Pulsed laser beam control by hot spot tracking

The use of a self-developed body-centered reference for precision pulsed laser hot spot tracking is examined through a numerical computer simulation based on a solution of the heat equation with time varying input flux. Tracking resolution and sensitivity in two commonly used infrared sensor bands are compared. To cope with inherent instability, a simple compensation scheme is suggested with a demonstrated improvement in boresight error drift. Optimum performance is obtainable by judicious selection of sensor wavelength bands and sampling times relative to laser pulse time.     

Microstructural characteristics and mechanical properties in laser beam welds of Ti6Al4V alloy

The microstructural characteristics and mechanical properties in laser beam welds of Ti6Al4V alloy were investigated. The microstructural characteristics in the heat affected zone and fusion zone change obviously after laser beam welding, which are strongly influenced by the welding conditions. The mechanical properties of the welds are evidently dependent on the microstructural characteristics, and the strengthening in the heat affected zone and fusion zone is mainly attributed to the formation of martensite.     

Phase control requirements of high intensity laser beam combining

Aiming at getting the general requirements of the beam combine for ignition scale laser facilities, the analytical expressions including the factors affecting the combine results are derived. The physical meanings of every part are illustrated. Based on these expressions, the effects of the factors, including the beam configuration, piston error, and tip/tilt error, are studied analytically and numerically. The results show that the beam configuration cannot affect the Strehl ratio (SR) of the combined beam, but it influences the FWHM of the main peak and the ratio of the main peak and the side peak. The beam separation should be no more than 1.24 times the individual beam width for the multibeam combine, and be close to the individual beam width for the two-beam combine as much as possible. The piston error can change the characteristics of the combine beam focus, including the peak intensity, the focal spot morphology, the fractional energy contained within a certain area, and the center of mass. For the two-beam combine, a piston error less than 2π/5 rad is suitable, and for the multibeam combine, the standard deviation of the piston error should be no more than 2π/10 rad. The tip/tilt error has a great influence on the combined results. It affects the superposition degree of the focal spots of the combined elements directly. A requirement of 0.5~1 μrad for the standard deviation of the tip/tilt error is adequate.     

Propagation modes and regimes of intense laser beam in magnetized plasma

This paper presents an analytical and numerical investigation of a circularly polarized beam propagating along the static magnetic field parallel to oscillating magnetic field in plasma at relativistic intensities. In the high intensity regime, such a magnetic field is created by the pulse itself. The authors have identified three regimes of propagation taking into account the relativistic mass correction. Based on WKB and paraxial ray theory, an appropriate expression for a dielectric tensor has been evaluated in the presence of an externally applied magnetic field. The natural electromagnetic modes are circularly polarized. Consequently, extraordinary and ordinary modes propagate, which are significantly affected due to the relativistic mechanism. The regimes are characterized by dimensionless power and beamwidth, characterizing the nature of propagation as steady divergence, oscillatory divergence and self-focusing. Numerical computations are presented and discussed for typical parameters of laser plasma interaction; defined through critical parameters, namely cyclotron-to-beam frequency (Ω _c ), plasma-to-beam frequency (Ω _p ) and beam power for arbitrary large intensities.     

Fracture toughness of laser welds in ship steel

Shipbuilders are showing increasing interest in laser welding as a means of reducing fabrication costs. The nature of the weld zone is very different to that in an arc weld. Careful research is needed to establish the safety of laser welded structures against a range of risk scenarios. This report deals with fracture toughness and defect tolerance. The laser weld is shown to exhibit very unusual behaviour. The lowest toughness is obtained when a crack is located at the heat affected zone (HAZ), but there is no apparent microstructural embrittlement at this location. It is proposed that the effect arises from an elevation of crack tip tensile stresses induced by the strength overmatch of the adjacent weld zone.     

Quality assurance of welds in ship structures

This paper describes procedures for predicting the growth rate of fatigue cracks in ship structure welds under sea-wave loading as well as the stress corrosion cracking behaviour of steel in marine structures. A computer program which can be applied to assess the residual life-time and to estimate the probability of failure of marine structures under wave action is presented. The main objective of applying this program is the development of an optimal inspection and maintenance policy for marine structures. Moreover, the presented computer program is of value to insurance companies in order to assess the safety level of a marine structure and consequently to estimate their financial risk.     

