Fiber Bragg Grating Sensor System Research Articles

Self-Healing Fiber Bragg Grating Sensor System Using Free-Space Optics Link and Machine Learning for Enhancing Temperature Measurement

This research investigates the integration of free-space optics (FSO) with fiber Bragg grating (FBG) sensors in self-healing ring architectures, aiming to improve reliability and signal-to-noise ratio in temperature sensing within sensor systems. The combination of FSO’s wireless connectivity and FBG sensors’ precision, known for their sensitivity and immunity to electromagnetic interference, is particularly advantageous in demanding environments such as aerospace and structural health monitoring. The self-healing architecture enhances system resilience, automatically compensating for failures to maintain consistent monitoring capabilities. This study emphasizes the use of intensity wavelength division multiplexing (IWDM) to manage the complexities of increasing the multiplexing number of FBG sensors. Challenges arise with the overlapping spectra of FBGs when multiplexing several sensors. To address this, a hybrid approach combining an unsupervised autoencoder (AE) with a convolutional neural network (CNN) is proposed, significantly enhancing the accuracy and efficiency of sensor signal detection. These advancements signify substantial progress in sensor technology, validating the effectiveness of the AE-CNN hybrid model in refining FBG sensor systems and underscoring its potential for robust and reliable applications in critical sectors.

Fiber Bragg grating-based flume tests for inversing the impact force coefficient of debris flow

In the piezoelectric sensor-based debris flow flume tests, the impact force coefficient (k) obtained from inversion has a significant dispersion range, which causes considerable uncertainty in the debris flow impact force. To reduce the dispersion range of k, this study designed a new measurement system by integrating the Fiber Bragg Grating (FBG) and the elastic cantilever beam as the core sensing unit. The impact of debris flows on the sensing unit causes the bending deformation of the cantilever beam, which subsequently causes the wavelength shift of the FBG fixed on the cantilever beam. The optical signals are converted into the dynamic strain signals of the cantilever beam using the photosensitive characteristic of the FBG. According to the mechanical characteristics of the cantilever beam and the Bingham model of debris flow, the method of inversing k using the strain signals was proposed. Based on the FBG sensor system, 45 tests were conducted under different working conditions. The inversion model obtained 38 sets of k by analyzing the test data. The standard deviation and dispersion coefficient of k were 0.4 and 0.25, respectively, making the k in the range of 0.9 to 2.8, which was superior to the k derived from the traditional measurement mode.

High-Resolution and Extended-Range Fiber Bragg Grating Sensing System Based on a Microwave Photonic Filter with Period-breaking Interrogation Method

High-Resolution and Extended-Range Fiber Bragg Grating Sensing System Based on a Microwave Photonic Filter with Period-breaking Interrogation Method

Temperature-Insensitive Fiber Bragg Grating Sensing System for Curvature Measurement Based on Optoelectronic Oscillator With the Enhanced Vernier Effect

Temperature-Insensitive Fiber Bragg Grating Sensing System for Curvature Measurement Based on Optoelectronic Oscillator With the Enhanced Vernier Effect

Microwave-photonic Vernier effect enabled high-sensitivity fiber Bragg grating sensors for point-wise and quasi-distributed sensing

This paper reports a sensitivity-improved fiber Bragg grating (FBG) sensor system based on microwave-photonic interferometry and the Vernier effect. An incoherent microwave photonics system based on a broadband light source is employed to interrogate the FBG sensor using the wavelength-to-delay mapping technique combined with interferometry. Specifically, the sensing FBG together with a reference FBG is used to construct a microwave photonics Michelson interferometer (MI). Changes in the Bragg wavelength of the sensing FBG subject to external perturbations are encoded into the spectral shifts of the microwave interferogram of the MI. A virtual interferometer is then generated from the sensing MI based on a computational Vernier effect modality. By superimposing the spectra of the sensing MI and the virtual interferometer, the Vernier effect is generated. By tracking the spectral shift of the Vernier envelope, it is shown that the measurement sensitivity of the sensing FBG is remarkably enhanced with an expected factor. Moreover, a quasi-distributed sensor system with enhanced sensitivity based on cascaded FBGs and the proposed virtual microwave-photonic Vernier effect technique is implemented, representing the first demonstration of a Vernier effect-enhanced FBG array sensor. Additionally, the possibility of employing the harmonic Vernier effect for further sensitivity enhancement is investigated, where a remarkable sensitivity enhancement factor up to 685 with a strain sensitivity of 94 MHz/µε is successfully demonstrated.

