Novel optical sensor for simultaneous measurement of liquid concentration and temperature

The paper describes a new optical sensor for simultaneous liquid concentration and temperature measurement. Temperature-dependent semiconductor absorption at the band edge is used as the principle of the temperature measurement, and the sensor exploits beam deviation caused by refraction due to the liquid concentration at the receiving end face of the optical device. The light intensity peak value and its deviation are detected by a charge-coupled device (CCD), and then the measured optical signal is reflected by a reflecting pyramid prism. The sensor probe is composed of an intrinsically pure GaAs single crystal, a reflecting pyramid prism, a partitioned water cell. Theoretical analysis and preliminary experimental results verify the feasibility of the proposed system.     

Performance evaluation of fiber Bragg grating sensors by digital holographic technique, strain gauge measurement

In view of the growing interest for non-destructive tests of materials, geodynamical monitoring and in general remote sensing, there is a great effort to bring practical optical sensors from research labs to industrial and environmental applications. In this paper, we employ digital holographic technique as an efficient tool for evaluating the strain measurement capability of fiber Bragg gratings (FBG). A cantilever beam has been employed as a test structure under loading test. The strain measurements results obtained by fiber-based sensors have been compared to those obtained by using full-field digital holographic technique and point-wise strain gauge sensors glued on the same cantilever beam. A simple theoretical model is also presented to interpret and compare the experimental results coming from different techniques.     

Strain and tilt measurement using multi-point diffraction strain sensor

A multi-point diffraction strain sensor (MDSS) was developed earlier by us for strain measurement with variable sensitivity and measurement range using a microlens array. The technique is now extended to measure both tilt and non-uniform strain with a sensitivity of 0.41 mε/pixel and 4.7 mrad/pixel. The validation was made through comparison of the strain measured using MDSS with that by a micro-Moiré interferometer incorporated with Gabor filtering method, while the tilt is compared with derivatives of the surface profile measured by a confocal microscope.     

Calibration of optical systems for the measurement of microcomponents

Optical coherent techniques are well suited for the measurement of microcomponents but give accurate results only when calibrated systems are used. This paper presents a strategy for the calibration that involves at first the development of micromechanical reference components deforming in a reproducible retraceable and precise way when submitted to standard loadings. These reference materials are then used for the calibration of the measuring systems. Guidelines for building and measuring reference materials together with methods for evaluating the measurement uncertainty are presented.     

A two-step optical flow method for strain estimation in elastography: Simulation and phantom study

Optical flow (OF) method has been used in ultrasound elastography to estimate the strain distribution in tissues. However the bias of strain estimation by OF has previously been shown to be close to 20%. The objective in this paper is to improve the performance of OF-based strain estimation, a two-step OF method with a local warping technique is proposed in this paper. The local warping technique effectively decreases the decorrelation of the signals, and hence improves the performance of strain estimation. Simulations on both homogeneous and heterogeneous models with different strains are performed. Experiments on a heterogeneous tissue-mimicking phantom are also carried out. Simulation results of the homogeneous model show that the two-step OF method reduces the bias of strain estimation from 23.77% to 1.65%, and reduces the standard deviation of strain estimation from 2.9 × 10⁻³ to 0.47 × 10⁻³. Simulation results of the heterogeneous model shows that the signals-to-noise ratio (SNR_e) of strain estimation is improved by 2.1 and 5.3 dB in the inclusion and background, respectively, and the contrast-to-noise ratio (CNR_e) is improved by 6.8 dB. Finally, results of phantom experiments show that, by using the proposed method, the SNR_e is increased by 4.0 dB and 8.9 dB in the inclusion and background, respectively, while the CNR_e is increased by 13.1 dB. The proposed two-step OF method is thus demonstrated capable of improving the performance of strain estimation in OF-based elastography.     

A full-field method for measuring residual stresses in optical fiber

A method for measuring the axial residual stress profile in axisymmetric optical fibers is presented. The procedure is based on integrated photoelasticity and a fringe shifting technique is used to measure the optical retardation. The radial distribution of the axial residual stress is reconstructed using the inverse Abel transform. The paper describes the operating principle, the experimental setup and the results obtained on a multimode fiber are also reported. The influence of the measurement errors is finally discussed.     

