三角形大带宽反射光谱光纤光栅的设计和制备

反射谱具有三角形形状的光纤光栅在光纤传感等领域中具有重要的应用前景。利用遗传算法设计出了产生三角形光谱分布的光纤光栅的耦合系数沿光纤的分布。计算表明,三角形光谱光纤光栅是可以实现的,其反射带宽可以通过光栅的啁啾量进行调节,常周期的三角光栅反射带宽小,变周期的三角光栅反射带宽大,其带宽同其啁啾量大致相等。实验中,采用光束扫描法制备了反射底宽为0．77nm三角形光谱的光纤光栅。     

非线性二次曝光法制作三角形光纤Bragg光栅

提出了一种利用非线性二次曝光技术制作三角形光纤Bragg光栅的新方法.该方法只使用普通Bragg位相模板,采用两次曝光技术,通过控制光纤光栅的本地谐振波长和反射率,达到控制光纤光栅谱形的目的.第一次直接对光纤曝光,用来控制光纤光栅谐振波长;第二次通过位相模板曝光,用来控制光纤光栅的反射率.考虑到光敏光纤感光特性曲线的多项e负指数变化规律,按照"分步写入光纤光栅设计软件"进行曝光分布设计,利用"分步曝光光纤光栅写入系统"进行曝光控制,使用信息产业部第46研究所生产的DCS-01型光敏光纤,得到了线性范围1.24 nm、反射率为0～64%的三角形光纤Bragg光栅.三角形光纤Bragg光栅作为光纤光栅传感解调器件,在光纤传感领域具有重要应用价值.     

Electro-optic properties of switchable gratings made of polymer and nematic liquid-crystal slices

We report the diffraction properties at wavelengths of 632.8 and 1550 nm for volume transmission gratings made of a sequence of continuously aligned nematic liquid-crystal layers separated by isotropic polymer slices. The gratings are generated by holographically curing a solution of liquid crystal diluted in an isotropic prepolymer by means of a laser beam at a wavelength of 352 nm with a total intensity of approximately 10 mW/cm2. A diffraction efficiency of 98% was measured, and an electric field as low as 5 V/microm switches off the phase grating. Measured angular spectra are fitted by use of the modified coupled-mode theory including the effects of grating birefringence.     

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.     

Optimization of top polymer gratings to improve GaN LEDs light transmission

We present a grating model of two-dimensional (2D) rigorous coupled wave analysis (RCWA) to study top diffraction gratings on light-emitting diodes(LEDs). We compare the integrated-transmission of the non-grating,rectangular-grating,and triangular-grating cases for the same grating period of 6μm,and show that the triangular grating has the best performance. For the triangular grating with 6-μmperiod, the LED achieves the highest light transmission at 6-μ gratingbottom width and 2.9-μm grating depth. Compared with the non-grating case, the optimized light transmission improvement is about 74.6%.The simulation agrees with the experimental data of the thin ploymer grating encapsulated flip-chip(FC) GaN-based LEDs for the light extraction improvement.     

FDTD analysis of infrared radiative properties of microscale structure aluminum surfaces

Metallic triangular grating structures have a wide range of applications. This study focuses on the radiative properties of triangular grating structures of aluminum in the mid-infrared wavelength to determine if we can tailor the infrared radiative properties by designing special geometrical details into triangular grating surfaces. Fabrication of triangular gratings within the range of micron scale is very difficult. Therefore, the influences of slight geometry modification, generated by fabrication errors, on radiative properties of triangular gratings are mainly investigated. The electromagnetic wave scattering from such surfaces is predicted by solving Maxwell's equations using the finite-difference time-domain (FDTD) method. The results show that the spectral reflectance varies with the dimensions of triangular gratings, which is due to the excitation of surface wave by confirmation of the EM fields. It is found that the spectral reflectance can also be reduced by increasing the height of slight geometry modification. The effect of oblique incident wave state is also investigated. An optical vortex is found due to the coupling of surface waves, the oblique incident wave and the scattering waves. This study helps to gain a better understanding of the radiative properties of metallic triangular gratings with slight geometry modification and will have an impact on triangular grating processing.     

High performance diffraction gratings made by e-beam lithography

Gratings are essential components in different high performance optical set-ups such as spectrometers in space missions or ultrashort-pulse laser compression arrangements. Often such kinds of applications require gratings operating close to the technological accessible limits of today??s fabrication technology. Typical critical parameters are the diffraction efficiency and its polarization dependency, the wave-front error introduced by the grating, and the stray-light performance. Additionally, space applications have specific environmental requirements and laser application typically demand a high damage threshold. All these properties need to be controlled precisely on rather large grating areas. Grating sizes of 200?mm or even above are not unusual anymore. The paper provides a review on how such high performance gratings can be realized by electron-beam lithography and accompanying technologies. The approaches are demonstrated by different examples. The first example is the design and fabrication of the grating for the Radial-Velocity-Spectrometer of the GAIA-mission of the ESA. The second grating is a reflective pulse compression element with no wavelength resonances due to an optimized design. The last example shows a three level blazed grating in resonance domain with a diffraction efficiency of approximately 86?%.     

