Multiple Fiber Bragg Grating Research Articles

Concatenated Bragg grating fiber-optic sensors for simultaneous measurement of curvature, temperature, and axial pressure

Abstract. This paper presents the development and evaluation of four sensors based on multiple fiber Bragg grating (FBG) constellations embedded in a silicon dioxide single-mode fiber (SMF) for simultaneous measurement of pressure, temperature, and bending curvature. We applied dimension and material variations – including core, cladding, and coating dimensions; coating material; and the number and arrangement of the FBGs – to optimize the reflected signal response and increase information density. A bootstrap-aggregated ensemble of decision trees was used to evaluate the sensor signal. The results show that adjusting the cladding-to-coating ratio led to significant improvements in pressure and bending prediction performance. Additionally, two combined FBGs were fabricated to form a fiber Bragg grating Fabry–Pérot interferometer, which enabled the detection of curvature with a root-mean-square error (RMSE) of 0.0034 L mm−1 (R2=1), axial pressure with an RMSE of 0.0564 bar (R2=0.99), and temperature with an RMSE of 0.0265 °C (R2=1). At the time of writing, there is no commercially available instrument that can perform these measurements simultaneously.

Shape and Hardness Perception of Robot Soft Finger Based on Fiber Bragg Grating

Due to the increasing demand for tactile sensing of robotic hands, this paper designs a soft robot hand that can realize object shape recognition and hardness detection. Taking advantage of the high sensitivity and wavelength division multiplexing of Fiber Bragg Gratings (FBG), a distributed detection method using a single fiber in series with multiple FBG is proposed, which can demodulate bending and pressure at the same time, reduce wiring and improve efficiency.

Experimental Demonstration of Peak Wavelength Measurement of Multiplexing Fiber Bragg Gratings Using Convolutional Neural Network

We propose a peak detection method for measuring fiber Bragg gratings (FBGs) using convolutional neural network (CNN) to improve the performances of wavelength division multiplexing. In wavelength division multiplexing, each FBG occupies a certain wavelength range; therefore, the number of FBGs that can be installed is limited by the wavelength band of the light source. To address this issue, methods for overlapping multiple FBGs of the same wavelength within a single occupied wavelength range have been studied. This contributes to improving the limit of multipoint FBGs manifold. However, this method results in the complex overlapping of multiple FBG reflectance spectra making it difficult to accurately measure the peak wavelengths of individual FBGs using conventional peak detection methods. Therefore, we developed a peak detection process using CNN, which is suitable for identifying unique feature data. Each FBG of the same wavelength was characterized to have a unique spectral shape by assigning a different full width at half maximum to each. We introduced noise-additive learning, a well-known method of data augmentation that increases tolerance to variations in the experimental signal. As a result, the standard deviation for peak wavelength detection significantly improved to 2.8 pm and the strain measurements with three complex overlapping FBGs were successfully demonstrated. The CNN model is the first to solve the problem of three overlapping FBGs for arbitrary wavelength changes. Furthermore, the developed peak detection process was found to be applicable to measurements that combined multiplexing of FBGs of either identical or different wavelengths.

A Microwave Photonics True-Time-Delay System Using Carrier Compensation Technique Based on Wavelength Division Multiplexing

A novel microwave photonic true-time-delay (TTD) system using carrier compensation technology is proposed and experimentally demonstrated. Wavelength division multiplexing combines ten lasers into a single beam. We separate one channel from the laser as a compensating carrier, and the compensation carrier is combined with the time-delayed optical signals to be detected. Meanwhile, sideband signals are amplified effectively thanks to carrier-suppressed double-sideband (CS-DSB) modulation. Therefore, the power of both the central optical carriers and sidebands is guaranteed, which produces a better beat frequency result than the TTD system without carrier compensation. The simulation results confirm that the signal amplitude has an order of magnitude improvement due to the compensation. With employing the delay fibers based on multiple-fiber Bragg gratings (MFBGs), the experimental delay and response time reach 90.160 μs and 160.80 ns. The proposed technique can find applications in time-delay beamforming of phased array antennas (PAAs).

