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.     

Genetic algorithm-based design method for multilevel anisotropic diffraction gratings

We developed a method for the design of multilevel anisotropic diffraction gratings based on a genetic algorithm. The method is used to design the multilevel anisotropic diffraction gratings based on input data that represent the output from the required grating. The validity of the proposed method was evaluated by designing a multilevel anisotropic diffraction grating using the outputs from an orthogonal circular polarization grating. The design results corresponded to the orthogonal circular polarization grating structures that were used to provide outputs to act as the input data for the process. Comparison with existing design methods shows that the proposed method can reduce the number of human processes that are required to design multilevel anisotropic diffraction gratings. Additionally, the method will be able to design complex structures without any requirement for subsequent examination by a human designer. The method can contribute to the development of optical elements by designing multilevel anisotropic diffraction gratings.     

Shadow Moiré method for the determination of the source position in three-dimensional shearography

Shearography is a full-field non-contact optical technique used for characterisation of surface strain. In a multi-component system, the displacement derivative components are measured using a number of illumination positions. These components are then transformed into a three-dimensional coordinate system, using a knowledge of the source positions. This process is highly sensitive to errors in the knowledge of the source position. Shadow Moiré, with either linear or circular gratings, can be used to measure angle of illumination, with the measurement sensitivity and accuracy variable by changing the grating pitch. Circular gratings have a measurement range determined by multiple fringe analysis and linear gratings have a different measurement range determined by sub-fringe analysis. In this paper vertical linear, horizontal linear and circular gratings are combined to extend the measurement range and the accuracy of the measurement of the source position in two directions. Using this method the source position was measured to an accuracy of ±3%.     

一种MEMS周期可调光栅衍射特性的实验研究

介绍了一种周期连续可调的微型可编程光栅.光栅采用静电梳齿驱动,不同驱动电压对应不同周期变化,从而引起特定级次衍射角发生变化.为了测得不同驱动电压下的衍射角变化,搭建了实验测量光学系统,利用激光三角法测量原理及图像处理的方法得出实验结果.测量结果与光栅测量参量计算角度变化基本一致,不同电压下的衍射角变化与不同的周期变化对应良好.     

DWDM Devices Based on Planar Waveguide Technologies

A review is presented on some of our recent results for designs, simulations and fabrication of several photonic integrated devices, such as arrayed-waveguide gratings (AWGs) and etched diffraction gratings (EDGs), based on planar waveguide technologies. Some novel designs for flat-top AWGs and EDGs with flat-top spectral responses are presented.     

高衍射效率亚波长结构Dammann光栅的设计

传统的Dammann光栅是基于标量衍射理论设计的二值相位等光强分束器件，其衍射效率的典型值为80%左右.基于严格耦合波分析理论和遗传算法，提出了一种设计亚波长结构Dammann光栅的新方法，且该新型二值相位光栅具有较高的衍射效率.同时利用自编的仿真程序包设计了多个光栅，并分析了制作误差对其衍射效率和光强均匀性的影响.仿真结果表明，用该方法所设计的Dammann光栅的衍射效率超过92%.     

平面全息光栅刻线密度的倍频式调整方法

刻线密度准确与否直接影响光栅色散及给定波长的衍射方向,进而影响光谱仪器结构设计.为了提高全息光栅刻线密度的制作精度,提出了平面全息光栅刻线密度的倍频式调整方法.将给定刻线密度的基准光栅放在干涉场曝光区域内,调节光束干涉角,干涉场曝光光束经基准光栅衍射后,根据调整光栅刻线密度的不同选择不同的衍射级次相互叠加形成莫尔条纹,...     

