Single Grating Research Articles

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.

Investigation of Transmission and Reflection of Single Mode Fiber Bragg Grating

The use of Single Mode Fiber Bragg Grating (SMFBG) has been increasing in recent years due to its compact size, low cost, fast response and immunity to electromagnetic interference. It is commonly integrated into medical devices for long-distance light transmission and collection due to its high flexibility, low propagation loss, compatibility and tolerance to electromagnetic interference. SMFBG is a device made of thin glass material that is used as a medium for transmitting information in the form of light signals sourced from lasers or LEDs from one location to another. It consists of 3 main components, namely core with a certain grating, blanket (cladding) and jacket (coating). The advantage of optical fiber is that the data when transmitted is converted into light so as to reduce the risk of data damage. Other advantages include very small size, minimal interference with electromagnetic waves, resistance to temperature changes, attenuation when the transmission process is small enough, and large enough bandwidth. The orientation of this literature review is to understand the concept of optical fiber, the concept of reflection and refraction, and how light propagates in optical fiber.

Simultaneous generation of first- to fourth-order OAM modes based on a cascaded preset-twist long-period fiber grating

Abstract We propose and demonstrate the simulation and fabrication of an all-fiber orbital angular momentum (OAM) mode converter capable of generating first- to fourth-order modes simultaneously, which is realized by inscribing a cascaded preset-twist long-period fiber grating (CPT-LPFG) in a six-mode fiber utilizing a CO2 laser. A new segmented Runge–Kutta method is proposed to simulate the preset-twist long-period fiber gratings. By calculating the twist angle and relative coupling coefficient for each pitch and then solving the coupled mode equations utilizing the Runge–Kutta algorithm. The simulation illustrates that the preset-twist method significantly improves the coupling coefficient of higher-order modes, thereby reducing coupling difficulty. In the experiment, by twisting the fiber at an angle of 1080° and fabricating cascaded gratings with periods of 745 μm, 310 μm, 204 μm, and 146 μm, it is feasible to generate first- to fourth-order OAM modes simultaneously, at wavelengths of 1635 nm, 1548 nm, 1460 nm, and 1334 nm, respectively. The insertion loss is less than 1 dB, and the mode purity is over 90 %. To the best of our knowledge, this is the first time that first- to fourth-order OAM modes are simultaneously generated utilizing a single long-period fiber grating.

Modified CFBG-based approach for chromatic dispersion compensation in high speed DWDM links.

Chromatic dispersion (CD) is a major concern in high speed fiber optics communication systems. This work presents an efficient optical compensation technique with reduced complexity to mitigate CD effects in single mode fiber (SMF) at high data rates. The proposed method includes a post-dispersion mitigation system using a single linear chirped fiber Bragg grating (CFBG) with tanh apodization. Here, various parameters of CFBG, such as grating length, effective refractive index, and apodization function, are optimized to achieve effective CD compensation. Then, the system is extended to a 16-channel WDM link with each channel carrying data at 25Gbps over 100km. The simulation results show that the proposed model enhances the system performance while minimizing overall system complexity. The proposed system is also compatible with the prevailing fiber optics networks. The results are consistent with the findings published in recent studies.

Au coated etched FBG SPR sensor for the detection of ethanol in aqueous solution

Over the past few years, there has been a growing need for reliable ethanol sensors in the pharmaceutical and food industries. These industries require constant monitoring to ensure that the acceptable ethanol levels are maintained. This study reports a new fiber optic sensor that uses etched single mode fiber Bragg grating (FBG) coated with the thin layer of 45 nm gold to produce Surface Plasmon Resonance (SPR). A thin layer of 45 nm gold improves the sensitivity of sensor by improving the evanescent wave around the fiber, resulting in better interaction with the analyte. The sensing performance of developed structure was examined by analyzing the change in wavelength shift observed by varying ethanol concentration in aqueous solution from 0 to 100 %. The performance of developed sensor in terms of sensitivity is 20 pm/RIU. Which is expected to play an important role in food and pharmaceutical industries to protect human health from detrimental affects due to high concentration of ethanol.

