大数值孔径微透镜阵列激光扫描

提出微透镜阵列与转镜相结合的大口径激光光束扫描方法。采用机械制模法制作微透镜阵列模板 ,然后采用模压法制作光学微透镜阵列 ,其子口径为 2 mm× 2 mm,数值孔径为 0 .2 ,阵列数为 8× 8。并进行了扫描测试实验 ,扫描角达± 6.56°。     

Parametric studies of multi-material laser densification

Fabrication of multi-material components via a laser-assisted layer-by-layer fabrication process has been numerically simulated and analyzed using a three-dimensional thermo-mechanical model. Effects of the chamber preheating temperature, laser scanning rate, initial porosity and thickness of each powder layer on the out-of-plane warping and residual thermal stresses of a nickel/porcelain workpiece have been investigated. It is found that warping and residual thermal stresses of the laser-densified multi-material workpiece are more sensitive to the chamber preheating temperature and the thickness of each powder layer than to the laser scanning rate and the initial porosity of the powder layer. The major mechanism responsible for these phenomena is identified to be related to the change of the temperature gradient induced by these laser processing parameters.     

Microlenses and Microlens Arrays Formed on a Glass Plate by Use of a CO(2) Laser

Microlenses and microlens arrays were formed directly on a surface of a glass plate by use of a CO(2) laser. When the surface of a glass plate is heated locally to a working point of the glass material by use of a focused CO(2) laser beam, it tends to become a hyperboloid owing to surface tension, which results in a microlens. A profile of the microlens was measured with an ultrahigh accurate three-dimensional profilometer (Model UA3P, Matsusita Electric Industrial Company Ltd.) that utilizes a specially designed atomic force microscope. An intensity profile and a spot diameter at the focus of the microlens were measured with a microscope and a CCD system utilizing a He-Ne laser as a light source. The focused spot FWHM diameter of 1.35 mum was obtained, and the modulation transfer function was derived from the spot profile. Microlens arrays were also fabricated and characterized.     

Three-dimensional computer model for simulating realistic solid-state lasers

We developed an accurate and fast three-dimensional computer model for simulating realistic solid-state laser systems. An iteration of the beam propagation calculation was developed to account for the counterpropagation of the laser beams in the saturated gain medium and eventually obtain the converged solution for the output beam. An analytic method was devised to account for the curved cavity mirror and the surface deformation of the gain medium induced by the temperature gradient due to pump absorption. The temperature gradient induced thermal lensing and stress birefringence is also properly included in the model calculation. This model has been validated and shown to be very accurate and efficient for modeling three-dimensional laser systems in a personal computer.     

Three-dimensional calculation of high-power, annularly distributed, laser-beam-induced thermal effects on reflectors and windows

Based on the three-dimensional transient heat conduction equation and the elastic stress-strain equation, the temperature rise, distortion, and equivalent stress distributions of a high-reflectivity silicon reflector and a white bijou window irradiated by a high-power sloped annularly distributed laser beam are simulated using a three-dimensional finite element model (FEM). The effects of laser intensity, output duration, beam obscure ratio, and laser intensity spatial gradient on the results are especially investigated. The effects of mirror and window thermal distortion on laser beam phase aberrations are also evaluated. This noncylindrosymmetric three-dimensional FEM can be used to evaluate high-power, high-energy, laser beam-induced thermal effects on optical components.     

Actual focal length of a symmetric biconvex microlens and its application in determining the transmitted beam waist position

The actual focal length of a three-dimensional continuous profile symmetric biconvex microlens with normal monochromatic plane wave illumination is theoretically determined using a full-field separation of variables method in the oblate spheroidal coordinate system. The investigations are performed for microlenses of 5, 10, and 20 wavelength diameters by calculating the electromagnetic field distributions inside of and adjacent to the microlenses. The importance and potential application of the microlens actual focal length in the design of microlens optical systems are demonstrated by showing that for normal monochromatic TEM00 mode Gaussian beam illumination, the transmitted beam waist position through a single microlens, calculated using Self's beam waist position transformation formula [Appl. Opt.22, 658 (1983)] with the microlens actual focal length, closely matches the exact value given by the separation of variables method.     

