Bragg Grating Research Articles

All-fiber, high-speed, and high-resolution dispersion measurements of chirped fiber Bragg gratings

All-fiber, high-speed, and high-resolution dispersion measurements of chirped fiber Bragg gratings

Spacing mode multi-wavelength erbium doped fiber laser based on a symmetrical long-period fiber grating.

Abstract We present a multi-wavelength erbium-doped fiber laser based on a symmetrical long-period fiber grating (LPFG) with spacing mode between single lasing mode and multiwavelength spectra. The LPFG was manufactured by thermal expansion technique using a LZM-100 glass processing system equipped with a CO2 laser, and was used as a wavelength-selective filter (WSF) in the laser ring. The laser can emit a single, double, triple, quadruple, quintuple, or sextuple lines in the range from 1546 to 1563 nm, that can be tunable by controlling the radius of curvature of the LPFG in the range 0-0.2662 m -1. A minimal spacing mode of 1.92 nm was observed in the multi-wavelength region; meanwhile the fiber laser offers an average spacing mode of 6.945 nm between the multi-wavelength region and the single emission. This laser has a 3 dB linewidth of 0.11 nm and a side mode suppression ratio (SMSR) of 55.56 dB. Finally, according to experimental results, the laser has high wavelength stability at room temperature for 88 min.

Fiber Bragg grating-based touch-slip sensor for surface roughness detection

To enable humanoid prosthetic hands to accurately identify and grasp objects, a touch-slip sensor based on fiber Bragg grating (FBG) was proposed in this paper. The sensor was designed with a double-layer sensing structure to detect three-dimensional force, sliding information, surface roughness, and compensate for ambient temperature using a reference grating. To analyze the relationship between the sensor surface structure parameters and FBG's vibration signal, the contact sliding model was introduced. By using a finite element simulation, the contact sliding model was validated, and the surface structure parameters of the sensor were determined. A series of experiments were conducted on the sensor’s three-dimensional force detection, temperature calibration, sliding measurement, and surface roughness detection. Using machine learning methods, a regression prediction model was created. The sensor could detect the surface roughness on the sample plate surface more accurately with a maximum error of 8.58×10−4. This sensor has the following advantages: a simple structure, low cost, high linearity, and quick response time. It provides a new design solution for the detection of touch-slips on humanoid prosthetic hands.

The Development of Multiplexing Capability for Reflective Matched Fiber Bragg Gratings Interrogation Technique and its Application in Real-Time Micro cracks Detection

The Development of Multiplexing Capability for Reflective Matched Fiber Bragg Gratings Interrogation Technique and its Application in Real-Time Micro cracks Detection

Simultaneous bending and temperature measurement based on a superimposed fiber grating sensor

A compact and novel superimposed fiber grating (SI-FG) sensor for simultaneous measurement of temperature and bending based on UV-induced FBG cascaded with a multimode fiber (MMF) embedded ultra-long period fiber grating (MMF-EULPFG) has been proposed and experimentally validated. The MMF-EULPFG acts as a temperature and bending sensing unit and was fabricated by periodically splicing a non-UV-non-photosensitive MMF and a UV-photosensitive single-mode fiber (SMF) which is so called cleaving-splicing method. Due to the periodic refractive index modulation, the 6-period SMF-MMF structure based MMF-EULPFG with a total length of 2.7 mm forms transmission spectra with a −17 dB dip. The proposed SI-FG sensor exhibits high bending and temperature sensitivities of 17.096 nm/m−1 and −0.238 nm/℃, respectively. The UV-induced FBG is fabricated in the initial segment of UV-sensitive fibers within EULPFG by UV exposure method. The dip of FBG transmission spectra also shows response to bending and temperature. By utilizing a matrix obtained from the experimental results, simultaneous measurement of temperature and bending can be achieved. Due to its compact structure and high sensitivity, the proposed novel SI-FG sensor can be used as a competitive all-fiber sensor for structural health monitoring and 3D-shape sensing.Index Terms: Optical fiber sensor, long period fiber grating, superimposed fiber grating, simultaneous measure.

