Short Fiber Bragg Grating Research Articles

Improved performance of a fiber-optic hydrogen sensor based on a controllable optical heating technology.

A novel, to the best of our knowledge, and compact fiber-optic hydrogen sensor based on light intensity demodulation and controllable optical heating technology is proposed and experimentally investigated. This system employs three photodetectors for optic signal transformation. The first PD is used to receive a little fraction of the amplified spontaneous emission (ASE) for calibration, and the second PD is utilized to detect optic signal reflected by a single mode fiber deposited with WO3-Pd2Pt-Pt composite film. The last PD is utilized to receive the optical power reflected by the short fiber Bragg grating (SFBG) with a central wavelength located in a steep wavelength range (the intensity decreases approximately linearly with the increase of the wavelength) of the ASE light source. A 980 nm laser and proportion integration differentiation (PID) controller were employed to ensure the hydrogen sensitive film working at an operating temperature of 60°C. This sensing system can display a quick response time of 0.4 s toward 10,000 ppm hydrogen in air. In addition, the detection limit of 5 ppm in air can be achieved with this sensing system. The stability of this sensor can be greatly enhanced with a controllable optical heating system, which can greatly promote its potential application in various fields.

A novel short fiber Bragg grating accelerometer based on a V-type dual mass block structure for low- and medium-frequency vibration measurements

A novel short fiber Bragg grating (FBG) accelerometer based on a dual mass block structure is presented in this paper. The sensor is established on a solid, symmetrical design comprising a V-type beam. The configuration in the FBG accelerometer causes the FBG fiber to distort twice as much when subjected to external vibration. The theoretical research and optimization of the FBG accelerometer's natural frequency and sensitivity are performed using MATLAB software, while static structural stress and modal simulation analysis are examined using ANSYS software. An experimental test is applied to evaluate the implementation of the sensors. The natural frequency of the sensor is 880 Hz, and its stability frequency response is flat in the 20–340 Hz low- to middle-frequency range. The linearity of the average sensitivity is approximately 66.95 pm/g, and the transverse anti-interference of the FBG fiber is around 6.53%. This work proposes a suitable method to observe low- and medium-frequency vibration signals in large-scale applications.

In-situ monitoring of internal temperature and strain of solid-state battery based on optical fiber sensors

As an emerging battery, solid-state battery has broad development and application prospects. The safety and stability of the battery cannot be monitored in real-time during the work process. In this work, we fabricated solid electrolytes, assembled solid-state battery and proposed a method to monitor internal real-time temperature and strain information by embedding short fiber Bragg gratings in the battery. The experimental results show that the maximum temperature change is 0.42 °C and the maximum strain change is 11.5 με during the charging and discharging processes of the battery, and the formation of lithium dendrites is observed. This method enables real-time online in-situ monitoring of temperature and strain in solid-state battery and explains the causes of strain at the microscopic level, which is critical for the safe use of the battery. • Optical fiber sensors embedded in solid-battery for in-situ measurements. • Real-time monitoring of temperature and strain. • To explain the formation of strain at the microscopic level. • To optimise of battery designs and to promote solid-battery safety improvement.

A flexible hinge accelerometer based on dual short fiber Bragg grating

Fiber Bragg gratings (FBGs) are a type of optical element with several advantages. In this study, we propose a vibration monitoring system consisting of a flexible hinge and a dual FBG. The novel design enables the dual short FBG to be tensile and compressed separately during vibration, while achieving sensitivity enhancement and self-temperature compensation. Through theoretical analysis and experiments, the resonant frequency was 150 Hz. The working frequency bandwidth of the accelerometer ranged from 10 to 100 Hz with a sensitivity of 1711 pm/g. The minimum resolution and detectable acceleration of the proposed system were 0.584 mg and 3 mg, respectively, which are significantly better than similar systems with a single FBG.

Highly Dense FBG Temperature Sensor Assisted with Deep Learning Algorithms.

In this paper, we demonstrate the application of deep neural networks (DNNs) for processing the reflectance spectrum from a fiberoptic temperature sensor composed of densely inscribed fiber bragg gratings (FBG). Such sensors are commonly avoided in practice since close arrangement of short FBGs results in distortion of the spectrum caused by mutual interference between gratings. In our work the temperature sensor contained 50 FBGs with the length of 0.95 mm, edge-to-edge distance of 0.05 mm and arranged in the 1500–1600 nm spectral range. Instead of solving the direct peak detection problem for distorted signal, we applied DNNs to predict temperature distribution from entire reflectance spectrum registered by the sensor. We propose an experimental calibration setup where the dense FBG sensor is located close to an array of sparse FBG sensors. The goal of DNNs is to predict the positions of the reflectance peaks of the reference sparse FBG sensors from the reflectance spectrum of the dense FBG sensor. We show that a convolution neural network is able to predict the positions of FBG reflectance peaks of sparse sensors with mean absolute error of 7.8 pm that is slightly higher than the hardware reused interrogator equal to 5 pm. We believe that dense FBG sensors assisted with DNNs have a high potential to increase spatial resolution and also extend the length of a fiber optical sensors.

