摆动刻蚀法制作高衍射效率凸面闪耀光栅

通过摆动离子束刻蚀方法,制作了用于短波红外高光谱成像光谱仪的凸面闪耀光栅。该方法通过在光栅子午方向上进行摆动刻蚀,解决了凸面光栅子午方向的闪耀角一致性问题。建立了摆动刻蚀模型来分析摆动速度、束缝宽度等工艺参数对槽型演化的影响,并计算了优化的刻蚀工艺参数。制备了基底尺寸为67 mm,曲率半径为156.88 mm,刻线密度为45.5 gr/mm,闪耀角为2.2°的凸面闪耀光栅,并对其表面形貌及衍射效率进行了测量。实验结果表明,摆动刻蚀法能够制作出闪耀角一致性好、衍射效率高的小闪耀角凸面光栅,满足成像光谱仪对光谱分辨率和便携性的使用要求。     

高质量全息光栅的制作

全息光栅由感光版记录高斯光束干涉条纹制得 ,干涉条纹的不等宽性降低了光栅品质。制取高质量全息光栅的关键是要用均匀平面波作光源。在分析干涉区内光强分布的基础上 ,给出了制作等周期正弦全息光栅的两种方法 :限制曝光区大小的方法和高斯光束强度均匀化的方法。     

高密度相位光栅的偏振选择性

高密度光栅具有与传统光栅不同的性质,其衍射特性往往是偏振相关的。本文针对1550nm波长TE/TM偏振入射光和0.5的光栅占空比,利用严格耦合波分析数值计算了不同光栅周期下0级及-1级的衍射效率。研究表明,相比周期为1550nm的光栅,当周期为1200nm时,偏振相关衍射效应明显增强,当光栅周期为890nm时,TE偏振光的衍射效率随着光栅深度呈正余弦变化,而TM偏振光的衍射效率始终集中在0级,具有偏振选择性。通过模式方法,利用模式中的有效折射率概念,研究了不同周期下被入射光所激发的两种光波模式通过光栅区域传播所累积的相位差;基于双光束干涉,模拟了0级和-1级的衍射效率。结果表明,利用严格耦合波分析的数值计算结果符合模式方法的理论预期,对于高密度相位光栅的偏振选择性给予了合理的物理机制解释。     

Thin and thick diffraction gratings: Thin matrix decomposition method

A brief review of the properties of transmission diffraction gratings is presented. Two types of gratings will be analyzed: thin and volume gratings explaining how the efficiency of the different orders that propagate inside the gratings can be calculated in both cases. For thin diffraction gratings the so-called amplitude transmittance method is applied in order to get the amplitude of the different orders, whereas in the case of volume gratings more complex methods are needed, such as Coupled Wave or modal theories. We will comment on the thin matrix decomposition method (TMDM), firstly proposed by Alferness, which gives a very intuitive approach and connects the properties of thin gratings to the properties of volume ones. The thin matrix decomposition method consists in dividing the volume grating in a number of thin gratings and applying the amplitude transmittance method to each thin grating. In this way the output of a grating will be considered as the input of the next and any individual grating can be treated by the amplitude transmittance method. The novelty of this work is that a comparison is made between the analytical expressions obtained by Alferness using the TMDM with the numerical results obtained using the coupled wave (CW) and rigorous coupled wave (RCW) theories for the efficiencies of the zero, first and second order when a plane wave incides onto a sinusoidal diffracion grating at the second on-Bragg replay angular condition.     

Blazing of smooth gratings

The diffraction efficiency of smooth weak gratings can depend significantly on the profile of the gratings as, in the diffraction process there exists wave coupling. By adjusting the grating profile it is possible to obtain enhancement or suppression of the diffraction efficiency. Possible applications include improvements in grating devices, grating profile reconstruction, and precise control of the slightest periodical disturbances. Some numerical examples are given.     

