Birefringence dynamics of poly{1-[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfonamido]-1,2-ethanediyl, sodium salt} cast films

Photoinduced birefringence creation/decay dynamics in poly{1-[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfonamido]-1,2-ethanediyl, sodium salt}cast films, has been characterized in the 5 to 100 mW writing laser power range at the wavelength of 514 nm. The maximum birefringence magnitude increased with laser beam power, being the largest value of 0.03, measured at 632.8 nm. Birefringence creation kinetics followed a biexponential behavior, where a slow process and a fast process could be clearly distinguished. The fast process accounted for more than 60% of birefringence. The time constants for both fast and slow processes decreased with increasing power of writing beam. Birefringence relaxation after removal of writing laser beam revealed to be independent of beam power and, was found to be ruled by two processes, described by a biexponential curve plus a constant term accounting for residual birefringence. A high residual birefringence signal of 85 ± 3% was attained, with the fast process contributing to 67% of decay from maximum value. The large values obtained for time constants for both birefringence creation and relaxation processes are believed to arise from stabilization of azo-group either by the formation of an ionic network, or by local short range interactions that are coming through as a result polyelectrolyte coiling due to its high degree of ionization.     

Ray-transfer-matrix model for accurate pulsed cavity ring-down measurement in the mismatching case

A theoretical model based on the ray-transfer matrix is developed for the pulsed cavity ring-down (CRD) technique to numerically investigate the influence of the geometric parameters of the pulsed-CRD arrangement on the CRD signal. By fitting the spatial distribution of the pulsed laser beam to that of the TEM(00) cavity mode, the geometric parameters are optimized to obtain perfect matching between the laser beam and the ring-down cavity. It is indicated by the numerical simulations that as long as the laser power exiting the ring-down cavity is fully collected, a single exponential-decay signal, identical to the perfectly-matched CRD signal, is obtained in the mismatching case to determine accurately the cavity decay time. Intensity fluctuations appear in the mismatched CRD signal if the laser power exiting the ring-down cavity is not fully collected. Both the conventional exponential decay fitting approach and a linear fitting procedure are employed to analyze these mismatched CRD signals and the latter is recommended to make an accurate pulsed-CRD measurement.     

球面扫描的数学模型及其计算机仿真

在检测球体表面时,需要用激光束对其进行扫描。研究了当用固定不动的激光束扫描球心固定的旋转球体时扫描点在球面上的移动模式。用微分方程组建立了扫描点与角速度矢量变化规律的数学模型,并对此模型及其控制曲面分别用仿真软件Simulink和三维计算机图形工具OpenGL进行了成功的计算机仿真。由于在此模型中激光束是固定不动的,因此,可使检测装置大大简化。     

Fuzzy logic model for bending angle in laser forming

Abstract

Laser forming is a thermoelastoplastic and complicated process. Finite element model simulation is time consuming and the analytical model is cumbersome for computing the bending angle. A model for bending angle in laser forming is constructed using adaptive fuzzy logic called adaptive network fuzzy inference system (ANFIS). The ANFIS model is trained with the published experiment data, in which the laser forming process parameters include thickness of the plate, laser power, laser beam diameter and scanning velocity. Trained ANFIS model is also tested on experiments not pertaining to previous training data. The performance of ANSIF model is optimised as a function of a type of membership function and number of membership functions. Optimised ANFIS model well predicts the results compared with the experiment data. Based on the established model, the analysis results of the process parameters show that a maximum bending angle can be achieved by choosing a reasonable laser beam diameter given other laser forming parameters.     

Simulation of temperature distribution in single metallic powder layer for laser micro-sintering

Simulation of temperature distribution in single metallic powder layer for laser micro-sintering (LMS) using finite element analysis (FEA) has been proposed, taking into account the adoption of ANSYS μMKS system of units, the transition from powder to solid and the utilization of moving laser beam power with a Gaussian distribution. By exploiting these characteristics a more accurate model could be achieved. The effects of the process parameters, such as laser beam diameter, laser power and laser scan speed on the temperature distribution and molten pool dimensions have been preliminarily investigated. It is shown that temperature increases with the laser power and decreases with the scan speed monotonously. For the laser beam diameter during single-track, the maximum temperature of the powder bed increases with the decrease in the laser beam diameter, but far from the center of the laser beam area, the temperature increases with the laser beam diameter. The molten pool dimensions in LMS are much less than that in classical selective laser sintering (SLS) process. Both molten pool length and width decrease with the laser beam diameter and the laser scan speed, but increase with the laser power. The molten pool length is always larger than the molten pool width. Furthermore, the center of molten pool is slightly shifted for the laser multi-track.     

