Showing papers in "Optics and Laser Technology in 1998"
TL;DR: In this article, the authors describe the fabrication of infrared optical modulators for use in high dynamic range and high frame rate scene generation, which is achieved by coating a suitable substrate with a thin film of vanadium dioxide via a reactive sputtering process.
Abstract: The continued proliferation of imaging infrared (I2R) missile systems has created the need for fully representative infrared scene generators. In order to test these I2R systems correctly a very large dynamic range is required of the scene generator. This cannot easily be produced by the standard approach of an array of suspended resistor heater elements. This paper therefore describes the fabrication of infrared optical modulators for use in high dynamic range and high frame rate scene generation. Modulation is produced by the heating of a variable reflectivity coating. This is achieved by coating a suitable substrate with a thin film of vanadium dioxide (VO2) via a reactive sputtering process. VO2 undergoes a semiconductor to metal phase transition at approximately 68°C and the associated change in reflectivity is exploited to create the modulator. The correct stoichiometry of the thin film of VO2 is critical in producing high frame rate devices. Both transmissive and reflective modulators are described.
60 citations
TL;DR: In this article, the effect of the supersonic gas jets on the cut quality is explained based on two dimensional steady state gas dynamic theory, computer simulation and shadowgraphic techniques.
Abstract: During laser cutting of stainless steels, titanium and aluminum alloys, a coaxial and high pressure inert gas jet is used to improve the cut edge quality. The process normally consumes a large amount of inert gas and has a poor tolerance to variation in process parameters. This is solely because the gas nozzles are mostly of the conical and convergent type in which the gas jets are subsonic. Based on two dimensional steady state gas dynamic theory, computer simulation and shadowgraphic techniques, the gas jet patterns from conical nozzles and the newly designed supersonic nozzles are analyzed. The distribution of pressure, momentum, gas density and existence of shock waves are predicted and mapped. Based on these characteristics, the effect of the gas jets upon the cut quality is explained. It is concluded that a supersonic gas jet offers the best flow characteristics for high pressure laser cutting.
48 citations
TL;DR: In this paper, an optical system for the evaluation of transient deformations is described, in which a ruby laser producing four pulses has been used to increase the temporal resolution, and four separate digital holograms are recorded in a few microseconds on three CCD sensors.
Abstract: An optical system for the evaluation of transient deformations will be described. In order to increase the temporal resolution, a ruby laser producing four pulses has been used. Four separate digital holograms (one hologram for each pulse) of the test object are recorded in a few microseconds on three CCD sensors. The Fourier method is used for the quantitative evaluation of the digital holograms. The phases are obtained from the complex amplitudes, and the deformation at different times is calculated from phase subtraction. Experimental results are presented.
47 citations
TL;DR: In this article, the authors used a 60 W-cw high power diode laser (HPDL) to determine the feasibility and characteristics of sealing the void between adjoining ceramic tiles with a specially developed grout material having an impermeable enamel surface glaze.
Abstract: Work has been conducted using a 60 W-cw high power diode laser (HPDL) in order to determine the feasibility and characteristics of sealing the void between adjoining ceramic tiles with a specially developed grout material having an impermeable enamel surface glaze. A two-stage process has been developed using a new grout material which consists of two distinct components: an amalgamated compound substrate and a glazed enamel surface; the amalgamated compound seal providing a tough, heat resistant bulk substrate, whilst the enamel provides an impervious surface. HPDL processing has resulted in crack free seals produced in normal atmospheric conditions. The basic process phenomena are investigated and the laser effects in terms of seal morphology, composition and microstructure are presented. Also, the resultant heat affects are analysed and described, as well as the effects of the shield gases, O2 and Ar, during laser processing. Tiles were successfully sealed with power densities as low as 500 W/cm2 and at rates up to 600 mm/min. Contact angle measurements revealed that due to the wettability characteristics of the amalgamated oxide compound grout (AOCG), laser surface treatment was necessary in order to alter the surface from a polycrystalline to a semi-amorphous structure, thus allowing the enamel to adhere. Bonding of the enamel to the AOCG and the ceramic tiles was identified as being principally due to van der Waals forces, and on a very small scale, some of the base AOCG material dissolving into the glaze.
