Small Laser Beam Research Articles

Accurate measurement of small Gaussian laser beam diameters using various rulings

A new analytical approach based on Fourier transform and convolution operations for measuring very small Gaussian laser beam diameters is proposed. In addition, a comparative study of various recently reported rulings is presented. A general rule of thumb approach to select a ruling for measuring small beam diameters is demonstrated. Finally, a new Ronchi ruling capable of measuring extremely small beam radius is proposed.

Automatic measurement of fiber orientation in softwoods by using the tracheid effect

Spiraled grain commonly occurs in softwood trees. Instead of running parallel to the pith, the grain runs spirally around the trunk like a helix. Since wood is an orthotropic material with higher shrinkage perpendicular to than parallel to the fibers, the log will twist when dried, and so will a plank or board cut from it. Several investigations have shown that the magnitude of twist in sawn wood is highly correlated to the fiber orientation, so by measuring the fiber orientation on green lumber, the risk of warp after drying can be indicated. Fiber orientation is also interesting for other purposes, for example stress grading and research applications. This paper concerns the tracheid effect, which utilizes the light conducting properties of the softwood tracheids to measure fiber orientation. A small circular laser beam was projected onto the wood surface. The light was transmitted in the wood and scattered back to form an elliptical shape extended in the direction of the fibers. The ellipse of light was registered with a CMOS camera, and the orientation of the ellipse's major axis was calculated. This method has a correlation coefficient of 0.99996 to manually aligned fiber orientation, and repeated measurements show a standard deviation of 0.2°. Calculation time for one 64×64 pixel image was 67 μs, which must be regarded as useful for industrial applications.

Light-scattering measurements of cavitation from ultrasound contrast agent bubbles

Ultrasound contrast agents are typically micron-sized gas bodies with stabilizing shell coatings. The shell prevents the gas bubble from dissolving. The shell also changes a bubble’s scattering properties and affects a bubble’s destruction. Optimization of the shell can be beneficial for diagnostic imaging and therapeutic ultrasound applications. Thus, understanding agent response to pulsed ultrasound is important. However, because the agents are so small, their response to pulsed ultrasound is difficult to measure. Until recently, the only tool available for measuring contrast agent dynamics was a very expensive high-speed camera. We have been investigating the use of light-scattering to measure aspects of cavitation from contrast agents in which a small HeNe laser beam scatters off a bubble and is focused onto a photomultiplier tube detector. Previous experiments have involved clusters of agents, where collective bubble oscillations and destruction were observed. Our current apparatus is designed to observe single-bubble oscillations from a diagnostic ultrasound system. The advantages for using light-scattering include small cost and the ability to monitor bubble oscillations for extended times. The technique will be described, and representative data samples will be shown.

大出力ＣＯ２レーザ・ＭＩＧハイブリッド溶接特性と開先裕度拡大効果―ハイブリッド溶接による厚鋼板の突合せ溶接特性（第１報）―

There are few applications of laser welding to thick plates joining. One reason is attributed to narrow gap tolerance due to a small laser beam diameter leading to unacceptable seam defects such as underfill and burn through. Another is easy formation of weld imperfection such as porosity and cracking in a keyhole—type of deeply penetrated laser weld beads. Hybrid welding with a laser beam and an arc has been receiving considerable attention as one of the methods to suppress such defects and imperfection. This paper reports fundamental investigation results of hybrid welding with combined high power CO2 laser and Metal Inert Gas (MIG) arc, and especially hybrid welding results of thick plates with such a wide gap that the majority of a laser beam can pass through. The behavior of laser-induced and arc plasmas near the molten pool during the hybrid welding was observed with a black-and-white CCD camera. Penetration depth were confirmed to depend upon the distance between laser beam irradiation location and MIG wire target. The different depths were interpreted in terms of the interaction between laser-induced plasma and molten metal, melt flows, and so on. Moreover, under the formation conditions of the deepest weld beads, the wide gap groove was fully filled with the molten metal. The observation suggests that the recoil pressure due to the laser-induced evaporation should force the molten metal fed with a MIG wire to fill the leading gap and a fully penetrated keyhole was formed during hybrid welding. Consequently, a sound hybrid weld bead could be produced in joining the plates of 22 mm thickness with 4 mm gap, and moreover some weld beads indicated the best grade of the inspection test result according to JIS Z3104.

