Influence of secondary treatment with CO_2 laser irradiation for mitigation site on fused silica surface

The ablation debris and raised rim, as well as residual stress and deep crater will be formed during the mitigation of damage site with a CO2 laser irradiation on fused silica surface, which greatly affects the laser damage resistance of optics. In this study, the experimental study combined with numerical simulation is utilized to investigate the effect of the secondary treatment on a mitigated site by CO_2 laser irradiation. The results indicate that the ablation debris and the raised rim can be completely eliminated and the depth of crater can be reduced. Notable results show that the residual stress of the mitigation site after treatment will reduce two-thirds of the original stress. Finally, the elimination and the controlling mechanism of secondary treatment on the debris and raised rim, as well as the reasons for changing the profile and stress are analyzed. The results can provide a reference for the optimization treatment of mitigation sites by CO_2 laser secondary treatment.     

Surface damage morphology investigations of silicon under millisecond laser irradiation

The surface damage morphologies of single crystal silicon induced by 1064 nm millisecond Nd:YAG laser are investigated. After irradiation, the damage morphologies of silicon are inspected by optical microscope (OM) and atomic force microscope (AFM). The plasma emission spectra of the damaged region are detected by the spectrometer. It is shown that surface oxidation and nitridation have occurred during the interaction of millisecond laser with silicon. In addition, the damage morphologies induced by 2 ms and 10 ns pulse width laser are compared. The damage morphology obtained by 2 ms laser is an evident crater. Three types of damage morphologies are formed at different laser energy densities. The circular concentric ripples are found surrounding the rim of the crater. The spacing of the ripples is 15 ± 5 μm. Two types of cracks are observed: linear crack and circular crack. The linear crack is observed in the center of the damaged region which propagates to the periphery of the damaged region. The circular crack is located at the rim of the crater. The damage morphology induced by 10 ns laser is surface layer damage. The periodic linear waves are generated due to the interference between the incident beam and the scattered beam. The spacing of the ripples is 1.54 μm which is close to the incident laser wavelength 1.064 μm. The linear crack is located at the center of the damaged region. Furthermore, for the same laser energy density, the dimension of the damaged region and the crater depth induced by 2 ms laser are greater than that of 10 ns laser. It indicates that the damage mechanism under millisecond pulse laser irradiation is strongly different from the case of nanosecond pulse laser.     

The interaction and the surface crack of single-crystal silicon induced by a millisecond laser

When the silicon material is irradiated by laser, it absorbs the laser energy leading to the temperature rise and the thermal stress. The damage effect includes melting, vaporation and thermal stress damage. Once the thermal stress exceeds the stress strength the crack will initiate. The silicon surface cracks induced by a millisecond laser are investigated. The experimental results show that three types of cracks are generated including cleavage crack, radial crack and circumferential crack. The cleavage crack is located within the laser spot. The radial crack and circumferential crack are located outside the laser spot. A two-dimensional spatial axisymmetric model of silicon irradiated by a 1064 nm millisecond laser is established. To assess what stresses generate and explain the generation mechanism of the different cracks, the thermal stress fields during laser irradiation and the cooling process are obtained using finite element method. The radial stress and hoop stress within the laser spot are tensile stress after the laser irradiation. The temperature in the center is the highest but the thermal stress in the center is not always highest during the laser irradiation. The cleavage cracks are induced by the tensile stress after the laser irradiation. The radial crack and the circumferential crack are generated during the laser irradiation.     

Numerical model and experimental demonstration of high precision ablation of pulse CO_2 laser

To reveal the physical mechanism of laser ablation and establish the prediction model for figuring the surface of fused silica, a multi-physical transient numerical model coupled with heat transfer and fluid flow was developed under pulsed CO_2 laser irradiation. The model employed various heat transfer and hydrodynamic boundary and thermomechanical properties for assisting the understanding of the contributions of Marangoni convention,gravitational force, vaporization recoil pressure, and capillary force in the process of laser ablation and better prediction of laser processing. Simulation results indicated that the vaporization recoil pressure dominated the formation of the final ablation profile. The ablation depth increased exponentially with pulse duration and linearly with laser energy after homogenous evaporation. The model was validated by experimental data of pulse CO_2 laser ablation of fused silica. To further investigate laser beam figuring, local ablation by varying the overlap rate and laser energy was conducted, achieving down to 4 nm homogenous ablation depth.     

