基于光学厚度的云红外辐射计算

 为了研究不同物理厚度和不同波段下云的红外辐射特性,提出了基于光学厚度的云红外辐射计算方法。综合考虑云的红外辐射的各个因素,建立了较为完善的云的红外辐射模型,引入光学厚度经验计算公式,并根据光学厚度针对中波和长波红外分别计算了云的发射率、反射率和透过率,进而得到云的红外辐射亮度分布。利用该计算方法,计算了中波和长波红外云的辐射亮度数值,计算结果表明：随着光学厚度增大,云的发射率和反射率增大,探测器接收到的云红外辐射增强。比较发现,该计算结果与实测数据有较好的一致性,该计算模型可以为云背景的红外特性分析、探测及仿真提供参考数据。     

混合型集成光学拾音器的结构设计

采用光学追踪法计算子圆铃光线主射到棱镜中的轨迹,并对计算结果进行了实验验证,实验结果与计算结果相一致,从而获取了拾音器的光学结构参数。     

不同光线入射角度下超声速湍流边界层气动光学效应的实验研究

利用基于纳米粒子的平面激光散射技术获取超声速(Ma=3.0)湍流边界层的密度分布,采用光线追迹方法计算其对应的光程差分布,并结合边界层气动光学相似律验证实验结果的可靠性.着重研究了光线入射角度对超声速湍流边界层气动光学效应的影响,并对其内在机理进行了分析.研究表明,气动光学效应对光线入射角度的依赖性源于光线在流场中的传输路径,传输路径的不同导致了光线在流场中的传输距离以及对应密度脉动互相关结果的差异.光线倾斜入射导致其在流场中传输距离增长,进而气动光学效应出现恶化.光线入射方向与壁面垂直方向之间的夹角越大,气动光学效应越显著,而且不同时刻的差异性增加,气动光学效应校正的难度增加.超声速湍流边界层中大量具有特定方向的涡结构导致了湍流边界层气动光学效应的各向异性.当光线倾斜向下游入射时,光线传播方向与流场中的涡结构具有较好的一致性,体现为此方向上密度脉动互相关系数较大,故气动光学效应比较严重.而当光线倾斜向上游入射时,相关系数较小,故气动光学效应较弱.     

梯度折射率球对在轴高斯波束散射的几何光学近似

基于梯度折射率(GRIN)球对平面波前向散射的几何光学近似(GOA),推导出GRIN球对在轴高斯波束散射的GOA.详细推导高斯波束在GRIN球中的散射角和相位公式,给出了算法实现步骤.在此基础上,利用扩展后的GOA计算了粒子的散射强度分布,并与广义洛伦兹一米理论(GLMT)的计算结果进行比较;由于表面波的影响,当波束的束腰半径大于等于粒子半径时散射强度分布与GLMT计算所得的结果相比存在误差.比较结果表明扩展后的GOA方法适用于束腰小于粒子半径的情况.对比GOA方法和GLMT的计算速度,结果表明GOA方法具有较高的运算速度.     

基于时域有限差分法的液晶光学特性模拟

研究了模拟光在液晶中传播的时域有限差分(FDTD)法。该方法采用完全匹配层吸收边界条件来截断计算区域,通过独立计算总场区域与散射场区域的左连接边界和右连接边界上的入射场场值来实现非周期结构的平面波波源引入,并提出了分裂场一维辅助FDTD方法来计算含有非均匀各向异性介质的边界上的入射场场值。实现了平面波源入射下具有非周期结构的液晶器件光学特性计算。对开态下扭曲向列相液晶盒的仿真结果表明,30°入射时散射区透射率的最大泄漏误差小于3%,可知该方法对非周期、非均匀各向异性介质光学特性分析是有效的。     