温度梯度溶液生长法制备x=0.2的Cd1-xZnxTe晶体及性能研究

利用温度梯度溶液生长法(TGSG)在较低生长温度下制备了掺Al和掺In的x=0.2的Cd1-xZnxTe晶体,晶体起始生长温度约为1223K,温度梯度为20～30K/cm,坩埚的下降速度为1mm/h。采用红外显微镜、傅里叶红外光谱仪、扫描电镜能谱仪(SEM/EDS)和I-V测试分别研究了晶体中的Te夹杂相、红外透过率、Zn组分分布和电阻率。结果显示CdZnTe晶锭初始生长区、稳定生长区的Te夹杂相密度分别为8.3×103、9.2×103/cm-2,比垂直布里奇曼法生长的晶体低约1个数量级,红外透过率分别为61%、60%。Al掺杂CdZnTe晶体的电阻率为1.05×106Ω.cm,而In掺杂CdZnTe晶体的电阻率为7.85×109Ω.cm。晶锭初始生长区和稳定生长区的Zn组分径向分布均匀。     

钛合金表面激光熔覆TiC/NiCrBSi涂层温度场有限元模拟

为在钛合金表面获得优质激光熔覆涂层,用有限元方法研究了激光熔覆工艺对熔池温度场分布和凝固后熔覆层组织的影响,考虑相变潜热、辐射对流散热以及温度对热物理性能的影响等因素,建立三维有限元模型模拟了Ti6Al4V合金表面激光熔覆TiC/NiCrBSi复合涂层过程中的温度场,并结合熔覆过程的温度场分布,对涂层的形貌、结合区、基...     

Design and fabrication of a freeform microlens array for a compact large-field-of-view compound-eye camera

In this research, a unique freeform microlens array was designed and fabricated for a compact compound-eye camera to achieve a large field of view. This microlens array has a field of view of 48°×48°, with a thickness of only 1.6 mm. The freeform microlens array resides on a flat substrate, and thus can be directly mounted to a commercial 2D image sensor. Freeform surfaces were used to design the microlens profiles, thus allowing the microlenses to steer and focus incident rays simultaneously. The profiles of the freeform microlenses were represented using extended polynomials, the coefficients of which were optimized using ZEMAX. To reduce crosstalk among neighboring channels, a micro aperture array was machined using high-speed micromilling. The molded microlens array was assembled with the micro aperture array, an adjustable fixture, and a board-level image sensor to form a compact compound-eye camera system. The imaging tests using the compound-eye camera showed that the unique freeform microlens array was capable of forming proper images, as suggested by design. The measured field of view of ±23.5° also matches the initial design and is considerably larger compared with most similar camera designs using conventional microlens arrays. To achieve low manufacturing cost without sacrificing image quality, the freeform microlens array was fabricated using a combination of ultraprecision diamond broaching and a microinjection molding process.     

Design and fabrication of polymeric micro-optical components using excimer laser ablation

Abstract

Numerical simulation was used to predict the profile of a three-dimensional aspherical microlens and a microprism array. Based on the simulated results, the desired micro-optical lens profile was obtained using excimer laser ablated polyimide. The simulation method applied to excimer laser ablation can significantly reduce the quantity of microablation experiments. Ablated microstructures with surface roughness R _a < 20 nm were successfully achieved for micro-optical components. The excimer laser ablation parameters included laser fluence, shot number, workstation velocity, and repetition rate. Numerical simulation can be applied to predict various profiles of microlens and microprism arrays with different dimensions, the desired geometries being formed by laser ablation.     

Shack Hartmann wave-front measurement with a large F-number plastic microlens array

A new plastic microlens array, consisting of 900 lenslets, has been developed for the Shack Hartmann wave-front sensor.The individual lens is 300 μm × 300μm and has a focal length of 10 mm, which provides the same focal size, 60 μm in diameter, with a constant peak intensity. One can improve thewave-front measurement accuracy by reducing the spot centroiding error by averaging a few frame memories of an image processor. A deformable mirror for testing the wave-front sensor gives anappropriate defocus and astigmatism, and the laser wave front is measured with a Shack Hartmann wave-front sensor. The measurement accuracy and reproducibility of our wave-front sensor are better than λ/20 and λ/50 (λ = 632.8 nm),respectively, in rms.     

