10.6μm激光诱导扩散中热致破坏的抑制

在半导体激光诱导扩散实验中,用连续波CO2 10．6μm激光聚焦后照射基片表面。为实现局部区域的选择扩散,激光光斑半径仅数十微米。要使曝光区温度达到扩散实验要求,必须使曝光区功率密度很高。另一方面,Si、InP等半导体材料对10．6μm波长激光的吸收系数随温度的升高而增大,这导致实验时容易产生热致破坏,损伤基片。在分析热致破坏的产生机理后,提出了在聚焦激光束照射下,曝光区温度的数值计算方法。计算结果表明,在半导体基片初始温度为室温时,以恒定功率的激光束照射基片,曝光区温度不能稳定在扩散试验需要的温度范围。在此基础上,提出了预热基片及对曝光区温度进行实时控制等抑制热致破坏的方法,有效地克服了这一困难。这对于用激光微细加工制作出高性能的单片光电集成电路(OEICs)器件有重要意义。     

Effect of angle of incidence on self-mixing laser Doppler velocimeter and optimization of the system

Based on the theory of speckle and self-mixing interference in laser-diode, three-facet cavity model is introduced to analyze laser Doppler effect based on self-mixing interference in the case of a rough surface, and numerical solution of the signal is obtained. Simulation results of speckle-modulated Doppler signal based on self-mixing effect and tracking accuracy at different incident angles are given using parameters employed in the experiment. Simulation results indicate incident angle of around 30° is most suitable when both tracking accuracy and signal amplitude are considered. Experimental waveforms agree well with simulation results, and similar conclusions as simulation predictions about changing trend of tracking accuracies of the system at different incident angles can be made. Combining with difference frequency analog phase-lock loop (PLL) technique and appropriate sampling time, a laser Doppler velocimeter with tracking accuracy better than 1.3% in the range of 10-470 mm/s is realized.     

基于计算机视觉的光学目标自动定位系统

本文运用计算机视觉检测技术,提出了一种新的光学目标自动定位系统,导出了自动定位数学模型,分析了定位精度.运用计算机视觉定位技术开发的ZYZ-620PCB自动定位钻机.定位钻孔精度优于25μm.实践表明,计算机视觉光学目标自动定位系统具有高精度、快速实时的优点,在工业领域必将有广泛的应用前景.     

Model and experimental analysis of oblique incident ultrasound in a tissue layer using doublet mechanics theory

The fundamental framework of doublet mechanics (DM) is used to analyze high-frequency ultrasound wave propagation in materials with discrete microstructure. Ultrasonic reflection coefficients were measured from a thin layer of tissue embedded between two glass substrates at oblique incidence. Theoretical calculations for the reflection coefficients of a multi-layered system at oblique angles are performed using both DM theory and the classical continuum mechanics theory (CCM). For example, at the frequency of 10 MHz at incident angle 8° in sample with 30 μm thickness, the discrepancy in the magnitude of the reflection coefficient between experimental results and theoretical prediction is 15.8% for DM but 79.0% for CCM; similar results at other frequencies and incident angle in the samples with 30 and 60 μm thickness have also been obtained, which demonstrates that the DM theory can better describe the wave propagation in tissue. The influence of the incident angles and tissue thickness are also discussed in this paper.     

Measurement of diffusion coefficients in polystyrene using xenon-129 NMR

A sample of polystyrene beads, 18 μm in diameter, has been sealed in an NMR tube under 10 atm of xenon gas. Two dimensional,¹²⁹Xe NMR spectra show cross peaks between the resonances corresponding to xenon in the free gas and the sorbed state, indicating that appreciable exchange occurs during the mixing time of the NMR experiment. Selective saturation of the free gas resonance attenuates the integrated intensity of the sorbed xenon resonance as a function of saturation time, thus allowing the accurate measurement of the exchange rates between the gas and the sorbed states. A model has been developed using a slightly modified form of Crank’s treatment of diffusion in a sphere which allows for the accurate determination of the diffusion coefficient for xenon in the sorbed state. The diffusion coefficient for xenon in polystyrene at 25°C is determined to be 2.9·10^?9 cm²/s.     

Evaluation of the echolocation model for range estimation of multiple closely spaced objects

Experimental evidence indicates that bats can use frequency-modulated echolocation to identify objects with an accuracy of less than 1 μs. However, when modeling this process, it is difficult to estimate the delay times of multiple closely spaced objects by analyzing the echo spectrum, because the sequence of delay separations cannot be determined without information on the temporal changes in the interference patterns of the echoes. To extract the temporal changes, Gaussian chirplets with a carrier frequency compatible with bat emission sweep rates are introduced. The delay time for object 1 (T(1)) is estimated from the echo spectrum around the onset time. The T(2) is obtained by adding the T(1) to the delay separation between objects 1 and 2. Further objects are located in sequence by this procedure. Here echoes were measured from single and multiple objects at a low signal-to-noise ratio. It was confirmed that the delay time for a single object could be estimated with an accuracy of about 1.3 μs. The range accuracy was less than 6 μs when the frequency bandwidth was less than 10 kHz. The delay time for multiple closely spaced objects could be estimated with a high range resolution by extracting the interference pattern.     