Taper array in silica glass for beam splitting

We proposed taper array in silica glass for beam splitting which was fabricated by water-assisted femtosecond laser direct writing technology and the subsequent heat treatment. We divided the array into many fabricating cells which were executed automatically in sequences as specified by the program that contained the information for the three-dimensional stage movements. Each cell could fabricated a rectangular cylinder. The size and distribution of the rectangular cylinder could be controlled by adjusting the position of the fabricating cells. Then the heat treatment should be used to reshape the rectangular cylinders into taper array. The experimental results show that the taper periodic microstructures in silica glass are uniform and smooth, and the tapers can divide the incident light into beam array. The results demonstrated that the combination of the water-assisted femtosecond laser direct writing technology and the heat treatment is accessible and practical for the high quality micro-optical elements. These micro-optical elements will have potential applications in fluorescence detection and beam splitter.     

Saturable-absorber-based spatial filtering of high-power laser beams

A saturable-absorber-based technique for spatial filtering of high-average-power laser beams is described. For a focused, radially symmetric beam having its highest intensity at the center, this saturable absorber behaves like a soft aperture with gradually increasing attenuation toward the beam edges, thus selectively transmitting the low divergence components that are confined close to the central axis of the propagating laser beam. This technique has been successfully used to reduce the divergence of a high-power, high-repetition-rate, tunable, narrowband, pulsed dye laser. Our results demonstrate how a judicious choice of operating parameters allows spatial filtering to be achieved with the introduction of a minimum absorption loss of the laser beam in the saturable absorber. Following a time-dependent analysis of a rate equation model describing the propagation and interaction of the laser beam with the saturable absorber, we have also obtained theoretical estimates for the extent of spatial filtering. Our theoretical estimates have been found to be in good agreement with our experimental observations.     

A Semi-analytic Approach for Coherent Laser Radar System Efficiency

Abstract

A nearest Gaussian approximation (NGA) is proposed to approximate any shape for a single mode laser beam by a Gaussian shape. The application considered is a determination of the system efficiency in heterodyne coherent laser radar (HCLR). For an actual beam its NGA is defined by three parameters: the waist spot size and location, and an amplitude coefficient. These parameters are computed by a maximization of the norm of the scalar product written for the actual and Gaussian beams. In the case of the truncated Gaussian beam, particularly relevant to HCLR, the waist location can be analytically calculated, and only two parameters remain unknown: the waist spot size and amplitude coefficient. Using numerical applications, it is shown the NGA is in good agreement with Fresnel integral solution. The NGA combines a good accuracy and capability of analytical solutions. It can treat a variation in system efficiency owing to a misalignment angle between the transmitter and local oscillator.     

Optimization of the LENS Rapid Fabrication Process for In-Flight Melting of Feed Powder

A model for in-flight melting of feed-powder particles propelled through a laser beam in the Laser-Engineered Net Shaping (LENS) process has been developed. The model is next incorporated in an optimization analysis to determine optimum LENS process parameters (laser power, particle velocity, and the angle between the laser-beam axis and particle trajectory), which maximize the probability for in-flight particle melting while ensuring the absence of melting of the surface of the substrate. A simple model, based on solution of the thermal energy conservation equation, is also developed to determine the laser-power threshold for melting of the substrate surface. The optimization analysis is then applied to Inconel 625 Ni-Cr-Mo superalloy. The results show that by maximizing the laser power and the residence time of the particles in the laser beam (increases with reductions in particle velocity and particle trajectory angle), the probability for in-flight particle melting can be greatly increased, i.e. relatively coarse (–30/+40 mesh size) particles can be melted by propelling them through the laser beam.     