Design and performance of a ZnSe tetra-prism for homogeneous substrate heating using a CO2 laser for pulsed laser deposition experiments

We report on the design and performance of a ZnSe tetra-prism for homogeneous substrate heating using a continuous wave CO(2) laser beam in pulsed laser deposition experiments. We discuss here three potential designs for homogenizing prisms and use ray-tracing modeling to compare their operation to an alternative square-tapered beam-pipe design. A square-pyramidal tetra-prism design was found to be optimal and was subjected to modeling and experimental testing to determine the influence of interference and diffraction effects on the homogeneity of the resultant intensity profile produced at the substrate surface. A heat diffusion model has been used to compare the temperature distributions produced when using various different source intensity profiles. The modeling work has revealed the importance of substrate thickness as a thermal diffuser in producing a resultant homogeneous substrate temperature distribution.     

Concurrent three-dimensional characterization of the refractive-index and residual-stress distributions in optical fibers

A three-dimensional index-stress distribution (3DISD) measurement method for determining concurrently the refractive-index distributions (RIDs) and residual-stress distributions (RSDs) in optical fibers is presented. The method combines the quantitative-phase microscopy technique, the Brace-K?hler compensator technique, and computed tomography principles. These techniques are implemented on a common apparatus to enable concurrent characterization of the RID and the RSD. Measurements are performed on Corning SMF-28 fiber in an unperturbed section and in a section exposed to CO₂ laser radiation. The concurrent measurements allow for the first accurate comparison of the collocated RID and RSD. The resolutions of the refractive index and stress are estimated to be 2.34×10^-5 and 0.35?MPa, respectively.     

Temperature distributions and thermal deformations of mirror substrates in laser resonators

For finite-thickness media with convective surface losses, the three-dimensional temperature distributions and thermal deformations of mirror substrates in laser resonators that are due to absorption of laser light with a Gaussian power-density profile are calculated by use of the well-known Green's function methods. Some expressions and theoretical profiles of the temperature distributions and thermal deformations as functions of the radius and the thickness of a mirror substrate are obtained. The results of the calculations show that the rise in temperature is closely related to the absorption coefficient of the medium as well as to the convective heat-transfer coefficient, that the initial thermal deformations of mirror surfaces increase quickly at the beginning of laser heating and that then the thermal deformations are insensitive to laser heating times. Meanwhile, thermal deformations of a silicon mirror are experimentally demonstrated by use of CO(2) laser irradiation. The experimental trends of thermal deformations are in agreement with the theoretical profiles.     

Multipoint diffraction strain sensor with variable sensitivity

In our earlier work, a multipoint diffraction strain sensor using a microlens array was developed for measurement of whole-field strains. The method is extended to a system with variable sensitivity and measurement range. In the present system, two collimated laser beams, 3 mm in diameter, symmetrically strike the grating attached to the specimen surface at prescribed angles. The diffracted wavefronts, magnified by a microscope objective, are sampled by a lenslet array with each microlens acting as an individual strain sensor. In-plane strain components over the full field can be measured by what is to our knowledge a new sensor with variable sensitivity by changing the distance from the microscope objective to the microlens array. Both a theoretical explanation and experimental verification are provided.     

