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%.

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.

High-Precision Magnetic Field Sensor Based on Fiber Bragg Grating and Dual-Loop Optoelectronic Oscillator

A novel fiber-optic magnetic field sensor with high interrogation speed and resolution by using an etched fiber Bragg grating(FBG)in conjunction with a dual-loop optoelectronic oscillator(OEO)is proposed and experimentally demonstrated.A commercial FBG is firstly dipped into mixed hydrofluoric acid solution to remove the cladding layer and then is embedded with the magnetic fluid(MF)as a sensing element.The central wavelength reflected from the FBG is related to the overall time delay of the dual-loop OEO,which determines the oscillating frequency of the OEO.Therefore,the magnetic field can be estimated by measuring the oscillating frequency shift of OEO.The experimental results show that the oscillating frequency linearly increases with the increment of the magnetic field,achieving the sensitivity of 16.3 Hz/Oe with a R-square of 0.991 in the range of 5 mT-10 mT.In addition,the maximum error is within±0.05 mT in the range of 7 mT-8 mT,which offers potentials in many fields where the high-precision magnetic field measurement is required.

Fiber Bragg Grating Strain Sensing Detecting Multi-Crack Damages under Vibrating Status

The multi-crack damages modal of simple supported beam has been build, at the vibrating status, the multi-damage detecting method of simple supported beam measured by fiber Bragg grating strain sensing array has been studied. From 0 hz to 200 hz, using exciter vibrating simple supported beam, with different damages, resonant frequency of simple supported beam has changed. So, when the damage appears in simple supported beam, the local rigidity will decrease, the resonant frequency of simple supported beam will be affected, the damage status of simple supported beam have been determined by this. The experimental result indicates that the resonant frequency of simple supported beam has changed when there is no damage, one damage, two damages, three damages on simple supported beam. According to this, the fiber Bragg grating strain sensing array can detect multi-crack damage of simple supported beam under vibrating status.

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.

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.

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.

Design of Fiber Magnetic Field Sensor Based on Fiber Bragg Grating Fabry-Perot Cavity Ring-Down Spectroscopy

A novel fiber magnetic sensor based on the fiber Bragg grating Fabry-Perot （FBG-FP） cavity ring-down technique with pulse laser injection is proposed and demonstrated theoretically. A general expression of the intensity of the output electric field is derived, and the effect of the external magnetic field on the ring-down time is discussed. The results show that the output light intensity and the ring-down time of the FBG-FP cavity are in the inverse proportion to the magnitude of the external magnetic field. Our results demonstrate the new concept of the fiber magnetic sensor with the FBG-FP cavity ring-down spectroscopy and the technical feasibility.

Research on the Surface Subsidence Monitoring Technology Based on Fiber Bragg Grating Sensing

In order to monitor the process of surface subsidence caused by mining in real time, we reported two types of fiber Bragg grating （FBG） based sensors. The principles of the FBG-based displacement sensor and the FBG-based micro-seismic sensor were described. The surface subsidence monitoring system based on the FBG sensing technology was designed. Some factual application of using these FBG-based sensors for subsidence monitoring in iron mines was presented.

Tilted Fiber Bragg Gratings:Principle and Sensing Applications

In this paper,the mode coupling mechanism of tilted fiber Bragg gratings(TFBGs)is briefly introduced at first.And a general review on the fabrication,theoretical and experimental research development of TFBGs is presented from a worldwide perspective,followed by an introduction of our current research work on TFBGs at the Institute of Modern Optics,Nankai University(IMONK),including TFBG sensors for single-parameter measurements,temperature cross sensitivity of TFBG sensors,and TFBG-based interrogation technique.Finally,we would make a summary of the related key techniques and a remark on prospects of the research and applications of TFBGs.

Polyimide-Coated Fiber Bragg Grating for Relative Humidity Sensing

A fiber-optic humidity sensor has been fabricated by coating a moisture sensitive polymer film to the fiber Bragg grating （FBG）. The Bragg wavelength of the polyimide-coated FBG changes while it is exposed to different humidity conditions due to the volume expansion of the polyimide coating. The characteristics of sensors, including sensitivity, temporal response, and hysteresis, were improved by controlling the coating thickness and the degree of imidization during the thermal curing process of the polyimide. In the relative humidity （RH） condition ranging from 11.3%RH to 97.3%RH, the sensitivity of the sensor was about 13.5pm/%RH with measurement uncertainty of ±1.5%RH.

Review of Femtosecond Laser Fabricated Fiber Bragg Gratings for High Temperature Sensing

This paper reviews high temperature sensing applications based on fiber Bragg gratings fabricated by use of femtosecond laser. Type II fiber Bragg gratings fabricated in the silica fiber can sustain up to 1200 ℃ while that fabricated in the sapphire fiber have the good thermal stability up to 1745 ℃.