DCG全息元件的简化处理及特性研究

本文提供了重铬酸明胶（ＤＣＧ）的一种简化处理步骤，最高衍射效率和透明度均达９７％，测量和分析了处理后ＤＣＧ光栅的衍射特性，讨论了高质量ＤＣＧ全息元件的应用。     

Simplified processing and characteristic investigation of DCG holographic elements

In this paper,the simplfied procedures of processing dichromated gelatin(DCG)holographic optical elements(HOE's) are presented.Both diffraction efficiency and transmis-sion of DCG grating made by the procedures have been achieved up to 97%.The diffractioncharacteristics of the DCG gratings are measured and analysed,the applications of DCG grat-ings with high quality are also discussed.     

一种新型透射式自聚焦光栅的设计及理论研究

结合普通透射光栅的分光特性和菲涅耳波带片的聚焦特性,提出一种应用于软X射线波段的新型透射式自聚焦光栅。根据菲涅耳衍射理论,对其衍射特性进行理论推导,证明该新型光栅存在一个与光栅平面垂直的焦平面,不同波长的光波聚焦于该焦平面的不同位置。在焦平面上,焦点位置随光子能量的变化是线性的,即能实现光子能量的线性测量。通过解析推导,得到在沿着焦平面和垂直于焦平面两个方向上光斑的展宽模式,发现该新型光栅在沿着焦平面的光斑展宽模式不同于普通光栅。根据基尔霍夫衍射公式,设置实用化参数,对其衍射模式进行了详细的数值模拟研究,并讨论了相应的测谱范围和谱分辨率。     

用菲涅耳衍射理论分析双光栅成像效应

从理论上对双光栅成像效应现象进行研究,分别从菲涅耳衍射理论和平面透射光栅的菲涅耳衍射公式出发,分析了物光波经过两个平行放置的平面透射光栅组成的系统衍射后的复振幅分布。分析结果表明,在理想状态下,当两个光栅的放置位置满足一定条件时,在第二个光栅后面可以观察到一个清晰的原物体图像。理论分析解释了两个光栅在双光栅成像效应中的作用,得到了双光栅成像效应发生时,两个光栅的空间频率、衍射光级数以及光栅放置位置之间的关系式,这与实验所得结果相一致。     

Inscription of high contrast volume Bragg gratings in fused silica with femtosecond laser pulses

We present volume Bragg gratings (VBGs) with a period of 1.075 μm inscribed in fused silica using a femtosecond laser and a phase mask. The femtosecond-inscribed VBGs can be used as reflecting elements with reflectivities of about 80% for a 1-mm-long grating. Due to the non-sinusoidal refractive-index shape, higher order Bragg resonances up to the 7th reflection order could be measured. Therefore, the Bragg gratings also reflect light in the visible-wavelength range.     

Measurement of a discontinuous object based on a dual-frequency grating

The dual-frequency grating measurement theory is proposed in order to carry out the measurement of a discontinuous object. Firstly, the reason why frequency spectra are produced by low frequency gratings and high frequency gratings in the field of frequency is analysed, and the relationship between the wrapped-phase and the unwrapping-phase is discussed. Secondly, a method to combine the advantages of the two kinds of gratings is proposed: one stripe is produced in the mutation part of the object measured by a suitable low frequency grating designed by MATLAB, then the phase produced by the low frequency grating need not be unfolded. The integer series of stripes is produced by a high frequency grating designed by MATLAB based on the frequency ratio of the two kinds of gratings and the high frequency wrapped-phase, and the high frequency unwrapping-phase is then obtained. In order to verify the correctness of the theoretical analysis, a steep discontinuous object of 600×600 pixels and 10.00~mm in height is simulated and a discontinuous object of ladder shape which is 32.00~mm in height is used in experiment. Both the simulation and the experiment can restore the discontinuous object height accurately by using the dual-frequency grating measurement theory.     

Holographic grating based Ebert spectrograph

In this paper we discuss the aberration properties and design procedure of in-plane Ebert spectrograph using conventional as well as holographic diffraction gratings. In both cases the gratings are situated at the well-known √3-position so that the spectrum can be recorded on a flat surface. It has been found that the holographic grating system has better resolution than the conventional grating system. The design parameters of a medium sized holographic grating spectrograph in which a concave spherical mirror is mounted in the off-axis configuration have been specified. The performance of the spectrograph has been evaluated by plotting spot diagram.     