Refractive Index Determination and Air-Void Characterization of BNNT Assembly Using Optical Fiber Bragg Grating Sensors

Refractive Index Determination and Air-Void Characterization of BNNT Assembly Using Optical Fiber Bragg Grating Sensors

Signal Amplification Study for Gold-Coated Tilted Fiber Bragg Grating With Avidin-Biotin Sandwich Assay in the Detection of SARS-CoV-2

Signal Amplification Study for Gold-Coated Tilted Fiber Bragg Grating With Avidin-Biotin Sandwich Assay in the Detection of SARS-CoV-2

Temperature Compensated Differential Pressure Sensor Based on a Glass Membrane With Femtosecond Laser Written Bragg Gratings

Temperature Compensated Differential Pressure Sensor Based on a Glass Membrane With Femtosecond Laser Written Bragg Gratings

Birefringence Free Spectral Tuning of Bragg Grating Based Fabry-Pérot Resonator in LiNbO₃ Waveguide Employing Counter- Propagating Cladding Modes

Birefringence Free Spectral Tuning of Bragg Grating Based Fabry-Pérot Resonator in LiNbO₃ Waveguide Employing Counter- Propagating Cladding Modes

A novel single mode fiber optic temperature sensor combined with the FLRDS technique

Abstract A single mode optical fiber loop was employed as a temperature sensor to observe changes in optical loss regarding to ringdown time (RDT) by high sensitive the fiber loop ringdown spectroscopy (FLRDS) technique which has real-time and fast response measurement capability due to allowing trapped light pulse multiple interactions with the measurands. Two different fiber loops of 455 m and 1205 m lengths were embedded one by one into a copper, circular and closed housing. Continuous monitoring of RDTs was carried out by changing the temperature in the range of 25-200 ℃ with the steps of 25 ℃ for the first time by using bare fiber without any modification as a temperature sensor. The FLRDS system for temperature sensing has simple design without extra components such as an optical time domain reflectometer (OTDR), long-period fiber grating (LPFG) or fiber Bragg grating (FBG) as sensorhead. The FLRDS system was diligently optimized to achieve the lowest baseline as %0.49. Since the RDT of the FLRDS system was changed due to the thermal expansion of the fiber, continuous monitoring of the temperature was the first time recorded by utilizing this kind of FLRDS temperature sensor. These kind of FLRDS temperature sensors have high potential to be employed in mining, nuclear facilities, railways, underwater structures, biomedical, medicine, structural health monitoring, transportation and communication applications with simple system setup, lower cost, higher sensitivity, portability, real-time and continuous monitoring for early detection.

Accuracy compensation method for 2D curve reconstruction of torsional FBG shape sensor of scraper conveyor

The straightness perception of scraper conveyor is a technology that helps to achieve intelligent operation of coal mine working faces. However, there is a problem of accuracy degradation caused by sensor torsion based on fiber Bragg grating (FBG) shape sensing technology. This paper proposes an accuracy compensation method based on torsion angle, and verifies the effectiveness of accuracy compensation through simulation and experimental research. Experimental tests have shown that when the torsion angle of the sensing substrate is 10° and 20°, the end accuracy of the reconstructed curve after accuracy compensation is increased by 2.77% and 1.93% compared to before compensation, respectively. This study indicates that compensation based on the torsion angle can significantly improve the accuracy of two-dimensional (2D) curve reconstruction, and it has broad engineering application prospects in fields such as straightness perception of scraper conveyors and monitoring of underwater pipelines.

Performance of an Anti-Phase Technology-Powered Microwave Ablation System on Ex Vivo Liver, Lung and Kidney: Analysis of Temperature Trend, Ablation Size and Sphericity.

Investigating the performance of the new Dophi™ M150E Microwave Ablation System, in terms of temperature distribution, ablation size and shape, reproducibility. The Dophi™ M150E Microwave Ablation System was tested on ex vivo liver, lung and kidney, at 6 different settings of time, power and number of MW antennas (single antenna: 50 and 100W at 5 and 10min; double antenna: 75W at 5 and 10min). The temperature distribution was recorded by Fiber Bragg Grating sensors, placed at different distances from the antennas. The ablation axes were measured and the sphericity index was calculated. The standard deviation of ablation axes was < 5mm, except at the highest energy and time setting for the lung. A maximum temperature rise of ~ 80°C was measured. The measured ablation axes are overall comparable with the manufacture's values, especially at lower power and with one MW antenna (average maximum difference is 7mm). The mean sphericity index of 0.95, 0.79 and 0.9 was obtained for the liver, lung and kidney, respectively, with a single antenna. With double antenna setup, the sphericity index was closer to 1 when 75W for 10min were used. Dophi™ M150E allows good reproducibility of ablation axes for all cases except in the lung at the highest energy level. With one antenna, an almost spherical ablation area for the liver and kidney was obtained. Using double antenna results in more homogeneous temperature distribution within the tissue compared to single antenna.

Flexible Arc-Shaped Micro-Fiber Bragg Grating Array Three-Dimensional Tactile Sensor for Fingertip Signals Detection and Human Pulse Monitoring.

