Engineering approach to in-situ bridge health monitoring with fiber bragg gratings

In this presentation the feasibility and capability of fiber Bragg gratings （FBG） employed in bridge health monitoring are demonstrated on a real bridge. FBG＇ s wavelength shift depending on strain variance has been tested. The technique of FBG installation on bridges has been developed. 12 FBG strain sensors and 3 temperature sensors have been successfully embedded in the prestressed concrete box girder during the construction of Heilongjiang Hulan River Bridge. The prestressing tension process and quasi-static loading process of the girder were monitored with those sensors before it was installed onto the bridge. After the bridge was completed, the FBG sensors embedded have been utilized to monitor the strain shift of the beam under quasi-static load, traffic load and temperature. The results show that the traffic fluxes, possible tatigue damage and deflection of the bridge can be revealed conveniently through strain measurements with these FBG sensors, which provide key information for structural health diagnosis. The fact that the FBG strain sensors have withstood the ordeal of harsh construction process and lasted for more than one year proves that their durability and stability can satisfy the requirements for bridge health monitoring. It is also shown that the FBG strain sensor is more adaptive to long-term structural health monitoring than the electric resistance strain gauge.

Fabrication of arbitrarily apodized fiber Bragg gratings with narrow bandwidth

A double-exposure fabrication method without any intensity shadow mask to fabricate arbitrarily apodized fiber Bragg gratings（ FBGs） with narrow bandwidth is demonstrated by controlling the total ultra violet （UV） irradiation along the grating by varying the speed of a translation stage. The UV source used is a stable continuous intracavity frequency-doubled argonion laser. The parameters （such as length, apodization profile, average index change）of FBGs can be easily changed with this method. The total UV irradiation is kept constant in the doubleexposure process because of the precise control of the exposure time, which ensures that the apodized FBG＇s bandwidth can be extremely narrow. The full width at half maximum （FWHM） bandwidth of the 2-cm-long apodized FBG fabricated by this method is 0. 15 nm with a maximum reflectivity of more than 95%.

Fiber Bragg Gratings in Small-Core Ge-Doped Photonic Crystal Fibers

This paper reports fiber Bragg gratings （FBGs） inscribed in a small-core Ge-doped photonic crystal fibers with a UV laser and a Talbot interferometer. The responses of such FBGs to temper- ature, strain, bending, and transverse-loading were systematically investigated. The Bragg wavelength of the FBGs shifts toward longer wavelengths with increasing temperature, tensile strain, and transverse-loading. The bending and transverse- loading properties of the FBGs are sensitive to the fiber orientations.

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.

Irradiation effect on strain sensitivity coefficient of strain sensing fiber Bragg gratings

The effect of irradiation on the strain sensitivity coefficient of strain sensing fiber Bragg gratings （FBGs） has been investigated through experiments. FBGs were fabricated in single mode fibers with 3 tool% Ge-concentration in the core and with a H2-1oading treatment. In experiments, the FBGs were subjected to y-radiation exposures using a Co6~ source at a dose-rate of 25 Gy/min up to a total dose of 10.5 kGy. The GeO defect in fiber absorbs photons to form a GeE＇ defect; the interaction with H2 is a probable reason for the y-radiation sensitivity of gratings written in hydrogen loaded fibres, The effect mechanism of radiation on the strain sensitivity coefficient is similar to that of radiation on the temperature sensitivity coefficient. Radiation affects the effective index neff, which results in the change of the thermo-optic coefficient and the strain-optic coefficient. Irradiation can change the strain sensitivity coefficient of FBGs by 1.48%-2.71%, as well as changing the Bragg wavelength shift （BWS） by 22 pm-25 pm under a total dose of 10.5 kGy. Our research demonstrates that the effect of irradiation on the strain sensitivity coefficient of FBG is small and that strain sensing FBGs can work well in the radiation environment.

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.

Experimental investigation of pre-irradiation effect on radiation sensitivity of temperature sensing fiber Bragg gratings

The effect of pre-irradiation on radiation sensitivity of fiber Bragg gratings （FBGs） is verified experimentally. FBGs are fabricated in photosensitive optical fibers and single mode fibers with Ce-concentration in a range from 3 mol% to 23.37 mol% in the core. In experiments, the FBGs are subjected to twice v-radiation exposures to a Co^60 source at a dose-rate of 0.1 Gy/s up to a total dose of 50 kGy. Pre-irradiation treatment can reduce the temperature sensitivity variation of FBGs by 18.12%-35.91%, as well as Bragg wavelength shift （BWS） by 8%-27.08 %. Our research demonstrates that pre-irradiation treatment is a feasible method to improve the radiation tolerance of FBGs.

Tunable Super-Structured Fiber Bragg Gratings with Perfect Sequences Based on m-Sequence

Recently, a tunable fiber Bragg grating(FBG) was developed by using stress-responsive colloidal crystals. In this paper, we have simulated the application of these nanoparticles into the super-structured fiber Bragg grating(SSFBG) written with perfect sequences derived from a short maximal-length sequence. A tunable SSFBG will be available to overcome the prohibitive temperature variation of the optical codecs. Nevertheless,we presented a method to implement coherent time spreading optical code-division multiple-access(OCDMA) where a unique code(or perfect sequence) can be reused and mixed with different wavelengths to obtain a tunable wavelength-division multiplexing(WDM)system. In order to maximize the binary throughput, we have selected a unique short maximal-length sequence composed of 7 chips that can be tuned with 7 different optical wavelengths. We found thousands of different tunable combinations that presented power contrast ratios(P/C) higher than 12 dB. When a WDM-OCDMA system used 2 different combinations simultaneously, the perfect binary detection with error correction codes was achieved successfully. The tunable SSFBG with colloidal crystals will be a simple and good alternative choice for fiber-to-the-home(FTTH) communications.

