Tunable fiber laser based fiber Bragg grating strain sensor demodulation system with enhanced resolution by digital signal processing

In this paper, a soft fiber Bragg grating (FBG) strain sensor was constructed of a rubber strip, FBGs and steel plates, which exhibits the advantages of high precision and a small size. A series of FBGs was uniformly pasted on a flexible rubber strip which can monitor the slope deformation by measuring the bending deformation of the rubber strip. Most notably, this sensor can be used to monitor horizontal displacement in the subsurface of the slope model. The relationships among the bending angle of the rubber strip, the strain of the rubber strip, and the subsurface deformation of the slope model were established. In addition, the subsurface deformation of the slope model can be obtained by the FBG strain sensor monitoring. Since a rigid-flexible structure was formed by uniformly pasting a series of steel plates on the other side of the rubber strip, the sensitivity of the FBG strain sensor was improved to be 1.5425 nm/°. The measurement results verify that the FBG strain sensor shows good performance, and the model test results demonstrate that the FBG strain sensor can be used for monitoring the subsurface deformation of the slope model.

To address the issue of extra-large structural deformation or strain in infrastructures such as bridges, buildings, railroads, and pipelines during catastrophic events, this study proposes a wide-range fiber Bragg grating (FBG) strain sensor utilizing a snake spring desensitization mechanism to share large parts of the strains. Initially, the axial stiffness of the snake spring desensitization mechanism was derived using the strain energy method, which was applied for stiffness calculation, range determination, and parameter design of the entire structure, where the snake spring and the FBG strain sensor were connected in series. Then, the snake springs were fabricated using 3D printing technology and assembled with the FBG sensor to construct a wide-range strain sensor. The wide-range sensor was subsequently calibrated, achieving a strain range of 10,000 με and a linearity coefficient above 0.9995. Finally, the sensor was installed in a pipeline for testing, yielding favorable results. These results demonstrate that the proposed sensor exhibits a wide strain monitoring range and can be effectively used for real-time structural safety analysis by continuously monitoring localized large structure strains.

The control of robots in electromagnetically noisy environments may benefit from the use of EMI-insensitive multi-component force sensors. Multi-component force sensing is usually done with local strain measurements on an elastic transducer. We propose to use fibre Bragg grating (FBG) strain sensors to perform these local strain measurements, taking advantage of their multiplexing capabilities and their immunity to electromagnetic interference. In this paper, we discuss the design and the calibration of a compact multi-component force sensor using an elastic transducing body and eight multiplexed fibre Bragg gratings. We demonstrate, for the first time, that multi-component force sensors based on multiplexed FBG strain sensors can be constructed.

Strain measurement has important applications in mechanical engineering, civil engineering, aerospace, earthquake monitoring, and other fields. Aiming at the problem of low sensitivity of existing fiber Bragg grating (FBG) strain sensors, a high-sensitivity FBG strain sensor of the substrate type with a sensitization structure is proposed. The sensitivity of the sensor is analyzed theoretically, the sensor is simulated by Solidworks and ANSYS software, and the structural parameters are optimized. According to the simulation results, the real sensor is developed, and the strain test system is built to test the performance of the sensor. The results show that the strain sensitivity of the sensor is 3.21p m/µε, which is about 2.7 times that of the bare FBG strain sensor, which is basically in line with the theoretical value. The fitting linear correlation coefficient is 0.9999, and the repeatability error is 3.9%FS. The research results provide a reference for developing the same type of sensors and further improving the sensitivity of fiber strain sensors.