Strong phase-controlled fiber Bragg gratings for dispersion compensation

Dispersion-compensating fiber Bragg gratings with approximately 99.9% reflectivity that are made by continuous apodization and phase control are demonstrated. These strong dispersion-compensating gratings provide precision second-order, third-order, or even more complex dispersion compensation, as well as sufficient transmission isolation to be used at add-drop stages without additional filtering. A 99.84% grating with a constant approximately 700-ps/nm dispersion and a 99.94% grating with dispersion varying linearly from 1000 to -1000 ps/nm are demonstrated.     

Fabrication of high-efficiency diffraction gratings in glass

We investigated a microfabrication process for optical gratings with periods of micrometer order that use ultrafast laser pulses in semiconductor-doped glass. ZnS- or PbS-doped SiO2-A12O3-B2O3-CaO-ZnO-Na2O-K2O glass was prepared by a melting method. Glass transmission diffraction gratings with a high refractive-index difference were fabricated with femtosecond laser pulses. The first-order diffraction efficiencies of these gratings were approximately 80%, and the first-order diffraction angles of these gratings were 8 degrees at telecommunication wavelengths.     

Five-port beam splitter of a single-groove grating

A single-groove grating for five-port TE-polarization beam splitting under normal incidence at the wavelength of 1550 nm is presented. The transmitted diffraction efficiency of the gratings is over 94.5% with uniformity better than 2%. A physical view of diffraction inside the grating is presented by the simplified modal method(SMM).Initial parameters of the grating profiles are obtained by use of SMM and then optimized by employing rigorous coupled-wave analysis and the simulated annealing algorithm.     

High-efficiency multilayer-coated ion-beam-etched blazed grating in the extreme-ultraviolet wavelength region

We describe a simple method to fabricate blazed gratings used in the extreme ultraviolet wavelength region. The method uses an argon and oxygen mixed-gas ion beam to directly etch the grating substrate through a rectangular profile photoresist grating mask. With this method the etched grating groove profile can be well controlled. An Mo/Si multilayer-coated specimen with a blaze angle of 1.9 degrees was fabricated and measured. At an incident angle of 10 degrees and a wavelength of 13.62 nm, the diffraction efficiency of the negative second order reaches 36.2%.     

High-average-power femtosecond fiber chirped-pulse amplification system

Efficient generation of 76-W average power of 400-fs pulses at 75-MHz repetition rate by use of a diode-pumped ytterbium-doped double-clad fiber-based chirped-pulse amplification system is demonstrated. The key element in the system is a diffraction grating compressor consisting of highly efficient transmission gratings in fused silica, allowing recompression at this high power level.     

多层介质膜脉宽压缩光栅的严格矢量分析

用于展宽和压缩激光脉冲的多层膜脉宽压缩光栅是由多层介质高反膜和位于其顶层的浮雕光栅构成。以设计的高反射多层膜为基础,利用傅里叶模式理论分析了其衍射场分布,给出了TE波自准直角入射的使用条件下,多层介质膜脉宽压缩光栅衍射效率的表达式。以-1级衍射效率为评价函数,分别讨论了HfO2和SiO2为顶层材料时,多层膜脉宽压缩光栅-1级衍射效率高于0.95的光栅结构参量范围。结果表明,在该条件下,选择HfO2为顶层材料时,光栅结构参量有较大的取值范围。给出了优化的光栅结构参量,并分析了光栅制作误差及其使用条件的宽容度,对光栅制作工艺和使用具有一定的指导意义。     

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.     

Surface profile optimization of antireflection gratings for solar cells

The solar cell efficiency can be improved by antireflection gratings. In this paper, the antireflection gratings with different symmetrical surface profiles are investigated by numerical simulations based on the Rigorous Coupled-Wave Analysis. Simulated results show that the antireflection performance of sharp profile such as quadratic profile has a significant improvement compared with triangular and parabolic profiles, while the top cutoff in the grating tip will severely influence the antireflection performance. Meanwhile, proper length of flat region between grating features in the nonclose-packed triangular antireflection grating can have better antireflection performance than the close-packed counterpart for the same grating period and height. Such antireflection gratings with different surface profiles may offer attractive solutions to current commercial silicon solar cell, as well as organic and other semiconductor material based solar cells.     

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%.     

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.     

Normal-incidence guided-mode resonant grating filters: design and experimental demonstration

Guided-mode resonant grating filters have numerous applications. However, in weakly modulated gratings designed for use at normal incidence, the filtering resonance of these subwavelength-period devices splits for angles of incidence that are even slightly off normal incidence. Strongly modulated gratings are designed that essentially overcome this practical problem near normal incidence. In addition, these gratings can have, by design, either broad or narrow spectral characteristics. An experimental demonstration (1.5-2.0-mu m wavelength range) of such a normal-incidence guided-mode resonant silicon grating upon a sapphire substrate is presented. The measured reflection resonance had a FWHM of 67-100 nm for angles of incidence of 0-8 degrees and peak efficiency of ~80% .     

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.     

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.