Showing papers in "Optics and Laser Technology in 1996"

Excitation of Hermite Gaussian modes in end-pumped solid-state lasers via off-axis pumping

Abstract: We demonstrate a method for the excitation of Hermite Gaussian modes (one transverse dimension) in end-pumped solid-state lasers. The excitation is performed by pumping off-axis with the help of a fibre-coupled laser diode. With increasing transverse separation of the fibre from the optical axis, the Hermite Gaussian modes can be successively observed. This laser has a variable M 2 -factor and can therefore be used as an M 2 -standard in calibration purposes. M 2 -factors between 1 and 190 are attained with this method.

Spectral and polarization analysis of optical images by means of acousto-optics

Abstract: This paper presents results of a theoretical and experimental study of Bragg anisotropic diffraction. The diffraction may be used for filtration of optical radiation and processing of images. Special attention is devoted to the investigation of diffraction, which provides spectral filtration simultaneously with polarization analysis of divergent optical beams. A tunable tellurium dioxide acousto-optical filter for processing of optical images is described in the paper. Some experimental results of the image analysis in visible light and in the near-infrared region of the spectrum are discussed.

Spectrally-resolved white-light interferometry as a profilometry tool

Abstract: Phase-shifting interferometry and white-light interferometry are reliable techniques for surface analysis in which the optical path difference has to be changed by some transducer to evaluate the phase. We present here a different procedure in which optical path modulation is completely avoided. This technique is based on the spectral analysis of white-light interferograms. By means of a spectroscopic device, a non-visible interferogram is split into its monochromatic components and absolute, unambiguous values of the phase are obtained along the spectral axis. Only one interferogram is required to obtain the profile of one-dimensional surfaces with nanometric resolution.

Overview of hybrid optical neural networks

Abstract: This paper reviews optical architectures for the implementation of hybrid neural networks Optics is mainly applied to implementing the matrix-vector or tensor-matrix multiplication In addition, the general background of neural networks as well as a brief discussion on holographic associative memory are also given

Application of a pseudogeometrical optical approach for calculation of the field formed by a focusator

Abstract: The present work deals with the application of pseudogeometric optics techniques to calculating a light field generated by a focusator of laser radiation into the rectangular domain We have derived an analytical expression for the principal term of the asymptotic expansion in the focal plane including the geometric-optics shadow domain

Phase difference determination by fringe pattern matching

Abstract: A method of phase difference determination in interferometry is presented. In this method, the phase difference between two interferograms is determined by fringe pattern matching with subpixel accuracy. The signal-to-noise ratio is significantly improved due to the region-based fringe pattern matching and its effect of averaging noise. The experiment shows that this method is useful for the determination of phase difference between two equi-spaced fringe patterns, and it has the advantages of high precision of measurement and high resistance to noise.

Two sub-pixel processing algorithms for high accuracy particle centre estimation in low seeding density particle image velocimetry

Abstract: This article presents two algorithms for spatial processing of low seeding density PIV (particle image velocimetry) images which lead to sub-pixel precision in particle positioning. The particle centres are estimated to accuracies of the order of 0.1 pixel, yielding 1% error in velocity calculation. The first algorithm discriminates valid particles from the rest of the image and determines their centres in Cartesian coordinates by using a two-dimensional Gaussian fit. The second algorithm performs local correlation between particle pairs and determines instantaneous two-dimensional velocities. The methods have been applied initially to simulated data. Gaussian noise and distortion has then been added to simulate experimental conditions. It is shown that, in comparison with conventional methods, the new algorithms offer up to an order of magnitude higher accuracy for particle centre estimation. Finally, the Gaussian fit approach has been used to map an experimental transonic flow field from the stator trailing edge wake region of a cascade with an estimated error of 1%. The experimental results are found to be in good agreement with previous theoretical steady-state viscous calculations.

