Optical Fiber Gratings Research Articles

Inscription and Thermal Stability of Fiber Bragg Gratings in Hydrogen-Loaded Optical Fibers Using a 266 nm Pulsed Laser

Fiber Bragg gratings (FBGs) have gained substantial research interest due to their exceptional sensing capabilities. Traditionally, FBG fabrication has required the use of pre-hydrogenated fibers and high-cost laser systems such as excimer lasers at 193 nm or femtosecond lasers. In this study, we present the first instance of FBG inscription in hydrogen-loaded, standard single-mode silica optical fibers using a more affordable 266 nm solid-state pulsed laser combined with a scanning phase mask lithography technique. We systematically explored the effects of pulse energy and scanning speed on the quality and spectral characteristics of the gratings, achieving reflectivities as high as 99.81%. Additionally, we tracked the spectral evolution during the FBG inscription process, demonstrating uniform growth of the core mode. We also investigated the stability of the core mode during a 24-h thermal annealing process up to 150 °C. The sensitivity was 10.7 pm/°C in the range of 0 to 130 °C. Furthermore, strain measurement was conducted based on the FBG annealed at 100 °C, showing a sensitivity of 0.943 pm/µε in the range of 0 to 1667 µε.

Real-time optical fibre near-infrared chromatic dispersion analyser using collocated optical fibre gratings.

We present what we believe to be a novel optical fibre device for real-time measurements of near-infrared chromatic dispersion of a liquid medium from 1100 nm to 1700 nm. This inline optical fibre chromatic dispersion analyser is based upon collocated fibre long-period gratings written adjacently in the core by femtosecond laser inscription yielding 8 attenuation bands associated with different cladding modes, yielding 8 independent measurements. This fibre device is tested on a series of chemical compounds associated with wines. The results for each compound yielded distinctive chromatic responses. A machine learning pipeline was developed that successfully clustered and classified spectral responses of the different chemicals to illustrate the distinctive responses. The limits of detection for these fibre devices ranged from 1×10-3 to 6×10-5 mol/L for the compounds associated with wines, demonstrating the potential usefulness of an optical fibre chromatic dispersion analyser is capable of monitoring in real-time effective chromatic dispersion that is not offered by any current technology.

Embedded passive guided wave tomography. Application to corrosion monitoring in multilayered pipes

Guided wave tomography techniques have been studied in the last decade to quantitatively image defects in waveguides such as plates or pipes. Recent developments based on the use of an autocalibration method, removing the need of a reference acquisition, enabled the absolute imaging of the structure and drastically reducing the sensitivity to environmental and operational conditions. Another interesting feat for the structural health monitoring (SHM) is the use of passive methods relying on the measurement of the ambient noise, related to the use of the structure and its environment, and signal processing based on a cross-correlation to recover signals equivalent to the ones measured when a sensor is emitting. Such system prevents from the need of cumbersome circuitry and energy consumption related to wave emission. It also enables the sole use of sensors that cannot emit waves or only with a poor actuation such as optical fiber sensors. A SHM system relying solely on Fiber Bragg Gratings (FBGs) in optical fibers can then be designed. Several FBGs can be enscribed in a single optical fibers, reducing further the size and weight of the system. The optical fibers being resistant to harsh environments such as extreme temperatures or ionizing environments and having a good integration capability, such a system meets the needs for an industrial application. This talk will hence show the principle of the passive guided wave tomography with embedded sensors in a structure. It will be exemplified in the case of a multi-layered structure, showing the possibility to apply this system in a large panel of cases.

Strain Measurement Technology and Precision Calibration Experiment Based on Flexible Sensing Fiber.

As the basic application of fiber optic sensing technology, strain measurement accuracy as a key index needs to be further calibrated and analyzed. In this paper, accuracy calibration experiments and the related analyses of two fiber-optic sensing technologies, the fiber-optic grating (FBG) and optical frequency domain reflectometry (OFDR), are carried out using a standard beam of equal strength and a mature resistive strain gauge (ESG). The fiber-optic single-point strain data for loading and unloading changes of the beams of equal strength show good continuity and linearity, with good cyclic stability, and the error in the strain test data is less than 2% after repeated loading. At the same time, using finite element theory to analyze the data and using the measured data error within 5%, a good strain test curve linearity is achieved and R2 is better than 0.998. After repeated loading and unloading tests, it is verified that the fiber grating and the distributed optical fiber in the strain test have good stability in repeatability accuracy. The calibration experiments and data analysis in this paper further illustrate the three sensing technologies in determining the strain test accuracy and the advantages and disadvantages of the indicators, and the development of the fiber optic sensing technology application provides basic technical support.

