Laser Beam Axis Research Articles

Cross-type optical separation of elastic oblate capsules in a uniform flow

The dynamic behavior of an elastic capsule with an initially oblate spheroidal shape during cross-type optical separation was numerically investigated. The penalty immersed boundary method was adopted for the fluid-membrane interaction, and the optical force calculation was conducted by using the ray optics method including the ray-surface intersection algorithm. The oblate elastic capsule of b/a = 0.5 with different surface Young's moduli and different initial inclination angles was considered. The oblate capsule with higher surface Young's moduli was less deformed, and was more migrated for each initial inclination angle. Unlike the oblate rigid particle, the initially inclined capsules with moderate inclination angles were similarly migrated since the oblate elastic capsule was deformed during rotation near the laser beam axis. The oblate capsules can be separated according to the surface Young's modulus, except for nearly non-inclined capsules. As the fluid velocity decreased, the migration distance increased. The maximum deformation parameter was insensitive to the fluid velocity. Furthermore, a new dimensionless number (Sec) was introduced to predict the migration distance of the oblate elastic capsule.

Femtosecond Laser Fabrication of Periodical Structures in Bulk of Transparent Dielectrics

We present results of experiments in development technology of precision micromachining of materials by focusing the femtosecond laser radiation into the bulk of transparent materials. Conditions of the realization and some key parameters of special regime of micromachining have been defined. It implements at certain ratio between scanning speed and repetition rate of femtosecond laser pulses. Under mentioned conditions the area destroyed by laser radiation moves along the optical axis near focus region in both directions, forming (during transverse scanning) in a bulk of the sample “periodical” structures. In results of our investigation we demonstrate precision cutting crystals, glasses, polymers and creating cylindrical cavities 1÷2 microns in diameter with aspect ratio more than 200 directed along the axis of laser beam.

Investigation into the effect of Ar ratio in He-Ar mixture side assisting gas on plasma properties during CO2 laser welding based on 3D reconstruction

In this paper, a set of synchronous CCD cameras were employed for image taking during CO2 laser welding with He-Ar mixed side assisting gas. The cameras recorded the projection of laser-induced plasma in different directions during welding process. 3D reconstruction based on algebraic reconstruction technology was performed through the projected 2D views of plasma. The volume, height on laser beam axis, and spatial inclination of plasma were extracted from the 3D reconstructed data as plasma characteristics. These characteristics were analyzed to investigate the effect of Ar ratio in He-Ar side assisting gas on plasma properties related to laser energy transfer as well as welding process stability. The experimental results demonstrate that an appropriate Ar ratio in the side assisting gas can help to suppress the plasma height interacting with incident laser and to increase the stability of welding process.

Experimental and numerical study of laser-assisted machining of Ti6Al4V titanium alloy

Laser-assisted machining combines several experimental parameters such as cutting speed, feed rate, depth of cut, laser power and distance between tool rake face and the laser beam axis. The optimization of these parameters is necessary to ensure the efficiency of assistance and to increase productivity. This paper focuses on the understanding of the physical phenomena during laser-assisted machining, and on optimising this process. This contribution is based on an experimental and a numerical study. The experimentalpart highlights the effects of the laser power as well as the distance between the tool rake face and the axis of the laser beam. As for the numerical part, it was performed on the ABAQUS/Explicit software.The proposed model improves the understanding of the physical phenomena of chip formation and the cutting force reduction when machining with laser assistance. In addition, this model allows a better optimization of laser and cutting parameters.Numerical findings generally corroborate experimental results and can lead to some other information difficult to catch experimentally.The main contention in the paper is that the distance between the axis of the laser beam and the tool rake face is the most important parameter that controls the reduction of the cutting force. This cutting force reduction can exceed 50%.