气动光学头罩热辐射效应数值仿真研究

气动热环境下高速飞行器的光学头罩由于受气动热效应作用,其温度急剧升高,产生严重的气动热辐射效应。为评估气动热环境下高速飞行器光学头罩热辐射对探测系统性能的影响,采用有限光线代表连续辐射的方法,并引入热瞳概念建立了气动光学头罩热辐射传输计算模型并对气动光学头罩自身干扰辐射光线在光学系统内的传输进行了数值仿真,考察了光学头罩温度场为非均匀分布时其自身干扰辐射在探测器接收面的辐照度分布。研究结果表明:由头罩顶点求得的热瞳是光学头罩热辐射能的公共入口,它可将光线追迹的时间减小至追迹全部光线所需时间的十几分之一;根据探测器接受面干扰辐射随时间的变化趋势可知在飞行数秒后头罩干扰辐射将淹没目标信号,气动光学头罩热辐射效应不容忽视。     

关于光学速调管长波长辐射的分析

由波荡器渡越辐射公式和电子通过wiggler边界产生的长波长辐射公式, 解析给出了电子通过光学速调管时产生的长波长辐射公式. 根据基本的电动力学公式数值计算出合肥同步辐射装置光学速调管的长波辐射, 并将其结果与解析公式的结果进行比较. 计算合肥同步辐射装置的光学速调管和弯转磁铁在红外波段范围内辐射的强度和通量, 表明光学速调管有很好的性能.本文说明, 光学速调管不但可以用来进行相干谐波的试验研究, 也可以兼用作储存环中的红外辐射光源.     

任意形状金属腔体内电磁脉冲的三维计算

提出了模拟任意形状腔体中的内电磁脉冲的三维直角坐标系时域有限差分(FDTD)算法。该算法采用FDTD共形网格技术模拟任意形状腔体的边界,可以解决腔体内非对称的边界问题。推导了射线斜入射的差分方程,进行了三维数值计算,并采用直角坐标系FDTD算法和柱坐标系FDTD算法计算了射线斜入射圆柱腔体产生的内电磁脉冲,二者吻合很好,验证了直角坐标系FDTD算法正确性。     

气动光学头罩光传输数值仿真

 气动热环境下光学头罩由于受气动热效应作用,其形状和光学性质都将发生改变,从而影响头罩内的光传输。建立气动光学头罩有限元分析模型,根据热光效应和弹光效应理论,对头罩的折射率场进行数值计算,采用四阶龙格-库塔法编制光线追迹程序对气动光学头罩内红外辐射光线传输进行了数值仿真,得到了头罩光学系统出射波面波像差和调制传递函数。研究结果表明：气动热环境下热光效应对光学头罩光传输的影响远比弹光效应大,且气动光学头罩成像质量受到气动热效应的影响严重下降,为了提高高速飞行器的制导精度,不能忽视气动热对头罩光传输的影响。     

碳纳米管光学天线的有效波长和谐振特性

将经典金属自由电子气模型应用于金属型碳纳米管, 基于光学天线有效波长理论, 得出了金属型碳纳米管光学天线响应的有效波长与碳纳米管介电特性之间的普适关系. 在对碳纳米管介电特性进行第一性原理计算的基础上, 以金属型4 Å碳纳米管为例, 进一步研究了金属型碳纳米管光学天线响应的有效波长与入射波长之间的关系, 以及金属型碳纳米管光学偶极子天线的谐振特性. 通过将已有传统金属光学天线和碳纳米管天线有效波长的研究结果进行对比, 验证了本文理论的正确性. 结果表明, 碳纳米管光学天线响应的有效波长与入射波长呈近似线性关系, 与传统金属材料构成的同直径光学天线相比, 碳纳米管天线显示出了更强的波长压缩能力, 并且在可见光到红外波段内易于发生谐振. 该研究方法可为碳纳米管光学天线研究提供新的思路.     

