Effect of recording conditions on the anisotropic diffraction of volume holographic gratings

The anisotropic Bragg diffraction of the volume holographic gratings in photorefractive crystals are investigated based on the model of anisotropic coupled-wave theory. The effect of the initial intensity ratio and the recording angles of the two recording waves on the anisotropic Bragg diffraction properties is discussed. It is shown that both the ratio of the initial intensity and the incident angles of the recording waves are selective action for the anisotropic Bragg diffraction efficiency of the volume holographic gratings, while these two recording conditions are not selective action for the isotropic Bragg diffraction. Furthermore, the Bragg phase matching condition of anisotropic diffraction is analyzed when the recording angles change.     

记录结构对体光栅各向异性衍射选择特性研究

以静态体光栅各向异性衍射理论分析模型为基础,将与体光栅厚度有关的记录光强调制度引入到各向异性耦合波方程中.利用耦合波理论分析了光折变晶体中体光栅的各向异性衍射性能,研究了在不同初始记录光强比的条件下,记录光入射角的改变对光折变体光栅各向异性布喇格衍射性能的影响.结果表明：记录光入射角和记录光初始光强比均对体光栅的各向异性衍射具有选择性;与此相比,记录光入射角和记录光初始光强比对于体光栅各向同性衍射不具备选择性.     

光栅周期包含的Bragg层数对Bragg型凹面衍射光栅的影响

单个衍射光栅周期所包含的Bragg周期层数是连续Bragg齿型凹面衍射光栅的主要参数之一,该参数可改变光栅齿结构,对凹面衍射光栅的分辨力.自由光谱范围及衍射效率有重要影响.本文通过理论分析与仿真模拟,对比了4种不同层数的Bragg型凹面衍射光栅的特性参数.研究结果表明:在衍射光栅尺寸不变的情况下,改变单个光栅周期包含的Bragg周期层数不会显著提高器件主衍射级次的分辨力;单个光栅周期包含的Bragg周期层数与光栅可衍射的级次数成正相关.单周期层数的Bragg凹面衍射光栅的主衍射级次效率最高,其可衍射的级次数最少,且其他衍射级次分散的能量最少;增加单个光栅周期所包含的Bragg周期层数会降低主衍射级次的自由光谱范围.该研究对于设计低插损、高分辨率、宽工作波段的波分复用器或光栅光谱仪具有重要的指导意义.     

New approaches for the fabrication of photonic structures of nonlinear optical materials

We revisited two different strategies to fabricate 1D photonic crystals of nonlinear optical dielectric materials based on ultrafast laser ablation of the surface of an RbTiOPO₄ crystal, and selective etching of ferroelectric domains of the surface of a periodically poled LiNbO₄ crystal. We evaluated their behaviour as Bragg diffraction gratings. We also presented the recent advances we developed in a new procedure of fabrication of 2D and 3D photonic crystals of KTiOPO₄ (KTP) grown on the surface of a KTP substrate by liquid phase epitaxial means within the pores of a silicon macroporous template. Optical, structural, morphological, and compositional characterization for the photonic crystals produced through this technique are presented.     

光纤Bragg光栅的饱和效应

光纤 Bragg光栅的饱和效应可引起二阶衍射,并使一阶衍射的峰值反射率下降.本文利用折射率变化模型和耦合模理论,模拟了饱和情况下 Bragg光栅的一阶和二阶衍射谱,并讨论了饱和系数对一阶和二阶衍射的 Bragg波长和峰值反射率的影响.数值模拟结果与参考文献中的实验结果相吻合.     

横向衍射技术测量光纤光栅的特性

提出了一种用Ｈｅ－Ｎｅ激光侧面照射光纤光栅，从另一侧探测光栅的布拉格衍射效率来测量光纤光栅光谱特性的技术，并用该技术测量了光栅的折射率变化的分布。     

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.     