Fabrication of High Quality Broadband Type IIA Chirped Fiber Bragg Gratings

Chirped fiber Bragg gratings have found many applications in optical communication and sensing systems. High quality filters based on chirped fiber Bragg gratings with reflection bandwidth of 2.6 and 32nm and high reflectivity are demonstrated experimentally with 2 and 4cm long phase masks, respectively. These filters with flat reflection band and high reflectivity are achieved by writing type IIA chirped Bragg gratings.

Dynamic Single/Dual-channel Filter with Tunable Fiber Bragg Gratings

The shift mechanism of Bragg wavelength with stress variation for a fiber grating is investigated in detail. The influence of strain change on reflection and bandpass is theoretically analyzed. By applying stress, the dynamic single/dual channel filter with tunable fiber Bragg gratings is achieved.

Multiwavelength Actively Mode-locked Fiber Laser with a Double-ring Configuration and Integrated Cascaded Sampled Fiber Bragg Gratings

A modified multiwavelength actively mode-locked fiber ring laser is proposed and experimentally demonstrated. In this kind of laser, stable multiwavelengths lasing is achieved by integrating cascaded sampled fiber Bragg gratings(SFBGs) into the laser cavity. To implement actively mode-locking technique, a double-ring cavity configuration is used to assure that the cavity lengths for all wavelengths lasing are identical. Thus, simultaneous mode locking of all wavelengths has been successfully achieved by using the same mode-locking signal.

Real-time monitoring of internal temperature of a lithium-ion battery using embedded fiber Bragg gratings

This study proposes a method for real-time monitoring of lithium-ion battery(LiB)internal temperatures through the temperature response of an embedded fiber Bragg grating(FBG)sensor.This approach overcomes the limitations of most methods that can only detect the external temperature at limited places by providing the advantages of sensing both the internal temperature and external temperature at multiple points simultaneously for precise condition monitoring.In addition,a numerical LiB cell model was developed to investigate the heat generation and temperature gradient using the finite element analysis method.The outcomes show that this model can be used to predict the temperature with less than 5%discrepancy(1.5°C)compared with experimental results.Thereby,this proposed method can be effectively used to monitor the safety and state of health of LiBs and other types of rechargeable batteries in real-time.

Femtosecond laser direct written fiber Bragg gratings with narrow bandwidth and high sideband suppression

Aiming for suppressing side-mode and spectrum broadening,a slit beam-shaping method and super-Gaussian apodization processing for femtosecond laser point-by-point(PbP)inscription technology of fiber Bragg gratings(FBGs)are reported here.High-quality FBGs,featuring narrow bandwidth of less than 0.3 nm,high reflectivity above 85%,low insertion loss(0.21 dB),and low cladding loss(0.82 dB),were obtained successfully.By a semi-automatic PbP inscription process,an array consisting of six FBGs,exhibiting almost no side-mode peaks with high suppression ability and narrow bandwidth,was fabricated along three independently developed single-mode fibers with an interval of 20 mm.

Fiber oscillator of 5 kW using fiber Bragg gratings inscribed by a visible femtosecond laser

We fabricate a pair of fiber Bragg gratings(FBGs)by a visible femtosecond laser phase mask scanning technique on passive large-mode-area double-cladding fibers for multi-kilowatt fiber oscillators.The bandwidth of high-reflection(HR)and lowreflection(LR)FBG is~1.6 nm and 0.3 nm,respectively.The reflection of the HR-FBG is higher than 99%,and that of the LR-FBG is about 10%.A bidirectional pumped all-fiber oscillator is constructed using this pair of FBGs,a record output power of 5027 W located in the signal core is achieved with a slope efficiency of~82.1%,and the beam quality factor M2is measured to be~1.6 at the maximum power.The FBGs are simply fixed on a water cooling plate without a special package,and the thermal efficiency of the HR-FBG and the LR-FBG is 2.76℃/kW and 1℃/kW,respectively.Our research provides an effective solution for robust high-power all-fiber laser oscillators.

Speckle-based interrogation system for quasi-distributed weak fiber Bragg gratings

A simple quasi-distributed fiber sensing interrogation system based on random speckles is proposed for weak fiber Bragg gratings(WFBGs) in this work. Without using tunable lasers or spectrometers, a piece of multimode fiber is applied to interrogate the WFBGs relying on the wavelength sensitivity of speckles. Instead of the CCD sensor, an InGaAs quadrant detector serves as the receiver to capture the fast-changing speckle patterns. A supervised deep learning algorithm of the multilayer perceptron architecture is implemented to process speckle data and to interrogate temperature changes or dynamic strains. The proposed demodulation system is experimentally demonstrated for WFBGs with 0.1% reflectivity.The experimental results demonstrate that the new system is capable of measuring temperature change with an accuracy of 1℃ and achieving dynamic frequency of 100 Hz. This speckle-based interrogation system paves a new way for distributed WFBGs sensing with a simple design.

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.

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%.

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.

Study of the Sensing Characteristics of Irradiated Fiber Bragg Gratings and Fabry-Perot Interferometers Under Gamma Radiation

The sensing characteristics of irradiated fiber Bragg gratings(FBGs)and Fabry-Perot interferometers(FPIs)were investigated under a 2MGy dose of gamma radiation.The study found that the pressure sensitivity of FP sensors after irradiation was stable,while the temperature sensitivity of FBG sensors was unstable,and both wavelengths displayed a shift.These findings offer the possibility for the application of FP pressure sensors in the gamma radiation environments,and FBG sensors require further research to be suitable for application in the nuclear radiation environments.