Direction Of Laser Beam Research Articles

Laser-induced plasma image velocimetry

A novel velocimetry method is proposed for point velocity measurement, which is based on tracking a laser-induced plasma in a flow. The plasma’s behaviour is first analysed spatially, temporally and spectrally in quiescent air. The dependence of this technique on the delay time between subsequent plasma images and the processing methods are described. It is found that, for optimized operation of the technique in a turbulent air jet (exit diameter 10.0 mm from a 480 mm long pipe; with an averaged velocity of 50 m/s at the jet exit resulting in Reynolds number of 34,000) with 100 µs time delay between plasma images, the systematic and random components of the velocity uncertainty are − 0.51 m/s and ± 3.6 m/s along the laser beam direction, and 1.25 m/s and ± 0.86 m/s along other directions perpendicular to the laser beam. These uncertainties are mainly caused by the asymmetric laser energy deposition during the formation of plasma, and the associated spatial resolution (in this realisation of the instrument) of 5 mm. The mean velocity measurements in the turbulent air jet flow are consistent with the reported flow behaviour in the literature for mean velocity: the turbulent intensity of axial velocity fluctuations is comparable to those in the literature but difference arises due to the limited spatial resolution. This velocimetry method is an alternative to traditional tracer-based velocimetry methods, because it does not require ‘seeding’ of particles or other substances in the flow. It also has the ability to measure local gas mixture composition, using laser-induced breakdown spectroscopy approach, simultaneously with flow velocity, but this aspect is not explored in the current study.Graphical abstractConcept of Laser-Induced Plasma Image Velocimetry (LIPIV) applied to a turbulent jet flow. The LIPIV technique measures the flow velocity vector by using the temporal displacement of the laser-induced plasma in a flowing fluid. For this reason, two sequential images of the plasma with time delay Δt are used.

A Normal Sensor Calibration Method Based on an Extended Kalman Filter for Robotic Drilling.

To enhance the perpendicularity accuracy in the robotic drilling system, a normal sensor calibration method is proposed to identify the errors of the zero point and laser beam direction of laser displacement sensors simultaneously. The procedure of normal adjustment of the robotic drilling system is introduced firstly. Next the measurement model of the zero point and laser beam direction on a datum plane is constructed based on the principle of the distance measurement for laser displacement sensors. An extended Kalman filter algorithm is used to identify the sensor errors. Then the surface normal measurement and attitude adjustments are presented to ensure that the axis of the drill bit coincides with the normal at drilling point. Finally, simulations are conducted to study the performance of the proposed calibration method and experiments are carried out on a robotic drilling system. The simulation and experimental results show that the perpendicularity of the hole is within 0.2°. They also demonstrate that the proposed calibration method has high accuracy of parameter identification and lays a basis for high-precision perpendicularity accuracy of drilling in the robotic drilling system.

Role of photostriction and photothemal effects in photon induced deflection of Si microcantilevers

Laser beam (He-Ne – 6328 A0) induced deflection in uncoated, Au, and Al coated Si microcantilevers (MCs) of various dimensions was studied, using an AFM head, to examine the induced stress contribution from photothermal and photostriction phenomenon. The laser beam was made to incident sequentially on front and back surfaces of the MC, by flipping it, and results were compared in terms of deflection magnitude, direction of bending and response time. In case of uncoated MCs, it was observed that the induced deflection was always in the direction of the incident laser beam with a fast response. In contrast, in metal (Au, Al) coated MCs, deflection direction was found to be dependent on laser beam exposure side with two orders of enhanced deflection sensitivity and slow response time. The former is attributed to photostriction effect while the latter is explained on the basis of photothermal effect in bimetallic MCs. Flipping experiments on bimettalic MCs, further revealed that stress contribution from these two source mechanisms crucially depends on MC dimensions and direction of laser exposure.

General Method for the Identification of Crystal Faces Using Raman Spectroscopy Combined with Machine Learning and Application to the Epitaxial Growth of Acetaminophen.

