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The essence of innovation, which is the successful combination of need and technical feasibility, was well illustrated by Born in his analogue computer for determining Fourier transforms of a number of different functions which occur in acoustics, optics and X-ray crystal analysis. After a brief description of the first photo-electric Fourier transformer, which involved rotating a sine grating behind a slit to select a range of spatial frequencies, subsequent developments of Born's basic idea are discussed and, in particular, its use in equipment for measurement of the optical transfer function (OTF). The widespread use of this equipment has led to several international exchanges of information on standards relating to image quality and, even after 20 years of intense technical development throughout the world, the performance of OTF systems based on Born's idea has not so far been surpassed.
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A sense organ is a non-linear operator operating on an external field. It performs a dual task: a. Data reduction by many orders of magnitude. b. Making sense, that is producing individual-invariant data out of the raw stream. A Wigner distribution function is a suitable operator for both hearing and vision fields. In hearing it leads to frequency , tone, perception. In vision it produces ray directions. Now tactile , auditory and visual information fit together into a single perception spectrum. Relations are shown to exist between the specification of eye , ear and tactile sense. These relations hold for many species of animals , including insects.
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In this paper we discuss the technique of laser fluorescence microscopy. First, we present the latest version of a laser microfluorometer, characterized by high spatial and temporal resolution, and by a high degree of versatility and automation due to microprocessor control. We also present our latest results in the application of this technique to the study of the interaction properties of the photosensitizing antitumor drug Hematoporphyrin Derivative (HpD) at cellular level.
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A single-exposure laser speckle photography technique is being developed for the visualization of velocity distributions in flow fields. Of particular interest is its application to the in vivo mapping of retinal blood flow. The basic principles of the technique are outlined, together with a mathematical model and some experimental results using rotating diffusers. Some preliminary results from a human retina are also presented.
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Techniques of dynamic light scattering have been applied to biological lens material in recent years by a number of authors. The enhanced scattering due to cataract, leading to eventual opacity, has been described by two related models - one ascribing the phenomenon to aggregations of the globular crystallins and the other to a phase separation. Our own observations of Brillouin scattering, and correlation studies of diluted material, indicate some degree of simple aggregation. However, work on natural and homogenised material (including human, bovine and fish lenses) shows very slow fluctuations which suggest more complex interactions.
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In clinical practice it is a frequent requirement to identify disease rapidly and more importantly to establish the onset and course of malignant disease. It is towards this end that many new diagnostic techniques are sought. Some techniques have been developed that rely on the examination and analysis of blood and more recently laser techniques have been developed to sort and measure blood samples.
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A new interferometric technique to measure deformations of the eye in vivo is described and its application to the diagnosis of glaucoma is discussed.
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A preliminary report is given on the implementation of a UK National Standard for the relative spectral responsivity of infrared detectors. This will overlap with existing NPL absolute spectral responsivity standards for the visible and near infrared and will cover the spectral range from 1 to 30 micrometres with a target accuracy in the primary standard of ±1% at wavelengths up to 18 micrometres and ±2% at longer wavelengths.
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The design and performance of a radiometer working in the 8-12μm wavelength range is described. The radiometer has a spatial resolution of lm radian and a noise equivalent resolution of 0.01°C.
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In conventional zoom systems with the aperture stop located to the rear of the moving components, the entrance pupil position varies greatly during zooming. A simple system of three positive elements, is described which can be used as a dual focal length quasi-zoom lens, which for a rear-positioned aperture stop, has a constant entrance pupil at the zoom extremes. The system may also be employed as a true zoom with limited excursion of the entrance pupil between the zoom extremes. The system is useful as a zoom relay stage in a periscope or where a prism or mirror is located at the entrance pupil to scan the line of sight.
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In the field of infrared optics (8-12 microns) there is an increasing requirement for continuous zoom telescopes operating with scanner systems. To date, zoom telescopes have for some applications been precluded due to excessive length. An exercise was undertaken to explore the possibility of length reduction. The study resulted in the design of a compact, high performance IR zoom telescope. This design and its development will be described including achromatisation, athermalisation and tolerancing aspects.
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A collimator is described which has residual chromatic and spherical aberration similar in magnitude to that of the human eye. The collimator can be used in experimental measurements of the MTF of telescopes and binoculars in order to assess the overall performance of eye plus instrument. The collimator can also be used as the basis for theoretical studies of the effect of eye aberrations on the performance of visual instruments. Examples are given of the effect which the optics of the eye can produce on the MTF of a well corrected telescope. The paper also considers some of the predictions about the visual system which can be made from a study of the through-focus MTF curves of the collimator, such as the magnitude of night myopia.
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This paper reviews the mechanisms by which laser damage occurs and summarises the forms of the resulting damage which have been observed, with examples at both 1.064 and 10.6 μm. Summaries of the laser induced damage threshold measurements made on a wide range of materials, windows, mirrors and optical components are presented at both wavelengths.
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The measurement of energy deposited in optical coatings on windows or mirrors is necessary for the non-destructive estimation of damaae thresholds which are of particular significance in laser optics. Such measurements are very difficult to perform by observing energy loss from a light beam incident on the optical element, since the absorbed energy is very low (ppm range) and at these levels absorption and scattering of the light are difficult to quantify separately. The technique described below offers a sensitive, non-invasive approach to calculating peak temperatures reached at the surface of the element under pulsed conditions. Results are reported for a variety of mirror surfaces and substrates.
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Biocular magnifiers are in widespread use in night vision equipment. These bioculars, used to view the outputs of image intensifier tubes, are typically of x5 magnification. This magnification, and consequent upper limit on biocular diameter, has precluded their use for applications such as the relaxed viewing of CRT displays. Conventional glass bioculars scaled-up in diameter could be used to magnify. small CRTs but would, have no size or cost advantage relative to a large diameter CRT. Recently, however, large diameter but compact biocular magnifiers have been designed, using inexpensive and lightweight optical components such as plastic aspheric and Fresnel lenses. The following describes these designs and exawines their characteristics.
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The basic concepts of image histogram modification are described, including in particular the technique of histogram equalisation or flattening, which is the basis for a large variety of transformations. A fast and versatile digital system is described which is capable of acquiring image histogram data, and under microprocessor control, of performing many transformations interactively. Images are input in the form of standard T.V. rate video signals and processed with < 150ns throughput delay. Transformation calculations are performed in well under a second.
