变波长读出条件下光折变局域体全息光栅的衍射性质

利用二维耦合波理论,研究了用短波长记录全息光栅而用通讯波段内的长波长读出时,用于90°结构垂直读出平板型的光折变局域体全息光栅的衍射性质.讨论了局域体全息光栅的几何尺寸对其衍射效率及光栅布喇格选择性的影响.结果表明,随着光栅的纵向和横向尺寸的增加,光栅的衍射效率也逐渐增加.当光栅的纵向和横向尺寸发生改变但光栅的总面积不变时,光栅衍射效率保持不变.此外,随着光栅尺寸的增加光栅的布喇格选择性越好.在利用短波长记录全息光栅而用长波长读出的光学器件设计过程中,为了获得最佳的衍射效率及其布喇格选择特性,应当根据要求合理地设计光栅的几何尺寸.     

用于WDM的多重体光栅曝光方法及层状体全息光栅的研究

本文针对层状多重体全息在红外通讯波段分光滤波的应用,分析了其布喇格匹配与衍射效率均匀性问题.给出了在LiNbO3∶Fe光折变晶体中制作层状多重体全息光栅的几种方法.利用双片光干涉在直接分层结构中写入了三层光栅,光栅层厚1 mm,层间距2 mm,每层复用了四个光栅.对这三层光栅的分析测试表明,12个光栅的衍射效率均匀,相邻通道间隔约3 nm.实验结果表明,这种层状结构的多重体全息光栅有希望用来组成具有大量通道数的WDM系统中的复用/解复用器件.     

导致光聚物全息存储布喇格偏移因素的研究

研究了光聚物高密度全息存储材料在全息记录过程中曝光量、曝光时间、厚度收缩比例、折射率调制度及记录角度等因素对布喇格偏移的影响规律,结果表明:曝光量一定时,布喇格偏移与曝光时间的关系不大,当曝光量不一定时布喇格偏移随曝光时间增加缓慢增加最后达到饱和;而布喇格偏移随厚度收缩比例和折射率调制度的增加以接近正比例的关系增加;记录光栅的倾斜度越大则布喇格偏移也越大.这些结论对光聚物高密度全息存储研究具有重要的意义.     

平面波导阵列布喇格光栅及其光辐照制作方法

本文提出了一种新的平面波导阵列布喇格光栅结构,并给出了在光敏感材料中制作这种波导光栅结构的全息光辐照方法.以LiNbO3:Fe晶体作为光敏感材料,用不同空间频率的双光束干涉条纹辐照后,通过数字全息技术对晶体的光致折射率分布测量和进行的导波衍射测试结果表明,利用这种光辐照方法在光敏感材料中制作平面波导阵列全息布喇格光栅是完全可行的.其显著的导波衍射现象表明,如果在平面波导阵列中制作多重布喇格光栅,有可能使其成为超密集波分复用（UDWDM）系统中波长复用/解复用的核心器件.     

光折变多重体光栅的制作及应用

根据光折变多重体光栅的滤波原理,在10×10×10 mm3的LiNbO3∶Fe晶体中利用循环曝光法写入了光栅波矢夹角为0.037°且衍射效率均衡的8重体光栅,实验上证实了光写入多重光折变体光栅应用于波分复用系统的可行性.利用LiNbO3∶Fe晶体制作了一个原理性双通道波分复用器,成功地实现了对相同方向入射的波长分别为635 nm和650 nm的两束光的有效分离,并根据布喇格条件,将光折变体光栅应用于光波波长的测量,测量精度可达10 pm量级.     

体积相位全息光栅的设计与制备

本文指出全息干版薄膜在曝光后处理过程中的收缩性和折射率变化对制备全息光栅的周期、倾斜度、布喇格入射角及衍射角的影响问题，并根据实验数据对光栅设计进行修正，再结合Ｋｏｇｅｌｎｉｋ衍射方程及折射率调制度和曝光量的线性关系，在实验中基本上能准确设计出指定周期、倾斜度和衍射效率的全息光栅。     