Performance assessment of discrete wavelet transform for de-noising of FBG sensors signals embedded in asphalt pavement

In recent years, the Fiber Bragg Grating (FBG) sensor technology has been increasingly utilized as an optical measurement system in various engineering applications, particularly for structural health monitoring (SHM) purposes. This trend can be attributed to the inherent benefits of FBG sensors, such as their small size, immunity to electromagnetic interference, resistance to corrosion, and high accuracy and sensitivity. Various factors cause noise in the FBG sensor signal, which has a significant effect on measurement precision. As a result, de-noising plays an important role in the use of FBG sensor systems. In this study, strain data collected from FBG sensors embedded in a road section were used to evaluate the performance of discretized wavelet transform (DWT) for denoising FBG signals. The presence of noise poses a significant challenge in accurately measuring low-amplitude strains and light loads. To address this issue, various approaches have been investigated, including the selection of appropriate mother wavelets, levels of decomposition, thresholding functions, and thresholding selection approaches, with the aim of identifying the optimal parameters for effective denoising. The results show that FBG signals could be denoised successfully and low amplitude strains appeared completely without any loss of valuable data.

Integrated Sensor-Optics Communication System Using Bidirectional Fiber and FSO Channels and Hybrid Deep Learning Techniques.

This paper introduces a new bidirectional integration approach that combines fiber sensor/free space optics (FSO) communication using an intensity and wavelength division multiplexer (IWDM) techniques-based long-distance fiber Bragg grating (FBG) sensor strain-sensing system. By implementing coarse wavelength division multiplexing (CWDM), the system achieves the simultaneous transmission of optical communication and fiber optical sensor (FOS) sensing signals, resulting in a highly capable, flexible, and cost-effective solution. The proposed FSO transmission technique addresses complex fiber cable installation concerns with topographical limitations. This bidirectional structure ensures the reliability and stability of the long-distance FBG sensor system, supported by extensive research and experimentation. A hybrid stacked gated recurrent units and long short-term memory (SGRU-LSTM) model is proposed to enhance strain measurement accuracy by predicting and measuring the central wavelength of overlapped strain-sensing FBG sensor signals. The results demonstrate the superiority of the proposed model in peak wavelength detection accuracy. The primary benefit of integrating communication and sensing is the significant reduction in construction costs by eliminating the requirement for two individual fiber optic systems, as the integration allows for a single system to fulfill both functions, resulting in more efficient and cost-effective implementation. Overall, this paper contributes to advancing long-distance FBG sensor systems by integrating fiber sensor/FSO communication and deep learning techniques, improving transmission distance, multiplexing capacity, measurement accuracy, system survivability, and cost-effectiveness.

Simultaneous Implementation of the High Reliability and Accuracy in a Fiber Bragg Grating Sensing System for Space Application

The complex space environment puts forward more stringent requirements for the reliability and accuracy of sensors and instrumentation. In this paper, a dual light sources multiplexing fiber Bragg grating (FBG) sensing system with high reliability and accuracy for space application is proposed. The proposed FBG sensing system has two independently driven light sources, which emit scanning laser light into the FBG sensors. The proposed system eliminates single-point failures (SPOF) compared with other system configurations. The five-year single channel reliability and the five-year all channels reliability are 0.9802 and 0.9349 respectively, which are the highest among the five systems. The reliability of the proposed system is improved by 14.1% compared with the common commercial FBG sensing system. Moreover, the experimental systems were established and two FBGs were tested six times on a constant temperature heating stage. The average coefficient of determination (COD) R <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> of FBG1 and FBG2 are 0.9112 and 0.9058 in the proposed system, which are 4.9% and 8.4% higher than those in the common commercial FBG sensing system owing to higher wavelength resolution. Therefore, the proposed dual light sources multiplexing FBG sensing system will provide higher reliability and accuracy performance in space applications.