Microstructure and mechanical properties of Inconel 718 electron beam welds

Abstract

Bead on plate, full penetration electron beam welds were produced in 2 mm thickness sheets of Inconel 718 in the solution treated condition. Welds were subjected to an aging treatment with and without post-weld solution treatment. Weld microstructures, high temperature tensile properties and stress rupture properties were evaluated. The as welded fusion zone showed a considerable amount of interdendritic niobium segregation and brittle intermetallic Laves phase. The tensile and stress rupture properties of the welds after post-weld aging treatment were found to be inferior in relation to the base metal. Post-weld solution treatment at 980°C was found to result in partial dissolution of Laves phase, some reduction in niobium segregation and the formation of δ phase needles around the Laves particles. The use of 980°C solution treatment was found to improve the weld properties to some extent, although not to the level of the base metal. The reasons for this behaviour are discussed, correlating microstructures, fracture features and mechanical properties.     

准分子激光前端光束形态控制技术研究

在高功率准分子激光系统中,前端光束形态直接决定着系统输出的光束形态,并直接与靶物理需求密切相关.主要介绍了窄脉冲准分子激光前端光束形态控制的实验研究进展,基于散射法获得了满足系统要求的部分相干源,直接利用三束激光脉冲堆积获得了平顶整形脉冲,并结合放大实验结果对上述实现方法进行了评价分析.     

Parts inspection by laser beam heat injection

When a pulse of laser radiation falls on a material surface, a certain amount of warming will occur which depends upon the surface properties of the material, particularly upon its absorptivity. The amount of warming will also depend upon what is underneath the surface and especially upon the amount of mass which is available for the thermal energy to expand into. If the warming process is monitored by an infrared detection system, one has a ready means of determining whether or not two parts are alike in their surface properties or in their internal features. An automatic, non-contact system is described which uses this principle for the inspection of solder joints on printed wiring boards at rates of up to 10 joints per second. With minor changes, the inspection system can be used to repair defective joints and to manufacture new ones by the reflow of solder paste or performs. Extensions of the inspection method are described with regard to the testing of bonds in metal sheets and to the measurement of paint thicknesses on metals.     

Beaconless stochastic parallel gradient descent laser beam control: numerical experiments

We apply a target-in-the-loop strategy to the case of adaptive optics beam control in the presence of strong atmospheric turbulence for air-to-ground directed energy laser applications. Using numerical simulations we show that in the absence of a cooperative beacon to probe the atmosphere it is possible to extract information suitable for effective beam control from images of the speckled and strongly turbulence degraded intensity distribution of the laser energy at the target. We use a closed-loop, single-deformable-mirror adaptive optics system driven by a target-in-the-loop stochastic parallel gradient descent optimization algorithm minimizing a mean-radius performance metric defined on the image of the laser beam intensity distribution formed at the receiver. We show that a relatively low order 25-channel zonal adaptive optical beam control system controlled in this way is capable of achieving a high degree of turbulence compensation with respect to energy concentration if the tilt can be corrected separately.     

Gaussian laser beam transformation into an optical vortex beam by helical lens

In this article, we investigate the Fresnel diffraction characteristics of the hybrid optical element which is a combination of a spiral phase plate (SPP) with topological charge p and a thin lens with focal length f, named the helical lens (HL). As incident a Gaussian laser beam is treated, having its waist a distance ζ from the HL plane and its axis passing through the centre of the HL. It is shown that the SPP introduces a phase singularity of pth order to the incident beam, while the lens transforms the beam characteristic parameters. The output light beam is analyzed in detail: its characteristic parameters and focusing properties, amplitude and intensity distributions and the vortex rings profiles, and radii, at any z distance behind the HL plane, as well as in the near and far field.     

Optimum control of the laser beam intensity profile with a deformable mirror

A new deformable mirror control system is developed. This system consists of a deformable mirror, a CCD camera, an image processor, a computer, and actuator drive power supplies. A genetic algorithm is adopted as a control algorithm to obtain an optimum surface profile of the deformable mirror. A circular cross-sectional Gaussian beam was transformed into a beam with a rectangular contour using this system. Although the transfer function of this system is complicated and unknown, this system can be used to obtain the optimum beam profile within the achievable limits of a deformable mirror.     

