利用深度图优化的自然光束体构建与绘制

为了模拟由散射形成的自然光束,提出基于深度图的光束体优化构建和实时绘制算法.首先把阴影体视作特殊的光束体参与计算,实现光束被物体遮挡的光照与阴影交错的效果;其次利用深度图在GPU中消除重叠的阴影体,降低填充率,优化复杂场景中多个阴影体与光束体相交情况的绘制效果.该算法将自然光束体构建中的一部分提前到预处理中完成,另一部分在GPU内核实现,提高了效率,再结合GPU上高效的散射计算绘制出动态光源产生的自然光束效果;此外,添加了大气中因散射可见的尘埃粒子的模拟,进一步增强了真实感.对动态光源下多个场景的实验结果表明,文中算法解决了散射光束中的遮挡与阴影问题,有效地模拟了光影交错的综合效果,并具有实时性.

具有散射效果的室内外光束实时绘制

依据空气散射的物理原理,提出一种高效的能够实时模拟动态自然光束的真实感绘制方法.首先推导出散射计算的解析表达式以取代传统的数值积分方法;然后基于GPU特点设计了一种有效的光束体表示,作为光束体和阴影体统一形式共同参与GPU着色器的运算,从而生成具有阴影效果的光束.该方法不仅可以模拟单个光源形成的光束,还可以实时模拟动态太阳光源产生的多个自然光束.由于采用了GPU加速计算,可以满足实时应用需求.

高功率激光合束器工作可靠性能评估方法研究

为降低高功率激光在单根光纤中的传输风险,提出基于多光纤传输的激光非相干空间合束19×1圆斑合束器。基于“内外同心圆+中心”同轴排列方式,采用光线追迹法进行合束器的光学设计,同时为实现合束器长期运行可靠性评估,基于多激光束同步照射光学元件的体吸收模型,采用有限元方法对合束器内部透镜的温度、热畸变和热应力等进行研究。结果表明,在10 kW激光照射30 mins下,合束器内部透镜的最高温度为381.11 K,远低于熔融石英1900 K的软化点温度,热畸变相关的最大光圈数为0.07,远小于1个光圈数的透镜加工公差,热应力最大值为14.02 MPa,小于熔融石英4.5 GPa的屈服应力。此外实验测得合束器输出总功率可达10.43 kW。本研究对评价其他高功率激光系统的长期运行可靠性提供了有效的方法参考。

An All-E-Beam Lithography Process for the Patterning of 2D Photonic Crystal Waveguide Devices

We present an all-e-beam lithography （EBL） process for the patterning of photonic crystal waveguides. The whole device structures are exposed in two steps. Holes constituting the photonic crystal lattice and defects are first exposed with a small exposure step size （less than 10nm）. With the introduction of the additional proximity effect to compensate the original proximity effect, the shape, size, and position of the holes can be well controlled. The second step is the exposure of the access waveguides at a larger step size （about 30nm） to improve the scan speed of the EBL. The influence of write-field stitching error can be alleviated by replacing the original waveguides with tapered waveguides at the joint of adjacent write-fields. It is found experimentally that a higher exposure efficiency is achieved with a larger step size;however,a larger step size requires a higher dose.

Surface Morphology and Photoluminescence of 1.3 μm Wavelength In(Ga)As/GaAs Quantum Dots

Self-organized In 0.5 Ga 0.5 As/GaAs quantum island structure emitting at 1.35 μm at room temperature has been successfully fabricated by molecular beam epitaxy (MBE) via cycled (InAs) 1/(GaAs) 1 monolayer deposition method. Photoluminescence (PL) measurement shows that very narrow PL linewidth of 19.2 meV at 300 K has been reached for the first time, indicating effective suppression of inhomogeneous broadening of optical emission from the In 0.5 Ga 0.5 As islands structure. Our results provide important information for optimizing the epitaxial structures of 1.3 μm wavelength quantum dot (QD) devices.