Frequency Division Multiplexing Fiber Bragg Grating Sensing System Based on a Dual-Loop Optoelectronic Oscillator

Frequency Division Multiplexing Fiber Bragg Grating Sensing System Based on a Dual-Loop Optoelectronic Oscillator

Utilizing a Tunable Delay Line Interferometer to Improve the Sensing Accuracy of an FBG Sensor System

This paper proposes a novel sensing system based on a tunable delay line interferometer. The tunable delay line interferometer has been used to interpret strain, bringing us high accuracy as well as tunability. The shifted wavelength of the fiber Bragg grating (FBG) sensor caused by the applied strain can be visualized by an optical power meter (OPM) instead of an optical spectrum analyzer (OSA) by converting it to a power change using a tunable delay line interferometer (TDI). Different free spectral ranges (FSRs) are assigned to the TDI to investigate the accuracy and operation range of the proposed system. Thus, we achieve high accuracy and sensitivity by adjusting the FSR to 0.47 nm. Experimental results show that the maximum output power variation corresponding to a strain of 10 με is about 0.9 dB when the FSR is set to 0.47 nm. The proposed system is also cost-effective regarding the equipment utilized for interrogation: a tunable delay line interferometer and an optical power meter.

Self-healing integration of fiber/FSO communication and sensor network for improving survivability

In this paper, we proposed a novel self-healing fiber/free-space optics (FSO) communication integrated with a sensor network for simultaneous transmission of optical communication signal and sensing signal as well as a function of fault protection by using a coarse wavelength-division multiplexer (CWDM). The combination of CWDM and fiber integrated with FSO transmission channel is incorporated to design a cost-effective, flexible network and increase the coverage of transmission distance. The experiment result proves that the fiber integrated with an FSO transmission channel achieves a clear constellation diagram and best error vector magnitude (EVM) value. On the other hand, the scanning-laser interrogation system is used for data acquisition in the strain-based fiber Bragg grating (FBG) sensor system. Moreover, the radial basis function neural network (RBFNN) algorithm is used to estimate the central wavelength of FBGs.

Hybrid of Free Space Optics Communication and Sensor System Using IWDM Technique

In this work, we designed a novel hybrid of free-space optics (FSO) communication and intensity and wavelength division multiplexing (IWDM)-fiber Bragg grating (FBG) sensor system over a single shared FSO transmission channel for both optical and sensor signal transmission. In the proposed system, we use one broadband light source and one FSO link for both optical and FBG sensor signal transmission, which significantly reduces the cost of system installation and complexity. Besides, the FSO transmission channel is used to solve the problem of geographical constraints when the environment is difficult to install the fiber. In addition, IWDM is one of the best multiplexing technique to increase the sensor number by more than two times the conventional wavelength division multiplexing (WDM) method. Besides, to solve the overlap spectra problem of IWDM based FBG sensor system, we proposed the stacked gated recurrent unit (SGRU) algorithm with the neural network (NN) profile method. The results prove that the proposed SGRU with the NN profile method improves the accuracy of wavelength detection techniques with the Gaussian profile method. Moreover, the FSO transmission channel achieves a clear eye diagram even the system is a hybrid of optical communication and FBG sensors system.

Highly Sensitive Fiber Bragg Grating Sensing System Based on a Dual-Loop Optoelectronic Oscillator With the Enhanced Vernier Effect

In this paper, a highly sensitive fiber Bragg grating (FBG) interrogation system based on a dual-loop optoelectronic oscillator (OEO) with the enhanced Vernier effect has been proposed and experimentally demonstrated. Light reflected from the FBG sensor is used as the optical source of the OEO. By introducing dispersion in the OEO loop, the sensor information encoded in the FBG wavelength can be interrogated by detecting the frequency of microwave signal generated by the OEO loop. The dispersive media in OEO has two paths, one path is formed with a roll of dispersion compensation fiber (DCF), and the other path is formed with a roll of single-mode fiber (SMF). By designing the length of two paths with slightly difference, the Vernier effect is generated in the OEO. Since the dispersion of the two paths is opposite, an enhanced Vernier effect can be realized, which can significantly enhance the sensitivity. The experimental results prove the possibility of applying the concept of enhanced Vernier effect to the OEO for temperature sensing. The sensitivity is about 0.926 kHz/°C and 0.0912 kHz/°C for a single-loop OEO with DCF and SMF, respectively. By employing the enhanced Vernier effect, the sensitivity can be improved to 141.13 kHz/°C, which is estimated about 152.4 and 1547.5 times higher than that of the two single loops, respectively. Furthermore, sensitivity can be further improved by using a dispersive link with larger dispersion.

