Bragg Wavelength Research Articles

Thermal properties of fluoride fiber Bragg gratings at high to cryogenic temperatures.

The thermal sensitivity of fiber Bragg gratings (FBGs) is extensively employed in diverse industrial and scientific applications. FBGs lie at the core of flexible, low-cost, and highly precise sensors, featuring stability in harsh environments and distributed sensing capability. This study assesses the thermal properties of FBGs in fluoride fibers within a temperature range of 4-373 K. Despite having higher thermal expansion coefficients, FBGs in the near-IR wavelength range do not exhibit high sensitivity at room or higher temperatures. However, the pronounced enhancement of their thermal sensitivity at longer Bragg wavelengths shows the potential for sensing applications in the light of the fluoride glass extended transmission range up to 4-5.5 µm. Most importantly, employing FBGs inscribed in fluoride fibers enables the further expansion of fiber-based sensors to cryogenic environments, as they exhibit a detectable sensitivity of 0.5-1.7 pm/K below 50 K. Overall, the exposure to low temperatures provides valuable information on glass stability and physical parameters, which is beneficial for the further development of photonic systems based on fluoride fibers.

Temperature-insensitive and cost-effective distributed NP-Doped optical fiber sensors

This paper presents the development of a cost-effective distributed optical fiber sensor for temperature-insensitive assessment of mechanical disturbances along an optical fiber cable. The proposed sensor system uses a nanoparticle (NP)-doped optical fiber with enhanced Rayleigh backscattering to provide higher sensitivity and spatial resolution using the transmission and reflection analysis (TRA) approach, where the transmitted and backscattered optical powers are analyzed as a function of the mechanical disturbance. In addition, Fiber Bragg Gratings (FBGs) are used as wavelength filters to provide the wavelength division multiplexing of the proposed device, which enable the use of 3 different NP-doped optical fiber sections for simultaneous detection of multiple curvature conditions in a cost-effective distributed sensing approach. The sensor characterization tests are performed by means of applying curvature angles from 360° to 1080° at different positions along NP-doped fibers (namely 25 mm, 100 mm and 175 mm) at 4 different temperatures of 25 °C, 30 °C, 40 °C and 50 °C. The results indicate the feasibility of the proposed approach, where the temperature variations lead only to a wavelength shift of the Bragg wavelength, whereas the mechanical disturbances (the curvatures) lead only to variations in the transmitted and reflected optical powers. Thus, by analyzing the transmitted and reflected optical powers in conjunction with the Bragg wavelength shift, it is possible to estimate both the mechanical disturbance amplitude (i.e., curvature angle) and the position along each NP-doped optical fiber section. Results indicate a relative error of around 3 % and 4 % for the mechanical disturbance location and absolute value, respectively. Moreover, the temperature cross-sensitivity in this case is below 2 % considering both amplitude and location of the mechanical disturbance. The proposed approach can be applied in structural health monitoring of different types of structures by integrating the fibers in the structures themselves with the possibility of measuring the strain distribution along the fibers (instead of in different points along the fiber) using a lower cost hardware when compared with similar distributed optical fiber sensing approaches.

A Study on the Simulation of Fiber Bragg Grating (FBG) as a Strain Sensor

Abstract: This study models, simulates, and characterizes the Fibre Bragg grating (FBG) in terms of maximum reflectivity, bandwidth, the effect of applied strain on the wavelength shift, λB, and the wavelength shift sensitivity with strain for an optical sensing system. This work uses a commercial FBG with a 1550 nm center wavelength to measure the spectral response of FBG to strain. The effective refractive index (1.46), the grating period (Λ) for 530 nm in the FBG performance, the fluctuations in refractive index (∆n) from 0.0002 to 0.0020, and the fiber grating length (L) from 1 to 10 mm are the parameters used in these simulations. The bandwidth and spectrum reflectivity can be obtained by analyzing the refractive index and grating length fluctuation. OriginPro Software and Microsoft Excel are used to perform simulations on the FBG. Data are generated using the Excel sheet, and visualisations are produced using OriginPro Software. The obtained results show that the bandwidth and spectral reflectivity are impacted by variations in the refractive index and grating length. Furthermore, the obtained results indicate that variations in the Bragg wavelength can be attributed to an elongation of the grating zone caused by the applied strain.