Investigation into absorption of the incident laser beam during Nd:YAG laser processing of metals

Abstract: Laser materials processing is highly affected by the existence of surface plasma. The absorption of surface plasma during drilling alters the power intensity distribution of the incident laser beam across the irradiated spot. The present study is carried out to measure the electron number density and temperature using a Langmuir probe while a mathematical formulation is conducted for the absorption coefficients due to electron-ion, electron-neutral atom collisions, inverse Bremsstrahlung, and photoionization processes. Consequently, a computer program is developed to compute the relevant absorption coefficients as well as the overall absorption coefficient. The laser power intensity distribution before and after the plasma absorption is computed at a plane 2.6 mm above the workpiece surface. It is found that 13% of the reduction occurs in the incident laser output power intensity at this plane in the plasma.

Twin-image elimination in optical scanning holography

Abstract: We propose a novel multiplexing technique to solve the twin image problem in optical scanning holography without the use of a spatial carrier, as commonly used in conventional off-axis holography. The technique involves simultaneously acquiring sine and cosine Fresnel zone-lens plate coded images by optical scanning. A complex addition of the two coded images will then be performed and decoded to give a twin-image rejection reconstruction. Computer simulations will be presented to demonstrate the validity of the idea.

Application of the holographic carrier fringe and FFT technique for deformation measurement

Abstract: This paper describes a holographic system based on the use of fibre optics and automatic spatial carrier fringe pattern analysis. Carrier fringes are generated by simply translating the object beam between two exposures. Single-mode optical fibres are used to transfer both the object and reference beams. The fast Fourier transform method is used to process the interferograms: it extracts phase from fringe patterns resulting from the interference of tilted wavefronts. The method is illustrated by measuring the deformation of an arbitrarily clamped, uniformly loaded circular plate. The results are given for the perspective plot of the out-of-plane deformation field, the maps of wrapped and unwrapped phase, and a contour map of the unwrapped phase.

Laser induced damage in GaAs at 1.06 μm wavelength: surface effects

Abstract: An experimental study of single shot damage to monocrystal GaAs by a free running laser pulse of 106 μm wavelength is described In order to see the role of surface effects on laser induced damage, samples of different surface finish were prepared The laser induced damage threshold (LIDT) of lapped GaAs samples is considerably lower than that of the mirror polished samples (ratio 1 2 ) Damage threshold results are analysed in terms of a thermal model incorporating the temperature dependent thermal and optical parameters of GaAs The combined effect of the enhanced surface absorption and high surface recombination velocity of photo-excited carriers significantly enhances the surface temperature rise and reduces the damage threshold value of GaAs The evolution of damage morphology is governed by the mechanical damage caused due to the polishing process

Heating effects induced by a pulsed laser in a semi-infinite target in view of the theory of linear systems

Abstract: The two-dimensional Laplace integral transform technique is used to solve the problem of heating a semi-infinite target induced by the surface absorption of a laser pulse. Mathematical expressions for the surface temperature and the temperature profile within the target are obtained considering cooling and temperature-dependent absorption coefficient at the front surface. The linear behaviour of the system is found to be established for all cases except for that of temperature dependent surface absorptance. As an illustrative example, computations are carried out on Al, Cu, Ag and Au targets using an empirical formula for the temporal distribution of a measured laser pulse.

Optical fibre temperature sensor in the cryogenic range

Abstract: The realization of an optical fibre sensor applied to cryogenic temperature measurement in harsh environments is presented. The measurement principle is based on the analysis of the decay-time of the fluorescence emitted by special doped crystals, the excited state lifetimes of which are greatly dependent on temperature. The sensor is intended to work in the presence of strong perturbations encountered on the testing benches of the liquid hydrogen and liquid oxygen turbo-pumps of the Ariane 5 Vulcan engine developed by `la Societe Europeenne de Propulsion (SEP)?.

Canonical quaternary signed-digit arithmetic using optoelectronics symbolic substitution

Abstract: A higher radix based signed-digit number system, such as the quaternary signed-digit (QSD) number system, allows higher information storage density, less complexity, fewer system components, and fewer cascaded gates and operations. An optoelectronics symbolic substitution scheme to handle the parallel quaternary signed-digit (QSD) arithmetic operations is proposed. A conversion algorithm is employed on the QSD numbers to simplify the addition process and reduce the number of the optical symbolic substitution rules. The optical addition operation of two QSD numbers is performed in one-step. An efficient shared content-addressable memory (SCAM)-based optical implementation of the QSD addition/subtraction operations employs a fixed number of minterms for any operand length. The canonical QSD number addition/subtraction scheme requires a significantly reduced number of minterms when compared with a similar previously reported technique.