High-efficiency continuous-wave Tm-doped fiber laser with a single fiber Bragg grating at 1942 nm

We report a continuous-wave Tm-doped fiber (TDF) laser with a single fiber Bragg grating (FBG). The Fresnel effect presenting in the passive fiber end-face and high-reflectivity FBG established the resonator. The peak wavelength of the laser was 1942.25 nm with a spectral linewidth of 194 pm. The maximum output power was 37.5 W, corresponding to a slope efficiency of 63.7% with a beam quality factor of M 2 ∼ 1.51. To the best of our knowledge, this is the highest slope efficiency of a TDF laser pumped by 793 nm laser diode.

Simultaneous Efficient Excitation of Multiple Higher-Order OAM Modes Using a Single Long-Period Fiber Grating

In this paper, a method for the simultaneous generation of multiple higher-order orbital angular momentum (OAM) modes in a few-mode fiber (FMF) using a single long-period fiber grating (LPFG) is proposed and demonstrated. The high diffraction order of the grating is used to achieve the resonant coupling of multiple modes, and the coupling coefficients of different order modes are balanced by a preset-twist method. This solves the two coupling problems of over-coupling for lower-order modes and difficult excitation of higher-order modes faced by multi-order mode generation, and overcomes the challenge of balanced excitation of multiple modes with high coupling efficiency. In the experiment, the simultaneous generation and flexible combination of first-order, second-order and third-order OAM modes are successfully achieved in a six-mode fiber, in which the mode order, resonant wavelength and wavelength interval can be flexibly controlled. By fabricating with grating periods of 2300 μm, 782 μm and 990 μm, respectively, high conversion efficiency of first-order and second-order modes, first-order and third-order modes, second-order and third-order modes can be generated simultaneously. In addition, for the first time, the first-order, second-order and third-order modes are generated simultaneously at a grating period of 1630 μm with conversion efficiencies of 98.61%, 99.11% and 98.89%, respectively. The mode profiles and interference patterns of different order modes show that multiple higher-order OAM modes with insertion loss less than 1 dB and mode purity over 95% are successfully generated.

Single-Shot Phase-Contrast Imaging with a Single Grating

In the field of X-ray phase-contrast imaging, a time-saving approach and preservation of details are crucial factors for obtaining phase-contrast images. In this manuscript, a single grating imaging system is proposed to perform the X-ray phase-contrast imaging. Instead of the time-consuming phase-stepping method, this system uses a single-shot algorithm to retrieve the distribution of samples’ attenuation and phase gradient. Unlike the single-shot Fourier transform algorithm, which truncates the high-frequency component of the image and reduces the spatial resolution, our method can retrieve the attenuation and phase information images with the same spatial resolution as the images acquired directly by the X-ray detector used. Furthermore, by using a large-size X-ray detector (29 cm × 23 cm), the imaging system can be configured as either a microscopic instrument or a normal large field-of-view imaging system. Finally, a series of experiments were performed to validate the feasibility of the proposed method.

Dual-Color Lasers in Interlayer-Free Solution Processed Polymeric Bilayer Devices.

Multiwavelength organic lasers have attracted considerable interest in recent years due to the cost efficiency, wide luminescence coverage, and simple processability of organics. In this work, by simply spin coating immiscible polymeric gain media in sequence, dual-wavelength (blue-green or blue-red) amplified spontaneous emission (ASE) was achieved in bilayer devices. The blue emission, water/alcohol-soluble conjugated polyelectrolyte, poly[(9,9-bis(3'-((N,N-dimethyl)-N-ethylammonium)-propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)]dibromide (PFN-Br), was used as the bottom layer. The commercially available nonpolar solvent soluble polymer poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) and its blend with poly(3-hexylthiophene) (P3HT) were used as the top active layers offering green and red emission, respectively. This novel compact configuration, without interlayers between the two active layers, offers potential for developing various applications. The carefully selected top and bottom layer polymers not only meet the conditions of immiscibility and different emission wavelength range but also have a common absorption band in UV, which allows simultaneous blue-green or blue-red dual-color ASE behaviors observed in the bilayer devices under the same 390 nm laser excitation. By introducing two-dimension (2D) square distributed feedback (DFB) gratings with different periods (300 nm for blue, 330 nm for green, and 390 nm for red) as cavities, single mode blue-green (Eth = 245 μJ cm-2) and blue-red (Eth = 189 μJ cm-2) lasers were achieved by focusing the excitation laser spot on different 2D DFB gratings area. Furthermore, we found it possible to gain sufficient light confinement for red emission along its diagonal direction (Λ ∼424 nm), whereas the 2D DFB gratings offer feedback for blue emission from the 300 nm period along the rectangle direction. Therefore, both blue and red lasers were eventually achieved in the same PFN-Br/F8BT:P3HT bilayer device on the single 2D DFB gratings with a period of 300 nm in this work.

