Position Of Laser Beam Research Articles

Sensing Characteristics for a Fiber Bragg Grating Inscribed Over a Fiber Core and Cladding

Femtosecond laser exposure is used to fabricate a fiber Bragg grating (FBG) over a fiber core cladding on a short section of photon-sensitive thin-core fiber (TCF) using a side-illumination technique. The cladding modes are excited in the fusion region due to the core mismatch between the leading-in single-mode fiber and the TCF, and one resonant cladding mode and core mode are then transmitted downstream as separate wavelengths by the FBG. Furthermore, the transmission intensities for the two resonant dips could be controlled by changing the focal-line position of laser beam. Because the cladding modes have unique field shapes, they react to perturbations both inside and outside the fiber, which makes them good candidates for measuring fiber bending and ambient refractive index variation.

Effects of welding parameters on weld geometry during high-power laser welding of thick plate

During high-power deep-penetration laser welding of thick plate, the geometrical characteristics of weld cross section were investigated under different welding conditions. The molten metal behaviors were observed by using the high-speed camera and X-ray imaging system to study the formation mechanism of weld cross section. With the increase of welding speed, the width and depth of weld seam decreased, and the geometry of weld cross section changed from big-head shape to needle-like shape. The change of focal position of laser beam led to the variation of molten metal behavior which thus resulted in different geometries of weld cross sections during laser welding. The accumulation of high-temperature molten metal at both the top surface of molten pool and the middle depth of molten pool could lead to a higher heat-transferring efficiency of the molten metal which could lead to a wider weld width. The stable molten metal behavior on the molten pool surface resulted in a low heat-transferring efficiency, and the sound appearance of weld seam could be obtained when the focal position was inside the metal.

Disk Laser Welding of ZE 41 Magnesium Alloy

The paper deals with disk laser welding of magnesium alloy with the content of rare earth elements. ZE 41 magnesium alloy with the thickness of 2.0 mm was used as experimental metal. TruDisk 4002 solid state laser with a wavelength of 1.03 microns and a maximum power of 2.0 kW was used for welding of magnesium alloy. Laser power which was applied ranged from 0.6 to 1.2 kW. The focal position of laser beam was fixed on the surface of magnesium alloy and the welding speed was set to 70 mm/s. Argon with flow rate of 12 l/min was used as shielding gas. Light microscopy, EDX microanalysis and micro hardness measurements across base metal - fusion zone interface were carried out in order to analyze the weld joints.

Process observation in fiber laser–based selective laser melting

The process observation in selective laser melting (SLM) focuses on observing the interaction point where the powder is processed. To provide process relevant information, signals have to be acquired that are resolved in both time and space. Especially in high-power SLM, where more than 1 kW of laser power is used, processing speeds of several meters per second are required for a high-quality processing results. Therefore, an implementation of a suitable process observation system has to acquire a large amount of spatially resolved data at low sampling speeds or it has to restrict the acquisition to a predefined area at a high sampling speed. In any case, it is vitally important to synchronously record the laser beam position and the acquired signal. This is a prerequisite that allows the recorded data become information. Today, most SLM systems employ f-theta lenses to focus the processing laser beam onto the powder bed. This report describes the drawbacks that result for process observation and suggests a variable retro-focus system which solves these issues. The beam quality of fiber lasers delivers the processing laser beam to the powder bed at relevant focus diameters, which is a key prerequisite for this solution to be viable. The optical train we present here couples the processing laser beam and the process observation coaxially, ensuring consistent alignment of interaction zone and observed area. With respect to signal processing, we have developed a solution that synchronously acquires signals from a pyrometer and the position of the laser beam by sampling the data with a field programmable gate array. The relevance of the acquired signals has been validated by the scanning of a sample filament. Experiments with grooved samples show a correlation between different powder thicknesses and the acquired signals at relevant processing parameters. This basic work takes a first step toward self-optimization of the manufacturing process in SLM. It enables the addition of cognitive functions to the manufacturing system to the extent that the system could track its own process. The results are based on analyzing and redesigning the optical train, in combination with a real-time signal acquisition system which provides a solution to certain technological barriers.

Precision absolute measurement and alignment of laser beam direction and position.