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The most common laser window and mirror materials for use at 10.6 µm will be discussed and compared. Metal mirros have been shown to yield high reflectance and high damage thresholds. Dielectrically coated mirrors yield ultra-high reflectances but lower damage thresholds due to coating defects. The window operational characteristics depend both on the coating and on the window substrate material. The range of available 10.6 μm transmitting substrates will be discussed, their parameters analysed and their operational faults highlighted.
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Techniques for determining the optical constants of thin absorbing films are described and applied to the evaluation of the optical constants of carbon and silicon semiconductor films prepared by magnetron sputtering and ion beam sputtering techniques. The results are discussed with emphasis on the nature and origin of the optical gaps.
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New algorithms and microcomputer-programs for generating original multilayer designs (and printing a spectral graph) from refractive-index input are presented. The programs are characterised TSHEBYSHEV, HERPIN, MULTILAYER-SPECTRUM and have originated new designs of narrow-stopband, non-polarising edge, and Tshebyshev optical filter. Computation procedure is an exact synthesis (so far as this is possible) numerical refinement not having been needed.
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The optical transfer function of interference coatings in quasi-monochromatic light appears suitable to directly perform 2-D low-pass, band-pass and high-pass spatial frequency filtering. Uniformity of the coating over an extended area allows for space-invariant operation. Both coherent and incoherent processing are possible. In multiple wavelength illumination the optical information is coded into colour channels, corresponding to spatial frequency bands. This allows for parallel imaging in multiple wavelength with different transfer functions, and can result in pseudocolouring effects if suitable coatings such as unblocked interference filters are employed.
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When glass is annealed under stressed conditions, its physical properties such as refractive index, density, etc. are subjected to change as compared with similarly annealed glass under stress free conditions. This phenomenon may be termed "thermal stress effect". In this paper various types of optical glasses were studied for this effect by interferometric method using beam shaped specimens annealed under externally bending conditions. In the annealing of a bulk of optical glass, when cooling the glass through the annealing range, thermal stress is generated within the bulk of glass, resulting an uneven distribution of refractive index due to thermal stress effect. Experiments were carried out for various types of optical glasses and their corresponding values were evaluated.
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The design of a simple pulse stacker is described. It enables pulses of variable duration in the nanosecond regime, and of predefined shape, to be synthesised from a short pulse. A pulse stacker of this type has been tested and used for laser compression experiments.
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A simple eye-position monitor is described, which has an application as an aid to communication by those without the muscular control necessary to operate a keyboard or microswitch. The output from this device is insensitive to head movements, and it controls the operation of a Sinclair ZX81 microcomputer with a speech synthesizer, Alternatively, if the output is directed to a modified voltmeter, it provides a low-cost portable unit for simple communication tasks.
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A method of unpolarized laser pulses shaping is reported. The basis of the method is the use of an hybrid optical bistable device with nematic liquid-crystals, similar to the one previously reported by us. A sample of the input light constrols, by an asymmetrical electronic comparator, a 1 x 2 electro-optical total switch. The output pulses are reshaped and maintain the same polarization properties as the input light. From triangular input light signals, symmetriacl and asymmetrical output pulses have been obtained. The minimum pulse width achieved was 0.1 msec. A representation of the output versus input light signals gives an hysteresys cycle in the asymmetrical case.
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Results of numerical calculations are presented and analyzed for pulse generation and propagation in one-and two-spacial dimensions in a medium consisting of a collection of two-level atoms which are resonantly swept-excited by an impulse excitation traveling at the speed of light in the medium. The conditions imposed for the calculation in one-spacial dimension are that T1 = T2, where T1 and T2 are the single atom relaxation and dephasing times, respectively, and that the gain, g, to loss, κ , ratio g/κ >> 1 , which determines the non-linear regime for pulse evolution. In addition, we impose that T2 >> τc, where τc is the characteristic superradiance cooperation time, so that the pulse evolves from conditions appropriate for superradiant pulse generation for sufficiently small values for the propagation distance z . We report and analyze calculational results for the transient regime of pulse buildup through the asymptotic regime of large propagation distance z where the pulse generated exhibits steady-state behavior with regard to pulse area, energy and intensity profile. Additional results of computations are presented which incorporate transverse effects and diffraction using a Gaussian radial gain profile for the initial condition and under imposed conditions comparable with those of the corresponding one-spacial dimension calculations. The results of the two sets of calculations are compared and discussed. We demonstrate and give predictive requirements for swept-gain pulse evolution from the superradiant state.
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A modified Monte-Carlo method is presented, suitable for evaluating the contribution of different orders of scattering to the power transmitted through a turbid atmosphere. The results of numerical calculations performed by using scalar scattering functions are also shown.
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A laser Doppler electrophoresis system for the measurement of cell mobilities in hospital laboratories has been developed. Standard inexpensive optical components are used together with a 2 mW HeNe laser and a photodiode detector. An equal path length fringe mode system is used. No elaborate antivibration precautions have been found necessary. Our electrophoretic measurement cell is robust and simple. Data are collected and analyzed by an existing Nova 3D minicomputer utilizing standard software used routinely for electrophysiological signal processing. Mobility spectra are presented from various human and animal blood cells and from a tissue culture cell line.
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A calibrated attenuator is demonstrated, operating on a well-known power distribution produced by diffraction. The diffracting object is a Ronchi ruling: Once its physical characteristics are known, the angular power spectrum provides a set of mutually calibrated sources over an extended intensity range. Precise measurements of the grating performance in the visible are referred, and scaling to NIR is demonstrated with generation of a number of calibrated sources. In pulsed laser operation the method is proved suitable to obtain a reliable ratioing to compute the peak value of the pulses, and to sample fixed attenuations.
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A simple compact interferometric method is described which allows to measure very slow movements. Application to the measurement of thermal dilatation of a semiconductor surface heated by a laser is discussed.
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The paper will describe the use of a TV based system to measure the position of fringes in the interferograms of OTF standard lenses, as formed in a Twyman-Green interferometer. The core of the system is the transfer, under computer control, of sampled image signals from the TV to the buffer store of a desktop computer. Data recovered from this memory is then analysed to determine the exact position of fringes within the interferogram pupil. Details of the computer subroutines used to evaluate fringe positions and to automatically renumber the fringes will be followed by a brief description of data fitting routines used to derive aberration polynomials and thence the lens Optical Transfer Function.
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The production of large precise optical components is usually a cyclical process of measurement and optical working which is repeated many times before the desired accuracy is achieved. The precision of testing limits the accuracy attainable in manufacture and reduces the efficiency of production cycles if the residual errors are comparable in size with the error of testing.
