Bragg Grating Research Articles

High Sensitivity Fiber Bragg Grating Humidity Sensors Made With Through-the-Coating Femtosecond Laser Writing and Polyimide Coating Thickening

High Sensitivity Fiber Bragg Grating Humidity Sensors Made With Through-the-Coating Femtosecond Laser Writing and Polyimide Coating Thickening

Frost-Resistance Assessment of C60 Concrete Based on a Fiber Bragg Grating Sensor

Frost-Resistance Assessment of C60 Concrete Based on a Fiber Bragg Grating Sensor

Remote monitoring of sleep disorder using FBG sensors and FSO transmission system enabled smart vest

Optical sensors, particularly fiber Bragg grating (FBG) sensors have achieved a fast ingress into the fields of medical diagnostic and vital signs monitoring. Wearable smart textiles equipped with FBG sensors are catching huge research attention in different applications for measurement and monitoring of physiological parameters. In this paper, we report a simple technique for remote monitoring of sleep disorder using a smart vest implemented with four FBG stress sensors located at different sides of the vest and free space optics (FSO) transmission system. The sleep disorder of the patient is monitored in real time through shifts in the original Bragg wavelengths of sensors by stress loading during random changes in patient’s sleeping postures. The reflected wavelength from a stress loaded sensor at a certain posture is transmitted over 0.5 km long FSO channel towards remote medical center, photodetected, and then can be processed in a PC to record the restlessness in a certain time interval in terms of total number of times sleeping postures are changed, total time spent at a certain posture etc. To correctly detect the stress loaded FBG sensor at the medical center, various parameters of FBG sensors and demultiplexer are carefully adjusted to minimize the power leakages from unloaded sensors that may result into errors in the detection. Maximum dynamic range around 45 dB has been achieved ensuring accurate detection. This study not only provides a cost-efficient and non-intrusive solution for monitoring the sleep disorder of patients but also can be used for real-time monitoring of various other ailments, such as lung, brain, and cardiac diseases in future.

Pd–Ta alloy films hydrogen sensors based on partially coated π -phase-shifted FBG

A fiber-optic hydrogen sensor based on Pd–Ta alloy films partially coated π-phase-shifted fiber-Bragg grating (FBG) is proposed and experimentally demonstrated. Three types of Pd–Ta alloy films with different Ta content were prepared via co-sputtering technology. Material characterization shows the transformation of Pd–Ta alloy to amorphous phase with the increase of Ta content, and the phase transition between α-PdTaHx and β-PdTaHx is inhibited at 2 % hydrogen concentration. Hydrogen sensing experiments show that in-situ temperature compensation is possible with Pd/Ta films partially coated on π-phase-shifted FBG. Moreover, the sensor's hysteresis phenomenon was eliminated with an increase in Ta content due to the first-order phase transition. Pd–Ta alloy films hydrogen sensor with a Ta content of 27 % is capable of in-situ temperature compensation, fast response, good stability, and reliability.

Multi-channel broadband optical chaos generation assisted by phase modulation and CFBG feedback

In this paper, we propose a novel and simple multi-channel broadband optical chaos generation scheme based on phase modulation and chirped fiber Bragg grating (CFBG). Firstly, phase modulation is introduced to generate more new frequency components to broaden the spectrum of the phase chaos. Meanwhile, the accumulated dispersion from CFBG distorts the intensity chaos, converts phase chaos to intensity chaos, and weakens the laser relaxation oscillation. This process would lead to energy redistribution in the power spectrum, effectively increasing the chaotic bandwidth. Then, the wavelength detuning between CFBG and the semiconductor laser is introduced to enhance the chaotic bandwidth further. The experiment results show that the 10 dB bandwidths of the five channels are up to 31.0 GHz, 34.3 GHz, 36.3 GHz, 40 GHz, and 40 GHz, respectively. Note that the maximum bandwidth of the PD in our experiment is limited to 40 GHz. In addition, the multi-channel chaotic signals obtained from the experiment system are used to generate multi-channel physical random numbers. After the post-processing operations, the total rate of five parallel high-speed physical random number generation channels is 4.64 Tbit/s (160 GSa/s × 5bit × 1 channel + 160 GSa/s × 6bit × 4 channels). As far as we know, this is the highest record of using external cavity feedback semiconductor lasers to generate random numbers, which has great potential to meet the security requirements of next-generation Tbit/s optical communication systems.