Optical separation of ellipsoidal particles in a uniform flow

The behavior of an ellipsoidal particle subjected to a vertical optical force by a loosely focused laser beam in a uniform flow was studied numerically. The fluid flow and the particle motion were separately solved and coupled using the penalty immersed boundary method, and the optical force was calculated using the dynamic ray tracing method. The optical force and optically induced torque on the ellipsoidal particle varied according to the aspect ratio and initial inclination angle. The ellipsoidal particle, whose major axis was initially aligned with the laser beam axis, was more migrated as the aspect ratio increased. The migration distance also depended on the initial inclination angle, even for a given ellipsoidal particle shape. As the laser beam power increased and the flow velocity decreased, the effect of the initial inclination angle increased. The ellipsoidal particles with different aspect ratios could be effectively separated if the rotation along the spanwise direction was suppressed. Moreover, the migration distance could be predicted analytically by introducing a new dimensionless number Sc to represent the ratio of the optical force to the viscous force for the ellipsoidal particles.

Dipolar distribution generated by laser-induced plasma (LIP) in air in earliest instants

We present an experimental investigation of the electric field potential and magnetic field generated by a laser-induced plasma in air, for time delays 0 ⩽ t ⩽ 50 ns. The laser used is a Nd : YAG, λ = 1064 nm, 10 ns at FWHM, and the irradiance applied is I = (1011–1012) W cm−2. We find that the collective effect of the charges in the plasma form a dynamic dipole distribution aligned with the laser beam axis. This experimental result is explained based on the mobility of the electric charges detected by mapping near the plasma with a coaxial cable probe. Shadowgraphy and fast photography techniques show that the plasma ionization front advances asymmetrically and mostly toward the lens. The intrinsic dipole moment is estimated by applying an external electric field. The magnetic diagnostic shows the presence of a current aligned with the laser beam that gives rise to an azimuthal magnetic field, corroborating the observed dipolar configuration.

Shape control of elemental distributions inside a glass by simultaneous femtosecond laser irradiation at multiple spots

The spatial distributions of elements in a glass can be modulated by irradiation with high repetition rate femtosecond laser pulses. However, the shape of the distribution is restricted to being axially symmetric about the laser beam axis due to the isotropic diffusion of photo-thermal energy. In this study, we describe a method to control the shape of the elemental distribution more flexibly by simultaneous irradiation at multiple spots using a spatial light modulator. The accumulation of thermal energy was induced by focusing 250 kHz fs laser pulses at a single spot inside an alumino-borosilicate glass, and the transient temperature distribution was modulated by focusing 1 kHz laser pulses at four spots in the same glass. The resulting modification was square-shaped. A simulation of the mean diffusion length of molten glass demonstrated that the transient diffusion of elements under heat accumulation and repeated temperature elevation at multiple spots caused the square shape of the distribution.

Obtaining Slots and Channels by using a 1070 nm Wavelength Laser

Slots and channels are sometimes necessary in machine building; the surfaces of the slots and channels could be obtained by means of a laser beam machining equipment. Essentially, the material removal from the workpiece needs the development of thermal phenomena of melting and vaporizing and of dynamic phenomena, respectively, by which the melted and vaporized material is moved from the machining zone. The paper includes the results of the researches concerning the possibilities to use 1070nm wavelength laser equipment whose output power is of 300W, in order to obtain slots and channels in metallic test pieces. The sizes of the parameters able to characterize the machined surface are influenced by the machining conditions. The theoretical analysis of the machining process highlighted the necessity of sending a compressed air jet along the laser beam axis and of an adequate selection of the machining parameters. Experimental researches were developed by changing the speed of the relative motion between the laser spot and the test piece. Empirical relations were established by mathematical processing of the experimental results. The empirical relations highlight the influence exerted by speed between the test piece and the laser beam on the depth of the channels thus generated.

