Strain-Insensitive Fiber Bragg Grating Composite Structure for Wide-Range Temperature Sensing

This paper reports on the design,fabrication,and temperature strain sensing performance of a fiber Bragg grating composite structure for surface mounted temperature measurements over a wide temperature range,with highly reduced strain cross-sensitivity.The fiber Bragg grating sensor is encapsulated in a polyimide tube filled with epoxy resin,forming an arc-shaped cavity.This assembly is then placed between two layers of glass fiber prepreg with a flexible pad in between and cured into shape.Experimental results,supported by finite element simulations,demonstrate an enhanced temperature sensitivity is 26.3 pm/°C over a wide temperature range of–30°C to 70°C,and high strain transfer isolation of about 99.65%.

Monitoring shear deformation of sliding zone via fiber Bragg grating and particle image velocimetry

Monitoring shear deformation of sliding zones is of great significance for understanding the landslide evolution mechanism,in which fiber optic strain sensing has shown great potential.However,the cor-relation between strain measurements of quasi-distributed fiber Bragg grating(FBG)sensing arrays and shear displacements of surrounding soil remains elusive.In this study,a direct shear model test was conducted to simulate the shear deformation of sliding zones,in which the soil internal deformation was captured using FBG strain sensors and the soil surface deformation was measured by particle image velocimetry(PIV).The test results show that there were two main slip surfaces and two secondary ones,developing a spindle-shaped shear band in the soil.The formation of the shear band was successfully captured by FBG sensors.A sinusoidal model was proposed to describe the fiber optic cable deformation behavior.On this basis,the shear displacements and shear band widths were calculated by using strain measurements.This work provides important insight into the deduction of soil shear deformation using soil-embedded FBG strain sensors.

Application of fiber Bragg grating sensor network in aluminum reduction tank shell temperature monitoring

A fiber Bragg grating temperature sensor network was designed to implement the real-time health monitoring of the aluminum reduction cell. The heat transfer process was simulated using software ANSYS, and an on-line shell monitoring system was established based on optical sensing technology. According to aluminum reduction cell heat transfer theory, the 2D slice finite element model was developed. The relationship between shell temperature and cell status was discussed. Fiber Bragg grating （FBG） was chosen as the temperature sensor in light of its unique advantages. The accuracy of designed FBG temperature sensors exceeds 2 ~C, and good repeatability was exhibited. An interrogation system with 104 sensors based on VPG （volume phase grating） filter was established. Through the long-term monitoring on running state, the status of the aluminum reduction cell, including security and fatigue life could be acquired and estimated exactly. The obtained results provide the foundation for the production status monitoring and fault diagnosis. Long-term test results show good stability and repeatability which are compatible with electrolysis process.

Measurement of Temperature and Residual Strain during Fatigue of a CFRP Composite Using Fiber Bragg Grating Sensors

Fatigue behaviour has important implications for engineering composite structures in sectors ranging from automotive to aerospace. Optical sensing technology displays excellent performance in these fields for monitoring. In this paper, temperature and residual strain during fatigue of a carbon fiber reinforced polymer（CFRP） are investigated. Four autoclaved CFRP beam specimens, with fiber Bragg grating（FBG） sensors and thermocouples embedded at selected locations, are subjected to three-point bending cyclic loading on the BOSE testing machine for fatigue testing. Thennocouples are used to measure the temperature while FBGs can sense the temperature and strain as well. Seven tests in total are conducted at different frequencies, and each test lasts for several days. From the experimental results, transient steep peaks of temperature increases （up to 2.3℃） are discovered at the beginning of the load. The following constant temperature increments are around 1.0℃, which is not relevant to frequencies from 0.1 Hz to 20 Hz and suspected due to fatigue. Residual strains of 1×10^-5-2×10^-5 during fatigue, fading away rapidly when unloading, are also reported. Embedded FBGs here are validated to sense temperature and strains in composite structures, which demonstrates promising potentials in structure monitoring fields. CFRP are verified to have an excellent performance during fatigue with low temperature increase and residual strain.

