Bragg Grating Research Articles

Experimental Determination of the Equivalent Moment of Inertia and Stresses of Aluminium Profiles with Thermal Breaks

This paper presents the results of experimental tests and computer simulations on the stiffness of composite aluminium mullions used in unitised façades. The elements analysed were subjected to bending in order to simulate the actual operating conditions of aluminium façades subjected to significant wind pressure or suction loads. The basic mechanical and physical properties of the materials from which the analysed type of aluminium façade is made (Aluminium EN AW-6060 in the T66 temper and polyamide PA66 25GF), the test method, and the results obtained are described. As a result of the tests, equivalent moments of inertia of the composite profile (aluminium profile with the thermal break) were determined, which are strongly dependent on the strength of the connection between the individual elements, the asymmetry of the cross-section, and the properties of the thermal break. Strain measurements carried out using FBG (Fiber Bragg Grating) strain sensors installed in the profiles under tests allowed for determining the actual stress values of the aluminium profiles under consideration. The results obtained were compared to theoretical (numerical) values, indicating discrepancies at higher load values. The methodology presented in this article is to be used to monitor the deformation of the aluminium façade mullions of HRB (High-Rise Buildings).

Low cross-sensitivity and sensitivity enhanced FBG sensor based on OCMI with three cascaded FBGs

This paper presents a highly sensitive, temperature-insensitive optical carrier microwave interferometry (OCMI) system using a cascaded three fiber Bragg grating (FBG) structure to generate an enhanced Vernier effect for sensing applications. The enhanced Vernier effect is created by superimposing the interferograms of two separate interferometers formed by pairing the sensing FBG with each reference FBG. Experimental and theoretical results show that in strain sensing, the sensitivity based on enhanced Vernier effect is -4.642 MHz/µε, which is 66.3 and 61.4 times higher than that of strain sensor based on two single interferometers used to generate the enhanced Vernier effect in this system respectively. Compared with the strain sensor based on traditional Vernier effect, this sensor has higher sensitivity, and in addition the amplification factor for measuring sensitivity can be easily adjusted by changing the spatial distance between the three cascade FBGs. Moreover, the temperature sensitivity is decreased from -38.318 MHz/°C to -71.384 kHz/°C with temperature compensation. The sensor exhibits high resolution, high sensitivity, and low cross-sensitivity, making it great potential for measuring small physical changes in complex environments.

Fiber Bragg grating (FBG)-based sensors: a review of technology and recent applications in structural health monitoring (SHM) of civil engineering structures

Fiber Bragg grating (FBG)-based sensors: a review of technology and recent applications in structural health monitoring (SHM) of civil engineering structures

High Performance GaSb-Based DBR Laser with On-Chip Integrated Power Amplifier via Gain-Match Design

We reported on a single-longitudinal-mode operated distributed Bragg reflector laser diode emitting at 1950 nm with an on-chip integrated power amplifier. Second-order Chromium–Bragg gratings are carefully designed and fabricated at the end of the ridge waveguide. Achieving a stable single-mode operation with a large injecting current range of 800 mA from 15 °C to 40 °C. The maximum side-mode suppression ratio (SMSR) is up to 42 dB. To increase the output power, an on-chip integrated master oscillator power amplifier (MOPA) is also introduced. MOPA-DBR lasers with different matching configurations between the gain peak and Bragg wavelength are fabricated, resulting in various amplification consequences. The best device is realized with 40 nm red-shifted between Bragg wavelength and photoluminescence (PL) peak. A power amplification of 5.6 times is achieved with the maximum output power of 45 mW. Thus, we put up the feasibility and key design parameters of on-chip integrated power amplification DBR lasers towards mid-infrared.

A Photonics-Based Bessel Beam Launcher with an Air-Filled Core and Cylindrical Bragg Gratings in the Cladding

A Photonics-Based Bessel Beam Launcher with an Air-Filled Core and Cylindrical Bragg Gratings in the Cladding

Damage and Failure Monitoring of Aerospace Insulation Layers Based on Embedded Fiber Bragg Grating Sensors.

Carbon fiber-reinforced polymer (CFRP) composites are widely used in aviation thermal insulation layers due to their high strength-to-weight ratio and excellent high-temperature performance. However, challenges remain regarding their structural integrity and durability under extreme conditions. This study first employed finite element simulation to model the damage evolution of CFRP laminated plates under axial tensile loads and their thermal decomposition behavior in high-temperature environments, providing a theoretical reference. Subsequently, experimental research was conducted on CFRP laminated plates embedded with fiber Bragg grating (FBG) sensors. In the tensile tests, FBG sensors accurately monitored the entire process from elastic deformation to damage propagation and eventual failure. In the high-temperature tests, despite a 75% reduction in tensile strength, FBG sensors effectively monitored damage evolution. Conclusively, the results demonstrate that FBG sensors possess reliable monitoring capabilities under complex conditions, making them a promising solution for the long-term structural health monitoring of aviation thermal insulation materials and paving the way for future developments in this area.

