Optic Beam Delivery System Research Articles

광섬유 빔 전달 시스템을 이용한 펨토초 레이저 재료 가공 임계치 연구

A femtosecond laser is used in various fields such as microscale machining, OLED repair, micro 3D structure fabrication, and eye surgery. Particularly, because of non-thermal property, ablation and ablation threshold are the most representative characteristics of femtosecond laser. The ablation system is accompanied by many optics, stage, or gantry. In the case of the gantry, an ordinary optic system delivers a beam where mirrors and lens are required. If the gantry moves to the sample, external stimulation such as vibration will occur. Vibration has an influence on optics such as transforming beam path and becomes an error that decreases accuracy, precision, and spatial resolution. Generally, Fiber Optic Beam Delivery System (FOBDS) is used to solve this issue. But in the case of the femtosecond laser, FOBDS is incompatible. Recently, another FOBDS model that is compatible with the femtosecond laser has been developed. In this paper, the ablation threshold was obtained by FOBDS and femtosecond laser. The results present a possibility of ablation without external stimulation.

Effect of Pre and Post Weld Heat Treatments on the Microstructure and Mechanical Properties of Fiber Optic Beam Delivery System Assisted Robotic Nd: Yag Laser Welded Ti-6Al-4V Alloy

Effect of Pre and Post Weld Heat Treatments on the Microstructure and Mechanical Properties of Fiber Optic Beam Delivery System Assisted Robotic Nd: Yag Laser Welded Ti-6Al-4V Alloy

Generic software tool for counteracting the dynamics effects of optical beam delivery systems

Laser micro-machining is a promising manufacturing solution for fabricating complex micro-engineering products in wide range of materials that incorporate different multi-scale functional features. Optical beam deflector systems are key components in laser micro-machining systems, and they are one of the main factors determining the processing speed and hence machining throughput. However, their performance is speed dependent and the negative dynamics effects have a direct impact on the laser micro-machining accuracy, repeatability and reproducibility. This article presents a generic software solution for minimising these negative dynamics effects, thus improving significantly the laser machining performance across the full range of available processing speeds. In particular, these improvements are achieved by introducing machine-specific compensations in machining vectors to counteract beam deflectors’ inertia regardless of their directions, length and set process speed. An empirical model was developed to obtain data about the actual dynamic response of the beam deflection system across the full range of available processing speeds, and then based on these data, the proposed generic software was implemented into a stand-alone ‘adaptive’ postprocessor. The generation of machine executable part programs is automated, and it is only necessary for the user to enter the selected scanning speeds and beam diameters. Experimental validation was conducted to demonstrate the capability of the proposed software tool. The results demonstrate that substantial improvements can be obtained in machining quality by maintaining a constant pulse distance throughout the machining operations, while the dimensional accuracy is maintained across the available processing speeds without sacrificing the machining efficiency.

Design of an Optical Fibre-based Receiver System for Atmospheric Sensing Lidar

In this paper we report on the design of an optical fiber-based light receiver which is meant for replacing the conventional beam steering secondary mirror of the Newtonian telescope of a biaxial Raman lidar. The influence of using optical fibre, acting as an aperture stop of constant diameter on the geometrical compression form factor; the transitional altitude and the resulting blur disk diameter have been determined. The viability of using a commercially available optical fibre as a receiver and the associated trade-offs have been discussed. A hot-fused fibre bundle-based receiver is proposed as a potential alternative for low altitude profiling to be enabled. These theoretical investigations have formed the basis for our current experimental work aimed at the development of all fibre-based reconfigurable beam delivery systems for mobile lidars. Full Text: PDF References A.M. Melesse et al., "Remote Sensing Sensors and Applications in Environmental Resources Mapping and Modelling", Sensors 51, 3209 (2007). CrossRef P. Wang et al., "A macrobending singlemode fiber refractive index sensor for low refractive index liquids", Phot. Lett Poland 2, 67 (2010). CrossRef J.R. Jenness et al., "Design of a lidar receiver with fiber-optic output", Appl. Optics 36, 4278 (1997). CrossRef J.V. Hey et al., "Determination of overlap in lidar systems", Appl. Optics 50, 5791 (2011). CrossRef K. Stelmaszczyk et al., "Analytical function for lidar geometrical compression form-factor calculations", Appl. Optics 44, 1323 (2005). CrossRef A.A.P. Boechat et al., "Bend loss in large core multimode optical fiber beam delivery systems", Appl. Optics 30, 321 (1991). CrossRef http://www.schott.com/china/chinese/download/284345_fo_light_it_right_e_without_tecspec.pdf DirectLink https://www.thorlabs.com/thorproduct.cfm?partnumber=APCH1500 DirectLink