Versatile 3D-printed LPFG: Characterization, challenges and insights

This manuscript studies a method to create Long Period Fiber Gratings in optical fibers using a cost-effective 3D-printed device equipped with interchangeable masks, allowing fast reconfiguration and non-permanent modifications. We investigate diverse configurations by analyzing their transmission spectra in the range from 1520 nm to 1570 nm, and measure the dip wavelength (λDip) evolution with the force applied to the device. A force-dependent blue-shift is observed, and the refractive index modulation is estimated up to 2.84×10−3 and 2.39×10−3 for the two examined gratings masks. Analysis of the λDip and the figure of merit suggest a threshold force beyond which the spectral characteristics tendency reverse. This is likely linked to the different elasticity between the optical fiber and the mask material and the way they deform. Although the method imposes certain limitations in bandwidth, it can produce LPFGs with good applicability in scenarios such as low Technology Readiness Level or swiftly deployable cost-efficient systems for optical filtering or sensing. Considering sensing applications, we present a simple model for real-time computing of displacement, strain, or pressure through spectrum monitoring. The model aligns with the observed experimental results, underlining its practicality for potential applications.

Recent advances in optical fiber grating sensors for detection of organic substances

The continuous development of society has led to the flourishing of environmental chemistry and biomedical fields, as well as to the research and development of novel functional materials and sensors. Some novel materials have excellent optical and biological/chemical properties and can be coupled with the optical fiber grating. The coupling between the materials and optical fiber grating can improve the performance due to the advanced materials can amplify the changing signals generated during the reaction with analytes of organic compounds. Despite the great achievements in the development of bio- and chemical-optical fiber grating sensors, there is no publicly available systematic review of the recent advances in the use of materials combined with optical fiber grating in organic substances sensing available in the literature since January 2016.Thus, we summarized the sensing principles of the bio- and chemical-optical fiber grating with and without surface plasmon resonance. We discussed the performance parameters such as sensitivity, selectivity, detection range, and low limit of detection for detection of organic substances including organic pollutants, life organics (glucose, hemoglobin, cancer markers, antibody-antigen, biotin and streptavidin, DNA, enzymes), microorganisms, and other organic substances. We analyzed the advantages and limitations of optical fiber grating sensors and presented challenges in their practical applications and future research directions.

Quasi-distributed optical fiber sensing for the coupled vibration analysis of high-speed train-bridge coupled system under earthquakes

Due to the extensive construction of high-speed rail lines passing through seismically active areas, the possibility of a sudden earthquake occurring during train operation is very high. Additionally, the earthquake will intensify the dynamic response of trains and bridges, causing serious bridge damage. In this work, the dynamic response behavior of the bridge structures in the coupled train-bridge system under the earthquake is investigated based on the 4-table shaking table system. Shaking table experiments are also conducted on a high-speed railroad model of simple-supported girder bridge at a scale of 1/10 with the simulated layers of China Railway Track System type II (CRTS II) ballastless track slabs. The strain responses of various components, including rails, track plates, base plates, and beams, were measured on the bridge using quasi-distributed fiber optic gratings, both with and without seismic actions. Additionally, the seismic isolation characteristics of the fasteners and the CA mortar layer were investigated. The dynamic performance of the coupled train-bridge system under earthquakes was studied in both the time and frequency domains.