47 citations
TL;DR: In this article, a simple technique using cavity stability conditions is demonstrated to measure the effective dynamic thermal lensing of a flashlamp-pumped Cr, Tm, Ho:YAG laser operated in a low repetition-rate regime of 4-7 Hz.
Abstract: A simple technique using cavity stability conditions is demonstrated to measure the effective dynamic thermal lensing of a flashlamp-pumped Cr, Tm, Ho:YAG laser operated in a low repetition-rate regime of 4–7 Hz. By measuring the flashlamp energy where a stable resonator becomes unstable, the effective focal length of the laser rod can be calculated. When no mode restricting aperture is used in the cavity, the thermal lens behaviour displays a significant deviation from the commonly assumed thin lens approximation.
44 citations
TL;DR: In this article, a different and modified configuration was presented in order to obtain the curvature fringes by incorporating phase shifting and image processing techniques, in which optical fibers were used for obtaining shear fringes in which they have used Wollaston prism as the shear element.
Abstract: We present here a different and modified configuration in order to obtain the curvature fringes by incorporating phase shifting and image processing techniques. Optical fibers are used for obtaining shear fringes in which we have used Wollaston prism as the shear element.
39 citations
TL;DR: In this article, a different procedure is adopted to measure birefringence directly as a single quantity in terms of the thicknesses of mica plates and the wavelengths at which they act as quarterwave or halfwave phase retarders.
Abstract: Birefringence of muscovite mica (ne−no) is usually calculated from the dispersion relations of the indices ne and no. As small differences between relatively large experimentally-measured values, the few available data for birefringence of mica are widely distinct and sometimes contradict basic experimental facts. A different procedure is adopted in this work to measure birefringence directly as a single quantity in terms of the thicknesses of mica plates and the wavelengths at which they act as quarterwave or halfwave phase retarders. Birefringence is found to decrease gradually from −0.0040 at 420 nm to −0.0046 at 700 nm in conformity with pertinent independent experimental results.
34 citations
TL;DR: In this paper, an estimation of the heat loss by conduction can be obtained from measurements of the surface temperature and an overall heat balance at the clad laser interaction zone through an inverse calculation of the boundary temperature from observed surface temperatures, along with the variation in the clad height expected during laser cladding.
Abstract: An estimation of the heat loss by conduction can be obtained from measurements of the surface temperature and an overall heat balance at the clad laser interaction zone. Through an inverse calculation of the boundary temperature from observed surface temperatures the powder catchment efficiency can be estimated along with the variation in the clad height expected during laser cladding. This method shows a possible way to monitor and control the clad height and profile as required by near net shape manufacturing methods based on laser cladding.
34 citations
TL;DR: In this article, the ten second order moments of a simple TEM 8,0 Hermite-Gaussian beam and a twisted beam generated from the TEM8,0 mode were determined.
Abstract: The general astigmatic beam can be characterized by its ten second order moments in first order approach. Most intensity moments, except the beam twist, can be determined by measuring the intensity in a reasonable number of positions around the waist of the beam. The beam twist is determined by applying a rotated cylindrical lens. The ten intensity moments of two kinds of astigmatic beam were determined: a simple astigmatic TEM 8,0 Hermite–Gaussian beam and a twisted beam generated from the TEM 8,0 mode. The experimental results were compared with the theoretical calculations and demonstrate that the ten second order moments of a beam can be determined in a rather simple way.
33 citations
TL;DR: In this article, the authors summarized the theory and highlighted the danger of using the wrong LIDT units in the context of potentially hazardous materials, laser safety eyewear and laser safety screens.
Abstract: The commonly used units of measurement for laser induced damage are those of peak energy or power density. However, the laser induced damage thresholds, LIDT, of all materials are well known to be absorption, wavelength, spot size and pulse length dependent. As workers using these values become divorced from the theory it becomes increasingly important to use the correct units and to understand the correct scaling factors. This paper summarizes the theory and highlights the danger of using the wrong LIDT units in the context of potentially hazardous materials, laser safety eyewear and laser safety screens.
32 citations
TL;DR: In this article, a portable 60 Wcw high power diode laser (HPDL) and a specially developed grout material having an impermeable enamel surface glaze have been tested in order to determine the mechanical, chemical and physical characteristics of the seals.
