Weak Fiber Bragg Gratings Research Articles

Investigation on temperature characteristics of weak fiber Bragg gratings in a wide range

A weak fiber Bragg grating (WFBG) is an ideal quasi-distributed optical fiber sensor. Special attention should be paid to the spectrum and sensing performance of the WFBG at extreme temperatures due to its poor reflection intensity. In this Letter, the temperature characteristics of the WFBG from −252.75°C to 200.94°C are experimentally investigated. Five WFBGs with reflectivity from ∼0.25% to ∼10% are used in the experiments. The reflectivity variations and wavelength shifts at different temperatures are studied. Experimental results show that the WFBG can survive and work at extreme temperatures, but the performance is affected significantly. The reflectivity is affected significantly by both cryogenic temperature and high temperature. The temperature responses of Bragg wavelengths in the wide temperature range are also obtained.

Partial Discharge Recognition Based on Optical Fiber Distributed Acoustic Sensing and a Convolutional Neural Network

Fiber-optic distributed acoustic sensing (FDAS) with phase-sensitive optical time-domain reflectometry (Φ-OTDR) is a promising technique for high-sensitivity measurement. In this paper, an improved Φ-OTDR system with a weak fiber Bragg grating (wFBG) array for partial discharge (PD) detection in cross-linked polyethylene (XLPE) power cables is demonstrated; and an event recognition method based on a convolutional neural network (CNN) model is proposed to identify and classify different types of events, including internal PD, corona PD, surface PD, and noise. A multiscale wavelet decomposition and reconstruction method is used to extract PD signals and a two-dimensional spectral frame representation of the PD signals is obtained by the mel-frequency cepstrum coefficients (MFCC). The experimental results based on 1280 training samples and 832 test samples have demonstrated high values of precision, sensitivity, and specificity for each event (up to 96.3%, 96.4%, and 98.7%, respectively), which means that the combination of multiscale wavelet decomposition and reconstruction, the MFCC and CNN may be a promising event recognition method for the FDAS systems.

Random Bragg-gratings-based narrow linewidth random fiber laser with a π-phase-shifted FBG

A new structure of short-cavity random fiber laser (RFL) with narrow linewidth lasing is proposed. A π-phase-shifted fiber Bragg grating (FBG) loop mirror was used in the RFL for spectral filtering and to suppress multi-mode oscillation due to the narrow transmission window. The random feedback of the RFL was implemented by a randomly dispersed weak reflection FBG array. The high gain of the erbium-doped fiber and a half-open cavity design results in a low lasing threshold. The linewidth of the laser was 225 Hz with 58 dB side-mode-suppression ratio. The laser threshold was 4.5 mW, and the optical signal-to-noise ratio was up to 63 dB.

Study of a novel distributed vibration sensing system based on weak fiber Bragg gratings: retraction

Study of a novel distributed vibration sensing system based on weak fiber Bragg gratings: retraction

Measurements of DIII-D poloidal field by fiber-optic pulsed polarimetry.

A new technique for measuring the spatial and temporal structure of the poloidal field is presented, whereby the magnetic field causes the polarization of light traveling through an optical fiber to rotate via the Faraday effect by an amount proportional to the strength of the field oriented along the fiber. In fiber optic pulsed polarimetry, changes in the polarization of the backscatter light from the fiber are detected, thereby permitting measurement of the field as a function of position along the fiber. In this proof-of-principle experiment, specially prepared single-mode fibers with weak fiber Bragg gratings were installed in the poloidal direction on the outside of the thermal blanket on DIII-D. Light at 532 nm from a mode-locked Nd:YAG laser was injected into the optical fibers. The laser repetition rate was 895 kHz with a pulse length of <10 ps, resulting in ∼1 μs temporal resolution. A photodetector system measured the Stokes polarization components necessary to determine the amount of polarization rotation. For this experiment, bandwidth limitations of the detectors resulted in a spatial resolution of ≈2 cm. The measured temporal and spatial distributions of the poloidal field are consistent with inductive probe measurements and Elastodynamic Finite Integration Technique reconstructions of the spatial distribution. This demonstrates the ability of this technique to provide real-time detection of the temporal and spatial variations of the poloidal field. Besides revealing more detailed information about the plasma, this new diagnostic capability can also help in detecting instabilities in real time, thereby enabling enhanced machine protection.