Type-II Bragg gratings with a 300 nm period fabricated using tightly focused femtosecond pulses and the phase mask technique

1000°C-resistant Bragg gratings (i.e., Type-II Bragg gratings) with a 300 nm period are fabricated in non-photosensitized silica-based optical fibers using tightly focused ultraviolet/visible (400 nm) femtosecond pulses and a phase mask. Five millimeter-long Bragg gratings with a -10 dB transmission dip and -0.1 dB off-resonance insertion loss are demonstrated. Inscription of such gratings is also possible through protective acrylate coatings on the fiber. The Bragg wavelength drift and changes in the transmission of the fiber Bragg gratings are monitored in the course of isothermal annealing performed at 1000°C for 120 hours.

Design of a Fiber Temperature and Strain Sensor Model Using a Fiber Bragg Grating to Monitor Road Surface Conditions

In this paper, the types and principles of operation of fiber sensors based on fiber Bragg gratings (FBGs) are investigated. The influence of strain and temperature on the characteristics of FBGs is considered, and a method for the simultaneous measurement of these parameters is presented. Laboratory studies were carried out in the temperature range from +18 °C to +135 °C with an incremental step of 5 °C, with the actual temperature not deviating by more than ±0.5 °C. From the data obtained, the Bragg wavelength–temperature relationships were plotted, which showed a linear increase in wavelength with increasing temperature. This study shows that the use of two FBGs with a different sensitivity to temperature and strain allowed for the simultaneous measurement of both parameters. Numerical models created in the MATLAB R2022b environment confirmed the high accuracy and precision of the measurements. The FBG-based sensors demonstrated a robust performance in harsh environments, withstanding temperatures of up to 160 °C and high humidity, making them applicable in various industries and sciences. This work confirms that FBGs are a promising tool for accurate temperature and strain measurements, providing reliable results in harsh environments.

Accurate high-temperature profile sensing with dense multipoint arrays of regenerated fiber Bragg gratings

A spectrally and spatially dense sensor array consisting of 15 regenerated fiber Bragg gratings (RFBGs) over a length of 30 mm is presented for precise and fast multipoint temperature sensing up to 700°C. For the first time, it could be shown that with a dense fiber Bragg grating (FBG)-based sensor array the accuracy requirements of Class 1 thermocouples could be achieved and even exceeded. This also represents the highest spatial density of FBG-based high-temperature multipoint sensing reported so far. The mitigation of broadband losses during the regeneration process was studied, revealing the advantages of the low broadband loss characteristics of the RFBGs, especially when larger numbers of measuring points are required. Low measurement uncertainties were achieved by a new, improved calibration methodology and by an analysis of interferences of an FBG with the side lobes of spectrally neighboring FBGs as well as their suppression by a suitable arrangement of the Bragg wavelengths within the array and corresponding data processing. The capabilities of this multipoint sensor technique were demonstrated by resolving the temperature profile within the calibration volume of a calibration furnace and by resolving the temporal and spatial temperature gradients within the flame of a Bunsen burner. The results are of great importance for fiber-optic sensing of high-temperature profiles in real-world applications.

The Fundamental of Fiber Bragg Grating (FBG) Sensor

An optical sensor known as an FBG is produced by laterally exposing a single mode fiber core to a strong UV laser light pattern on a regular basis. The exposure causes a sustained increase in the refractive index (ncore) of the fiber's core, resulting in fixed index modulation called grating (λ). The grating inside the fiber optic core must reflect a certain wavelength of incoming light while transmitting all other wavelengths. This wavelength is referred to as the Bragg wavelength or Bragg linked to grating period. FBG functions as intended when a Bragg reflector is built on an optical fiber by taking use of the regular fluctuations in the refractive index of the single mode fiber core. When light travels through the FBG, certain wavelengths will be transmitted and others will be reflected. When the strain or temperature around the grating varies, a shift in the wavelength of the light reflected is observed.