Crystal morphology is one of the key crystallographic characteristics that governs the macroscopic properties of crystalline materials. The identification of crystal faces, or face indexing, is an important technique that is used to get information regarding a crystal's morphology. However, it is mainly limited to single crystal X-ray diffraction (SCXRD) and it is often not applicable to products of routine crystallizations becasue it requires high quality single crystals in a narrow size range. To overcome the limitations of the SCXRD method, we have developed a robust and convenient Raman face indexing method based on work by Moriyama et al. This method exploits small but detectable differences in Raman spectra of crystal faces caused by different orientations of the crystallographic axis relative to the direction and polarization of the excitation laser beam. The method requires the compilation of a Raman spectral library for each compound and must be built and validated by SCXRD face indexing. Once the spectral library is available for a compound, the identity of unknown crystal faces (from any crystal that is larger than laser beam) can be inferred by collecting and comparing the Raman spectra to spectra within the library. We have optimized this approach further by developing a machine-learning algorithm that identifies crystal faces by performing a statistical comparison of the spectra in the Raman library and the Raman spectra of the unknown crystal faces. Here, we report the development of the Raman face indexing method and apply it to three different epitaxial systems: Acetaminophen (APAP) grown as an overlayer crystal on d-mannitol (MAN), d-galactose (GAL), and xylitol (XYL) substrates. For each of these epitaxial systems, the crystals were grown under various experimental conditions and have a wide range of sizes and quality. Using the Raman face indexing method, we were able to perform high-throughput indexing of a large number of crystals from different crystallization conditions, which could not be achieved using SCXRD or other analytical techniques.

Calibration method for laser beam direction and zero point of laser displacement sensor

Calibration method for laser beam direction and zero point of laser displacement sensor

Dynamics and density distribution of laser-produced plasma using optical interferometry

Dynamic evolution and spatio-temporally resolved density profiles of laser-produced plasma in air were investigated using optical interferometry. A series of interferograms were obtained with a pulse energy of 50 mJ and delay times from 50 ns to 2650 ns. With increasing delay time, the expansion profiles of the shock wave change from a flat ellipsoid to a spheroid. The phase shift has been extracted using a two-dimensional (2D) fast Fourier transformation algorithm and the radial distribution of the refractive index is calculated using the inverse Abel transformation assuming that the plasma is axisymmetric along the direction of the incident laser beam. Interferograms of the 2D expansion and evolution of the plasma plume and the shock wave were obtained by exploiting the spatial dependence of the refractive index. The electron densities in the plasma region and the air densities in the compressed air region were calculated from the refractive-index distributions obtained. Our results provided further understanding of the expansion and the dynamic evolution of the laser-produced plasma and the shock wave and of the spatio-temporal evolution of the density of plasma in air.

Dense yttria-stabilized zirconia obtained by direct selective laser sintering

This work demonstrated the possibility to manufacture dense yttria-stabilized zirconia (YSZ) parts by direct laser beam sintering using a commercial 3D SYSTEMS machine. To achieve this, several requirements of the process itself had to be met. First, an efficient laser-matter interaction had to be achieved. Indeed, a major issue to overcome was the intrinsic nonabsorbance property of the YSZ powder at the Nd:YAG laser wavelength. This issue was solved by adding a small amount of graphite to the YSZ powder that resulted in a simple mechanical blending of the two compounds. The graphite adjunction allowed an increase in the absorbance value from 2% to approximately 60%. The chemical composition of the powder blend and the density and the particle size distribution of the starting material used in this work enabled adequate heat transfers due to the appropriate thermophysical properties. Finally, a set of machine parameters that allows to manufacture parts with a high relative density, good geometrical accuracy, and mechanical strength was experimentally determined. Ultimately, it was found that the direct SLS allows the reproducible manufacture of simple YSZ parts with a relative density of 96.5%.

