Multipoint diffraction strain sensor with variable sensitivity

In our earlier work, a multipoint diffraction strain sensor using a microlens array was developed for measurement of whole-field strains. The method is extended to a system with variable sensitivity and measurement range. In the present system, two collimated laser beams, 3 mm in diameter, symmetrically strike the grating attached to the specimen surface at prescribed angles. The diffracted wavefronts, magnified by a microscope objective, are sampled by a lenslet array with each microlens acting as an individual strain sensor. In-plane strain components over the full field can be measured by what is to our knowledge a new sensor with variable sensitivity by changing the distance from the microscope objective to the microlens array. Both a theoretical explanation and experimental verification are provided.     

Simple three-dimensional laser angle sensor for three-dimensional small-angle measurement

A simple three-dimensional (3D) laser angle sensor for 3D measurement of small angles based on the diffraction theorem and on ray optics analysis is presented. The possibility of using position-sensitive detectors and a reflective diffraction grating to develop a 3D angle sensor was investigated and a prototype 3D laser angle sensor was designed and built. The system is composed of a laser diode, two position-sensitive detectors, and a reflective diffraction grating. The diffraction grating, mounted upon the rotational center of a 3D rotational stage, divides an incident laser beam into several diffracted rays, and two position-sensitive detectors are set up for detecting the positions of +/-1st-order diffracted rays. According to the optical path relationship between the three angular motions and the output coordinates of the two position-sensitive detectors, the 3D angles can be obtained through kinematic analysis. The experimental results show the feasibility of the proposed 3D laser angular sensor. Use of this system as an instrument for high-resolution measurement of small-angle rotation is proposed.     

The laser strain gauge: micro machining of diffraction gratings using an Excimer laser

The laser strain gauge is a recently developed instrument for the single point measurement of surface strain to a resolution of 1 μm. It measures the deformation of a diffraction grating adhered to or etched into the surface of various substrates. In comparison with a conventional electrical resistance strain gauge the new gauge offers greater thermal stability and avoids the inconvenience of electrical connections. When gratings are etched directly into the surface of a component absolute strain measurements over its lifetime are possible.
High frequency (1000 lines/mm) diffraction gratings can be etched into the surface of various substrates including carbon steels, polymers and ceramics using an Excimer laser. This paper describes the optical beam delivery system necessary for this application and demonstrates the feasibility of this exciting new technique.     

胶粘剂对表贴式FBG传感器应变传递系数的影响

利用光纤光栅（FBG）传感器对基体表面应变进行测量,通常利用胶粘剂将光纤光栅传感器粘贴在基体表面,使其与基体协调变形。但胶粘剂在不同表面的粘接性能不同,对应变测量所造成的影响也就存在差异。针对这一问题,本文通过实验,简化应变传递模型,对比FBG传感器粘贴在等强度梁以及标准树脂试件的应变测量结果。结果表明:胶粘剂在基体表面的剪切强度越大,表面粘贴式FBG传感器的应变传递系数也越大。      

Techniques for Neutron Stress Determination with High Spatial Resolution

A number of sub-surface strain measurement problems require spatial resolutions of the order of 10^?1 mm to be meaningful. The use of neutron diffraction can be both time effective and otherwise advantageous compared to synchrotron diffraction if measurement directions and beam shapes are chosen such that scattered intensity is maximized. The first point refers to those directions and specimen orientations for which the gage volume orientation can be kept constant. They can be chosen such that preferred orientation boosts the diffracted intensity. The second point refers to the shaping of the incident beam for maximizing the illuminated volume in the material even for curved specimens. Guidelines and limitations of such high spatial resolution measurements are discussed.     

测量复合材料残余应力场的新方法

本文建立了一维表面光栅衍射光强分布与表面应变之间的理论关系,再利用弹性理论,即可求出材料中的残余应力,从而开拓了残余应力的测试方法;在测试技术方面本方法是在试样表面制作一维表面光栅,在激光束的照射下,形成一列清晰的衍射光斑,当试样背面剥层释放残余应力而发生形变时,引起表面光栅栅距d的变化,反映为放大了一列衍射光斑间距的变化,用电荷耦合器件将光信息快速地转换为电信号,以及A/D转换和微机控制的光信息处理系统,可高精度(m)、快速(s)地测试表面应变和残余应力的定量结果,这是一种测试分析表面应变和残余应力及其之间关系的新方法.      

Spectrograph based on a single diffractive element for real-time measurement of circular dichroism

In this study, a novel and simple diffractive spectrographic method for real-time measurements of circular dichroism (CD) is considered from a theoretical and experimental approach. A demonstrator prototype of the CD spectrograph has been developed and its performance has been compared with a commercial phase-modulation CD spectrometer. The main element of the device is a polarization holographic grating, recorded in a thin photosensitive organic film, by two interfering opposite circularly polarized beams. A peculiarity of this grating is that the amplitude of the +1 (-1) order of diffraction is proportional to the right (left) circular polarization component of the incoming beam. Here we demonstrate that the CD spectrum of a specimen can be easily evaluated from the intensities of the diffracted beams. A white light beam passing through the specimen is diffracted from the grating and the intensities of the +/-1 orders of diffraction are measured. Due to the spectral selectivity of the grating, the CD at each wavelength can be evaluated at the same time using two linear array detectors.     

