FBG Array Research Articles

Intrusion identification using GMM-HMM for perimeter monitoring based on ultra-weak FBG arrays.

Intrusion identification has been an intractable task for perimeter security. One of the primary challenges is to possess high identification rate over a long-distance range monitoring. This paper proposes an intrusion identification scheme based on ultra-weak fiber Bragg grating (UWFBG) arrays. The scheme is acquired by the combination of a Gaussian mixture model (GMM) and a hidden Markov model (HMM). The time dependencies are obtained by the analysis of relevant sensors in UWFBG arrays from the procedure of intrusions. The features extracted from vibration signals with time dependencies are used as the input of GMM-HMM. The GMM-HMM simultaneously analyzes features and time dependencies to identify intrusion. Experimental demonstration verifies that the proposed scheme can identify three intrusions (walking, knocking and climbing) and two non-intrusions (heavy truck passing and wind blowing) with the average identification rate of 98.2%. By the comparison tests with other six classifiers, the proposed GMM-HMM scheme shows a solid performance in the integrated evaluation for intrusion identification.

Femtosecond laser auto-positioning direct writing of a multicore fiber Bragg grating array for shape sensing.

A multicore fiber Bragg grating (MC-FBG) array shape sensor is a powerful tool for a variety of applications. However, the efficient fabrication of high-quality MC-FBG arrays remains a problem. Here, we report for the first time, to the best of our knowledge, a new method of directly writing FBG arrays in a seven-core fiber (SCF) through the protective coating using femtosecond laser auto-positioning point-by-point technology, which is accomplished by image recognition and micro-displacement compensation. An MC-FBG array consisting of 140 individual FBGs with a grating length of 2 mm was successfully inscribed into seven cores of a 440 mm-long SCF. Each core contained 20 wavelength-division-multiplexed (WDM) FBGs with wavelengths ranging from 1522.11 nm to 1579.28 nm. In other words, the MC-FBG array consisted of 20 WDM nodes with an interval of 2 cm along the fiber, and each node contained seven identical FBGs integrated in parallel into the fiber cross-section. Moreover, the fabricated MC-FBG array exhibited a strong orientation dependence in bend sensing, with a maximum sensitivity of 55.49 pm/m-1. Subsequently, 2D and 3D shape sensing were demonstrated using the fabricated MC-FBG array, with maximum reconstruction errors per unit length of 4.51% and 10.81%, respectively. Hence, the MC-FBG arrays fabricated using the proposed method are useful in many applications, such as posture monitoring, smart robotics, and minimally invasive surgery.

Heterogeneous-frequency-dual-pulse chain and weak FBG array for quasi-distributed acoustic sensing with improved response bandwidth.

By continuously injecting four groups of heterogeneous frequency dual pulses into the sensing fiber with weak fiber Bragg gratings (WFBGs), a quasi-distributed acoustic sensing method based on frequency-division multiplexing is proposed. Each group of pulses generates interference signals with different carrier frequencies after being reflected by the WFBGs. Through the discrete Fourier transform phase demodulation method, each carrier frequency interference signal is demodulated and then the phase is spliced. The feasibility of this method is theoretically analyzed, and a detection with a bandwidth of 2 kHz is realized on a 70 km sensing fiber with a spatial resolution of 10 m.

Ultimate Spatial Resolution Realisation in Optical Frequency Domain Reflectometry with Equal Frequency Resampling.

A method based on equal frequency resampling is proposed to suppress laser nonlinear frequency sweeping for the ultimate spatial resolution in optical frequency domain reflectometry. Estimation inaccuracy of the sweeping frequency distribution caused by the finite sampling rate in the auxiliary interferometer can be efficiently compensated by the equal frequency resampling method. With the sweeping range of 130 nm, a 12.1 µm spatial resolution is experimentally obtained. In addition, the sampling limitation of the auxiliary interferometer-based correction is discussed. With a 200 m optical path delay in the auxiliary interferometer, a 21.3 µm spatial resolution is realised at the 191 m fibre end. By employing the proposed resampling and a drawing tower FBG array to enhance the Rayleigh backscattering, a distributed temperature sensing over a 105 m fibre with a sensing resolution of 1 cm is achieved. The measured temperature uncertainty is limited to ±0.15 °C.