Laser surface hardening of thin steel slabs

The transformation hardening of steels by surface heating by a c.w. laser beam has been studied. We examined the surface treatment of thin steel slabs by a suitable mathematical model of the thermal transient induced by laser beam heating. The laser parameters for surface hardening of such samples and the resulting microstructures are discussed. Hardening depths calculated from the mathematical model fit well with experimental results.     

Some characteristics of an extremely-short-external-cavity laser diode realized by butt coupling a Fabry-Perot laser diode to a single-mode optical fiber

When butt coupling a Fabry-Perot laser diode to an extremely closely spaced waveguide (separation less than or equal to a few times the Rayleigh range of the laser beam), there is a trade-off between the optimal power coupling and the variation of the coupled laser diode's operational characteristics. Changes in the butt-coupling configuration parameters influence the coupling efficiency, as well as the strength of the feedback into the laser diode. Using a previously reported phenomenological model that treats the butt-coupled laser diode as an extremely short external-cavity (ESEC) device, we quantitatively describe how the butt-coupling parameters can be used to control the output power, threshold current, wavelength, and relative intensity noise of the ESEC laser diode. Our analyses are supported by experimental results. The importance of choosing the correct coordinate plane for evaluation of the overlap integrals that are used in the model is also discussed.     

Robust optimisation of Nd: YLF laser beam micro-drilling process using Bayesian probabilistic approach

Nd: YLF laser beam machining (LBM) process has a great potential to manufacture intricate shaped microproducts with its unique characteristics. Continuous improvement (CI) effort for LBM process is usually realised by response surface methodology, which is an important tool in Design of Six Sigma. However, when determining the optimal machining parameters in CI for LBM process, model parameter uncertainty is typically neglected. Performing worst case analysis in CI, this paper presents a new loss function method that takes model parameter uncertainty into account via Bayesian credible region. Unlike existing CI methods in LBM process, the proposed Bayesian probabilistic approach is based on seemingly unrelated regression which can produce more precise estimations of the model parameters than ordinary least squares in correlated multiple responses problems. An Nd: YLF laser beam micro-drilling process is used to demonstrate the effectiveness of the proposed approach. The comparison results show that micro-holes produced by the proposed approach have better quality than those of existing approaches in terms of robustness and process capability.     

Study on Laser Transformation Hardening of HT250 by High Speed Axis Flow CO2 Laser

In this article, laser transformation hardening of HT250 material by high speed axis flow CO2 laser was investigated for first time in China. Appropriate laser hardening parameters, such as laser energy power P(W), laser scanning rate V(m/min), were optimized through a number of experiments. The effect of the mentioned parameters on the hardened zone, including its case depth, microhardness distributions etc., were analyzed. Through the factual experiments, it is proved that axial flow CO2 laser, which commonly outputs low mode laser beam, can also treat materials as long as the treating parameters used are rational. During the experiments, the surface qualities of some specimens treated by some parameters were found to be enhanced, which does not coincide with the former results. Furthermore in the article, the abnormal phenomenon observed in the experiments is discussed. According to the experimental results, the relationship between laser power density q and scanning rate V is shown in a curve and the corresponding formulation, which have been proved to be valuable for choosing the parameters of laser transformation hardening by axial flow CO2 lasers, was also given.     

Determination of the longitudinal profile of a focused Nd:YAG Gaussian beam from second-harmonic generation in a thin KTP crystal

We present an original experiment describing a focused Gaussian laser beam by using second-harmonic generation (SHG) in a thin strip of a nonlinear optical material, in this case KTP doubling of an Nd:YAG laser at 1.064 μm. The dependence of the SHG efficiency on the fundamental beam radius allows the determination of beam parameters by harmonic-power measurements. The characterization of a telescope shows the good precision of this method in radii in the range of 10-100 μm. The average accuracy is 4% for the radius determination and 1.5% for the beam-waist localization.     