Experimental study on thermo-mechanical responses of pre-bored grouted planted piles with different restraint conditions

Geothermal energy piles (GEPs) are an innovation technology of vertical ground heat exchangers (GHXs) which can be used for the heating and cooling of building combined with ground source heat pumps. The main barriers to adoption are the inconvenient installation and a lack of the knowledge regarding the potential effect of temperature changes on pile foundations compared to traditional GHXs. The installation process can be facilitated by using pre-bored grouted planted (PGP) GEPs, which simplify the installation of heat exchange pipes and hence reduce the installation time. It is desirable to investigate the thermo-mechanical responses of this GEP for understanding the potential effects of the temperature changes.A PGP GEP was installed in a building during construction that incorporated two heating tests with different conditions (a short-term Test 1 without building load, and a long-term Test 2 with strong structural restraints and building load) to investigate the behavior of the PGP GEP. A fiber Bragg grating (FBG) sensor system was used to evaluate temperature and strain behaviors of the test pile and two adjacent boreholes. The results show that the nonuniform spatio-temporal response of temperature induced the nonuniform thermal-induced stress along the pile depth. The end-restraints had significant effects on the thermo-mechanical response over the full pile. A comparison between the PGP GEP and other types of GEPs were conducted, and the similarities and differences between them were discussed.

Development of a Fault Detection Instrument for Fiber Bragg Grating Sensing System on Airplane.

This study develops a fault detection device for the fiber Bragg grating (FBG) sensing system and a fault detection method to realize the rapid detection of the FBG sensing system on airplanes. According to the distribution of FBG sensors on airplanes, the FBG sensing system is built based on wavelength division multiplexing (WDM) and space division multiplexing (SDM) technologies. Furthermore, the hardware and software of the fault detection device and the relevant FBG demodulator are studied in detail. Additionally, in view of the similar features of the healthy FBG sensor in the same measuring point, a rapid fault diagnosis method based on a synthetical anomaly index is proposed. The features (light intensity I, signal length L, standard deviation of original sample and energy value in time-domain P) of FBG sensors are extracted. The aggregation center value of the above feature values is obtained through the loop iteration method. Furthermore, the separation degrees of features are calculated and then form the synthetical anomaly index so as to make an effective diagnosis of the state of the FBG sensor. Finally, the designed fault detection instrument and proposed fault detection method are used to monitor the 25 FBG sensors on the airplane, the results indicated that three faulty and two abnormal FBG sensors on the airplane are identified, showing the effectiveness of the proposed fault detection method.

Pico-strain resolution fiber-optic sensor with white-light interferometry.

The fiber Bragg grating (FBG) sensor is well known for simple fabrication, absolute measurement, inherent multiplexing capability, etc. To date, most FBG sensors that use a broadband light source for demodulation can only achieve resolution at the µɛ level. In this Letter, we propose a white-light-driven self-reference sensing system with FBGs, using a Mach-Zehnder interferometer (MZI) and a white light source, to realize multiplexed high-resolution vibration sensing with a very simple system configuration. A strain resolution of 35p ε/Hz at 1 kHz is demonstrated, which is several orders of magnitude better than the current FBG sensor systems with white light sources. The performance of the sensing system is analyzed, and multiplexing capability is also experimentally evaluated; there is no observable cross talk.

Event Recognition System Based on Fiber Bragg Grating and mRMR-CWCs-SCN

Traditional algorithm lacks the ability to generalize across different application scenarios and how to set the stopping conditions of network iteration in Stochastic Configuration Network (SCN) is a problem. This paper proposes an event recognition system based on minimum redundancy, maximum relevance (mRMR) and a modified Stochastic Configuration Network (CWCs-SCN) for the fiber Bragg grating (FBG) sensor system. mRMR is used to analyze the importance of 14th time-frequency domain features in different application scenarios. After that, a modified SCN (CWCs-SCN) neural network is used to solve the stopping condition of the network iteration. The experiments demonstrated that the average recognition accuracy of four kind of human intrusion signal can reach to 96.7%, which is better than others traditional methods. In addition, a more compact neural network is obtained compared to SCN. To verify the generalization ability of the proposed model, it is applied to gear fault diagnosis with an accuracy of 96.3%. This imply that the proposed method in this paper has a wide application prospect.