Development of a Scaled Down Test-Rig for Wheel–Rail Contact Thermal Experiments Using Optical Sensors

Abstract Rail wheel contact measurement is crucial for several reasons. First and foremost, it directly affects the safety of passengers, crew, and the general public. Accurate measurements help identify irregularities in wheel-rail contact, such as wheel defects, wear, or track anomalies, which can lead to derailments or accidents if left unaddressed. An inventive technique for measuring the temperatures at rail wheel contact at various speeds is presented in this research. The novel approach uses a 1:5 scaled-down test rig model of a wheel and rail with Fiber Bragg Grating (FBG) sensor to combine the experimental and finite element analysis simulation to determine the rail wheel contact temperature. By employing the various data acquisition and data analysis techniques rail wheel contact temperature at different speeds ranging between 10kmph to 40kmph was determined 1538.735 nm to 1538.831nm with centre wavelength of 1538.438 nm. The results illustrate the possibilities of the downsized test rig with experimental observations at varying speeds by examining the benefits of FBG sensors over traditional sensors. The experimental results are used to determine the equivalent wavelength shift. Fibre Bragg Grating (FBG) sensor design and simulation are done with the Grating MOD optical tool. For this temperature range and Bragg's wavelength of 1538.438 nm, the sensitivity of Fiber Bragg Grating is observed to be 13.6pm/°c.

Residual Stress Analysis at the Conductor-Insulator Interface During the Curing Process of Hair-Pin Motors.

The curing process of hair-pin motor stator insulation is critical, as residual stress increases the risk of partial discharge and shortens a motor's lifespan. However, studies on the stress-induced defects during insulation varnish curing remain limited. This research integrates three-dimensional numerical simulations and experimental analysis to develop a curing model based on unsaturated polyester imide resin, aiming to explore the mechanisms of residual stress formation and optimization strategies. A dual fiber Bragg grating (FBG) sensor system is employed for simultaneous temperature and strain monitoring, while curing kinetics tests confirm the self-catalytic nature of the process and yield the corresponding kinetic equations. The multi-physics simulation model demonstrates strong agreement with the experimental data. The results show that optimizing the curing process reduces the maximum stress from 45.1 MPa to 38.6 MPa, effectively alleviating the stress concentration. These findings highlight the significant influence of the post-curing temperature phase on residual stress. The proposed model offers a reliable tool for stress prediction and process optimization in various insulating materials, providing valuable insights for motor insulation system design.

Sensitivity calibration method of FBG strain sensor in high and low temperature conditions

Abstract Strain and temperature sensitivity coefficients of fiber Bragg grating (FBG) sensors are of vital importance to measuring accuracy, especially in varying temperature conditions. To improve the measurement accuracy of the FBG strain sensor, its strain and temperature sensitivity coefficients must be calibrated before use. In this study, we designed a substrate FBG strain sensor using the metal packaging method and illustrated its packaging process. A sensitivity calibration method for FBG strain sensors under different high-low temperatures was proposed. Additionally, an equal-intensity cantilever beam with the same material as the application structures of the FBG strain sensor, which affords loads within the range of −2500 to 3000 μϵ was designed and manufactured. A modified coefficient of strain δ was introduced to modify the strain results according to the calibration principle of an equal-intensity cantilever beam. Furthermore, a FBG unstressed temperature compensation method was proposed to compensate for the temperature of the FBG strain sensor. To verify the performance of the proposed sensitivity calibration method, a series of calibration experiments under −55 °C, −20 °C, 0 °C, 20 °C, 50 °C, and 70 °C were implemented, which proved the excellent performance of the proposed calibration method. Finally, the temperature and strain sensitivity coefficients of the FBG strain sensor in the temperature range of −55 °C to 70 °C are achieved at 0.3022 nm °C−1 and 0.5039 pm μϵ −1. The proposed sensitivity calibration method in high-low temperatures is simple and easy to implement. Meanwhile, the calibrated FBG strain sensor has prospective applications for strain measurement in structural health monitoring of aircraft, especially under varying temperature conditions.