Rapid Directional Solidification with Ultra-High Temperature Gradient and Cellular Spacing Selection of Cu-Mn Alloy

The detailed laser surface remelting experiments of Cu-31.4 wt pct Mn and Cu-26.6 wt pct Mn alloys on a 5kW CO2 laser were carried out to study the effects of processing parameters (scanning velocity, output power of laser) on the growth direction of microstructure in the molten pool and cellular spacing selection under the condition of ultra-high temperature gradient and rapid directional solidification. The experimental results show that the growth direction of microstructure is strongly affected by laser processing parameters. The ultra-high temperature gradient directional solidification can be realized on the surface of samples during laser surface remelting by controlling laser processing parameters, the temperature gradient and growth velocity can reach 10^6K/m and 24.1mm/s, respectively, and the solidification microstructure in the center of the molten pool grows along the laser beam scanning direction. There exists a distribution range of cellular spacings under the laser rapid solidification conditions, and the average spacing decreases with increasing of growth rate. The maximum,λmax, minimum, λmin, and average primary spacing,-↑λ, as functions of growth rate, Vb, can be given by, λmax=12.54Vb^-0.61,λmin=4.47Vb^-0.52, -↑λ=9.09Vb^-0.62, respectively. The experimental results are compared with the current Hunt-Lu model for rapid cellular/dendritic growth, and a good agreement is found.     

Solidification Simulation of Investment Castings of Single Crystal Hollow Turbine Blade

The three-dimensional solidification simulation of the investment castings of single crystal hollow turbine blade at the withdrawal rates of 2 mm/min, 4.5 mm/min and 7 mm/min has been performed with the finite element thermal analysis. The calculated results are in accordance with the experimental ones. The results show that with the increase of withdrawal rate the concave curvature of the liquidus isotherm is larger and larger, and the temperature gradients of the blades increase. No effects of withdrawal rate on the distribution of the temperature gradients of the starter and helical grain selector of the blades are observed at withdrawal rates of 2 mm/min, 4.5 mm/min and 7 mm/min. The relatively high temperature gradient between 500℃/cm and 1000℃/cm in the starter and helical grain selector is obtained at three withdrawal rates.     

Validation of temperature imaging by H2O absorption spectroscopy using hyperspectral tomography in controlled experiments

This paper describes a preliminary demonstration and validation of temperature imaging using hyperspectral H2O absorption tomography in controlled experiments. Fifteen wavelengths are monitored on each of 30 laser beams to reconstruct the temperature image in a 381 mm × 381 mm square room-temperature plane that contains a 102 mm × 102 mm square zone of lower or higher temperature. The hyperspectral tomography technique attempts to leverage multispectral information to enhance measurement fidelity. The experimental temperature images exhibit average accuracies of 2.3% or better, with pixel-by-pixel standard deviations of less than 1%. In addition, even when the internal zone is only 4 K cooler than the surroundings, its presence is still detectable; statistical analysis of the associated experimental image reveals a 98% confidence that the internal zone is in fact cooler than the surroundings.     

Influence of thermal deformations of the output windows of high-power laser systems on beam characteristics

By using the well-known Green's function methods, we study the three-dimensional temperature distributions and thermal deformations of the output windows of unstable optical resonators induced by an incident annular laser beam. Some expressions and theoretical profiles of the temperature distributions and thermal deformations as functions of the radius and of the thickness of optical windows are obtained. Moreover, the influence of the thermal deformations of sapphire, silica, and silicon windows within unstable optical resonators on the Strehl ratio and on the far-field laser intensity distribution is also discussed. Under conditions of 50-kW intense laser irradiation during 5 s, the maximum thermal deformation in sapphire, silica, and silicon substrates is 1.993, 0.393, and 6.251 microm, respectively. Under the same conditions the Strehl ratio of sapphire is higher than that of silica.     

Evaluation of Residual Stresses Using Laser-Generated SAWs on Surface of Laser-Welding Plates

In this paper, residual stresses in laser-welding plates are studied by both numerical simulation and experiment based on laser ultrasonics. First, a three-dimensional finite-element model is developed to predict temperature distributions and thermo-structure response during the laser-welding process of an aluminum alloy plate, and the residual stresses around the joint are described from structure analysis. After that, experiments based on surface acoustic waves generated by a pulsed laser are carried out to determine the velocity distribution of SAWs around the joint, from which the distribution of main residual stresses are calculated according to acoustoelastic theory. By comparing the thermal-structure model results with the measurements, it is found that the numerical simulation results are in good agreement with the experimental data.