亚波长金属光栅的表面等离子体激元共振特性

亚波长金属光栅在共振波长处有光场局域增强、异常透射等现象,为深入认识其共振机制,本文研究了亚波长金属光栅的表面等离子体激元（SPP）共振特性。通过研究不同金属光栅的几何结构以及金属介电常数对SPP共振波长的影响,获得了3种共振波长的基本物理机制。采用周期边界元法进行数值模拟,在边界积分方程的基础上结合平面波展开方法来处理任意形状的周期性结构。模拟结果表明,3种共振波长可以分别由金属的材料、金属光栅周期和金属光栅厚度所调谐。该研究为微纳米光学器件的设计提供了依据。     

软X射线透射光栅制作技术

金透射光栅广泛用于真空紫外、X射线、物质波的衍射和光谱测量。分别介绍了美国麻省理工学院空间微结构实验室和中国科学技术大学国家同步辐射实验室的透射光栅制作技术及其新进展。麻省理工学院空间微结构实验室为完成 AXAF巨型空间天文台上的高能透射光栅谱仪经历了近 2 0年的发展 ,其光栅制作工艺日趋成熟 ,代表了当前世界最高水平。中国科学技术大学国家同步辐射实验室为了满足惯性约束核聚变实验研究的需要 ,在国内较早地开展了透射光栅的研制工作 ,在提高光栅质量的同时不断提高线密度     

Beam splitting, combining, and cross coupling through multiple superimposed volume-index gratings

We investigate the diffraction properties of multiple-superimposed index gratings from a vector synthetic viewpoint using coupled wave analysis with Bragg and non-Bragg readout. The vector synthetic gratings consist of two volume-index gratings with different grating wave vectors, which can be obtained in photorefractive materials, in optical fibers or in Bragg cells with two driving acoustic waves. This structure can realize beam splitting, beam combining, and beam cross coupling, etc. Analytic expressions for the diffraction efficiency as well as the amplitude of the waves involved are obtained. The results are presented and discussed.     

Edge-enhanced imaging with polyvinyl alcohol/acrylamide photopolymer gratings

We demonstrate edge-enhanced imaging produced by volume phase gratings recorded on a polyvinyl alcohol/acrylamide photopolymer. Bragg diffraction, exhibited by volume gratings, modifies the impulse response of the imaging system, facilitating spatial filtering operations with no need for a physical Fourier plane. We demonstrate that Kogelnik's coupled-wave theory can be used to calculate the transfer function for the transmitted and the diffracted orders. The experimental and simulated results agree, and they demonstrate the feasibility of our proposal.     

Brewster incidence for metallic gratings

This paper is a first report on a theoretical and numerical study undertaken in order to understand the grating anomalies linked with an important absorption of energy. It is shown that metallic gratings can exhibit anomalies which consists of total absorption of a plane wave.     

低串扰的多波长光纤光栅色散补偿器

分析了光纤光栅色散补偿系统中的串扰，并比较了几种常用的多波长光纤光栅色散补偿器的串扰特性，结合串联的窄带光纤光栅和取样光纤光栅的优点，提出一种抑制光纤光栅色散补偿系统串扰的方法.它通过在邻近信道间引入随机时延差，既能改善取样光纤光栅的线性串扰特性，又能抑制交叉相位调制效应和四波混频等非线性串扰.使用该方法可以得到具有低串扰的多波长光纤光栅色散补偿器.     

A comparison of the optical path and differential equation methods for optically thin phase gratings

It is shown that the optical path method may lead to an approximate solution of the wave equation valid under a wide range of conditions for slanted phase gratings of arbitrary profile. Analytic solutions are given for sinusoidal and sawtooth gratings, and the accuracy of the approximations is checked by comparison with numerical solutions. The conditions under which sawtooth gratings may yield 100% efficiency are clarified.     

Periodic and solitary waves in optical fiber Bragg gratings with dispersive reflectivity

The system of partial differential equations for moving optical solitons in fiber Bragg gratings are studied. The traveling wave reductions are used to look for solutions of the system of equations. The compatibility conditions for the overdetermined system of equations are analyzed and discussed. Periodic and solitary waves in optical fiber gratings are found and illustrated.     

