Research on transmission character of side polished fiber

Transmission characteristics of the side polished fiber were studied by experimental method.The side polished fibers with different depth and length were implemented,and the corresponding wavelength dependent loss was measured.Based on wheel fabrication,the side polished fibers were achieved with the low insertion loss and cost.Meanwhile,they can be artificially controlled for the use of evanescent field area and easy to system integration.

Structural Deformation Monitoring of Flight Vehicles Based on Optical Fiber Sensing Technology:A Review and Future Perspectives

Structural deformation monitoring of flight vehicles based on optical fiber sensing(OFS)technology has been a focus of research in the field of aerospace.After nearly 30 years of research and development,Chinese and international researchers have made significant advances in the areas of theory and methods,technology and systems,and ground experiments and flight tests.These advances have led to the development of OFS technology from the laboratory research stage to the engineering application stage.However,a few problems encountered in practical applications limit the wider application and further development of this technology,and thus urgently require solutions.This paper reviews the history of research on the deformation monitoring of flight vehicles.It examines various aspects of OFS-based deformation monitoring including the main varieties of OFS technology,technical advantages and disadvantages,suitability in aerospace applications,deformation reconstruction algorithms,and typical applications.This paper points out the key unresolved problems and the main evolution paradigms of engineering applications.It further discusses future development directions from the perspectives of an evolution paradigm,standardization,new materials,intelligentization,and collaboration.

Stimulated Brillouin scattering-induced phase noise in an interferometric fiber sensing system

Stimulated Brillouin scattering-induced phase noise is harmful to interferometric fiber sensing systems. The localized fluctuating model is used to study the intensity noise caused by the stimulated Brillouin scattering in a single-mode fiber. The phase noise structure is analyzed for an interferometric fiber sensing system, and an unbalanced Michelson interferometer with an optical path difference of 1 m, as well as the phase-generated carrier technique, is used to measure the phase noise. It is found that the phase noise is small when the input power is below the stimulated Brillouin scattering threshold, increases dramatically at first and then gradually becomes flat when the input power is above the threshold, which is similar to the variation in relative intensity noise. It can be inferred that the increase in phase noise is mainly due to the broadening of the laser linewidth caused by stimulated Brillouin scattering, which is verified through linewidth measurements in the absence and presence of the stimulated Brillouin scattering.

A hardening load transfer function for rock bolts and its calibration using distributed fiber optic sensing

Confinement of rock bolts by the surrounding rock formation has long been recognized as a positive contributor to the pull-out behavior,yet only a few experimental works and analytical models have been reported,most of which are based on the global rock bolt response evaluated in pull-out tests.This paper presents a laboratory experimental setup aiming to capture the rock formation effect,while using distributed fiber optic sensing to quantify the effect of the confinement and the reinforcement pull-out behavior on a more local level.It is shown that the behavior along the sample itself varies,with certain points exhibiting stress drops with crack formation.Some edge effects related to the kinematic freedom of the grout to dilate are also observed.Regardless,it was found that the mid-level response is quite similar to the average response along the sample.The ability to characterize the variation of the response along the sample is one of the many advantages high-resolution fiber optic sensing allows in such investigations.The paper also offers a plasticity-based hardening load transfer function,representing a"slice"of the anchor.The paper describes in detail the development of the model and the calibration/determination of its parameters.The suggested model captures well the coupled behavior in which the pull-out process leads to an increase in the confining stress due to dilative behavior.

Deformation and failure characteristics of sandstone under uniaxial compression using distributed fiber optic strain sensing

This paper investigates the deformation and fracture propagation of sandstone specimen under uniaxial compression using the distributed fiber optic strain sensing(DFOSS)technology.It shows that the DFOSS-based circumferential strains are in agreement with the data monitored with the traditional strain gage.The DFOSS successfully scans the full-field view of axial and circumferential strains on the specimen surface.The spatiotemporal strain measurement based on DFOSS manifests crack closure and elastoplastic deformation,detects initialization of microcrack nucleation,and identifies strain localization within the specimen.The DFOSS well observes the effects of rock heterogeneity on rock deformation.The advantage of DFOSS-based strain acquisition includes the high spatiotemporal resolution of signals and the ability of full-surface strain scanning.The introduction to the DFOSS technology yields a better understanding of the rock damage process under uniaxial compression.