Optical Trapping and Orientation Manipulation of 2D Inorganic Materials Using a Linearly Polarized Laser Beam

AbstractBecause inorganic nanosheets, such as clay minerals, are anisotropic, the manipulation of nanosheet orientation is an important challenge in order to realize future functional materials. In the present study, a novel methodology for nanosheet manipulation using laser radiation pressure is proposed. When a linearly polarized laser beam was used to irradiate a niobate (Nb6O174-) nanosheet colloid, the nanosheet was trapped at the focal point so that the in-plane direction of the nanosheet was oriented parallel to the propagation direction of the incident laser beam so as to minimize the scattering force. In addition, the trapped nanosheet was aligned along the polarization direction of the linearly polarized laser beam.

A quick method to fabricate large glass micromodel networks

The study of multi-phase fluid flow in microfluidic devices provides an opportunity for researchers to characterize effective factors and mechanisms in microscale. The application of microtechnology in various fields of science and engineering has always raised different technical problems that require further research and developments. This paper aims to address the associated challenges with micromodel studies that employ the chemical (wet) etching method for making glass micromodels. To overcome these challenges, recent advances in chemical etching are modified and combined to decrease the construction time and costs. The general steps in the chemical etching process are masking, etching, and bonding. First, the common masking step [projecting the designed pattern (regular or irregular) on the acid-resistant layer] in this work is simplified by engraving the mirror plates with a CO2 laser. This new technique skips the required facilities for photoresist layer deposition and UV lithography by using direct laser beam machining of pre-coated soda lime glasses (mirror). Moreover, the variety of mirror products and flexible functionality of laser machines make it possible to create larger size models with any desirable flow pattern. Next, the general composition of solution used in the etching step and the operating conditions of thermal bonding step are modified based on recent investigations in literature to enhance the mechanical strength of the micromodel. Finally, the fabricated model with this procedure is applied in a two-phase flow visualization study to examine the practical features under experimental conditions.

Radiation Pressure Induced Hierarchical Structure of Liquid Crystalline Inorganic Nanosheets

Although hierarchical assemblies of colloidal particles add novel structure-based functions to systems, few local and on-demand colloidal structures have been developed. We have combined the colloidal liquid crystallinity of two-dimensional inorganic particles and laser radiation pressure to obtain a large hierarchical and local structure in a colloidal system. The scattering force of the laser beam converted the parallel nanosheet alignment to the direction of the incident laser beam. At the focal point, the nanosheet orientation depends on the electric field of the polarized laser beam. In contrast, a giant tree-ring-like nanosheet texture of more than 100 μm, and which is independent of the polarization direction, was organized at the periphery of the focal point. This organization resulted from a cooperative effect between the liquid-crystalline nanosheets, which indicates an effectiveness of optical manipulation to construct hierarchical colloidal structures with the aid of interparticle interactions.

Automated System for Femtosecond Laser Writing of Photonic Structures

An automated system is reported in this work with the goal of fabricating photonic structures by the direct-write technique with femtosecond laser pulses. This technique uses a fine focused laser beam into transparent substrates and the translation of the substrate parallel or perpendicular to the propagation direction of the incident laser beam. Control procedures related to laser pulses and the sample translation were implemented and integrated, giving rise to a fully automated femtosecond writing system. Functional straight-line and curved waveguides have been successfully achieved with the automated writing system, where single-mode propagation is observed from their near-field profiles when 635-nm light is injected and guided. In addition, the possibility of writing 2D and 3D photonic structures is confirmed by tracing a logarithmic spiral structure (2D) and a fan-out structure (3D). These results show that the developed system is fully operational in practice for the purpose of fabricating a variety of photonic structures with potential applications in the modern communication networks and integrated photonic circuits.