Laser-Based Optical Wireless Communications for Internet of Things (IoT) Application

Internet of Things (IoT) is enabled by the integration of communication and sensor systems that are used to collect important information from objects around the world. In this article, we proposed the integration of laser-based optical wireless communication (OWC) with a fiber sensor system for IoT applications. OWC can be utilized in a harsh environment to send and receive communication and sensor signals in a wireless means. Moreover, fiber sensors are important to achieve real-time, accurate, and smart monitoring in IoT applications. As compared to mechanical and electrical sensors, optical fiber sensors have numerous benefits, including the tolerance to electromagnetic interference (EMI), small size, lightweight, high bandwidth, low cost, provide distributed sensing, high sensitivity, and electronic devices are not required at the sensor point. Moreover, we proposed a deep variational autoencoder (DVAE) model to estimate the strain changes and peak wavelength of multiple fiber Bragg grating (FBG) sensors using only the spectrum of FBGs obtained from the real experiment. The result showed that our proposed DVAE model can estimate the strain changes and peak wavelength of FBGs with low root mean squared error (RMSE) and high estimation accuracy. Furthermore, the performance of the proposed integration of OWC with the fiber sensor system achieves a clear eye diagram and excellent bit error rate (BER) curve.

Submarine Optical Fiber Sensing System for the Real-Time Monitoring of Depth, Vibration, and Temperature

Ocean observation becomes increasingly important as the ocean climate changes diversely and the marine disasters (such as tsunamis, typhoon, and earthquakes) occur frequently, which typically requires widespread and reliable monitoring techniques. In such a scenario, this paper presents a submarine optical fiber sensing system to realize real-time monitoring of the environmental parameters. The system consists of an undersea optical interrogation module together with multiple fiber Bragg grating (FBG)-based sensors, particularly for the measurement of depth, vibration, and temperature. The experimentally demonstrated sensitivities of the pressure, temperature, and vibration sensors are -1.993 nm/MPa, 0.08 nm/°C, and 0.139 nm/g (g = 9.8 m/s2), corresponding to the resolutions of 0.25 kPa, 0.006°C, and 0.004 g, respectively, based on the interrogation resolution of ~0.5 pm. To verify the feasibility and reliability of the proposed submarine sensing system, a prototype was developed and a proof test under the sea was conducted in an area close to Pearl River Estuary in China. The achieved results from the sea test show promising accuracy that is comparable to the commercially available electric-based sensors. Good characteristics of the surface water wave were observed by conducting the fast Fourier transform of the measured depth change, which shows a dominant frequency of ~0.25 Hz. The system provides the flexibility of replacing various optical fiber sensors easily and the capability of real-time monitoring in a remote way. The demonstrated submarine sensing system could find potential applications in real-time monitoring of the undersea ecosystem and the environmental evolution where multiparameter sensing is in demand.

Multiple fiber Bragg grating sensing system based on active mode-locking fiber laser

In this paper, a new multiple fiber Bragg grating (FBG) sensing system based on active mode-locking fiber laser is proposed. The active mode-locking fiber laser is composed of a linear cavity with an Fabry–Perot laser diode (F-P LD) as a reflective cavity mirror, and the erbium-doped fiber as the gain medium. Meanwhile, the F-P LD is also used as the modulating element of the fiber laser. Multiple FBGs cascaded in a long fiber are used as both the sensors in the system and the components for wavelength selection in the active mode-locking fiber laser. The capacity of the proposed sensing system to interrogate multiple FBGs in wavelength and spatial domain is investigated. The proposed sensing system has the characteristics of low cost, good stability, good compatibility, and can be used in quasi-distributed multi-point sensing.

On the feasibility of using an optical fiber Bragg grating array for multi-point dose measurements in radiation therapy

Fiber Bragg gratings (FBGs) have proven to be a valuable dosimeter in nuclear environment where radiation doses reach up to a hundred of kiloGray (kGy). Multiple FBGs can be written in a single fiber to allow multi-point detection which would prove very useful for radiotherapy dosimetry. The purpose here is to adapt this already existing technology to provide a novel dosimeter for radiotherapy measurements. The proposed real-time dosimeter consists of twenty 4 mm-long FBGs, equally distributed over 20 cm. FBGs are written through the coating of a standard polyimide-coated silica fiber with the phase-mask technique and femtosecond pulses. The wavelength dependant variation of each FBG is recorded at 1 kHz with a commercially available interrogator. The use of gamma radiation (clinical radiotherapy accelerator) induces a linear shift (0.070 ± 0.006 pm/Gy) of the FBG’s reflected wavelength, which is independent of the dose rates (2.8-11.6 Gy/min) and the energy (6-23 MV). A statistical error of 0.03 pm is obtained on data points therefore limiting the detectable dose to 0.4 Gy. A dose profile of 6 and 23 MV radiotherapy accelerator is also measured. The presented FBGs dosimeter allows for real-time dose measurement in 2D and the small size of its detector makes it a versatile tool. The length and spacing of FBGs can be easily modified to increase both the spatial resolution and the amount of dose point.