Effectiveness of Fiber Bragg Grating monitoring in the centrifugal model test of soil slope under rainfall conditions

Centrifugal model testsare playing an increasingly importantrolein investigating slope characteristics under rainfall conditions. However, conventional electronic transducers usually fail during centrifugal model tests because of the impacts of limitedtest space, high centrifugal force, and presence of water, with the result that limited valid data is obtained. In this study, Fiber Bragg Grating(FBG) sensing technology is employed in the design and development of displacement gauge, an anchor force gauge and an anti-slide pile moment gauge for use on centrifugal model slopes with and without a retaining structure. The two model slopes were installed and monitored at a centrifugal acceleration of 100 g. The test results show that the sensors developed succeed in capturing the deformation and retaining structure mechanical response of the model slopes during and after rainfall. The deformation curvefor the slope without retaining structure shows a steepresponse that turns gradualfor the slope with retaining structure. Importantly, for the slope with the retaining structure, results suggest that more attention be paid to increase of anchor force and antislide pile moment during rainfall. This study verifies the effectiveness of FBG sensing technology in centrifuge research and presents a new and innovative method for slope model testing under rainfall conditions.

A simple model of suppressing stimulated Brillouin scattering in optical fiber with frequency-modulated laser

A simple model is developed to study the mechanism of stimulated Brillouin scattering（SBS） suppression with frequency-modulated laser in optical fiber. By taking into account the laser frequency distribution along the fiber induced by frequency modulation, the average effective Brillouin gain is calculated to determine the SBS threshold. Experimental results show agreement with the numerical analysis. The application for SBS suppression in interferometric fiber sensing system is also discussed in this paper. The results show that the maximum input power can be increased effectively by frequency modulation method.

Feasibility study on sinkhole monitoring with fiber optic strain sensing nerves

Anthropogenic activity-induced sinkholes pose a serious threat to building safety and human life nowadays.Real-time detection and early warning of sinkhole formation are a key and urgent problem in urban areas.This paper presents an experimental study to evaluate the feasibility of fiber optic strain sensing nerves in sinkhole monitoring.Combining the artificial neural network(ANN)and particle image velocimetry(PIV)techniques,a series of model tests have been performed to explore the relationship between strain measurements and sinkhole development and to establish a conversion model from strain data to ground settlements.It is demonstrated that the failure mechanism of the soil above the sinkhole developed from a triangle failure plane to a vertical failure plane with increasing collapse volume.Meanwhile,the soil-embedded fiber optic strain sensing nerves allowed deformation monitoring of the ground soil in real time.Furthermore,the characteristics of the measured strain profiles indicate the locations of sinkholes and the associated shear bands.Based on the strain data,the ANN model predicts the ground settlement well.Additionally,micro-anchored fiber optic cables have been proven to increase the soil-to-fiber strain transfer efficiency for large deformation monitoring of ground collapse.

Development of a monitoring and warning system based on optical fiber sensing technology for masonry retaining walls and trees

Hong Kong has a long history of applying masonry retaining walls to provide horizontal platforms and stabilize man-made slopes.Due to the sub-tropical climate,some masonry retaining walls are colonized by trees.Extreme weather,such as typhoons and heavy rains,may cause rupture or root failure of those trees,thus resulting in instability of the retaining walls.A monitoring and warning system for the movement of masonry retaining walls and sway of trees has been designed with the application of fiber Bragg grating(FBG)sensing technology.The monitoring system is also equipped with a solar power system and 4G data transmission devices.The key functions of the proposed monitoring system include remote sensing and data access,early warning,and real-time data visualization.The setups and working principles of the monitoring systems and related transducers are introduced.The feasibility,accuracy,serviceability and reliability of this monitoring system have been checked by in-site calibration tests and four-month monitoring.Besides,a two-level interface has been developed for data visualization.The monitoring results show that the monitored masonry retaining wall had a reversible movement up to 2.5 mm during the monitoring period.Besides,it is found that the locations of the maximum strain on trees depend on the crown spread of trees.

Distributed multicore fiber sensors

Multicore fiber(MCF)which contains more than one core in a single fiber cladding has attracted ever increasing attention for application in optical sensing systems owing to its unique capability of independent light transmission in multiple spatial channels.Different from the situation in standard single mode fiber(SMF),the fiber bending gives rise to tangential strain in off-center cores,and this unique feature has been employed for directional bending and shape sensing,where strain measurement is achieved by using either fiber Bragg gratings(FBGs),optical frequency-domain reflectometry(OFDR)or Brillouin distributed sensing technique.On the other hand,the parallel spatial cores enable space-division multiplexed(SDM)system configuration that allows for the multiplexing of multiple distributed sensing techniques.As a result,multi-parameter sensing or performance enhanced sensing can be achieved by using MCF.In this paper,we review the research progress in MCF based distributed fiber sensors.Brief introductions of MCF and the multiplexing/de-multiplexing methods are presented.The bending sensitivity of off-center cores is analyzed.Curvature and shape sensing,as well as various SDM distributed sensing using MCF are summarized,and the working principles of diverse MCF sensors are discussed.Finally,we present the challenges and prospects of MCF for distributed sensing applications.

