Fiber Bragg Grating Sensor System Research Articles

Novel real-time acousto-ultrasonic sensors using two phase-shifted fiber Bragg gratings

A novel real-time acousto-ultrasonic sensor system using a phase-shifted fiber Bragg grating rather than a normal fiber Bragg grating was investigated. The spectrum of the phase-shifted fiber Bragg grating was simulated and analyzed, which indicates that a phase-shifted fiber Bragg grating has superior properties when compared to a normal fiber Bragg grating. Based on theoretical considerations, a novel real-time acousto-ultrasonic sensor system was proposed. Two identical 5-mm phase-shifted fiber Bragg gratings, with slightly different Bragg wavelengths, were used as the filter and the sensor. An amplified spontaneous emission light source and an apodized fiber Bragg grating were connected to illuminate the phase-shifted fiber Bragg gratings. The design allows the strain resulting from the ultrasonic wave to be precisely received and converted to the fluctuation of the output voltage. An ultrasonic wave generated in a carbon fiber–reinforced plastic plate using a macrofiber composite actuator was detected by both the phase-shifted fiber Bragg grating sensor system and a sensor system based on a normal fiber Bragg grating and arrayed waveguide grating. Comparison of the temporal and spectral responses of these two systems indicates that the phase-shifted fiber Bragg grating sensor system has a higher sensitivity than the fiber Bragg grating system. The results show that the phase-shifted fiber Bragg grating system does not require data averaging for noise reduction to measure the Lamb wave modes in a carbon fiber–reinforced plastic plate.

高速高分辨率光纤布拉格光栅传感系统的解调技术

高速高分辨率光纤布拉格光栅传感系统的解调技术

Wavelength Shift Detection Methods Based on Cross-Correlation Analysis for Signal Demodulation of Fiber Bragg Grating Sensing System

In order to improve the signal demodulation performance of fiber Bragg grating (FBG) sensing system, a wavelength shift detection method based on cross-correlation analysis is proposed to improve the accuracy of the measurement. The simulation results indicate that the absolute error of the measurement can be between 2 pm - 3 pm within the range of SNR of 14 dB-7 dB. Under the condition with high SNR, the absolute error can be approximately reduced to 0.5 pm. Hence, this demodulation algorithm can reduce the influences of the noises effectively and help to further study the features of the FBG sensor demodulation system.

FBG Sensors Encapsulated into 7-Wire Steel Strand for Tension Monitoring of a Prestressing Tendon

This paper proposes a novel method for in-service evaluation of prestress forces of 7–wire prestressing strand which is frequently employed for prestressed concrete (PSC) bridge girders. The proposed method incorporates the fiber Bragg grating (FBG) sensor system. The straight king wire of the 7-wire prestressing strand is replaced with an instrumented steel tube in which the FBG sensor is embedded. Since the strain of the strand can easily be measured using the in-strand FBG sensor, it is possible to monitor the prestress force of the strand during the in-service state as well as during the jacking process. To demonstrate the effectiveness of the proposed strand with the FBG sensor system, the proposed method is applied to 1 m, 7 m, 64 m and 43 m long strand prototypes to evaluate the prestress forces, and they are respectively tested under different conditions. The experimental results show that the proposed in-strand FBG sensor system is viable for monitoring the prestress forces of the prestressing strand.

DYNAMIC MONITORING THE STRAIN OF COMPOSITES DUE TO HIGH SPEED COLLISIONS USING FIBER BRAGG GRATING NETWORK

This paper describes the Fiber Bragg Grating (FBG) sensor system for a dynamic monitoring strain due to high speed collision. The authors investigate whether or not this FBG system can be used for the real time detection of both the impact and the damage generated by high speed collisions. A three-component strain sensor system was preliminary designed for real-time detection the location of collision and the damage extent such as the ranges of the elastic deformation and the plastic deformation. The basic optical and electronic theories about this system, as well as the mechanism and mechanics model of FBG sensors, are expressed in this paper.DOI: http://dx.doi.org/10.5755/j01.mech.18.1.1277

Performance comparison of Raman/erbium-doped-fiber hybrid-amplification-based long-distance fiber Bragg grating sensor systems

A study regarding the performance comparison of three different types of long-distance fiber Bragg grating (FBG) sensors based on bidirectional Raman and erbium-doped fiber (EDF) amplification has been carried out to determine the most power-efficient and accurate configuration. The three sensor system configurations studied were a bidirectional hybrid Raman amplifier with (Type I) an EDF located after the distributed Raman amplification section, (Type II) an EDF located before the distributed Raman amplification section, and (Type III) without an EDF. The FBG sensor systems are based on a demodulation scheme employing radio frequency power measurements of a beat signal, in which a sine-modulated amplified spontaneous emission from a directly modulated reflective semiconductor optical amplifier is employed as a broadband light source. The results of our experimentation showed that the sensor system with the EDF located prior to the Raman amplification section provided the best performance in regards to its pump power efficiency, electrical signal-to-noise ratio, and measurement accuracy.

