Fiber Bragg Grating Array Research Articles

Fiber Bragg Grating Sensors for Performance Evaluation of Fast Magnetic Resonance Thermometry on Synthetic Phantom.

The increasing recognition of minimally invasive thermal treatment of tumors motivate the development of accurate thermometry approaches for guaranteeing the therapeutic efficacy and safety. Magnetic Resonance Thermometry Imaging (MRTI) is nowadays considered the gold-standard in thermometry for tumor thermal therapy, and assessment of its performances is required for clinical applications. This study evaluates the accuracy of fast MRTI on a synthetic phantom, using dense ultra-short Fiber Bragg Grating (FBG) array, as a reference. Fast MRTI is achieved with a multi-slice gradient-echo echo-planar imaging (GRE-EPI) sequence, allowing monitoring the temperature increase induced with a 980 nm laser source. The temperature distributions measured with 1 mm-spatial resolution with both FBGs and MRTI were compared. The root mean squared error (RMSE) value obtained by comparing temperature profiles showed a maximum error of 1.2 °C. The Bland-Altman analysis revealed a mean of difference of 0.1 °C and limits of agreement 1.5/−1.3 °C. FBG sensors allowed to extensively assess the performances of the GRE-EPI sequence, in addition to the information on the MRTI precision estimated by considering the signal-to-noise ratio of the images (0.4 °C). Overall, the results obtained for the GRE-EPI fully satisfy the accuracy (~2 °C) required for proper temperature monitoring during thermal therapies.

Bandwidth-Enhanced Quasi-Distributed Acoustic Sensing With Interleaved Chirped Pulses

In this letter, to break through the response bandwidth limit of quasi-distributed acoustic sensing system, a novel interleaved chirped pulses (ICP) approach is proposed on the basis of pulse compression ${\Phi }$ -OTDR with coherent detection. As shown in the theoretical analysis, for ultra-weak fiber Bragg grating (UWFBG) array with fixed positions and intervals, the space between any two FBGs can be used to multiplex probing pulses, without occupying extra detection bandwidth. The cross-correlation peak of probe pulses can be separated from each other in the time domain with matched filtering. By injecting probe pulses which are evenly interleaved as positive-chirped and negative-chirped, crosstalk of the cross-correlation peaks can be suppressed. In the proof-of-principle experiment, the response bandwidth is 3 times as that of the traditional single pulse probing scheme, all with the same detection bandwidth; 860 m sensing range with 166.4 kHz response bandwidth has been achieved, with 5 m spatial resolution and 1.8 p ${\varepsilon /\sqrt {Hz}}$ strain sensitivity.

Femtosecond Pulse Structuring of Multicore Fibers for Development of Advanced Fiber Lasers and Sensors

In this paper we investigate the fiber Bragg grating (FBG) arrays selectively inscribed in a multicore fiber for a different sensor and laser applications. Particularly, wavelength-switchable and tunable fiber laser was realized based on uniform and non-uniform FBGs precisely positioned in the selected cores. A quasi-distributed 3D shape sensor based on FBG array inscribed in a multicore fiber with helically twisted side cores was fabricated and applied for shape reconstruction of papillotome.

High Speed, Localized Multi-Point Strain Measurements on a Containment Vessel at 1.7 MHz Using Swept-Wavelength Laser-Interrogated Fiber Bragg Gratings

Dynamic elastic strain in ~1.8 and 1.0 m diameter containment vessels containing a high explosive detonation was measured using an array of fiber Bragg gratings. The all-optical method, called real-time localized strain measurement, recorded the strain for 10 ms after detonation with additional measurements being sequentially made at a rate of 1.7 MHz. A swept wavelength laser source provided the repetition rate necessary for such high-speed measurements while also providing enough signal strength and bandwidth to simultaneously measure 8 or more unique points on the vessel’s surface. The data presented here arethen compared with additional diagnostics consisting of a fast spectral interferometer and an optical backscatter reflectometer to show a comparison between the local and global changes in the vessel strain, both dynamically and statically to further characterize the performance of the localized strain measurement. The results are also compared with electrical resistive strain gauges and finite element analysis simulations.