Post-gel polymerisation shrinkage profiling of polymer biomaterials using a chirped fibre Bragg grating

A strain profile measurement technique using a chirped fibre Bragg grating (CFBG) sensor by implementing an integration of differences (IOD) method is reported in this paper. Using the IOD method the spatial distribution of strain along the length of the CFBG is extracted from its power reflectance spectra. As a proof of concept demonstration, the developed technique is applied to measure the polymerisation shrinkage strain profile of a photo-cured polymer dental composite which exhibits a non-uniform strain distribution attributed to the curing lamp characteristics. The result from the CFBG technique is compared with that of an FBG array embedded in the dental composite and is correlated with the degree of conversion of the material which also depends on the curing lamp intensity distribution. This technology will have significant impact and applications in a range of medical, materials and engineering areas where strain or temperature gradient profile measurement is required in smaller scales.

Surface Structure State Perception System Based on FBG Array

Surface Structure State Perception System Based on FBG Array

Simultaneous Temperature Estimation of Multiple Gratings Using a Multi-Layer Neural Network

This paper introduces a method to do simultaneous temperature estimations of multiple gratings in an FBG array. The method involves training a multi-layer neural network using simulated training data. The network is then used to estimate the temperature changes of multiple gratings simultaneously using only the experimentally obtained spectrum of the FBG array. The versatility of the method is seen from the results of three different setups. That is the broadband lightsource with OSA, the broadband lightsource with spectrometer, and incoherent optical frequency domain reflectometry. The method can estimate the temperature changes with high accuracy and low root mean squared error (RMSE) for the setups under consideration. Finally, the method is shown to be capable of simultaneous estimation of temperature changes for 19 FBGs using the BLS setup with a maximum absolute error of $6.64~\mathrm {\textbf {K}}$ and an RMSE of $1.69~\mathrm {\textbf {K}}$ in only 1.74 ms/FBG.

Optical Fiber Tag Based on an Encoded Fiber Bragg Grating Fabricated by Femtosecond Laser

A type of encoded FBG array for use as an optical fiber tag is inscribed using a femtosecond laser line-by-line technique. Each tag included two labels and several codes formed by grating fragments with a predetermined length. Encoded information was characterized by measuring the backscattered optical signal. Local spectra from the internal grating fragments were then used to enlarge the encoding capacity or encrypt the encoded FBG. The insertion loss of the device is as low as <0.1 dB.

Design, Instrumentation and Usage Protocols for Distributed In Situ Thermal Hot Spots Monitoring in Electric Coils using FBG Sensor Multiplexing.

Random wound coils are a key operational element of most electric apparatus in modern industrial systems, including low voltage electric machines. One of the major current bottlenecks in improved exploitation of electrical devices is the high sensitivity of their wound components to in-service thermal stress. The application of conventional thermal sensing methods (e.g., thermocouples, resistance temperature detectors) for thermal condition monitoring of current carrying random wound coils can impose considerable operational limitations due to sensor size, EMI sensitivity and the existence of electrically conductive material in their construction. Another substantial limitation exists in distributed sensing applications and is caused by what is often a considerable length and volume of conventional sensor wiring leads. This paper reports the design of a fiber optic FBGsensing systemintended for enabling real-time distributed internal thermalcondition monitoring within random wound coils. The procedure of random wound coil instrumentation with the FBG sensing system is reported in a case study on an IEEE standard wound coil representative of those utilized in electrical machines. The reported work also presents and discusses important practical and technical aspects of FBG sensing system implementation and application, including the FBG array geometry design, sensing head and fiber packaging, the sensor array installation and calibration procedure and the use of a commercial interrogation system for obtaining thermal measurements. Finally, the in situ multiplexed FBG sensing system thermal monitoring performance is demonstrated in representative static and dynamic thermal conditions.