RETRACTED: Study of a novel distributed vibration sensing system based on weak fiber Bragg gratings

A novel vibration sensing system based on distributed weak fiber Bragg gratings is demonstrated to overcome the drawbacks of the conventional methods used in optical fiber sensing networking, such as insufficient quantity of multiplexed fiber Bragg gratings in wavelength-division multiplexing systems and low signal intensity in conventional time-division multiplexing systems. By mixing wavelength-division multiplexing technology and time-division multiplexing technology, the physical parameters of the fiber Bragg grating locations can be measured, but the distributed measurement along the whole optical fiber cannot be realized. The proposed novel sensing system detects the vibration events along the optical fiber link according to the signal’s intensity interference variation caused by the reflected signals with the adjacent weak fiber Bragg gratings. The position of the vibration events can be accurately determined by using time-division multiplexing technology. The experimental results verify that this novel system is more convenient for realizing vibration detection and demodulation along the whole optical fiber than before, and the system locating accuracy can be up to 20.5 m, with the capability of detecting various signals with 0.2 V of the driven signals as the bottom limit of voltage and a frequency range from 3 to 1000 Hz.

TDM Interrogation of Identical Weak FBGs Network Based on Delayed Laser Pulses Differential Detection

We report a time- and wavelength multiplexing sensing network based on nearly identical ultraweak fiber Bragg gratings (FBGs). In the multiplexing scheme of the network, each wavelength channel is distinguished by time delays of the two probe laser lines, and every channel works as an optical time domain reflectometry, and the response of each FBG is represented by two delay pulses corresponding to the two laser lines. The Bragg wavelengths of the FBGs are interrogated by calculating the ratio of two peak power of returned laser lines. Compared with a conventional wavelength scanning system, the proposed system can achieve relatively high interrogation speed. A proof of concept experiment with nine FBGs, at three wavelength channels was demonstrated, and fast measurements for static strain were achieved, with the sensitivity around $ - 0.0023\;{\text {dB}}/{{\mu \varepsilon }}$ and dynamic range over $2500\;{ {\mu \varepsilon }}$ for three channels.

High-speed demodulation of weak fiber Bragg gratings based on microwave photonics and chromatic dispersion.

A high-speed demodulation technique based on microwave photonics and chromatic dispersion is proposed for distributed weak fiber Bragg gratings (FBGs). A broadband light modulated with a frequency-swept microwave is reflected by FBGs, and the reflected signal mixes with the original microwave to produce a beat frequency. A dispersion compensation fiber (DCF) changes the beat frequency within the FBG wavelength range. With a crossing microwave sweep, all wavelengths of cascade FBGs can be quickly decoded by measuring the change of the beat frequency. The Chirp-Z + Hanning window algorithm helps to analyze the beat frequency in a single cycle. By adopting the single-peak filter as a reference, the DCF thermal drift can be also eliminated. Therefore, the wavelength detection accuracy is improved, and a higher demodulation speed of 40kHz can be realized. We have demonstrated 105FBGs with 0.1% reflectivity and 1m interval, with the achieved accuracy of 8pm.

Synchronous Reflectivity Measurement of a Weak Fiber Bragg Grating Array Using Frequency-Shifted Interferometry

We demonstrate a method of reflectivity measurement for a weak fiber Bragg grating (FBG) sensor array based on frequency shifted interferometry. In the scheme, the measured weak FBG sensor array was connected after a referenced weak FBG, by using frequency shifted interferometry, the reflectivity of the referenced FBG and the reflection amplitude spectrum of each FBG from the sensor array were obtained, respectively, and then each grating reflectivity was iteratively calculated by using the spectral shadowing removal method. A sensor array of 60-weak FBGs with an average peak reflectivity around −25 dB was experimentally investigated. The obtained peak reflectivity ranged from −37.7 to −21.7 dB with an average value of −26.0 dB. Error analysis indicates the average reflectivity measurement stability for all FBGs was ±1.93 dB. The results reveal that this paper presents a feasible, simple, and low-cost method of reflectivity measurement for large-scale weak FBG sensor arrays.

Distributed Weak Fiber Bragg Grating Vibration Sensing System Based on 3 \xd7 3 Fiber Coupler

A novel distributed weak fiber Bragg gratings (FBGs) vibration sensing system has been designed to overcome the disadvantages of the conventional methods for optical fiber sensing networking, which are: low signal intensity in the usually adopted time-division multiplexing (TDM) technology, insufficient quantity of multiplexed FBGs in the wavelength-division multiplexing (WDM) technology, and that the mixed WDM/TDM technology measures only the physical parameters of the FBG locations but cannot perform distributed measurement over the whole optical fiber. This novel system determines vibration events in the optical fiber line according to the intensity variation of the interference signals between the adjacent weak FBG reflected signals and locates the vibration points accurately using the TDM technology. It has been proven by tests that this system performs vibration signal detection and demodulation in a way more convenient than the conventional methods for the optical fiber sensing system. It also measures over the whole optical fiber, therefore, distributed measurement is fulfilled, and the system locating accuracy is up to 20 m, capable of detecting any signals of whose drive signals lower limit voltage is 0.2 V while the frequency range is 3 Hz‒1 000 Hz. The system has the great practical significance and application value for perimeter surveillance systems.