Beam alignment for scanning beam interference lithography

By interfering two small diameter Gaussian laser beams, scanning beam interference lithography (SBIL) is capable of patterning linear gratings and grids in resist while controlling their spatial phase distortions to the nanometer level. Our tool has a patterning area that is up to 300 mm in diameter. The motive for developing SBIL is to provide the semiconductor industry with a set of absolute metrology standards, but the technology is easily adaptable to other important applications such as the making of high precision optical encoders. In this article, we describe a system for carrying out automated beam alignment for SBIL. Our design goals require tight alignment tolerances, where beam position and angle alignment errors must be controlled to ∼10 μm and ∼10 μrad, respectively. We describe our system setup, and discuss the so-called iterative beam alignment principle, focusing specifically on deriving a mathematical formalism that can guide the development of similar systems in the future. Repeatability experiments demonstrate that our system fulfills the alignment requirements for nanometer-level SBIL writing.

Topographical distribution of pinprick and warmth thresholds to CO 2 laser stimulation on the human skin

We studied the topographical distribution of laser sensory thresholds on the human hairy skin, using a small laser beam for pinprick and a large beam for warmth sensations. The threshold for pinprick sensation correlated positively with the distance from the brain, suggesting that Aδ nociceptors, the fibers which convey pinprick sensation, are more dense at proximal than at distal body sites. This finding adds information to skin biopsy studies of epidermal free nerve endings which showed a similar gradient, but could not differentiate small myelinated from unmyelinated fiber afferents. Possibly because of a diffuse low density of warmth receptors, laser warmth thresholds showed no trend.

Polarimetric study of the optically induced dynamical behaviour in nematic liquid crystal films

We report the photopolarimetric study of the molecular director dynamics observed in a homeotropically aligned nematic liquid crystal layer under the action of a small angle incident s-polarised laser beam. For increasing light intensities several interesting dynamical regimes have been observed. In order to accomplish this aim, we propose a new experimental apparatus which, using a pump-probe technique and a new polarimeter (four-detector polarimeter), allows us to measure with a good resolution the variation in the polarisation state of the laser probe during the molecular reorientation induced by a laser beam in the sample. A preliminary description of the dynamical regimes, based on the analysis of the Fourier spectra and the autocorrelation functions of the measured time series, have been reported.

A Novel Technique to Measure Laser Beam Spot Sizes

In this letter we report a novel technique to measure small laser beam spot sizes. We use the open aperture z-scan technique as a tool to measure the laser beam spot size. This technique measures small spot sizes with accuracy better than 10%.

Vacuum-Ultraviolet Light Sources and Their Applications for Processings. Ablation Using VUV-UV Multiwavelength Excitation.

A novel technique for precision microfabrication of hard materials such as fused quartz and SiC has been developed by using laser ablation with VUV-UV multiwavelength excitation. In this process, VUV laser beam of small laser fluence (several tens mJ/cm2) and UV laser beam of large laser fluence (several J/cm2) are directed to samples at the same time. The essential mechanism for insulators such as fused quartz is strong absorption of UV laser beams to the excited state formed by VUV laser beams (Excited-State Absorption: ESA). However, direct photo-ionization by the VUV laser beam is important for wide bandgap semiconductors such as SiC. Advantages of this process are discussed in comparison of the conventional single wavelength VUV laser ablation.