An active temperature compensated fiber Bragg grating vibration sensor for high-temperature application

An active temperature compensated fiber Bragg grating(FBG)vibration sensor with a constant section cantilever beam is proposed for the simultaneous measurement of temperature and vibration,and the sensor is verified by a temperature compensation feedback system.The high-temperature vibration sensor is composed of a quartz cantilever beam and a femtosecond Bragg grating.The feedback control demodulation system of active temperature compensation can adjust the laser wavelength to stabilize the grating offset point and realize simultaneous measurement of temperature and vibration.On this basis,the performance of the sensor is tested and analyzed within the range of 20-400℃by setting up a high-temperature vibration test system.The experimental results show that the sensitivity of the sensor is about 132.33 mV/g,and the nonlinearity is about 3.33%.The sensitivity between the laser wavelength and temperature is about 0.01307 nm/℃.In addition,the active temperature compensated fiber Bragg grating vibration sensor has the advantages of a simple structure,stable performance,easy demodulation and high sensitivity.Moreover,the sensor can achieve high temperature vibration signal monitoring and has good practical application value.

Temperature Sensor Based on Fiber Bragg Grating Embedded in SNS Fiber Structure

A novel fiber temperature sensor based on multimode interference theory is proposed and experimentally demonstrated.The sensing head is formed by a fiber bragg grating(FBG)connected with single-mode-no core-single-mode fiber(SNS)fiber structure which consists of two sections of single mode fiber and no-core fiber.Using such a structure,not only the reflective measurement can be realized,but also the need for a gold-plated film can be avoided at the end of the fiber to enhance the reflected light signal.More importantly,the sensitivity is increased by 4 times as compared with the conventional FBG temperature sensor according to the experimental results,and it also provides the development space for multi-parameters monitoring.

Temperature Compensation for Double Fiber Bragg Grating Sensors

A novel double fiber Bragg grating（FBG） strain sensor configuration is presented. Temperature compensation method is based on double FBG moored on a rhombus frame. Through the theoretical analysis, the relation between relative shift of Bragg wavelength and the strain applied on the sensor is obtained, and the analytical expression of strain sensitivity coefficient is also given. The experiment results show that： in the strain range of 0-0. 8 mm, the relation between the relative shift of Bragg wavelength and applied strain is linear, and the dispersion of double FBG wavelength at the range of -25℃- 60℃ is 0-0. 002 nm. The strain sensitivity of the displacement sensor configuration is 0. 171 nm/με, and is nearly twice than that of single FBG sensor.

Novel Apodized Fiber Bragg GratingApplied for Medical Sensors:Performance Investigation

Sensors play an important role in shaping and monitoring human health.Exploration of methods to use Fiber Bragg Grating(FBG)with enhanced sensitivity has attracted great interest in the field of medical research.In this paper,a novel apodization function is proposed and performance evaluation and optimization of the same have been made.A comparison was conducted between various existing apodization functions and the proposed one based on optical characteristics and sensor parameters.The results evince the implementation of the proposed apodization function for vital sign measurement.The optical characteristics considered for evaluation are Peak Resonance Reflectivity level,Side Lobes Reflectivity level and FullWidth HalfMaximum(FWHM).The proposed novel apodization novel function has better FWHM,which is narrower than the FWHM of uniform FBG.Sensor characteristics like a quality parameter,detection accuracy and sensitivity also show improvement.The proposed novel apodization function is demonstrated to have a better shift in wavelength in terms of temperature and pulse measurement than the existing functions.The sensitivity of the proposed apodized function is enhanced with a Poly-dimethylsiloxane coating of varying thickness,which is 6 times and 5.14 times greater than uniform Fiber Bragg grating and FBG with the proposed novel apodization function,respectively,enhancing its utilization in the field of medicine.

