructed. Pre- and post-processing techniques have allowed us to improve the optical quality, as well as the optical damage threshold, of these components. In addition we have copied computer generated holograms (CGHs) onto DCG, thereby recording higher efficiency holograms.
We report on the design and experimental characterization of an holographic deflection system which enables slowly reconfigurable point-to-point interconnections. In this system collimated beams emerging from single-mode fibres at wavelength λl are deflected by a matrix of thin photothermoplastics holograms, recorded at λ2. We use presently Fresnel holograms for connecting two 12 x 12 2D arrays (with a 4 mm separation between neighbouring channels). Besides the improvement of the cyclability of the thermoplastics, the main problem to overcome are the aberrations (mainly astigmatism), due to high λ1/λ2 ratio (> 2). A suitable choice of both recording and reconstruction configurations allows to minimize astigmatism and other aberrations have been evaluated with a ray-tracing program. The predictions are in good agreement with the following experimental datas : λ1 = 1,334 nm, λ2 = 647 nm, mean deflection angle of 23°, input/output distance of 50 cm, maximum connection loss into a 600 μm silica fibre 16.5 dB (10 dB for diffraction efficiency, 5 dB for coupling efficiency and 1.5 for the aberrations). The crosstalk is lower than - 29 dB. Improvements allowing to achieve larger capacity and lower wavelength sensitivity are suggested.
Dynamic holographic architectures for connecting processors in parallel computers have been generally limited by the response time of the holographic recording media. In this paper we present a different approach to dynamic optical interconnects involving spatial light modulators (SLMs) and volume holograms. Multiple-exposure holograms are stored in a volume recording media, which associate the address of a destination processor encoded on a spatial light modulator with a distinct reference beam. A destination address programmed on the spatial light modulator is then holographically steered to the correct destination processor. We present the design and experimental results of a holographic router for connecting four originator processors to four destination processors.
A phase-aberrated pump beam was photorefractively coupled to a divergent signal beam in BaTi03 at 514.5 nm. Pump-to-signal ratios were set to induce large modulation depths. No phase cross-talk was observed in the amplified signal beam or in the depleted pump beam. However, phase aberrations present on the pump beam did deleteriously affect energy coupling.
Spatial filtering and hologram formation using coherent and incoherent imaging techniques are investigated experimentally in photorefractive materials. Spatial filtering involves imaging appropriate filter designs into a BaTiO3 crystal in which a Fourier-transform hologram has already been written. White-light filter images erase selected frequency components of the hologram preventing them from appearing in the reconstruction. A similar imaging technique is also used to create holograms in photorefractive materials. Standard computer-generated holograms are imaged into the crystal, changing the index of refraction of the material by the photorefractive effect in proportion to the local intensity. The resulting index modulation pattern forms a phase hologram with the imaging and spatial frequency characteristics of the original computer-generated transmittance. Because the spatial filters and holograms used in these experiments may be electronically produced and incoherently displayed, these systems can be updated easily without the usual constraints of coherent spatial light modulators. Characteristics of these systems and their application to real-time computer display are reported.
We discuss the use of holography to perform the temporal reversal of picosecond pulses by recording thick holograms in photorefractive materials. Experimental results are given for an asymmetrical pulse shape.
Holography, and particulary in-line far-field holography, has proven to be a valuable technique in particle size and velocity measurement. As significant refinements of the technique are discovered new applications are developed. Recent advances in the field are reviewed and a selected bibliography to papers since 1979 is provided
Optical interrogation of double exposure records in speckle metrology or particle image velocimetry is extended beyond the commonly applied generation of Young's diffraction fringes. The combination of two step optical processing (to produce autocorrelation functions) with spatial multiplexing (to generate a whole field of such functions in parallel) presents a purely optical alternative to digital electronic processing. Special concepts are proposed to evaluate records directly as to statistical parameters or to facilitate screening procedures on large sets of records.
Holographic and Speckle Pattern Interferometry utilising continuous wave lasers has so far been useful only as a laboratory tool. A major drawback limiting its introduction to industrial applications is the degree of environmental stability needed to obtain high quality fringes. Application of a pulsed laser to Holographic Interferometry has been demonstrated by several workers to show significant improvement in stability and scope of application. In a similar way, a pulsed laser can produce speckle correlation fringes on a single frame from a CCD TV camera, thus eliminating environmental sensitivity problems and rigid body motion. Results obtained with Electronic Speckle Pattern Interferometry (ESPI) incorporating a pulsed YAG laser are reported. Vibration and static deformation fringes have been produced for a clamped metal plate. With a double pulse laser, the first pulse can he positioned at any time during the vibration cycle. The firing of the second pulse depends on the scanning time of a single field of the CCD camera. Due to the pulsing action of the laser the vibration amplitude fringe pattern can he animated on the TV screen to gain a visual impression of the surface motion. A new technique has been developed for extracting deformation and shape information from interferometric fringes. It involves a significant improvement in environmental stability and reduced computational effort. Problems due to noise and data ambiguity are eliminated with the extraction of phase information from only two digitised interferograms. Continuous wave, stroboscopic and pulsed laser illumination are all amenable to this one technique. Results employing the new fringe processing algorithms are presented. Comparisons are made with data obtained with a computer Finite Element prediction for the same target.
A method for the detection and measurement of surface defects is described, which includes the use of a holographic contouring system and the application of image digitation and automated computer analysis for quantitative information. Calibration tests on defects of known sizes have been effected; the possibility of applying the method to studying the state of artwork conservation is examined.
The windowpane of a tourist bus is usually part of the whole coachwork and is therefore a structural element. Using double-pulsed holography it might be possible to determine if a windowpane is properly mounted on the bus chassis in order to reduce the risk for windowcracks. In order to expose incorrect mounting of the windowpane, holographic vibration analysis is used. Primarily the vibration behavior of the windowpane has to be known. Double-pulsed holography is used to investigate vibration behavior of a windowpane mounted to a modern tourist bus. The size of the busfront is: height = 2.8m and width = 2.3m. Three different vibration techniques are used: idling the motor, exciter vibration of the chassis and direct impact of the windowpane. Using the exciter, the resonance frequencies of the windowpane are measured. Theoretical calculations of the resonance frequencies of a plate clamped on four edges, considering known parameters of the windowpane, are compared with the holographic experimentally obtained resonance nodal interferometric patterns. The holographic recordings of the windowpane corresponds well with the theoretical nodal lines for a four edge clamped plate.
A non-contact method of analysis of rotating components has been developed using pulsed laser ESPI with precision triggering and a high resolution tv-system for image recording. Engineering problems over a very wide range of rotational speeds up to 12500 r.p.m. and tangential velocities of 150 ms-1 may be analysed. The use of fibre optics extends the technique to components with limited optical access.
Optical circuits of new holographic interferometers, illuminating the investigated zone in several directions simultaneously are considered. Using the devices, interference and schlieren patterns of three-dimensional gas flows are obtained.