Ultra-broadband and high extinction ratio silicon TM-pass polarizer based on grating-assisted antisymmetric multimode waveguide

We propose an all-silicon TM-pass polarizer using a grating-assisted antisymmetric multimode waveguide (GAMW). The GAMW design facilitates the conversion of the undesired TE0 mode into the back-reflected TE1 mode, enabling efficient transmission of the desired TM0 mode with low insertion loss (IL) and high extinction ratio (ER). Furthermore, by cascading multiple GAMW structures, the device can filter TE0 mode over a broader spectral range effectively. The simulation results indicate that the single-GAMW-based polarizer achieves IL < 0.17 dB and ER > 40 dB in the wavelength range from 1480 nm to 1620 nm. When cascaded, the device expands the bandwidth to 275 nm (1435 nm–1710 nm), maintaining IL < 1 dB and ER > 40 dB simultaneously. The bandwidth can be further improved by cascading more GAMW structures working at different central Bragg wavelength. Limited by the measuring equipment, the measured wavelength range is between [1470 nm, 1630 nm]. The results of experiment demonstrate that the fabricated single-GAMW-based polarizer achieves IL < 0.65 dB and ER > 28.4 dB in 144 nm bandwidth ranging from 1470 nm to 1614 nm. For the cascaded-GAMW-based polarizer, IL < 0.83 dB and ER > 28.8 dB is achieved throughout the entire measuring wavelength range.

Underwater temperature and pressure monitoring for deep-sea SCUBA divers using optical techniques

The safety of SCUBA divers remains at high risk in deep-sea owing to multiple factors such as dangerous surrounding, rely upon technical equipment necessary for life support, decreased underwater navigation, and communication infrastructure. Gradual decrease and increase in water temperature and pressure corresponding to depth are among the most common problems that cause most of the fatalities in deep-sea diving. Therefore, different gadgets for accurate measurement of vital parameters, reliable navigation, and seamless communication are of prime importance. In this paper, we propose an all-optical technique for local and remote monitoring of underwater temperature and pressure for deep-sea SCUBA divers based on fiber Bragg grating (FBG) sensors and underwater optical communication-single mode fiber (UWOC-SMF) integrated transmission system. The proposed technique is implemented using two FBG temperature and pressure sensors fixed over diver’s suit and UWOC-SMF integrated transmission system for simultaneous local and remote monitoring of underwater temperature and pressure. Remote monitoring of underwater temperature and pressure is achieved at ship station through a remotely operated underwater vehicle (ROV) and UWOC-SMF integrated transmission system by means of shifts in the original Bragg wavelengths of sensors due to temperature and pressure variations. The performance of the sensors is analyzed for pressure and temperature in the range of 0 to 6.4 MPa (≈0 to 655 mH2O) and 40 to −2°C, respectively corresponding to different depths. The results show that the proposed technique can work well in the deep ocean over a range of pressures and temperatures of 0–7 MPa and 40 to −2°C while achieving a temperature sensitivity of 4.3 p.m./°C and a pressure sensitivity of 30.5 p.m./MPa. Clear spectra of reflected signals from FBG sensors at ship station are achieved after signal transmission over UWOC-SMF hybrid link.

Machine learning-augmented multi-arrayed fiber bragg grating sensors for enhanced structural health monitoring by discriminating strain and temperature variations