A Novel Demodulation Method for Fiber Bragg Grating Sensor System Based on the LabView Software

The theory of fiber Bragg grating demodulation system based on unbalanced Mach-Zehnder (M-Z) interferometer is introduced. A novel design for demodulation of fiber Bragg grating sensor system based on the LabView Software is proposed and demonstrated. The system is obtained by making use of piezoelectric ceramic (PZT) technology achieving phase carrier modulation. Carrier signal is collected by data acquisition card(DAQ), and the phase modulation of signal is detected by discrete fourier transform(DFT). It realizes real-time demodulation for multiplexed sensing system.

An Exact Probability Density Function for Intensity-Based Output Noise Propagating Through a Fiber Optic Sensor Demodulation Process

Fiber optic interferometry is a common architecture in many optical sensing strategies and typically requires a demodulation scheme for phase (signal) extraction. For sensing applications, signal-to-noise and other statistical metrics are of great importance in characterizing system performance. In the context of a specific demodulation algorithm employing three-channel inputs, we analytically compute a probability density function of the demodulator output noise, given an arbitrary distribution of input intensity noise and arbitrary noise correlation among the three channels. We compare the analytical formulations with previously validated simulation data from a fiber Bragg grating sensor system, and we find excellent agreement within the specific example of Gaussian input noise.

Genetic algorithm-support vector regression for high reliability SHM system based on FBG sensor network

Structural Health Monitoring (SHM) based on fiber Bragg grating (FBG) sensor network has attracted considerable attention in recent years. However, FBG sensor network is embedded or glued in the structure simply with series or parallel. In this case, if optic fiber sensors or fiber nodes fail, the fiber sensors cannot be sensed behind the failure point. Therefore, for improving the survivability of the FBG-based sensor system in the SHM, it is necessary to build high reliability FBG sensor network for the SHM engineering application. In this study, a model reconstruction soft computing recognition algorithm based on genetic algorithm-support vector regression (GA-SVR) is proposed to achieve the reliability of the FBG-based sensor system. Furthermore, an 8-point FBG sensor system is experimented in an aircraft wing box. The external loading damage position prediction is an important subject for SHM system; as an example, different failure modes are selected to demonstrate the SHM system's survivability of the FBG-based sensor network. Simultaneously, the results are compared with the non-reconstruct model based on GA-SVR in each failure mode. Results show that the proposed model reconstruction algorithm based on GA-SVR can still keep the predicting precision when partial sensors failure in the SHM system; thus a highly reliable sensor network for the SHM system is facilitated without introducing extra component and noise.

Ultra-Long Distance Fiber Bragg Grating Sensor System

A novel tunable fiber ring laser configuration with a combination of Raman amplification and erbium-doped fiber amplification (EDFA) is proposed for realizing high signal-to-noise ratio (SNR), ultra-long distance (300 km) fiber Bragg grating (FBG) sensor systems. The hybrid Raman–EDF amplification configuration can enhance the optical SNR of FBG sensor signals significantly, so the optical power reflected by the fiber Bragg grating can be compensated. A good optical SNR of 15 dB is achieved for a 275 km transmission distance, which is the best performance to our knowledge, and about 4 dB at 300 km. In the static strain experiments with fiber lengths of 100,200,250,300 km, the linearity is greater than 0.999. This system can be widely used in the ultra-long-distance sensing applications such as railways, oil(gas) pipelines and coastline.