Analysis and optimization of fabrication of continuous-relief diffractive optical elements

The fabrication of continuous-relief diffractive optical elements by direct laser beam writing in photoresist is analyzed. The main limitation and tolerances are identified, and their influence on optical performance is quantified. Fabricated structures show rounded profile steps resulting from the convolution of the desired profile with the writing beam. This leads to a reduction in diffraction efficiency. Optimization techniques are presented to minimize this effect. Scaling the profile depth by a factor of mu > 1 increases the first-order diffraction efficiency for blazed elements. This method is also applied to suppress the zeroth diffraction order in computer-generated holograms. A nonlinear compensation of the exposure data for the Gaussian beam convolution results in an 18% increase of the diffraction efficiency for a blazed grating with a 10-mum period to a value of 79%.     

Photo-rewritable flexible LCD using indium zinc oxide/polycarbonate substrates

This work presents a photo-rewritable transmissive flexible-LCD based on the alignment effect of the photo-induced adsorption of azo dye on flexible indium zinc oxide/polycarbonate (IZO/PC) substrates. Images can be displayed without applying an external field and rewritten by changing the direction of the writing laser beam while the cell temperature is controlled. By using IZO/PC substrates, the writing and erasing processes can be achieved within 1 min with a high contrast.     

Diffractive light attenuators with variable transmission

Abstract

A new application of diffractive optical elements (DOEs) for continuous or multistage adjustment of optical radiation intensity is described. The diffractive attenuators are linear or circular gratings (amplitude or phase) with constant period and diffraction efficiency that varies across the grating. The zero order of diffraction is used as the output and transmitted through the grating without angular deviation. The diffractive attenuators, in distinction to conventional analogues, allow one to change the intensity of the light beam according to predetermined function and have no limitations for power of the regulated light beam. These elements can be used in optical systems as a beam splitter with adjusted splitting coefficient. The experimental results on a circular diffractive attenuator fabricated by direct laser writing on a chromium film are presented. The range of transmission variation was 20 times within a 340° angle of attenuator turn. The possibility to use a phase diffractive attenuator as a light radiation modulator for a powerful technological laser is discussed.     

Simulation of inverse heat conduction problems in fusion welding with extended analytical heat source models

The paper presents bounded volume heat sources and the corresponding functional-analytical expressions for the temperature field. The power density distributions considered here are normal, exponential and parabolic. The sources model real heat sources like the welding arc, laser beam, electron beam, etc., the convection in the weld pool as well as the latent heat due to fusion and solidification. The parameters of the heat source models are unknown a priori and have to be evaluated by solving an inverse heat conduction problem. The functional-analytical technique for calculating 3D temperature fields in butt welding is developed. The proposed technique makes it possible to reduce considerably the total time for data input and solution. It is demonstrated with an example of laser beam welding of steel plates.     

基于微透镜阵的外腔谱组束系统光束质量分析

针对基于微透镜阵的光纤激光外腔谱组束系统的光束质量评价问题,利用高斯光束的传输变换理论,建立了评价谱组束激光光束质量的理论模型.通过数值模拟,详细分析了各相关参数对谱组束光束质量的影响,结果表明:在基于微透镜阵的谱组束系统中,离焦量、微透镜焦距及模场半径是影响谱组束激光光束质量的主要因素;阵列宽度对谱组束激光光束质量影...     

Laser beam melting of aluminum alloys with hull-core build strategy by using an initial meso-scale simulation

Laser beam melting (LBM) of aluminum alloys is gaining a wide popularity in different industrial applications as an alternative technology for the production of individual and complex parts. A long build time and the high amount of experimental work for optimizing or finding new process parameters are two of the current challenges for reaching an industrial maturity. This paper proposes an efficient way to determine new process parameters for aluminum alloy aluminum-silicon10-magnesium with highest build-up rates by using a 3D finite element model on the mesoscopic level. High laser power in combination with the hull-core build strategy was used to increase the build-up rate without impairing the part accuracy. The influences of high laser power, laser diameter and scan speed on the melt pool were studied by using a thermal simulation of single laser tracks. Based on the simulation results the process window could be derived and was tested on a laser beam melting (LBM) system. The achieved reduction of the build time of up to 31 % without loss in part accuracy proved the novel approach for the prediction of the required process window as an efficient method to reduce costly and time-consuming experimental work.