Infrared radiative properties of two-dimensional square optical black holes

Optical black hole (OBH) is a special optical structure, in which the dielectric function or refractive index of media becomes gradually larger from the outside to the core. The circular optical black holes (COBH) have been proved remarkably useful for broadband omnidirectional light absorption. The main goal of this paper is to propose an alternative square structure of OBH due to fabrication consideration. The infrared radiative properties of two-dimensional SOBHs are numerically studied with geometric optics approximation (GOA) and the finite-difference time-domain (FDTD) method. A critical wavelength is found in this paper: when the wavelength is smaller than half of the inner core side length, the Poynting vectors obtained by the FDTD method agree well with the ray trajectories calculated by the GOA, and the absorptance is nearly 100% with a proper refractive index gradient. While the wavelength is increasing, the net energy flows do not agree with the ray trajectories and the absorptance will be decreased. The reason is attributed to the diffraction effect, which is discussed in detail in this paper.     

Finite element method for radiation heat transfer in multi-dimensional graded index medium

In graded index medium, ray goes along a curved path determined by Fermat principle, and curved ray-tracing is very difficult and complex. To avoid the complicated and time-consuming computation of curved ray trajectories, a finite element method based on discrete ordinate equation is developed to solve the radiative transfer problem in a multi-dimensional semitransparent graded index medium. Two particular test problems of radiative transfer are taken as examples to verify this finite element method. The predicted dimensionless net radiative heat fluxes are determined by the proposed method and compared with the results obtained by finite volume method. The results show that the finite element method presented in this paper has a good accuracy in solving the multi-dimensional radiative transfer problem in semitransparent graded index medium.     

Least-squares finite element method for radiation heat transfer in graded index medium

To avoid the complicated and time-consuming computation of curved ray trajectories, a least-squares finite element method based on discrete ordinate equation is extended to solve the radiative transfer problem in a multi-dimensional semitransparent graded index medium. Four cases of radiative heat transfer are examined to verify this least-squares finite element method. Linear and nonlinear graded index are considered. The predicted dimensionless net radiative heat fluxes are determined by the least-squares finite element method and compared with the results obtained by other methods. The results show that the least-squares finite element method is stable and has a good accuracy in solving the multi-dimensional radiative transfer problem in a semitransparent graded index medium, while the Galerkin finite element method sometimes suffers from nonphysical oscillations.     

Discontinuous finite element method for radiative heat transfer in semitransparent graded index medium

To avoid the complicated and time-consuming computation of curved ray trajectories, a discontinuous finite element method based on discrete ordinate equation is extended to solve the radiative transfer problem in a multi-dimensional semitransparent graded index medium. Two cases of radiative heat transfer in two-dimensional rectangular gray semitransparent graded index medium enclosed by opaque boundary are examined to verify this discontinuous finite element method. Special layered and radial graded index distributions are considered. The predicted dimensionless net radiative heat fluxes and dimensionless temperature distributions are determined by the discontinuous finite element method and compared with the results obtained by the curved Monte Carlo method in references. The results show that the discontinuous finite element method has a good accuracy in solving the multi-dimensional radiative transfer problem in a semitransparent graded index medium.     

Power propagation in homogeneous isotropic frequency-dispersive left-handed media

We study transmission at a boundary between a right-handed medium (RHM: epsilon>0, mu>0) and a frequency dispersive left-handed medium [LHM: epsilon(omega)<0, mu(omega)<0 for some omega], both homogeneous and isotropic. In order to account for the dispersion, two types of signal spectra are considered. The first consists of two discrete frequencies, while the second is Gaussian. Explicit expressions for the time-domain fields are obtained, from which the time-averaged Poynting vectors and hence power flow vectors are calculated. In both cases, we find that waves refract at negative angles at a RHM-LHM interface.     

Transient coupled heat transfer in a rectangular medium with black surfaces

The main objective of this paper is to extend to two-dimensional (2-D) medium the ray tracing-node analyzing method, which has already been successfully used to solve one-dimensional (1-D) problem of coupled heat transfer in a semitransparent medium. For simplicity, an infinitely long rectangular semitransparent medium with four black opaque surfaces is chosen as our studying object. A control volume method in the implicit scheme is adopted for discretizing the partial transient energy equation. In combination with spectral band model, the radiative heat source term is calculated using the radiative transfer coefficients (RTCs), which are deduced by the ray tracing method. The Partankar's linearization method is used to linearize the radiative source term and the opaque boundary condition, and the linearized equations are solved by the ADI method. Effects of absorption coefficient, refractive index and conductivity on transient cooling process in the 2-D gray rectangular medium are investigated under the condition that the radiation and convection processes cool one side of the rectangular medium while heat the remaining three sides.     