Magnetic scattering with spin-momentum locking: Single scatterers and diffraction grating

Simultaneous manipulation of charge and spin density distributions in materials is the key element required in spintronics applications. Here we study the formation of coupled spin and charge densities arising in scattering of electrons by domains of local magnetization producing a position-dependent Zeeman field in the presence of the spin-momentum locking typical for topological insulators. Analytically and numerically calculated scattering pattern is determined by the electron energy, domain magnetization, and size. The spin-momentum locking produces strong differences with respect to the spin-diagonal scattering and leads to the scattering asymmetry with nonzero mean scattering angle as determined by only two parameters characterizing the system. To extend the variety of possible patterns, we study scattering by diffraction gratings and propose to design them in modern nanostructures based on topological insulators to produce desired distributions of the charge and spin densities. These results can be useful for engineering of magnetic patterns for electron optics to control coupled charge and spin evolution.

High Spatial Resolution Sensing of Temperature and Strain Without Cross Sensitivity Based on Chaotic Brillouin Dynamic Grating

A distributed fiber-optic sensing system based on chaotic Brillouin dynamic grating (BDG) without cross-sensitivity of temperature and strain is proposed and experimentally demonstrated. The chaotic BDG is stimulated by interaction of two chaotic pump beams with a certain frequency difference along the slow axis of polarization-maintaining fiber (PMF) and detected by the continuous probe light along the fast axis. The generated chaotic BDG has the characteristic of single, permanent, and localized grating. The grating length is determined by the low coherence length of the chaotic laser, which ensures that the spatial resolution of the chaotic BDG sensing system can reach the centimeter scale. Two parameters, i.e., Brillouin frequency shift (BFS) and birefringence frequency shift (BireFS) are measured to achieve the temperature and strain simultaneous discrimination. In the experiment, a 2.3-cm spatial resolution non-cross-sensitivity measurement was demonstrated over a 100-m long PMF, where temperature and strain discriminative accuracy can reach 0.3 °C and 11 με, respectively.

High-resolution astronomical spectrograph design method with a single echelle grating.

The paper presents a method for calculation of the main paraxial and energy parameters of a high-resolution astronomical spectrograph with a single echelle grating in a wide spectral range, without cross-dispersion elements. We consider two variants of the system design, namely, with a fixed grating (spectrograph) and with a movable grating (monochromator). Based on the analysis of the spectral resolution dependence on the echelle grating characteristics and the collimated beam diameter, the limits of the achievable maximum spectral resolution of the system are determined. The results obtained in this work can simplify the choice of the starting point for spectrograph designing. As an example of the presented method application design of the spectrograph for the Large Solar Telescope-coronagraph LST-3, operating in the spectral range of 390-900nm, with spectral resolving power of R=200,000 and minimum diffraction efficiency of the echelle grating I g>0.68, is considered.

Neutron interferometry using a single modulated phase grating.