Lumped Rayleigh reflectors.

Distributed acoustic sensing (DAS) via optical fibers makes use of Rayleigh backscattering for the detection of acoustic waves that interact with the fiber along its entire length. The random nature of Rayleigh backscattering leads to nonuniform performance along the fiber and, occasionally, to complete signal fading. In addition, distance-dependent signal-to-noise (SNR) degradation is always present due to propagation loss. In contrast, using arrays of discrete reflectors [such as weak fiber Bragg gratings (FBGs) with equal center wavelengths] offers deterministic performance which can be designed to be uniform along the fiber. Here we describe an approach for implementing Rayleigh-based discrete reflectors that can offer enhanced detection performance in selected regions. It is based on enclosing sections of the fiber in acoustically insulated boxes to create lumped Rayleigh reflectors. Besides diminishing the randomness in detection sensitivity, the method enables increasing the detection SNR far beyond the typical value for Rayleigh-based DAS and obtaining sensitivities comparable with discrete reflectors. The proposed method was successfully tested via both simulation and experiment.

A Liquid-Level Sensing Technique Based on Differential Detection of Correlation Peaks From Broadband Chaos

A highly sensitive liquid-level sensing technique is proposed and experimentally demonstrated. Two narrowband fiber Bragg gratings (FBGs) with high reflectivity are used to filter out the two light signals from the broadband chaos. Two weak FBGs are served respectively as the liquid-level sensors at two different sensing points. The change of liquid-level will induce the wavelength shift of the weak FBGs, which can be demodulated through calculating the amplitude difference in the logarithm of two cascaded correlation peaks in the cross-correlation spectrum. Adopting the differential calculation of the cascaded correlation peak amplitudes can avoid the complex wavelength detection and enhance the robustness against the power variation of the broadband chaos. Our method can also support the simultaneous multiplexing and locating with high resolution from the time delays of the corresponding correlation peaks. Experimental results show that the liquid level changes linearly with the relative peak amplitude difference in the logarithm and the sensitivity is around 0.019/mm. The sensing resolution of liquid-level can reach at least 3 mm, and the relative resolution is around 0.055. Moreover, the real-time fiber fault monitoring can be achieved with a spatial resolution of around 2.8 cm, which improves the survivability in harsh environment.

Performance Optimization Design for a High-Speed Weak FBG Interrogation System Based on DFB Laser.

A performance optimization design for a high-speed fiber Bragg grating (FBG) interrogation system based on a high-speed distributed feedback (DFB) swept laser is proposed. A time-division-multiplexing sensor network with identical weak FBGs is constituted to realize high-capacity sensing. In order to further improve the multiplexing capacity, a waveform repairing algorithm is designed to extend the dynamic demodulation range of FBG sensors. It is based on the fact that the spectrum of an FBG keeps stable over a long period of time. Compared with the pre-collected spectra, the distorted spectra waveform are identified and repaired. Experimental results show that all the identical weak FBGs are distinguished and demodulated at the speed of 100 kHz with a linearity of above 0.99, and the range of dynamic demodulation is extended by 40%.

Are optical fibers really uniform? Measurement of refractive index on a centimeter scale.

Many applications of optical fiber, such as specialized fiber Bragg gratings (FBGs), require high uniformity of a fiber's refractive index (RI) along its length. We show here that the mode effective index of most fibers is not constant, even on a short length scale. To help improve fiber manufacturing and selection, we demonstrate a technique for characterizing a meter length single-mode optical fiber's effective RI over a centimeter scale with a precision of 3×10-6 RI units (RIUs) and an absolute accuracy of 2×10-4 RIU. By writing several weak probe FBGs as frequency references and then measuring the frequency deviation of these probe FBGs along the length of the fiber with an optical frequency domain reflectometer, the RI distribution of the effective mode index may be found. We validate our measurements on reference and fibers under test with theoretical simulations.