+1/-1相位掩模板和532nm激光下低损耗As_2S_3硫系光纤布喇格光栅的制备

采用改进型Sagnac干涉光栅写入系统,利用532nm准带隙光曝光源和带+1/-1衍射级的相位掩模板,在两种不同直径的低损耗As_2S_3硫系玻璃光纤上刻写布喇格光栅,并研究曝光期间光栅的动态特性.实验表明,As_2S_3光纤布喇格光栅透射峰值随光纤直径的减小而增强;在曝光过程中,布喇格波长先是较快地向短波长方向移动,随着曝光时间的延长,布喇格波长缓慢地向长波长方向回复.曝光时间为800~1 000s时,在包层直径为140μm的As_2S_3光纤上获得质量良好的布喇格光栅光谱,其透射峰值可达-2.6dB,带宽为0.37nm.对As_2S_3硫系光纤纤芯的光敏性分析结果表明,折射率调制幅度和平均折射率变化随曝光时间分别可达到10-4和10-3数量级.     

体积相位全息光栅的设计与制备

本文指出全息干版薄膜在曝光后处理过程中的收缩性和折射率变化对制备全息光栅的周期、倾斜度、布喇格入射角及衍射角的影响问题,并根据实验数据对光栅设计进行修正,再结合Kogelnik衍射方程及折射率调制度和曝光量的线性关系,在实验中基本上能准确设计出指定周期、倾斜度和衍射效率的全息光栅。     

声光技术的进展

一、声光效应和声光器件 当声波在介质中传播时,由于物质的光弹效应,使介质的折射率发生周期性变化,从而也像相位光栅那样使光衍射.当超声波频率较低,声光互作用区较短时,入射光常被衍射成多级衍射光,这就是喇曼-奈斯衍射.如果超声波频率很高,光沿着适当的角度射入介质并通过较多的超声波阵面,则高次衍射光消失,只剩下0级和1级光,这就是布喇格衍射,这时入射光与衍射光的角度应满足布喇格方程即     

基于体全息光栅的光学相控阵放大级设计

为了扩展光学相控阵的角度扫描范围,设计了基于体全息光栅的放大级,研究了光栅结构参数及其制作误差对性能的影响规律,提出了容差优化方法。研究结果表明:光栅厚度和折射率调制度是影响衍射效率的主要参数,光栅周期和光栅厚度是影响角度选择性的主要参数。光栅出入射角由光栅周期和光栅矢量倾斜角决定,在保持角放大率不变时,可以通过在出入射面内旋转介质调节这两个参数。实际制作中,光栅周期误差和光栅矢量倾斜角误差会导致衍射角偏离设计值,读出光波长越长,光栅周期误差的影响越小;光栅后面介质折射率越高,光栅矢量倾斜角误差影响越小。增加光栅厚度设计值可以减小光栅厚度误差对衍射效率的影响,而减小光栅厚度设计值可以减小折射率调制度误差对衍射效率的影响,实际制备时,需结合系统需求进行综合设计。     

Study on measuring refractive index modulation of volume phase holographic gratings

In this paper, we present an approach to measure the refractive index modulation of volume phase holographic gratings (VPHG) by use of its angular selectivity, with the advantage of avoiding the diffraction efficiency errors induced by noise. Based on Kogelnik's coupled wave theory for volume holographic gratings, the wave vectors matching relationship for reflective VPHG at different readout wavelengths is deduced and the diffraction efficiency as a function of readout angle is also obtained. Then an all-fiber experimental system is designed to measure corresponding diffraction efficiency with different readout angles for VPHG recorded in dichromate gelatin. By seeking the Bragg angle where diffraction efficiency is maximum and the blank angle where diffraction efficiency drops to the minimum value, the refractive index modulation can be calculated. The experiment results proves the feasibility of the method.     

折射率直流分量调制对光纤Bragg光栅的影响

从耦合模理论出发,推导了折射率直流分量调制函数与光纤Bragg光栅相位之间的关系式,分析了折射率直流分量调制对光纤Bragg光栅时延特性的影响.利用传输矩阵法进行了数值仿真,仿真结果与理论分析一致.在理论分析基础上,提出采用同一块均匀相位掩膜版灵活制作具有不同线性或非线性时延特性的啁啾光纤光栅的方法,可用于高速光通信系统中不同色散和色散斜率的补偿.     