A new concept for the self-diagnosis of embedded fiber Bragg grating (FBG) strain sensors was developed, simulated and experimentally tested. This concept is based on a magnetostrictive metallic layer directly coated on the fibre cladding over the grating segment of the FBG sensor, so that an on-demand external magnetic field in a millitesla scale can produce a controllable artificial strain as an indication signal for the remote optical interrogator. The relationship between the pre-defined magnetic field and its induced Bragg wavelength shift characterizes this validation concept. Any deviation of the local bonding state of the interfaces from the initial or/and any change of shear strain transferring mechanism from composite matrix to the optical fibre core will result in alterations in this sensitive relationship, and thus triggers an immediate alert for a further inspection. The finite element method is used to simulate the strain of this configuration as result of different values of the magnetic field in order to optimize the geometrical sensor parameters. The simulations are verified by experiments results. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Tunnel excavation induces the stress redistribution of the surrounding rock. Structural cracks may develop in the secondary lining due to this stress redistribution and bias pressure, consequently affecting the overall construction safety of the tunnel. This paper aims to achieve real-time monitoring of the excavation stability of the lining structure by integrating two monitoring technologies: structural deformation monitoring and fiber grating strain monitoring. Additionally, it proposes a method to simultaneously measure the thermal strain and applied stress-strain of the structure. By analyzing the displacement and deformation of the lining structure, its stability can be preliminarily evaluated in the short term. To achieve long-term real-time monitoring and a more accurate assessment of the tunnel structure's stability, the paper introduces fiber Bragg grating (FBG) strain sensor monitoring technology. First, based on the geological stratigraphy information obtained from the exploration, a simulation model of the tunnel under different section bias angles is established. The displacement and stress concentration areas of the lining structure are then analyzed to optimize the sensor deployment array and provide a theoretical basis for the sensor arrangement. FBG strain sensors are installed on the surface of the structure to measure thermal strain and loading stress-strain, whereas FBG temperature sensors measure local temperature. The findings indicate that following tunnel excavation, the maximum daily strain differences at K107+043 and K107+240 were 126.87 µε and 209.38 µε, respectively. After a period of rock disturbance, the average daily strain differences due to applied stress-strain were 16.8 µε and 12.65 µε, respectively. The thermal strain was close to the daily strain difference. Therefore, after the rock disturbance subsided, the strain fluctuations in the lining structure were mainly caused by local temperature changes, and the surrounding rock tended to stabilize. This offers a viable method for evaluating structural stability post-tunnel excavation.

The investigation of fluid-structure interaction during the impact of a flexible plate on a water surface has received much attention. Measurement of highly transient, distributed strain of the plate during the slamming event is of great interest. Multiplexed fiber Bragg grating (FBG) strain sensors provide a promising solution for such measurement since these sensors are inherently waterproof and are immune to electromagnetic interference. However, in order to monitor the highly transient, distributed strain responses (up to 20 kHz), high-speed simultaneous interrogation of multiplexed FBG sensors is required, which is challenging by using commercial optical interrogators. We present a tunable-wavelength-filter-based optical interrogation system for high-speed simultaneous interrogation of multiplexed FBG strain sensors and demonstrate its application for structural monitoring of a flexible plate during the slamming event. The interrogation system employs a piezoelectric-transducer-controlled Fabry–Perot tunable filter. By operating the tunable filter at its resonant frequency and demodulating the sensor signal based on a peak tracing method, we demonstrated an interrogation speed of 100 kHz, an interrogation range of 98 nm, and an interrogation resolution of 5 pm. For proof-of-performance, the interrogation system was used to monitor the vibrational responses of a cantilever plate under impact loading and the measurement of vibration modes up to 6.785 kHz was demonstrated. Finally, the slamming experiments were carried out with six multiplexed FBG strain sensors mounted on a flexible plate. The dynamic strain measurement of the plate during the slamming event was successfully demonstrated by using the high-speed FBG interrogation system.

For a surface mounted fiber Bragg grating (FBG) strain sensor, this study used an improved bonding method that distributes cement only on the two ends of the grating and leaves the grating free from cement. According to a recent numerical study, for a conventional surface mounted FBG strain sensor its relationship between average strains (within its glued grating) and voltage signals is nonlinear and changeable depending on the variation patterns of strain gradients (within its glued grating) in a strain history. By contrast, the relationship for the improved bonding one is linear because of the constant zero strain gradient within the glue-free grating. To experimentally verify the linear relationship for the improved bonding fiber Bragg grating (IBFBG), this study set up a cantilever with both the IBFBG and a resistance strain gauge surface mounted in close proximity, and the vibration responses were induced by impactions of a free-falling steel ball on the cantilever's surface. Experimental results indicate that dynamic calibrations of an IBFBG by using a commonly used resistance strain gauge are possible because both sensors in close proximity have their changes of voltage signals almost proportional to each other.

Fiber Bragg Grating (FBG) strain sensors represent a cutting-edge technology in the field of optical sensing, providing precise measurements of strain, temperature, and other physical parameters in various environments. This article explores the fundamental principles, applications, and advantages of FBG strain sensors, highlighting their scientific and industrial significance.

Optical fiber Bragg grating (FBG) has been recognized as an outstanding high-performance local monitoring sensor and is largely applied in structural health monitoring (SHM). This paper proposes a series of fiber-reinforced polymer- (FRP-) packaged optical fiber Bragg grating strain sensors to completely meet the requirements of rough civil engineering infrastructures, and their sensing performance under normal environment and harsh environment is experimentally investigated. It is experimentally and theoretically proved that FRP-packaged FBG strain sensors maintain excellent sensing performance as the bare FBG sensor under a harsh environment, and their durability is significantly enhanced due to the FRP materials. These FRP-packaged FBG strain sensors are successfully applied in the SHM system of Aizhai Bridge.