AbstractFiber optic sensors (FOS) in long-term structural health monitoring (SHM) have drawn significant attention due to their pivotal role in detecting defects and measuring structural performance in diverse infrastructures. While using FOS, temperature variation due to environmental factors is still considered one of the major challenges to isolating sensing parameters. To address this issue, we reported a machine learning (ML)-augmented multi-parameter sensing system that enables simultaneous detection of strain and temperature effects based on one single fiber Bragg gratings (FBGs) sensor for SHM. The initial phase entailed designing, fabricating, and characterizing a novel FBG sensor in the laboratory, incorporating a set of four FBGs, each distinguished by distinct Bragg wavelengths. In the next phase, ML algorithms are employed to separate temperature effects from strain variations. As a proof of concept, mechanical loading tests are conducted on the sensor, exposing the FBG portion to various temperature conditions. In the final phase, data collected from a post-tensioned concrete bridge embedded with both strain and temperature FBG sensors are utilized, and the developed ML models are applied to observe real-environment outcomes. Despite the limited feature points of collected FBG spectrums, the developed ML models effectively address cross-sensitivity issues induced by temperature perturbations. The long-term benefit of using FOS is that it will enable a better understanding and utilization of aging infrastructure. This will potentially reduce embodied carbon of infrastructure in the future and assist in the global efforts to achieve Net-Zero.

Fiber Bragg grating guided laser interferometer-based highly sensitive vacuum pressure sensor

Vacuum sensing and metrology pave the way for promising solutions to fulfill the scientific and technological demands of various contemporary industries and research fields. This study introduces an innovative vacuum pressure sensor, employing a fiber Bragg grating (FBG) guided Michelson interferometer. The sensor works on the principle of interferometric measurement of precisely gauging the displacement of an elastic diaphragm with pressure variation connected to a vacuum chamber in terms of interference fringe counts due to arm-length variation of the interferometer. The elastic silicone diaphragm and stainless steel cantilever, being critical components of the sensor, were examined using finite element analysis and subsequently demonstrated experimentally. The diaphragm’s position is continuously monitored in real time through the Bragg’s wavelength of the FBG, continuously updating the interferometer after each 15 ms for the accurate measurement of fluctuating vacuum pressures. The strain-induced shift in the FBG’s Bragg wavelength follows a linear trend with pressure variation, exhibiting a sensitivity of 12.7 pm/mbar. With a dynamic range spanning 0.05–100 mbar, the sensor demonstrates a sensitivity of 16.073 fringe counts/mbar and a notable resolution of 0.3364 mbar. Moreover, the sensor exhibits good repeatability, with a hysteresis of up to 2.59% during full span cyclic operation. The coupling of the interferometer with FBG makes it a unique secondary standard solution for precision vacuum measurement.

High Sensitivity CH4 and CO2 Gas Sensor Using Fiber Bragg Grating Coated with Single Layer Graphene

This article outlines the development of a Fiber Bragg Grating (FBG) intended for use as a sensor for CH4 and CO2 gases. Following fabrication, the FBG was effectively treated with a layer of Graphene Material through a modified RF Sputtering process. This coating procedure involved introducing argon gas into the chamber and subjecting the FBG, securely held by two vacuum stages, to a temperature range of 27°C to 600°C by adjusting the power supplied to the cathode and anode, ranging from 0 to 125 Watts. Subsequently, the FBG was employed as a key sensing element within an experimental setup aimed at measuring gas concentrations within a confined space. The assessment involved analyzing the reflected signal of the FBG using an Optical Interrogator System, which demonstrated a shift in the Bragg wavelength of the reflected signal corresponding to varying gas concentrations. This study indicates promising outcomes for the Graphene-coated FBG as a gas sensor. The sensor’s sensitivity was evaluated based on the Bragg wavelength shift resulting from gas presence within the chamber. The Graphene-coated FBG exhibited sensitivities of 3.3 ppm for CH4 and 3.7 ppm for CO2, surpassing those reported in prior research efforts.

Optimization and analysis of apodized fiber Bragg grating properties for quasi-distributed sensing