A Nonlinear Mirror Structure to Improve the Performance of the Distributed Fiber Temperature Sensor Based on Raman Backscattering

A novel nonlinear mirror structure which can increase the optical signal-to-noise ratio of a distributed fiber Raman temperature sensor is proposed, and 6 dB improvement of the optical signal-to-noise ratio is obtained. With the assistance of the nonlinear mirror, we demonstrate that the spatial resolution of the sensor is improved from 3 m to 1 m, and the temperature accuracy is improved from ±0.6℃ to ±0.2℃. The theoretical analysis and the experimental data are in good agreement.

Preparation and Properties of Fiber Optic Oxygen Sensing Material by Thermo-polymerization Method

A new kind of fiber optic oxygen sensing material based on the fluorescence quenching of Ru（bpy）3Cl2 was prepared by the themo-polymerization method. The ruthenium dye was immobilized in N, N-methylene bisacrylamide（MBBA） polymer by physically trapping while MBBA was covalently crosslinked on the glass micro-beads by NaHSO3-O2-MnSO4 initiator system. The lock-in amplifyication technology was used for the detection of their sensing properties. The influences of indicator concentration, glass micro-beads diameter, post polymerization time, concentration and reaction time of glutaraldehyde on the properties of sensing materials were studied. To optimize the influencing factors to the sensing materials, the indicator concentration of 0.7 g/L, glass micro-beads diameter of 0.3 mm, post polymerization time of 5 h were achieved. The immobilization stability of ruthenium dye and the performance of the sensing materials were improved by the new polymerization system. An absolute detection limit of 3×10-6 （V/V） and the response time of 10 s were obtained. This kind of sensing materials has good stability and their life time is 2 years.

RESEARCH ON TRANSVERSE STRESS CHARACTER OF SINGLE MODE OPTICAL FIBER

This paperdetails experimental work done to quantify stress measurements made optically utilizing ordinary single mode optical fibers. Strain-induced changes of birefringence for ordinary single mode optical fiber responses are characterized against standard stress measurements in a well understand configuration. The experimental scheme for this work and the results are presented in detaial. In this paper, POssible applications for this transverse stress character of single mode fibers are also proposed.

Experiment Study of Fiber Optic Sensing in Railway Security Monitoring

Aiming at some security problems in railway running and the application condition of existing technology, this paper studies some issues of using fiber optic sensing technology in railway security monitoring. Through field experiment measuring the strain of the rail and analyzing the experiment data, the method of diagnosing the health condition of rail and wheel is investigated.

A Novel Temperature-Compensated, Intensity-Modulated Fiber Bragg Grating Sensor System

An intensity-modulated, fiber Bragg grating （FBG） sensor system based on radio-frequency （RF） signal measurement is presented. The RF signal is generated at a photodetector by two modulated optical signals reflected from the sensing FBG and a reference one. Bragg wavelength shift of the sensing FBG changes intensity of the RF signal by changing phase difference between the two optical signals, with temperature effect being compensated automatically by the reference FBG, Strain measurement with a maximum sensitivity of -0.34 μV/με has been achieved.

Pure Bending Characteristic of Tilted Fiber Bragg Grating

a novel structure of the pure macro- bending sensor based on the tilted fiber Bragg grating （TFBG） is proposed. The TFBG located in the half circle with the different diameters is bent at a constant angle with respect to the tilted grating planes. With the variations of the curvature, the core-mode resonance is unchanged and the transmission power of cladding modes detected by the photodiodes varies linearly with curvature, while the ghost mode changes by the form of two-order polynomial. So we can use the transmission power of ghost mode or other cladding modes to detect bending curvature as shape sensor. From a practical point of view, the sensor proposed here is simple, low cost and easy to implement. Moreover, it is possible to make a temperature-insensitive shape sensor due to the same temperature characteristic between the core mode and the cladding modes.