Neutron grating interferometry provides information on phase and small-angle scatter in addition to attenuation. Previously, phase grating moiré interferometers (PGMI) with two or three phase gratings have been developed. These phase-grating systems use the moiré far-field technique to avoid the need for high-aspect absorption gratings used in Talbot-Lau interferometers (TLI) that reduce the neutron flux reaching the detector. We first demonstrate, through theory and simulations, a novel phase grating interferometer system for cold neutrons that requires a single modulated phase grating (MPG) for phase-contrast imaging, as opposed to the two or three phase gratings in previously employed PGMI systems. The theory shows the dual modulation of MPG with a large period and a smaller carrier pitch P, resulting in large fringes at the detector. The theory was compared to the full Sommerfeld-Rayleigh diffraction integral simulator. Then, we proceeded to compare the MPG system to experiments in the literature that use a two-phase-grating-based PGMI with best-case visibility of around 39%. The simulations of the MPG system show improved visibility in comparison to that of the two-phase-grating-based PGMI. An MPG with a modulation period of 300 µm, the pitch of 2 µm, and grating heights with a phase modulation of (π,0, illuminated by a monochromatic beam produces visibility of 94.2% with a comparable source-to-detector distance (SDD) as the two-phase-grating-based PGMI. Phase sensitivity, another important performance metric of the grating interferometer, was compared to values available in the literature, viz. the conventional TLI with the phase sensitivity of 4.5 × 103 for an SDD of 3.5m and a beam wavelength of 0.44nm. For a range of modulation periods, the MPG system provides comparable or greater theoretical maximum phase sensitivity of 4.1 × 103 to 10.0 × 103 for SDDs of up to 3.5m. This proposed MPG system appears capable of providing high-performance PGMI that obviates the need for the alignment of two phase gratings.

In-Fiber Thermally Diffused Coupler and Fiber Bragg Grating Inscribed in Twin-Core Fiber for Sensitivity-Enhanced Vector Bending Sensing

A vector bending fiber sensor based on core-by-core inscribed fiber Bragg gratings in a twin-core fiber has been proposed and experimentally demonstrated. An in-fiber integrated vector bending sensor is realized by using the thermal diffusion technique to fabricate the coupler. The characteristics of the coupler fabricated by thermal diffusion are simulated and experimented. By inscribing fiber Bragg gratings with different reflection wavelengths in the two cores of a symmetrical twin-core fiber, the curvature sensitivity can be enhanced by tracking the wavelength difference between the fiber Bragg gratings of the two cores. The measured bending sensitivity of the fiber Bragg grating ranges from −161.6 pm/m−1 to +165.5 pm/m−1. The differential sensitivity of the two cores is twice that of a conventional single grating, and the temperature-induced crosstalk is also reduced. The bending sensor proposed in this paper has the advantages of high integration, enhancing the sensitivity and two-dimensional orientation recognizability, and reducing temperature crosstalk, which can be a promising candidate for structural health monitoring or wearable artificial electronics applications.

Temperature-compensated fiber Bragg grating sensor based on curvature sensing for bidirectional displacements measurement

Measurement of displacement, deformation, curvature, and other physical parameters is of great importance for adequate Structural Health Monitoring (SHM) of large surfaces. Fiber Bragg Grating (FBG) sensors have become a focus of research in academia and engineering for structural monitoring applications. This paper presents the study and characterization of a displacement FBG sensor based on curvature sensing of an array of 4 FBGs, as well as possible applications for structural monitoring, developing the sensing part, and analyzing the signals obtained to develop a sensor with good sensitivity and resolution, capable of measuring bidirectional displacements. The sensor prototype is presented to measure displacements in a range of ±30 mm with a single grating, with a sensitivity of up to 74 pm/mm, linearity to R2 = 0.999, and hysteresis error up to 1.27 %, with temperature compensation for displacement monitoring in case of temperature variations.

Performance characteristics of single Bragg grating fiber under various filter impulse functions and grating filter profiles

Abstract This work has demonstrated the performance characteristics of single Bragg grating fiber under various filter impulse functions and grating filter profiles. Single fiber Bragg grating (FBG) power propagation, grating index change, coupling coefficient and apodization variations versus grating length variations is demonstrated at the central wavelength of 1550 nm by using both Hamming, Blackman impulse filter function and Hamming, Blackman grating profile. As well as the single FBG power spectrum, delay, dispersion, transmittivity/reflectivity are simulated and clarified against wavelength variations using both Hamming, Blackman impulse filter function and Hamming, Blackman grating profile. The Hamming impulse function filter and Hamming grating profile have clarified better performance than Blackman impulse function filter and Blackman grating profile.