Antireflection coating on germanium for dual channel (3–5 and 7.5–10.6 μm) thermal imagers

The dual channel thermal imager, operating in the 3–5 and 7.5–10.6 μm wavelength bands, is one of the latest achievements in instrumentation for target recognition and acquisition. While the 3–5 μm band is utilised for detecting hot objects such as engine exhausts of vehicles and fighter planes, the 7.5–10.6 μm band is employed for human bodies and objects at ambient temperatures. Many substrates are available which transmit in both these wavelength bands and their transmission can be enhanced by providing a suitable antireflection coating. In this paper, a broad band antireflection coating on germanium substrate is reported. The design approach involves achieving a continuously varying refractive index from that of the incident medium to the substrate. The continuously varying refractive index profile may be generated by using a sequence of thin layers of high and low refractive index materials. In this design a continuous refractive index profile is approximated by using a 13-layer stack of thorium fluoride and germanium as low and high index coating materials respectively. This coating conforms to environmental stability standards and shows an average transmission of 91% in 3–5 μm band and 94.5% in 7.5–10.6 μm band with a peak of 97% at 9 μm on 10 mm thick germanium substrate. Polycrystalline germanium has 2.5% absorption for a 10 mm thick substrate.     

Prediction of the reverberation time in high absorbent room using a modified-diffusion model

A modification of the diffusion model’s boundary condition, based on the Eyring absorption coefficient, to account for high walls absorption is proposed. Numerical comparisons are carried out for three geometrical configurations (a proportionate room, a corridor and a flat enclosure). Comparisons with the statistical theory and a ray-tracing software show that the modified boundary condition increases the accuracy of the diffusion model in term of reverberation time in all the simulated configurations. An experimental comparison in the case of a non-uniformly absorbent room (a reverberation chamber covered with patches of glass wool) is also carried out. The modified-diffusion model results match well with the ray-tracing ones. Both models are in agreement with the experimental data for most of third octave bands (discrepancy close to or below 10%). However, some discrepancies up to 40% can also be observed in a few octave bands, probably due to experimental considerations and to the modal behaviour of the room at low frequencies.     

入射角对Al0.5Ga0.5As-AlAs分布布拉格反射器反射光谱的影响

采用传输矩阵法对Al0.5Ga0.5As-AlAs材料的发光二极管分布布拉格反射器进行入射角的反射光谱研究,计算发现反射偏振光p和s随入射角的增大呈“V”形变化,在49.8°处有最小反射值。不同入射介质[以空气和限制层(Al0.7Ga0.3)0.5In0.5P材料]下的反射光谱受入射角的影响差异很大,其中入射角对空气入射介质的反射谱影响较小,由0°入射的反射率88.13%降至45°的84.94%,反射峰值波长蓝移仅10 nm;但入射角对(Al0.7Ga0.3)0.5In0.5P入射介质的反射谱影响很大,仅从0°到45°入射,反射率降幅就超过45%,反射峰值波长蓝移超过127 nm。为了减缓这种影响,提出了多波长布拉格反射器结构设计。计算表明多波长分布布拉格反射器在0ο~45°的入射角内比传统的分布布拉格反射器有更好的光谱特性,这对提高发光二极管的出光效率有现实意义。     

Widely continuous-tunable 2.789-4.957 μm twin- MgO：PPLN cascaded optical parametric oscillator

An all-solid-state mid-infrared optical parametric oscillator with wide tunability by using two identical multi-grating periodically poled 5-mol.-% MgO-doped lithium niobate cascaded is reported. The pump laser is an acousto-optically Q-switched Nd：YAG laser with 150-ns pulse width at repetition rate of 10 kHz. Wide tunability from 2.789 to 4.957μm at the idler wavelength is achieved by varying the temperature from 40 to 200 ~C and translating the grating periods from 26 to 31 μm with a step of 0.5μm. When the incident pump average power is 8.15 W, the maximum idler output average power is 2.23 W at 3.344 μm and the optic-optic conversion efficiency is about 27.4%.     