A combined global-local approach for delamination assessment of composites using vibrational frequencies and FBGs

In this paper, a combined global-local strategy was developed for structural health monitoring (SHM) of fiber reinforced polymer (FRP) composites to assess the internal delamination damage, which has combined vibrational frequency shifts as the global index and the fiber Bragg grating (FBG) wavelength shifts as the local index. The delamination detection was carried out in two steps. The first step was based on the global damage index, which is the changes in multiple modes of frequencies measured by a non-contact scanning laser Doppler vibrometer (SLDV) system. Machine learning (ML) algorithms including support vector machine (SVM) and extreme learning machine (ELM) algorithms were used to initially predict the delamination interface, location, and size through frequency shifts. Numerical and experimental verification results show that SVM has a higher prediction accuracy than ELM when only a small number of samples exist, and the classification of SVM is better than its regression function to be used in the prediction of discrete delamination interfaces. To verify the damaged area predicted by frequency changes and further update the delamination edges with better accuracy, the second step has taken the changes in the wavelength of multiple FBG sensors as the local damage index. Multistage loads were applied at equidistant positions of the FRP specimens to induce deformations and the wavelength shifts in FBGs were used to determine the boundary of delamination. The results showed that delamination in FRP composites can be assessed more precisely using the global-local two-step SHM strategy, which can compensate for the shortcomings of only using vibration-based or FBG-based monitoring techniques. Besides, the SVM algorithm showed excellent predictive performance in both steps and had great potential in delamination damage monitoring.

High-Peak Power Frequency Modulation Pulse Generation in Cascaded Fiber Configurations with Inscribed Fiber Bragg Grating Arrays

We explored the dynamics of frequency-modulated (FM) pulses in a cascaded fiber configuration comprising one active and one passive optical fiber with multiple fiber Bragg gratings (FBGs) of different periods inscribed over the fiber configuration length. We present a theoretical formalism to describe the mechanisms of the FM pulse amplification and pulse compression in such fiber cascades resulting in peak powers up to ~0.7 MW. In combination with the decreasing dispersion fibers, the considered cascade configuration enables pico- and sub-picosecond pulse trains with a sub-terahertz repetition rate and sub-kW peak power generated directly from the continuous optical signal.

Multi-peak detection algorithm based on wavelength feature recognition in FBG sensor networks

We propose an improved Gaussian curve fitting method based on the Hilbert transformation (HTG) to tackle the ineffectiveness of the traditional peak-seeking algorithm in detecting the multi-peak Fiber Bragg grating (FBG) reflection spectra. A five-point sliding filter is used to process the FBG reflection spectral signal, de-noise and smooth the noise, and select the optimal threshold point by the Hilbert transformation (HT). The sub-spectra of the multiple FBG reflection spectral signals were derived and the initial positioning of the spectral peaks were achieved. The Levenberg–Marquardt (LM) algorithm is used to extract the Bragg wavelength from the segmented sub-spectral signals as well as optimize the Gaussian curve fitting coefficients. The HTG-LM algorithm is then proposed, and is optimized and utilized to achieve precise positioning of the spectral peaks. The theoretical analysis and experimental results showed that the proposed HTG-LM algorithm could dynamically detect the multiple reflection spectra of the FBG sensing system with good stability, and at the same time, reduce the amount of peak-seeking data, which is highly beneficial to improve the signal demodulation rate. The peak detection accuracy of the proposed algorithm is better than 1 pm and the precision is better than 4 × 10 − 7 pm, which indicates that this HTG-LM algorithm provides an accurate demodulation algorithm for the FBG sensor networks. As a result, it is a promising multi-peak detection algorithm proposed by this paper to be applied to the FBG sensing systems.