Novel dynamic wavefront control scheme for ultra-fast beam smoothing

In inertial confinement facilities, the irradiation uniformity of the lasers is highly required to suppress the laser plasma instabilities. In order to realize the ultrafast smoothing of the focal spot, a novel scheme by using an optical Kerr medium and a high-power pump laser is proposed. The principle of the ultrafast beam smoothing scheme is to change the refractive index of the Kerr medium with the pump laser, which appends a spatiotemporal wavefront to the main laser beam in the beamline. The dynamic wavefront modulation of the main laser beam further makes the speckles within the focal spot redistributed rapidly and complicatedly, which contributes to the smoothing of the focal spot. A Gaussian beam with a temporal profile of a Gaussian pulse train is obliquely incident on the optical Kerr medium at a small angle. As a result, the spherical wavefront of the main laser beam is rapidly changed in the direction perpendicular to the propagation direction of the main laser beam. Thus the transverse and the radial redistribution of the speckles within the focal spot are both generated simultaneously. Comparing with the simple radial smoothing scheme, the spherical phase of the main laser beam always changes perpendicularly to the propagation direction in the novel scheme, and thus achieving a more stable beam smoothing effect. Besides, the phase gradient in the center region of the main laser beam changes greatly over time, making the irradiation uniformity on the focal plane further improved. The optimal deflection angle in the optical Kerr medium of the pump laser is obtained. By controlling the deflection angle of the pump laser, the spatial period of the pump laser in the transverse direction is set to be equal to the waist diameter of the main laser, which is identical with one color cycle in the typical smoothing by spectral dispersion technique. Moreover, a relatively low control precision of the deflection angle of the pump laser is required.

Annular laser beam based direct metal deposition

The paper provides a description of a direct annular laser beam based metal deposition head, which makes use of axial material feeding and variation of the laser beam intensity distribution (LBID) on the surface of the workpiece. The application and advantages of the developed head are presented by processes of direct metal droplet, metal wire and powder deposition. Due to the achieved process symmetry, the stability and robustness of the processes are increased. This is the most clearly seen in the case of metal droplet deposition, which is an inherently unstable process. In the case of wire deposition, a precise pre-set of the proportion of energy input into the workpiece surface is enabled, which has been shown to be an important parameter affecting the stability and process outcome. In the case of laser powder deposition, the benefits of the developed head are reflected in achieved powder catchment efficiency greater than 80 % and in the head’s ability to influence the deposited layer properties by adjusting the LBID.

Cutting thin glass by femtosecond laser ablation

The femtosecond laser ablation process for cutting thin aluminoborosilicate glass sheets of thickness 100 μm was investigated with emphasis on effective cutting speed (Veff) and mechanical strength of diced samples. The process parameters including the laser fluence (F), overlap ratio (r) of the laser beam and polarization direction were varied at a fixed pulse repetition rate f = 1 kHz to find the optimal process condition that maximizes Veff and edge strength. A three-point bending test was performed to evaluate the front-side and back-side bending (edge) strength of the laser-cut samples. Veff was proportional to F unless r exceeded a critical value, at which excessive energy began to be delivered at the same spot. The front-side edge strength was bigger than the back-side strength because of the back-side damages such as chipping. Good edge strength, as high as ∼280 MPa (front-side) and ∼230 MPa (back-side), was obtained at F = 19 J/m2, r = 0.99, with laser polarization vertical to the cutting path.

Femtosecond-laser-induced damage initiation mechanism on metal multilayer dielectric gratings for pulse compression

The femtosecond-laser-induced damage behaviors of metal multilayer dielectric gratings (MMDG) for pulse compression are explored. The grating ridge of this type of MMDG consists of a layer of HfO2 sandwiched between two SiO2 layers. The initial damage position is on the HfO2 layer of the ridge which opposite to the laser beam direction. A theoretical model is constructed to explain the femtosecond-laser-induced damage initiation mechanism on the MMDG, and the model can simulate the evolution of the electron density in the conduction band and the change of the dielectric constants of HfO2 and SiO2 in the sandwiched grating structure. The dramatic increase in the imaginary part of the dielectric constant of the middle HfO2 layer indicates that it strongly absorbs laser energy, resulting in damage to the MMDG. The experimental results and theoretical calculation agree very well with each other.

“Open-Loop” Tracking Interferometer Measurement Using Rotary Axes of a Five-Axis Machine Tool

The tracking interferometer, or the laser tracker, is a laser interferometer with a steering mechanism to change the laser beam direction to automatically follow a retroreflector. Many researchers have studied its application to the multilateration to measure the retroreflector's three-dimensional position. This paper shows that the multilateration measurement can be done by regulating the laser beam toward the command retroreflector position, assuming that the machine tool's positioning error is reasonably small. The machine's rotary axes are used to regulate the laser beam direction. The proposed scheme enables a user to perform the multilateration measurement by using a laser interferometer and the machine's rotary axes only, without requiring any specialized tracking mechanism. An experiment is presented to investigate its measurement performance. The paper's emphasis is on the assessment of its measurement uncertainty, introduced by the elimination of automated tracking mechanism.