Mechanically induced long period gratings in different silica multi-layered optical fibers

This work presents a thorough comparative investigation of mechanically induced long period gratings (MILPGs) realized in different unconventional multi-layered silica optical fibers: two double cladding fibers (DCF), with progressively three layered (PTL) and W-type structures, and a solid core photonic crystal fiber (PCF). A periodic interdigitated grooved structure, based on the stereo-lithography 3D printing technique, is developed for the grating formation in these fibers with simplicity and cost-effectiveness. Multiple grating periods are developed in the range 480–660 μm, resulting in attenuation bands with depth within 15–25 dB and wide range of tunability over the wavelength range of 1100–1700 nm. The results demonstrate effective management of transmission spectra evolution as a function of the applied weight, with minimal alteration in wavelength position when testing the fiber with or without its coating. Polarization dependent properties of these gratings are also investigated demonstrating a negligible impact. This is the first time that such a broad investigation is conducted about MILPGs in the fibers mentioned earlier with broad scouting in fabrication parameters, opening to the application of such devices for sensing and communication purposes.

Superlens-Assisted laser nanostructuring of Long Period optical fiber Gratings (LPGs) for enhanced refractive index sensing

We introduce an innovative approach to enhance refractive index sensing utilizing Long Period Optical Fiber Gratings (LPGs) processed through microsphere-assisted superlens laser nanostructuring technology. This method involves the self-assembly of a silica microsphere monolayer on the outer surface of LPGs, followed by pulsed laser irradiation to create nanoholes (with a diameter of 300–500 nm) on the surface, for forming the nanohole-structured LPGs (NS-LPGs). The sensing capabilities of NS-LPGs, featuring two different nanohole densities (5 % and 7.9 %), were experimentally evaluated in sucrose and glycerin solutions. The results revealed a notable improvement in sensitivity, with increases of 16.08 % and 19.57 %, respectively, compared to conventional LPGs. This suggests that higher nanohole density contributes to greater enhancement in refractive index sensing. Furthermore, the permanent nanohole structures showed improved durability and lifespan in various environments compared to surface-coating-based LPGs. Further enhancements can be achieved by refining the nanostructuring density and controlling the dimensions of the nanoholes.

A two component friction balance based on ultrathin fiber Bragg grating pairs

We have designed a two component friction balance based on an ultrathin fiber Bragg grating pairs to monitor external friction resistance, and statically calibrated it with weights. Friction resistance can cause the support beam of the fiber optic friction balance to tilt, while also driving the strain of the fiber optic grating. Therefore, the change in the center wavelength of the grating can be used to measure external frictional resistance. The experimental results show that the static calibration output of the friction balance is stable, reproducible. The wavelength change of the grating is proportional to the size of the load, and the two exhibit a good linear relationship. The range of the fiber optic friction balance is ±0.03 N. The sensitivity of the balance sensor in the X component is about 62.28 nm/N, and the sensitivity in the Y component is about 25.36 nm/N. In addition, we also conducted tests on the built-in temperature compensation of the fiber Bragg grating pairs.

Electron radiation impact on Long Period Gratings in different optical fibers

Electron radiation impact on Long Period Gratings in different optical fibers

Application of fiber Bragg grating sensing technology and physical model in bridge detection

In order to understand the application of fiber Bragg grating sensing technology and physical models in bridge detection, the author proposes an application research based on fiber Bragg grating sensing technology and physical models in bridge detection. The author first introduced the principle of fiber optic sensors, then analyzed the technology of demodulating fiber optic gratings, and discussed the application of fiber optic sensing technology in bridge detection, effectively improving the monitoring effect and safety performance of bridges. Secondly, in response to the problem that electrical sensors are difficult to meet the long-term monitoring requirements of bridge structures, it is proposed to use fiber optic grating vibration sensors to monitor the vibration status of bridges, and the feasibility of the scheme is demonstrated through theoretical and experimental results. FBG sensor parameters: Select a cylindrical mass block with a diameter of 3 mm and a height of 4 mm, weighing 0.6 g, a fiber length of 65 mm, and a pre stretching amount of 1.3 mm. Fiber Bragg grating sensors not only meet the requirements of frequency measurement range and temperature adaptation range, but also have advantages such as resistance to electromagnetic interference, simple appearance structure, convenient deployment, and good long-term stability. They have good engineering application prospects.

π-FBG Fiber Optic Acoustic Emission Sensor for the Crack Detection of Wind Turbine Blades.

