Yttrium Aluminum Garnet Laser Beam Research Articles

Impulse Generation and Stabilization Performance of a Doughnut–Spherical Laser Launch System

A conceptual laser launch system of a spherical vehicle propelled by a doughnut-shaped laser beam is expected to accelerate the spherical vehicle continuously to the orbit velocity along a laser beam. Once lateral offset between the axis of the laser beam and the center of the sphere is generated, lateral force is generated to recede the offset. In this study, a pulsed Neodymium-doped Yttrium Aluminum Garnet laser beam of a doughnut-shaped cross section is focused on a 10-mm-diameter aluminum sphere. The impulse components, parallel and lateral to the laser beam, are first investigated experimentally using a pendulum method. As a result, the ratio of the doughnut beam radius to the target radius is found to have a major impact on the impulse. For a relatively small value of at 0.56, at atmospheric pressure, the impulse is enhanced due to the focusing of the toroidal blast wave, whereas the shock focusing effect is negligible for a larger value of at 0.86. On the other hand, must be larger than 0.83 to generate the restoring lateral force.

Fundamental research of laser micro welding for practical use of titanium eyeglass frames

Micro welding of titanium eyeglass frames requires sound strength, good exterior appearance and stable production, which suit micro laser welding. The objective of this work is to fundamentally investigate butt welding conditions with a pulsed yttrium aluminium garnet laser beam for exterior appearance and joint strength. Eleven factors of welding conditions which influence joint strength were evaluated from the viewpoint of quality engineering. It was found that two factors of defocused distance and processing of welding surface of the welding parts led to fluctuation of the joint strength. Therefore, sub-millimetre scale control of the laser irradiated point along the optical axis and the removal of impurities lead to brittle metallic compounds on the surface and produce sound strength of the joints. Furthermore, the optimized laser condition is applied to fabrication of components with the shape of titanium eyeglass frames. The qualities of the obtained joints were confirmed by a stripping test, cyclic bending test, metallographic observation and hardness determination. It was revealed that the laser welding method is more reasonable than conventional resistance brazing methods.

CuInSe2 semiconductor formation by laser annealing

One industrially relevant fabrication method for CuInSe2 absorber layers begins with electrodeposition, which is highly resource efficient and easily up-scalable, followed by a high temperature annealing step. Commonly a furnace is used for annealing, although it is possible to reduce the duration of this step by 2–3 orders of magnitude using laser heating. Past work demonstrated that rastering a 1064nm, continuous wave, neodymium-doped yttrium aluminium garnet laser beam over an electrodeposited precursor film with 1s dwell time, promoted CuInSe2 formation, grain growth, and gave a photovoltaic device with 1.6% power conversion efficiency. However, this device showed inhomogeneous current collection correlating approximately to the Gaussian flux profile of the laser beam. This work demonstrates how deviations in the incident laser flux on the precursor lead to local temperature variation in the film. The temperature affects the rates of atomic diffusion, grain growth and chemical reactions, with the hottest region having the largest crystallites and a stoichiometric and constant composition through the film depth. Spatially resolved photoluminescence yield shows a positive correlation to the temperature profile caused by the Gaussian flux. It is expected that with a homogeneous laser beam flux, spatial variations in absorber layer properties would be eliminated, leading to uniform device properties.

Random lasers in dye-doped polymer-dispersed liquid crystals containing silver nanoparticles

Abstract We report on the observation of random lasers for the first time in dye-doped polymer-dispersed liquid crystals (PDLCs) containing nano-sized liquid crystal droplets and silver nanoparticles. The dye-doped polymer-dispersed liquid crystal containing silver nanoparticles film is exposed by a collimated 532 nm Nd: YAG (yttrium aluminum garnet) laser beam, so that it is quickly cured. Under the excitation of a frequency-doubled Nd: YAG (yttrium aluminum garnet) laser operating at a wavelength of 532 nm, random lasing from dye-doped PDLCs containing Ag nanoparticles is observed as a result of cooperative effect due to light scattering of nano-sized liquid crystal droplets and the local field enhancement capabilities around silver nanoparticles. We show that the threshold of the random lasing is about 0.95 μ J / pulse which is lower than the lasing threshold of dye-doped polymer-dispersed liquid crystals containing nano-sized liquid crystal droplets. The linewidth of the lasing peaks is shown to be 0.2 nm. We also propose a possible mechanism to explain the random lasing from dye-doped polymer-dispersed liquid crystals containing silver nanoparticles.