Fracture Monitoring of Composite Wind Turbine Blade Using High-Speed Bragg Grating Sensor System

This paper introduces a FBG (fiber Bragg grating) based AE (acoustic emission) sensing system for use in health monitoring of composite wind turbine blades. In this study, a multiplexing high speed FBG sensor system was developed with a spectrometer-type demodulator based on a linear photo detector. Pencil break test was performed using an FBG sensor and the results were compared with the results of piezo-based AE sensor. For the performance test of fracture sensing in composite materials, a down-scaled wind turbine blade was fabricated and drop impact tests were performed. Arrayed 4 FBG sensors were installed in the skin of wind turbine blade and impacts of 15 J energy were applied by a drop weight. The frequency characteristics of impact induced AE signals were examined with short-time Fourier transform focused on the leading waves. Finally, the onset of fractures in composite structure was successfully assessed using arrayed FBG sensors.

Dynamic strain measurement using two wavelength-matched fiber Bragg grating sensors interrogated by a cascaded long-period fiber grating

In this work, we describe a fiber Bragg grating (FBG) sensing system using two wavelength-matched FBG sensors for static and dynamic strain measurement. A cascaded long-period fiber grating (CLPG)-based demodulation technique has been used to interrogate the two wavelength-matched FBG sensors. Experimental results of static strain measurement show that the proposed system has a strain resolution of 1 με. This system has also been used for dynamic strain measurement. An eddy current displacement meter-based system has been used as a comparison measurement. Experimental results of dynamic strain measurement have proved that the FBG sensing system has a good performance in the measurement of dynamic strain. The results of static and dynamic strain measurement indicate that the sensing system using two wavelength-matched FBG sensors is superior to the single FBG sensor system.

Remote (250 km) Fiber Bragg Grating Multiplexing System

We propose and demonstrate two ultra-long range fiber Bragg grating (FBG) sensor interrogation systems. In the first approach four FBGs are located 200 km from the monitoring station and a signal to noise ratio of 20 dB is obtained. The second improved version is able to detect the four multiplexed FBGs placed 250 km away, offering a signal to noise ratio of 6–8 dB. Consequently, this last system represents the longest range FBG sensor system reported so far that includes fiber sensor multiplexing capability. Both simple systems are based on a wavelength swept laser to scan the reflection spectra of the FBGs, and they are composed by two identical-lengths optical paths: the first one intended to launch the amplified laser signal by means of Raman amplification and the other one is employed to guide the reflection signal to the reception system.

Low-Cost, High Sensitivity, Signal Processing-Enhanced Fiber Bragg Grating Sensing System for Condition-Based Maintenance Application

This paper presents a low-cost, high sensitivity fiber Bragg grating sensing system specifically designed to measure the amplitude of weak signals at audio frequencies. Low cost is obtained using for the optical part an intensity based interrogation scheme, with time and wavelength divi- sion multiplexing to create a sensor network that minimizes the expense per sensing point. Then, performances are dramatically improved using sequential signal processing techniques, which clear noise inside and outside the sensor, and provide an exceptionally performing spectral estimation. This way, the sensor gains 66 dB of signal-to-noise performance, and is able to measure vibra- tions as low as few picostrains, with a frequency resolution of 0.05 Hz. Example of application to multipoint condition-based maintenance application in industrial machineries is given. In this case, the proposed optical sensor is used to detect very weak signals buried into noise coming from ball bearings and rotating elements that are about to break, and can raise a warning alert well before critical damages occur

Femtosecond laser-inscribed fiber Bragg gratings for strain monitoring in power cables of offshore wind turbines

A fiber Bragg grating sensor system used for monitoring the effects of strain on the power cable of an offshore wind turbine is presented. The Bragg grating structure was inscribed into coated nonphotosensitive standard telecommunication fibers using an IR femtosecond laser and the point-by-point writing technique. Because of the presence of the protective coating of the fiber, the mechanical stability of the resultant sensor device is better than that of a sensor consisting of a bare fiber. A system containing this sensing element was to our knowledge for the first time successfully installed and tested in an offshore wind turbine prototype (REpower 6M, REpower Systems, AG, Germany) in February 2010, near Ellhöft (Germany). The fabrication process of the fiber Bragg gratings, measurement results of the online monitoring, and a comparison between the sensor signal and commonly used sensing techniques are presented.