求解介质内热辐射传递的双向统计蒙特卡罗法

基于热辐射传输的光路可逆性原理,提出了求解介质内热辐射传递的双向统计蒙特卡罗法(BSMC法)。该方法采用等温等权抽样,利用能束传输路径的可逆关系进行辐射传递的双向统计计算,充分利用了能束跟踪的计算信息。以二维矩形区域内吸收性介质的热辐射传递为例,介绍了BSMC的求解过程,分析了其计算误差。通过数值模拟,从辐射传递因子计算结果的倒易性满足程度与辐射平衡温度场两方面,将BSMC法与传统的蒙特卡罗法(TMC法)进行了比较。结果表明,在相同的计算量下,BSMC法比TMC有更高的模拟精度。     

Numerical analysis of propagation characteristics of electromagnetic wave in lossy left-handed material media

The propagation characteristics of electromagnetic wave in lossy left-handed materials (LHM) are studied using finite-difference time-domain (FDTD) method base on auxiliary differential equation (ADE) technology. The LHM medium is realized with lossy Drude models for both the negative electric permittivity and the negative magnetic permeability. The discretized ADE-FDTD equations are derived in detail. The incident wave used in the simulation is a multiple cycle m-n-m pulses source. The term of Poynting's vector E_xH_y was calculated. These numerical results demonstrate conclusively that the phase velocity direction of electromagnetic wave propagation and the direction of the Poynting vectors are anti-parallel in LHM. The amplitude of electric field is reduced with the enhancive distance of LHM slab. It is also demonstrated that the energy of electromagnetic wave in the LHM slab is obviously attenuated, and the attenuation of energy becomes stronger with the angular plasma frequency ω_p increasing. These results indicate that LHM stealth is effective in theory, and reasonable selection of the large negative index of refraction can greatly enhance its effectiveness.     

Radiative properties of optical board embedded with optical black holes

Unique radiative properties, such as wavelength-selective transmission or absorption, have been intensively studied. Historically, geometries for wavelength-selective of light absorption were developed based on metallic periodical structures, which were only applied in the case of TM wave incidence due to the excitation of surface plasmons. In this paper, we develop an alternative approach to selective wavelength of light absorption (both TE and TM waves), based on an optical board periodical embedded with optical black holes. Numerical work was carried out to study such structure’s radiative properties within the wavelength range of 1-100 μm. The electromagnetic wave transmission through such a structure is predicted by solving Maxwell’s equations using the finite-difference time-domain (FDTD) method. Spectral absorptance varies with the period of optical black holes. When the incidence wavelength is much larger than the inner core radius, most of the light energy will be transmitted through the inner core. Otherwise, the energy will be mainly absorbed. Numerical results of the radiative properties of the optical board with different incidence wavelengths are also obtained. The effect of the oblique incidence wave is investigated. This study helps us gain a better understanding of the radiative properties of an optical board embedded with optical black holes and develop an alternative approach to selective light absorption.     

Discrete curved ray-tracing method for radiative transfer in an absorbing-emitting semitransparent slab with variable spatial refractive index

A discrete curved ray-tracing method is developed to analyze the radiative transfer in one-dimensional absorbing-emitting semitransparent slab with variable spatial refractive index. The curved ray trajectory is locally treated as straight line and the complicated and time-consuming computation of ray trajectory is cut down. A problem of radiative equilibrium with linear variable spatial refractive index is taken as an example to examine the accuracy of the proposed method. The temperature distributions are determined by the proposed method and compared with the data in references, which are obtained by other different methods. The results show that the discrete curved ray-tracing method has a good accuracy in solving the radiative transfer in one-dimensional semitransparent slab with variable spatial refractive index.