Optimum placement of thin-layer elements in a horizontal sedimentation tank purification of drinking water

Experimental studies of the effectiveness of water distribution methods in a thin-layer sump were carried out on model and semi-industrial installations. When designing sedimentation tanks, it is usually recommended to take η = 0.03-0.07. According to our data, when η > 0.05, it is not possible to ensure uniform operation of all slots (holes), which worsens the hydrodynamic conditions in the settling zone. Experiments have also established that double gratings with a relative area of holes of 2n provide almost the same flow distribution as single gratings. In this case, the experimental values of β 1 were 2.6 and 2.7, respectively, i.e. differed by no more than 5%. With this in mind when installing double gratings with reflectors, the relative area holes with a sufficiently efficient flow distribution can be accepted and η = 0.06-0.1 instead of η = 0.03-0.05 recommended for single grates, which is essential to reduce the risk of clogging them and improve the reliability of sewer sedimentation tanks. The obtained dependences with sufficient accuracy for practice can be used to calculate double gratings and gratings with reflectors at values η=2η1 , where η1 is the relative area of the slots in a single grating, as well as when calculating partitions with round (square) holes, taking into obtained formulas bs = r holes, and ns = n holes.

Dispersion correction in the advanced volume holographic filter.

Thick volume Bragg gratings (VBG) have been used for wavefront selectivity in various applications such as data storage, endoscopy, or astronomic observation. However, a single thick grating is also selective in wavelength, severely limiting the spectral throughput of the system. Recently, our group introduced a two element Advanced Volume Holographic Filter (AVHF) where the first, dispersive Bragg grating is coupled to a thick VBG such that it dramatically improves the spectral bandwidth, and ultimately enhances the signal to noise ratio of polychromatic sources. Still, the two grating AVHF configuration introduced wavelength dispersion which prevents usage of the filter in imaging systems. Here, we present a solution to this problem by introducing a third diffraction grating that compensates for the dispersion of the two initial gratings. Using both simulation and experimental implementation of a visible-based, broadband AVHF system, the spectral dispersion was improved by a factor of up to 41 × compared to our previous system, re-collimating the output filtered beam. This new AVHF system can be utilized in imaging applications with noisy environments requiring filtration of a polychromatic source.

Detection of hazardous chemical using dual-wavelength Raman spectroscopy in the ultraviolet region

This study proposes a stand-off Raman spectroscopy system using dual-wavelength in the ultraviolet (UV) region to detect hazardous chemicals. The Raman spectrum generated by the UV excitation source avoids solar background noise during daytime for chemical detection as the spectrum is in the solar blind range. Wavelengths of 213 and 266 nm by 5th and 4th harmonics are generated from Nd:YAG laser. However, Raman spectra of chemicals exhibit different signal-to-noise ratios for both the excitation wavelengths; therefore, to detect such chemicals, Raman spectra by two sources are required. Raman spectra were acquired using a dual-wavelength laser and spectrometer with a single grating and detector at the wavelengths of 213 and 266 nm simultaneously. The Raman spectra of sulfuric acid, 2-chloroethyl ethyl sulfide, and dimethyl methylphosphonate were acquired and analyzed, thus highlighting the application of dual-wavelength Raman spectroscopy. For efficient chemical detection in the field, we have ensured that the system developed in this study is robust.

Artifacts reduction in high-acutance phase images for X-ray grating interferometry.

X-ray grating-based techniques often lead to artifacts in the phase retrieval process of phase objects presenting very fast spatial transitions or sudden jumps, especially in the field of non-destructive testing and evaluation. In this paper, we present a method that prevents the emergence of artifacts by building an interferogram corrected from any variations of the object intensity and given as input in the phase retrieval process. For illustration, this method is applied to a carbon fiber specimen imaged by a microfocus X-ray tube and a single 2D grating. A significant reduction of artifacts has been obtained, by a factor higher than 10. This evaluation has been performed experimentally thanks to the Confidence Map tool, a recently developed method that estimates the error distribution from the phase gradient information.