Development of FBG Sensing System for Outdoor Temperature Environment

We have experimentally designed a practical pass-through type FBG temperature sensor. The objective of this study is to design and built a prototype outdoor fibre Bragg grating (FBG) temperature sensor system. Its performance is evaluated at different times of the day. In order to reduce the optical losses of the FBG system, the shortest optical fiber path used to connect the FBG system is 55.5m. It has a total connector loss 4.0dB and fibre loss of 0.3dB thus giving a total loss of the system as 4.3dB. The FBG sensor system is connected to the tunable laser source (TLS) and optical spectrum analyzer (OSA). The TLS is used to provide a broadband light source via a fibre optic cable of wavelength 1550nm. The OSA is used to display the transmission and reflection spectrum to give the Bragg wavelength _B, bandwidth and power dip. The output spectrum can be obtained through direct connection to the FBG. Result of transmission and reflection spectrum show the sensitivity which is calculated from the slope of the graph. The FBG temperature sensor system has an average sensitivity of 9.1 pm (°C)-1 based on the transmission spectrum. It has an average sensitivity of 10.6 pm (°C)-1 based on the reflection spectrum.

A High Sensitive Micro-Seismic Fiber Bragg Grating(FBG) Sensor System

Abstract A high sensitive micro-seismic fiber Bragg grating (FBG) sensor system based on narrow line width distributed feedback (DFB) laser is realized by using filtering demodulation. Sensitivity of micro-seismic FBG sensor lies on the structure of the sensor shell and the shape of the reflectivity spectrum of FBG. Experiments on the sensor system's frequency character have been done. Results show that this sensor has the smallest acceleration of 1mm/s 2 . The experiments on coal mine exhibit that the sensor can be used to measure micro-seismic sigmals. So the FBG sensor is feasible to have practical use to measure micro-seismic signals because the sensor has high sensitivity and is easy to be fabricated, it has good low frequency response and the interrogation system is simple, low cost.

A Fiber Bragg Grating Sensor System for Train Axle Counting

Railway signaling facilitates two main functions, namely, train detection and train control, in order to maintain safe separations among the trains. Track circuits are the most commonly used train detection means with the simple open/close circuit principles; and subsequent adoption of axle counters further allows the detection of trains under adverse track conditions. However, with electrification and power electronics traction drive systems, aggravated by the electromagnetic interference in the vicinity of the signaling system, railway engineers often find unstable or even faulty operations of track circuits and axle counting systems, which inevitably jeopardizes the safe operation of trains. A new means of train detection, which is completely free from electromagnetic interference, is therefore required for the modern railway signaling system. This paper presents a novel optical fiber sensor signaling system. The sensor operation, field setup, axle detection solution set, and test results of an installation in a trial system on a busy suburban railway line are given.

A Fiber Bragg Grating-Based All-Fiber Sensing System for Telerobotic Cutting Applications

A fiber Bragg grating (FBG)-based strain sensing system for minimally invasive telerobotic cutting applications is presented in this paper. Investigations assume that a scissor blade can be approximated as a uniformly tapered cantilever beam. A replica of the scissor blade is produced and strain characterization has been carried out using an FBG sensor system. Results are validated against measurements obtained using conventional electrical resistance strain gauges. The scissor blade experiences both direct and lateral forces during cutting, hence the system is characterized for a direct load range of 0-30 N and a lateral load range of 0-10 N. The results show a very good linear response for direct loading and some sensitivity to lateral loading. An actual sensorized scissor blade prototype is also characterized and results compared with that of the replica blade. The FBG interrogation system used was a macro-bend fiber filter-based ratiometric system. The use of FBGs together with macro-bend fiber-based interrogation system eliminates the influence of temperature on the sensing system and hence temperature independent strain information from the blade is obtained. The results obtained using the macro-bend fiber filter are compared with that of a commercial interrogation system and found to be in agreement. By implementing an all fiber sensing system based on fiber Bragg gratings and macrobend fiber filter interrogation system, remote operation of telerobotic cutting applications can be made more cost effective while providing a competitive accuracy and resolution solution.

Tunable erbium-doped fiber ring laser with signal-averaging function for fiber-optic sensing applications

This work proposes a novel erbium-doped fiber ring laser as a sensing control center (CC) for long-haul fiber Bragg grating (FBG) sensor systems. The tunable erbium-doped fiber laser with signal averaging functionality not only provides intense and stable laser light, but also suppresses the effect of noise on the system performance. A sample 30/60 km FBG sensor array is connected to the fiber ring laser to demonstrate experimentally the feasibility and effectiveness of the laser as a CC. The experimental results indicate that the signal averaging operation inside the proposed setup increases the system signal-to-noise ratio (SNR).