Distributed Weak Fiber Bragg Grating Vibration Sensing System Based on 3 × 3 Fiber Coupler

A novel distributed weak fiber Bragg gratings （FBGs） vibration sensing system has been designed to overcome the disadvantages of the conventional methods for optical fiber sensing networking, which are： low signal intensity in the usually adopted time-division multiplexing （TDM） technology, insufficient quantity of multiplexed FBGs in the wavelength-division multiplexing （WDM） technology, and that the mixed WDM/TDM technology measures only the physical parameters of the FBG locations but cannot perform distributed measurement over the whole optical fiber. This novel system determines vibration events in the optical fiber line according to the intensity variation of the interference signals between the adjacent weak FBG reflected signals and locates the vibration points accurately using the TDM technology. It has been proven by tests that this system performs vibration signal detection and demodulation in a way more convenient than the conventional methods for the optical fiber sensing system. It also measures over the whole optical fiber, therefore, distributed measurement is fulfilled, and the system locating accuracy is up to 20m, capable of detecting any signals of whose drive signals lower limit voltage is 0.2 V while the frequency range is 3 Hz - 1 000 Hz. The system has the great practical significance and application value for perimeter surveillance systems.

2D and 3D Shape Sensing Based on 7-Core Fiber Bragg Gratings

A fiber-optic shape sensing based on 7-core fiber Bragg gratings(FBGs)is proposed and experimentally demonstrated.The investigations are presented for two-dimensional(2D)and three-dimensional(3D)shape reconstruction by distinguishing bending and twisting of 7-core optical fiber with FBGs.The curvature and bending orientation can be calculated by acquiring FBG wavelengths from any two side cores among the six outer cores.And the shape sensing in 3D space is computed by analytic geometry theory.The experiments corresponding of 2D and 3D shape sensing are demonstrated and conducted to verify the theoretical principles.The resolution of curvature is about 0.1 m^(-1) for 2D measuring.The error of angle in shape reconstruction is about 1.89°for 3D measuring.The proposed sensing technique based on 7-core FBGs is promising of high feasibility,stability,and repeatability,especially for the distinguishing ability on the bending orientation due to the six symmetrical cores on the cross-section.

Response Analysis of Ultrasonic Sensing System Based on Fiber Bragg Gratings of Different Lengths

In recent years, fiber Bragg gratings （FBGs） have been widely used in ultrasonic detection for practical structural health monitoring in light of its unique advantages over the conventional sensors. Although FBGs have been successfully tested in ultrasonic inspection, the effect of the grating length on the sensitivity of the FBG ultrasonic sensing system is yet to be analyzed. Hence, using the simulation model, the main influencing factor on the sensitivity of the ultrasonic sensing system with different lengths gratings was first investigated. In the following experiment, the ultrasonic responses of the sensing system with six different lengths FBGs were obtained, respectively. The theoretical analysis and the experimental results would be useful for sensitivity improvement of the FBG-based ultrasonic and acoustic emission sensing system.

Fiber loop ring-down cavity integrated U-bent single-mode-fiber for magnetic field sensing

A novel magnetic field sensing system based on the fiber loop ring-down technique is proposed in this paper. In the fiber loop, a U-bent single-mode-fiber structure coated with magnetic fluid(MF) serves as the sensing head, and an erbium-doped fiber amplifier(EDFA) is introduced to compensate for the intrinsic loss of the cavity. The ring-down time of the system varies with the change of applied magnetic field due to the tunable absorption coefficient and refractive index of the MF. Therefore, measurement of the magnetic field can be realized by monitoring the ringdown time. The experimental results show that the performance of the system is extremely dependent on the interrogation wavelength, because both the gain of the EDFA and the loss of the sensing head are wavelength dependent.We found that at the optimal wavelength, the ratio of the gain to loss attained its maximum. The sensing system was experimentally demonstrated and a sensitivity of-0.5951 μs∕Oe was achieved.

Experimental research on cholesterol solution concentration sensing based on tilted fiber Bragg grating

The cholesterol solution concentration sensing characteristics based on tilted fiber Bragg grating(TFBG) are investigated by means of theoretical analysis and experiments. We prepare two groups of cholesterol solutions with the same concentration range and different refractive index ranges. The sensitivity of the two groups of solutions was 11.83 pm·m L/mg and 124.79 pm·m L/mg, respectively. The results show that the sensitivity of cholesterol solution can be improved by adjusting the refractive index range. This conclusion is valuable for measuring the concentration of fat-soluble solution.

An interferometric phase shift fiber Bragg grating sensing system with greatly reduced background phase noise

We presented an interferometric phase shift fiber Bragg grating （FBG） sensor, which inherited the advantages of FBG sensors, and, at the same time, the greatly reduced full-width-at-half-maximum bandwidth brought longer coherent length, higher sensitivity, and lower phase noise. Experiments show that at least a 7 dB reduction of phase noise can be achieved compared to FBG sensors interrogated by interferometer with the same optical path difference.

Regenerated fiber Bragg grating for fiber laser sensing at high temperatures

Thermally regenerated low-reflectivity fiber Bragg gratings（RFBGs）, as one mirror of a resonant cavity, have been introduced as linear-cavity fiber lasers combining with fiber saturable absorbers. The output of lasing presents an optical signal-to-noise ratio of 50 dB and temperature sensitivity coefficient of 15.36 pm∕℃ for the heating process and 15.46 pm∕℃ for the cooling process. The lasing wavelength variation and power fluctuation at 700℃ are less than 0.02 nm and 0.21 dB, respectively. The RFBG-based fiber laser sensing has displayed good linearity for both the temperature rising and cooling processes, and favorable stability at high temperatures.