Abstract: Ceramic tiles sealed using a portable 60 Wcw high power diode laser (HPDL) and a specially developed grout material having an impermeable enamel surface glaze have been tested in order to determine the mechanical, chemical and physical characteristics of the seals. The work showed that the generation of the enamel surface glaze resulted in a seal with improved mechanical and chemical properties over conventional epoxy tile grouts. Both epoxy tile grout and laser generated enamel seals were tested for compressive strength, surface roughness, wear, water permeability and acid/alkali resistance. The enamel seal showed clear improvements in strength, roughness and wear, whilst being impermeable to water, and resistant (up to 80% concentration) to nitric acid, sodium hydroxide and detergent acids. The bond strength and the rupture strength of the enamel seal were also investigated, revealing that the enamel adhered to the new grout and the ceramic tiles with an average bond strength of 45–60 MPa, whilst the rupture strength was comparable with the ceramic tiles themselves. The average surface roughness of the seals and the tiles was 0.36 and 0.06 μm, respectively, whilst for the conventional epoxy grout the average surface roughness when polished was 3.83 μm and in excess of 30 μm without polishing. Life assessment testing revealed that the enamel seals had an increase in actual wear life of 2.9 to 30.4 times over conventional epoxy tile grout, depending upon the corrosive environment.
TL;DR: The merit of the proposed non-contact tracker for calculating the angle of rotation of a HMD (head-mounted display) is that the computer will change the spatial view to match the user's head posture and keep the `immersion' sensation at high accuracy and precision.
Abstract: In this study a non-contact tracker for calculating the angle of rotation of a HMD (head-mounted display) is proposed. The merit of our proposed method is that the computer will change the spatial view to match the user's head posture and keep the `immersion' sensation at high accuracy and precision. Our results are validated using moire fringes as an excellent HMD tracker.
ARCO1
TL;DR: In this article, a technique based on two-dimensional fast Fourier fringe analysis for the automated evaluation of the inclination angle of parallel fringe patterns is described and numerical results of the angle evaluation error using this algorithm are discussed.
Abstract: In this paper we describe a technique based on two-dimensional fast Fourier fringe pattern analysis for the automated evaluation of the inclination angle of parallel fringe patterns. Analysis of noise-free theoretical parallel fringe patterns is presented and numerical results of the angle evaluation error using this algorithm are discussed. We also demonstrate an application of the method to the measurement of the inclination angle of experimental parallel fringe patterns.
TL;DR: In this article, five and six-temperature models for the CO 2 -N 2 -He system were used to describe the process of the dynamic emission in the TEA CO 2 laser.
Abstract: Five and six-temperature models for the CO 2 –N 2 –He system are used to describe the process of the dynamic emission in the TEA CO 2 laser. All physical constants and relaxation rates related to these models are examined to estimate the output pulse parameters as a function of the input parameters. The two pumping processes implemented; empirical function and differential equation show a good agreement with the experimental data.
TL;DR: In this paper, an excitation and optical extraction geometry suitable for compact high power gas lasers is described, where multiple slab discharges are established in a diffusion-cooled radial electrode array.
Abstract: An excitation and optical extraction geometry suitable for compact high power gas lasers is described. Multiple slab discharges are established in a diffusion-cooled radial electrode array. Each gain channel is independently driven from a common RF source via a resonant-cavity power distribution system. Radial excitation augmented with multi-channel self-injection phase locking provides stable optical power extraction at good efficiency. The concept is easily scalable to very high powers while dramatically reducing unit size and cost.
TL;DR: In this paper, a nonlinear spatial filtering is realized due to the electro-optical effect in spatial light modulators (SLMs) for optical inspection of photolithography masks, wavefront sensor, etc.
Abstract: Nonlinear spatial filtering is realized due to the electro-optical effect in spatial light modulators (SLMs). Optical methods with nonlinear filtering are suggested for various optical applications such as optical inspection of photolithography masks, wavefront sensor, etc. A fast optical response of the SLM allows optical inspection with a video (or faster) rate. The optical system proposed is tolerant to the vibrations and temperature variations because of the dynamic properties of the SLM.
TL;DR: This paper briefly discusses laser forming and the development of a basic process monitoring and control system used to overcome variability problems due to the complex nature of the lasers themselves and the manner in which they interact with material.