Determination of the Postmortem Interval Using Fiber Bragg Grating Sensors

Fiber Bragg grating(FBG)sensors are often used in monitoring activities and to ensure that environmental parameters satisfy industrial requirements.They offer crucial safety measures in the early detection of hazards due to their greatly reduced size,low weight,flexibility,and immunity to electromagnetic interference.These characteristics make FBGs suitable also for use in relation to the human body for in vivo measurements and long-term monitoring.In this study,recent developments are presented with regard to the utilization of these sensors to measure the so-called post-mortem interval(PMI).Such developments rely on numerical simulations based on the Matlab software and monitoring of the rectal temperature,which is one of the main parameters for estimating the PMI.First,the Matlab software is used to solve the Henssge equation for different ambient temperatures and for different body masses;then a Bragg grating sensors is used for post-mortem dating.The results and their accuracy are discussed.

Numerical Analysis of Fiber Bragg Grating and Long Period Fiber Grating Undergoing Linear and Quadratic Temperature Change

The coupled-mode equations for fiber Bragg grating （FBG） and long period fiber grating （LPFG） undergoing linear and quadratic temperature change were given. The effects of temperature gradient and quadratic temperature change on the reflectivity spectrum of fiber Braggs grating and the transmission spectrum of long period fiber grating were investigated using the numerical simulation, and the dependence relationships of the central wavelength shift, the full-width-athalf-maximum, and the peak intensity upon temperature gradient were also obtained. These relationships may be used to design a novel fiber optical sensor which can simultaneously measure the temperature and temperature gradient.

Modified simulated annealing evolutionary algorithm for fully distributed fiber Bragg grating temperature sensing

In this paper, we present a simple and fast spectra inversion method to reconstruct the temperature distribution along single fiber Bragg grating （FBC） temperature sensor. This is a fully distributed sensing method based on the simulated annealing evolutionary （SAE） algorithm. Several modifications are made to improve the algorithm efficiency, including choosing the most superior chromosome, setting up the boundary of every gene according to the density of resonance peaks of the reflection spectrum, and dynamically modifying the boundary with the algorithm running. Numerical simulation results show that both the convergence rate and the fluctuation are significantly improved. A high spat-ial temperature resolution of 0.25 mm has been achieved at the time cost of 86 s.

Advances in femtosecond laser direct writing of fiber Bragg gratings in multicore fibers:technology,sensor and laser applications

In this article,we review recent advances in the technology of writing fiber Bragg gratings(FBGs)in selected cores of multicore fibers(MCFs)by using femtosecond laser pulses.The writing technology of such a key element as the FBG opens up wide opportunities for the creation of next generation fiber lasers and sensors based on MCFs.The advantages of the technology are shown by using the examples of 3D shape sensors,acoustic emission sensors with spatially multiplexed channels,as well as multicore fiber Raman lasers.

Demodulation System for Fiber Bragg Grating Sensors Using Digital Filtering Technique

A discrimination measurement method and demodulation technique for fiber Bragg grating (FBG) sensors were presented using digital filtering technique. The system can control a tunable fiber Fabry-Perot filter with sawtooth wave voltage generated by digital clock to interrogate FBG sensors. Using the analogue digital converter (ADC), the reflected FBG signals were sampled with synchronous digital clock. With the aid of digital matched filtering technique, the sampled FBG signals were processed to obtain the maximum signal-to-noise ratio (SNR) and the Bragg wavelength shift from the FBG signals was recovered. The results demonstrate that this system has a scanning range of 1 520 nm-1 575 nm,and the wavelength detection accuracy is less than 2 pm with 1.5 Hz scanning frequency.

Resistance Spot Welding Method for Metal-Based Fiber Bragg Grating Sensors

Aiming at the requirements of installation and work environment of metal-based fiber Bragg grating(FBG)sensor,a resistance spot welding method is presented.Firstly,the effect of welding parameters on welding quality is discussed theoretically,and a suitable resistance spot welding method for the metal-based FBG sensor is proposed for the first time.Then through serial resistance spot welding tests,the feasibility and practicability of the method are verified,and optimal welding parameters for two different tested metals are obtained.Fatigue performance test validates FBG sensors installed by the proposed method with good fatigue properties and long-term stable measurement performance.The research results can provide technical guidance for engineering structure long-term safety monitoring.

