Characterization of single and double fiber-coupled diffusing spheres

A fiber-optic sensor, consisting of an optical fiber with and without a 1.59-mm-diameter spherical ceramic tip, inserted into a 19-mm-diameter Spectralon sphere has been characterized. This sensor is evaluated as a large-area omnidirectional sensor. An optical transport measurement system that rotates the sphere about two axes has been designed. The system measured the UV transport efficiency at 351 nm of light impinging on the sphere with an f-6 cone angle. When a bare fiber was placed at the center of the target sphere, the detection sensitivity was biased in the forward direction. The peak of the sensitivity of the inverse integrating sphere shifted from front to back as the bare fiber was withdrawn from the sphere and the numerical aperture of the fiber viewed more of the scattering volume. The response function with respect to the angle of incidence of the dual sphere was much more uniform than that obtained with a bare fiber in the center of the sphere.     

A Compact Fiber-Optic Flow Velocity Sensor Based on a Twin-Core Fiber Michelson Interferometer

A novel fiber-optic flow velocity sensor based on a twin-core fiber Michelson interferometer has been proposed and demonstrated. The sensor only is a segment of twin-core fiber acting as cylinder cantilever beam. The force exerted on the cylinder by the slow flow speeds of order mm/s of the fluid with unknown velocity bends the fiber, which corresponds to the shift of the phase of the twin-core in-fiber integrated Michelson interferometer.     

Pointwise fiber Bragg grating displacement sensor system for dynamic measurements

A method for setting up a fiber Bragg grating (FBG) sensor which can measure the pointwise, out-of-plane or in-plane dynamic displacement is proposed. The proposed FBG sensor is reusable. A multiplexing demodulation system based on a single long-period fiber grating is used in this study. The experimental results of the steady-state motion for a multilayer piezoelectric actuator and the dynamic response of a cantilever beam subjected to impact loadings are presented. These results indicate that the proposed displacement sensor has the ability to measure the out-of-plane dynamic displacement with high sensitivity. Measurements for a piezoceramic plate excited by high frequency show that the proposed displacement sensor also has the ability to provide the in-plane dynamic displacement up to 20 kHz.     

用光纤干涉仪检测I形复合材料梁腹板/翼缘连接处的分层

介绍了一种在超声回弹波频谱分析基础上用光纤干涉仪来检测I形复合材料梁腹板/翼缘连接处分层的方法, 利用超声发射器在I形梁中产生应力波, 用表面粘贴的光纤干涉仪来接收应力波产生的输出信号, 对此信号进行频谱分析可找到I形梁的分层位置。理论分析和实验都表明了此方法探测复合材料梁腹板/翼缘连接处分层的可行性。      

用光纤干涉仪检测I形复合材料梁腹板/翼缘连接处的分层

介绍了一种在超声回弹波频谱分析基础上用光纤干涉仪来检测I形复合材料梁腹板/翼缘连接处分层的方法,利用超声发射器在I形梁中产生应力波,用表面粘贴的光纤干涉仪来接收应力波产生的输出信号,对此信号进行频谱分析可找到I形梁的分层位置。理论分析和实验都表明了此方法探测复合材料梁腹板/翼缘连接处分层的可行性。     

Self-compensating fiber optic flow sensor system and its field applications

A self-compensating fiber optic flow sensor system based on the principle of broadband white-light interferometers and cantilever beam bending is described. The fiber optic sensor system uses two fiber ferrule sensors that are bonded on either side of a cantilever beam to measure the flow rate by monitoring the air-gap changes caused by the bending of the cantilever beam. Cross sensitivity of the temperature and pressure dependence of the sensor can be compensated for automatically. The prototype sensor system was constructed, laboratory characterized, and field tested. The results from the field testing have demonstrated high resolution, repeatability, and stability for on-line detection of the flow rates of fluids.     