Electric Machine Bearing Health Monitoring and Ball Fault Detection by Simultaneous Thermo-Mechanical Fibre Optic Sensing

This article reports the use of fiber Bragg grating (FBG) sensors for electric machines' in-service bearing condition monitoring. The proposed method enables simultaneous extraction of thermal and mechanical information on the bearing operational status for health monitoring and fault diagnosis purposes. The work first reports the instrumentation, calibration and the measurements interpretation methods of an FBG array sensor fitted to the drive-end bearing of an operating inverter driven induction motor. Experimental tests are then undertaken on a purpose build test rig to evaluate the proposed in-situ sensing scheme's performance under healthy and faulted bearing conditions. The results demonstrate that the measurements acquired by individual FBG sensors fitted in the bearing architecture contain concurrent thermal and mechanical information, which can be clearly differentiated to enable understanding of the examined motor bearing's thermal only and mechanical only operating conditions. Furthermore the in-situ measurements obtained in fault conditions are shown to contain clearly defined time and frequency domains fault signatures, which can enable unambiguous fault diagnosis and trending.

Application of Fibre Bragg Grating Sensors in Strain Monitoring and Fracture Recovery of Human Femur Bone

Fibre Bragg Grating (FBG) sensors are gaining popularity in biomedical engineering. However, specific standards for in vivo testing for their use are absolutely limited. In this study, in vitro experimental tests were performed to investigate the behaviors and applications of gratings attached to intact and fractured thighbone for a range of compression loading (<300 N) based around some usual daily activities. The wavelength shifts and the corresponding strain sensitivities of the FBG sensors were measured to determine their effectiveness in monitoring the femoral fracture healing process. Four different arrangements of FBG sensors were selected to measure strains at different critical locations on the femoral sawbones surface. Data obtained for intact and plated sawbones were compared using both embedded longitudinal and coiled FBG arrays. Strains were measured close to the fracture, posterior linea aspera and popliteal surface areas, as well as at the proximal and distal ends of the synthetic femur; their responses are discussed herein. The gratings on the longitudinally secured FBG arrays were found to provide high levels of sensitivity and precise measurements, even for relatively small loads (<100 N). Nevertheless, embedding angled FBG sensors is essential to measure the strain generated by applied torque on the femur bone. The maximum recorded strain of the plated femur was 503.97 µε for longitudinal and −274.97 µε for coiled FBG arrays, respectively. These project results are important to configure effective arrangements and orientations of FBG sensors with respect to fracture position and fixation implant for future in vivo experiments.

An Enhanced Distributed Acoustic Sensor With Large Temperature Tolerance Based on Ultra-Weak Fiber Bragg Grating Array

An enhanced distributed optical fiber sensor based on broadband ultra-weak fiber Bragg grating array (UWFBG) for dynamic strain measurement with large temperature tolerance range was proposed and experimentally demonstrated. The reflectivity of UWFBG is about -50 dB, and the intensity of the reflected signal is 17 dB higher than the Rayleigh back-scattering. Using an unbalanced Michelson interferometer (MI) and 3 × 3 coupler phase demodulation technique, multiple vibration events can be detected simultaneously. We experimentally demonstrated the effectiveness of the technique by monitoring dynamic strain with a large temperature range of 20-200 °C. The minimum phase detection of the system is 1.02 × 10 - 3 rad/√Hz, and the signal-to-noise ratio (SNR) is above 59.2 dB @100 Hz at different temperatures. Meanwhile, the system has a wide frequency response from 10 Hz to 4.5 kHz.

A Coherent Optical Frequency-Domain Reflectometer Based on a Self-Sweeping Fiber Laser for Sensing Applications

The first results are presented that demonstrate the applicability of a coherent optical frequency-domain reflectometer based on a self-sweeping fiber laser for sensing. An array of various fiber Bragg gratings (FBGs) was used as the sensing line. It is shown that FBGs with a reflection spectrum that lies outside the scan region of ​​our source can be used in the developed reflectometer. Using the frequency-domain reflectometry technique, it is possible to perform spatial separation of FBG sensors, even with the use of the same reflection wavelengths. The possibility of measuring the temperature has been experimentally demonstrated.