A flexible arc-shaped micro-Fiber Bragg Grating (mFBG) array three-dimensional tactile sensor for fingertip signal detection and human pulse monitoring is presented. It is based on a three mFBGs array which is embedded in an arc-shaped poly (dimethylsiloxane) (PDMS) elastomer, which can effectively discriminate the normal force, left force, and right force by monitoring the reflected intensity variation of the three mFBGs. Different from the traditional FBG sensors, this sensor measures force by detecting changes in light intensity, effectively avoiding the wavelength cross-sensitivity impact of temperature variations on the sensor performance. This design strategy simplifies the sensor structure, reduces the system complexity and signal interrogation cost, and enhances reliability and practicality. Through systematic experiments, we successfully validated the sensor's superior performance, achieving a minimum detection force of 0.01 N and providing robust data support for practical applications. In addition, the sensor has been used to monitor human pulse accurately. The successful fabrication and experimental validation of this sensor lay a foundation for its widespread application in fields such as robot perception and human vital signal detection.

Suppressing Side‐Scattering on Laser‐Written Bragg Gratings for Back‐Reflection Engineering in Fibers

AbstractLaser direct writing (LDW) is versatile in structuring fibers with micro‐sized functional elements such as fiber Bragg grating (FBG) and backscattering centers by finely manipulating back and side scattering from laser‐induced refractive index modified (RIM) points. However, the side‐scattering is a lesser‐explored property in laser‐structured fibers. In this work, a concise physical model is established to understand the side‐scattering as a combined effect of microstructure and geometry of RIM points. Based on a single‐pulsed LDW method, the parametric decoupling between scattering loss (α) and coupling strength (κ) coefficients of FBGs is reported, whose cross‐section is customized to have a flattened ellipse with thoroughly positive RIM, enabling controllable reflectivity from −21.33 dB to −0.0018 dB while maintaining narrow bandwidth and low loss. Exemplarily, a designed FBG realizes ultra‐low loss of 0.008 dB with a resonance attenuation of 10.81 dB, exhibiting a record‐breaking κ/α of 2083. Using this FBG as the high‐reflective mirror of a home‐made bismuth‐doped fiber laser, narrow‐band lasing with a high optical signal‐to‐noise ratio of ≈43 dB is achieved, demonstrating flexibilities of the proposed approach in customizing both back‐ and side‐scattering in fibers and opening up wide opportunities for combining multifunctional components into optical fibers and realizing all‐fiber networks.

Strain Measurement for Composite Materials During Mechanical Testing Based on Fiber Bragg Grating Monitoring

Composite materials are widely used as repair materials in the oil and gas sector, particularly for vessels, piping, and pipelines. Inspection and monitoring of these composite repairs is a critical aspect of ensuring their integrity and long-term performance. The reason being that failure of composite repairs may lead to loss of production and incur additional costs for re-repair or equipment replacement. Conventionally, a visual inspection is generally performed to observe any irregularities on the external surface of the composite repairs. This will usually be followed by a series of Non-destructive Tests (NDT) to identify the extent of the damage. However, these NDT methods, merely allowing off-line testing in a local manner with complicated and heavy equipment, require extensive and time-consuming labour. Structural Health Monitoring (SHM) based on Fibre Bragg Grating (FBG) that utilizes strain-based signals is one of the best options for monitoring the damage mechanism in composite due to its unique advantages of light weight, high stability and reliability, a long-life cycle, and the ability to capture the online reading. In this paper, the application of the FBG monitoring approach was used to examine the strain characteristics of composite materials based on mechanical tests (tensile and flexural). Three types of samples were prepared to simulate the type of damage mechanism: healthy, delamination, and disbondment. Based on the results, it was shown that the FBG sensor has potential for detecting and distinguishing the types of failure mechanisms in composites, including cracks, delamination, and disbondment. Hence, it was suggested that the FBG sensor technology could be used for site deployment in industry.

A double clad ASE Re-injected hybrid TDFA and HDFA amplifier with ±1.44 dB GF

Abstract Current developments in wireless communications accentuate the exigency of high gain, low noise figure (NF) and minimum gain fluctuations (GF) based optical amplifier in 1.9–2.15 μm eye safe wavelength band. Hybrid thulium (Tm) doped fiber amplifier (TDFA) and holmium (Ho) doped fiber amplifier (HDFA) are the perfect candidates to extend the bandwidth towards 2 μm long wavelengths. In this work, amplifier spontaneous emission (ASE) re-injected double clad TDFA-HDFA hybrid amplifier using Fiber bragg gratings (FBGs) is presented for the amplification of 64 wavelength division multiplexed (WDM) channels covering 63 nm wavelength window. Further, performance of proposed amplifier with and without ASE reinjection is evaluated in terms of Gain, NF, GF and optical signal to noise ratio (OSNR). Effects of single and double clad on the Gain are studied and optimization of Tm and Ho doped fiber length, pump powers, and input power has also been accomplished. The GF as low as ±1.44 dB, Gain &gt;28 dB, NF &lt; 5.08 dB, output power 4.2 W are achieved in ASE re-injected configuration using 785 nm pump at 55 dBm in TDFA and 1,900 nm pump at 20 dBm in HDFA. It is noticed that double clad and ASE reinjection in TDFA and HDFA offer better GF.