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It has been shown previously that Electronic Speckle Pattern Interferometry can be used to compare the shape of nominally identical components. Techniques for calibrating the measurements and for converting the fringe data to surface shape information are discussed here and the accuracy of the measurements has been established.
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A technique using a laser to align the parallelism of a nest of rotational axes or surfaces 10 metres apart to within one-half arc minute will be described. The parallelism of rotational axes, 10 centimetres apart, of two cylinders can be aligned and measured with standard metrological instrumentation. The parallelism can also be aligned and measured with optical instrumentation, such as plane mirrors, an auto-collimator, or a telescope. When the sizes of the cylinders are enlarged and the separation is increased 100 fold, these approaches are impracticable because an auto-collimator of twice the aperture diameter and structure of the 200 inch telescope at Mount Palomar would be required.
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This paper describes a system capable of measuring and storing the cylindrical coordinates describing the surface of a small object (< 10 mm). Two T.V. cameras are used to view an illuminated section of the surface and the z and r coordinates are extracted from the video signals and stored on floppy disc. The resolution of these coordinates is ≈ 32 μm. The θ coordinate is supplied by rotating the object so that a new section of the surface is analysed. The process is repeated until the whole surface has been covered.
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The truncated and statistical nature of photon correlation functions obtained from laser Do-,opler anemometry experiments and the ill-conditioned problem of their inversion, has led to a wide variety of approaches to the extraction of reliable information from the data. We have performed direct experimental comparisons (using identical data) of the four widely used methods of data reduction, ranging from simple to highly sophisticated. Our comparisons covered most forms of correlogram likely to be encountered experimentally and we draw conclusions on the practicability, limitations and error magnitudes of current techniques.
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The electric field and fringe spacing dependence of the stationary energy transfer between two writing beams in volume phase-hologram formation with photoconductive electro-optic materials (KNb03:Fe2+, Bi12(Si,Ge)020,...) has been studied. Experimental results are compared with the theoretical expressions describing the influence of the different photoinduced space-charge fields in photorefractive media. A peak exponential gain of Γ= 15 cm-1 has been reached in KNb03:Fe2+ for the proper choice of experimental parameters such as recording wavelength, fringe spacing, applied electric field etc.). Results of the electric field dependence of the wavefront reflectivity ρ in degenerate four-wave mixing experiments are reported and discussed in terms of a similar theoretical treatment as the two-wave mixing experiments. It is shown, that peak reflectivities of ρ = 25 percent have been reached for KNb03:Fe2+, that conjugate complex wavefronts can be generated and that distorted optical wave-fronts can be effectively corrected by four-wave mixing experiments in KNb03:Fe2+.
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The work described in this paper, extends to finite magnetic fields, the study of the exceptionally large value of x(3) found for moderate radiation intensities close to resonance with interband transitions in Indium Antimonide (InSb). The intensity dependence of both interband magneto-absorption and dispersion, for transitions between individual identifiable Landau levels, was examined experimentally. The intensity dependence of the magnetic field dependent oscillations, in non-resonant interband Faraday rotation, provided a sensitive technique for the study of changes in magneto-dispersion. An intensity induced rotation corresponding to a maximum refractive index of - 9 x 10-5 W-1 (x(3) ca 10-1 esu, or ca 10-9 m2 V-2) was observed. This effect exhibited resonant enhancement as the pump photon energy approached that of both the fundamental bandgap and an acceptor transition and showed saturation at increased incident intensity. The effect was determined to be primarily the result of the blocking of the interband transition by photoexcited carriers. The intensity dependence of the magneto-absorption showed saturation effects for radiation intensities as low as 6 W cm-2, with the peak absorption coefficients being reduced by nearly a factor of two for an order of magnitude increase in intensity. The results are compared with two new microscopic models, the direct saturation model, which assumes a non-thermal distribution of the photoexcited carriers, and the dynamic Burstein-Moss model, which assumes a thermalised distribution. Comparison with experimental results yields values for the electron-hole dephasing time of T2 of the order of 3 ps and an interband relaxation time of TI of the order of 100 ns. In thin samples, a value of T1 some two orders of magnitude shorter was obtained, due to carrier diffusion and rapid surface recombination.
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Calculational results and analysis are presented and discussed for the effects of coherent pump dynamics, propagation, transverse and diffraction effects on superfluorescent (SF) emission from an optically-pumped three-level system. The full, co-propagational aspects of the injected pump pulse together with the SF which evolves are explicitly treated in the calculation. It is shown that the injection pulse initial characteris-tics can have a strong effect on the SF emission.
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During some initial investigations of bistability,in an optical cavity containing calcium vapour, a hitherto unrecorded modification of the resonant mode separation was found. This unusual phenomenon occurs symmetrically on either side of the absorption line and is shown to result from the dispersive character of the cavity.
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The optical extinction theorem due to P. P. Ewald, C. W. Oseen and C. G. Darwin is used completely to describe the action of a dielectric slab as a Fabry-Perot interferometer. The analysis is very comparable to that given by Born and Wolf. It is then shown that by generalising the optical extinction theorem to the nonlinear regime essentially the same Fabry-Perot cavity action is regained but in this case the refractive index and hence the cavity tuning depends on the input intensity. This way a multistable output/input relation is derived. It is shown further how the extinction theorem will also handle the problem of 'standing waves' in the cavity. The possibility of optical turbulence in a Fabry-Perot cavity is briefly mentioned.
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We report the observation of large intensity dependent refractive index changes in cadmium mercury telluride at 175K and at 10.6 μm wavelength. The effects are observed by measuring changes in the half power width of a Gaussian laser beam focused through a thin sample of the material.
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Transverse effects on the profile of an intense off-resonant cw light beam, propagating through a gazeous cell of length ℓ, are numerically displayed in both cases of the very small absorption length (α-1 « ℓ ) and the intermediate case (α-1 ~ ℓ). As predicted by the theory, self-focusing and spatial ringings are obtained. Moreover for αℓ ~ 1, these distorsions generally appear as a recurrent process.
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The theory of phase conjugation by Brillouin enhanced 4 wave mixing is developed and the effects of phase mismatching and detuning the probe beam from the centre ofathe Brillouin resonance are investigated. Extremely high reflection coefficients (R > 106) are shown to be possible when one pump beam is much stronger than the second, and when the pump beam intensities, the probe beam frequency and the interaction length all take certain critical values.