Chirped Integrated Bragg Grating Design

We analyze the two classic methods for chirped Integrated Bragg Gratings (IBGs) in Silicon-on-Insulator technology using the transfer matrix method based on the effective refractive index (neff) technique, which translates the geometry of an IBG into a matrix of neff depending on the wavelength. We also implement a procedure that allows engineering of the chirped IBG parameters, given a required bandwidth (BW) and group delay (GD). Finally, a complementary method for designing chirped IBG is proposed, showing a significant improvement in the bandwidth of the device or a moderation in the variation of the geometrical parameters of the grating.

Structural Damage Detection Based on Structural Macro-Strain Mode Shapes Extracted From Non-Stationary Output Responses

Abstract Long-gauge Fiber Bragg grating (FBG) strain sensors have been widely employed because of their broader measuring range and higher sensitivity. However, current structural damage detection methods using macro-strain modal parameters are based on structural frequency response function or stationary power spectrum density, which are not applicable to non-stationary responses. To overcome this limitation, an improved method is proposed in this paper for structural damage detection based on structural macro-strain responses under unknown multi-point non-stationary excitations. First, a new concept of macro-strain energy spectrum transmissibility (MEST) is proposed using structural non-stationary macro-strain responses, and it is derived that MEST at a certain system pole equals the ratio of macro-strain mode shape. Then, the singular value decomposition (SVD) technique is adopted for the MEST matrix to identify structural natural frequencies and macro-strain mode shapes. Finally, two damage detection indicators are constructed based on the identified normalized macro-strain mode shape (NMMS). The first indicator is the difference in structural NMMS before and after structural damage. The second one is based on the curvatures of structural NMMS, which can be used for structures without intact baseline. Numerical verifications are conducted to identify beam-type structural damage under multi-point nonstationary excitations or vehicle loads. Five damage scenarios with different measurement noise levels are investigated, and damage detection results validate the effectiveness of the proposed method.

Enabling ultra-high bit rate transmission with CFBG as dispersion compensator in an OptiSpan 240 km DWDM network

Abstract In this paper, a dispersion compensation technique for an ultra-long haul optical network utilizing a chirped fiber Bragg grating (CFBG) is presented. A high bit-rate signal of 40 Gbps is inputted into each channel. The CFBG, employed for dispersion compensation, is positioned after demultiplexing, enabling effortless network upgrades. The grating parameters of the implemented CFBG are mathematically analyzed and optimized to counteract net dispersion of over 3153 ps/nm across the channel. The design is verified using the OptiSystem software, resulting in a successful transmission up to 240 km with an average Q-factor of 8.09, utilizing an amplification gain of 49.5 dB. Additionally, the achieved optical signal-to-noise ratio (OSNR) level using the symmetrical amplification technique surpasses the acceptable value, with an average bit error rate of 10−16.

Research and Fabrication of the Chirped Moiré Fiber Bragg Grating With Tunable Channels Based on Temperature Control

Research and Fabrication of the Chirped Moiré Fiber Bragg Grating With Tunable Channels Based on Temperature Control

A Multiplexing Optical Temperature Sensing System for Induction Motors Using Few-Mode Fiber Spatial Mode Diversity

Induction motors are widely applied in motor drive systems. Effective temperature monitoring is one of the keys to ensuring the reliability and optimal performance of the motors. Therefore, this paper introduces a multiplexed optical temperature sensing system for induction motors based on few-mode fiber (FMF) spatial mode diversity. By using the spatial mode dimension of FMF, fiber Bragg grating (FBG) carried by different spatial modes of optical paths is embedded in different positions of the motor to realize multipoint synchronous multiplexing temperature monitoring. The paper establishes and demonstrates a photonic lantern-based mode division sensing system for motor temperature monitoring. As a proof of concept, the system demonstrates experiments in multiplexed temperature sensing for motor stators using the fundamental mode LP01 and high-order spatial modes LP11, LP21, and LP02. The FBG sensitivity carried by the above mode is 0.0107 nm/°C, 0.0106 nm/°C, 0.0097 nm/°C, and 0.0116 nm/°C, respectively. The dynamic temperature changes in the stator at different positions of the motor under speeds of 1k rpm, 1.5k rpm, 2k rpm with no load, 3 kg load, and 5 kg load, as well as at three specific speed–load combinations of 1.5k rpm_3 kg, 1k rpm_0kg, 2k rpm_5 kg and so on are measured, and the measured results of different spatial modes are compared and analyzed. The findings indicate that different spatial modes can accurately reflect temperature variations at various positions in motor stator winding.