Sub-kHz Linewidth Fiber Laser via Weak Fiber Bragg Grating Enabled Self-Injection Locking

Sub-kHz Linewidth Fiber Laser via Weak Fiber Bragg Grating Enabled Self-Injection Locking

Finger Tapping Test Evaluated by Embedded Fiber Bragg Gratings

Finger Tapping Test Evaluated by Embedded Fiber Bragg Gratings

Erbium-Doped Fibers Designed for Random Single-Frequency Lasers Operating in the Extended L-Band

The paper presents the results of developing Er-doped optical fibers for creating random single-frequency lasers in the wavelength range of 1570–1610 nm. The possibility of broadening the luminescence band of Er3+ ions in silicate glasses in the long-wavelength region of the spectrum by introducing a high concentration of P2O5, as well as by additional doping with Sb2O3, is investigated. It is found that both approaches do not improve the dynamics of luminescence decay in the L-band. In addition, Er2O3-GeO2-Al2O3-SiO2 and Er2O3-GeO2-Al2O3-P2O5-SiO2 glasses were studied as the core material for L-band optical fibers. The developed fibers exhibited high photosensitivity and a high gain of 5 and 7.2 dB/m, respectively. In these fibers, homogeneous arrays of extended weakly reflecting Bragg gratings were recorded directly during the fiber drawing process. Samples of arrays 5 m long and with a narrow reflection maximum at ~1590 nm were used as the base for laser resonators. Narrow-band random laser generation in the wavelength region of 1590 nm was recorded for the first time. At a temperature of 295 K, the laser mode was strictly continuous wave and stable in terms of output power. The maximal power exceeded 16 mW with an efficiency of 16%.

Study on spectral characterization of non-uniform PT symmetry Bragg gratings with duty-cycle modulation

Abstract It is a great challenge to improve the detection efficiency of optical structures. In order to increase the reflection effect of Bragg grating, a non-uniform PT-symmetric Bragg grating structure based on duty cycle modulation is proposed. The grating exhibits PT symmetry by quantum doping technology and is optimized by analyzing the structural parameters which have great influence on the grating. Two methods of Odd-function Duty-cycle Modulation (ODC) and Even-function Duty-cycle Modulation (EDC) are proposed to obtain the non-uniform grating structure under duty cycle modulation. The results show that the reflectivity of the grating is obviously enhanced under odd-function duty cycle modulation. Our method improves the reflection characteristics of the Bragg grating to a great extent, and is conducive to designing more efficient reflectivity detection devices.

Spectral tailoring of silicon grating-assisted contra-directional couplers.

Grating-assisted, contra-directional couplers (GA-CDCs), owing to their four-port operations, can offer several important advantages over traditional, single waveguide-based Bragg gratings. However, how to flexibly design the spectral responses of GA-CDCs has been much less studied. We report the spectral tailoring methodology of GA-CDCs to achieve arbitrary, physically realizable, complex spectral responses. Silicon GA-CDCs with various customized responses are demonstrated using the methodology, including sidelobe-suppressed filters, single- and multi-channel flattop filters, sawtooth- and triangle-shaped filters, and three-channel photonic Hilbert transformers.

PLC-AWG-based FBG demodulation system for high-frequency vibration measurement.

A fiber Bragg grating (FBG) demodulation system based on arrayed waveguide gratings (AWGs) is proposed. We designed the key parameters of the AWG, prepared the AWG chip based on a silica-on-silicon planar light wave circuit (PLC) platform, and integrated the AWG with a PD array. Eight AWG output channels were selected and the output signals from the photodiode (PD) array was connected to the board with gold wire bonding. The demodulation circuitry consists of transimpedance amplifiers (TIAs), analog-to-digital converters (ADCs), and a main control chip. The signal from the PDs are synchronously captured and amplified and then transmitted at high speed through an Ethernet interface. The total volume of the demodulation system is 200 × 100 × 60 mm3. Experiments show that the wavelength demodulation accuracy of the system is 4.24 pm, the demodulation rate is more than 200 kHz, and the average error of the demodulation acceleration is better than 22.8 mg. The proposed demodulation system can be applied in the field of high-frequency vibration sensing of FBGs.

ТЕМПЕРАТУРАНЫҢ ҚИҒАШ БРЭГГ ТОРЛАРЫНЫҢ ПАРАМЕТРЛЕРІНЕ ӘСЕРІ

The presented article states that various control methods based on optical technologies are currently relevant and are used in areas other than engineering and science. It is said that there is interest in Bragg gratings, including oblique Bragg gratings. The history of the oblique Bragg grating is briefly discussed, and its definition is given. Focusing on their features, the main formulas were reviewed in detail. Brief details of the conducted research are given, and data on experiments performed in certain temperature ranges are given. Sets of spectral characteristics are given, and graphs of temperature dependence of Bragg and host wavelengths of OBG are given. At the same time, the temperature sensitivity and coefficients of OBG were calculated and the results were presented in the form of a table.