Process optimization controller for robotic laser machining

This work details the performance assessment of a high-bandwidth optical range sensor for local stand-off control of a robotically manipulated optical fiber laser beam delivery system. Consistent, repeatable high quality materials processing, using fiber optically delivered Nd3+-YAG laser radiation requires precise local control of stand-off distance. Unless specifically designed for laser processing applications, many robots produce a jerking incremental trajectory and usually suffer from position dependent displacement of the end effector. With the addition of a focal height servomechanism performance of the system can be considerably improved. The high-bandwidth optical range sensor described herein allows the use of such robots for fiber optically delivered laser beams enabling high quality laser cutting and welding operations using equipment that may already exist. Radiation from a low power laser diode is focused onto a workpiece surface; light reflected from the surface is collected through a main lens and directed into an astigmatic lens, which focuses the signal onto a quadrant photodiode. A differential amplifier is used to generate an output signal that determines the magnitude and direction of any workpiece displacement. The system facilitates a measuring range of ±5 mm; its effectiveness in controlling the stand-off distance for laser materials processing applications is assessed.

Fibre optic beam delivery system for high peak power laser PIV illumination

Diffractive optical elements (DOEs) are used to couple Q-switched and frequency-doubled Nd:YAG laser beams into optical fibres to achieve significantly increased damage thresholds, enabling fibre optic beam delivery for high-speed particle image velocimetry (PIV) measurements. For single fibre delivery systems, the maximum pulse energy density that can be transmitted is increased by a factor of 5-10 compared with the best that can be achieved using conventional optics, of up to 10 mJ in a m core diameter fibre at 532 nm. We also applied the DOE arrangement to our previously developed bundle delivery system, comprising nineteen m core diameter fibres. On testing, over 1000 pulses with energies of 30 mJ were successfully transmitted, with no indication of damage. This allows fibre delivery to become a practical option for many air-flow PIV applications, as demonstrated here with measurements of flow in a square duct.

Dot Matrix Hardening of Steels Using a Fiber Optic Coupled Pulsed Nd:YAG Laser

ABSTRACT This paper describes a novel process called “Dot Matrix Hardening” as applied to Ol, D2 and AISI 4340 steels. This process uses a pulsed laser (particularly an Nd:YAG laser) to create a uniform distribution of transformation-hardened spots to cover only a certain percentage of the desired surface. Due to significantly reduced energy input, wear resistance can be imparted to thin and intricate parts without distortion. In addition, with the use of a coupled fiber optic beam delivery system, this process provides greater flexibility compared to conventional CO2 laser hardening for a number of applications. The use of an Nd:YAG laser also eliminates the need of absorptive coating required for hardening with a CO2 laser. With optimized processing parameters, a relatively uniform hardened layer is obtained within the hardened spot, with a thickness of about 60 um and hardness values around 800 HV100. The sliding wear test results show that the wear resistance of Ol samples with only 20-40% area coverage of laser-hardened spots is similar to the 100% covered laser dot hardened sample as well as the furnace hardened (Re 60) sample.

Influence of hydrogen on the colour center formation in optical fibers induced by pulsed UV-laser radiation.: Part 2: All-silica fibers with low-OH undoped core

Up to now only high-OH fibers were suitable for pulsed UV-laser applications, since low-OH material suffered from preexisting UV-absorbing colour centers and excessive generation of these defects during UV-irradiation. By loading commercial low-OH fused silica fibers with hydrogen, we were able to reduce the amount of initial absorption sites and to influence the generation of colour centers during irradiation. In our experiments we studied the transmission behaviour of hydrogen loaded low-OH fibers for 266 nm pulsed laser radiation. Spectroscopic experiments explain many aspects of colour center creation and annealing and allow the determination of kinetic data for the underlying reactions. Based on this work it is possible to realize fiber optic beam delivery systems for special applications, where low-OH material shows superior performance over high-OH material.