Experimental characterization of bi-directional terahertz emission from gold-coated nanogratings

The THz radiation emission of Au-coated nanogratings (fused silica substrate, 30 nm Au layer thickness, 500 nm grating constant) upon fs laser irradiation (785 nm, 150 fs, 1 kHz, ≤1 mJ/pulse) is observed in both directions along the laser beam axis (forward and backward) and for both, illumination of the Au/air or the Au/silica interface. THz radiation along the laser beam propagation is emitted in a narrow solid angle of about 15° fwhm independent on the laser pulse fluence, the angle of incidence and the nanograting profile. The bar width and groove depth of the nanograting as well as the angle of laser beam incidence strongly affect the THz radiation yield. The energy of single THz light pulses is measured absolutely (2 fJ in the 0.3–0.38 THz range) using a highly sensitive and fast superconducting transition edge sensor. The bi-directional emission of THz radiation is in agreement with the model assumption of surface plasmon polaritons propagating simultaneously on both Au layer interfaces (Au/air and Au/silica).

Measurement of the Transient Velocity of Nanoparticles in Flowing Nanofluids by Using the Method of Dynamic Laser Speckles

Experimental equipments are designed for the zero-crossing method of dynamic laser speckles to investigate the moving nature of nanoparticles in flowing nanofluids. After the experiments the velocity of nanoparticles in flowing nanofluids is determined according to its relationship with the zero-crossing rate of speckle intensity fluctuation. The results show that the zero-crossing rate is very sensitive to the velocity of nanoparticles when the velocity of nanofluids ranges from 17.6 mm/s to 60.2 mm/s. A TEM10 mode–like laser beam is designed and taken as an incident light to detect the moving direction of nanoparticles in flowing nanofluids. The moving direction of nanoparticles is parallel to the horizontal axis of the TEM10 mode-like laser beam when the zero-crossing rate reaches maximum. Experimental results show that the moving direction of nanoparticles can also be detected by using the zero-crossing method.

Beam Riding Performance of Asymmetrically Propelled Laser Vehicle

To improve the in-air flight performance of a laser propulsion vehicle, it is helpful to develop a computational model that can reproduce full dynamics, including impulsive vehicle motion driven by interaction with a blast wave. The authors have developed a three-dimensional hydrodynamics code coupled with six-degree-of-freedom equations of motion of a laser propulsion vehicle for analyzing flight dynamics through numerically simulating flowfield interacting with unsteady motion of the vehicle. Using ray tracing, asymmetrically deposited energy corresponding to laser offsets was initially added to the flowfield around the vehicle to estimate beam riding mechanics against lateral or angular offset, and a combination of them for a single pulse. The centering performance of the lightcraft is excellent for lateral and combined offsets, but the recentering force becomes weak with a large angular offset. If the vehicle inclines toward the axis of the incident laser beam, an angular restoring moment is generated to reduce the angular offset. Also, flight dynamics simulations were performed for multiple pulses by integrating aerodynamic responses based on single-pulse results, and spiral-flight motion and deviation from the laser path were successfully reproduced. When the laser axis is rotated following the gyro motion of the vehicle, the angular restoring moment is maintained for a longer time, and the vehicle is able to fly to a higher altitude.

Optical levitation of a non-spherical particle in a loosely focused Gaussian beam

The optical force on a non-spherical particle subjected to a loosely focused laser beam was calculated using the dynamic ray tracing method. Ellipsoidal particles with different aspect ratios, inclination angles, and positions were modeled, and the effects of these parameters on the optical force were examined. The vertical component of the optical force parallel to the laser beam axis decreased as the aspect ratio decreased, whereas the ellipsoid with a small aspect ratio and a large inclination angle experienced a large vertical optical force. The ellipsoids were pulled toward or repelled away from the laser beam axis, depending on the inclination angle, and they experienced a torque near the focal point. The behavior of the ellipsoids in a viscous fluid was examined by analyzing a dynamic simulation based on the penalty immersed boundary method. As the ellipsoids levitated along the direction of the laser beam propagation, they moved horizontally with rotation. Except for the ellipsoid with a small aspect ratio and a zero inclination angle near the focal point, the ellipsoids rotated until the major axis aligned with the laser beam axis.