Impact of Moisture Absorption on Optical Fiber Sensors: New Bragg Law Formulation for Monitoring Composite Structures

In recent decades, the aviation industry has increasingly adopted composite materials for various aircraft components, due to their high strength-to-weight ratio and durability. To ensure the safety and reliability of these structures, Health and Usage Monitoring Systems (HUMSs) based on fiber optics (FO), particularly Fiber Bragg Grating (FBG) sensors, have been developed. However, both composite materials and optical fibers are susceptible to environmental factors such as moisture, in addition to the well-known effects of mechanical stress and thermal loads. Moisture absorption can lead to the degradation of mechanical properties, posing a risk to the structural integrity of aircraft components. This research aims to quantify and monitor the impact of moisture on composite materials. A new formulation of the Bragg equation is introduced, incorporating mechanical strain, thermal expansion, and hygroscopic swelling to accurately measure Bragg wavelength variations. Experimental validation was performed using both uncoated and polyimide-coated optical fibers subjected to controlled hygrothermal conditions in a climate chamber. The results demonstrate that uncoated fibers are insensitive to humidity, whereas coated fibers exhibit measurable wavelength shifts due to moisture absorption. The proposed model effectively predicts these shifts, with errors consistently below 2.6%. This approach is crucial for improving the performance and reliability of HUMSs in monitoring composite structures, ensuring long-term safety in extreme environmental conditions.

Fiber Bragg Grating-Based Tunable Wavelength Orbital Angular Momentum Beam Generation with Varying Polarization

This paper introduces and theoretically analyze the generation of orbital angular momentum (OAM) modes with different polarization in a few-mode optical fiber using short-period gratings (Bragg gratings) at telecommunication wavelengths. The modal characteristics of the supported modes have been studied using a full-vector true-mode analysis. Additionally, a appropriately designed raised-cosine apodized grating is employed to couple the HE11 core mode to the counter-propagating cladding mode. In contrast to the long-period grating based OAM mode generation, the present scheme is free from periodic back-coupling of optical power into the fundamental core mode which drastically enhances the OAM mode purity. The ±1-order OAM beam has been generated by combining the first higher order core modes with a π/2 phase shift between them. This paper also shows that by combining different higher order modes, it is possible to obtain a linearly polarised OAM mode or a circularly polarised OAM mode. Finally, I also report that in fibers with larger core radius, the higher order OAM mode of topological charge ±2 or higher can also be generated using similar approach of using short-period gratings. The topological charge of the OAM mode is verified by observing both the coaxial and off axial interference fringes. Received: 13 March 2024 | Revised: 3 June 2024 | Accepted: 25 November 2024 Conflicts of Interest The author declares that he has no conflicts of interest to this work. Data Availability Statement Data sharing is not applicable to this article as no new data were created or analyzed in this study. Author Contribution Statement Avijit Koley: Conceptualization, Methodology, Software, Validation, Formal analysis, Data curation, Writing - original draft, Writing - review & editing.

Single-longitudinal mode quadruple wavelength C+L-band erbium-doped fiber laser based on the pairs of reflective fiber bragg gratings

Abstract Flatness of lasing wavelengths of a multi-wavelength fiber laser (MWFL) is an important design constraint that is considered attractive in various applications of photonics and optical communication systems. In this work, we propose a single-longitudinal mode (SLM) quadruple wavelength C+L-band Erbium-doped fiber laser (EDFL) with high output power flatness. It is implemented with a short piece of Erbium-doped fiber (EDF) pumped by conventional 980 nm laser diode and four pairs of reflective fiber Bragg gratings (FBGs) through numerical simulations. The reflectivities of FBGs are adjusted such that SLM quadruple wavelengths are obtained at the output of EDFL with flatness of 0.9 dB, optical signal-to-noise ratio (OSNR) in the range of 44.5–53.2 dB, and linewidths (LWs) in the range of 5.4–7.3 MHz. Slope efficiency (SE) of around 24% is achieved considering the total power of all lasing wavelengths generated. Moreover, the power variation around 0.1 dB for lasing wavelengths of 1540.2 nm and 1605.7 nm is noticed for twelve iterations each repeating after five-minute interval. The proposed SLM quadruple wavelength EDFL has promising application prospects for distributed sensing and optical communication systems due to excellent performance metrics.

Multipoint Thermal Sensing System for Power Semiconductor Devices Utilizing Fiber Bragg Gratings

This study investigates the feasibility of using fiber Bragg grating (FBG) sensors for multipoint thermal monitoring of several power semiconductor devices (PSDs), such as insulated gate bipolar transistors (IGBTs), and rectifiers assembled on a common heatsink in a three-phase inverter. A novel approach is proposed to integrate FBG sensors beneath the baseplates of the IGBT modules, avoiding the need for invasive modifications to the device structure. By strategically positioning multiple FBG sensors, accurate temperature profiles of critical components can be obtained. The experimental results demonstrate the effectiveness of the proposed method, with the temperature measurements from FBG sensors closely matching those obtained using thermal infrared (IR) cameras within ±1.1 °C. This research highlights the potential of FBG sensors for reliable and precise thermal management in power electronic systems, contributing to improved performance and reliability.