Generation of millimeter-wave in optical pulse carrier by using an apodized Moiré fiber grating

A novel scheme is proposed to transform a Gaussian pulse to a millimeter-wave frequency modulation pulse by using an apodized Moiré fiber Bragg grating in radio-over-fiber system. The relation between the input and output pulses is analyzed theoretically by Fourier transformation method and the requirements for the proposed fiber grating are presented. An apodized Moiré fiber Bragg grating is designed and its characteristics are studied. It is shown that the proposed device is feasible, and the new scheme is believed to be an effective solution for the generation of millimeter-wave sub-carrier in future radio-over-fiber systems.     

Talbot effect in metallic gratings under Gaussian illumination

Metallic gratings can be found in applications such as optical metrology. Due to their fabrication process, the surface presents a certain roughness. In this work, the effect of roughness on Talbot effect has been analyzed when the grating is illuminated with a Gaussian beam. A model based on Fresnel regime is used in order to determine the intensity distribution in the near field. Contrast of the self-images is obtained and it is found that it decreases in terms of the distance between the grating and the observation plane. When the autocorrelation function of roughness presents a Gaussian behaviour, the diffracted beams are still Gaussian although some of their properties change. For example, the width of the diffracted beams increases with respect to the case of the standard chrome on glass gratings. On the other hand, the power of each diffracted beam is independent on the roughness properties of the surface.     

Excimer laser-induced etching of sub-micron surface relief gratings in fused silica using phase grating projection

Phase grating projection using a Schwarzschild objective is presented in conjunction with a process that allows laser-induced etching of transparent materials at the interface of liquids, in order to generate periodical surface relief structures with sub-micron resolution in fused silica. The achieved sinusoidal gratings exhibit a period of 780 nm and a depth of up to 180 nm, having a roughness lower than 5 nm r.m.s. The depth and roughness of the gratings are related to the applied laser fluence and pulse number. In addition to the grating formation, an overall removal of material at higher laser fluences was found. Received: 3 December 2001 / Accepted: 5 December 2001 / Published online: 11 February 2002     

Use of steel substrates in diffractive optics: Near field of high surface quality steel tape gratings

Steel tape is used in optical applications due to its mechanical properties. However, roughness of steel surfaces affects to wave propagation. The effect can be observed using diffraction gratings with low periods. In such a case, self-images of the grating appear at certain periodic distances from the grating (Talbot planes). When standard steel is used, the contrast of Talbot self-images strongly decreases with the distance from the grating. In this work, we prove that controlling the surface quality of steel, it is possible to improve its optical behavior and, as a consequence, high quality surface steel represents a good choice for diffractive optics when the use of chrome-on-glass masks is not indicated. As an example, we have manufactured a diffraction grating over a high quality steel surface by means of an ablation process with a nanosecond pulsed laser. The contrast of the self-images for these gratings decreases very slowly with the distance, in comparison with the self-images obtained with standard steel tape 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%.     

飞秒激光用多层介质膜脉宽压缩光栅的设计

以薄膜光学的干涉理论和衍射光学的傅里叶模式理论为基础，给出了0.8μm飞秒激光器用多层介质膜脉宽压缩光栅的理论设计；设计采用H3L(HL)^9H0.5L2.4H的多层介质膜为基底，当刻蚀后表面浮雕结构的占宽比为0.35，线密度为1480线/mm，槽深为0.2μm，顶层HfO₂的剩余厚度为0.15μm时，对于Littrow角度(36.7°)和TE波模式入射的衍射光栅其-1级衍射效率达到95％以上. 关键词： 飞秒激光 脉宽压缩光栅 多层介质膜     

Flat Filter Based on a Chirped Super Moiré Grating

In this paper, super Moiré grating (SMG) with multiple superimposed gratings, in the best of our knowledge, is studied at first time. A flat multichannel transmission filter based on a chirped SMG with three superimposed gratings can be achieved, which benefits wavelength-division multiplexing (WDM) system. At the same time, a reflection filter can also be obtained in a chirped SMG, which can be used as an optical add/drop multiplexer filter for WDM system.     