Development of a laser-induced plasma probe to measure gas phase plasma signals at high pressures and temperatures

The ability of laser induced breakdown spectroscopy (LIBS) technique for on line simultaneous measurement of elemental concentrations has led to its application in a wide number of processes. The simplicity of the technique allows its application to harsh environments such as present in boilers, furnaces and gasifiers. This paper presents the design of a probe using a custom optic which transforms a round beam into a ring (Donut) beam, which is used for forming a plasma in an atmosphere of nitrogen at high pressure (20bar) and temperature (200°C). The LIBS experiments were performed using a high pressure cell to characterize and test the effectiveness of the donut beam transmitted through the LIBS probe and collect plasma signal in back scatter mode. The first tests used the second harmonic of a Nd:YAG laser, pulse width 7ns, to form a plasma in nitrogen gas at five different pressures (1, 5, 10, 15 and 20bar) and three different gas temperatures (25, 100 and 200°C). The uniqueness of this probe is the custom made optic used for reshaping the round laser beam into a ring (Donut) shaped laser beam, which is fed into the probe and focused to form a plasma at the measurement point. The plasma signal is collected and collimated using the laser focusing lens and is reflected from the laser beam axis onto an achromatic lens by a high reflection mirror mounted in the center section of the donut laser beam. The effect of gas pressure and temperature on N(I) lines in the high pressure cell experiment shows that the line intensity decreases with pressure and increases with temperature. Mean plasma temperature was calculated using the ratios of N(I) line intensities ranging from 7400K to 8900K at 1bar and 2400K to 3200K at 20bar for the three different gas temperatures. The results show that as a proof of principle the donut beam optics in combination with the LIBS probe can be used for performing extensive LIBS measurements in well controlled laboratory environment as well as harsh and demanding environments of practical devices at both high pressures and temperatures.

Acoustooptical visualisation of biological media using multislit diaphragms

The influence of special multislit diaphragms placed in front of the receiver cathode on the signal value and signal-to-noise ratio in acoustooptical visualisation of scattering biological media is studied. Using such diaphragms the visualisation of a bulk lightabsorbing and sound-transmitting object in the form of a rectangular parallelepiped with the size 5 × 5 mm in the plane, perpendicular to the laser beam axis, and with the size 2.5 parallel to this axis is carried out. The object was merged into scattering media of thickness Lc = 62 mm along the direction of propagation of the optical radiation with the parameter μLc varying from 0 to 46 (μ being the extinction coefficient of the medium).

Absorption and scattering of laser radiation by the diffusion flame of aviation kerosene

The absorption coefficient of the radiation of a repetitively pulsed Nd : YAG laser with an average output power up to 6 W and of a cw ytterbium optical fibre laser with an output power up to 3 kW was measured in the diffusion flame of aviation kerosene burning on a free surface in the atmospheric air. The absorptioncoefficient as a function of flame length, radiation power, and radiation intensity, which was varied in the ∼103 — 5×104 W cm-2 range, was obtained for two distances (1 and 2 cm) between the laser beam axis and the surface. The coefficient of radiation absorption by kerosene flame was compared with that in ethanol and kerosene — ethanol mixture flames. The radiation power scattered by a small segment of the kerosene flame irradiated by Nd : YAG laser radiation was measured as a function of longitudinal and azimuthal coordinates. An estimate was made of the total scattered radiation power.

Formation of silver nanoparticles inside a soda-lime glass matrix in the presence of a high intensity Ar+ laser beam

Formation and motion of the silver nanoparticles inside an ion-exchanged soda-lime glass in the presence of a focused high intensity continuous wave Ar+ laser beam (intensity: 9.2 × 104 W/cm2) have been studied in here. One-dimensional diffusion equation has been used to model the diffusion of the silver ions into the glass matrix, and a two-dimensional reverse diffusion model has been introduced to explain the motion of the silver clusters and their migration toward the glass surface in the presence of the laser beam. The results of the mentioned models were in agreement with our measurements on thickness of the ion-exchange layer by means of optical microscopy and recorded morphology of the glass surface around the laser beam axis by using a Mirau interferometer. SEM micrographs were used to extract the size distribution of the migrated silver particles over the glass surface.