Application of acousto-optic interaction for holographic conversion of light fields

Abstract: This paper represents a survey of research on the problem of acousto-optic conversion of optical images. This method of conversion permits us to retain all information that is contained in both amplitude and phase spatial modulation of a light wave. Particular attention is given to acousto-optic visualization of phase objects and registration of optical wavefronts. Information possibilities of the acousto-optic method are illustrated by numerical calculations as well as by some experiments recently performed in this area.

Multi-channel slab CO2 laser excitation with resonant cavities

Abstract: A unique way of driving a multi-channel RF excited slab laser is presented. Resonant cavity techniques are employed to provide high power splitting and impedance transformation. Uniform and isolated power division was observed in eight discharge channels, stacked in a radial array, referred to as the Zodiac geometry. Impedance matching networks were not necessary as the RF cavity splitter creates a near ideal voltage source capable of driving any impedance. With 8 out of the 24 electrodes driven in the radial array, optical powers in excess of 1.2 kW have been observed at efficiencies of 12%.

Thermal processes taking place in the bone during CO2 laser irradiation

Abstract: The present study examines the drilling and heating of a bone due to pulsed CO2 laser irradiation. To obtain temperature profiles inside and at the surface of the bone, electron-kinetic and Fourier theories are used. The study is extended to include an experimental investigation into the measurement of the penetration speed. This is achieved using a fibre-optic system. In order to reduce the degree of burning around the hole side, helium was used as the assisting gas during drilling. It is found that the penetration speeds predicted from the theory are in good agreement with the experimentally obtained values.

The small signal gain and the saturation intensity measurement of the nitrogen-ion laser

Abstract: A series oscillator-amplifier has been fabricated and optimized in order to determine the small signal gain and the saturation intensity of the N2+ amplifying medium. Two plasmas, which are composed of a mixture of helium and nitrogen gases, have been produced under pressures of 2–5 atm by applying an electrical transverse discharge. The time delay between the two plasmas has been controlled by the gas pressure. To optimize the gain, the entrance time of the oscillator's output pulse into the amplifier was synchronized with the maximum population inversion in the amplifier. The gain coefficient ( p out P in ) was obtained from 6 to 124, by varying the intensity of the input beam at the optimum pressure. The small signal gain and the saturation intensity have been calculated as 0.04, 0.07, 0.10 cm−1 and 7.77, 13.33, 19.97 (kW cm−2) at 1.25, 1.70, 2.20 atm, respectively. The dominant wavelength of the nitrogen-ion laser corresponding to the B2∑u(v = 0) → X2∑g(v′ = 1) transition, 427.8 (nm) wavelength, was used in this work.

Display and applied holography in museum practice

Abstract: Possible applications of holography for cultural heritage study, demonstration and restoration, particularly in practical museum applications, are discussed. In this respect two main applications of holography are of interest: display holography and technical holography. The latter includes holographic NDT methods, three-dimensional measurements of objects, memory systems, and so on.

Measurement of the double layer capacitance of aluminium samples by holographic interferometry.

Abstract: In the present investigation, holographic interferometry was utilized for the first time to measure the double layer capacitance of aluminium samples during the initial stage of anodization processes in an aqueous solution without any physical contact. The anodization process (oxidation) of the aluminium samples was carried out chemically in different sulphuric acid concentrations (0.5–3.125% H2S04) at room temperature. In the meantime, a method of holographic interferometry was used to measure the thickness of anodization (oxide film) of the aluminium samples in aqueous solutions. Along with the holographic measurement, a mathematical model was derived in order to correlate the double layer capacitance of the aluminium samples in solutions to the thickness of the oxide film of the aluminium samples which forms due to the chemical oxidation. The thickness of the oxide film of the aluminium samples was measured by real-time holographic interferometry. Consequently, holographic interferometry is found to be very useful for surface finish industries, especially for monitoring the early stage of anodization processes of metals, in which the thickness of the anodized film as well as the double layer capacitance of the aluminium samples can be determined in situ. In addition, a comparison was made between the obtained data of the double layer capacitance from the holographic measurements and the double layer capacitance data obtained from measurements of electrochemical impedance spectroscopy. The comparison indicates that there is good agreement between the data from both techniques.