An Endoscope With 2 DOFs Steering of Coaxial Nd:YAG Laser Beam for Fetal Surgery

This paper reports a rigid endoscope that provides an endoscopic view and transmits a high-power infrared laser beam coaxially to arbitrary points in the view to photocoagulate body tissue, mainly for treatment of twin-to-twin transfusion syndrome. The endoscope system consists of a hot mirror for coaxial transmission of visible light and a neodymium-doped yttrium aluminum garnet laser beam, and an optical galvano scanner for steering the laser beam from the endoscope tip. The optical system of the endoscope is designed for high-power laser transmission, and achieves a small laser beam. The camera and galvano mirrors are calibrated to control the laser spot automatically by pointing a target on the endoscopic image. We evaluated the laser transmission efficiency, firing angle, positioning accuracy, and feasibility of photocoagulation by in vitro experiment. The laser transmission efficiency was 34%. However, the endoscope provided changeable firing angles of 24° in a horizontal direction and 22° in a vertical direction. Furthermore, the positioning accuracy was sufficiently high because the errors in both air and water were mostly smaller than the diameter of target vessels of 1 mm. As an in vitro experiment, we successfully coagulated rabbit's mesenteric vessels by changing the firing angle.

Nearly complete isobar suppression by photodetachment

The efficiency of selective suppression of negative ions by photodetachment in a gas-filled radio frequency quadrupole ion cooler was investigated with a new detection method. A neodymium doped yttrium aluminum garnet laser beam at 1064 nm was used to remove Co− ions in the radio frequency quadrupole cooler and the remaining ions were then probed by photodetachment and neutral particle detection. More than 99.99% suppression of the Co− ions was observed. Under identical conditions, only 20% of a Ni− beam was suppressed. The results demonstrate that this isobar suppression technique can lead to nearly complete elimination of certain isobaric contaminants in negative ion beams, opening up new experimental possibilities in nuclear and atomic research and accelerator mass spectrometry.

Laser-direct photoetching of metal thin film for the electrode of transistor

We report that metal thin films can be directly photoetched by a pulsed neodymium doped yttrium aluminum garnet laser beam irradiating the film surface. This process utilizes a laser-induced thermoelastic force, which plays a role to detach the film from the underlying layer. High-fidelity patterns at the micrometer scales have been fabricated using a spatially modulated laser beam. A zinc-tin-oxide thin film transistor with photoetched Al source and drain electrodes exhibited an on/off ratio higher than 105 and a very low off-current level. This indicates that the metallic layer is completely etched out by this process, making an additional cleaning or etching step unnecessary.

Visualization of refraction and attenuation of near-infrared laser beam due to laser-induced plume

This study was performed to obtain a fundamental knowledge of the interaction between a near-infrared laser beam and an induced plume during welding. The plume was characterized by spectroscopy, and the effect and mechanism of the laser-induced plume on the refraction and/or attenuation were investigated by high-speed video observation of the plume and the probe laser, and power meter measurement of the fiber probe laser beam of 1090 nm wavelength which passed horizontally through the plume formed during bead-on-plate welding of an 8-mm-thick type 304 plate with a 1.5 kW yttrium-aluminum-garnet (YAG) laser beam. The plume induced by YAG laser at the focus position grew about 20 mm toward the incident laser beam and was identified to be nonionized metallic vapor of 3600 K in average temperature on the basis of the spectroscopic analyses. The high-speed observation images and the power measuring results revealed that the rapid movement and the low brightness of the probe laser beam, seen after the plume, were caused by refraction due to density difference between the plume and its environment and Rayleigh scattering due to ultrafine particles, respectively. The maximum refraction and attenuation were 1.2 mrad and 3%, which were much lower than the beam divergence and the same levels as power variation in the incident YAG laser beam under argon (Ar) shielding gas, respectively. Moreover, the YAG laser-induced plume hardly affected the reduction in weld penetration under the defocused conditions since the penetration of type 304 weld made without Ar shielding gas was deeper than that produced with Ar shielding gas although the former plume was longer than the latter one. This reason was interpreted in terms of the greater effect of surface tension-driven melt flows. Consequently, the wavelengths of near-infrared laser beams were desirable for laser welding in the case of the laser-induced plume of less than 20 mm height, owing to their weak optical interaction to the plume.