Design, Fabrication and Test of a Soft X-Ray Even-Order Transmission Grating

Most transmission gratings in the x-ray region work with their first orders and dispersion is limited by the line density achievable with current fabrication technology. We present a novel design of a two-dimensional x-ray transmission grating. The grating works with higher dispersion using its second orders, and the influence from first and third orders can be suppressed. A grating according to the novel design is fabricated and its diffraction performance is tested in comparison with a traditional x-ray transmission grating with the same line density. The novel grating could be especially useful when high dispersion is desired while the fabrication of high-density gratings becomes more difficult.     

基于Talbot-Moiré法测量透镜焦距的莫尔条纹的图像处理

利用Ronchi光栅的Talbot效应和Moiré条纹测量了长焦透镜焦距,并根据焦距与莫尔条纹斜率之间的定量关系可求得透镜焦距.为获得条纹斜率,用CCD采集莫尔条纹图像后,利用数学形态学滤波方法进行图像滤波平滑等预处理,有效地增强了图像的对比度.通过二值化、细化获得单像素宽连通的条纹,通过对条纹进行标记,为条纹斜率的直线拟合计算提供了可靠的数据,采用这些数据计算的焦距的误差为0.10%.     

Analysis of the Chirped Super Moiré Gratings Based on Acousto-Optical Superlattices

There are multiple passbands in a chirped Bragg grating based on acousto-optic superlattice. The structure can be regarded as an acousto-induced super Moiré grating (AI-SMG). The analysis shows that the passbands of the AI-SMG are produced by several pairs of neighboring chirped gratings. With suitable structure parameters selected, the passbands can be non-larenzian-shape, narrow flat-top, with ripple less than 0.5dB and steep slope. The devices can be used as tunable narrow-band multichannel transmission filters. This analysis is useful for the designs and applications of the devices.     

Focused ion beam Moiré method

A focused ion beam (FIB) Moiré method is proposed to measure the in-plane deformation of object in a micrometer scale. The FIB Moiré is generated by the interference between a prepared specimen grating and FIB raster scan lines. The principle of the FIB Moiré is described. The sensitivity and accuracy of deformation measurement are discussed in detail. Several specimen gratings with 0.14 and 0.20 μm spacing are used to generate FIB Moiré patterns. The FIB Moiré method is successfully used to measure the residual deformation in a micro-electro-mechanical system structure after removing the SiO₂ sacrificial layer with a 5000 lines/mm grating. The results demonstrate the feasibility of this method.     

Analytical and numerical study on grating depth effects in grating coupled waveguide sensors

The in-coupling process for grating-coupled planar optical waveguide sensors is investigated in the case of TE waves. A simple analytical model based on the Rayleigh–Fourier–Kiselev method is applied to take into account the depth of the grating coupler, which is usually neglected in the modeling. Analytical expressions are derived both for the position and width of the in-coupling peaks to illustrate the effects of grating depth on the guided mode resonances in grating coupled waveguide sensors. Numerical computations verify the model for shallow gratings both in terms of peak shape and position and provide the limitations for the analytical formulas.     

一种设计环形汇聚光栅反射镜的新方法

提出了一种利用数学变换来快速设计环形汇聚光栅反射镜的方法.通过分析具体的物理场景,抽象出已有条形汇聚光栅的"线"汇聚特性与所要设计的"点"汇聚特性在数学上对应的变换关系,然后用该数学变换对条形汇聚光栅进行外形上的变换,外形变换后的条形光栅即为可以实现"点"汇聚的环形光栅.用有限元算法对设计的环形汇聚光栅进行仿真,仿真证明采用该方法设计的环形光栅可以很好地实现高反、高汇聚.采用这一方法,设计了直径为29.788μm的环形光栅反射镜,当垂直入射的径向偏振光从设计的环形光栅表面反射回来后将发生汇聚,汇聚焦点位于环形光栅表面10μm处.经计算,反射镜的数值孔径为0.8302,反射率为0.9163,在焦点所在的汇聚面上,汇聚光栅电场分布的半高宽为1.5548μm.     