Fiber optic measurement system for determination of extended object position and bends in 3D space

Subject of Research. Fiber optic measurement systems are widely used in various industries. Most of these systems are fiber optic devices for detecting physical quantities. The development and creation of a measurement system for determination of the location and bends of extended objects in 3D space are actual at the moment. We propose the implementation of the sensitive part of the fiber optic measurement system based on seven single-mode fibers with an array of fiber Bragg gratings fixed to each other. Method. The method for determining the position and bends of extended objects in space is based on finding the level of axial deformation (compression, tension) of the fiber Bragg gratings during bending, and provides the calculation of the curvature direction and magnitude. Further reconstruction of the curve in space is realized by solving a system of differential equations with given initial conditions containing Frenet-Serre formulas. Main Results. The paper presents experiment results on writing fiber Bragg grating arrays into single-mode optical fibers, taking into account the features of the optical scheme and the source spectrum. The design and prototype of the fiber optic measurement system are developed. The results on the reconstruction of the shape based on experimental data are obtained. Practical Relevance. A special feature of this system is the fiber Bragg grating arrays formed in a wide range of wavelengths. Each Bragg grating corresponds to its own reflection wavelength, providing a sensitive part 1 m long. The geometry of the structure is based on seven fiber-optic armored microcables twisted with a given lay length during manufacturing, and provides the measurement of the kink value and direction.

High-sensing-resolution distributed hot spot detection system implemented by a relaxed pulsewidth.

Reliable sensing and accurate location of a weak and small hot spot are critical for applications in industrial infrastructure monitoring. We propose and experimentally demonstrate a practical and reliable distributed hot spot detection method using ultra-weak fiber Bragg gratings (UWFBGs) array and optical time-domain reflectometry (OTDR) based interrogator. To reliably detect the hot spots, the grating spacing of the sensor array is decreased to a similar size of the hot spot. All UWFBGs within a fiber section (FS) are considered as one sensing element, and the wavelength-division multiplexing technique is introduced to reduce crosstalk between adjacent FSs. To retrieve the sensing information, the proposed FS spectrum interrogation method based on OTDR technology is numerically analyzed and experimentally demonstrated. The interrogator exploits the reflection spectrum of each FS instead of each grating, enabling the low-speed hardware implementation of the whole demodulation method. Experimental results show that the expected hot spot can be successfully detected with a sensing resolution of 10 cm and a location resolution of 1 m over a range of 2 km by exploiting 10-ns pulsewidth. Besides, the temperature measurement can be demonstrated with a temperature sensing precision of ± 1°C and a measurement time of 1.5 s, which are meaningful for the early warning of centimeters-sized fire source in some oil and gas pipelines monitoring applications.

Disturbed partial discharge detection system based on an improved Φ-OTDR assisted by a wFBG array.

Partial discharge (PD) detection is an effective method for evaluating the insulation status of power cables. In this paper, a disturbed acoustic sensing (DAS) system based on phase-sensitive optical time-domain reflectometry (Φ-OTDR) for online PD detecting is proposed and demonstrated. To detect the ultra-small fiber strain induced by PD, a weak fiber Bragg grating (wFBG) array is introduced into the sensing fiber to increase signal-to-noise ratio (SNR) of the system. Besides, the sensing fiber is packaged with aluminum tape to enhance the sensitivity. Consistent PD signals in a 10 kV cross linked polyethylene (XLPE) power cable with artificial defects have been observed, and characteristics of the signals are analyzed.

Combining SDAE Network with Improved DTW Algorithm for Similarity Measure of Ultra-Weak FBG Vibration Responses in Underground Structures.