Application of a novel detection approach based on non-dispersive infrared theory to the in-situ analysis on indicator gases from underground coal fire

Coal mine fires,which can cause heavy casualties,environmental damages and a waste of coal resources,have become a worldwide problem.Aiming at overcoming the drawbacks,such as a low analysis efficiency,poor stability and large monitoring error,of the existing underground coal fire monitoring technology,a novel monitoring system based on non-dispersive infrared(NDIR)spectroscopy is developed.In this study,first,the measurement principle of NDIR sensor,the gas concentration calculation and its temperature compensation algorithms were expounded.Next,taking CO and CH_(4) as examples,the liner correlation coefficients of absorbance and the temperature correction factors of the two indicator gases were calculated,and then the errors of concentration measurement for CO,CO_(2),CH_(4) and C_(2)H_(4) were further analyzed.The results disclose that the designed NDIR sensors can satisfy the requirements of industrial standards for monitoring the indicator gases for coal fire hazards.For the established NDIR-based monitoring system,the NDIRbased spectrum analyzer and its auxiliary equipment boast intrinsically safe and explosion-proof performances and can achieve real-time and in-situ detection of indicator gases when installed close to the coal fire risk area underground.Furthermore,a field application of the NDIR-based monitoring system in a coal mine shows that the NDIR-based spectrum analyzer has a permissible difference from the chromatography in measuring the concentrations of various indicator gases.Besides,the advantages of high accuracy,quick analysis and excellent security of the NDIR-based monitoring system have promoted its application in many coal mines.

Characteristics of High-power GaAs Laser Beams and Their Coupling with Fibers

The beams of 980nm high-power LDs are analyzed, and the reasons that aspect ratio of LD beams is high are explained. It is certified by the test that cylindrical lens can efficiently compress the perpendicular divergence angle of the beam. Some typical and popular lensed fibers were compared and analyzed according to coupling characteristics. The factors which affect the coupling efficiency and tolerance of the wedged-shaped GRIN tipped lensed fiber are pointed out, and some methods to reduce the coupling loss of the lensed fibers are proposed finally.

Influence of welding processes on microstructure, tensile and impact properties of Ti-6Al-4V alloy joints

Titanium alloys have been successfully applied for aerospace, ship and chemical industries because they possess many good characteristics such as high specific strength, superior corrosion resistance and excellent high temperature resistance. Though these alloys show reasonable weldability characteristics, the joint properties are greatly influenced by the welding processes. Weld thermal cycle of the processes will control the weld metal solidification and subsequent phase transformation and resultant microstructure. The welded joints of Ti-6Al-4V alloy were fabricated by gas tungsten arc welding （GTAW）, laser beam welding （LBW） and electron beam welding （EBW） processes. The joints fabricated by EBW process exhibit higher strength compared with the GTAW and LBW joints; but the joints by GTAW process exhibit higher impact toughness compared with the LBW and EBW joints. The resultant tensile and impact properties of the welded joints were correlated with the weld metal microstructures.

Vibration Spectra of Quasi-confined Optical Phonon Modes in an Asymmetric Wurtzite AlxGa1-xN/GaN/AlyGa1-yN Quantum Well

Based on the dielectric continuum model and Loudon＇s uniaxial crystal model, the properties of the quasi. confined （QC） optical phonon dispersions and the electron-QC phonons coupling functions in an asymmetric wurtzite quantum well （QW） are deduced via the method of electrostatic .potential expanding. The present theoretical scheme can naturally reduce to the results in symmetric wurtzite QW once a set of symmetric structural parameters are chosen. Numerical calculations on an asymmetric AlN/GaN/AIo,15 Gao.85N Wurtzite Q W are performed. A detailed comparison with the symmetric wurtzite QW was also performed. The results show that the structural asymmetry of wurtzite QW changes greatly the dispersion frequencies and the electrostatic potential distributions of the QC optical phonon modes.

Optical Characterization of GaN Grown by Plasma Source MBE

By using spectroscopic photoconductance, transmittance and luminescence methods,the optical characterization of GaN grown by plasma source MBE have been evaluated. The imperfection of the epitaxial layer deduced from the measured results have been discussed. The transient responses of the photoconductive detectors have been measured. Two time constants of 0.17 ms and 6.85 ms at room temperature are deduced from the measured results. The origins have also been discussed.