Variable Wavelengths Fiber Bragg Gratings Written with A Phase Mask and Four Prisms

1　Ｉｎｔｒｏｄｕｃｔｉｏｎ　　Ｓｉｎｃｅｔｈｅｆｉｂｅｒｗａｓｐｌａｃｅｄｄｉｒｅｃｔｌｙｂｅｈｉｎｄｔｈｅ ｐｈａｓｅｍａｓｋ ,ｗｈｉｃｈｗａｓｏｒｉｇｉｎａｌｌｙｄｅｍｏｎｓｔｒａｔｅｄｂｙＫ .Ｏ .Ｈｉｌｌｅｔａｌ.ｉｎ 1 993[1] ,ｔｈｉｓｍｅｔｈｏｄｈａｓｂｅｅｎｕｓｅｄｗｉｄｅｌｙｆｏｒｗｒｉｔｉｎｇｆｉｂｅｒＢｒａｇｇ ｇｒａｔｉｎｇ .Ｈｏｗｅｖｅｒ ,ｔｈｅｃｏｎｔａｃｔｔｅｃｈｎｉｑｕｅｏｆｐｈａｓｅｍａｓｋｃａｎｏｎｌｙｉｎｓｃｒｉｂｅｔｈｅｆｉｂｅｒＢｒａｇｇ ｇｒａｔｉｎｇｗｉｔｈ…     

半导体微结构物理效应及其应用讲座第1讲布拉格衍射效应在半导体光电子器件中的应用与发展

文章对布拉格衍射效应在半导体光电子材料和光电子器件，特别是在光纤通信中的光电子器件的应用和发展进行了详细的介绍．文中以布拉格光栅衍射效应的光反馈和选模功能为主线，逐一介绍了X射线双晶布拉格衍射技术在半导体材料、特别是在量子阱和应变量子阱超薄层晶体生长和质量控制方面的作用，介绍了内建布拉格光栅对光通信用光信号源的发展所起的重要作用，波导光栅在复用和解复用过程中的作用，以及光纤布拉格光栅在全光纤通信系统中的应用及发展等．     

Phase diffraction gratings based on a Si(400) crystal

The X-ray optical properties of diffraction gratings fabricated on the basis of a Si(400) crystal with a period of D = 1 μm are studied by triple-crystal X-ray diffractometry. The diffraction gratings are manufactured both by silicon profiling during the process of plasma chemical etching and by forming a phase-shift grating on the surface of a Si crystal. The principal difference in the diffraction properties of these gratings is demonstrated. The presence of an Au phase-shift grating is shown to lead to the formation of a two-dimensional diffraction pattern, whereas Si profiling leads to the formation of only a one-dimensional diffraction pattern.     

Detection of higher nonlinear harmonics of volume photorefractive gratings in reflection geometry

A comprehensive technique has been used to detect diffraction from the second and the third nonlinear spatial harmonic components, along with the fundamental harmonic of volume photorefractive gratings recorded in a channel waveguide in a LiNbO₃ substrate. For the first time to our knowledge, efficient diffraction from the higher spatial harmonic components of a photorefractive grating has been detected in reflection geometry. The dependence of a Bragg wavelength shift on the order of nonlinearity is also reported.     

Analysis of Interdigital Electro-optic Bragg Diffraction Grating

Some integrated optics devices can be made based on the interdigital electro-optic Bragg diffraction grating. The point-matching method is extended to the analysis of interdigital electro-optic Bragg diffraction gratings. This method provides a simple and fast analytic expression of the electric field in the structure. The field distributions are used to calculate the optical and electrical characteristic parameters of the gratings. The effects of finite conductor thickness have been taken into account in the analysis. It is shown that the simulation results agree well with the measured data.     

Influence of the characteristic curve of the material on the diffraction efficiency of holographic volume phase gratings

The behaviour of the diffraction efficiency of holographic gratings is discussed on the basis of the multiwave coupled-wave approach, taking into consideration the photoinduced change in the refractive index of the recording medium. The cases of Bragg and off-Bragg reconstruction in Bragg and mixed diffraction are considered. The influence of the higher harmonics of the refractive index modulation profile on the diffraction efficiency is shown.     

基于宽光谱应用的波长解复用多重体光栅的光写入特性

针对光通信中波长覆盖范围日益增宽的特点,从理论上研究了用于宽光谱范围波长解复用器件的多重体光栅的布喇格匹配和各波长对应光栅衍射效率的均匀性.计算及实验结果均表明,由于记录介质的色散效应将导致多重体全息光栅读出过程中明显的布喇格失配,并且在写入过程中获得的均匀折射率调制度的多重体光栅,在读出过程中各通信波长对应光栅的衍射效率将不均匀为此,提出了改进多重体光栅的光写入方法,给出了利用角度复用法写入透射型多重体光栅时解复用波长与写入角度的关系,以及写入体光栅过程中记录介质的折射率调制度修正公式.     