Abstract: Laser forming is a process that uses the energy of relatively high powered lasers to cause permanent deformation to components by inducing localised thermal stresses. It is envisaged that this material processing technique will find a number of commercial applications. This paper briefly discusses laser forming and the development of a basic process monitoring and control system used to overcome variability problems due to the complex nature of the lasers themselves and the manner in which they interact with material. It then goes on to show how the basic control system was modified, using increased feedback data sampling, time delays and a modified control algorithm which takes account of the forming rate in addition to the error. The effect of these developments is then illustrated by a series of tests which show the modifications significantly improve process tolerances.
TL;DR: A new method for meauring one's blink in the new human–machine interfaces has been developed using a diagonal-box checker search to find the feature subset and features that are needed so that it can save much searchtime.
Abstract: A new method for meauring one's blink in the new human–machine interfaces have been developed in this paper. The major part of eyeball tracking technology is the feature extraction of the eyes and in the experiment we use a diagonal-box checker search to solve the problem. Prior to diagonal-box checker search, we find the feature subset and features that are needed so that we save much searchtime. The opening-and-closing action of the eyes results in other controlling commands that will be used for driving the robots to working or dialing the phone system.
TL;DR: Efficient parallel schemes for carry-propagation-free addition of modified signed-digit trinary numbers are presented and some dc bias is necessary to increase the signal-to-noise ratio of the optical circuit.
Abstract: Efficient parallel schemes for carry-propagation-free addition of modified signed-digit trinary numbers are presented. The necessary minterms for implementation using an optical programmable logic array area are derived. The proposed schemes require only a truth table of 25 entries compared with an earlier scheme of 625 entries. The proposed schemes are amenable to optical implementation. Experimental results using an optical programmable logic array are demonstrated. The experiments show the problems of noise and crosstalk. This suggests some dc bias is necessary to increase the signal-to-noise ratio of the optical circuit.
TL;DR: In this article, an interferometric fiber-optic temperature sensing system employing a light emitting diode (LED) as the broadband optical source and a single mode fiber coil which was fabricated in a definite length as the sensor head is described.
Abstract: An interferometric fibre-optic temperature sensing system employing a light emitting diode (LED) as the broadband optical source and a single mode fibre coil which was fabricated in a definite length as the sensor head is described. A reference fibre line transmitting back and forth along the same path as the sensing transmitting fibre is used, so the change caused by the environmental temperature fluctuation of the fibre path can automatically be compensated. The sensitivity of the sensing system can be easily improved by using the long length of the sensing fibre. The experiment results of the sensing gauge length versus the sensitivity are given. Two typical wavelengths (1300 and 1550 nm). LED sources are used, it is shown that the sensitivity of the system between the two wavelengths is different. The experimental curve of the resolution characteristic of the system related to the length of sensing fibre coil is also discussed.
TL;DR: In this article, a comparison of theoretically-predicted optimum cutting speeds for decorative ceramic tile with experimentally-derived data is presented, where four well-established theoretical analyses are considered and applied to the laser cutting of ceramic tile, i.e., Rosenthal's moving point heat-source model and the heat balance approaches of Powell, Steen and Chryssolouris.
Abstract: This paper presents a comparison of theoretically-predicted optimum cutting speeds for decorative ceramic tile with experimentally-derived data. Four well-established theoretical analyses are considered and applied to the laser cutting of ceramic tile, i.e. Rosenthal's moving point heat-source model and the heat balance approaches of Powell, Steen and Chryssolouris. The theoretical results are subsequently compared and contrasted with actual cutting data taken from an existing laser machining database. Empirical models developed by the author are described which have been successfully used to predict cutting speeds for various thicknesses of ceramic tile.
TL;DR: In this paper, the authors used a scanning probe microscope/laser combination to perform nanostructuring on insulator and metal surfaces in air, which enabled processing of structures with a lateral resolution of approximately 10 nm.
Abstract: We report preliminary results of using a scanning probe microscope/laser combination to perform nanostructuring on insulator and metal surfaces in air. This technique enables processing of structures with a lateral resolution of approximately 10 nm. In this paper we present our last structuring results with both scanning tunnelling and scanning force microscopy. Some possible interaction mechanisms responsible for the structuring will be discussed.