An accessible method of embedding fibre optic sensors on lithium-ion battery surface for in-situ thermal monitoring

Presented is a proof of concept to evaluate the suitability of a mounting technique for fibre optic sensors (FOSs) to measure Lithium-ion Battery (LIB) temperature. Two mounting techniques for the optical fibre, i.e. containing multiple Fibre Bragg Gratings (FBGs), placed on the LIB are considered. The FOS results are compared with K-type thermocouples, which are also mounted on the LIB. The first mounting technique attaches the FBGs on to the battery surface using a binding agent, while for the second technique, a “guide-tube” is used for alleviating longitudinal strain in the FOS induced by LIB expansion without the complexity of metal or glass tubes typically used for signal decoupling. The first mounting technique is calibrated over a 40 °C temperature variation to derive the sensitivity factor for each FBG; the other technique uses the free fibre sensitivity obtained by the manufacturer. The LIB is subject to 10 Watts constant power (CW) discharge, inducing 14 °C surface temperature increase. The guide-tube technique proposed in this study improves measurement accuracy from± 4.25 °C to + 2.06 °C when both mounting techniques are directly compared to the thermocouples. The proposed mounting technique is found to be a feasible method for cell-level temperature monitoring applications.

Simultaneous Measurement of Vibration and Temperature Using Frequency-Scanned Parallel Phase-Shifting Interferometry

In order to simultaneously detect high-frequency dynamic parameters and low-frequency quasi-static parameters through spectrum acquisition, a novel frequency-scanned parallel phase-shifting interferometry is proposed in this work. A Michelson interferometer (MI) composed of a 3 × 3 coupler is used for high-frequency vibration measurement. Different from conventional 3 × 3 fiber coupler based demodulation methods, a frequency swept laser source with flat intensity output is used. The time-varying vibrations will be encoded on the phase shifted spectra measured at two return ports of the coupler. Using a modified phase shifting algorithm, the optical path difference (OPD) changes along optical frequency can be retrieved, thereby significantly improving the sampling rate of the dynamic MI. Thanks to the full-spectrum scanning, the asymmetric parameters of non-ideal 3 × 3 couplers can be directly measured. Moreover, it provides the possibility of spectral demodulation of multiple quasi-static parameters. Another sensing channel containing multiple fiber Bragg grating (FBG) sensors is used to achieve quasi-distributed temperature measurement. Experimental results demonstrate its effectiveness in dynamic/static multi-parameter measurement.

Multiple Cladding Fiber Bragg Gratings Inscribed by Femtosecond Laser Point-by-Point Technology

Femtosecond laser point-by-point (PbP) technology can be used as a direct writing method to inscribe fiber Bragg gratings (FBGs) into the cladding of optical fibers. When the refractive index modulated area of cladding FBGs (CLFBGs) is close to the corecladding interface, the CLFBGs exhibit characteristics similar to conventional core-based FBGs. When bending was applied, the reflected power of the CLFBGs changed significantly and showed an orientation dependence on the fiber bending direction. The CLFBG inscription method provides a new opportunity to fabricate vector sensing devices, which can be realized by parallelly inscribed multiple CLFBGs. Compact CLFBGS with a low insertion loss (less than 0.05 dB) have application value in the fields of optical fiber sensors, communications, and lasers.

Curvature, twist and pose measurements using fiber Bragg gratings in multi-core fiber: A comparative study between helical and straight core fibers

This paper presents a technique to acquire measurements of curvature, twist and pose for two multi-core fibers; one with straight cores and the other with helical cores. Both the fibers have multiple fiber Bragg grating (FBG) sensors inscribed in the cores and the fibers are placed in known configurations in order to compare their measurement accuracy. For the curvature measurements both the fibers are placed in constant curvature slots; for the twist measurements, a set of twists are applied to each fiber and for the pose measurements the fibers are placed in molds of different shape. The mean curvature errors are 0.22 and 0.13 m−1, in the helical and straight core fiber respectively. For the twist measurement the mean errors are 26.57°/m and 146.50°/m in the helical and straight core fiber, respectively. Lastly, the pose measurement consists of position and orientation where the orientation is represented in the axis-angle form. The mean position errors are 0.49 and 0.27 mm, the mean axis orientation errors are 0.12° and 0.26° degrees and the mean angle orientation errors are 1.10° and 1.18°, for the helical and straight core fiber, respectively. The results show that the twist measurement error is significantly low with the helical core fiber, thus helical core fiber is better suited than straight core fiber for applications where twist measurements are required.