Spectrally and angularly resolved measurements of three-halves harmonic emission from laser-produced plasmas

Spectra of three-halves harmonic emissions (3/2) from laser-produced plasmas were measured at different angles, including both forward and backward sides, from the direction of incident laser beams. The /2 emitted from carbon–hydrogen (CH) targets was observed to be larger than that from aluminum (Al) targets with the same incident laser intensity, which supports the argument that the two-plasmon decay (TPD) instability could be inhibited by using medium-Z ablator instead of CH ablator in direct-drive inertial confinement fusion. Besides, the measured /2–incident intensity curves for both materials suggest relatively lower threshold of TPD than the calculated values. In experiments with thin Al targets, the angular distribution of the blue- and red-shifted peaks of /2 spectra were obtained, which shows that the most intense blue- and red-shifted peaks may not be produced in paired plasmons, but the spectra produced by their ‘twin’ plasmons were not observed. Because may have been influenced by other physical processes during their propagation from their birth places to the detectors, the mismatches on emission angle, wavelength shift, and threshold may be qualitatively explained through the assumption that small-scale light filaments widely existed in the corona of laser-produced plasmas.

Atmospheric multiple scattering of a vertically directed laser beam

Vertical laser beams are often used at ground-based cosmic ray observatories employing the fluorescence technique for characterizing the height-dependent properties of the atmosphere, as well as for calibration and telescope alignment studies. The light flux received at a detector from a laser is typically assumed to be only singly scattered out of the beam, with no possibility for the multiple scattering of photons initially scattered in other directions back into the detector’s field of view. We present the results of a new simulation for the scattering of light from a vertically-directed laser beam, and derive a parametrization for the multiple scattered signal expected at a detector from such a source as a function of the prevailing atmospheric conditions. The parametrization is then used to estimate the increase in the reconstructed height-dependent aerosol loading when recovered using a laser-based technique.

Fatigue crack growth rate and tensile strength of Re modified Inconel 718 produced by means of selective laser melting

The paper presents results of mechanical tests, namely fatigue crack growth (FCG) rate and tensile tests – of Inconel 718 produced by Selective Laser Melting (SLM) in pure form and with Re addition. SLM method was used to manufacture “comb like” structures, simulating small parts with thin walls, of which final mini-samples were cut out (two types of samples for tensile and one for the FCG rate test were used). A fraction of samples underwent a standard procedure of a heat treatment designed for Inconel 718 alloy. The influence of samples orientation to the laser beam direction, samples size and heat treatment on the tensile strength, yield strength and elongation to fracture were investigated. FCG rate tests were carried out using mini-samples with notches. Cyclic loading of samples was synchronized with CDD camera trigger for registering images of samples surfaces at the moments of maximal loading. Digital Image Correlation (DIC) was used to determine near crack tip displacement fields. The results of DIC measurements were analyzed using the inverse method to automatically determine the stress intensity factor and crack tip coordinates. Additionally, fracture surfaces SEM observations, X-ray Diffraction (XRD) analyses, X-ray fluorescence (XRF), light microscopy (LM) and Transmission Electron Microscopy (TEM) observations have been done to understand mechanical properties variation revealed during mechanical testing.

Integration of Single-Photon Emitters into 3C-SiC Microdisk Resonators

We demonstrate the integration of bright, fully polarized single-photon emitters readily created by thermal oxidation of cubic silicon carbide (SiC) into microdisk resonators. The resonators are created by a direct laser beam writing lithography technique that is used to align the position of the resonator to a preselected single defect. Quality factors as high as 1900 are measured. We show the presence of whispering gallery modes in the emission spectrum of a single defect and an increase in the detected emission intensity. The experimental work is supported by numerical calculations of the electric field distribution in the resonators.