TL;DR: A development of the “locales” approach to position estimation to achieve the desired objective of high accuracy PIV measurements, which makes use of the fact that by considering all the possible digital representations of the Gaussian particle profile, regions of indistinguishable position can be derived.
Abstract: The analysis of digital PIV data, either derived from CCD technology or through film and then scanned, typically involves two quantization steps: spatial and intensity quantization. The all-optical systems do not introduce these sources of error. For systems which make use of digital technology however, it is of crucial importance to have reliable error bounds and a sufficiently accurate estimate of particle position, taking into consideration both types of quantization. The accuracy demanded by aerodynamicists from PIV has been a major barrier to its practical application in the past. The more recent approach of using the Gaussian profile of the particle images to yield sub-pixel accurate position estimates has resulted in robust measurements being taken to an accuracy of 1/10th pixel and 1% in velocity for the in-plane velocity, in hostile industrial environments. A major problem for 3D PIV estimation has historically been that the out-of-plane velocity error was of the order of 3–4 times larger than in-plane. The out-of-plane velocity estimate can be derived from the change in the ratio of amplitude to variance—known as the depth factor—of the Gaussian form, as a particle traverses the beam profile. However, such measurements are crucially dependent not only on an accurate position estimate but also on an equally accurate estimate of the amplitude and variance. The accuracy of the Gaussian profile fit using a Nelder–Meade optimisation method as developed until now however, is not capable of providing the required accuracies. Therefore, this paper presents a development of the “locales” approach to position estimation to achieve the desired objective of high accuracy PIV measurements. This approach makes use of the fact that by considering all the possible digital representations of the Gaussian particle profile, regions of indistinguishable position can be derived. These positions are referred to as “locales”. By considering the density, distribution, and shape of these locales, the available precision can be estimated and an accurate (no worse than 0.5% error for a typical PIV image) in-plane velocity, accuracy can be obtained; while at the same time providing estimates of the depth factor with an error of approximately 0.8%. This paper describes the implementation of an efficient algorithm to provide velocity estimates to an accuracy of at least 0.5% in-plane, together with a discussion of the required constraints imposed on the imaging. The method was validated by creating a synthetic PIV image with CCD-type noise. The flow being analysed is that of flow past the near wake of a cylinder at a Reynolds Number of 140,000. This image was then analysed with the new method and the velocity estimates compared to the CFD data for a range of signal-to-noise ratios (SNR). For a realistic SNR of 5, the accuracy of the method is confirmed as being at least 0.5% in-plane. Finally, the algorithm was used to map an experimental transonic flow field of the stator trailing edge region of a full-size annular cascade with an estimated error of 0.5%. The experimental results are found to be in good agreement with a previously reported steady state viscous calculation and PIV mapping.
TL;DR: A quaternary signed-digit number representations-based arithmetic unit performs parallel one-step addition, multiplication and division using the symbolic substitution technique to reduce the number of the computationrules involved in the computation rules.
Abstract: A quaternary signed-digit number representations-based arithmetic unit is proposed. The arithmetic unit performs parallel one-step addition (subtraction), multiplication and division. We use the symbolic substitution technique to reduce the number of the computation rules involved in the computation rules. Fast parallel nonrecoded quaternary signed-digit multiplication is proposed using our proposed one-step quaternary signed-digit adder. Also, parallel quaternary signed-digit division is performed in constant time by exploiting an iterative conversion algorithm where in every iterative step a negation operation, an addition operation and two multiplication operations are performed. The execution times of the proposed QSD operations are proportional to log2 n, where n are the length of operands.
TL;DR: In this paper, the authors demonstrate image processing techniques such as edge enhancement and phase contrast by using an optically addressed liquid crystal spatial light modulator (OASLM) in the frequency plane of an optical 4f processor.
Abstract: In this paper we demonstrate image processing techniques such as edge enhancement and phase contrast by using an optically addressed liquid crystal spatial light modulator (OASLM) in the frequency plane of an optical 4f processor. The transfer function of the device is derived on the basis of the Jones formalism. Faced with a lack of a general theory for such non-linear optical processors, we show that an analogy to the propagation of optical pulses in fibres is helpful for the understanding of the image processing operation.
TL;DR: In this article, a unidirectional electrooptic modulator based on an asymmetric highly multi-mode waveguide coupler is proposed, which can achieve high switching performance in a guided-wave coupler.