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Astronomical instrumentation has improved steadily with time as technological progress has allowed the demands of astronomers to be satisfied. There can be little doubt that astronomers will continue to seek improved performance but in some respects the performance achieved is already close to physical limits while in others technology or available funding are the limiting factors. Resolution has always been important to astronomers, not only because of the additional structural detail that improved resolution may reveal but also because of the improvement in ability to detect and study faint unresolved objects in the presence of background radiation. The purpose of this paper is to review the resolution currently obtainable at wavelengths shorter than 100 μm and to examine the extent to which improved resolution could be obtained with only modest extensions of current technology. The review is confined to consideration of telescope resolution and does not include the performance of detectors.
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Light propagation through the atmosphere is disturbed by atmospheric turbulence. This limits the high angular resolution in astronomical imaging. Active optics is a method to overcome this problem. It allows a real-time optimization of the resolving power. An active mirror was developed which consists of an electrostatically deformable membrane. The polymer foil is actuated by an electrode array with 63 hexagonal elements arranged in a ring structure. The sensitivity is 0,05 microns per volt. It works up to 4 kHz without resonances. The maximum local tilt of the membrane is 3 microns per 5 millimeter. For an atmospheric tilt compensation of the wavefront the mirror housing is in a gimbal mount. Piezo-electric actuators provide a total mirror angular movement up to 20 Hz and angular sensitivity of 77 arcsecs per kV. With this active mirror device and a multi-microprocessor control unit the stabilisation of the star-speckle pattern positions and the deconvolution of the speckle patterns are possible. Two control methods are in development and analyzed in comparison. One is working with a modified shearing interferometer as a wave-front sensor and a feedback with cross-talk compensation. The other is sensing the optical information in the image plane by a diode-array and computing the wave-front from its Fourier modulus. The compensating phase distribution is generated by an expansion of the turbulence phase distortions into modes of a set of basis functions, i.e. Zernike-polynomials and Karhunen-Loeve-functions. By applying a modal control concept to the adaptive optical system, the electrodes of a membrane mirror are controlled in parallel with compensated cross-talk. The coefficients of the approximating functions, each of which corresponds to a mode on the mirror surface, are fed back to the actuators by a modal control matrix. The system presented in this paper is designed for the 0,75 m RC-telescope with alt-az mount at the Landessternwarte in Heidelberg, F.R.G..
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The variation with wavelength of spherical aberration in Schmidt cameras can be mitigated by a number of different methods. The more usual one is to balance this spherochromatism against longitudinal chromatic difference of focus, by introducing a positive vertex power at the corrector surface. An alternative is to make an achromatic corrector of two materials with different dispersions. A further method described here is to use a negative vertex power or chromatic aberration at the corrector, which induces a compensating spherochromatism by changing the incidence height at the mirror surface. Various other aberrations can be corrected. Examples include a solid camera for use with a charge-coupled device as detector in a spectrometer.
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A sensitometer has been commissioned for the photometric calibration of spectroscopic and wide field astronomical plates. A set of illuminated apertures of various diameters illuminate a corresponding set of apertures all of equal diameter located immediately in front of emulsions. Twenty calibration spots in irradiance steps of 1.27 are exposed on to each plate. Burr-free apertures of precise diameter were photo-etched in thin foil. During periodic calibrations, a fibre-optic feed system is coupled to a photon counting photomultiplier. The instrument is spectrally neutral and spot irradiance ratios are calibrated to within 1%.
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Acousto-optic filters possess some attractive properties for applications to astronomical spectrophotometry. Tuning over a wide spectral range is achieved by variation of the frequency of an acoustic wave propagated through the crystal which causes diffraction of the incident collimated light in a narrow range of wavelengths. Spectral scanning either continuously through the spectrum or by switching to a programmed set of wavelengths can be accomplished easily and rapidly since there are no moving parts. Spectral performance characteristics of a commercially available filter are given and an outline is presented of a prototype astronomical spectrometer which is controlled by a microcomputer.
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The current state of the art and immediate future of CO2 rangers is seen in the context of Nd:YAG laser history : first generation - establishing the technology/separate apertures, second generation - development and applications/the transceiver configuration, third generation - integration, maturity and production. A CO2 ranger product range and outline parameters are indicated and the technology summarised. Advantages and disadvantages, current and projected, and the rangefinding performances are compared with the Nd:YAG baseline. Likely applications are indicated, stand-alone and integrated, and requirements and methods of optical integration are shown in principle.
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A simple effective achromatic half wave plate has been designed at Spectron by computer analysis of birefringent networks based on Poincare sphere representation. By observing some simple rules and applying selection criteria based on rotational properties of half wave plates, some potentially successful plate configurations have been identified. The most promising of these, a three plate combination, has been chosen for manufacture and is now available from the Spectron product range.
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The operating characteristics of CW atomic mercury lasers are described. Operating at 546.1nm the laser is optically pumped by DC mercury discharge lamps. The laser has high perceptual brightness and potentially low noise and high stability. Experimental and theoretical aspects of the laser and its performance are discussed and the effectiveness of various optical coupling schemes considered. Evidence for absorption loss due to mercury dimers is presented and some results are given for direct measurement of this loss by tunable dye laser.
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To study the pulse-broadening mechanism in an injection mode-locked TEA CO2 laser oscillator, a train of Gaussian pulses with variable widths were injected in the slave-oscillator. Theoretical analyses and experimental data are presented of the AM mode-locked master-oscillator with an additional c.w. low-pressure gain cell in the cavity and of the broadening mechanism on the injected pulse in the slave-oscillator. Injection mode-locking is now a commonly used method for generating high-energy nanosecond pulses. Due to the absence of inserted modulators, which have in general relatively low damage thresholds and small apertures, this technique allows the generation of high-power short pulses.
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This work is concerned with the possibility of getting optical amplification in the uv/visible region by means of doped alkali-halide crystals, pumped by a Nitrogen laser. The Amplified Spontaneous Emission (ASE) technique is adopted with the continuous recording of the luminescence intensity as a function of either the pump intensity or the pumped length. In the latter case the pumping beam inhomogeneity is taken into account. A CsI crystal doped with In+ and T1+ was investigated at room temperature. The fast component of the emission (~10 ns) at 430 nm showed a gain of about 1.5 with a pumping of 104 W/cm on a length of 8mm. There is experimental evidence that the active element is thallium after investigation on CsI:T1+ and CsI:In+ crystals. Negative results of amplification for KI:T1+ and KI:Ag were obtained.