A 3-D-Printed Wearable Sensor Based on Fiber Bragg Gratings for Shoulder Motion Monitoring

A 3-D-Printed Wearable Sensor Based on Fiber Bragg Gratings for Shoulder Motion Monitoring

Polymer-based cascaded waveguide Bragg grating for blood glucose sensing.

Waveguide Bragg grating (WBG) blood glucose sensing, as a biological sensing technology with broad application prospects, plays an important role in the fields of health management and medical treatment. In this work, a polymer-based cascaded WBG is applied to glucose detection. We investigated photonic devices with two different grating structures cascaded-a crossed grating and a bilateral grating-and analyzed the effects of the crossed grating period, bilateral grating period, and number of grating periods on the sensing performance of the glucose sensor. Finally, the spectral reflectance characteristics, response time, and sensing specificity of the cascaded WBG were evaluated. The experimental results showed that the glucose sensor has a sensitivity of 175 nm/RIU in a glucose concentration range of 0-2 mg/ml and has the advantages of high integration, a narrow bandwidth, and low cost.

Fiber Bragg grating sensing system for temperature measurements based on optically injected DFB-LD with an OEO loop

A novel fiber Bragg grating (FBG) sensing system, based on an optically injected distributed feedback laser diode (DFB-LD) with an optoelectronic oscillating (OEO) loop, is proposed and experimentally demonstrated for temperature measurements with high and tunable sensitivity. The FBG sensor device works as an edge filter to adjust the optical power of the injected beam in response to temperature variations. The optically injected DFB-LD works at Period-one (P1) oscillating state, and the central wavelength of the oscillating mode of the DFB-LD can be tuned by the variable power of the injected beam. Furthermore, an OEO loop is implemented to improve the signal quality of the generated P1 microwave signal. Hence, the sensing parameter of temperature is converted to the frequency variation of the generated P1 microwave signal in the proposed sensing system. In the proof-of-concept experiment, a series of P1 microwave signals are generated while different temperatures are applied to the FBG sensor. The sensitivity of the proposed FBG sensing system for temperature measurements can be tuned from 0.44322 GHz/°C to 1.25952 GHz/°C. The stability and repeatability experiments are also performed, demonstrating the high measurement accuracy (0.0629°C) and low error of the system. The proposed FBG-based sensing and interrogation system exhibits high sensitivity, large tunability, good linearity, and flexible sensing generality.

Steel corrosion process in ultra-high performance concrete monitored by fiber bragg grating sensor

The steel corrosion in Ultra-High Performance Concrete (UHPC) would cause degradation of structural performance or even structural failure. It is an urgent need to monitor and predict the steel corrosion in UHPC, however it is still a tough task. In this paper, Fiber Bragg Grating (FBG) sensors are applied for monitoring the steel corrosion process in UHPC, by combining with steel bar, which is equidistant winding with the sensors, and connected with power supply for accelerating corrosion. The UHPC specimens embedded with steel bars are surrounded by sodium chloride (NaCl) solution, and the steels are artificially corroded with different current densities for investigation. The steel corrosion in Original C60 and Marine Concrete 60 (Marine C60) are applied for comparison. The volume expansion coefficient for monitoring the steel corrosion process, is carried out by FBG sensors, and further comprehensively monitored by three grating sensors. The results suggest that the steel corrosion process in UHPC is slowly with current density lower than 100 μA/cm2, and largely increases with current density higher than 500 μA/cm2, the volume expansion coefficient of corroded steel in UHPC is in range of 2.02–2.62 in this test, which is much different from Original C60 and Marine C60.

In-fiber chirped Fabry-Perot cavity for temperature sensing.

Measurement resolution and dynamic range of conventional optical fiber sensors are often mutually restricted. In this work, an in-fiber chirped Fabry-Perot cavity (interferometer) is proposed, for the first time to our knowledge, to resolve the conflict between the resolution and dynamic range. The chirped Fabry-Perot interferometer is constructed by two chirped fiber Bragg gratings inscribed in the opposite directions, resulting in a gradually varied (i.e., chirp) cavity length for different reflection wavelengths. As such, the interference spectrum exhibits high figure of merit (FOM) and large free spectrum range (FSR) at long and short wavelength regions, respectively, enabling high-resolution and large-dynamic-range measurement simultaneously. Temperature tests are then carried out to confirm the validity of the solution. The proposed sensing schema may be developed further and find vital applications in biomedicine fields such as endosomatic temperature monitoring of living bodies. The proposed concept of chirped Fabry-Perot interferometer can provide breakout ideas for other sensing scenarios where high-resolution and large-dynamic range are demanded and can be further generalized to other measurands or even free-space interference metrologies.