Tomographic reconstruction of the retina using a confocal scanning laser ophthalmoscope

Retinal imaging with a confocal scanning laser ophthalmoscope (cSLO) involves scanning a small laser beam over the retina and constructing an image from the reflected light. By applying the confocal principle, tomographic images can be produced. However, the thickness of such slices, when compared with the retinal thickness, is too large to give useful 3D retinal images. In this study an algorithm has been developed which fits a double Gaussian curve to the axial intensity profiles generated from a stack of image slices. The underlying assumption is that the laser light has mainly been reflected by two structures in the retina, the internal limiting membrane and the retinal pigment epithelium. From the fitted curve, topographic images and novel thickness images of the retina can be generated. The technique has been applied to three normal volunteers and seven patients with macular pathology (cystoid macular oedema and macular hole) demonstrating the clinical value of the technique. The improvement in accuracy achieved by using a double rather than a single Gaussian is also demonstrated. Keywords: confocal, scanning laser ophthalmoscope, reconstruction, tomographic imaging

Study of a wide-angle laser ranging system for relative positioning of ground-based benchmarks with millimeter accuracy

A wide-angle airborne laser ranging system (WA-ALRS) is developed at the Institut Geographique National (IGN), France, with the aim of providing a new geodesy technique devoted to large (100 km2) networks with a high density (1 km−2) of benchmarks. The main objective is to achieve a 1-mm accuracy in relative vertical coordinates from aircraft measurements lasting a few hours. This paper reviews the methodology and analyzes the first experimental data achieved from a specific ground-based experiment. The accuracy in relative coordinate estimates is studied with the help of numerical simulations. It is shown that strong accuracy limitations arise with a small laser beam divergence combined with short range measurements when relatively few simultaneous range data are produced. The accuracy is of a few cm in transverse coordinates and a few mm in radial coordinates. The results from ground-based experimental data are fairly compatible with these predictions. The use of a model for systematic errors in the vehicle trajectory is shown to be necessary to achieve such a high accuracy. This work yields the first complete validation of modelization and methodology of this technique. An accuracy better than 1 mm and a few mm in vertical and horizontal coordinates, respectively, is predicted for aircraft experiments.

An in-depth analysis of the 'Elymat' technique for characterizing metallic microcontamination in silicon: Experimental validation for iron contamination in p-type wafers

Application of the on-line Elymat technique for measuring bulk carrier recombination lifetime is numerically analysed using a complete three-dimensional carrier transport simulation including the Dember electric field, the characteristics of the laser beam, the transport parameters of the wafer and Shockley-Read-Hall recombination kinetics. We found a strong dependency of the measured lifetime on the shape and power of the laser beam as well as on the deep energy level of the impurity in the gap, owing to nonlinear effects not described by the classical model. Our numerical simulations show that the use of a small laser beam permitting the approximation of point-like excitation gives the most reliable results. The comparison between our numerical simulations and experimental results for iron-contaminated p-type samples with a resistivity between 1 and 18 Omega cm shows that the donor level of Fe-B pairs at Ec-0.3 eV and not the classical acceptor level at Ev+0.1 eV is the predominant site of recombination of Fe-B pairs in p-type silicon in the above doping range. First results for the electron and hole capture cross sections for this defect will be given. Based on these results, the sensitivity of the technique is shown to be less than 1011cm-3 in the case of Fe-B pairs corresponding to less than 10 PPT (parts per trillion).

Droplet Sizing Using the Shifrin Inversion

A method for measuring droplet size distributions was investigated with an emphasis on limitations related to measurements in real spray environments. The method stores a photographic image of a plane of droplets within a spray and is capable of evaluating particle size distributions within the spray, one small region at a time. The method complements droplet velocity measurements made using Particle Image Velocimetry. In a typical experiment, a plane of the spray was illuminated by a laser light sheet and photographed. After processing, a small laser beam scanned the film and a diffraction pattern was generated for each region illuminated by the small laser. The diffraction pattern was inverted using a Shifrin inversion to solve for the particle size distribution within the illuminated region. In this paper we will discuss some of the limitations of this method and indicate one approach which seems to allow for improved inversions using data signal processing.

Significance of Bruch's membrane in the creation of iatrogenic chorioretinal venous anastomosis.