Sensitivity Enhancement of Fiber-Optic Accelerometers Using Thin-Cladding Fiber Bragg Gratings

We propose and demonstrate a method for improving the sensitivity of fiber Bragg grating (FBG) based accelerometers. The method employs a short FBG inscribed in single mode fiber (SMF) with cladding substantially thinner than 125 μm. The resultant stress concentration at the FBG significantly improved the axial strain response compared with 125-μm-diameter fiber. A 50-μm-diameter FBG fixed on a cantilever beam was used to experimentally demonstrate this sensitivity enhancement. The accelerometer has a working frequency bandwidth from 0.5 Hz to 30 Hz with a sensitivity of 2150±30 pm/g, amounting to 5-fold sensitivity enhancement compared to 125-μm-diameter fiber.

Wear measurement based on the length variation of a sacrificial FBG.

A high-precision wear measurement method with temperature stability achieved by measuring the length variation of a fiber Bragg grating (FBG) is proposed. The adoption of the optical frequency-domain reflectometry (OFDR) technology makes the spatial resolution of this measurement method reach 15.13 µm, and the offline and online measurement accuracies are 30 µm and 100 µm, respectively. The systematic error of the FBG length measuring system is within 30 µm. Because the length measurement is done with a short FBG instead of a much longer fiber, the measurement error induced by the time-varying temperature or strain is significantly reduced in the proposed method. The spatial resolution and accuracy of this method is suitable for wear measurements of various parts in the mechanical field, such as bearings, gears, and pistons.

High spatial resolution temperature field measurements by apodized short fiber Bragg grating sensors

To realize the high spatial resolution measurement of the temperature field, a temperature sensor array composed of eight apodized short fiber Bragg gratings with grating length of 1 mm and spatial spacing of 2 mm are fabricated. To suppress the side lobes, apodization methods by single-slit diffraction are designed. The sensor array is proposed to measure the real-time temperature distribution. The experimental results show that the temperature-sensing array can accurately measure the temperature field distribution with high spatial resolution and rapid response.

Characterization of 100 Micron Long Fiber Bragg Gratings and Applications in Ultrafast Shock Wave Experiments

Ultrashort Fiber Bragg Gratings (FBGs) may be very useful whenever highly localized pressure measurements are desired. Obviously, very short FBGs will exhibit lower, as well as spectrally broader, reflections; however, these deficiencies may be outweighed by the advantage of the potential for highly localized measurement. In addition, with such short gratings, their accurate localization along the fiber is of great importance. In this paper, commercial 100 μ m FBGs are characterized, and their location along the fiber is accurately determined by two methods: Spectral and white-light interferometry. We also demonstrate the advantage of ultrashort gratings in a shock-wave experiment; when a solid target is hit by a projectile, the resultant pressure waves may change rapidly in both location and time. In this paper, the suitability of a 100 μ m grating to this scenario is demonstrated, revealing features in good agreement with theory—in particular, an approximately 0.7 mm long constant pressure region. Although these pressure waves may be monitored by ordinary (∼6–8 mm long) FBGs, due to the high-pressure gradients involved, the FBG response will be very complicated and data retrieval may be ambiguous. Thus the advantage of ultrashort FBGs is demonstrated.

In-Line Mach-Zehnder Interferometer With D-Shaped Fiber Grating for Temperature-Discriminated Directional Curvature Measurement

A high-sensitivity curvature sensing configuration is implemented by using a fiber Mach-Zehnder interferometer (MZI) with D-shaped fiber Bragg grating (FBG). A segment of D-shaped fiber is fusion spliced into a single mode fiber at both sides, and then a short FBG is inscribed in the D-shaped fiber. The fiber device yields a significant spectrum sensitivity as high as 87.7 nm/m−1 to the ultralow curvature range from 0 to 0.3 m−1 , and can distinguish the orientation of curvature experienced by the fiber as the attenuation dip producing either a blue or red wavelength shift, by virtue of the asymmetry of D-shaped fiber cladding. In addition, by tracking both resonant wavelengths of the MZI and embedded FBG, the temperature and curvature can be measured simultaneously.