高密度栅测应变

将高密度光栅应用于力学参量的测量中。通过测得试件变形前后正交光栅衍射点阵的偏移移量，可逐点得到试件因载荷作用而引起的面内应变、离面位移和刚体传动分量。     

Compact sensor for measuring two-dimensional tilt using a two-dimensional transmission grating and the Talbot effect

This paper proposes a tilt sensor that measures the small two-dimensional tilt of a plane reflective object using the Talbot effect. It is an extension of a previously proposed one-dimensional tilt sensor. The light beam reflected from the object impinges on a hexagonal grating, and the intensity of the diffracted wave is detected on an image sensor located at a Talbot distance from the grating. The diffraction intensity displaces due to the tilt of the object. The displacement is calculated by the Fourier transform method to obtain the two-dimensional tilt. This sensor is very simple and compact. The principle of the sensor is explained for a grating with a general pattern. An experiment using a hexagonal grating shows its validity. Discussions are given for making it more practical.     

Optical temperature sensor and thermal expansion measurement using a femtosecond micromachined grating in 6H-SiC

An optical temperature sensor was created using a femtosecond micromachined diffraction grating inside transparent bulk 6H-SiC, and to the best of our knowledge, this is a novel technique of measuring temperature. Other methods of measuring temperature using fiber Bragg gratings have been devised by other groups such as Zhang and Kahrizi [in MEMS, NANO, and Smart Systems (IEEE, 2005)]. This temperature sensor was, to the best of our knowledge, also used for a novel method of measuring the linear and nonlinear coefficients of the thermal expansion of transparent and nontransparent materials by means of the grating first-order diffracted beam. Furthermore the coefficient of thermal expansion of 6H-SiC was measured using this new technique. A He-Ne laser beam was used with the SiC grating to produce a first-order diffracted beam where the change in deflection height was measured as a function of temperature. The grating was micromachined with a 20 microm spacing and has dimensions of approximately 500 microm x 500 microm (l x w) and is roughly 0.5 microm deep into the 6H-SiC bulk. A minimum temperature of 26.7 degrees C and a maximum temperature of 399 degrees C were measured, which gives a DeltaT of 372.3 degrees C. The sensitivity of the technique is DeltaT=5 degrees C. A maximum deflection angle of 1.81 degrees was measured in the first-order diffracted beam. The trend of the deflection with increasing temperature is a nonlinear polynomial of the second-order. This optical SiC thermal sensor has many high-temperature electronic applications such as aircraft turbine and gas tank monitoring for commercial and military applications.     

Slope measurement of bent plates using double grating shearing interferometry

A grating-based shearing interferometeric setup for slope measurement of bent plates has been proposed. The specimen under test is illuminated by a collimated beam from the laser. Light reflected from the specimen passes through two identical holographic gratings placed in tandem. The grating frequency has been so chosen that the diffracted orders from each grating are separated out distinctly. Two first-order beams diffracted from each of the gratings superpose in space. In the resulting interferogram, the fringes due to slope information of the object are visualized. Mathematical formulation for experimental determination of slope values has been undertaken. Validation of the experimental results with theoretical predictions in case of cantilever beam provides good correlation. The main advantage of the technique has been the realization of very compact geometry without the need for spatial filtering arrangement commonly associated with the grating-based techniques used to date.     

大量程光栅位移测量累积误差的修正

提出了一种自动测量光栅栅距修正累积误差的方法。栅距测量是基于高阶累积量估计光栅传感器输出的两路莫尔条纹信号的时间延迟而得到的,该方法能够实现每个栅距的测量,通过对每个光栅栅距的误差进行修正来减少累积误差,为大量程高精度测量奠定了基础。实验采用长为500 mm的50线/mm的光栅传感器,该传感器包含栅线25 000条,实现栅距测量分辨力为3 nm,达到了纳米级测量。该方法抗干扰能力强,适合在生产现场应用。     

Planar reflection grating lens for compact spectroscopic imaging system

A compact spectroscopic imaging device consisting of a planar reflection grating lens, a probe fiber array, and a two-dimensional image sensor was proposed and discussed. Reflected or luminescent lights from a subject are coupled to the probe fibers, guided to fiber output ends, radiated into the air, diffracted by the grating lens with wavelength-dependent angle, and focused onto lines on the image sensor. Two-dimensional intensity distribution on the image sensor can give one-dimensional spectrum distribution along a specified direction. A grating lens was designed with a fiber array and a CCD image sensor for 100-nm wavelength range and 10-mm fiber array width. A spectral resolution of 5 nm and a spatial resolution of 0.25 mm were experimentally confirmed.     