Temperature-sensing characteristics of polarization-maintaining fiber Bragg grating inscribed directly by 800-nm femtosecond laser pulses

Abstract A temperature-sensing system based on a polarization-maintaining fiber Bragg grating (PM-FBG) with dual-wavelength peaks was proposed and demonstrated experimentally. First-order bow-tie- and panda-type PM-FBGs with 3-mm gratings were inscribed by a 800-nm femtosecond laser by the point-by-point method, in which the laser was focused onto the fiber core through the fiber coating and cladding. The FBG arrays were directly fabricated by the femtosecond laser writing technique. The PM-FBGs with dual-wavelength reflection-spectrum peaks were scanned by a cost-effective interrogator system using modulated-grating Y-branch (MG-Y) tunable laser to sample the wavelength center position. In the temperature-sensing experiment, the sensitivity of the bow-tie-type PM-FBG exceeded 14 pm/°C within the 30–90 ℃ range. The proposed PM-FBGs and its interrogation system can be widely used in the field of fiber sensing and fiber laser.

Surface intrusion event identification for subway tunnels using ultra-weak FBG array based fiber sensing.

A scheme is proposed for the identification of surface intrusion events, from signals detected by an ultra-weak fiber Bragg grating array in a subway tunnel. The spectral subtraction and the root mean square of the power spectral density are combined to extract event signals. The local characteristics-scale decomposition and the multi-scale permutation entropy are employed subsequently for feature extraction, which can improve the event recognition rate from the perspective of multi-scale analysis. Experimental demonstration verifies that the proposed scheme can identify four common events. Among the events, the discrete pulse construction and the continuous pulse construction on the ground surface are intrusion events, the subway train traveling in the tunnel and the lorry passing on the ground surface are non-intrusion events. The average recognition rate of 96.57% is achieved, which can satisfy actual application requirements.

Durable shape sensor based on FBG array inscribed in polyimide-coated multicore optical fiber.

The paper presents a novel three-dimensional quasi-continuous shape sensor based on an FBG array inscribed by femtosecond laser pulses into a 7-core optical fiber with a polyimide protective coating. The measured bending sensitivity of individual FBGs ranges from 0.046 nm/m-1 to 0.049 nm/m-1. It is shown that the sensor allows for reconstructing 2- and 3-dimensional shapes with high accuracy. Due to the high value of the core aperture and individual calibration of each FBG we were able to measure the smallest reported bending radii down to 2.6 mm with a record accuracy of ∼1%. Moreover, we investigate the magnitude of the errors of curves reconstruction and errors associated with measurement of curvature radii in the range from 2.6 to 500 mm. The main factors affecting the accuracy of measurements are also discussed. The temperature resistance of both the inscribed FBG structures and of the protective coating, along with the high mechanical strength of the polyimide, makes it possible to use the sensor in harsh environments or in medical and composite material applications.

Enhanced Distributed Fiber Optic Vibration Sensing and Simultaneous Temperature Gradient Sensing Using Traditional C-OTDR and Structured Fiber with Scattering Dots.

We present results demonstrating several beneficial effects on distributed fiber optic vibration sensing (DVS) functionality and performance resulting from utilizing standard single mode optical fiber (SMF) with femtosecond laser-inscribed equally-spaced simple scattering dots. This modification is particularly useful when using traditional single-wavelength amplitude-based coherent optical time domain reflectometry (C-OTDR) as sensing method. Local sensitivity is increased in quasi-distributed interferometric sensing zones which are formed by the fiber segments between subsequent pairs of the scattering dots. The otherwise nonlinear transfer function is overwritten with that of an ordinary two-beam interferometer. This linearizes the phase response to monotonous temperature variations. Furthermore, sensitivity fading is mitigated and the demodulation of low-frequency signals is enabled. The modification also allows for the quantitative determination of local temperature gradients directly from the C-OTDR intensity traces. The dots’ reflectivities and thus the induced attenuation can be tuned via the inscription process parameters. Our approach is a simple, robust and cost-effective way to gain these sensing improvements without the need for more sophisticated interrogator technology or more complex fiber structuring, e.g., based on ultra-weak FBG arrays. Our claims are substantiated by experimental evidence.

Research on Strain Distribution Detection Method of Freeform Surface Based on FBG Array

In order to obtain the strain distribution of the free-form surface to be tested and reduce the difficulty of assembling large-scale curved structures, a free-form surface strain distribution detection method based on FBG array was designed. The system consists of a laser, a coupler, a demodulator, and an FBG array. The strain distribution and displacement offset of the test specimen under five different force conditions are simulated and analyzed. The simulation results show that the strain distribution is related to the position, size and surface structure of the applied force. The experiment was tested on a 3 mm aluminum plate and compared to the simulation data. l. The experimental results show that the maximum strain is 24.7µϵ and the minimum strain is 2.8µϵ. The difference in strain at the same curvature position is significant. It verifies the feasibility of solving the displacement field distribution by the strain field distribution.