Development of narrow-linewidth diode lasers by use of volume holographic transmission gratings

We present two diode laser setups that employ volume holographic transmission gratings to provide optical feedback. The advantage of this kind of grating is high diffraction efficiency and the possibility to place optical elements on both sides of the grating. This allows for advanced external cavities and adjustable feedback efficiency. The first setup is a diode laser in the Littman configuration with the transmission grating replacing the conventional reflection grating. The second setup improves the frequency selectivity by substituting the feedback mirror with a passive resonator. This grating-enhanced external cavity diode laser (GECDL) achieves excellent frequency stability. A prototype of the GECDL setup demonstrates an intrinsic linewidth of 7 kHz and an operation range that covers the full amplification profile of the laser diode.     

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.     

High-order diffraction suppression using modulated groove position gratings

Conventional diffraction gratings composed of a series of equally spaced slits suffer from wavelength overlapping caused by high-order diffraction. Here modulated groove position gratings (MGPGs) are proposed to significantly suppress the high diffraction orders. Both numerical solution and experimental results demonstrate the effectiveness of MGPGs. The suppression ratio is determined by the number of grating grooves used. By using an MGPG with 10,000 grooves, a suppression ratio as high as 18,000 can be obtained. In addition, the minimum linewidth is kept to 1/4 of the grating period, which enables grating realization with high line density employing today's nanofabrication technology. Our results should be of great interest in both diffraction grating theory and applications, particularly due to MGPGs' applicability in a wide wavelength range and realizability with high line density.     

Opto-electronic displacement gauge based on speckle interferometer

Modern technology demands precise distance measurements. People have used many phenomena for this purpose but there is always a need for less expensive and more reliable devices. In this paper we present a simple opto-electronic distance measurement gauge based on the phenomenon of speckling phenomenon. In the proposed setup we substitute diffraction gratings, which are commonly used in displacement gauges, with a rough surface. When illuminated by a coherent and monochromatic beam the surface scatters light to form a speckle field, whose intensity variations are read by one or more detectors. These changes are related to displacement of the rough surface and, therefore, can be utilized for displacement measurements. The presented setup has some important advantages over the grating approach: lower mechanical tolerances on parts and being based directly on the wavelength of light instead of the period of a grating. Also, it is less expensive, so can offer a viable solution for medium accuracy distance measurements.     

用飞秒激光在透明介质体内形成衍射光栅

用数值孔径为0.65的40倍显微透镜紧聚焦能量稍高于能量损伤阈值的200 fs的钛宝石激光脉冲,分别在ZK6玻璃和K9玻璃内部导致局部折射率改变,在透明介质内部形成透射式相位衍射光栅．在635 nm的He-Ne激光照射下,通过对光栅衍射效率的数值拟合计算,得出了的光栅条纹宽度与实际测量基本吻合,这从侧面证明了折射率改变区域为超高斯型的假定．还对折射率改变区域的性质进行了分析．在实验上观察到脉冲间的相互影响将导致折射率改变区域范围的增大．从理论上给出了增加1级衍射效率的途径．     

SiC/W/Ir multilayer-coated grating for enhanced efficiency in 50-100 nm wavelength range in Seya-Namioka mount

A broadband [SiC/W/Ir](2) multilayer coating was deposited onto a diffraction grating to enhance the grating efficiency in the 50-100 nm wavelength range in a Seya-Namioka mount. The holographic ion-beam etched grating had a laminar profile with 1200 lines/mm. The coating was designed by using the subquarterwave multilayer theory. The measurement results show that the efficiency in the -1st diffraction order is greater than 9.3% throughout the wavelength range of 50-100 nm, which is obviously higher than that of single-layer gratings with SiC, Ir, or Au coatings.