An optical device for measuring bending strain to 5000 microstrain and compatible with optical fiber installations

An optical sensor is described which can be attached to a structure and used as a gage for measuring bending strain. This device can be adjusted to maximize the gage factor for predetermined strain ranges. The sensor consists of glass capillaries coated on the outer surfaces with an optical absorbing layer followed by a reflecting layer. A mechanical strengthening layer can be included to extend the range of strain response. A source laser beam from an optical fiber is injected into one end of the gage. The light remaining in the beam after traveling through the gage is collected via another optical fiber. The optically active layer is adjusted during manufacture to provide a predetermined gage factor. For a given thickness of the absorber layer, the detected light is proportional to the amount of bending. Thus, by rigidly affixing the sensor to a structural member, the strain experienced by the member can be monitored.     

Fiber Bragg grating temperature sensor with controllable sensitivity

We demonstrate a fiber Bragg grating (FBG) sensor with controllable sensitivity by connecting two metal strips that have different temperature-expansion coefficients. By changing the lengths of the metal strips we successfully controlled and improved the temperature sensitivity to 3.3 times of that of bare FBG.     

复合材料I形梁分层的光纤超声探测技术数值研究

针对纤维增强塑料多界面、高阻尼以及体波模式多样性对超声无损检测的影响, 介绍了一种利用光纤超声探测技术结合小波信号分析检测I 形纤维增强塑料梁中分层的方法。该方法利用超声发射器在梁中产生超声应力波, 用表面粘贴的光纤干涉仪来探测梁中应力波的传播情况, 然后利用小波的多分辨率特性对此信号进行时频分析, 结合梁中超声波的传播特性即可确定分层的位置。理论分析和模拟结果都表明了此方法探测复合材料梁腹板/翼缘连接处分层的可行性。      

Evaluation of multi-layered graphene surface plasmon resonance-based transmission type fiber optic sensor

Graphene is a zero band-gap semi-metal with remarkable electromagnetic and mechanical characteristics. This study is the first ever attempt to use graphene in the surface plasmon resonance (SPR) sensor as replacement material for gold/silver. Graphene, comprised of a single atomic layer of carbon, is a purely two-dimensional material and it is an ideal candidate for use as a biosensor because of its high surface-to-volume ratio. This sensor is based on the resonance occasion of the surface plasmon wave (SPW) according to the dielectric constants of each metal film and detected material in gas or aqueous phase. Graphene in the SPR sensor is expected to enlarge the range of analyte to bio-aerosols based on the superior electromagnetic properties of graphene. In this study, a SPR-based fiber optic sensor coated with multi-layered graphene is described. The multi-layered graphene film synthesized by chemical vapor deposition (CVD) on Ni substrate was transferred on the sensing region of an optical fiber. The graphene coated SPR sensor is used to analyze the interaction between structured DNA biotin and Streptavidin is analyzed. Transmitted light after passing through the sensing region is measured by a spectrometer and multimeter. As the light source, blue light which of 450 to 460 nm in wavelength was used. We observed the SPR phenomena in the sensor and show the contrary trends between bare fiber and graphene coated fiber. The fabricated graphene based fiber optic sensor shows excellent detection sensitivity of the interaction between structured DNA and Streptavidin.     

Microprocessor-Based Temperature Monitoring System Using Optical Fibers

This paper describes a microprocessor-based temperature monitoring instrumentation system based on phase modulation principle using interferometric optical sensors system, where the phase of a beam through a temperature sensing fiber (placed in a hot chamber) is compared with that of a beam from a reference fiber placed out side the hot chamber. The sensor system consists of a He-Ne laser source, two beam splitters, two single-mode optical fibers, and two light dependent resistor (LDR)-based timer circuits as detectors. The frequencies of the timer circuits change according to the change in temperature of the hot chamber. A 8085 microprocessor-based system is used to sample the frequencies of the timer circuits.      

Measurement of low-frequency ultrasonic wave in water using an acoustic fiber sensor.