Through-the-coating femtosecond laser inscription of very short fiber Bragg gratings for acoustic and high temperature sensing applications

Very short Type I and Type II Bragg gratings, on the order of 100 µm in length, are written through the protective polyimide coating of high NA and standard single mode silica optical fibers with infrared femtosecond pulses and a phase mask. By exploiting the transverse walk-off of apertured diffracted beams produced by the phase mask and a slit placed proximate the mask, complex grating structures are fabricated and characterized. These gratings are suitable for structural health monitoring based on acoustic measurements or localized high-temperature measurements.

Fiber Bragg gratings inscriptions in multimode fiber using 800 nm femtosecond laser for high-temperature strain measurement

A short fiber Bragg grating (FBG) is successfully written in a multimode fiber (MMF) with core and cladding diameters of 50μm and 125μm using 800nm femtosecond laser side-illumination technique. Three-type grating inscriptions can be realized at the different positions over the core of MMF by moving the focal-line position of laser beam. Both fundamental mode and higher-order modes of MMF are excited at the core-mismatch junction, resulting in two well-defined resonances in transmission. The strain measurement with a single core resonance mode is demonstrated experimentally at the ultra-high temperature. The result indicates that the strain sensitivity improved to 5.24pm/µε at the temperature of 600–900°C, making it as a good candidate for the strain measurement at the high temperature environments.

Double Cantilever Beams Accelerometer Using Short Fiber Bragg Grating for Eliminating Chirp

An accelerometer based on the shorter fiber Bragg grating (FBG) is demonstrated and experimentally researched. The reflective optic spectra of FBG under inhomogeneous strain are researched by using the coupled-mode theory; the theoretical analysis demonstrates that the features of the reflective optic spectra are closely related with the chirping coefficient and the length of FBG under inhomogeneous strain. On these basics, an FBG accelerometer based on the double cantilever beams with small strain gradient is pointedly proposed. Experimental results show that the accelerometer has good response characteristic, and it provides broad flat frequency range from 4 to 30 Hz, corresponding sensitivity range from 7.76 to 10.8 pm/ms−2, while the flatness is less than 3 dB, the cross-axis sensitivity is less than 3.89% of the main-axis sensitivity, and the temperature sensitivity can be effectively eliminated by the self-symmetrical structure.

Optimal Design of Short Fiber Bragg Grating Using Bat Algorithm With Adaptive Position Update

We propose a new method to optimally design short triangular-spectrum fiber Bragg gratings (TS-FBGs), using the metaheuristic bat algorithm (BA). In addition, we propose a novel modification, i.e., adaptive position update (APU), to optimize the index modulation within a predetermined range. The discrete layer-peeling algorithm is used to efficiently initialize BA. The APU improves the search performance and accuracy of the BA algorithm in fiber Bragg grating (FBG) design application. Using the proposed method, a 0.2-nm-bandwidth TS-FBG, operating in the C-band, is designed. The short-length FBG with chirp-free structure prevents complex phase modulation for practical considerations. In addition, a dual-wavelength TS-FBG of 0.75-nm bandwidth operating in the eye-safe region has been designed and provided the most accurate optimum design length. The latter is well suited for FBGs inscribed in high-birefringent fibers, which are utilized in multiwavelength fiber laser systems. For both cases, the accuracy and search performance of APU-BA are shown to be better than those of previously reported algorithms in the literature.

FBG Incorporated Side-open Fabry-Perot Cavity for Simultaneous Gas Pressure and Temperature Measurements

A dual-parameter sensing structure based on a fiber Bragg grating (FBG) incorporated side-open Fabry-Perot cavity (FPC) is proposed and experimentally demonstrated for simultaneous measurements of gas pressure and temperature. The side-open FPC consisted of a micro-piece of chemical etched side-hole fiber acts as a pressure sensing unit. Two samples with different cavity lengths of 60 μm and 100 μm were fabricated and tested for gas pressure and temperature sensing in the experiment. Linear gas pressure responses of 4.063 pm/kPa and 4.071 pm/kPa in a large measuring range of 0-1000 kPa were obtained, respectively, which agree well with the theoretical analysis. Meanwhile, the temperature cross-sensitivities were measured as 214 Pa/° and 204 Pa/°. As a local temperature monitor, the incorporation of a short FBG in the leading single mode fiber was used to compensate the temperature cross-sensitivity of the sensor. This kind of compact and highly sensitive fiber sensor is appropriate for simultaneous gas pressure and temperature sensing in harsh micro environment.