微细腔内CO_2对CH_4/H_2O重整反应特性影响

基于镍基催化剂下表面反应机理,采用数值模拟方法,深入研究了在一定水碳比下,CO_2含量的变化以及在固定的CH_4/H_2O/CO_2比例下催化壁面温度,质量流量对微细腔内甲烷重整反应的影响。结果表明提高CO_2/CH_4的比例能够明显提高CO_2、CH_4转化率,提高CO的含量。CH_4/CO_2基元反应的产物在低温条件下CO和H_2O,水蒸气转化率和H_2产量降低,CO_2含量增加降低水蒸气的消耗;在高温条件下,由于产生CO和H_2,使氢气含量增加但水蒸气转化率降低.     

Reduction of post-weld shift in semiconductor laser packaging

A reduction of post-weld shift (PWS) in semiconductor laser packaging using a laser welding technique is presented. It is found that the PWS and its association with the power coupling loss of the laser packaging can be reduced under proper pressure constraint. Characteristics of defect mechanisms in laser welding techniques for semiconductor laser packaging are also investigated experimentally. The results in the stainless-to-Kovar joints show that the surface cracks are dependent on the Au thickness on the Kovar material. The low solubility of gold in the Kovar is identified as the defect mechanism for surface cracks. Preliminary reliability data demonstrated that these laser packages without crack defects in the welded joints are reliable.     

Incident laser modulation of a repaired damage site with a rim in fused silica rear subsurface

Local CO2 laser treatment has proved to be an effective method to prevent the 351-nm laser-induced damage sites in a fused silica surface from exponentially growing, which is responsible for limiting the lifetime of optics in high fluence laser systems. However, the CO2 laser induced ablation crater is often surrounded by a raised rim at the edge, which can also result in the intensification of transmitted ultraviolet light that may damage the downstream optics. In this work, the three-dimensional finite-difference time-domain method is developed to simulate the distribution of electrical field intensity in the vicinity of the CO2 laser mitigated damage site located in the exit subsurface of fused silica. The simulated results show that the repaired damage sites with raised rims cause more notable modulation to the incident laser than those without rims. Specifically, we present a theoretical model of using dimpled patterning to control the rim structure around the edge of repaired damage sites to avoid damage to downstream optics. The calculated results accord well with previous experimental results and the underlying physical mechanism is analysed in detail.     

A conduction model for deep penetration laser welding based on an actual keyhole

Using a specially designed experimental setup and properly choosing the sample material and the process parameters, we obtained a clear stable keyhole with a high-speed camera. On the basis of the actual keyhole profile, a conduction model with a cylindrical surface heat source has been developed under the assumption of the keyhole per thin layer being cylindrical. The model is numerically solved by the finite-difference method, the temperature field around the keyhole and the heat flux lost on the keyhole wall can be obtained. The effects of such factors as the shape and the size of the keyhole, the welding speed on the shape of the melt pool are studied. By comparing the laser intensity absorbed on the keyhole walls with the heat flux lost there, the mechanism of energy balance on the keyhole walls was investigated.     

Crystallization over the liquid layer

In this article, a new phenomenon of high-speed crystallization of metals in a low-temperature plasma formed as a result of the effect of a short laser pulse is considered. The mechanism of the way the reaction occurs on the surface of the melt formed under the effect of the laser pulse on an amorphous substrate is described. The main factors affecting the crystallization process are described. Primary attention is paid to laminar convection and the latent heat of crystallization.     

Rapid crystallization of copper on the own liquid substrate

The phenomenon of rapid crystallization of metals in low-temperature laser plasma is described. Possiblemechanism of the crystallization on the melt surface formed upon laser exposure is described. The major factors affecting the crystallization process are analyzed.     