Wind power is growing rapidly as a green and clean energy source. As the core part of a wind turbine, the blades are subjected to enormous stress in harsh environments over a long period of time and are therefore extremely susceptible to damage, while at the same time, they are costly, so it is important to monitor their damage in a timely manner. This paper is based on the detection of blade damage using acoustic emission signals, which can detect early minor damage and internal damage to the blades. Instead of conventional piezoelectric sensors, we use fiber optic gratings as sensing units, which have the advantage of small size and corrosion resistance. Furthermore, the sensitivity of the system is doubled by replacing the conventional FBG (fiber Bragg grating) with a π-phase-shifted FBG. For the noise problem existing in the system, this paper combines the traditional WPD (wavelet packet decomposition) denoising method with EMD (empirical mode decomposition) to achieve a better noise reduction effect. Finally, small wind turbine blades are used in the experiment and their acoustic emission signals with different damage are collected for feature analysis, which sets the stage for the subsequent detection of different damage degrees and types.

Periodic liquid crystalline waveguiding microstructures

Different methods allowing for creating optical waveguides with liquid–crystal (LC) cores, in which molecules form periodic patterns with precisely controlled periods, are reported. The first one is based on reversible photoalignment with high-resolution selective illumination and allows to control the period of LC molecules inside silica microcapillaries. The second method employs microstructures formed in PDMS, allowing to obtain both: LC-core waveguides and a set of specially designed periodic microelectrodes used for the periodic reorientation of molecules. Using both methods, we successfully controlled the period of the patterned alignment in the range from about 500 µm and scaled it down to as small as 20 µm. We performed experimental studies on waveguiding phenomenon in such structures, in view to obtain transmission spectra typical to optical fiber gratings. Since the results achieved in experimental conditions differed from those expected, the additional numerical simulations were performed to explain the observed effects. Finally, we obtained the waveguiding in a blue phase LC, characterized by naturally created three-dimensional periodicity with periods smaller than one micrometer. In such a structure, we were able to observe first-order bandgap, and moreover, we were able to tune it thermally in nearly the whole visible spectral range.

All-Fiber Narrow-Bandwidth Mode-Locked Laser Based on Polarization-Dependent Helical Long-Period Grating

As a crucial component of nonlinear polarization rotation (NPR) mode locking, optical fiber gratings offer advantages such as polarization modulation capability, stability, fiber compatibility, and preparation maturity, making them a vital technological foundation for achieving NPR mode locking. Here, a polarization-maintaining fiber helical long-period grating (PMF-HLPG) was designed and fabricated as a polarizer using the CO2-laser direct-write technique to realize the NPR effect. A homemade fiber Bragg grating (FBG) was also introduced into the laser system to enable a narrow-bandwidth lasing output and wavelength tunability. Based on the PMF-HLPG and FBG mentioned above, an all-fiber mode-locked laser with a spectra bandwidth of 0.15 nm was constructed to generate stable short pulses with a fundamental repetition rate of 12.7122 MHz and a pulse duration of 30.08 ps. In particular, its signal-to-noise ratio is up to 84.5 dB, showing the high stability of the laser. Further, the operating wavelength of the laser can be tuned from 1559.65 nm to 1560.29 nm via heating the FBG while maintaining its mode-locked state with stability. The results indicate that the PMF-HLPG could be used as a polarizer to meet the NPR mechanism for ultrashort pulse laser applications in optical communication, optical sensing, and biomedical imaging.

In Situ Monitoring of Curing Reaction in Solid Composite Propellant with Fiber-Optic Sensors.

Curing activity in the preparation of solid composite propellants determines the performance of solid rocket motors in operation. Limited by the lack of effective monitoring tools, the complete curing behavior and thermal-induced curing kinetics are rarely disclosed. It is still a challenge to monitor in situ and in real-time the physical and chemical cross-linking reaction during the curing of propellant. Herein, we demonstrate a promising approach based on optical fiber capable of being implanted inside the propellant to monitor the internal stress evolution during the curing process, by taking hydroxyl-terminated polybutadiene propellant as an example. Attributed to the strain and temperature sensitivity of a pair of optical fiber gratings, the thermal-assisted physico-chemical cross-linking states of curing process have been demonstrated in detail. By tracking the stress-induced wavelength shifts of fiber gratings and calculating the curing mechanism function, the complete curing roadmap, including the viscous flow stage, gel stage, hardening stage can be clearly revealed, and the curing completion times are obtained as 154, 81, and 40 h, at the curing temperatures of 60, 70, and 80 °C, respectively. The apparent activation energy of this curing system obtained by calculation is 73.88 kJ/mol. This flexible fiber-based sensor provides an effective tool for unraveling the cure kinetic mechanism, and paves a universal pathway to guide the preparation and applications of versatile composite materials for solid rocket motors.