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The second harmonic beam in a phase matched frequency doubler crystal travels in the direction of phase matching. If the wavelength of a focused fundamental beam is changed, the phase matching direction is changed and the harmonic beam is deflected. The deflection angle and number of pixels are derived. Experimental results ona LiI03-crystal are presented. With electro-optic wavelength tuning very fast scanners (up to 1010 pixels/sec) can be constructed. Changes of the phase matching direction can also be obtained via the electro-optic effect. This effect is investigated, and it is found that this effect is too small for the usual non-linear electro-optic crystals, but can be of practical importance for the non-linear organic crystals recently discovered.
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oppler-free two-photon absorption and two-photon dispersion spectra of Rb87 have been taken by FM spectroscopy. Sensitivity achieved is comparable to fluorescence detection. The technique can be readily applied where fluorescence is difficult to detect.
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When a sample is illuminated by pulsewise radiation, the concurrent heating at the surface gives rise to a pulsating component in the specimen's thermal infrared reradiation. It appears possible to observe surface temperature oscillations as small as 10-4 - 10-6 K, using current technology in infrared detection; this corresponds to the absorption in less than a monolayer. With monochromatic irradiation, absorption spectra of opaque objects can be obtained; it also becomes possible to examine subsurface structures. We review briefly the background of photothermal radiometry, and we present some experimental results to demonstrate the technique's versatility, including some new results from studies of photosynthesis in live plants.
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This paper describes a method of producing p-n junctions in silicon using an ArF laser to dissociate triethyl boron. Junction depths of 0→0.8 μm and peak electrically active dopant concentrations of 8 x 1018 → 2 x 1020 have been measured.
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The use of picosecond pulse trains in performing high resolution atomic spectroscopy is described. Methods which rely in producing Hertzian coherence and optical coherence in atoms are compared and contrasted.
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A technique which uses Photothermal Deflection Spectroscopy to detect photoinduced dichroism is described. Some preliminary results are presented and discussed.
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Picosecond continuua generated by focussing amplified picosecond pulses into various media have been studied. We have measured both spectral and temporal characteristics of these continuua and discuss the implications of such continuum sources to spectroscopy. The characteristics of the dye amplifier used to generate such continuua are also discussed.
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The fiber optic gyroscope is now within the high sensitivity predicted by theoretical studies. We review the different steps involved in the progresses realized over the last five years by the various research teams working on this subject around the world. Measurement of rotation rates of 10-2 deg/h have been demonstrated. That corresponds to measurement of phase shift of 10-7 rad in this interferometric system, five orders of magnitude better than the earliest results. The first major breakthrough was the detailled analysis of the effects of fiber birefringence and of mechanical stability of the system, using the fundamental principle of reciprocity of modal propagation. The need for single mode filtering has led to a so-called minimum reciprocal configuration which improves dramatically the sensitivity of the system when it is used in addition of biasing detection techniques. We present the various technological approaches which have been used (bulk optics, all-fiber optics and integrated optics) and we analyse the residual parasitic phenomena which had to be reduced (thermal and acoustic noise, Faraday and optical Kerr effects). Experimental data of an all-fiber system and of a brass board using a single multifunction integrated optic circuit are reported. Finally we discuss the current problems and the future directions of research which should lead to an actual navigation instrument.
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A quasi-continuous process for production of large core low-loss step index all-silica fibers has been developed by combining axial plasma deposition and lateral plasma deposition. Named A.L.P.D. (Axial Lateral Plasma Deposition) this process allows industrial production of 2 db/km fibers (0,85 micron) which is almost equivalent to loss limit of silica fibers estimated from the characteristics of intrinsic infrared absorption and Rayleigh Scattering losses. Furthermore thanks to pure fused silica core these fibers show an excellent radiation hardness and high power throughput capability.
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A general integral equation for the total scattered magnetic field is presented. Thus,the coupling phenomena between N identical dielectric waveguides can be interpreted as a multiple scattering process. The application to the case of two identical dielectric waveguides allows to obtain a numerical behaviour where the control of the degree of coupling is also known.
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Pressure induced changes in optical pathlength are being utilized to realize a hydrophone consisting of glass fibers in a Mach-Zehnder interferometric arrangement. In our experimental setup we use single-mode fiber optical power dividers and a simple control system that maintains the polarization of both light signals so that interference with constant contrast is obtained after superposition. We use a modified phase-tracking detection system to maintain maximum interferometer sensitivity and show a design for a sensor-body with high acoustical sensitivity.
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The use of light backscattering in a fiber is suggested as a probe for hot radiation points in nuclear environments. Preliminary experiments show the feasibility of the method.
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A method and apparatus is described to monitor non-destructively the turbidity of near white materials based on edge-losses from a small illuminated area. Measurements will be shown on paper and plastic; on teeth during carious demineralization; on the whiteness of the human eyeball and on milk, changing the fat concentration.
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A first principles derivation is given of the evolution equation for nonlinear pulse propagation in an optical fibre. It avoids many of the difficulties encountered in some other theories and emphasises that waveguide effects can be included in a natural and logical manner. It is preceded by a brief review of the principal features of solitons in optical guides, and a critical discussion is given of previous theoretical descriptions of them. The numerical results reported here include an investigation of the competitive roles of waveguide and material dispersion and some examples of the behaviour of solitary wave solutions of the nonlinear Schrodinger equation.
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The paper presents a review of the basic principles of a number of optical fibre sensors, with discussion on the advantages and disadvantages of various types. In order to give an overview of what is rapidly becoming a very broad subject, the author has concentrated on the major classes of sensor of practical interest.
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In the recent years, a number of methods have been developped to overcome the limitations of the classical coherent and incoherent image processing methods. A review of the main trends in this area is presented, indicating the principles of low-noise coherent processing, of incoherent processing of complex quantities and of white light processing ; it is seen that optical processing can now in a number of cases perform nonlinear operations, or shift invariant operations while still keeping the advantage of parallelism.
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The coherent optical filtering techniques provide a general concept for the classification of patterns. This paper describes the design and testing of a hybrid optical-digital image processing system and the development of methods for a statistical expansion of the correlation signal shape because a simple correlation signal intensity measurement is in most of the applications insufficient. Six different algorithms for correlation signal evaluation are investigated. A feature reduction is achieved by a multivariate analysis. For alpha-numeric patterns modified by binary random noise, rotation, scaling and shearing high classification results have been obtained.