基于弱光纤光栅阵列的分布式振动探测系统

基于弱光纤光栅阵列的分布式振动探测系统

Crosstalk and Ghost Gratings in a Large-Scale Weak Fiber Bragg Grating Array

Large-scale distributed fiber Bragg grating (FBG) sensor networks have a wide range of applications. However, serious crosstalk could occur in a near-identical serial FBG sensor network because of multiple reflections. Since crosstalk cannot be observed directly and separately from signal, experimental investigation of crosstalk is challenging. In this work, we point out the similarity and symmetry between crosstalk and ghost gratings, and by way of measuring the spectrum of ghost gratings, we experimentally study the characteristics of crosstalk in a serial array of 3010 near-identical FBGs using a commercial FBG sensor interrogation system. We show that ghost gratings drop much more rapidly in power as one goes downstream, and they are more homogeneous in spectral distribution than real gratings. The peak wavelengths of real gratings vary within ±100 pm while the peak wavelengths of the ghost gratings they generate vary within ±20 pm only. Spectral ripples are observed for ghost gratings and real gratings severely affected by crosstalk. These ripples reduce the FBG wavelength reading accuracy to ±5 pm. The work constitutes the first experimental study on crosstalk in large-scale FBG arrays.

Interrogation of weak Bragg grating sensors based on dual-wavelength differential detection.

It is shown that for weak Bragg gratings the logarithmic ratio of reflected intensities at any two wavelengths within the spectrum follows a linear relationship with the Bragg wavelength shift, with a slope proportional to their wavelength spacing. This finding is exploited to develop a flexible, efficient, and cheap interrogation solution of weak fiber Bragg grating (FBGs), especially ultra-short FBGs, in distributed sensing based on dual-wavelength differential detection. The concept is experimentally studied in both single and distributed sensing systems with ultra-short FBG sensors. The work may form the basis of new and promising FBG interrogation techniques based on detecting discrete rather than continuous spectra.

High-Speed Distributed Sensing Based on Ultra Weak FBGs and Chromatic Dispersion

A high-speed quasi-distributed fiber optic sensing technique is proposed. A sensing link consisting multiple identical fiber Bragg gratings (FBGs) was interrogated by a pulsed light source, and a dispersive unit at the receiver end was used to convert the wavelength shifts of the reflected pulses into time-delay changes. This Bragg wavelength-dependent time delay can be accurately calculated by a fast cross-correlation-based algorithm, enabling distributed high-speed sensing. This technique is experimentally demonstrated using a sensing link containing 2014 FBGs with a reflectance of about −35 dB. A wavelength resolution of 9.03 pm was achieved without any data averaging. Dynamic strain was applied to one of the FBGs, and corresponding wavelength variation was measured at a sampling rate of 20 kHz, obtaining a dynamic resolution of $\surd $ Hz.

Optical fiber with distributed Bragg-type reflector

Optical fiber (OF) with a relatively high level of the backward signal at a certain wavelength is developed and studied. An increase in the backward signal is reached due to the recording of multiple weak fiber Bragg gratings (FBGs) in the course of fiber pulling. A scheme that is simpler than the direct FBG recording through a phase mask is proposed and implemented. Fibers with FBG recorded with different ratios of the region with recorded gratings to the total length of OF are fabricated. An increase in the level of the backward signal relative to the level of the Rayleigh scattering by more than 30 dB is reached. Results of experimental study of the proposed OFs and examples of practical application are presented.

Spot event detection along a large-scale sensor based on ultra-weak fiber Bragg gratings using time-frequency analysis.

A simple scheme for interrogating a 5 m long photonics device and its potential applications to quasi-distributed fiber sensing is proposed. The sensor consists of an array of 500 identical, very weak fiber Bragg gratings (FBGs). The gratings are 9 mm long and have been serially written in cascade along a single optical fiber. The measurement system is based on a combination of optical time domain reflectometry and frequency scanning of the interrogating pulse. The time-frequency analysis is performed by launching an optical pulse into the sensor and retrieving and analyzing the back-reflected signal. The measurement of the temperature, length, and position of spot events along the sensors is demonstrated with good accuracy. As both spatial and temperature resolution of the method depend on the input pulse duration, the system performance can be controlled and optimized by properly choosing the temporal duration of the interrogating pulse. A spatial resolution of 9 mm (ultimately dictated by one grating length) has been obtained with an 80 ps optical pulse, while a temperature resolution of less than 0.42 K has been demonstrated using a 500 ps incident pulse. The sensor proposed proves to be simple, robust, and polarization insensitive and alleviates the instrumentation complexity for distributed sensing applications.