Effects of strain on the optical and magnetic properties of Ce-doped ZnO

The magnetic and optical properties of Ce-doped ZnO systems have been widely demonstrated, but the effects of different strains of Ce-doped ZnO systems remain unclear. To solve these problems, this study identified the effects of biaxial strain on the electronic structure, absorption spectrum, and magnetic properties of Ce-doped ZnO systems by using a generalized gradient approximation + U (GGA + U) method with plane wave pseudopotential. Under unstrained conditions, the formation energy decreased, the system became stable, and the doping process became easy with the increase in the distances between two Ce atoms. The band gap of the systems with different strains became narrower than that of undoped ZnO without strain, and the absorption spectra showed a red shift. The band gap narrowed, and the red shift became weak with the increase of compressive strain. By contrast, the band gap widened, and the red shift became significant with the increase of tensile strain. The red shift was significant when the tensile strain was 3%. The systems with −1%, 0%, and 1% strains were ferromagnetic. For the first time, the magnetic moment of the system with −1% strain was found to be the largest, and the system showed the greatest beneficial value for diluted magnetic semiconductors. The systems with −3%, −2%, 2%, and 3% strains were non-magnetic, and they had no value for diluted magnetic semiconductors. The ferromagnetism of the system with −1% strain was mainly caused by the hybrid coupling of Ce-4f, Ce-5d, and O-2p orbits. This finding was consistent with Zener's Ruderman–Kittel–Kasuya–Yosida theory. The results can serve as a reference for the design and preparation of new diluted magnetic semiconductors and optical functional materials.     

Enhancement of measurement accuracy in fiber Bragg grating sensors by using digital signal processing

Wavelength detection accuracy in fiber Bragg grating (FBG) sensors can be increased by use of digital signal processing after photo-detection. Finite impulse response and infinite impulse response algorithms were implemented and used to improve the wavelength detection accuracy of peak detection and time-zero-crossing techniques. The wavelength detection resolution can be improved by an order of magnitude compared with that obtained directly from an optical spectrum analyzer/tunable laser.     

Hetero-core fiber Bragg grating for simultaneous measurement of strain and bending based on cladding mode ratiometric detection

A simple but practical method using multimode-single mode fiber Bragg grating (FBG) for cladding mode based simultaneous measurement of strain and bending is proposed and investigated experimentally. Experimental results show that the intensity ratio between cladding mode and FBG resonance of hetero-core FBG show monotonous changes following with the increase of deflection, by which strain and bending discrimination can be achieved by measuring the wavelength shift of FBG resonance and ratiometrically detecting the intensity changes between cladding modes and FBG resonance.     

Review of surface profile measurement techniques based on optical interferometry

With the fast development of modern science and technology, two or three-dimensional surface profile measurement techniques with high resolution and large dynamic range are urgently required. Among them, the techniques based on optical interferometry have been widely used for their good properties of non-contact, high resolution, large dynamic measurement range and well-defined traceability route to the definition of meter. A review focused on surface profile measurement techniques of optical interferometry is introduced in this paper with a detailed classification sorted by operating principles. Examples in each category are discussed and analyzed for better understanding.     

A camera calibration method for large field optical measurement

A camera calibration method is presented for large field optical measurement, where the camera is close to the ground and the control points can only be located close to the ground, too. In such conditions, the camera's optical center and the control points are approximately coplanar. Only a single image of these control points captured by the camera in measurement state is used in the method. Neither to distribute the control points in space rationally nor to calibrate the camera's intrinsic parameters in laboratory in advance is needed. By the presented method, the camera's principal point position, focal length, radial and transverse tangency lens distortion coefficients, and the camera's position and attitude parameters can be estimated precisely. Then the calibration results can be used for precise large field optical measurement in the conditions that the camera's longitudinal tangency lens distortion can be neglected or the objects’ movement field is close to the ground, which is usually factual in practical applications. The presented camera calibration method has been successfully used in applications, such as automatic landing of UAV (Unmanned Aerial Vehicle) based on optical measurement guidance, to calibrate the cameras precisely.     

Investigation of a coated optical fiber strain sensor embedded in a linear strain matrix material

A theoretical model has been developed to study the mechanical behaviors of the interface between an embedded optical fiber with coating material and a linear strain matrix. The results show that the longitudinal stress and strain in the fiber optic sensor are different from that distributed in the host material and depend on the strain distribution and embedded length of the optical fiber as well as the material properties of the fiber coating. The distribution of interfacial shear strain between the coating and the glass fiber and the distribution of strain/stress of the glass fiber are given.     

Novel method for high accuracy figure measurement of optical flat

Phase Measuring Deflectometry (PMD) is a non-contact, high dynamic-range and full-field metrology which becomes a serious competitor to interferometry. However, the accuracy of deflectometry metrology is strongly influenced by the level of the calibrations, including test geometry, imaging pin-hole camera and digital display. In this paper, we propose a novel method that can measure optical flat surface figure to a high accuracy. We first calibrate the camera using a checker pattern shown on a LCD display at six different orientations, and the last orientation is aligned at the same position as the test optical flat. By using this method, lens distortions and the mapping relationship between the CCD pixels and the subaperture coordinates on the test optical flat can be determined at the same time. To further reduce the influence of the calibration errors on measurements, a reference optical flat with a high quality surface is measured, and then the system errors in our PMD setup can be eliminated by subtracting the figure of the reference flat from the figure of the test flat. Although any expensive coordinates measuring machine, such as laser tracker and coordinates measuring machine are not applied in our measurement, our experimental results of optical flat figure from low to high order aberrations still show a good agreement with that from the Fizeau interferometer.     