Nanoscale Probing of Dielectric Interfaces with Single-Molecule Excitation Patterns and Radially Polarized Illumination

We present new results on the theoretical and experimental study of the excitation patterns acquired upon scanning a single molecule through the focal region of a radially polarized laser beam when the emitter is located at a glass–air interface and in bulk glass. Our calculations show that the presence of a dielectric interface, which coincides with the focal plane of a radially polarized laser beam, strongly changes the field intensity distribution within the focus, which results in the modification of the single molecule excitation pattern obtained upon scanning a molecule through the focal region. By changing the dielectric properties of the surrounding media in situ we show experimentally that the same individual molecule possessing the same orientation with respect to the laser beam axis exhibits different shapes of excitation patterns, depending on whether it is located on the dielectric interface or in medium with constant refractive index.

Study on Attraction of Laser to Arc Plasma in Laser-TIG Hybrid Welding on Magnesium Alloy

This paper presents the results of investigation on the attraction of laser to the electric arc plasma in laser-tungsten inert gas (TIG) hybrid welding of magnesium alloy AZ31B plates. By comparably estimating the characteristics of arc plasma, including the shape, the electron temperature, and density of the arc plasmas in hybrid welding and single TIG welding, three interactions between laser and arc plasma in hybrid welding are managed to be distinguished. The influences of laser power, intensity of arc current, and the distance between laser beam axis and tungsten electrode tip ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">D</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">la</sub> ) on the attraction are analyzed. Moreover, a new mechanism for attraction is proposed by a physical model in the viewpoint of electromagnetic interaction between the currents. It is thought that there exists a current in the welding keyhole in hybrid welding, and the electromagnetic force between this current and arc plasma current is the driving force of the attraction. Attraction happens when the driving force overcomes the stiffness of the electric arc.

Measurement of Concentration Distribution of Hydrogen Gas Flow by Measuring the Intensity of Raman Scattering Light

An experimental study to visualize and measure the concentration distribution of hydrogen gas flow using the Raman scattering was performed. A Nd:YAG laser of wavelength at 355 nm was used, and the beam pattern was transformed into a rectangle and a sheet beam was formed. The Raman scattered light was observed at a right angle with respect to the laser beam axis using a gated ICCD camera and an interference filter. Shadowgraph images were obtained at the same condition. The Raman scattering light image from atmospheric nitrogen was first acquired and the function of Raman scattering light acquisition and the background light suppression was confirmed. Next, images of the Raman scattering light image and shadowgraph of hydrogen gas discharged from a nozzle into the atmosphere were acquired. The two obtained Raman images were compared and the spatial concentration distribution of the flow of the hydrogen gas at different flow rates was calculated. This method is effective for visualizing the gas flow and measuring the concentration distribution of the Raman active molecules, such as hydrogen gas.

Laser repetitive pulse heating of tool surface

Laser heating of a cemented carbide tool is considered and the temperature field as well as phase changes in the heated region is modeled. Temperature rise, liquid layer thickness, and mushy size are predicted numerically. A control volume approach is introduced to solve the governing equations of heat transfer and phase change. Consecutive pulses with the duty cycle of 60% are accommodated in the simulations in line with the experimental conditions. An experiment is carried out to treat the cemented carbide tool surfaces using the CO 2 laser delivering consecutive pulses. The treated surfaces and their cross-sections are examined using the scanning electron microscope (SEM). It is found that the temperature gradient is high along the laser beam axis resulting in cracks at the irradiated surface. The rapid solidification of the surface causes compact structures with very fine grains in the surface region of the laser irradiated spot.