Long tapered fiber with array of FBG

Abstract A tapered optical fiber with an adiabatic transition from single-mode to a few-mode part has been fabricated with an array of fiber Bragg gratings being inscribed during the drawing process. We demonstrate that the propagating optical signal remains single-mode along the tapered fiber FBG array even in a multimode part of it. The spectral width of the reflected signal from the tapered optical fiber is 4 nm. The level of the reflected signal is up to 5% at some wavelengths.

Fundamental research on attitude measurement of catheter using the FBG array sensor

Fundamental research on attitude measurement of catheter using the FBG array sensor

Simultaneous distributed static and dynamic sensing based on ultra-short fiber Bragg gratings.

Distributed static and dynamic sensing is demonstrated with an ultra-short fiber Bragg grating (USFBG) array. The USFBGs serve as the sensors and reflection mirrors at the same time. Distributed static sensing is performed by demodulating the strain-induced or temperature-induced wavelength shift of each USFBG. Dynamic vibration sensing is realized based on phase variation between two adjacent USFBG reflected pulses. Static temperature and dynamic vibration are applied to the sensing ultra-short FBG array simultaneously. The experiments show that the temperature measurement from 30 °C to 80 °C is achieved successfully. And dynamic sensing of nε scale vibration and 12.5 kHz acoustic wave are demonstrated at a sampling rate of 50 kHz.

Multiplexed static FBG strain sensors by dual-comb spectroscopy with a free running fiber laser.

We demonstrate a novel and compact FBG interrogation system for multiplexed static strain sensing with a free running mode-locked fiber laser. Multiplexed FBG array in cascading are interrogated by coherent dual-comb pulses generated from a single fiber laser. Dual-comb spectroscopy is achieved with the fiber laser to precisely detect the strain-induced spectral shifts of the FBG sensors. Multi-point strain measurements are performed to characterize the proposed system, where a large dynamic range of 520 με with 0.5 με resolution is achieved.

High-speed interrogation system of multi-encoding weak FBGs based on FDML wavelength swept laser

A high-speed and high-capacity interrogation system, which uses Fourier domain mode locked (FDML) and multi-encoding weak FBGs, is proposed. As a wavelength-swept laser, the FDML transforms the wavelength domain into the time domain to achieve WDM; furthermore, its ultra-fast scanning facilitates the multiplex of WDM combinations in the form of TDM. The application of FDML enables the time division of these two multiplexing methods using only one scanning. Next, a multi-encoding weak FBG array is deigned; each FBG can be distinguished and interrogated by its unique code at high repetition rate. Theoretically, the array maintains the high-speed interrogation of the WDM while breaking through the bandwidth limitation of the wavelength-swept laser to expand the multiplexing capacity using the TDM. A proof-of-concept system consisting of 35 multi-encoding weak FBGs with 5-wavelength by 7-time codes is built to demonstrate the sensing performance. In the temperature and vibration experiments, an interrogation linearity above 0.9993 and a 100-kHz interrogation speed are achieved.

The study on the novel distributed vibration sensing system based on the adjacent FBGs reflected lights interference

Due to the different polarization states of the reflected light on the two arms of conventional Michelson interferometer, polarization fading of the interferometric signal occurs when environmental perturbations and noise are existing. To solve this problem, in this paper, we design a novel distributed FBG array vibration sensing system based on the interference caused by the reflected signals of the adjacent FBGs. By controlling the pulse width of the input pulse, The reflected light of the adjacent FBGs can interfere on the 3*3 coupler, and we can judge the outside invasion by analyzing the characteristics of the interference signal. The system can demodulate the driving signal with the frequency range of 10 Hz–50 Hz. Comparing with conventional vibration acceleration deteceing methods, the proposed system has the higher sensitivity, wider bandwidth, less response time and better spatial resolution. Meanwhile, the proposed system has sufficient sensing areas which can serve as the high-capacity interferometric seismic detection system.