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An opto-electronic hybrid system for automated near real-time processing of microscopic samples has been developed. Spatial frequency plane filtering techniques adapted to a commercial microscope are used for fast preprocessing. The noise problem was substantually reduced by use of either a moving point-like light source or by a spatially extended light source. Calculations and experiments show the conditions under which the influences of the extension of the source for both methods onto high-pass, band-pass and gradient filtering can be neglected. The system has been used for the automated evaluation of cytological smears and for detection of asbestos fibers in membrane filters.
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Coherent optical processors have interesting potential applications for pattern recognition or optical computing. A real-time cross-correlator using the dynamic holographic properties of Bi12 Si 020 (B.S.0.) crystals will be presented. A photo-conductor liquid crystal light valve transducer allows real-time data input in the processor via incoherent to coherent light conversion. A classical two-lenses optical configuration is used to simultaneously record and read the dynamic phase-volume hologram photoinduced by the photorefractive B.S.0 crystal placed in the focal plane of the Fourier transform lens. Some specific properties of the dynamic correlator such as cycle time constant, spatial resolution, beam intensities will be reported. Experimental cross-correlation results of various types of information written on the light valve will be demonstrated.
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Demonstration of holograms made by students from the Physics Department of Napier College, Edinburgh. One of the holograms shown illustrates the relationship between the orthoscopic and pseudoscopic images when both images are virtual.
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A description is given of a high resolution electro-optical device that supplies, in real time, values of surface tension of a drop of macromolecular substances as a function of time up to the equilibrium (pendant drop method). The optical system has a high magnification and high free working distance; the detection system consists of an optical scanner and a self scanned device. The application to automated drop inspection is discussed in relation to the other form of inspection systems.
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A microscopic set-up to observe fast moving solidifying particles during stir casting is described. The set-up consists of a Ruby laser and a frequency doubled Nd3+:YAG laser, a model device of an actual stir casting apparatus filled with a transparent organic alloy, melting at a low temperature and three different recording systems. Using a neopentyl alcohol alloy as a model substance for metal alloys, the crystallization process is studied by direct observation. Pulses from both lasers are used to provide a sufficiently short exposure time to take (simultaneously) holograms, microphotographs and videorecordings of the fast moving (flow rates up to 10 ms-1) small particles with sizes in the order of 10-103 μm. Primarily solidified particles appear to have equiaxed dendritic shapes. The longest diameter of these particles attain a maximum for low stirring rates and high cooling rates. After a period of stirring, some of the dendrite tips grow and transform the particle shapes into more spherical ones. At this stage the morphology of the solidified particles shows a good similarity with stir casted metal alloys.
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An holographic camera composed of two ruby lasers was built at ISL. It provides double exposure holograms with an adjustable time interval ranging from few ns to infinity. Various aircraft structures were first tested at ISL in laboratory conditions: honeycomb panels, wings ... The industrial tests on a military aircraft in maintenance checking were performed in a hangar of the SNIAS at Saint-Nazaire: wings, trap-door of the rear landing gear, air-brake... Electromechanical shocks were used to make the structure vibrate and to allow a fast trigger of the lasers. This avoids disturbance due to ambiant noises and vibrations.
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A holographic non destructive test procedure as a diagnostics tool in art conservation and preservation of ancient wooden panel paintings is presented. A brief discussion of the reability of the method is given. Some examples of experimental results are reported; they show that the defects can be detected in real and non real time on laboratory models, at varying depths,through ambient drift.The examination is made combining sandwich holography and real time holography, without any modification of holographic set up.
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Holographic interferometry has been used to measure the deflection of thin CFRP cross-ply plates under very small pure bending loads. The paper describes the design and calibration of the optical system, the method of mounting and loading the specimen, and the computation of curvature values from the fringe data, together with a brief section on the subsequent derivation of the bending stiffness. Results obtained, both for the composite material and for a control plate of aluminium alloy, are shown to be in good agreement with theory, and further applications to thick plates and those of unknown construction appear feasible.
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This paper describes the formation of an arbitrary blurring followed by the use of a filter in a frequency plane to eliminate the blur. The complex space filters produced by the computer-generated method enables one to construct many new and fascinating coding and decoding systems for dilute objects. In this paper several CGH filters, inverse filters, Wiener filters and the Wiener filter of the exponential type, have been developed. All these filters have been experimentally tested as to their capabilities for cryptology and good results have been obtained.
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It is now over 20 years since Ferranti plc introduced optically projected map displays into operational aircraft navigation systems. Then, as now, it was the function of the display to present an image of a topographical map to a pilot or navigator with his present position clearly identified. Then, as now, the map image was projected from a reduced image stored on colour micro film. Then, as now, the fundamental design problems are the same.In the exposed environment of an aircraft cockpit where brightness levels may vary from those associated with direct sunlight on the one hand, to starlight on the other, how does one design an optical system with sufficient luminance, contrast and resolution where in the daytime sunlight may fall on the display or in the pilot's eyes, and at night time the display luminance must not detract from the pilot's ability to pick up external clues? This paper traces the development of Ferranti plc optically projected map displays from the early V Bomber and the ill-fated TSR2 displays to the Harrier and Concorde displays. It then goes on to the development of combined map and electronic displays (COMED), showing how an earlier design, as fitted to Tornado, has been developed into the current COMED design which is fitted to the F-18 and Jaguar aircraft. In each of the above display systems particular features of optical design interest are identified and their impact on the design as a whole are discussed. The use of prisms both for optical rotation and translation, techniques for the maximisation of luminance, the problems associated with contrast enhancement, particularly with polarising filters in the presence of optically active materials, the use of aerial image combining systems and the impact of the pilot interface on the system parameter are all included.Perhaps the most interesting result in considering the evolution of map displays has not been so much the designer's solutions in overcoming the various design problems but that both the pilot and his environment are themselves an integral part of the system optical design.
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An ultra-violet laser damage and diagnostic facility has been set up. Laser induced damage threshold measurements have been made at 193nm and 248nm on a range of UV substrate/ window materials such as fused silica, MgF2, CaF2 and on various excimer laser mirrors with multi-layer dielectric coatings.