Highly Integrated FP/FBG Sensor for Simultaneous Measurement of High Temperature and Strain

An inline fiber-optic micro-Fabry-Perot (MFP) cavity and a short fiber Bragg grating (SFBG) is overlapped to form an integrated MFP/SFBG sensor for simultaneous measurement of temperature and strain under high temperature (300 °C). The FP cavity is fabricated on a GeO 2 -doped photon sensitive fiber using the 157-nm laser micromachining technique and the SFBG is written at the same position of the MFP cavity using the standard 248-nm laser method. As the MFP cavity and SFBG have different sensitivity coefficients to temperature and strain, they can be utilized for realization of simultaneous measurement of dual parameters.

Fiber Bragg grating based investigation of residual strains in ABS parts fabricated by fused deposition modeling process

Fused deposition modeling (FDM) is a process by which functional parts can be rapidly produced by sequential deposition of fused material layers. Various severe inaccuracies such as curl and warp distortions and delaminations are attributed to residual stresses and strains build up during the fabrication process. Their magnitude is largely influenced by the selected process parameters. This study investigates the magnitude of the solidification induced residual strains in FDM fabricated parts using different processing parameters. Two important parameters were considered: (a) layer thickness and (b) deposition orientation. The developed residual strains at the end of the fabrication process were recorded using an optical sensor with a short fiber Bragg grating (FBG) embedded at the midplane of FDM built specimens. The same specimens were additionally subjected to a thermal cycle where the coefficient of thermal expansion (CTE) of the ABS material was measured using the FBG sensor.

Optimal design of short fiber Bragg gratings with triangular spectrum

Based on the mutated adaptive quantum-behaved particle swarm optimization (MAQPSO) algorithm and the discrete layer peeling (DLP) algorithm, we propose a method to design triangular-spectrum fiber Bragg gratings (TS-FBGs). In this method, the DLP algorithm is used to generate an appropriate initial range of the index modulation, and the MAQPSO technique is applied to optimize the index modulation within this range. The quantum-behaved particle swarm optimization algorithm with mutation operation can improve the search performance of this algorithm. We purposely use a linear weighting factor for the fitness value function in view of the linear edge triangular-spectrum. This dramatically accelerates the convergence in the numerical implementation. Using the proposed method, a 0.2 nm bandwidth TS-FBG is designed and the performance is shown to be better than previously reported results in the literature. Then, a 2.5 nm bandwidth TS-FBG is reconstructed; it is indicated that a short grating length can be used to realize the TS-FBG with chirp-free structure so that complex phase modulation is avoided in fabrication. Finally, a triple-channel TS-FBG with sampled-free structure is successfully designed and optimized. We also present an analysis on the tolerance of the design method over possible fabrication error.

Power-Referenced Optical Fiber Refractometer Based on a Hybrid Fiber Grating

An in-fiber structure for power-referenced refractometry based on a hybrid fiber grating is proposed and demonstrated experimentally. The hybrid fiber grating is formed by superimposing a short fiber Bragg grating (FBG) on the rear part of a much longer tilted-FBG (TFBG) so that it can work in the reflection mode. The reflected optical power from the hybrid fiber grating can be changed with the variation of the surrounding refractive index. Experimental results show that maximum sensitivity of 228.24 dB/R.I.U. has been achieved within the refractive index range of 1.33-1.46, and the measurement is nearly insensitive to temperature.

Writing wide bandwidth nonchirped fiber Bragg gratings with high sidelobe suppression ratio by linearly scaling apodization

A linearly scaling optical apodization method for laser beam power profile modulation is induced. Changing an amplitude mask position between the beam-focusing cylindrical lens and the fiber can linearly scale the vertical apodization profile; this, in turn, enables the optimization of the grating apodization. By using this method, an apodized fiber Bragg grating (FBG) that is designed as an uncooled pump laser diode stabilizer with the specifications of 4.5% reflectivity, 1.6 nm bandwidth, and the side-lobe suppression ratio (SLSR) of 30 dB was UV-imprinted with a tiny Gaussian amplitude mask. Using the same apodization method, a nonchirped-type FBG edge filter was written with a very smooth spectrum. The grating specifications of 0.5-dB attenuation bandwidth of 3.6 nm and 96.5% reflectivity were achieved. The half-spectrum ascending bandwidth in the longer wavelength side is 2.2 nm with 27-dB SLSR. The linearly scaling apodization method is highly beneficial for optimizing the apodization process of UV-writing shorter FBGs with strong apodization.