An acoustic fiber sensor for measurement of ultrasonic waves, which used the approximate Raman-Nath diffraction effect where light diffraction waves were generated in an optical fiber by strain due to the ultrasonic waves, was proposed and examined. In order to characterize the acoustic fiber sensor as a basic study, measurements of low-frequency ultrasonic waves in water were examined using a step index fiber operating as a detection sensor. The results showed that characteristics of detected signals agreed with the theoretical prediction based on Fraunhofer diffraction. This indicates that our proposed fiber sensor can be used for the detection of low-frequency ultrasonic waves as well as the transmission of light diffraction signals.     

Measurement of low-frequency ultrasonic wave in water using an acoustic fiber

An acoustic fiber sensor for measurement of ultrasonic waves, which used the approximate Raman-Nath diffraction effect where light diffraction waves were generated in an optical fiber by strain due to the ultrasonic waves, was proposed and examined. In order to characterize the acoustic fiber sensor as a basic study, measurements of low-frequency ultrasonic waves in water were examined using a step index fiber operating as a detection sensor. The results showed that characteristics of detected signals agreed with the theoretical prediction based on Fraunhofer diffraction. This indicates that our proposed fiber sensor can be used for the detection of low-frequency ultrasonic waves as well as the transmission of light diffraction signals.     

Liquid-level sensor with a high-birefringence-fiber loop mirror

A novel liquid-level sensor with a high-birefringence-fiber loop mirror (HBFLM) based on a uniform-strength cantilever beam (UCB) is proposed and demonstrated. Part of the high-birefringence fiber is pasted onto the central surface of the UCB. A hollow suspending pole is utilized to apply force at the end of the beam. The applied force varies with the change of the liquid level, leading to a change of transmission intensity. Thus the variation of liquid level can be determined via the laser wavelength within the quasi-linear transmission range of the HBFLM filter. Its sensitivity, resolution, and linear measurement range reach 0.047/cm, 10 mm, and 140 mm, respectively. The advantages of the sensor include simple structure, high sensitivity, low cost, and good repeatability, etc. The sensing signal can be directly detected by a photodetector and does not require complicated demodulation devices.     

Optical fibre-based sensors for distributed strain monitoring of asphalt pavements

Strain distribution of asphalt pavement varies in transverse and longitudinal directions, and distresses, such as cracks, ruts and settlements, often occur randomly, which can be efficiently measured by distributed optical fibre sensing technology. As bare optical fibre is weak to resist shear and torsion forces during pavement construction, the protective technique is required. Therefore, a flexible asphalt-mastic packaged optical fibre sensor was developed in this research for distributed strain monitoring of asphalt pavement. Theoretical analysis on strain transfer of the optical fibre-based sensors embedded in asphalt pavement was conducted to improve the design of the protective layer and remove the strain transfer error. Afterwards, laboratory tests on the asphalt concrete beam were carried out to validate the performance of the sensor. Finally, the proposed sensors were applied to detect the in situ performance of urban asphalt pavement under temperature and traffic loads. The results indicate that the proposed optical fibre sensor detects the distributed strain of asphalt pavement effectively, and the in situ data show significant effects of temperature and traffic loads on asphalt concrete course. This research contributes to the full-scale monitoring and health assessment of large-span pavement.     

Femtosecond laser-inscribed fiber Bragg gratings for strain monitoring in power cables of offshore wind turbines

A fiber Bragg grating sensor system used for monitoring the effects of strain on the power cable of an offshore wind turbine is presented. The Bragg grating structure was inscribed into coated nonphotosensitive standard telecommunication fibers using an IR femtosecond laser and the point-by-point writing technique. Because of the presence of the protective coating of the fiber, the mechanical stability of the resultant sensor device is better than that of a sensor consisting of a bare fiber. A system containing this sensing element was to our knowledge for the first time successfully installed and tested in an offshore wind turbine prototype (REpower 6M, REpower Systems, AG, Germany) in February 2010, near Ellh?ft (Germany). The fabrication process of the fiber Bragg gratings, measurement results of the online monitoring, and a comparison between the sensor signal and commonly used sensing techniques are presented.     