Abstract: A unidirectional electrooptic modulator based on an asymmetrical highly multi-mode waveguide coupler is proposed. Firstly, the energy distributions of all the modes within two highly multi-mode waveguides are analyzed with eigenstate theory. Then, to achieve high switching performance in a guided-wave coupler, a dumping wall is placed on the larger waveguide of an asymmetric pair of waveguides, so that the larger waveguide has a dumping effect. This dumping effect makes the asymmetric highly multi-mode waveguide coupler possess a highly efficient unidirectional coupling process. Due to the large cross-section of the waveguide, a new modulation electrode structure is built. Based on this dumping effect, the unidirectional coupling process is theoretically modeled. Furthermore, in both scenarios, i.e., the unmodulated state and the electrooptically modulated state, the unidirectional coupling efficiencies are studied. As a result, not only can a high unidirectional coupling efficiency of 100% be achieved, but a high electrooptic modulation depth more than 90% can also be implemented in theory, and a high thermooptic modulation depth more than 90% under a low modulation voltage of 30 volts is also experimentally achieved. This device can be used for multi-mode optical interconnection systems such as data communication and fiber sensor networks.
TL;DR: In this article, an in-process vibration-assisted laser cladding method was designed for improving the shrinkage cavities of pulsed Nd:YAG laser ceramic-metal composite cladding on Al-alloys.
Abstract: An in-process vibration-assisted laser cladding method was designed for improving the shrinkage cavities of pulsed Nd:YAG laser ceramic–metal composite cladding on Al-alloys. The substrate was vibrated during the cladding process. A signal generator in this study externally modulated the frequency, amplitude, and wave shape of the vibration. An acceleration sensing device was used to monitor the vibration condition to ensure the modulation was correct. The feasibility of using the in-process vibration-assisted method for improving the shrinkage cavities of pulsed Nd:YAG laser SiC+Al–Si cladding on an Al-alloy was then experimentally evaluated. The experimental results show that the in-process vibration-assisted method significantly improves the degree of cavity shrinkage when the penetration depth is not very great. Analysis of the data shows that with vibration-assisted cladding, the depth of penetration increases with frequencies between 200 and 1500 Hz. The results were especially significant for 800 mV modulation vibration.
TL;DR: In this article, a simple and improved method is proposed for testing laser beam collimation by a new type of grating, comprising concentric equilateral triangles and the mirror images of these triangles.
Abstract: A simple and improved method is proposed for testing laser beam collimation by a new type of grating, comprising concentric equilateral triangles and the mirror images of these triangles. Although this type of grating possesses one-dimensional periodicity in a particular direction and inherent defects in periodicity, the moirefringe pattern formed by placing an exact replica of such a grating at its self-imaging plane shows that the self-imaging phenomenon takes care of the defects in the periodic objects.
TL;DR: In this paper, a diffractive optical element with 16 levels was designed by YG amplitude-phase retrieval and iteration algorithm to convert a given singlemode He-Ne laser beam into a ring-shaped intensity distribution.
Abstract: To convert a given single-mode He–Ne laser beam into a ring-shaped intensity distribution, a diffractive optical element with 16 levels was designed by YG amplitude-phase retrieval and iteration algorithm. This beam shaper is investigated experimentally and compared with the results of the computer design. The results show good conformity, and the measured diffraction efficiency is 87.2%. In addition, the diffractive effect is observed when the distance between diffractive optical element and CCD is changed.
TL;DR: In this paper, several types of fluorocarbon polymer (FCP)-coated silver hollow glass waveguides have been fabricated for Er:YAG laser delivery by using the improved wet chemical technique and dynamic coating procedure.
Abstract: Several types of fluorocarbon polymer (FCP)-coated silver hollow glass waveguides have been fabricated for Er:YAG laser delivery by using the improved wet chemical technique and dynamic coating procedure. The straight losses of 2 m long 700 μm∅ and 540 μm∅ waveguides are 0.4 and 1.0 dB, respectively. The transmission losses of these waveguides are below 1.5 dB even when the waveguides are bent to 180° with the bending radius of either 20 or 15 cm. The waveguides with the small diameters of 320 and 200 μm have also been developed for clinical treatment, which exhibit low enough transmission losses for Er:YAG laser light.