Influence of hydrogen on the colour center formation in optical fibers induced by pulsed UV-laser radiation. Part 1: all silica fibers with high-OH undoped core

Creation of colour centers in fused silica fibers is the main problem during transmission of high-power UV-laser radiation, that leads to solarization and hence decreasing transmission. By loading commercial fused silica fibers with hydrogen, we found a way to improve the transmission properties through the reduction or suppression of solarization. The transmission behaviour of hydrogen loaded high-OH fibers for 266 nm pulsed laser radiation has been studied. Spectroscopic experiments explain many aspects of colour center creation and annealing and allow the determination of kinetic data for the underlying reactions. Based on this work it is possible to realize fiber optic beam delivery systems for frequency quadrupled Nd:YAG lasers and for other high power UV laser sources like excimer-lasers. These results may also be useful for the improvement of bulk fused silica for UV applications.

Characteristics of Nd:YAG laser welded high carbon steels

A quantitative study of the relationship between the laser process parameters and the mechanical properties of welded high carbon steels was performed utilizing a Lumonics Nd:YAG pulsed laser, operating at 1.06μm, and a robotically manipulated fiber optic beam delivery system. A gage plate (0.88 mm thick) was butt welded with a constant power of 200 W and a He shielding gas was used at a pressure of 5 × 104 Pa. The welding performance of the Nd:YAG laser was strongly affected by the translation velocity, pulse length and pulse repetition frequency (PRF). The effects of varying these process parameters were quantified by measuring the sample's hardness profile, weld width, weld penetration, and tensile strength. Furthermore, micrographic examinations were conducted at the welded joints. It was shown that by increasing the pulse length and pulse repetition frequency a deeper weld penetration and a wider bead width were achieved; moreover, the weld region became tougher. Hardness profiles transverse across the weldment and as a function of weld depth were greatly reduced with increasing PRF. The quality of the laser weld was dominated by the rapid quenching rate which resulted in the weld structure being completely modified near the heat affected zone.

Microstructural analysis and oxidation behavior of laser-processed Fe-Cr-AI-Y alloy coatings

Dense crack-free coatings of Fe-Cr-Al-Y quaternary alloy were produced on stainless steel 316L substrates using a continuous wave Nd-YAG solid-state laser coupled with a fiber optic beam delivery system. Experiments were performed at a laser power between 0.6 and 2.4 kW, process speed in the range 0.053 to 0.423 cm/s, powder feed rate fixed at 0.083 g/s, and focused multimode laser beam with a diameter of 0.5 cm. Various microanalysis techniques demonstrated that the coatings were metallurgically bonded to the substrate and possessed thicknesses between 0.35 and 4.64 mm, refined columnar microstructures with grain sizes of 15 to 150 µxm, increased concentration of key alloying elements, and appreciably high microhardness up to 409 kg/mm2. The laser-processed microstruc-tures comprised a body-centered cubic (bcc) ferrite(α phase) crystal structure with a relatively large lattice parameter compared to α-Fe due to the enhanced dissolution of varying amounts of Cr, Al, Ni, and Y, depending on the dilution from the substrate material. Oxidation tests conducted in air at temperatures of 1100 ° to 1200 ° for 95 hours revealed the formation of an approximately 5-µm-thick dense α-Al2O3 oxide scale of a rhombohedral (hexagonal) crystal structure. The α-Al2O3 scale exhibited remarkable high-temperature resistance and strong adherence to the coating surface. Extensive oxidation of the uncoated stainless steel substrate produced a porous and heavily spalled alloy oxide scale about 60-µm thick consisting of FeCr2O4 and NiCr2O4 with cubic and tetragonal crystal structures, respectively. The retention of the bcc α phase and the insignificant grain growth after oxidation are indicative of the thermal stability of the laser-processed coating microstructures. The obtained results demonstrate that Fe-Cr-Al-Y alloy coatings exhibiting fine-grained hard mi-crostructures, high-temperature oxidation resistance, and strong adherence to stainless steel can be developed by means of laser processing.

Beam delivery characteristics of optical fibres for carbon monoxide lasers

An experimental investigation has been conducted of the propagation characteristics of several types of optical fibres which are candidates for use in optical beam delivery systems for carbon monoxide lasers. Both solid core (chalcogenide) fibres and hollow core (with dielectric or dielectric coated metal) waveguides have been investigated. Such experiments have included an assessment of both the power transmission characteristics, and the effects of waveguide transmission on the optical quality of the beam at the exit of the fibre. The experiments indicated lowest loss for the chalcogenide solid core fibre with a value of 0.4 dB m −1.