An apodized fiber Bragg grating (FBG) is introduced with a proposed apodization function for the effective quasi-distributed sensing estimation of the temperature and the strain. FBG features such as reflectivity, side lobes, and bandwidth have been optimized for the designed apodized grating to upgrade the effectiveness of FBG for properly measuring the variations in the Bragg wavelength. Based upon the simulation, a comparative analytical study on FBG properties with different apodization function profiles has been demonstrated to achieve the optimum profile with high reflectivity, narrow bandwidth and minimal range of side lobes for quasi-distributed sensing estimation of parameters. A strong linearity has been noted for the sensitivity of designed FBG with different apodization profiles for the temperature and the strain estimation subsequently. It has been reported that the obtained sensitivity of measurands for the FBG with proposed apodization profile are higher as compared with the other profiles. The wavelength division multiplexing (WDM) based quasi-distributed network of four optimized FBGs have been implemented for different apodization profiles to illustrate the impact of apodization to prevent overlapping between neighbouring FBG spectrums in the sensing network with spatial resolution of 2 nm. The maximal detectable temperature/strain sensitivity estimation of 153 °C/1439 μϵ have been obtained for the proposed apodized FBG along with minimal detectable temperature/strain ranges of −102 °C/−1345 μϵ in the quasi-distributed network. The achieved ranges with optimum resolution can be implemented effectively in quasi-distributed based sensing application for condition observation of civil infrastructures in any complex circumstances.

Enhancing data sparsity in spectral signals using wavelet decomposition for improved compression and storage efficiency

Wavelet decomposition (WD) is integrated into the Compressive Sensing (CS) model to enhance data sparsity. This approach aims to ensure efficiency and quality in the received data after compression and reconstruction processes. The proposed WD-CS model is developed for reflected spectra signals from Fiber Bragg Gratings-based extensometers, which were subjected to induced deflections simulating the underground soil movement. WD decomposes the input signal before compressive measurement, followed by transferring and storing the compressed data in the cloud. The spectral data were reconstructed using Orthogonal Matching Pursuit (OMP) followed by wavelet reconstruction. The impact of wavelet decomposition on the quality of compression and reconstruction is assessed and compared against that of the standard CS model. The findings indicate that the WD-CS model can improve data sparsity by a factor of three and compressibility by a factor of ten without degenerating the reconstructed data quality. The error in the Bragg wavelength shift of the reconstructed spectra is minimal extracted using Pseudo-high Resolution Interrogation (PHRI) method. The proposed WD-CS model is useful in improving data compression for FBG spectra as well as storage efficiency, which make it potentially applicable in FBG-based sensor networks for long-term structural health monitoring applications.

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.

Customized femtosecond laser-inscribed superstructure fiber Bragg grating: A novel approach to decoupling temperature and strain.

This paper introduces a novel technique to simultaneously measure temperature and strain using a single 5 mm femtosecond laser-inscribed superstructure fiber Bragg grating (SFBG). This SFBG enables improved spectral capabilities, resulting in side-resonances within the optical spectrum. The characteristics of the device depend on three key factors: the length of the SFBG, the inscribed FBG’s periodic perturbations, and the length of the uninscribed sections that compose the entire structure. Initially, mathematical expressions were derived to calculate the separation between the central peak and a resonance on each side, which plays a crucial role in determining the sensor’s response. Then, a series of complete simulations using OptiGrating software, and a theoretical analysis were performed to comprehend the functioning of this superstructure. Subsequently, the fabrication process employed a femtosecond laser following a point-by-point inscription. Finally obtaining a comprehensive study on the principle, design, simulation, and implementation of the proposed SFBG. The sensitivities of the central Bragg wavelength and the distance between central peak and first-order resonance when applying temperature and strain were calibrated. The results yielded sensitivities for the central Bragg peak of 10.58 pm/°C and 1.1598 pm/µɛ, while the peak-to-resonance distance had sensitivities of 0.3495 pm/°C and 0.03052 pm/µɛ. The different response of both magnitudes makes it possible to distinguish the contributions of strain and temperature within short distances, having the possibility to improve measurements in sensing applications. This study finally provides a critical insight into the design process, as it aligns with the theoretical expectations.