Doppler frequency up conversion of electromagnetic waves in a slotline on an optically excited silicon substrate

The Doppler frequency up conversion of microwaves in a slotline on an optically excited silicon substrate was experimentally observed. An array of 24 optical fibers with different lengths was used to effectively tilt the wave front of a 532 nm neodymium-doped yttrium aluminum garnet laser beam with a pulse duration of 33 ps. The tilted laser beam produced electron-hole surface plasma whose boundary moved at a relativistic velocity of about c/3.4 (c is the speed of light) along the slotline. The experiments showed that microwaves reflected at the moving boundary of the plasma in the slotline are converted to millimeter waves with a frequency up conversion ratio of 3.82.

Laser–material interaction and process sensing in underwater Nd:yttrium–aluminum–garnet laser welding

Laser–material interaction and process sensing technology for local-dry underwater Nd:yttrium–aluminum–garnet laser beam welding were studied. The optical emissions induced by laser–water interaction were detected with an infrared (IR) optical sensor and observed with a charge coupled device camera. It was found that under laser irradiation, a kind of water–vapor plasma formed immediately above the water surface if the water depth was more than 3 mm. This plasma severely reduced the laser power reaching the workpiece. The water above the workpiece surface must be excluded by forming a local-dry cavity so as to perform underwater laser welding. In local-dry underwater laser welding, a coaxial gas-shielding nozzle was used to form a local-dry cavity. The IR optical signal and weld bead shape variations with various shielding conditions of the local-dry cavity were investigated. The experimental results showed that the IR signals could reflect well the status of the local-dry cavity and predict the weld quality. The optimal shielding condition could be determined by IR signal stability.

Laser forming of thin foil by a newly developed sample holding method

A sample holding method is newly developed for laser forming, in which the target material is held between two transparent plates. During laser beam scanning on the target, the target foil is kept flat. After the scanning is finished, the foil is detached from the holder; the foil bends toward the beam scanned surface. Experiments are carried out with a Nd:yttrium–aluminum–garnet laser beam for a spot size of 25μm and for a traverse speed of 10 mm/s. A bending angle of about 30° is obtained for stainless steel (AISI 304) foil of 10 μm in thickness with a power of 0.3 W and a scanning repetition number of 20. On the other hand, if the laser beam is focused and scanned on the convex side along the ridge line of a curved foil, it bends toward the concave side and the bending angle increases. The increment angle of about 20° for a prebent angle of 45° is obtained for stainless steel foil 20 μm in thickness under the conditions of the power of 1.3 W and scanning repetition number of 10. If the sample is rotated counterclockwise with the increase in bending angle, a total angle of 90° or more is easily obtained. A microcube, of which side length is about 500 μm, and other objects of complicated shapes are made on the basis of the bending characteristics obtained experimentally.

Specific energy for pulsed laser rock drilling

Application of advanced high power laser technology to oil and gas well drilling has been attracting significant research interests recently among research institutes, petroleum industries, and universities. Potential laser or laser-aided oil and gas well drilling has many advantages over the conventional rotary drilling, such as high penetration rate, reduction or elimination of tripping, casing, and bit costs, and enhanced well control, perforating and side-tracking capabilities. The energy required to remove a unit volume of rock, namely the specific energy (SE), is a critical rock property data that can be used to determine both the technical and economic feasibility of laser oil and gas well drilling. When a high power laser beam is applied on a rock, it can remove the rock by thermal spallation, melting, or vaporization depending on the applied laser energy and the way the energy is applied. The most efficient rock removal mechanism would be the one that requires the minimum energy to remove a unit volume of rock. Samples of sandstone, shale, and limestone were prepared for laser beam interaction with a 1.6 kW pulsed Nd:yttrium–aluminum–garnet laser beam to determine how the beam size, power, repetition rate, pulse width, exposure time and energy can affect the amount of energy transferred to the rock for the purposes of spallation, melting, and vaporization. The purpose of the laser rock interaction experiment was to determine the optimal parameters required to remove a maximum rock volume from the samples while minimizing energy input. Absorption of radiant energy from the laser beam gives rise to the thermal energy transfer required for the destruction and removal of the rock matrix. Results from the tests indicate that each rock type has a set of optimal laser parameters to minimize specific energy (SE) values as observed in a set of linear track and spot tests. As absorbed energy outpaces heat diffusion by the rock matrix, local temperatures can rise to the melting points of the minerals and quickly increase observed SE values. Tests also clearly identified the spallation and melting zones for shale samples while changing the laser power. The lowest SE values are obtained in the spalling zone just prior to the onset of mineral melt. The laser thermally spalled and saw mechanically cut rocks show similarity of surface microstructure. The study also found that increasing beam repetition rate within the same material removal mechanism would increase the material removal rate, which is believed due to an increase of maximum temperature, thermal cycling frequency, and intensity of laser-driven shock wave within the rock.