3-D finite element modeling of laser cladding by powder injection: effects of laser pulse shaping on the process

This paper introduces a 3-D transient finite element model of laser cladding by powder injection to investigate the effects of laser pulse shaping on the process. The proposed model can predict the clad geometry as a function of time and process parameters including laser pulse shaping, travel velocity, laser pulse energy, powder jet geometry, and material properties. In the proposed strategy, the interaction between powder and melt pool is assumed to be decoupled and as a result, the melt pool boundary is first obtained in the absence of powder spray. Once the melt pool boundary is obtained, it is assumed that a layer of coating material is deposited on the intersection of the melt pool and powder stream in the absence of the laser beam in which its thickness is calculated based on the powder feedrate and elapsed time. The new melt pool boundary is then calculated by thermal analysis of the deposited powder layer, substrate and laser heat flux. The process is simulated for different laser pulse frequencies and energies. The results are presented and compared with experimental data. The quality of clad bead for different parameter sets is experimentally evaluated and shown as a function of effective powder deposition density and effective energy density. The comparisons show excellent agreement between the modeling and experimental results for cases in which a high quality clad bead is expected.     

Thermal stress cleaving of silicon wafer by pulsed Nd：YAG laser

The applied laser energy absorbed in a local area in laser thermal stress cleaving of brittle materials using a controlled fracture technique produces tensile thermal stress that causes the material to separate along the moving direction of the laser beam. The material separation is similar to crack extension, but the fracture growth is controllable. Using heat transfer theory, we establish a three-dimensional （3D） mathematical thermoelastic calculational model containing a pre-existing crack for a two-point pulsed Nd：YAG laser cleaving silicon wafer. The temperature field and thermal stress field in the silicon wafer are obtained by using the finite element method （FEM）. The distribution of the tensile stress and changes in stress intensity factor around the crack tip are analyzed during the pulse duration. Meanwhile, the mechanism of crack propagation is investigated by analyzing the development of the thermal stress field during the cleaving process.     

双预电离对TEA CO2激光器放电特性的影响

一些低增益弱线CO2激光光谱在分离某种同位素方面是非常有用的。在成比例的混合气体中很难获得这些谱线的输出,须适当提高CO2和N2的含量,但结果给主电极的辉光放电带来了困难。通过对改装后的双预电离放电回路的参量进行调节,实现了对预电离的单独控制,有利于增强预电离效果,使高含量的CO2混合气体的放电和出光成为可能。理论和实验证明了双预电离能较好地改善TEA CO2激光器的放电性能,尤其是有望实现用于分离某些同位素的弱线输出。     

The damage mechanism of fused silica surface induced by 355 nm nanosecond laser

In order to understand the physical mechanism, time-resolved dynamics of 355 nm nanosecond laser induced entrance and exit surface damage on fused silica was investigated by using shadow graphic technique. The results show that the damage mechanism is different between the entrance and exit surface during nanosecond laser interaction with fused silica. The plasma and shock waves in air is relatively higher at the entrance surface. The entrance surface damage is reduced because plasma shielding limits energy deposition. Without plasma shielding, the exit surface damage is more serious for more laser energy deposition in material. And without the stress of plasma and shock waves, the material is ejected easily at rear surface. These are confirmed by damage micrograph at the entrance and exit surface.     

Two localized CO₂ laser treatment methods for mitigation of UV damage growth in fused silica

Two methods:high-power,short-time,single-shot irradiation(Method A) and low-power,long-time,multi-shot irradiation(Method B) are investigated to mitigate the UV damage growth in fused silica by using a 10.6-μm CO2 laser.To verify the mitigation effect of the two methods,the laser induced damage thresholds(LIDTs) of the mitigated sites are tested with a 355-nm,6.4-ns Nd:YAG laser,and the light modulation of the mitigation sites are tested with a 351-nm continuous Nd:YLF laser.The mitigated damaged sites treated with the two methods have almost the same LIDTs,which can recover to the level of pristine material.Compared with Method A,Method B produces mitigated sites with low crater depth and weak light modulation.In addition,there is no raised rim or re-deposited debris formed around the crater edge for Method B.Theoretical calculation is utilized to evaluate the central temperature of the CO2 laser beam irradiated zone and the radius of the crater.It is indicated that the calculated results are consistent with the experimental results.     