Calculation of shear angles in holographic gratings recorded in bleached silver-halide emulsions

Shearing phenomena in holographic gratings formed in bleached silver-halide emulsions due to chemical processing are evaluated. These effects are introduced because of the inability of emulsion-thickness changes to explain the anomalous detuning of the peak diffraction efficiency from the expected Bragg condition. Experimental results obtained with transmission and reflection gratings are presented and discussed considering shearing effects, and the values of the shear angles calculated from the Bragg-angle deviation measurements are also provided. In order to separate the shearing phenomenon and the shrinkage of the emulsion layer, a rehalogenating bleach without fixing is used because this bleach bath introduces a minimal change in the thickness of the emulsion. The influences of exposure, spatial frequency and slant angle on shear angles are briefly analyzed and discussed.     

Two-dimensional non-spatial filtering based on holographic Bragg gratings

The filter made up of two gratings performs as a two-dimensional non-spatial filtering. This paper reports that the volume Bragg gratings are fabricated by interfering two collimated coherent laser beams in photopolymer. Diffraction efficiency of a single grating is up to 78% in Bragg's condition, then a two-dimensional non-spatial filter, which consists of two volume Bragg gratings and a half-wave plate, enables the laser beam filtered in two dimensions with the diffraction efficiency of 54%. The Bragg's condition and effect of polarisation on performances of the two-dimension filter are also discussed.     

Dynamic Gain Equalizer Based on the H-PDLC Volume Phase Grating

The structure and Bragg diffraction characteristics of volume phase gratings based on H-PDLC technology are presented, and the principles and simulation aided design of dynamic gain equalizers with the gratings are discussed.     

X-ray point- and line-projection microscopy and diffraction

A theoretical framework is presented to treat both imaging and diffraction experiments performed with point-focus and line-focus X-ray sources, with particular emphasis on two-dimensional and planar X-ray waveguides. In particular, point-projection and line-projection microscopy has been approached within the Huygens-Fresnel formalism; point-projection and line-projection diffraction, such as spatially-resolved Bragg/Laue diffraction of crystalline samples in a regime of dynamical scattering, has been treated both by means of the Huygens-Fresnel formalism and of the Takagi-Taupin dynamical theory. Both in point- and line- projection geometry, simply rotating the investigated crystalline samples, it is possible to switch from Fresnel self-imaging to Bragg/Laue diffraction conditions. This means to image, within the same experiment, either morphological features, with a sub-micrometric resolution, out of the exact diffraction condition, or the structure order on an atomic scale if placing the sample in diffraction.     

Diffraction properties of ultrashort pulsed beams with arbitrary temporal profiles by a reflection volume holographic grating

The diffraction properties of reflection volume holographic gratings are studied when the gratings are illuminated by an ultrashort beam with arbitrary temporal profiles. The modified Kogelnik's theory of coupled waves is used. Simple analytical expressions for spectral and temporal distribution of the transmitted and diffracted are obtained. The dependences of the diffraction bandwidth, the Bragg selectivity bandwidth and the total diffraction efficiency of the reflection volume grating on the temporal profiles of the input ultrashort pulse are investigated. Study shows that the bandwidth of the reflection volume holographic gratings varies with the grating parameters more slowly than the transmitting volume holographic gratings. For three different temporal profiles, numerical results about diffraction properties of the reflection volume holographic gratings are given and compared with the transmitting volume holographic gratings.     

Total internal reflection diffraction grating in conical mounting

The main conditions and parameters for obtaining surface relief total internal reflection diffraction gratings in conical mounting are presented. Calculated and experimental investigations reveal that there are ranges of grating periods, incidence angles, diffraction angles and gratings depths for which such gratings could be obtained, both for TE and TM polarizations. With optimized grating parameters the diffraction efficiency of the total internal reflection diffraction gratings can be greater than 90%.