Terahertz polarization beam splitter based on photonic crystal and multimode interference

We design a compact terahertz(THz) polarization beam splitter. Both plane wave expansion method and finite-difference time-domain method are used to calculate and analyze the characteristics of the proposed device. The designed polarization beam splitter can split TE-polarized and TM-polarized THz waves into different propagation directions. The simulation results show that the extinction ratios are larger than 18.36 dB for TE polarization and 13.35 dB for TM polarization in the frequency range from 1.86 THz to 1.91 THz, respectively. The designed polarization beam splitter has the advantages of small size and compact structure with a total size of 4.825 mm×0.400 mm.

Single nanoparticle trapping based on on-chip nanoslotted nanobeam cavities

Optical trapping techniques are of great interest since they have the advantage of enabling the direct handling of nanoparticles. Among various optical trapping systems, photonic crystal nanobeam cavities have attracted great attention for integrated on-chip trapping and manipulation. However, optical trapping with high efficiency and low input power is still a big challenge in nanobeam cavities because most of the light energy is confined within the solid dielectric region. To this end, by incorporating a nanoslotted structure into an ultracompact one- dimensional photonic crystal nanobeam cavity structure, we design a promising on-chip device with ultralarge trapping potential depth to enhance the optical trapping characteristic of the cavity. In this work, we first provide a systematic analysis of the optical trapping force for an airborne polystyrene （PS） nanoparticle trapped in a cavity model. Then, to validate the theoretical analysis, the numerical simulation proof is demonstrated in detail by using the three-dimensional finite element method. For trapping a PS nanoparticle of 10 nm radius within the air-slot, a maximum trapping force as high as 8.28 nN/mW and a depth of trapping potential as large as 1.15 × 105 kBTmW-1 are obtained, where kB is the Boltzmann constant and T is the system temperature. We estimate a lateral trapping stiffness of 167.17 pN. nm-1 . mW-1 for a 10 nm radius PS nanoparticle along the cavity x-axis, more than two orders of magnitude higher than previously demonstrated on-chip, near field traps. Moreover, the threshold power for stable trapping as low as 0.087 μW is achieved. In addition, trapping of a single 25 nm radius PS nanoparticle causes a 0.6 nm redshift in peak wavelength. Thus, the proposed cavity device can be used to detect single nanoparticle trapping by monitoring the resonant peak wavelength shift. We believe that the architecture with features of an ultracompact footprint, high integrahility with optical waveguides/cir- cuits, and efficient trapping demonstrated here will provide a promising candidate for developing a lab-on-a-chip device with versatile functionalities.

Nonlinear Interaction of Elliptical Laser Beam with Collisional Plasma：Effect of Linear Absorption

In the present work,nonlinear interaction of elliptical laser beam with collisional plasma is studied by using paraxial ray approximation.Nonlinear differential equations for the beam width parameters of semi-major axis and semi-minor axis of elliptical laser beam have been set up and solved numerically to study the variation of beam width parameters with normalized distance of propagation.Effects of variation in absorption coefficient and plasma density on the beam width parameters are also analyzed.It is observed from the analysis that extent of self-focusing of beam increases with increase/decrease in plasma density/absorption coefficient.

A novel polarization splitter based on three-core photonic crystal fibers

A novel polarization splitter based on photonic crystal fibers(PCFs) with three cores of high birefringence is proposed.The 45o linearly polarized light is launched into a core.After a coupling length(about 1500 μm),the xand y-polarized light beams are separated into different cores.When the light is launched into another core,the xand y-polarized light from different cores can be obtained and the degree of separation can be also adjusted.The polarization splitter is highly flexible and adjustable.The length of the polarization splitter is about 1500 μm which is just the coupling length.So it has appreciate significance of manufacturing mini-type photonic apparatus in integrated optics.