For the construction of high-precision optical assemblies, direction and position measurement and control of the involved laser beams are essential. While optical components such as beamsplitters and mirrors can be positioned and oriented accurately using coordinate measuring machines (CMMs), the position and direction control of laser beams is a much more intriguing task since the beams cannot be physically contacted. We present an easy-to-implement method to both align and measure the direction and position of a laser beam using a CMM in conjunction with a position-sensitive quadrant photodiode. By comparing our results to calibrated angular and positional measurements we can conclude that with the proposed method, a laser beam can be both measured and aligned to the desired direction and position with 10 μrad angular and 3 μm positional accuracy.

Imaging and Analysis of Single Optically Trapped Gold Nanoparticles Using Spatial Modulation Spectroscopy.

The extinction cross sections and spectra of single nanoparticles can be directly measured by moving the particle in and out of a tightly focused laser beam. This technique, known as spatial modulation spectroscopy, yields detailed information about the size, shape, and environment of the particles. These experiments are typically done on particles immobilized on a substrate. Here we demonstrate for the first time the use of spatial modulation spectroscopy to interrogate single, optically trapped nanoparticles in solution. Gold nanoparticles as small as 15 nm were trapped and imaged. The experiments were performed by modulating the position of the probe laser beam while scanning it over the trapped particle with a galvo-scanning mirror system. This technique opens up the possibility of precisely measuring the optical properties of single nanoparticles in liquid environments, free from the influence of a surface.

Influence of feed rate and spindle speed on referencing laser tool-setters

The use of CNC machines has been increasingly intensive in industry. Sophisticated characteristics and resources have been implemented into these machines in order to achieve higher production rates and better dimensional quality. One of these resources is the laser tool-setter, used to automatically measure the main dimensions (diameter and length) of the tools used in the machining process by means of the interruption of a laser beam located in a predetermined position. However, the feed rate and the spindle speed used during referencing of the tool-setter (when the tool approaches the laser beam position) can influence the results of the measurements of a tool-setter due to the processing delay of the information by the CNC machine controls. This paper discusses these influences and identifies the most adequate values for the feed rate and the spindle speed to reach an optimized measurement of the tools. For that purpose, two experimental procedures were conducted to obtain the values of tool diameter and length under diverse feed rate and spindle speed settings (a nominal O16 mm × 15 mm length referencing pin mounted into the tool holder was used). Statistical analyses were performed to identify the most adequate conditions based on the results of the experiments. These analyses indicated that the minimum tool measurement variations occurred when the spindle speed remained idle and the feed rate was kept at values typically around 0.25–9.5 mm/min.

Microstructure and mechanical properties of a laser welded low carbon–stainless steel joint

This article reports on an experimental investigation to understand and subsequently control the alloying composition in laser welding of austenitic stainless steel and low carbon steel. An energy dispersive spectrometer was used to analyse the alloying composition, while microscopy and tensile tests were used to study the microstructure, and mechanical performance of the welded joint respectively. The investigation revealed that above a certain specific point energy the material within the melt pool is well mixed and the laser beam position can be used to control the mechanical properties of the joint. This finding was confirmed using a numerical model based on computational fluid dynamics (CFD) of melt pool dynamics. Strategies to control the composition leading to dramatic changes in hardness, microstructure and mechanical properties of the dissimilar laser welded joint are discussed.

Relationship between spatter formation and dynamic molten pool during high-power deep-penetration laser welding

The spatter and the molten pool behavior, which were the important phenomena concerned with the welding quality, were observed and studied by using the high-speed camera and the X-ray transmission imaging system during laser welding under different welding parameters. The formation mechanism of spatter and the corresponding relationships between the spatter and molten pool behavior were investigated. The increase of laser power could cause more intense evaporation and lead to more spatter. When the focal position of laser beam was changed, different forms of spatter were generated, as well as the flow trends of molten metal on the front keyhole wall and at the rear molten pool were changed. The results revealed that the behavior of molten pool, which could be affected by the absorbed energy distribution in the keyhole, was the key factor to determine the spatter formation during laser welding. The relatively sound weld seam could be obtained during laser welding with the focal position located inside the metal.

Effect of laser beam position on mechanical properties of F82H/SUS316L butt-joint welded by fiber laser

A dissimilar butt-joint between reduced activation ferritic/martensitic steel F82H and SUS316L austenitic stainless steel was made by 4kW fiber laser and the influence of laser beam position on its mechanical properties before and after post-weld heat treatment (PWHT) was examined at room temperature. From the nano-indentation measurements and the microstructural observations, it is found that the micro hardness of weld metal partially decreases after PWHT by shifting beam position to SUS316L because its phase seems to move from only the martensitic phase to the mixture of austenitic and martensitic phases. In addition, Charpy impact test suggests that the impact energy slightly increases by shifting beam position before PWHT and obviously increases after PWHT due to the release of residual stress. Moreover, the tensile test indicates that the tensile strength of weld metal is higher than that of SUS316L and the fracture occurs at the base metal of SUS316L regardless of laser beam position.