In-Fiber Hybrid Structure Sensor Based on the Vernier Effect for Vector Curvature and Temperature Measurement

A vector curvature and temperature sensor based on an in-fiber hybrid microstructure is proposed and experimentally demonstrated. The proposed scheme enables the dimensions of the Fabry–Perot and Mach–Zehnder hybrid interferometer to be adjusted for the formation of the Vernier effect by simply changing the length of a single optical fiber. The sensor is fabricated using a fiber Bragg grating (FBG), multimode fiber (MMF), and a single-hole dual-core fiber (SHDCF). The sensor exhibits different curvature sensitivities in four vertical directions, enabling two-dimensional curvature sensing. The temperature and curvature sensitivities of the sensor were enhanced to 100 pm/°C and −25.55 nm/m−1, respectively, and the temperature crosstalk was minimal at −3.9×10−3 m−1/°C. This hybrid microstructure sensor technology can be applied to high-sensitivity two-dimensional vector curvature and temperature detection for structural health monitoring of buildings, bridge engineering, and other related fields.

Fatigue and residual strain monitoring for thermoplastic composite using embedded FBG inscribed in highly‑birefringent side-hole elliptical core optical fiber

Monitoring of thermoplastic carbon reinforced composites is critical for the safe operation and optimization of composite structures. Strain measurement during their manufacturing and operation is often achieved with embedded Fiber Bragg Gratings (FBG). However, conventional FBGs can only measure axial strain. To measure multiaxial strain in thermoplastic composites, FBGs inscribed in highly-birefringent side-hole elliptical core optical fiber (SH2 FBG) are proposed. SH2 FBG were used to monitor composite produced by automated tape laying process, first − to measure residual in-plane and out-of-plane strain during the manufacturing stage, second − in plane transverse strain during cyclic loading. It was demonstrated that the sensors can be used to measure multiaxial residual strain, observe changes in material stiffness, and calculate the area of the mechanical hysteresis loop. As a result − it was shown that embedding SH2 FBG can be an effective and robust method for monitoring the carbon fiber reinforced thermoplastic composite.

Stress transfer law in laboratory hydraulic fracturing experiments

During the true triaxial hydraulic fracturing experiments, the compression stress applied to the specimen surface cannot be transferred to the interior immediately, resulting in inconsistency with in-situ stress conditions. To quantitatively analyze the stress transfer process from the surface to the interior of the specimen, an experimental method for monitoring the inside stress was proposed based on Fiber Bragg Grating (FBG) sensing technique, based on which the true triaxial stress loading experiments were conducted on a concrete-like specimen of 30 cm × 30 cm × 30 cm. The results show that the stresses inside the specimen require a certain loading time to reach the uniform state. The loading time required for stress transfer from the surface to the interior of the specimen decreases with the increase of compression stresses. The stress transfer process in rock materials is determined by creep. The closure of microcracks results in stress redistribution inside the specimen during creep. Moreover, a 3-D Burgers model is modified to describe the stress transfer process. Finally, the stress transfer phenomenon during hydraulic fracturing was verified by coal fracturing experiments. This study can help to understand the stress transfer mechanism, providing guidance for standardizing the laboratory simulation of in-situ stress.

Fabrication of high temperature chirped FBG by thermal stretching of regenerated FBG

This paper presents an innovative method of high temperature sustainable chirped fiber Bragg grating (FBG) fabrication from the uniform regenerated FBG or their combination written by a single uniform phase mask. A thermal stretching technique is employed that modifies the grating structure of uniform FBGs, resulting in chirped FBGs capable of withstanding high temperatures without compromising their sensing capabilities. Thermal regeneration of uniform FBG was carried out at 850oC by applying step annealing schedule. Thermal stretching of single regenerated FBG and dual regenerated FBG was carried out for the fabrication of high temperature chirped FBG. For both approaches, the FBG was stretched by applying strain ~1200 µε along the length of the grating. In the single FBG approach, the chirped grating with reflection intensity 4500 (A.U.) and bandwidth ~ 7.1nm was fabricated by applying thermal gradient from 860oC to 900oC across the length of strained thermally regenerated FBG. In double grating approach, the chirped FBG with reflection intensity 6000 (A.U.) and bandwidth 12.7nm was fabricated. The fabricated chirped FBG can sustain temperature upto 900 oC. This innovative method not only extends the operating range of chirped FBGs to high temperatures but also opens up new possibilities for fabricating chirped FBGs of longer length and bandwidth using low-cost uniform FBGs.