Production of high average power UV by second-harmonic and sum-frequency generation from copper-vapor lasers

Progress in high average power UV generation by nonlinear frequency conversion of the output of copper-vapor lasers (CVL's) is reviewed. The specific parameters controlling the efficiency of nonlinear frequency conversion using CVL's are highlighted, with CVL beam quality and matching the optical beam delivery system to the characteristics of the nonlinear crystal being identified as the most significant issues. Recent experimental studies of second harmonic generation (SHG) with single-CVL oscillators and CVL oscillator-amplifier systems show that by careful optimization of the CVL pump laser and beam delivery systems, it is now possible to generate multiwatt average powers in the UV with high optical conversion efficiency (up to 35%) and overall electrical efficiency (approaching 0.1%).< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The laser strain gauge: micro machining of diffraction gratings using an Excimer laser

AbstractThe laser strain gauge is a recently developed instrument for the single point measurement of surface strain to a resolution of 1 μm. It measures the deformation of a diffraction grating adhered to or etched into the surface of various substrates. In comparison with a conventional electrical resistance strain gauge the new gauge offers greater thermal stability and avoids the inconvenience of electrical connections. When gratings are etched directly into the surface of a component absolute strain measurements over its lifetime are possible.High frequency (1000 lines/mm) diffraction gratings can be etched into the surface of various substrates including carbon steels, polymers and ceramics using an Excimer laser. This paper describes the optical beam delivery system necessary for this application and demonstrates the feasibility of this exciting new technique.

Curvature induced mode coupling in large core optical fibres with step refractive index profiles

Abstract A simple ray model is developed to approximate the output beam angle increase caused by a symmetric bend with variable curvature in a large core optical fibre beam delivery system of the type commonly used in laser material processing. The results are compared with experiments using curved fibres in the shape of half-cycle cosines with different rates of curvature change. The theoretical model was supported by experiment, and is sufficiently simple for application in the design of an optical fibre beam delivery system.

Curvature induced mode coupling in large core optical fibres with step refractive index profiles

A simple ray model is developed to approximate the output beam angle increase caused by a symmetric bend with variable curvature in a large core optical fibre beam delivery system of the type commonly used in laser material processing. The results are compared with experiments using curved fibres in the shape of half-cycle cosines with different rates of curvature change. The theoretical model was supported by experiment, and is sufficiently simple for application in the design of an optical fibre beam delivery system.

In-situ laser material process monitoring using a cladding power detection technique

Progress in laser material processing may require real-time monitoring and process control for consistent quality and productivity. We report a method of in-situ monitoring of laser metal cutting and drilling using cladding power monitoring of an optical fibre beam delivery system—a technique which detects the light reflected or scattered from the workpiece. The light signal carries information about the quality of the process. Experiments involving drilling and cutting of two samples, a thin aluminum foil and a 2-mm thick stainless steel plate, confirmed the effectiveness of this method.

Workpiece position sensing by means of a fiber optical beam delivery system

We report a method of measuring workpiece position in laser material processing and machining that uses a fiber optical beam delivery system. The method is based on a measurement of the power propagating in the cladding of the optical fiber in the direction from the workpiece to the laser. The power reflected from the workpiece and coupled into the cladding is shown to be a function of the position of the workpiece relative to the beam waist formed by the output optics of the beam delivery system.

Bi-directional cladding power monitor for fibre optic beam delivery systems

A technique is described for the measurement of the power propagating in either the forward or the reverse direction in the cladding of an optical-fibre beam delivery system. The technique is based on the use of a novel fibre component which is analogous to a directional coupler, except that it transfers power from the cladding of the system fibre (rather than its core) to the monitor fibre. Applications are described for the use of the bi-directional cladding power monitor for determining the efficiency with which light is coupled into the beam delivery system, for monitoring its integrity, and when used in material processing applications, for monitoring the power back-coupled into the fibre from the workpiece.

Fibre optic beam delivery: launching efficiency measurement by dual-detector cladding power monitoring

In the design and operation of high-power fibre optic beam delivery systems, a primary consideration is the efficiency with which power can be launched from the laser source into the core of the fibre. Techniques for the measurement of launching efficiency, based on monitoring power guided by the fibre cladding, are discussed, and advantages over direct measurement methods are indicated. An extension of the cladding power monitoring technique is introduced, which involves the use of two cladding power detectors separated by a controlled bend in the fibre. It is shown that it is hence possible to normalize the cladding power measurement by the core power, and thus to determine the launch efficiency independent of the laser power.