We have proposed and constructed a system to multiplex fiber Bragg grating (FBG) strain sensors with same reflection wavelength, adopting the synthesis of optical coherence function technique (SOCF), which has realized dynamic strain measurement. The ability to control the coherence peak position of SOCF allows random access in the spatial domain. In this paper, the function of random access strain measurement has been realized by switching the coherence peak position at every FBG, synchronizing with FBG spectrum measurement, to use the sampling rate to the full. In addition, we have proposed and confirmed the novel method for improving FBG interval by simply modulating the LD intensity.

We present a very simple and versatile fiber optic sensing platform for real time and continuous liquid level measurement. It includes a proper mass suspended in the liquid with one end fixed to a Fiber Bragg Grating (FBG) strain sensor, while the other fiber termination is fixed to a rigid support. When the liquid level increases inside the tank, the load exerted by the suspended mass on the optical fiber is reduced as a consequence of the increase of the buoyant force acting on the immersed mass, thus leading to a reduction of the strain applied to the FBG. Here, just as proof of principle, we exploited an aluminum cylindrical mass having a radius of 8mm, and experimentally demonstrated the capability of the realized platform to measure the water level inside a beaker over a range of 25 cm with a sensitivity of ~27pm/cm and a resolution of ~0.1% of the full scale reading (FSR).

As Oil & Gas Pipeline of Long Distance has potential mechanical risks caused by geologic hazards, based on sensing technique, Fiber Bragg Grating (FBG) strain sensor, for which unstressed grating temperature compensation method is adopted, is designed to solve problems of FBG temperature and strain cross sensitivity. According to experimental tests, strain sensitive coefficients of the sensor before and after sticking on pipeline sample are 1.20 pm/με and 1.09 pm/με in separate. Finally, applied to a landslide area for monitoring pipeline stress, the sensor got the impact of Wenchuan Earthquake on May 12, 2008 on the pipeline.

This article presents the structural assessment of an existing old steel railway bridge in Malaysia using fibre optic sensors. The steel bridge was built more than a century ago and the bridge structural drawings and other documentations were lost over the years. Keretapi Tanah Melayu Berhad (KTM), the Malaysian rail operator was upgrading the railway line to cater for heavier and faster locomotives. The bridge is subjected to a potential risk of structural health deterioration or fatigue due to corrosion and long service period in such a high humidity country. Full scale static and dynamic tests using locomotives were carried out to assess the structural integrity of the steel bridge. Two types of optical sensors were used. Discrete and distributed fibre optic sensors were mounted on critical steel panels to monitor the change in strain of these members under dynamic and static loads. The discrete sensor, Fibre Bragg Grating (FBG) strain sensors were able to measure the change in strain at 1000 Hz frequency, and were used during the dynamic test where a locomotive travelled through the bridge at maximum speed of 40 km/hour. Distributed fibre optic (DFOS) sensors were mounted on the bottom chord of the steel truss to measure the maximum tensile strain when the locomotive was placed at the centre of the bridge. The results from the two different fibre optic sensors (FBG and DFOS sensors) were then compared and the results showed good agreement to strain measurements. © International Conference on Smart Infrastructure and Construction 2019, ICSIC 2019: Driving Data-Informed Decision-Making.

Sensing properties of fiber Bragg grating (FBG) strain sensors produced in photosensitive optical fibers was the main task of the presentation. Several Bragg gratings were made by the scanning phase mask method. The standard telecom matched-clad fibers were used in the experiment but it was necessary to hydrogenate them before the UV illumination. We have hydrogenated our fibers for up to 30 days under the pressure of 130 bar. The double frequency 244 nm 100 mW Coherent argon ion laser was used. Bragg gratings spectral characteristics were measured, filter central wavelength and FWHM determined. Grating were stretched and their strain sensitivity determined. FBG was also attached to the surface of elastic bar. It was bended in the grating squeeze and stretch configurations. The linear response of the Bragg wavelength shift was observed in all experiments. Total wavelength shift of 10 nm was easily measured by our monochromator system in the range ±4355 με. The experimental results were compared to theoretical data obtained from simple model bar equations. We have found good agreement between theory and experiment. FWHM values remained nearly unchanged. We believe the model simplicity as the source of small discrepancies.