Prevention of welding defect by side gas flow and its monitoring method in continuous wave Nd:YAG laser welding

In the Nd:yttrium–aluminum–garnet (YAG) laser welding of thick plates, reduction of porosity and monitoring the keyhole/molten metal behavior are very important issues to assure the high quality welding. The authors applied a side gas flow to prevent the porosity in the bead on plate welding with 6 kW Nd:YAG laser equipment. At the same time, a reflected Ar+ laser in the same axis as the Nd:YAG laser beam and the light emission from the weld were measured as monitoring signals. Under the optimum side gas condition, pores in the weld metal could significantly decrease, the penetration depth increased slightly, and the bead width became narrower. Under that condition, moreover, the generation of spatters was quite few. An acceptable limit of the transverse misalignment of the side gas nozzle position was about 1 mm. The misalignment could be detected by using the above mentioned monitoring signals.

Formation of GaAs nanocrystals by laser ablation

The pulsed laser ablation method has been used to form GaAs nanocrystals. A quadrupled frequency Nd:yttrium–aluminum–garnet laser beam is focused onto a GaAs single crystal target, and a nitrogen flowing gas is sent at the neighborhood of the target in order to transfer in an ethanol bath, the nanoparticles grown in gas phase. The composition of the particles is close to stoichiometry and transmission electron microscopy analyses highlight zinc-blende GaAs nanocrystals with a rather well defined size: 5–8 nm diameter. The low temperature photoluminescence and photoluminescence excitation spectra show quantum confinement of about 870 meV via an emission band in the visible range (500–560 nm) for the GaAs nanocrystals produced by laser ablation without any postannealing treatment.

High-resolution edge Thomson scattering measurements on the Alcator C-Mod tokamak

A high-resolution Thomson scattering diagnostic is in operation on the Alcator C-Mod tokamak, measuring radial profiles of electron temperature and density at the plasma edge. Photons are scattered from a Nd–yttrium–aluminum–garnet laser beam pulsed at 30 Hz (1.3 J, 8 ns pulse), and are measured by a filter polychromator with four spectral channels. The polychromator measures Te in the range of 15–800 eV and ne of 0.3–3×1020 m−3. Twenty scattering volumes are located about the last closed flux surface, spaced for a nominal resolution of 1.3 mm in midplane radial coordinates. High resolution is essential for measuring edge Te and ne profiles on C-Mod, since these quantities exhibit gradient scale lengths as small as 2 mm in H mode. The steep profiles at the H mode edge are fit to a parameterized pedestal function for ease of analysis. Measured profiles are compared with edge profiles from electron cyclotron emission and visible continuum diagnostics.

Laser cladding technology to small diameter pipes

A laser cladding method which produces a highly corrosion-resistant material coating layers (cladding) on the austenitic stainless steel (Type 304 SS) pipe inner surface was developed to prevent SCC (stress corrosion cracking) occurrence. This technology is applicable to a narrow and long distance area from operators, because of the good accessibility of the YAG (Yttrium–Aluminum–Garnet) laser beam that can be transmitted through an optical fiber. In this method a mixed paste metallic powder and heating-resistive organic solvent are firstly placed on the inner surface of a small pipe, and then a YAG laser beam transmitted through an optical fiber irradiates to the pasted area. A mixed paste will be melted and form a cladding layer subsequently. A cladding layer shows as excellent corrosion resistance property. This laser cladding (LC) method had already applied to several domestic nuclear power plants and had obtained a good reputation. This report introduces the outline of laser cladding technology, the developed equipment for practical application in the field, and the circumstance in actual plant application.