Quantifying structural status and locating structural anomalies are critical to tracking and safeguarding the safety of long-distance underground structures. Given the dynamic and distributed monitoring capabilities of an ultra-weak fiber Bragg grating (FBG) array, this paper proposes a method combining the stacked denoising autoencoder (SDAE) network and the improved dynamic time wrapping (DTW) algorithm to quantify the similarity of vibration responses. To obtain the dimensionality reduction features that were conducive to distance measurement, the silhouette coefficient was adopted to evaluate the training efficacy of the SDAE network under different hyperparameter settings. To measure the distance based on the improved DTW algorithm, the one nearest neighbor (1-NN) classifier was utilized to search the best constraint bandwidth. Moreover, the study proposed that the performance of different distance metrics used to quantify similarity can be evaluated through the 1-NN classifier. Based on two one-dimensional time-series datasets from the University of California, Riverside (UCR) archives, the detailed implementation process for similarity measure was illustrated. In terms of feature extraction and distance measure of UCR datasets, the proposed integrated approach of similarity measure showed improved performance over other existing algorithms. Finally, the field-vibration responses of the track bed in the subway detected by the ultra-weak FBG array were collected to determine the similarity characteristics of structural vibration among different monitoring zones. The quantitative results indicated that the proposed method can effectively quantify and distinguish the vibration similarity related to the physical location of structures.

Demodulation of a weak fiber Bragg grating array using a fiber delay line.

We demonstrate the use of a fiber delay line (FDL) to demodulate a weak fiber Bragg grating (WFBG) array. The FDL is composed of a fiber-ring cavity, by which the delay time is matched with the interval length of the adjacent WFBGs. Two pulses reflected by the former WFBG with delay and a pulse reflected by the rear WFBG without delay overlap and interfere. The emitted sounds can be given by the interference signals of a 5-WFBG array with the FDL. Using the FDL, it is demonstrated that the simple structure can be used to demodulate the WFBG array.

A machine learning approach for simultaneous measurement of magnetic field position and intensity with fiber Bragg grating and magnetorheological fluid

Abstract This paper presents the simultaneous assessment of magnetic field intensity and position using a fiber Bragg grating (FBG) array immersed in magnetorheological (MR) fluid. The applied magnetic field leads to a variation of the MR fluid yield stress, which results in an axial strain on the FBG. As a well-known behavior of FBGs, the axial strain leads to a Bragg wavelength shift on the FBGs, which, in this case, is proportional to the magnetic field intensity and position. An array with 4 FBGs was used and characterized with respect to both magnetic field position and intensity. Then, a k-nearest neighbors’ algorithm was proposed to classify the magnetic field position through the wavelength shift of the FBGs, where the magnetic field intensity is estimated from the FBG closest to the magnetic field position previously detected. Results show the feasibility of the proposed approach, where the algorithm accuracy is 100% for the best case and 86% for the worst case of magnetic field position, whereas a relative error lower than 5% was obtained on the magnetic field intensity estimation.

Continuous delay tunability using a combination of three types of fiber Bragg gratings, wavelength conversion, and wavelength multicasting with a frequency comb

We experimentally demonstrate a delay subsystem that utilizes a combination of three types of fiber-Bragg gratings (FBGs), wavelength conversion, and wavelength multicasting using a frequency comb source. While the linearly chirped FBG together with a channelized dispersion compensation one, provide the continuous delay, an array of standard FBGs utilizes the multicast copies to extend the delay in steps determined by the array geometry. Following the characterization of the group-delay dispersion performance of the FBGs, a 10 Gbaud quadrature phase-shift keying (QPSK) signal is continuously delayed by 2.2 ns and discretely delayed in 3 steps of ∼2 ns, to provide a total gap-less delay of 8.1 ns. The delayed signal is finally recovered to its original wavelength with an optical signal-to-noise ratio (OSNR) penalty of ∼2.3 dB. Furthermore, we show that the penalty can be reduced to 1.2 dB after equalization and report delaying a 20 Gbaud QPSK signal. The demonstrated high-quality delay performance is quite scalable. The continuous delay can be extended to tens of nanoseconds, limited only by the performance of the used components, while the discrete delay is determined by the number of multicast copies and propagation distance between the reflections in the array of FBGs.