Hydrogen Permeability of a Foil of Pd–Ag Alloy Modified with a Nanoporous Palladium Coating

Thin films with the composition Pd–23% Ag are obtained via magnetron sputtering. The magnetron sputtering of Pd–50% Zn films with subsequent diffusion annealing and etching of the active component is used to modify the surfaces of palladium–silver films to improve their hydrogen permeability. Modifying the surfaces of the resulting Pd–Ag films using a nanoporous palladium coating with a predominant distribution of particles ranging from 0 to 50 nm allows a hydrogen flux density of up to 0.4 mmol s^?1 m^?2 to be achieved for sufficiently thin palladium membranes (<10 μm) under conditions of low temperature (<90°C) and pressure (<0.6 MPa). Experimental evidence is gathered that under these conditions, the velocity of hydrogen transport is limited by dissociative–associative processes at membrane boundaries and can be greatly (by an order of magnitude) increased, due to acceleration of the limiting stage of the process via the formation of a palladium nanoporous coating on the film’s surface.     

一种具有异常透射功能的新型惠更斯超表面

提出了一种具有独立调控透射幅值与相位能力的新型惠更斯单元结构,能够实现透射幅值在0~0.9、透射相位在0~2π的任意调控。基于广义斯涅耳定理对惠更斯单元进行排列,设计并制备出具有异常透射功能的超表面结构。全波仿真结果表明超表面能够使频率为10 GHz的正入射电磁波产生约60°的偏折。近场测试结果与仿真结果基本一致,进一步证实了超表面结构设计的正确性。     

声光调制光谱相机的成像漂移

研究了基于声光可调滤波机理的光谱相机在成像时由声光晶体的色散而产生的图像漂移现象。利用色散补偿法和图像位移补偿法,理论计算并实验测量了声光可调滤波器（AOTF）在可见光（488～644 nm）波段内由晶体外衍射角所引起的图像漂移,并进行了优化实验。采用色散补偿法,调整入射光为准平行光,入射光波长为488～644 nm时,在晶体出射面添加0.6°的光楔,晶体外衍射角的变化量可由0.066 50°降低到0.004 2°,即图像漂移量由162.1μm降低到10.9μm;采用图像位移补偿法,不添加光楔,入射光波长为488～644 nm时,图像水平漂移量可从468μm降低到0.658μm,漂移量在一个像元内。实验表明：基于提出的两种方法可忽略成像漂移对图像的影响,有效提高了基于AOTF机理的光谱相机的成像分辨率。     

基于原始波形测量的脉冲电场探测器

介绍了脉冲电场微分测量和原始波形测量的基本原理,设计研制了一系列不同灵敏度的脉冲电场原始波形测量系统,分析了脉冲电场探测器的理论修正模型。该测量系统主要包括天线模块、积分器模块、放大和驱动模块以及光电传输模块,利用同轴型TEM小室对测量系统进行了时域标定。结果表明:测量系统的前沿响应时间小于1.0 ns,系统输出脉冲平顶在10.0μs内下降不超过5%,测量系统输出幅度与电场强度在20 dB的动态范围内呈线性关系,该系列探测器可以用来测量最小10 V/m、最大100 kV/m的电场强度,满足高空电磁脉冲标准环境的测量要求。     

Energy dependence of electron inelastic scattering mean-free-paths using synchrotron radiation photoelectron spectroscopy

The energy dependence of electron inelastic scattering mean-free-paths (mfp's) has been measured for thin films of Au, Ag and Mg for energies up to 150 eV by the overlayer method using synchrotron radiation photoemission. After the initial sharp drop in mfp at low energies (not seen in our data for Mg) mfp's for all three materials remain almost constant over the remaining range of kinetic energy. Some of the essentials of this technique are discussed; in particular the choice of substrate material, overlayer growth mechanisms and geometry of electron spectrometer are considered in detail. Increases in the metal-related 2p emission on oxidation of films of Mg and Al indicate that the mfp in the oxides is typically at least twice as great as in the metal and this ratio is energy-dependent. The dangers of using the “universal curve” of mfp versus energy are highlighted by this work.     

反射式液晶空间光调制器电控光束偏转

为获得较大的光束电控偏转范围,使用空间分辨力高达8 m的反射式液晶空间光调制器实现了对入射632 nm激光的电控偏转。利用双光束共焦干涉方法测量了液晶空间光调制器的电控相位延迟特性,最大相移量可达3。根据二元光学理论和器件的电控相位延迟特性,设计了周期台阶相位模式和相应的加载灰度图,以最大衍射效率实现对入射光束的闪耀电控偏转。结果表明:相位模式台阶数为8时,可以实现10 mrad的光束偏转,闪耀级次衍射效率可达46%。利用二元光学的衍射模型对影响衍射效率的关键因素进行了分析,认为器件较低的填充因子和周期台阶模式相位重置点诱导的指向矢回程区是限制光束衍射效率的主要因素。     