Effect of wavelength-dependent attenuation on strain measurement using pulsed neutron diffraction

The measurement of stresses in engineering components and structures by neutron diffraction has traditionally been limited to depths of a few centimetres. However, recent developments in instrumentation open up the possibility of deeper measurements. It has been suggested that a systematic error may occur when measuring stresses deep in materials due to wavelength-dependent attenuation of the incident and/or diffracted beam. A series of experiments to evaluate the magnitude of this effect on ENGIN, a pulsed neutron strain scanner will be presented in this paper. It was found that the pseudo-strains induced by the attenuation of the diffracted beam were negligible, whereas there was a small but still noticeable effect produced by the attenuation in the incident beam. The possible causes of this shift and the consequences for deep measurements are discussed.     

From the reflected spectrum to the properties of a fiber Bragg grating: a genetic algorithm approach with application to distributed strain sensing

We describe a genetic algorithm approach to solve an inverse problem in optics, which determines the characteristics of a fiber Bragg grating from its reflected spectrum. The validity of the proposed method is demonstrated by use of a Bragg sensor for the measurement of nonlinear strain acting on a uniaxial aluminum test specimen.     

Comparison between point and long‐gage FBG‐based strain sensors during a railway bridge load test

Strain is a key parameter in laboratory and bridge load testing. The selection of a strain sensor depends on several factors, including the aim of the test and the specimen material. The application of the right sensor is vital to obtain accurate readings, especially in the case of heterogeneous materials such as concrete. This paper focuses on long‐gage and point fiber Bragg grating‐based strain sensors and their possible applications on concrete elements. First, strain sensors are described, after which long‐gage and point fiber Bragg grating strain sensors are compared in a concrete specimen test, a concrete column test and static and dynamic load tests on a concrete railway bridge. Results show that although it is advisable to use long‐gage sensors when monitoring heterogeneous materials, there are some particular cases were both sensors type can provide accurate strain measurements.     

Distributed measurement of static strain in an optical fiber with multiple bragg gratings at nominally equal wavelengths

The Fourier transform relationship between the reflected light froma Bragg grating and the complex spatial modulation of the Bragg grating is used to produce a distributed strain sensing system. A tunable external cavity diode laser along with a reference reflector in anoptical fiber are used to produce a measurement of the phase and amplitude of the reflected light from the modulated Bragg grating as a function of wavelength. The system is demonstrated with 22 Bragg gratings in a single fiber on a cantilever beam and compared with foil strain gauge readings.     

Theory of Electron Beam Moiré

When a specimen surface carrying a high-frequency line grating is examined under a scanning electron microscope (SEM), moiré fringes are observed at several different magnifications. The fringes are characterized by their spatial frequency, orientation, and contrast. These features of the moiré pattern depend on the spatial frequency mismatch between the specimen grating and the raster scan lines, the diameter of the electron beam, and the detailed topography of the lines on the specimen.A mathematical model of e-beam moiré is developed that expresses the spatial dependence of the SEM image brightness as a product of the local intensity of the scanning beam and the local scattering function from the specimen grating. Equations are derived that give the spatial frequency of the moiré fringes as functions of the microscope settings and the spatial frequency of the specimen grating. The model also describes the contrast of several different types of moiré fringes that are observed at different magnifications. We analyze the formation of these different fringe patterns, and divide them into different categories including natural fringes, fringes of multiplication, fringes of division, and fringes of rotation.     

Determination of longitudinal and transverse shifts of three-dimensional Gaussian beams reflected at a grating near an anomaly

Spatial displacements of three-dimensional Gaussian beams diffracted at a reflection grating are studied theoretically and numerically. The complex diffraction coefficients (amplitudes and phases) of the grating diffracted plane waves calculated by a rigorous method for conical diffraction are the basis for this investigation. The classical analytical formula for the longitudinal shift depending on the gradient of the reflection phase is generalized to the simultaneous analytical treatment of the longitudinal Goos—Hänchen like shift as well as a transverse shift. A second method uses the full integration on the whole spectrum of plane waves of the diffracted beam.     

Diffraction properties of volume holographic gratings for an ultrashort pulsed beam with different polarization states readout

The diffraction properties of volume holographic gratings are studied when the gratings are illuminated by an ultrashort pulsed beam with different polarization states. The developed coupled wave theory of Kogelnik is used. Considering the dispersion effect of the grating media, solutions for the diffracted and transmitted intensities, diffraction efficiencies and the bandwidths of the gratings are given in transmission volume holographic gratings and reflection volume holographic gratings. The bandwidths of the gratings are reduced by the dispersion effect of the grating media. They also have different influences on the diffraction of an ultrashort pulsed beam with different polarization states. For different values of the ratio of the spectral bandwidth of the input pulse to that of the grating, the changes of the spectral and temporal distributions of the diffracted intensities, as well as the diffraction efficiencies of the gratings are shown.