体全息光学元件布拉格偏移特性的研究及其应用

本文研究了体全息光学元件的布拉格(Bragg)偏移及补偿特性,得出了在某些调制度下,在偏移布拉格条件下仍可以得到较高的衍射效率的结论.文中也讨论了体全息光学元件布拉格偏移的容忍性并分别讨论了体全息元件布拉格偏移所允许的角度偏移和再现波波长偏移的性质,得出了在大调制度下体全息光学元件的布拉格偏移容忍性大的结论.同时指出实现体全息光学元件高衍射效率的变波长再现的途径,并给出相应的实验结果.     

First order phase change detection using planar waveguide Bragg grating refractometer

Solid-to-liquid and gas-to-liquid phase changes in water and ordered-to-isotropic phase changes in a nematic liquid crystal are detected with an optical sensor. A planar Bragg grating defined purely by refractive index modulation is covered with a water or liquid crystal overcladding and the temperature is controlled to trigger phase changes. Measurement of the Bragg wavelength allows changes of effective refractive index to be detected and discontinuities in behaviour caused by phase transitions can be clearly identified.     

High-frequency modulation of light in diffraction by the Bragg grating with a refractive index traveling wave

The diffraction by a Bragg grating with high-frequency modulation of the refractive index of the medium is analyzed theoretically. The effect of the grating parameters and parameters of an rf signal on the efficiency of modulation of optical signals at frequencies above 10 GHz is investigated.     

Holographic solitons

We propose a new kind of an optical spatial soliton: the holographic soliton. This soliton consists of two mutually coherent field components that interfere, induce a periodic change in the refractive index, and simultaneously are Bragg diffracted from the grating. Holographic solitons are formed when the broadening tendency of diffraction is balanced by phase modulation that is due to Bragg diffraction from the induced grating. Holographic solitons are solely supported by cross-phase modulation arising from the induced grating, not involving self-phase modulation at all.     

Design, analysis and optimization of silicon-on-insulator photonic crystal dual band wavelength demultiplexer

A highly efficient photonic crystal dual band wavelength demultiplexer (DBWD) using silicon-on-insulator (SOI) substrates is proposed for demultiplexing two optical communication wavelengths, 1.31 μm and 1.55 μm. Demultiplexing of two wavelength channels is obtained by modifying the propagation properties of guided modes in two arms of Y type photonic crystal structure. Propagation characteristics of proposed DBWD are analyzed utilizing 3D finite difference time domain (FDTD) method. Enhancement in spectral response is further obtained by optimizing the Y junction of demultiplexer giving rise to high transmission and extinction ratio for the wavelengths, 1.31 μm and 1.55 μm. Hence it validates the efficiency of proposed optimized DBWD design for separating two optical communication wavelengths, 1.31 μm and 1.55 μm. Tolerance analysis was also performed to check the effect of variation of air hole radius, etch depth and refractive index on the transmission characteristics of the proposed design of SOI based photonic crystal DBWD.     

Single-step writing of Bragg grating waveguides in fused silica with an externally modulated femtosecond fiber laser

For the first time to our knowledge, high-strength (>30 dB) first-order Bragg grating waveguides were fabricated in bulk fused silica glass in a single-scanning step by modulating a high-repetition-rate femtosecond fiber laser with an external acousto-optic modulator. The modulation induced a waveguide segmentation by delivering controlled bursts of laser pulses to define an array of partially overlapped refractive index voxels. With appropriate choice of modulation frequency and sample scanning speed, low loss waveguides could be formed at high writing speeds to yield sharp Bragg spectral resonances tunable over the 1300 to 1550 nm telecom band. Effects of acousto-optic modulation duty cycle on propagation loss and grating strength are characterized. This modulation method offers facile control and integration of multiwavelength Bragg grating devices to enhance overall functionality of optical circuits in three-dimensional geometries.     

Characterization of fiber Bragg grating for maximum reflectivity based on modulation depth of refractive index

This study proposed the modeling of fiber Bragg grating as a sensor for the optimum reflectivity with minimum discontinuities at the end of center wavelength. Recent advancement in optical sensor technology, make it necessary to generate the optimum results with minimum error. In this analysis, dependence of grating output spectra on various parameters has been studied for minimum bandwidth, side lobes and maximum reflectivity. In previous studies effect of length and other parameters has been studied. In this paper author has included the effect of modulation depth also. Study shows that grating length and modulation depth of refractive index of core are very critical factors which significantly alter the grating characteristics. Here low absorption third window of optical communication is used for the sake of low attenuation. All the simulation has done using GratingMOd software using transfer matrix method.