Photonic crystal fiber Mach-Zehnder interferometer with microholes ablated by a femtosecond laser for refractive index sensing

A new structure of the photonic crystal fiber（PCF）based Mach-Zednder interferometer（MZI）is fabricated and presented.The structure has microholes ablated by a femtosecond laser.The fringe visibility can be enhanced more than 10 dB compared with the interferometer without a microhole.The interferometer is characterized by sodium chloride solutions for refractive index（RI）sensing.The RI sensitivities are greatly increased by the hole fabrication since it directly changes the cladding modes of the PCF.For the interferometer sensor with two holes,the RI sensitivity is 157.74 nm/RIU,which is 5 times than that of the sensor without a microhole.Microholes ablation with a femtosecond laser on PCF can increase the sensor＇s sensitivity dramatically.Femtosecond laser has a wide application prospect in the field of performance improvement of the sensors.

Ultra-high resolution strain sensor network assisted with an LS-SVM based hysteresis model

Optical fiber sensor network has attracted considerable research interests for geoscience applications.However,the sensor capacity and ultra-low frequency noise limits the sensing performance for geoscience data acquisition.To achieve a high-resolution and lager sensing capacity,a strain sensor network is proposed based on phase-sensitive op-tical time domain reflectometer(φ-OTDR)technology and special packaged fiber with scatter enhanced points(SEPs)ar-ray.Specifically,an extra identical fiber with SEPs array which is free of strain is used as the reference fiber,for com-pensating the ultra-low frequency noise in theφ-OTDR system induced by laser source frequency shift and environment temperature change.Moreover,a hysteresis operator based least square support vector machine(LS-SVM)model is in-troduced to reduce the compensation residual error generated from the thermal hysteresis nonlinearity between the sensing fiber and reference fiber.In the experiment,the strain sensor network possesses a sensing capacity with 55 sensor elements.The phase bias drift with frequency below 0.1 Hz is effectively compensated by LS-SVM based hyster-esis model,and the signal to noise ratio(SNR)of a strain vibration at 0.01 Hz greatly increases by 24 dB compared to that of the sensing fiber for direct compensation.The proposed strain sensor network proves a high dynamic resolution of 10.5 pε·Hz-1/2 above 10 Hz,and ultra-low frequency sensing resolution of 166 pεat 0.001 Hz.It is the first reported a large sensing capacity strain sensor network with sub-nεsensing resolution in mHz frequency range,to the best of our knowledge.

Responses of calcareous sand foundations to variations of groundwater table and applied loads

The long-term settlement of calcareous sand foundations caused by daily periodic fluctuations has become a significant geological hazard,but effective monitoring tools to capture the deformation profiles are still rarely reported.In this study,a laboratory model test and an in situ monitoring test were conducted.An optical frequency domain reflectometer(OFDR)with high spatial resolution(1 mm)and high accuracy(10-6)was used to record the soil strain responses to groundwater table and varied loads.The results indicated that the fiber-optic measurements can accurately locate the swelling and compressive zones.During the loading process,the interlock between calcareous sand particles was detected,which increased the internal friction angle of soil.The foundation deformation above the sliding surface was dominated by compression,and the soil was continuously compressed beneath the sliding surface.After 26e48 h,calcareous sand swelling occurred gradually above the water table,which was primarily dependent on capillary water.The swelling of the soil beneath the groundwater table was completed rapidly within less than 2 h.When the groundwater table and load remain constant,the compression creep behavior can be described by the Yasong-Wang model with R2¼0.993.The daily periodically varying in situ deformation of calcareous sand primarily occurs between the highest and lowest groundwater tables,i.e.4.2e6.2 m deep.The tuff interlayers with poor water absorption capacity do not swell or compress,but they produce compressive strain under the influence of deformed calcareous sand layers.

Experimental study on the deformation and failure mechanism of overburden rock during coal mining using a comprehensive intelligent sensing method

Understanding the spatiotemporal evolution of overburden deformation during coal mining is still a challenge in engineering practice due to the limitation of monitoring techniques. Taking the Yangliu Coal Mine as an example, a similarity model test was designed and conducted to investigate the deformation and failure mechanism of overlying rocks in this study. Distributed fiber optic sensing(DFOS), highdensity electrical resistivity tomography(HD-ERT) and close-range photogrammetry(CRP) technologies were used in the test for comprehensive analyses. The combined use of the three methods facilitates the investigation of the spatiotemporal evolution characteristics of overburden deformation, showing that the mining-induced deformation of overburden strata was a dynamic evolution process. This process was accompanied by the formation, propagation, closure and redevelopment of separation cracks.Moreover, the key rock stratum with high strength and high-quality lithology played a crucial role in the whole process of overburden deformation. There were generally three failure modes of overburden rock layers, including bending and tension, overall shearing, and shearing and sliding. Shear failure often leads to overburden falling off in blocks, which poses a serious threat to mining safety. Therefore, realtime and accurate monitoring of overburden deformation is of great significance for the safe mining of underground coal seams.