Development of real‐time vibration measurement system by fiber Bragg gratings with pulse light

AbstractWe developed a vibration measurement system by fiber Bragg gratings (FBGs) with a pulse light. The pulse light is produced by modulating a broadband light with an optical fiber switch at a modulation frequency of 40 kHz. This system enables to separate and detect reflection signals from multiple FBGs like a pulse train. This system achieves multipoint and real‐time vibration measurement by FBGs with a time resolution of 25 µs.

A multi-peak detection algorithm for Fiber Bragg Grating sensing systems

Abstract Aiming at the problem that traditional peak-seeking algorithms cannot directly detect multiple reflections of Fiber Bragg Grating (FBG) sensing systems, this paper proposes a multi-peak detection algorithm. A seven-point moving average filter is used to process the FBG reflection spectral signal, reduce the high-frequency noise in the spectral signal, and determine the optimal segmentation spectral threshold point through Hilbert transform. The derived spectral signal obtains multiple FBG reflection spectral signal’s sub-spectra and achieves the initial positioning of the spectral peak. The Levenberg-Marquardt (LM) algorithm is used to extract the Bragg wavelength from the segmented sub-spectral signal, as well as optimizing the Gaussian curve fitting coefficients. The optimized Gaussian-LM algorithm is then utilized to achieve precise positioning of the spectral peak. The influence of different spectral segmentation thresholds on FBG peak-seeking accuracy was analyzed. Theoretical analysis and experimental results showed that the proposed algorithm could dynamically detect multiple reflection spectra of the FBG sensing system with good stability while reducing the amount of peak-seeking data, which is highly beneficial to improve the signal demodulation rate. The average error of the algorithm is 4.34 pm, i.e., it has high accuracy compared to the comparison algorithm. As a result, it is shown that the multi-peak detection algorithm proposed in this paper can be applied to the distributed FBG sensing systems.

Cold Crack Monitoring and Localization in Welding Using Fiber Bragg Grating Sensors

A study on the design, framework, experimental arrangement, data acquisition, and subsequent analysis of cold crack detection using fiber Bragg grating (FBG) sensors during welding is presented. Cold crack induced acoustic emission (AE) events are detected by multiple FBG sensor configurations and using a commercial FBG AE interrogation system. The crack locations are theoretically predicted using a method of linear localization based on the differences in the time of arrival of the AE events at different FBG sensors. The approach is experimentally validated by employing packaged and unpackaged FBG sensor arrays in a welding experiment with gas tungsten arc welding on ultrahigh toughness high hardness armor steel. Frequency analysis of the AE events is also carried out to understand the cold crack characteristics. The weld region of the steel plate is diced into 1-mm cross-sectional pieces, and the positions of the cold cracks are identified using scanning electron microscope imaging and then cross correlated with the measured AE events, and thereby confirming the correct identification of cold crack location using the FBG sensing method.

Low-cost multiple FBG interrogation technique for static applications.

In this Letter, a low-cost interrogation technique for multiple fiber Bragg gratings (FBGs) is demonstrated. The design utilizes the major advantage of the distributed feedback lasers, which is tuning their emitted wavelength by current modulation and temperature variation. The sensitivity of this method and the influence of optical losses on the measurement have been carefully studied, and an accuracy of 5 pm is achieved.

External Corrosion Detection of Oil Pipelines Using Fiber Optics.

Oil flowlines, the first “pipeline” system connected to the wellhead, are pipelines that are 5 to 30.5 cm (two to twelve inches) in diameter, most susceptible to corrosion, and very difficult to inspect. Herein, an external corrosion detection sensor for oil and gas pipelines, consisting of a semicircular plastic strip, a flat dog-bone-shaped sacrificial metal plate made out of the same pipeline material, and an optical fiber with Fiber Bragg Grating (FBG) sensors, is described. In the actual application, multiple FBG optical fibers are attached to an oil and gas pipeline using straps or strips or very large hose clamps, and, every few meters, our proposed corrosion detection sensor will be glued to the FBG sensors. When the plastic parts are attached to the sacrificial metals, the plastic parts will be deformed and stressed; thus, placing the FBG sensors in tension. When corrosion is severe at any given pipeline location, the sacrificial metal at that location will corrode till failure and the tension strain is relieved at that FBG Sensor location, and therefore, a signal is detected at the interrogator. Herein, the external corrosion detection sensor and its design equations are described, and experimental results, verifying our theory, are presented.