随机森林结合CatBoost的近红外光谱药品鉴别

药品质量关乎人民健康和国家命脉,随着社会经济的飞速发展对药品质量的快速、有效鉴别具有极其重要的作用。光谱分析技术具有较高的准确性、较快的分析速度且对样品不存在污染等突出优点,广泛应用在化工、石油以及医药等重要的领域。为了解决传统药品鉴别模型存在的鉴别精度低、鉴别速度不能满足实际需求且鉴别模型稳定性差的问题,采用光谱仪采集药品的近红外光谱数据达到对药品无污染鉴别的目的。结合随机森林和CatBoost对药品进行分类鉴别,以实现快速且准确的鉴别。首先采用随机森林（RF）对光谱仪采集的光谱数据进行有效特征波长的筛选,从而将药品光谱数据中的无关波长去除、筛选出最能表征样品属性的特征波长,然后以极限学习机（ELM）作为CatBoost的弱分类器分析筛选的特征波长对药品的属性鉴别。由于ELM仅只含有一个隐含层且无需多次迭代寻优保证了鉴别模型运行速度更快,CatBoost通过集成弱分类器以改善模型鉴别准确性。为对所提出的药品鉴别模型性能进行有效评估,采用随机抽取训练集的方式构造不同规模药品光谱数据并分别上进行独立实验且以10次运行结果的均值作为其最终结果,并通过与CatBoost、持向量机（SVM）、反向传播网络（BP）、ELM、波形叠加极限学习机（SWELM）和Boosting进行对比,进一步对模型的性能进行评估。从不同规模训练集的分类结果可看出,随着训练集样本的增加分类精度最高为100%且预测标准偏差趋于0。实验结果表明,所建立RF-CatBoost鉴别模型在不同规模的药品数据集上较对比方法具有更高的分类准确率、更快的速度且其鲁棒性更强,能够广泛应用于药品类别的准确鉴别,从而实现药品质量的有效监督。     

体布拉格光栅外腔半导体激光器的光栅旋转角度容差

采用步进电机控制光栅角度,对200 m和600 m圆柱透镜准直的体布拉格光栅（VBG）外腔单管C封装半导体激光器进行了系统研究。实验结果表明:激光二极管（LD）驱动电流越大,准直效果越好,VBG的角度调整容差越小;快轴方向的准直效果越好,慢轴方向的光栅调节角度容差越大;对于衍射效率28%、厚1.4 mm的光栅,LD快轴发散角为7.3 mrad时,快轴方向的角度容差不大于3.2 mrad,慢轴方向的角度容差较快轴大约一个数量级。     

Hybrid diffusion approximation in highly absorbing media and its effects of source approximation

A modified diffusion approximation model called the hybrid diffusion approximation that can be used for highly absorbing media is investigated. The analytic solution of the hybrid diffusion approximation for reflectance in two-source approximation and steady-state case with extrapolated boundary is obtained. The effects of source approximation on the analytic solution are investigated, and it is validated that two-source approximation in highly absorbing media to describe the optical properties of biological tissue is necessary. Monte Carlo simulation of recovering optical parameters from refiectant data is done with the use of this model. The errors of recovering μ＇s and μ＇s are smaller than 15% for the reduced albedo between 0.77 and 0.5 with the source-detector separation of 0.4-3 mm.     

Investigation of some nuclear engineering materials in terms of gamma ray buildup factors at experimental energies used in nuclear physics experiments

ABSTRACT

In shielding studies, the gamma ray buildup factor is referred to an attenuation correction when the narrow beam transmission geometry conditions are not met. This is the case if there is a beam divergence and a considerably thick material is used as shield. In the present work, a linear logarithmic interpolation based on five parameter geometric progression (G-P) fitting method was used to calculate energy absorption (EABF) and exposure buildup factors (EBF) at discrete energies for some standard nuclear engineering materials (SNEM) such as concrete, Pb glass, brass, bronze and stainless steel, and for some recently developed shielding alloys and glasses. Calculations were made at different penetration depths (1–20?mfp) using photons emitted from the standard radioactive sources such as ¹³³Ba (0.356 MeV), ¹³⁷Cs (0.662 MeV), ⁶⁰Co (1.173, 1.332 MeV) and ²²Na (0.511, 1.275 MeV). Results have been compared with MCNP6.1 Monte Carlo simulation code wherever possible and a reasonable agreement (Relative difference around 10% up to 10?mfp except for Pb glass) has been achieved. Results also showed that the gamma ray buildup correction is necessarily required since buildup factors are greater than unity even at lowest penetration depth, i.e. 1?mfp. The reported data on EABF and EBF for the energy values of radioactive sources may be of practical use in shielding calculations and estimations.