Effect of nonionic surfactant addition on Pyrex glass ablation using water-assisted CO2 laser processing

Pyrex glass etching using laser ablation is an important technology for the microfluid application to lab-on-a-chip devices but suffers from the formation of surface crack. In this article, the addition of nonionic surfactant to water for glass ablation using water-assisted CO₂ laser processing (WACLAP) has been investigated to enhance ablation rate and to eliminate conventional surface defects of cracks in air. WACLAP for Pyrex glass ablation can reduce thermal-stress-induced crack with water cooling and hydrophilic nonionic surfactant to water can enhance ablation performance. Compared to pure water, the 15% weight percent Lauramidopropyl Betaine surfactant solutions for WACLAP can enhance ablation rate from 13.6 to 25 μm/pass of Pyrex glass ablation at a linear laser energy density of 2.11 J/cm, i.e., 24 W power, 114 mm/s scanning speed, and obtain through-wafer etching at 3.16 J/cm for 20 passes without cracks on the surface. Effect of surfactant concentration and linear energy density on WACLAP was also examined. The possible mechanism of surfactant-enhanced phenomenon was discussed by the Newton’s law of viscosity of surfactant solution.     

Theoretical and experimental investigation into laser melting of steel samples

Among the available laser applications, laser melting has turned out to be a powerful technique for the production of mechanically improved surfaces. To enhance the understanding of the laser melting process investigations into modeling of the heating mechanism initiating the laser melting are necessary. In the present study, a mathematical modeling of the laser melting process is introduced and power require ments for the laser melting are predicted as functions of workpiece properties and velocity. Maximum melt width is predicted analytically and compared with the experimental results. In this regard, an experiment is conducted to melt the mild steel samples with a cw CO₂ laser at different power settings and workpiece velocities. It is found that the melt variables predicted from theory are in agreement with the experimental results.     

Nanoscale cavitation instability of the surface melt along the grooves of one-dimensional nanorelief gratings on an aluminum surface

Femtosecond laser nanostructuring at low fluences produces a one-dimensional quasiperiodic grating of grooves on an aluminum surface with a period (≈0.5 μm) that is determined by the length of a surface electromagnetic wave. The structure of the grooves of the surface nanograting is formed by regular nanopeaks following with a period of about 200 nm. Some nanopeaks manifest craters at their tops. It is suggested that nanopeaks are formed due to the frozen nanoscale spallative ablation of a nanolayer of an aluminum melt in quasiperiodic regions corresponding to interference maxima of the laser radiation with the surface electromagnetic wave. The periodicity of the appearance of nanopeaks along grooves is due to the previously predicted mechanism of cavitation deformation of the melt surface in the process of macroscopic spallation ablation. However, in this case, cavitation is coherent (similar to a near-critical spinodal decay) rather than spontaneous.     

CO_2激光发射装置光轴平行度的快速检测方法

设计了一个装调检测辅助系统,可完成外场条件下CO_2激光发射装置光轴平行度的快速检测。介绍了辅助系统的工作原理和检测装调过程,对影响检测精度的误差因素进行了分析和计算,着重分析了由辅助系统的装调和结构变形引起的误差,检测精度为5.3″。与大口径平行光管检测法进行了对比,结果表明,相对误差不大于1″。     

激光诱导熔石英表面损伤修复中的气泡形成和控制研究

基于熔石英材料在CO_2激光作用下的温度分布和结构参数变化的计算结果,对熔石英损伤修复中的气泡形成和控制进行了研究.针对损伤尺寸介于150—250μm之间的损伤点,提出了一种能够有效控制气泡形成的长时间低温预热修复方法.基于低温下熔石英材料结构弛豫时间常数较长的特点,该方法在不引起熔石英材料结构发生显著变化的同时,能够解吸附表面和裂纹处所附着的气体和杂质,可有效降低裂纹闭合过程中气泡形成的概率.实验结果表明,长时间低温预热修复方法的成功修复概率可达到98%.