Investigation of bending effects in chirped FBGs array in multicore fiber

The research and development of a measurement system for remote determination and monitoring of the location and bends of extended objects in 3D space are of current interest. In this work the results of study of bending effects in chirped fiber Bragg gratings array in multicore fiber are presented. The determination of the dependence of grating period change in each core on the bending radius of the fiber allows to reconstruct remotely optical fiber shape. The obtained investigation results will be used to determine the fiber-optic light guide shape changes without using methods of optical frequency domain reflectometry. The work presents experimental results with the multicore optical fiber with the array of chirped fiber Bragg gratings. The experimental optical scheme and the reflection spectra prior to and after the optical fiber bending are presented.

Mode characteristic manipulation of random feedback interferometers in Brillouin random fiber laser.

The Brillouin random fiber laser (BRFL) suffers from high intensity noise that comes mainly from longitudinal mode beating at different mode frequencies. In this Letter, we propose and demonstrate that the mode characteristic of BRFL can be manipulated by distributed random feedback, which acts as the longitudinal mode filter. A theoretical model is developed for the first time, to the best of our knowledge, to analyze the mode characteristics of BRFL with different lengths of a weak fiber Bragg grating (FBG) array. In experiment, a single FBG, weak FBG array (reflection of $ - {40}\;{\rm dB}$-40dB) at various lengths, and a Rayleigh scattering fiber are used to provide the random feedback. Both theoretical analysis and experimental results show that single longitudinal mode operation can be realized with the distributed random feedback interferometer, leading to a stable temporal intensity output of the BRFL in the time domain.

Dynamic Quasi-Distributed Ultraweak Fiber Bragg Grating Array Sensing Enabled by Depth-Resolved Dual-Comb Spectroscopy

Distributed sensing plays an important role in many industrial applications. Sensor arrays consisting of identical ultraweak fiber Bragg gratings can be applied to such scenarios by monitoring the optical spectral response of the sensors. Yet, the spatial resolution, the sampling speed, and the supported number of sensors in the array are mostly determined by the interrogation scheme. While dual-optical frequency comb techniques had been successfully demonstrated as a high-resolution spectroscopic technique in metrology, it had been beyond the reach of industrial sensing applications because of its high complexity and costs. It is proposed and demonstrated here that, enabled by a simple fiber-optic, dual-comb spectroscopy setup consisting of a fiber laser without frequency stabilization, interrogation of an ultraweak fiber sensor array can be realized. High spatial and spectral resolution as well as kilohertz interrogation speed is achieved in both static and dynamic measurements. It is expected that our depth-resolved dual-comb scheme can measure hundreds of sensors response in less than 1 ms. It is also flexible to deal with targets closely spaced or farther apart from each other. This shows the potential of applying dual-comb spectroscopy to more and more industrial application areas.

Monitoring temperature and vibration in a long weak grating array with short-pulse generation using a compact gain-switching laser diode module

Quasi-distributed temperature sensing and single point vibration sensing were performed. Ultrashort pulses generated by a gain-switching laser were used to interrogate a fiber Bragg gratings (FBG) array sensor. Temperature changes were measured down to 1°C with sub-centimeter spatial resolution. The advantages of our fast interrogation setup were exploited, as the higher frequency limit of a dynamic measure that can be sensed is limited by the time needed to generate the optical pulse and to acquire the data from the sensor. The experimental approach described in this paper can sense mechanical vibrations up to a frequency of 245 kHz and a strain resolution as low as 1.2 µɛ.

Random lasing in a short Er-doped artificial Rayleigh fiber

We report on random lasing observed with 5-m-long Er-doped fiber comprising an array of weak fiber Bragg gratings (FBGs) inscribed in the fiber core and uniformly distributed over the whole fiber length. The laser design ensures domination of dynamical population inversion grating over the FBGs in total distributed feedback enabling laser stabilization and nonlinear laser line filtering of the natural laser Lorentzian linewidth down to sub-300-Hz range, both observed in the experiment.