Study of particle size effects on an optical fiber sensor response examined with Monte Carlo simulation

Optical fiber sensors based on the total light transmittance are widely used to measure the volume fraction of particles in suspensions. However, the sensor response depends not only on the volume fraction but also on the particle size. The particle size effect is studied for a sensor configuration consisting of two linear arrays of fibers on each of two blocks: the emitting and receiving blocks. These two linear arrays are arranged with three adjacent fibers (one fiber on the first array, two fibers on the second array) forming a perfect triangle. The almost superimposition of the calculated sensor response versus the extinction factor for different particle sizes allows for the application of single- curve models. Two single-curve models that describe the sensor response for all particle sizes ranging from 36 to 200 microm are proposed. The models are validated by Monte Carlo simulation for different particle sizes and are valid within a detectable volume fraction. The single-curve models proposed provide an easier approach to creating a database for sensor calibration for suspended sediment concentration measurements.     

A New Flexible Optical Fiber Goniometer for Dynamic Angular Measurements: Application to Human Joint Movement Monitoring

The electronic measurement of the angle between two planes is generally performed by using the so-called electrogoniometers. The major drawback in using such devices is the presence of a fixed hinge that imposes a fixed center of rotation. This can cause problems when measuring the bending angle in some joints, such as Cardan or human joints, which have a variable rotation center. Based on an optical fiber, a sensor measuring the relative angle in a rotating joint has been developed. This joint makes use of the intensity modulation of a laser beam propagating in a single-mode optical fiber, due to the changes of its polarization status originated by the rotation of contiguous portions of the fiber, where controlled birefringence has been induced by the joint rotation. A prototype of this sensor has been developed with a range of the relative angle of 90$^{circ}$ , a resolution of less than 0.01$^{circ}$, and a standard deviation of 0.1$^{circ}$. The main advantages of this innovative sensor are lightness, flexibility, high speed of reaction, and high accuracy. This paper describes the development of the proposed sensor, with particular reference to the applications of human joint movement monitoring. Additionally, the equipment implemented for the test is illustrated, and results from laboratory tests are reported and discussed.      

Simultaneous sensing of the strain and points of failure in composite beams with an embedded fiber optic Michelson sensor

A fiber optic Michelson sensor was embedded in composite beams to sense the internal strain and points of failure of the composite structures. The bending deformation and matrix cracking were investigated by four-point bending tests of cross-ply composite beams with the embedded fiber optic sensor. The failure points of composite beams were detected by using both a PZT sensor and a fiber optic sensor in order to investigate the fiber optic failure signals. The failure due to matrix cracks in a composite beam was confirmed by the edge replica method. The digital processing of the fiber optic signal was carried out to determine the strains and failure points of composite beams. The failure points were observed from the processed failure signal by high-pass filtering. The initial failure strain of the composite beam was measured and processed from the fiber optic strain signal after low-pass filtering.     

Foundations for low-loss fiber gradient-index lens pair coupling with the self-imaging mechanism

A fiber-optic collimator that emits a Gaussian beam with its beam waist at a certain distance after the exit face of the lens is labeled a self-imaging collimator. For such a collimator, the waist of the emitted Gaussian beam and its location are partly dependent on the properties of the gradient-index (GRIN) lens. Parameters for the self-imaging collimator are formulated in terms of the parameters of a GRIN lens (e.g., pitch, core refractive index, gradient index, length) and the optical wavelength. Next, by use of the Gaussian beam approximation, a general expression for the coupling power loss between two self-imaging-type single-mode fiber (SMF) collimators is, for the first time to our knowledge, derived as a function of three types of misalignment, namely, separation, lateral offset, and angular tilt misalignment. A coupling experiment between two self-imaging collimators with changing separation distance is successfully performed and matches the proposed self-imaging mechanism coupling loss theory. In addition, using a prism, lateral offset, as well as angular tilt, misalignments are experimentally simulated for a two self-imaging collimator coupling condition by a single collimator reflective test geometry. Experimental results agree well with the proposed loss formulas for self-imaging GRIN lenses. Hence, for the first time to our knowledge, the mathematical foundations are laid for employing self-imaging-type fiber collimators in SMF-based free-space systems allowing optimal design for ultra-low-loss coupling.