Iatrogenic retinal to choroidal vein anastomoses, as a method of bypassing retinal venous occlusions has been reported in dogs in which Bruch's membrane is poorly formed. In order to determine whether chorioretinal venous anastomoses can be induced in an animal with a Bruch's membrane that is well developed as in humans, pigmented rats were chosen. A high intensity, small spot size argon green laser beam of 514 nm was used to induce the anastomosis. Three out of 5 rat eyes developed a retinal vein to choroidal vein anastomosis. The success rate of iatrogenic retinal to choroidal vein anastomoses in the rat was comparable to that obtained in the dog. This study suggests that Bruch's membrane was not significant in the creation of an iatrogenic chorioretinal venous anastomosis.

Kinetic simulations of ultra-intense laser plasma interactions

Kinetic simulations are used to investigate the interaction of an ultra-intense, small spot-size laser beam with a preformed plasma. The beam is sufficiently intense to drive the electrons relativistically and to impart a pressure in excess of hundreds of Mbars to the plasma. Key features of the interaction include generation of electrons and inward-directed ions with energies in the Mev range and strong hole boring which enhances the absorption. The simulations also show self-generated magnetic fields of order 108 Gauss, a Rayleigh-Taylor like instability of the light-plasma interface, as well as self-focusing of the laser beam and strong Raman backscatter.

Kinetic simulations of ultra-intense laser plasma interactions

Kinetic simulations are used to investigate the interaction of an ultra-intense, small spot-size laser beam with a preformed plasma. The beam is sufficiently intense to drive the electrons relativistically and to impart a pressure in excess of hundreds of Mbars to the plasma. Key features of the interaction include generation of electrons and inward-directed ions with energies in the Mev range and strong hole boring which enhances the absorption. The simulations also show self-generated magnetic fields of order 108 Gauss, a Rayleigh-Taylor like instability of the light-plasma interface, as well as self-focusing of the laser beam and strong Raman backscatter.

脳組織の活動電位分布のレーザー光による光化学的計測

Optical measurement of cortical activity in the brain using laser beam has the following advantages compared to white light: (1) laser beam is easy to be focused on the cell stained with voltage-sensitive dye (the beam waist is nearly equal to the wavelength): (2) the light is monochromatic, then, the overlap between the excitation and the emission from the dye stained in the cell is negligible: (3) the optical measurement system of the cortical activity is a small size and a simple design. We propose the system of the laser image mapping applied to the intact brain with the voltage-sensitive dye. An optical fiber with a short length can be used for transmitting the laser light. The beam spot emerged out of the fiber can be controlled by using SELFOC micro-lens. The optical image mapping of cortical activity from a few cells or aggregated cells is produced from the small or large laser beam spot size.

Nonlocal Transverse Dependence of Molecular Reorientation Induced in a Nematic Liquid Crystal by a Guassian Laser Beam

Abstract We present an analytical treatment of the nonlocal transverse dependence of the molecular reorientation induced by a Gaussian laser beam in a nematic liquid crystal. In the limit of small laser beam width compared to the film thickness, the induced molecular reorientation's transverse dependence exhibits broadening effect. These results are significant in opto-optical switching and self-phase modulation bistability.

Spray Characterization With A Nonintrusive Technique Using Absolute Scattered Light

A technique to measure the size and velocity of particles is discussed, and results are presented. In this technique two small laser beams of one color identify the center of a laser beam of a different color. This defines a region of almost uniform intensity where the light scattered by the individual particles can be related to their sizes. A variation of this technique that uses two polarizations of the same color of laser beam is also presented. Results are presented for monodisperse, bimodal, trimodal, and polydisperse sprays produced by the Berglund-Liu droplet generator and a pressure nozzle. Size distributions obtained at three different ranges for the same spray show excellent self-consistency in the overlapping regions. Measurements of a spray of known characteristics exhibit errors in the order of 10%.

Characterization of small area laser beam annealing

The structure of small area (∼10 micron diameter) spots produced by Q-switched pulsed laser irradiation of n-type silicon covered by a thin film of sputtered aluminum have been studied. Spatially resolved SIMS analysis has been combined with SEM observations to characterize the resulting microstructure. Careful analysis of the SIMS results indicate the spots display subsurface alloying and deep (∼1 μm) aluminum penetration.