Fiber Bragg gratings operating across arbitrary wavelength ranges

We demonstrate that fiber Bragg gratings in polymer optical fibers can lead to reflection peaks in any wavelength range when exciting high-order propagation modes, which can enhance the design of sensing systems for specific applications.

Strength properties of femtosecond-induced defects and weak Bragg gratings for distributed optical fiber sensors

One of the possible ways to increasing the sensitivity of distributed fiber sensors is to create point defects or weak Bragg gratings in a sensitive optical fiber serving as artificial reflectors. Such reflectors can be created by means of femtosecond laser radiation. However, after the inscribing such structures, a noticeable decrease in the mechanical reliability of the sensor fiber can be observed, which can be critical for the fabrication and operation of sensor cables. The paper reports the results of a study of the point defects and weak fiber Bragg gratings strength characteristics created for distributed sensor systems, depending on the parameters of the laser radiation used to inscribe them.

In-fiber interferometry sensors for refractive index

Compact interferometers based on waveguiding structures have found countless applications in refractive index measurements, chemical sensing, as well as temperature and pressure measurements. The most common fiber devices are based on Mach–Zehnder interferometry and Michelson interferometry—two design concepts that can readily be implemented using simple fiber optic components, such as mode splitters and combiners, fiber optic gratings, and fiber tapers, among others. Fiber interferometry can also be conducted based on the Sagnac effect and the Young (double-slit) interferometer. In this review, we examine and compare over 400 fiber optic interferometers as well as more than 60 fiber optic refractive sensors based on fiber optic cavities. Even though many of the devices show temperature-, strain-, and pressure-sensitivity, we focus our review on refractive index measurements, as these are the most common applications. Many devices were characterized by their inventors using their sensitivity to refractive index changes. While the sensitivity is an important characteristic of the device, it does not easily relate to the smallest resolvable refractive index change or the limit of detection when applied to chemical measurements. Instead, we propose here that one should use the figure of merit, which is defined through the refractive index sensitivity and the width of an interferometer fringe. Using simple assumptions, we were able to mathematically relate the sensitivity and the figure of merit to common design parameters, such as the length of the interferometer arms, the operating wavelength, refractive indices of the fiber and the sample, as well as an overlap parameter, which describes the fraction of the guided wave in the sensing arm that interacts with the sample. We determined this overlap parameter for each reviewed device from the reported interferograms. Our meta-analysis provides for the first time simple and easily applicable guidance to increase the figure of merit of fiber optic interferometers and fiber optic cavities with regard to their ability to detect small refractive index changes. A high figure of merit allows measuring very small refractive index changes such as those of gases at different pressures or of very dilute solutions.

Applications of Optical Fiber in Label-Free Biosensors and Bioimaging: A Review.

Biosensing and bioimaging are essential in understanding biological and pathological processes in a living system, for example, in detecting and understanding certain diseases. Optical fiber has made remarkable contributions to the biosensing and bioimaging areas due to its unique advantages of compact size, immunity to electromagnetic interference, biocompatibility, fast response, etc. This review paper will present an overview of seven common types of optical fiber biosensors and optical fiber-based ultrasound detection in photoacoustic imaging (PAI) and the applications of these technologies in biosensing and bioimaging areas. Of course, there are many types of optical fiber biosensors. Still, this paper will review the most common ones: optical fiber grating, surface plasmon resonance, Sagnac interferometer, Mach-Zehnder interferometer, Michelson interferometer, Fabry-Perot Interferometer, lossy mode resonance, and surface-enhanced Raman scattering. Furthermore, different optical fiber techniques for detecting ultrasound in PAI are summarized. Finally, the main challenges and future development direction are briefly discussed.