High-Precision Positioning of Laser Beams for Vibration Measurements

Predicting the vibratory response of structures with complex geometry can be challenging especially when their properties (geometry and material properties) are not known accurately. These structures can suffer also from high modal density, which can result in small changes in structural properties creating large changes in the resonant response. To address this issue, structural properties could be accurately identified, or the structural response could be experimentally measured. Both these approaches require collecting measurements of higher order vibration modes, which have complicated shape. Consequently, high-accuracy positioning of laser beams is necessary for vibrometers based on laser Doppler velocimetry. This paper presents a methodology to address this challenge. The architecture involves a single-point vibrometer, a motion controller, translating/rotating stages, and special application software for alignment and edge detection. A key novelty of this technology is that the beam of the vibrometer is used for both detecting the edges and for measuring the vibration. Using a motion controller, the system can automatically place/scan and measure the surface of the structure with a positioning resolution of 1 μm. Experimental results are provided to demonstrate the new technique.

Changes of the centroid position of laser beams propagating through an optical system in turbulent atmosphere

Abstract In this paper, the effects of atmospheric turbulence, initial field amplitude, optical system and thermal blooming on the centroid position of laser beams propagating through the atmosphere are studied in detail. With the average over the ensemble of the turbulent medium, the centroid position is independent of turbulence. However, the centroid position depends on the centroid positions at the source plane and in the far-field, and the elements of ray-transfer-matrix. The physical reason why the centroid position changes on propagation is that the far-field centroid position is not located on the propagation z-axis due to the field phase distortion and the decentred intensity. The centroid position of laser beams with the spherical aberration and the decentred intensity is examined analytically. When laser beams with the decentred intensity propagate through the atmosphere, the effect of thermal blooming on the centroid position is investigated by using the four-dimensional (4D) computer code of the time-dependent propagation of high power laser beams through the atmosphere.

Current Progresses of Laser Assisted Machining of Aerospace Materials for Enhancing Tool Life

A higher strength and heat resistance are increasingly demanded from the advanced engineering materials with high temperature applications in the aerospace industry. These properties make machining these materials very difficult because of the high cutting forces, cutting temperature and short tool life present. Laser assisted machining uses a laser beam to heat and soften the workpiece locally in front of the cutting tool. The temperature rise at the shear zone reduces the yield strength and work hardening of the workpiece, which make the plastic deformation of the hard-to-machine materials easier during machining. The state-of-the-art, benefits and challenges in laser assisted machining of metallic materials are summarized in this paper, and the improvement of tool life is discussed in relation to laser power, beam position and machining process parameters.

Numerical Simulation of Feeding Powder Laser Cladding Temperature Field Based on ANSYS

The numerical simulation method of LCRM Using Coaxial Inside-Beam Powder Feeding is put forward in this page. LCRM conduction model is established, the life and death unit to be used to simulate the real laser cladding process. Calculation results show that the molten pool temperature will gradually increase with the number of cladding layers increase; the highest temperature at the point of application of the laser beam position later; in the cladding process, cooling rate is great in high temperature period, and in low temperature section, cooling rate relatively slight.

Weldability of dissimilar joint between F82H and SUS316L under fiber laser welding

As one of the high beam quality heat sources, 4kW fiber laser was applied for joining between reduced activation ferritic/martensitic steel, F82H and SUS316L austenitic stainless steel, and the microstructural analyses and Vickers hardness measurements were carried out before and after post-weld heat treatment (PWHT). The microstructure of joint can be divided into four regions which are base metal of F82H, heat affected zone (HAZ) in F82H, weld metal (WM) and base metal of SUS316L. Also, it is revealed that the high-power fiber laser can be employed for constructing butt joint between F82H and SUS316L by applying PWHT and shifting the laser beam position to SUS316L, where the distance between the contact face and beam should be set as a range from radius to diameter of laser beam.