Enhanced methods to get high average power by combining yttrium–aluminum–garnet laser beams and their characteristics in processing materials

We investigated two related topics as the first stage of our research: the characteristics of welding with multiple yttrium–aluminum–garnet (YAG) laser beams transmitted through optical fibers and combined at the focal point; and the study of methods of combining YAG laser beams. Each laser beam must be inclined in such a combined system. The plume and the penetration changed their direction from the vertical direction of the specimen to the parallel one of an irradiated beam, if the peak power or energy of the irradiated YAG laser beam exceeds a threshold value. In the two continuous wave YAG laser beams irradiation the form of plume and welding penetration were the same at the average angle of inclination of the two beams of 15° and at typical total power levels as those with a single and vertically irradiated beam to the specimen surface, but were split above the angle of inclination of 30° and total average power of 3.3 kW. Two keyholes would be formed under the split penetration condition. The three split plumes were observed for irradiation by three combined beams. The second stage of our research was the development of an integration optics based on these experiments, which could focus the laser beams transmitted through optical fibers with a condensing lens. We studied all position welding characteristics using the developed integration optics and obtained the result that the overhead welding has the maximum fusion volume among the three orientations—horizontal, vertical, and overhead position welding. The result will be attributed to the direct reach of laser beam to the unfused bottom metal due to the molten metal being pulled down from around the keyhole by gravity. The integration optics is too heavy and large for an articulated robot to hand it and move with high speed. This difficulty comes from combining laser beams at the exit sides of optical fibers. Then, we developed a new launching optics of laser beams at an entrance side of an optical fiber in order to overcome the disadvantage. The development of the launching optics is described.

Laser-induced electron acceleration in a counterpropagating rf field.

Laser-driven electron acceleration by means of two counterpropagating waves is investigated, where the comoving field is an intense laser light beam, the other is a rf wave beam. A striking feature of the interaction is the presence of a ponderomotive potential propagating with phase velocity slower than the speed of light, allowing one to achieve wave-particle matching. Electron trajectories in the combined field are derived analytically, following the plane-wave and single-particle approximations. Special attention has been given to analyzing the phase slippage of an electron in the field, and the potential of accelerating electrons by means of this scheme. The results show that a neodymium-doped yttrium aluminum garnet laser beam with wavelength 1.06 \ensuremath{\mu}m and electric field intensity 4\ifmmode\times\else\texttimes\fi{}${10}^{10}$ V/cm is capable of accelerating electrons from 43 to 258 MeV over 0.27 m by using a rf field of wavelength 10.6 cm and an electric field of ${10}^{5}$ V/cm. The averaged accelerating gradient is 1038 MeV/m.

Photodeposition sputtering of titanium and TiN thin films

The interaction of any energy-carrying beam with solids leads to the degration of the solid crystals. The bombarded surface is sputtered and many effects on the atomic, microscopic and macroscopic scales can be observed. Very few experiments have been performed on the photodeposition sputtering of thin films, and the results obtained have been attributed to a variety of processes. In this work the results of the sputtering of and the morphological changes in titanium and TiN thin films induced by a point-focused neodymium-doped yttrium aluminium garnet laser beam are presented. The morphological changes, the shape and the size of the damaged area as functions of the laser beam density and the film thickness were investigated by scanning electron microscopy and profilometry studies. The results of the laser-induced damage have shown that the applied laser power density (in the range from 2 × 10 7 to 9 × 10 9W m −2) and the film thickness (from 22 to 2200 nm) were two very important parameters which strongly influenced the growth of the damage (exfoliation, hydrodynamic and thermal effects). The sputtering yield is a function of the laser beam power density. Its value increases with increasing power density and after reaching a maximum begins to fall owing to the plasma screening effect of the target.

Enamel surface roughness and dental pulp response to coaxial carbon dioxideneodymium: YAG laser irradiation

The purpose of this study was to evaluate the effect of a coaxial carbon dioxide/neodymium : yttrium aluminium garnet laser beam on enamel surface roughness and the dental pulps of mongrel dogs. In four dogs, four maxillary left posterior teeth were irradiated at 16 cm source-tooth distances. Two teeth were irradiated with 16 W CO 2,/16 W Nd : YAG and the remaining two with 16 W CO 2,/40 W Nd : YAG. Two maxillary right teeth were untreated controls. In addition, mandibular premolars were irradiated at the same distance and power levels, extracted, and analysed for surface roughness. Significant differences in surface roughness were found between control samples and either power level, but not between enamel surfaces at the two power levels. Maxillary teeth were removed at 10 days postoperatively, sectioned and stained (H > E). The reaction of pulpal cells to irradiation was scored. Data analysis revealed statistically significant differences between the control and lower power Nd : YAG groups and between the control and higher power Nd : YAG groups. The difference in pulpal response between both laser groups approached significance.