Digital volume gratings

An optical element constructed by stacking a set of binary-phase grating sheets can simulate the functions of optically recorded volume gratings. Our electromagnetic numerical study also shows that if one of the grating sheets is replaced by another one with different grating period, power spectrum of the diffracted wave changes completely with extra diffraction orders. This property will claim strong advantage in security document applications. Analysis of alignment error reveals interesting phenomena concerning to how misalignment affects diffraction efficiency.     

Boundary effects in the diffraction of light by volume gratings: II. Phase transmission gratings

The two-wave approximation of the method of characteristics modes is used to analyze theoretically the diffraction of light by volume phase transmission gratings for arbitrary proportions between the average refractive index of the grating and the refractive indices of the ambient media. Simple expressions are derived for calculating the amplitudes of the diffracted waves in the case where the angle of incidence of the primary wave coincides with the Bragg angle. The roles of the front and back boundaries of the grating are discussed separately, and a new method for separating the contribution of the boundary diffraction, which is due to the periodic inhomogeneity of the grating boundaries, is proposed. The experimental possibility of selecting partial Bloch waves in the measurements of the diffraction efficiency of gratings that have high reflection coefficients of both boundaries is demonstrated.     

Coupled wave analysis for out-of-phase mixed thick hologram gratings

A coupled wave analysis is given for mixed volume hologram gratings. The effect of spatial dephasing between the absorption and phase grating is considered and evaluated numerically for a number of cases. The analysis is motivated by the photorefractive effect in electro-optic crystals.     

二维光栅与周期性缝隙阵列组合薄膜结构的杂散光抑制

传统方格型二维光栅与周期性缝隙阵列的组合薄膜结构具有雷达和光学波段双带通的电磁特性. 但由于其杂散光能量集中分布而严重制约它在高精度探测以及成像领域中的应用. 本文提出了一种全新的组合薄膜结构, 即由满足一定约束条件的圆孔型二维光栅和十字缝隙阵列构成. 基于Fraunhofer衍射理论建立组合薄膜结构标量衍射模型, 通过对比两种组合薄膜结构的衍射光分布, 理论分析与实验测试均表明: 圆孔型光栅与十字缝隙阵列组合薄膜结构不仅能够提高其光学透过率, 而且还使其杂散光分布均匀, 降低了其杂散光总比率, 从而有效抑制杂散光, 进一步增强了二维光栅与周期性缝隙阵列组合薄膜结构在实际光学系统中的可靠性. 关键词： 二维光栅 组合薄膜结构 衍射光强 杂散光     

Investigation on the bistability of linearly chirped nonlinear Bragg gratings

Based on the nonlinear coupled mode theory, by using the reversely recursive transmission matrix method, the switching characteristics of linearly chirped nonlinear Bragg gratings (LC-NLBG) have been investigated numerically. The results show that, introducing spatial chirp in fiber gratings is helpful to widen the bistable wavelength range, and the influence of the chirp on the switching-on threshold and the on-off ratio of the bistable switching are obvious, the chirp-dependent bistability is due to the difference of the axial distribution of forward wave intensity in the gratings.     

Unidirectional transmission realized by two nonparallel gratings made of isotropic media

We realize a unidirectional transmission by cascading two nonparallel gratings (NPGs) made of isotropic, lossless, and linear media. For a pair of orthogonal linear polarizations, one of the gratings is designed as a polarizer, which is a reflector for one polarization and a transmitter for the other; another grating is designed as a polarization converter, which converts most of one polarized incident wave into another polarized transmitted wave. It is demonstrated by numerical calculation that more than 85% of the incident light energy can be transmitted with less than 1% transmission in the opposite direction for linearly polarized light at normal incidence, and the relative bandwidth of the unidirectional transmission is nearly 9%. The maximum transmission contrast ratio between the two directions is 62 dB. Unlike one-way diffraction grating, the transmitted light of the NPGs is collinear with the incident light, but their polarizations are orthogonal.