ROBUST POSITIONING OF LASER BEAMS USING PROPORTIONAL INTEGRAL DERIVATIVE AND BASED OBSERVER-FEEDBACK CONTROL

High-precision positioning of laser beams has been a great challenge in industry due to inevitable existence of noise and disturbance. The work presented in this study addresses this problem by employing two different control strategies: Proportional Integral Derivative (PID) control and state feedback control with an observer. The control strategies are intended to stabilize the position of a laser beam on a Position Sensing Device (PSD) located on a Laser Beam Stabilization (or, laser beam system) system. The laser beam system consists of a laser source, a Fast Steering Mirror (FSM), a PSD and a vibrating platform to generate active disturbance. The traditional PID controller is widely used in industry due to its satisfactory performance, various available tuning methods and relatively straightforward design processes. However, design of filters to obtain the derivative signal is challenging and can unexpectedly distort the dynamics of the system being controlled. As an alternative, use of an Observer-Based State Feedback (OBSF) method is proposed and implemented. The state-space model of the laser beam system is utilized and an observer is applied to estimate the state of the system, since all the state variables cannot be measured directly. For observer design, eigenvalue assignment and optimal design methods are used and compared in terms of system performance. Also a comparative analysis between the PID and OBSF controllers is provided. Simulations and experimental results show that the OBSF controller rejects disturbance better and has a simpler design procedure.

Research on Gear Repairing Technology by Laser Cladding

Wearing gears with complex tooth surface are difficult to repair. A test method of laser cladding was made to repair the damaged gears which were made by steel 45, with synchronous powder feeding and gear axial scanning method, and Ni45 powder as cladding material. The relative position of laser beam and the gear surface were designed to solve the interference problems in laser cladding process. It analyzed the morphology of the cladding layer in the different test conditions to get optimum parameters, and determined the cladding thickness and overlapping ratio, and optimal sequence by made a comparison of cladding quality in different condition. Microscopic metallographic analysis and hardness testing were made for the cladding layer to analyze the performance of cladding layer. Got high strength cladding layer on damaged gears tooth surfaces. Laser cladding method can repair the gear surfaces and raise the wear resistance.

Laser-Based Kinematic Calibration of Robot Manipulator Using Differential Kinematics

This paper proposes a novel systematic technique to estimate entire kinematic parameter errors of robot manipulator. Small errors always exist in link length and link twist for physical manipulators, which affect the precision in kinematic equations leading to calculate wrong joint angle values in inverse kinematic equations. In order to solve these problems, the proposed technique employs a structured laser module (SLM), a stationary camera, the Jacobian matrices, and an extended Kalman filter (EKF). The SLM is attached to the end-effector of the manipulator arm and the stationary camera is used to determine an accurate position where the laser comes out. Variances between actual and measured positions of laser beams are represented by the Jacobian matrices formulated from differential kinematics. Then, the EKF is used to estimate kinematic parameters. Effectiveness of the proposed technique is verified with 7 DOF humanoid manipulator arm by computer simulation and 4 DOF manipulator by actual experiment.

Amplification of Surface-Enhanced Raman Scattering in Photonic Crystal Fiber Using Offset Launch Method

In this paper, we report the amplification effect of surface-enhanced Raman scattering (SERS) in a solid-core photonic crystal fiber. Gold nanoparticles (AuNPs) are self-assembled on the inner walls of the cladding air holes through a multilayered deposition procedure. Offset launch method is for the first time employed to introduce extra enhancement in the intensity of SERS signals, as compared to the conventional core launch method. A theoretical analysis on modal field distribution for both launching conditions is carried out to account for such improvement. It shows that by adjusting the launching position of laser beam from the solid fiber core to an air hole in the cladding, overlap of the excited mode with AuNPs has been increased significantly. The SERS probe is demonstrated to achieve a detection limit as low as 10−7 M in concentration, which shows a competitive performance for molecule analysis.

Spatial distribution of ions in a linear octopole radio-frequency ion trap in the space-charge limit

We have explored the spatial distribution of an ion cloud trapped in a linear octopole radio-frequency (rf) ion trap. The two-dimensional distribution of the column density of stored silver dimer cations was measured via photofragment-ion yields as a function of the position of the incident laser beam over the transverse cross section of the trap. The profile of the ion distribution was found to be dependent on the number of loaded ions. Under high ion-loading conditions with a significant space-charge effect, ions form a ring profile with a maximum at the outer region of the trap, whereas they are localized near the center axis region at low loading of the ions. These results are explained quantitatively by a model calculation based on equilibrium between the space-charge-induced potential and the effective potential of the multipole rf field. The maximum adiabaticity parameter \eta_max is estimated to be about 0.13 for the high ion-density condition in the present octopole ion trap, which is lower than typical values reported for low ion densities; this is probably due to additional instability caused by the space charge.