Tunable gain clamping L-band erbium-doped fiber amplifier based on FBG and BBM

An improved structure for the L-band Erbium-doped fiber amplifier (EDFA) with gain clamping is suggested to address gain instability resulting from variations in the number of optical channels and the compromised gain clamping performance due to misalignment in the center wavelength of the grating. Here, a linear resonant cavity is formed by a broad-band mirror (BBM) and fiber Bragg grating (FBG), and a variable optical attenuator (VOA) is used for cavity loss adjustment. Experimental results show that the L-band EDFA solves the problem that the center wavelength of the narrowband grating pairs needs to be consistent. The 20 dB gain bandwidth is 42 nm(1582 ∼ 1623 nm). For signal at 1605 nm, the gain can be tuned from 28.8 to 34.8 dB, and a 40 dB (−50 to −10 dBm) gain clamped dynamic range is obtained at the gain of 28.8 dB, with stability of ± 0.25 dB, for which noise figure (NF) is less than 5.3 dB.

Design of an enhanced spatial resolution indoor plantar strain and gait analysis setup for biomedical applications

In the recent years, measurement of plantar strain and gait analysis has gained huge attention and plays a pivotal role in monitoring posture related ailments or Diabetic Foot Ulcer (DFU). Fibre Bragg Grating Arrays (closely spaced FBGs), another category of optical sensors, are employed in this study to understand the strain and gait of human foot. These arrays offer critical advantages such as enhanced sensitivity, data acquisition, multiplexing abilities, and sensor location effect compensation. For experimentation, five arrays were distributed among different regions (Upper, Medial, and Heel) of foot and their strain patterns for six volunteers (three male and three female) were recorded for eighty seconds. The data was analysed in two ways, Combined (data of array sensors were averaged to target the whole area) and Individual (for independent analysis of each sensor in array). Incoherent transitions and strain patterns of FBGs within the same arrays observed in individual analysis, explains enhanced resolution capabilities of the arrays. Strain mapping of sensor behaviour also confirmed the identification of various forms of gait i.e., Heel Strike, Flat Foot, Heel Off, and Zero Contact. The average standard deviation values for the arrays was reported below 0.16.

1.7 W holmium-doped fluoroindate fiber laser at 3920 nm.

A monolithic fiber laser emitting 1.7 W at 3920 nm is experimentally demonstrated in a Ho3+:InF3 fiber. The cavity comprises a pair of highly reflective fiber Bragg gratings written in the active fiber with the femtosecond phase-mask scanning technique and is spliced to the pump diode with a robust silica-to-fluoride fiber splice. This work is an important step toward high-power all-fiber laser operating in the vicinity of 4 µm.

Deep learning-assisted structural health monitoring: acoustic emission analysis and domain adaptation with intelligent fiber optic signal processing

Abstract The Acoustic Emission (AE) technique is a well-established method for detecting and locating cracks as they occur. One of the key challenges in fracture mechanics is determining the cracking levels during crack propagation. By linking the AE signature with crack geometry, we can determine the stages of crack growth. The classification of cracking levels is established by correlating micro and macro crack damage to the stable and unstable propagation stages. In this paper, a deep learning architecture based on Convolutional Neural Network is proposed to automatically extract features from the AE signal and classify cracking levels. The performance of this Deep Learning (DL) approach is evaluated using a simulated and experimental Hsu-Nielsen Pencil-led break test conducted in Finite Element Modeling and laboratory setup via COMSOL and a novel High-finesse Fiber Bragg Gratings Fabry-Perot Interferometer sensing system respectively. The results demonstrate the effectiveness of the DL approach for cracking level identification with high accuracy. The dataset was artificially augmented, and a comparison was made between balanced and imbalanced datasets for training purposes. In addition to gaining a deeper understanding of the fracture process, a finite element model was created to differentiate between active and passive AE sources by simulating the Pencil-led break test and crack propagation under cyclic loading conditions respectively. The analysis of the simulation model reveals that the energy associated with AE hits, as calculated from fracture mechanics, increases. This is the primary distinction between active and passive AE sources.

Multi-order orbital angular momentum mode generators based on integrated long period fiber gratings

Multi-order orbital angular momentum mode generators based on integrated long period fiber gratings