光学平面绝对检验方法的研究

应用两种方法对三个高精度平面进行了测试。第一种方法是Fritz的三面互检法,它利用Zernike多项式的特性拟合三个面四次组合测量得到的干涉图,然后求出三个面的Zernike多项式系数,从而得到三个面的面形偏差。第二种方法是奇偶函数法,根据函数的奇偶性,把平面的面形函数分解为四类:偶奇、奇偶、偶偶和奇奇函数,分别求出各分量,从而得到三个面的三维面形偏差。对两种方法都编制了理论模拟和实测程序,并进行了实验,实现了无参考面的高精度平面面形测试。     

Superposition of fiber Bragg and LPG gratings for embedded strain measurement

When a fiber Bragg grating strain sensor is embedded inside a structure, the interaction of the sensor with the host material can lead to spurious results if the radial strain is neglected. In this article, we use numerical simulations to show that the axial and radial strains can be simultaneously measured with a single fiber in which a Bragg grating and a long-period grating are superimposed. Moreover, we present an optimal architecture of the sensor.     

Optical measurement of the dynamic strain field of a fan blade using a 3D scanning vibrometer

Understanding the origin of the stress and strain distribution is crucial to increase the durability of components under dynamic loading. Numerical simulations based on finite element (FE) models help with this understanding but must be validated by real measured data. Updating the FE model using the measured data is often the next step in the design process. In this paper the recently developed 3D-scanning laser doppler vibrometer (3D-SLDV) is used to measure the 3D-displacement of a fan blade, which is then used to calculate the dynamic strain distributions. The measurement principle and experimental setup are discussed thoroughly. The experimental results are validated by using a FE model on the one hand and strain gage measurements on the other. It is shown that this technique is capable of measuring normal strain far below 1 microstrain. This technique has the potential to fill in the gap of accurately measuring small (full-field) normal and shear strains at both low and high frequencies, where other optical techniques (and strain gages) would certainly fail.     

形貌与微变形全场光学同时测量方法

在实际工程应用中,对材料形貌和结构变形等参量的检测是必不可少的,而且往往需要进行多参量同时测量。针对该背景,采用数字散斑干涉与数字条纹投影相结合的测量方法, 设计了一种集成光路,通过在数字散斑干涉实验光路中引入一个投影设备,实现物体表面形貌和微变形的同时测量。所提出的方法具有全场非接触测量的优点,且测量光路简单、操作方便、效率高、可靠性强。该方法的形貌测量分辨率优于10 μm,形变测量分辨率优于30 nm。     

Simultaneous strain and temperature measurement of advanced 3-D braided composite materials using an improved EFPI/FBG system

Simultaneous strain and temperature measurement for advanced 3-D braided composite materials using fibre-optic sensor technology is demonstrated, for the first time. These advanced 3-D braided composites can virtually eliminate the most serious problem of delamination for conventional composites. A tandem in-fibre Bragg-grating (FBG)/extrinsic Fabry–Perot interferometric sensor (EFPI) system with improved accuracy has been used to facilitate simultaneous temperature and strain measurement in this work. The non-symmetric distortion of the optical spectrum of the FBG, due to the combination of the FBG and the EFPI, is observed for the first time. Experimental and theoretical studies indicate that this type of distortion can affect the measurement accuracy seriously and it is mainly caused by the modulation of the periodic output of the EFPI. A simple method has been demonstrated to improve the accuracy for detection of the wavelength-shift of the FBG induced by temperature change. A strain accuracy of ±20μ and a temperature accuracy of ±1°C have been achieved, which can meet the requirements for practical applications of 3-D braided composites.     

Repeatability and reproducibility study of thin film optical measurement system

Investigation on surface roughness of thin films is an important issue in manufacturing engineering because the performance of a coated film is significantly affected by the surface roughness of thin films. A fast and flexible optical measurement system to measure surface roughness of hard coatings deposited by cathodic arc evaporation is developed in this work. The objective of this work is to examine the repeatability and reproducibility (R&R) of an optical measurement system. Percentage of equipment variation, appraiser variation and R&R is 7.25%, 1.42% and 7.39%. Thin film optical measurement system developed is acceptable according to the measurement systems analysis and the R&R technique.     

基于应变片的自由活塞斯特林制冷机位移测试研究

压缩活塞与排出器的位移大小及相位差对自由活塞斯特林制冷机的制冷性能具有极其重要的影响。文中通过对目前各种位移测试方法优缺点分析,结合自由活塞式斯特林制冷机自身特点,提出了采用应变片响应涡旋柔性弹簧形变来测量活塞和排出器位移的新方法。通过分析、设计和标定,建立了一套基于应变片的自由活塞斯特林制冷机位移测量系统。实验测试结果表明,该位移测量系统是可靠的,具有体积小、投资省等优点,且不需要特别的安装空间。