Application of Fiber Bragg Grating Sensor System for Simulation Detection of the Heart Rate

Fiber Bragg grating (FBG) is also widely used to detect the heart because it can be received in the form of pressure that results in changes in strain. FBG was chosen because it has a high sensitivity to strain. Heart rate detection can be done in several ways such as using a telescope, but using FBG has a high degree of accuracy and is sensitive to strain. Heart detection is still developing because of the serious problem with human life behavior so that efforts are needed to find other ways to more easily detect the heart. Currently, heart detection can be done without having to go to the hospital, such as by using a cellphone, watch, and others. This study aims to design and measure changes in the output power of FBG and to analyze the effect of strain change on FBG by loudspeaker vibration. The Bragg wavelengths used are 1310 nm and 1550 nm with a power of 1 mW as a diode laser source and the output is measured by an optical power meter. The highest change in output power at a wavelength of 1310 nm Bragg is equal to 0.471 μW, while at a wavelength of 1550 nm it is equal to 0.032 μW. The highest shift of the Bragg wavelength is at the Bragg 1310 nm wavelength, which is 0.598 nm, while the Bragg wavelength of 1550 nm is 0.552 nm. The highest change in strain was at 1310 nm Bragg wavelength valued at 576.186 με, while at 1550 nm Bragg wavelength was 432.113 με. This shows that the response at the Bragg wavelength of 1310 nm is more sensitive than the Bragg wavelength of 1550 nm.

Reliable self-healing FBG sensor network for improvement of multipoint strain sensing

In this paper, we proposed and designed reliable self-healing fiber Bragg grating (FBG) sensor network for improvement of multipoint strain sensing. We use an optical switch (OSW) to reconfigure the sensor network and improve the self-healing function during fault happen in the sensor network. In this work, to prove and validate the detection performance of our proposed Bragg wavelength detection technique even when the number of overlap spectra of FBGs increases, we conduct three experiments and collect three different cases of strain data (namely case 1, case 2, and case 3) by applying different strain steps to FBGs. In case 1 experiment, strain was applied to only FBG1 sensor, while the other four FBGs keeps fixed. In case 2 experiment, strain was applied to FBG1 and FBG2 sensors at the same time and with different strain steps. In case 3 experiment, strain was applied to FBG1, FBG2 and FBG3 sensors at the same time and with different strain steps. As a result, in all cases of the experiment, three situations of spectra were introduced between FBGs like non-overlapped, partially overlapped and completely overlapped spectra. To solve this overlap problem, we used deep learning technique to accurately identify the Bragg wavelength of FBGs in the condition of the partially or fully overlapped spectra. Therefore, our proposed FBG sensor system can improve the reliability and detection accuracy of the sensor system even the number of overlaps FBGs spectra increases.

Large dynamic-range fiber Bragg grating sensor system for acoustic emission detection.

A distributed feedback (DFB) fiber laser and fiber Bragg gratings (FBGs) are configured to demodulate the wavelength shifts of FBG dynamic strain sensors. The FBG sensors act as sensing units to detect the dynamic strain and the demodulators while the DFB fiber laser only acts as a narrow-linewidth light source. As the reflective spectrum of the FBG sensor changes due to dynamic strains, the output is subsequently converted into a corresponding intensity change and detected directly by a photodetector. The 0.2 nm linewidth FBG sensor can detect the impact signal with a frequency of up to 300 kHz with a maximum of 29.17 µɛ, which is comparable with the detecting result of the piezoelectric transducer sensor. Moreover, the directional response of the FBG sensor is maximized when the direction of acoustic wave propagation is parallel to the optical fiber. The relation between the sensitivity and the FBG spectrum linewidth is presented, and the detectable strain range versus different FBG linewidths is also discussed.