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Single frequency laser source phase noise constitutes a limitation in single-mode optical fiber coherent detection systems such as heterodyne communication receivers and optical fiber interferometric sensors. We investigate the phase noise affecting the beat photocurrent produced by the coherent detection of a single frequency laser field with a time-delayed and frequency-shifted image of itself in a heterodyne interferometer. The photocurrent spectrum is discussed as a function of the source lineshape and interferometer decorrelation time. An experimental study is retorted consisting of a single-mode fiber Mach-Zender interferometer employing a single-frequency laser and an acousto-optical modulation for frequency-shifting one of the interferometer arms. Results are in good agreement with theory.
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The eikonal and other approximation methods used to describe light scattering are compared with the exact theory for long absorbing fibres. Fibre diameters in the range 0.2 - 4.0 μm are considered. The real and imaginary refractive index values considered are 1.05, 1.10, 1.15 and 0.001, 0.01, 0.1, 1.0 respectively. The implications of these results to fibre sizing are discussed.
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A technique was developed for measuring tooth displacements, caused by extraoral orthodontic forces. By using speckle-interferometry as measurement technique and a macerated dry human skull as test object, a reliable investigation procedure is developed. Resolution of total displacement into vector components is realized by simple trigonometry. These observations showed to be very interesting for the understanding of the biomechanics of clinical headgear forces. For the purpose of orthodontic research speckle interferometry was applied and is expected to be a promising technique.
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Encouraging preliminary results are presented from a feedforward stabilized active optical filter. Its function is to greatly reduce the shot-to-shot and day-to-day pulse energy fluctuations commonly suffered by single shot Q switched lasers and their associated electro-optic gates and switchouts.
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Work on scintillation of laser light over open grassland has been motivated by the need to provide safety clearance for the use by the Army of various laser equipments (eg rangefinders). The scintillation strength may be characterised by the parameter n, n2 = 0.5 Cn2 K7/6 Lll/6 Here Cn is the Turbulent Refractive Index Constant K is the wavenumber of the radiation L is the range. For values of n2 (in weak scintillation n2 is the variance of the log irradiance) greater than 0.5, ie for ranges 0.5Km for visible or near IR lasers in Summer conditions the scintillation may no longer be described by the weak fluctuation theory (due to Rytov) . For medium and strong scintillation it is not at present possible to predict either the mean irradiance or the mean square irradiance fluctuations by analytical formulae. However, it is possible to obtain an integral expression for the mean irradiance I . In the present investigation this expression has been evaluated by numerical integration on a 1906A ICL computer. The mean square fluctuation of irradiance is more difficult to calculate, as it involves expressions for the fourth order moment of the field. In this case, it has not yet been possible to even derive closed expressions for the solution of the differential equation for the fourth moment of the field. However a number of possible numerical approaches have yielded results which compare favourably with experimental results in some cases. The results of some of these computations will be presented later.
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Light deflection is obtained by diffraction from a transient index modulation established as a grating of variable frequency by the interference of two light beams in an optical material. We describe a technique for altering the grating frequency by shifting only the wavelength of the control beams and the use of a novel optical system to maintain the Bragg condition over a wide range of frequencies in order to achieve light deflection without loss in efficiency. A computer simulation permits us to test various designs and to select the best configurations with emphasis on those that exhibit large angular deflections with minimum Bragg detuning. This new method of light deflection allows either discrete or continuous light scanning or modulation. The experimental results confirm the calculations and we have obtained angular deflection changes larger than 11 degrees from a wavelength shift smaller than 30 nm while maintaining the Bragg condition.
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For general non-dispersive optical systems, the existence of quasimonochromatic planar sources with different states of partial polarization that generate identical intensity distributions at the output plane of the system has been investigated. By using the procedure outlined by the authors in previously reported works, the common characteristic and the relationship that must be satisfied by the coherence matrix of all these sources have been determined.
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we present two models for light propagation in absorbing and scattering media, which are both based on the KUBELKA - MUNK theory. The models allow, within the general restrictions of the approach, the evaluation of light flux gradients across layered objects with specular reflection at the object boundaries; and the evaluation of optical pathlengths and pathlength distributions for the transmitted, reflected or absorbed light.
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Chirped grating lenses have been successfully designed, fabricated and evaluated for the first time on Ti-indiffused LiNbO3 waveguides. The design procedure, the expected and measured performances, the materials and fabrication processes developed, and the performance limitations imposed by the waveguide parameter and by the fabrication tolerances will be presented.
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An experimental technique which combines the accuracy of holographic interferometry and the ease of a television video display which may be used under service conditions for in-plane strain measurement is extremely attractive. Electronic Speckle Pattern Interferometry (ESPI) is a technique which displays this potential and development for application to rotating structures is at present taking place at The City University. The latest results achieved showing interference fringes recorded at tangential velocities up to nearly 5 ms-1 are presented.
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The use of a pulsed ruby laser in recording holographic interferograms of specific surface displacements of the human body is demonstrated. Technical particularities of the technique are discussed and attention is paid to the safety requirements of the measurement method.
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A virtual fringe analysis method can be applied to determine displacement vectors from speckle photographs. A diminishing of the visibility shifts dark and bright fringes from minima and maxima of the cosine. Then not only the bright fringes but the dark ones do give erroneous results. The analysing goemetry can increase a reduction of the visibility. This paper deals the fringe visibility of virtual fringes and its effect to the fringe shifts.
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Calculations of light scattering from transparent spherical particles under various polarization conditions have been made by the Mie theory with a view to assessing optical drop sizing techniques. Some interesting features revealed by detailed calculations are reported and illustrated by experimental observations. In forward scatter, there are fine details of the scattering pattern which are critically dependent on drop size. In back scatter, the intensity is found to be a rapidly varying function of particle diameter but under certain conditions of circular polarization, the scattering pattern is very regular and may be explained by a type of diffraction theory.
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On completion, the Joint European Torus (JET) will he the largest nuclear fusion experiment constructed. We are currently developing a novel system to measure radial electron temnerature/density profiles and central ion temperatures within the plasma using a 10Hz rep-rate CO2 laser. The large dimensions and expected neutron fluxes represent significant constraints on this design. Specifically the input and collected scattered radiation have to he relayed between shielded lasers/detectors and the torus, ~ 30m away, and the alignment maintained remotely. This system, containing ~ 75 mirrors and designed with the aid of a computer ray-tracing program, is described together with some details of the spectrometer and laser.
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This paper describes the analysis of photon correlation spectroscopy decay curves by a significant new method based on the fitting of sums of positive exponentials by the S-exponential sum fitting method. The method fits a positive exponential sum to a given data set providing a best weighted least squares fit. No initial setting of any of the parameters is required and the number of exponential coefficients does not have to be preset in the program but is determined by the number of components apparent above the noise level. Results will be discussed for application in scattering systems which may be single or multiple component. Systems generating single, double or multiple exponential decay functions derived from computer simulation or photon correlation exneriments are considered and fitting analysis with varying noise levels.