Broad band beam splitter based on the double-groove fused silica grating

In this paper, we propose a broad band 1 × 3 beam splitter operating in the telecommunication wavelength band under normal incidence, this device consisting of a double-groove fused silica grating layer is designed with using the inverse mathematical method and rigorous vector diffraction theory. It is shown from our calculations that the device presents excellent beam splitter ability for TE polarization light with the average diffraction efficiencies is more than 95% over ∼100 nm wavelength range, moreover, the uniformity of our beam splitter is better than 2% in the whole wavelength band. Furthermore, the physical understanding of the diffraction behaviors taking place inside the beam splitter gratings can be explained by the modal method.     

Light trapping in amorphous silicon solar cells with periodic grating structures

We report on the design of amorphous silicon solar cells with the periodic grating structures. It is a combination of an anti-reflection structure and the metallic reflection grating. Optical coupling and light trapping in thin-film solar cells are studied numerically using the Rigorous Coupled Wave Analysis enhanced by the Modal Transmission Line theory. The impact of the structure parameters of the gratings is investigated. The results revealed that within the incident angles of ? 40^° to + 40^° the reflectivity of the cell with a period of 0.5 μm, a filling factor of 0.1 and a groove depth of 0.4 μm is 4%–22.7% in the wavelength range of 0.3–0.6 μm and 1%–20.8% in the wavelength range of 0.6–0.84 μm, the absorption enhancement of the a-Si layer is 0.4%–10.8% and 20%–385%, respectively.     

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.     

Interferometry based on Lau effect with a grating registered in a photorefractive crystal

The use of a grating in the form of modulation of birefringence in a BSO crystal in a novel Lau interferometer set-up is proposed. The system behaves as a conventional Lau interferometer implemented by using two amplitude gratings. Moreover, it permits control of the visibility and provides a contrast inversion of the interferometric fringes by rotating a polarizer. Two alternative experimental arrangements with similar performance are presented. A theoretical approach is outlined and the predicted behavior is confirmed by results obtained under different experimental conditions.     

方位调制相位光栅Lau效应理论及实验研究

本文根据部分相干理论研究了方波调制相应光栅的Lau效应,在实验中观察到了理论所预言的高对比度Lau条纹.     

Random period arc-induced long-period fiber gratings

We report the fabrication of arc-induced long-period fiber gratings with strong random variations in the period. Long-period fiber gratings with standard deviations in the period from 8.50 to 36.98 μm were fabricated. The spectral position of the resonant bands is determined by the average period value, being similar to that observed in a long-period fiber grating with a fixed period equal to the average period of the random grating. Moreover the notch bands keep the shape characteristics like wideband and depth compared with a long-period grating with a constant period. In addition, their sensitivity to external parameters such as ambient refractive index is not too different with that of fixed period long-period gratings.     

Diffraction gratings of isotropic negative-phase velocity materials

Diffraction of electromagnetic plane waves by the gratings made by periodically corrugating the exposed planar boundaries of homogeneous, isotropic, linear dielectric-magnetic half-spaces is examined. The phase velocity vector in the diffracting material can be either co-parallel or anti-parallel to the time-averaged Poynting vector, thereby allowing for the material to be classified as of either the positive- or the negative-phase velocity (PPV or NPV) type. Three methods used for analyzing dielectric gratings — the Rayleigh-hypothesis method, a perturbative approach, and the C formalism — are extended here to encompass NPV gratings by a careful consideration of field representation inside the refracting half-space. Corrugations of both symmetric as well as asymmetric shapes are studied, as also the diversity of grating response to the linear polarization states of the incident plane wave. The replacement of PPV grating by its NPV analog affects only nonspecular diffraction efficiencies when the corrugations are shallow, and the effect on specular diffraction efficiencies intensifies as the corrugations deepen. Whether the type of the refracting material is NPV or PPV is shown to affect surface wave propagation as well as resonant excitation of surface waves.