Reflection and refraction by interfaces of uniaxial crystals

Abstract: We present a simple and complete approach to determining the directions and amplitudes of rays propagating through arbitrary systems of homogeneous and/or uniaxial materials. In the process we extend the work of Gu and Yen to full generality.

Pulse behaviour of a compact RF discharged CO2 slab laser

Abstract: We present the measurements made on a medium power, 35 cm long, radio-frequency discharge CO 2 slab laser operated in a pulsed regime obtained by switching the RF discharge. In this way, pulses with peak power up to 480 W at pulse repetition frequencies in the range 1 to 10 kHz have been obtained. Pulse energy and average extracted power have been experimentally characterized with respect to the pulse repetition frequency and to the pulse duration, and the pulse shape and characteristic times have been measured. We also investigated the propagation properties of the beam emerging from our laser.

Improvement of coupling efficiency between a 0.82 μm wavelength laser diode and a VAD single-mode fibre by conical microlenses

Abstract: Conical microlenses have been fabricated directly on the end faces of single-mode fibres which demonstrate −1.8 dB coupling loss to a 0.82 μm laser diode. Experimental results on coupling loss, lateral and axial offsets are also reported.

Laser-induced damage in InSb at 1.06 μm wavelength—a comparative study with Ge, Si and GaAs

Abstract: This paper deals with the study of laser-induced damage in n-type single crystal InSb irradiated with a Nd: glass laser of 1.06 μm wavelength and 300 μs pulse duration. Samples of different surface quality were prepared by mechanical lapping and polishing by diamond paste. Evolution of damage morphological features observed at different power densities is discussed. Damage threshold results are analysed in terms of a thermal model taking into account the temperature dependence of various physical parameters and using the finite difference method of calculation. A comparative study of laser induced damage in InSb, Ge, Si and GaAs irradiated under similar conditions is also presented.

Phase conjugation, edge detection and image broadcasting using two-beam coupling configurations in photorefractive KNbO3

Abstract: Two-beam coupling configurations are employed to obtain the phase conjugate image and edge extraction of an object using photorefractive KNbO 3 : Fe. Also, the self-organization of a beam in the material into a hexagonal spot array is utilized to broadcast an input object to the location of each of the spots.

Optical testing of hard disks

Abstract: A device and methodology for surface testing of hard disk blanks are presented. A beam of laser light is scanned in a spiral pattern across the disk, and surface defects are diagnosed from the modulation of the scattered light. Since acceptable polish marks also scattered significant levels of light, it was necessary to make use of the directional characteristics of the scattered light in the disk test device.

Discretized amplitudemodulated phase-only filter

Abstract: The amplitude-modulated phase-only filter (AMPOF) effectively utilizes both amplitude and phase information to obtain a narrower and larger autocorrelation peak. To make it readily suitable for real-time target recognition applications, we consider discretizing both its amplitude and phase. Accordingly, both amplitude and phase of the AMPOF are ternerized to yield improved performance statistics of the correlator.

Correlation between roughness and porosity in rocks

Abstract: The porosity of rocks is a very important parameter in the determination of the performance of oil wells. Optical methods allow us to study surface roughness and different materials that have surface properties with random characteristics. Therefore, we have extended these applications to porosity analysis. In our method, we have used the speckle produced by the scattered light from a porous rock, illuminated by a laser beam, and found a linear relationship between the decorrelation of the speckle intensity distribution and the porosity magnitude. In this paper we present the results for samples extracted from oil wells in Argentina.