Non-invasive FBG-based contact lens for continuous intraocular pressure monitoring

The intraocular pressure (IOP), or pressure inside the eye, is one of the key indicator for glaucoma diagnosis and treatment. The present work shows the design and fabrication of a non-invasive Fiber Bragg Grating (FBG) based contact lens for continuous IOP monitoring. By employing wet etching to reduce the FBG diameter, FBG based contact lens sensitivity and mechanical flexibility are enhanced. The contact lens is made of an optical fiber with a small core diameter that has an inscribed FBG of length 3 mm and is etched for a length of 40 mm to facilitate coiling the eFBG during fabrication. The soft contact lens was fabricated using flexible thin film technology with a transparent and flexible etched Fiber Bragg Grating (eFBG) integrated within flexible polydimethylsiloxane (PDMS) for sensing corneal deformations brought about by IOP variations. Experimental studies on the bionic eyeball model show that the Bragg wavelength shift is highly linear (r2 = 0.9694), with a very good sensitivity of 49.243 pm/mmHg in the IOP pressure range of 0 to 50 mmHg. Due to its ease of fabricating, simplicity of design, and ability to continuously measure pressure with good repeatability and stability, the proposed sensor thus offers a potential solution for IOP monitoring.

Polydimethylsiloxane-coated Fiber Bragg Grating as a Bend Sensor

The fiber Bragg grating sensor is often demonstrated as a bend sensor. However, the demonstrated sensor in previous studies was limited to low sensitivity, thus reducing the sensing performance. Hence, this work demonstrated the polydimethylsiloxane-coated fiber Bragg grating sensor for sensitivity enhancement. The proposed sensor with a diameter of 5 cm was bent by adjusting the translation stage from 0.1 cm to 0.9 cm distance in forward and backward direction. The results showed that the Bragg wavelength shifted towards the longer wavelength region when the distance decreased. The polydimethylsiloxane-coated fiber Bragg grating sensor shows a sensitivity of 1743000 A.U, 8 times higher than the uncoated fiber Bragg sensor with a sensitivity of 284000 A.U. The presence of the polydimethylsiloxane layer has successfully improved the sensor performance.

High performance TM-pass polarizer using multimode Bragg grating waveguide

A novel ultra-broadband TM-pass polarizer with high polarization extinction ratio (PER) and low reflection has been proposed and demonstrated by utilizing multimode Bragg grating waveguide (MBGW) and two tapered waveguides. By optimizing the period of the MBGW, the injected TE0 mode is coupled into the backward TE2 mode and effectively leaked into the cladding. Meanwhile, the injected TM0 mode propagates through the polarizer without any negative impact. The operation bandwidth can be significantly expanded by cascading multiple MBGW structures, each of which operates at a different central Bragg wavelength. The simulation results indicate that the designed polarizer can achieve an insertion loss (IL) below 0.24 dB and a PER above 39 dB simultaneously across a bandwidth of 300 nm (1400 nm∼1700nm), while the reflected signal is below −9.1 dB. The experiment results demonstrate that the fabricated polarizer can realize an IL below 0.56 dB and a PER above 33 dB in a 160 nm bandwidth ranging from 1470 nm to 1630 nm. Due to limitations in the equipment used, measurements for other wavelength ranges are not conducted. With these merits, the proposed device would find significant applications in optical communication systems.

High-Q Tamm plasmon-like resonance in spherical Bragg microcavity resonators.

This work proposes what we believe to be a novel Tamm plasmon-like resonance supporting structure consisting of an Au/SiO2 core-shell metal nanosphere structure surrounded by a TiO2/SiO2 spherical Bragg resonator (SBR). The cavity formed between the core metal particle and the SBR supports a localized mode similar to Tamm plasmons in planar dielectric multilayers. Theoretical simulations reveal a sharp absorption peak in the SBR bandgap region, associated with this mode, together with strong local field enhancement. We studied the modification of a dipolar electric emitter's radiative and non-radiative decay rates in this resonant structure, resulting in a quantum efficiency of ∼90% for a dipole at a distance of r=60n m from the Au nanosphere surface. A 30-layer metal-SBR Tamm plasmon-like resonant supporting structure results in a Q up to ∼103. The Tamm plasmon-like mode is affected by the Bragg wavelength and the number of layers of the SBR, and the thickness of the spacer cavity layer. These results will open a new avenue for generating high-Q Tamm plasmon-like modes for switches, optical logic computing devices, and nonlinear applications.