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Using a "free" propagation model, we examine the statistical properties of speckle patterns in confocal scanning optical microscopes. Both reflection and transmission geometries are considered. Expressions for the contrast in both in and out of focus planes are derived in terms of the statistical parameters of the rough surfaces and the geometry of the optical system. The results are evaluated numerically and compared to experimental data.
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Complex multifocal holographic lenses can be recorded in a single plate. Performance in accordance to each individual optical element function can be chosen through proper illumination of the plate. The basic aspects involved in their performance are reviewed and experimental results presented.
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Optical elements which direct light by diffraction rather than by reflection have hitherto been very little used because of their chromatic aberration and low efficiency. However, with the more widespread use of lasers there is an increasing number of applications for which chromatic aberration is no longer a problem. With a thick hologram or a blazed surface relief pattern it is possible to achieve useful efficiencies so it now seems reasonable to expect a wider use of diffracting optics. Using holographic techniques it is feasible to produce components of excellent optical quality which are both cheaper and lighter than their conventional counterparts.
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Dichromated gelatin has been shown to be a useful photosensitive holographic recording material (1,2,3). We are investigating some characteristics of dichromated gelatin, with particular attention to the diffraction efficiency (typically 95 - 98%), reflection bandwidth and incident angle sensitivity. The dichromated gelatin preparation and processing techniques that affect these parameters will be included. Applications for these holographically produced reflection filters include head-up displays, laser protection eyewear and holographic laser beam combiners.
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An acousto-optic deflector is essentially a diffractive device, and hence the scan generated by the deflector is a function of the optical wavelength used. This has meant that for multiwavelength operation, such as colour Laser displays, separate deflectors have been used for each wavelength. The extension of their use, to include more than one wavelength, would clearly have a number of practical advantages. Described in this paper is a novel holographic method which allows the deflector to be used with more than one wavelength. Also reported is an assessment of the practicality of the method.
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The set-up consists of a double-grating system which realises 2 successive auto-correlations of a grille, that is to say a two-dimensional repartition of opaque and transparent zones. The flux transmission of this set-up is identical to the classical slit spectrometer but the equivalent slit height can be up to one meter for a 1 meter focal length objective. Contrarily to a one-grating grille spectrometer previously proposed, no important background substraction by demodulation is needed. This system is then usable in spectral ranges where gratings are usually used.
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The paper presents a new approach to rule a long diffraction grating, in which a short ruled portion of the ruling grating blank is used as a new dividing element of the engine to continue its ruling . The main ruling errors are analysed and some formulae for the calculation of these errors are derived. The actual resolution of a specimen is 90% of its theoretical value. The tentative results have verified the correctness of the principle links to this method.
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This paper briefly describes the principles of the recording and read-out of optical video discs, and presents a method to inspect pit geometries. The pit structure on the disc containing the video information is considered to act as a biaxial grating. It is shown that when continuous grooves are recorded on a disc, diffraction experiments give the dimensions of these grooves by comparing them to results of rigorous theory of diffraction by a finitely conducting uniaxial periodic structure with polygonal height profile. This measuring technique yields a precision of 3 nm in depth, 25 nm in width and 3° in slope.
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The scattering of partially coherent radiation by structures such as diffraction gratings is analysed theoretically. The far-zone intensity and degree of coherence patterns are obtained in terms of relevant source parameters such as coherence length, beam width, phase curvature, and the properties of the diffracting structure. Particularly noteworthy is the fact that for relatively low source correlation lengths, far-zone coherence measurements can detect evidence of the diffracting structures even though the radiant intensity may be completely diffuse. This phenomenon has potential application to the coding and detection of optical information. Recent experiments do indeed corroborate these predictions.
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A variation of the Mach-Zehnder interferometric arrangement was used to show that both reversing and non reversing wave fronts produce peaks of the coherent function in the far zone when a beam splitter (such as a semi-mirror or a grating) is illuminated by diffuse laser light. The reversing case produces a much narrower coherent angle than the non-reversing case, which, in turn, can be much narrower than the diffuse radiant intensity pattern. A beam splitter hidden behind such a diffuser and detectable only by means of a coherent optical setup has potential applications for optical coding.
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Since the advent of the Cooley-Tukey FFT algorithm, many optical designers who have used the method to compute the optical transfer function with desk-top computers would welcome the availability of even faster algorithms. There has, of course, been a steady improvement in FFT techniques in the past decade or so, but it seems that Winograd's algorithm is the most encouraging yet. In this paper we report on the application of this new algorithm to OTF calculation and compare it with some other methods of computing the OTF.
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A generalized dispersion relation is given which is not restricted by the demand of analyticity of the optical constant. Sum rules are also given with the aid of the generalized dispersion relation.
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When electron beams of low energy (E < 25 eV (say), λ > .25nm) pass through matter, effects should occur analogous to optical activity and rotary dichroism in light-optics. The possible effects are described, and are discussed on the basis of Born's explanation of optical activity. In light-optics optical activity is observed more easily than is rotary dichroism; the opposite is expected in electron-optics. The possibility of observing these effects in electron-scattering from asymmetric molecules is discussed in relation to experiments, performed and proposed.
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We report on an IOSA based on a titanium diffused optical waveguide layer in lithium niobate with geodesic lenses. Lenses were designed by the method of Sottini et al but with essential modifications to smooth over regions of extreme curvature. Ray-tracing analysis showed that the deviation from the ideal profile introduced by this smoothing had negligible effect. Several lens fabrication methods were experimentally evaluated -ultrasonic impact grinding, computer-controlled diamond tool grinding and diamond turning, followed by polishing with large area flexible tools, or with small pads under microprocessor time and position control. The most reliable method was diamond turning followed by polishing with a small pad on a specially constructed computer controlled polishing machine. Data is also presented on factors affecting overall spectrum analyser dynamic range such as in-plane scattering and acousto-optic third-order intermodulation.
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The resolution of hydrated biological specimens by soft x-ray microscopy is considered. This is set by damage to the specimen caused by secondary electrons and free radicals. Using synchrotron radiation the limit would be about 10 nm. By using short lived (~ 1 ns) sources as produced by a laser generated plasma, the Rayleigh limit of 2.3 nm should be possible.
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