Effect of nitrogen environment on NdFeB thin films grown by radio frequency plasma beam assisted pulsed laser deposition

NdFeB is a very attractive material for applications in electrical engineering and in electronics, for high-tech devices where high coercive field and high remanence are needed. In this paper we demonstrate that the deposition of nitrogen doped NdFeB thin films by pulsed laser deposition, in the presence of a nitrogen radiofrequency plasma beam, exhibit improved magnetic properties and surface morphology, when compared to vacuum deposited NdFeB layers. A Nd:YAG pulsed laser (3ω and 4ω) was focused on a NdFeB target, in vacuum, or in the presence of a nitrogen plasma beam. Substrate temperature (RT-850 °C), nitrogen gas pressure, and radiofrequency power (75–150 W), were particularly varied. The thin films were investigated by means of X-ray diffraction, atomic force microscopy, scanning electron microscopy, spectroscopic-ellipsometry, and vibrating sample magnetometry.     

Characterization of fiber laser welds in X100 pipeline steel

Recent developments in steel grades for pipeline applications have lead to new high strength grades, such as API5L-X100. Higher strength enables the use of thinner walled pipe, with greater operating pressures and reduced overall costs. The base material presents good toughness values; however there are still limitations regarding the weldability of this material. Research is needed to develop appropriate welding procedures, avoiding typical problems like: cold cracking and toughness reduction in the weld area, and to achieve high productivity and economical feasibility. In the present paper an X100 steel grade welded by high power fiber lasers and TIG is studied and the microstructures developed are analysed aiming to contribute to a better understanding of the transformations induced in this material by the thermal cycle associated with fusion welding.     

Fracture toughness improvement of crack tip by irradiation with electron beam

By irradiation on the crack tip of a thin 30 CrMnSi steel plate with electron beam to modify its texture, the fracture toughness can be improved. The effect of such factors as the power density, the diameter of electron beam and the duration of irradiation on the fracture toughness are studied. The fracture toughness was measured with the $\text{R-}\text{curve}$ method.     

Electrical control of shape of laser beam using axially symmetric liquid crystal cells

This work demonstrates the electrical tuning of laser beam shape using an axially symmetric dye-dope liquid crystal (ASDDLC) device that is fabricated using a photo-alignment method. Various beam shapes can be obtained by linearly polarized Gaussian laser beams through an ASDDLC device under various applied voltages. The far-field intensity patterns generated by laser beams of selected shapes under various applied voltages are simulated, and the results are consistent with experiment. A rotatable petal-shaped beam is obtained by controlling the polarization of the output donut-shaped beam. The tenability of beam shape of light with a wavelength of 1064 nm, which is commonly used in biomedical applications, is also demonstrated.     

Efficient global optimization applied to wind tunnel evaluation-based optimization for improvement of flow control by plasma actuators

A kriging-based genetic algorithm called efficient global optimization (EGO) was employed to optimize the parameters for the operating conditions of plasma actuators. The aerodynamic performance was evaluated by wind tunnel testing to overcome the disadvantages of time-consuming numerical simulations. The proposed system was used on two design problems to design the power supply for a plasma actuator. The first case was the drag minimization problem around a semicircular cylinder. In this case, the inhibitory effect of flow separation was also observed. The second case was the lift maximization problem around a circular cylinder. This case was similar to the aerofoil design, because the circular cylinder has potential to work as an aerofoil owing to the control of the flow circulation by the plasma actuators with four design parameters. In this case, applicability to the multi-variant design problem was also investigated. Based on these results, optimum designs and global design information were obtained while drastically reducing the number of experiments required compared to a full factorial experiment.     

Superresolution laser beam shaping

The superresolution technique is well known for its ability to compress the central diffractive spot that is smaller than the Airy diffractive spot. In this paper, we extend the superresolution technique for different laser beam shaping. A complete set of superresolution diffractive elements is developed for the flat-top beam shaping, the single-circle beam shaping, and the novel circular Dammann grating. Five phase plates, corresponding to each of its applications, have been made by use of micro-optics technology. Experiments that are presented are in good agreement with the theoretical results. The superresolution technique presented in this paper should be highly interesting for the wide applications of laser beam shaping.