Packaging Effects on Fiber Bragg Grating Sensor Performance

In this paper, the effects of packaging material and structure of fiber Bragg grating sensor performance are investigated. The effects of thermal expansion coefficient of different embedding materials on the temperature sensitivities of the FBG sensors are studied both theoretically and experimentally with good agreement, which provides a means for selection of FBG packaging material to achieve desired temperature sensitivity. We also demonstrate a 4-point bending structured FBG lateral force sensor that measures up to 242N force with well-preserved reflection spectrum, whereas for 3-point bending structure, multiple-peaks start to occur when applied force reaches 72N.

High sensitivity micro-displacement sensor based on fiber Bragg grating and amplification substrate

A micro-displacement sensor based on fiber Bragg grating(FBG) is proposed. The device consists of a pair of FBGs with different central wavelengths fabricated by femtosecond laser phase mask method and a metal substrate with lever structure. The displacement is amplified by lever structure and it converts into axial tension of FBG, which has a high displacement sensitivity. The amplification factors obtained by theoretical analysis and finite element simulation are 2.67 and 2.50, respectively. The experimental results show that in the range of 0-50 μm the shift of FBG center wavelength is linearly related to the displacement of measured object and displacement sensitivity reaches 121 pm/μm. In addition, the cascaded FBG is used to compensate the temperature.

Experimental Measurements of the Sensitivity of Fiber-optic Bragg Grating Sensors with a Soft Polymeric Coating under Mechanical Loading,Thermal and Magnetic under Cryogenic Conditions

The strain and temperature sensing performance of fiber-optic Bragg gratings （FBGs） with soft polymeric coating, which can be used to sense internal strain in superconducting coils, are evaluated under variable cryogenic field and magnetic field. The response to a temperature and strain change of coated-soft polymeric FBGs is tested by comparing with those of coated-metal FBGs. The results indicate that the coated-soft polymeric FBGs can freely detect temperature and thermal strain, their At variable magnetic field, the tested results indicate accuracy and repeatability are also discussed in detail. that the cross-coupling effects of FBGs with different matrixes are not negligible to measure electromagnetic strain during fast excitation. The present results are expected to be able to provide basis measurements on the strain of pulsed superconducting magnet/cable （cable- around-conduit conductors, cable-in-conduit conductors）, independently or utilized together with other strain measurement methods.

Low Temperature Characteristics of FBG Temperature Sensors with Small-size Packaging

Two small-size package methods, thin stainless steel tube based package method and plating gold based package method for fiber Bragg grating(FBG) temperature sensors, are designed. The characteristics of the FBG temperature sensors in the environment temperature between 203K and 243K are investigated. The wavelengths of the FBG temperature sensors as a function of the temperature are examined experimentally. The experiment results of the two different small-size package sensors in identical conditions are compared with each other. The transmission spectrum of the sensors is analyzed at the temperature of 273K and 213K. The temperature sensitive factors of the sensors are derived analytically and are verified by experiments. Linear fitting to the wavelength-temperature curve is carried out.

Interrogating a Fiber Bragg Grating Vibration Sensor by Narrow Line Width Light

A method to interrogate fiber Bragg grating vibration sensor by narrow line width light is demonstrated. The interrogation scheme takes advantage of the intensity modulation of narrow spectral bandwidth light, such as distributed feedback laser, when a reflection or transmission spectrum curve of an fiber Bragg grating （FBG） moves due to the strain which is applied on the sensor. The sensor＇s response to accelerating frequency and amplitude is measured by experiment. The factors which have impacts on the sensitivity of the interrogation system are also discussed.

Magneto-Optic Fiber Bragg Gratings with Application to High-Resolution Magnetic Field Sensors

Magneto-optic fiber Bragg gratings （MFBG） based on magneto-optic materials have a lot of potential applications for sensing and optical signal processing. The transmission and reflection spectra of guided optical waves in the MFBG are investigated. According to the sensitivity of MFBG spectral lines to the magneto-optic coupling intensity varying with applied magnetic field, a novel magnetic field sensor of high-resolution up to 0.01 nm/（kA/m） is predicted.