Design of Optical Logic Gates Using Self-collimated Beams in 2D Photonic Crystal

Optical logic gates are elementary components for optical network and optical computing. In this paper, we propose a structure for AND, NAND, XNOR and NOR logic gates in the two dimensional photonic crystal which utilizes the dispersion based self-collimation effect. The self-collimated beam is splitted by the line defect and interfered with other self-collimated beam. This interference may be constructive or destructive based on their phase difference. This phenomenon is employed to realize all-optical logic gates. The gates are demonstrated numerically by computing electromagnetic field distribution using the finite difference time domain （FDTD） method. The results ensure that this design can function as AND, NAND, XNOR and NOR logic gates. The size of the structure is about 10 μm× 10 μm which in turn results in an increase in the speed and all the gates are realized in the same configuration. The ON-OFF contrast ratio is about 6 dB.

Design of a fiber polarizer based on an asymmetric dual-core photonic crystal fiber

We propose a novel optical polarizer based on an asymmetric dual-core photonic crystal fiber(PCF) with triangular lattice air-holes.The fiber is designed as that the effective indices of modes in the two cores are matched at one polarized state but mismatched at another polarized state.As a result,one of the polarization states is coupled to the other core and transferred into a high-order mode.The transmission properties of the polarizer are investigated by the semi-vectorial beam propagation method(SV-BPM).Numerical results demonstrate that a device length of 11.3 mm shows extinction ratio as low as-20 dB with bandwidth as great as 80 nm ranging from 1.51 mm to 1.59 mm.

Investigation of Weakly Relativistic Ponderomotive Effects on Self-Focusing During Interaction of High Power Elliptical Laser Beam with Plasma

This paper presents an investigation of weakly relativistic ponderomotive effects on self-focusing during interaction of high power elliptical laser beam with plasma. The nonlinear differential equations for the beam width parameters of elliptical laser beam have set up by using Wentzal–Krammers–Brillouin(WKB) and paraxial approximations. These equations have been solved numerically by using fourth order Runge–Kutta method to study the variation of these beam width parameters against normalized distance of propagation. Effects of variation in laser beam intensity,plasma density and electron temperature on the beam width parameters are also analyzed.

A novel polarization splitter based on octagonal dualcore photonic crystal fiber

A novel polarization splitter based on octagonal dual-core photonic crystal fiber(O-D-PCF) is proposed. The impacts of several fiber parameters on the coupling characteristics of the polarization splitter are investigated by full-vectorial finite element method(FV-FEM) in detail. Through optimizing the fiber configuration, a 4.267-mm-long polarization splitter with a bandwidth of 37 nm is achieved, and its extinction ratio(ER) is as high as 81.2 d B at the wavelength of 1.55 μm. Compared with the hexagonal dual-core photonic crystal fiber(H-D-PCF) based polarization splitter, both ER and bandwidth of the O-D-PCF based one are effectively improved.

Dual-core photonic crystal fiber polarization splitter based on lead silicate glass

A dual-core photonic crystal fiber(PCF)polarization splitter based on lead silicate glass is proposed.The characteristics of the polarization splitter are analyzed using full-vector finite element method.Compared with the silica glass PCF polarization splitter with the same structure,it is shown that the new material polarizer can realize splitting with less coupling loss and higher extinction ratio.When the wavelength is 1 550 nm and the PCF length in the beam splitter is 688μm,the coupling loss is only 0.001 9 d B,and the extinction ratio for the input core is-64.1 d B.

Coherence preservation during light-surface plasmon polaritons-light transformation

We constructed a four-layer system composed of a prism,a silver film,an air layer and a lithium niobate crystal.Initially we used two coherent light beams to excite surface plasmons.The surface plasmons were then decoupled into light in the photorefractive crystal where a holographic grating was recorded.The two beams remained coherent through light to surface plasmons to light transformation.Studying the characteristics of the holographic grating we found out that the thickness of the grating was to the order of hundreds of microns.The thick holographic grating suggests that the holographic recording in the photorefractive materials was induced by the leaky waves rather than by surface plasmon polaritons directly.