Search Results (28)

The packaging process of fiber Bragg gratings (FBGs) directly determines the strain transfer efficiency, chirp occurrence, and sensing performance of the sensors. At present, relevant theoretical and experimental studies on the surface roughness of packaging substrates remain scarce. In this paper, combined with the theoretical model of interfacial debonding driving force and the sensing mechanism of FBGs, the FBG sensing performance under different substrate surface roughness conditions was investigated. Experimental results show that an excessively high substrate surface roughness will induce FBG chirp when the external strain reaches 1.143 × 10⁻³, leading to the failure of strain transfer. In contrast, an excessively low surface roughness will weaken the interfacial coupling, thus reducing the strain response capability and cyclic stability of the sensor. The substrate surface treated by sandblasting with 150# abrasive exhibits the optimal comprehensive performance: The strain response capability of the FBG reaches 6.99994 × 10⁻⁶ pm/ε with a linear fitting coefficient of 0.99994, presenting excellent linear response and cyclic stability. This study clarifies the optimal range of substrate surface roughness for FBG packaging and can provide theoretical and technical references for the packaging design and optimization of high-performance FBG sensors. Full article

The simultaneous measurement of strain and temperature using Fiber Bragg Grating (FBG) sensors presents a significant challenge due to the intrinsic cross-sensitivity of the Bragg wavelength. While recent studies have increasingly employed “black-box” machine learning algorithms to address this ambiguity, such approaches often overlook the physical limitations of the sensor’s spectral response. This paper challenges the assumption that advanced algorithms alone can compensate for data that is physically ambiguous. We propose a “Sensor-Algorithm Co-Design” methodology, demonstrating that robust discrimination is achievable only when the sensor architecture exhibits a unique, orthogonal physical signature. Using a rigorous Transfer Matrix Method (TMM) and 4 × 4 polarization analysis, we evaluate three distinct architectures. Quantitative analysis reveals that a standard Quadratically Chirped FBG (QC-FBG) functions as an “ill-conditioned baseline” failing to distinguish measurands due to feature space collapse ($K_{cond}>4600$). Conversely, we validate two robust co-designs: (1) An Amplitude-Modulated Superstructure FBG (S-FBG) paired with an Artificial Neural Network (ANN), utilizing thermally induced duty-cycle variations to achieve high accuracy ($~3.4 °\mathrm{C}$ error) under noise; and (2) A Polarization-Diverse Inverse-Gaussian FBG (IG-FBG) paired with a 4 × 4 K-matrix, exploiting strain-induced birefringence ($K_{cond}\approx 64$). Furthermore, we address the data scarcity issue in AI-driven sensing by introducing a Physics-Informed Neural Network (PINN) strategy. By embedding TMM physics directly into the loss function, the PINN improves data efficiency by 2.2× compared to standard models, effectively bridging the gap between physical modeling and data-driven inference, addressing the critical data scarcity bottleneck identified in recent optical sensing roadmaps. Full article

The growing need for efficient and safe high-energy lithium-ion batteries (LIBs) in electric vehicles and grid storage necessitates advanced internal monitoring solutions. This work presents a comprehensive simulation model of a novel integrated optical sensor based on ethylene carbonate-filled photonic crystal fiber (EC-PCF). The proposed design synergistically combines a chirped fiber Bragg grating (FBG) and an extrinsic Fabry–Pérot interferometer (EFPI) on a multiplexed platform for the multifunctional sensing of refractive index (RI), temperature, strain, and pressure (via strain coupling) within LIBs. By matching the RI of the PCF cladding to the battery electrolyte using ethylene carbonate, the design maximizes light–matter interaction for exceptional RI sensitivity, while the cascaded EFPI enhances mechanical deformation detection beyond conventional FBG arrays. The simulation framework employs the Transfer Matrix Method with Gaussian apodization to model FBG reflectivity and the Airy formula for high-fidelity EFPI spectra, incorporating critical effects like stress-induced birefringence, Transverse Electric (TE)/Transverse Magnetic (TM) polarization modes, and wavelength dispersion across the 1540–1560 nm range. Robustness against fabrication variations and environmental noise is rigorously quantified through Monte Carlo simulations with Sobol sequences, predicting temperature sensitivities of ∼12 pm/°C, strain sensitivities of ∼1.10 pm/$\mathsf{\mu }$$\mathsf{\epsilon }$, and a remarkable RI sensitivity of ∼1200 nm/RIU. Validated against independent experimental data from instrumented battery cells, this model establishes a robust computational foundation for real-time battery monitoring and provides a critical design blueprint for future experimental realization and integration into advanced battery management systems. Full article

We demonstrate the fabrication of the fiber Bragg grating (FBG) in a self-developed Yb-doped seven-core fiber using two femtosecond laser direct writing methods: a grating array inscription method and a plane-by-plane inscription method. The array fabrication method uses the femtosecond laser to directly write a parallel fiber grating array in the core. The plane-by-plane method is implemented by adding a diaphragm in the optical path to precisely control the length of the refractive index modulation line along the femtosecond laser incident direction. Combined with femtosecond laser scanning, a uniform refractive index modulation plane can be inscribed in the core in a single scanning. Based on these methods, we successfully fabricate high-quality high-reflection FBGs and chirped FBGs in each core of the large-core multicore fiber (MCF) with 14 μm core diameters. Both fabrication methods achieve FBGs with reflectivity above 97% at the central wavelength. We report for the first time the fabrication of high-quality, high-reflectivity FBGs in large-core Yb-doped seven-core fibers using the femtosecond laser plane-by-plane inscription method. This work provides a feasible scheme for fabricating FBGs in MCF. Full article

Adhesive joining has the severe limitation that damages/defects developed in the bondline are difficult to assess. Conventional non-destructive examination (NDE) techniques are adequate to reveal disbonding defects in fabrication and delamination near the end of service life but are not helpful in detecting and monitoring in-service degradation of the joint. Several techniques suitable for long-term joint integrity monitoring are proposed. Fiber Bragg grating (FBG) sensors embedded in the joint are one of the promising candidates. It has the advantages of being close to the damage and immune to environmental attack and electromagnetic interference. Damage and disbonding inside an adhesive joint will give rise to a non-uniform strain field that may bring about peak splitting and chirping of the FBG spectrum. It is shown that the evolution of the full spectral responses can closely reveal the development of damages inside the adhesive joints during tensile and fatigue failures. However, recording and comparing the successive full spectra in the course of damage is tedious and can be subjective. An energy modulation interrogation technique is proposed using a pair of tunable optical filters. Changes in the full FBG spectral responses are modulated by the filters and converted into a conveniently measurable voltage output by photodiodes. Monitoring damage development can then be easily automated, and the technique is well-suited for practical applications. Filter spectrum width of 5 nm and initial overlap with the FBG spectrum to give 40% of the maximum output voltage is found to be optimal for measurement. The technique is tested on embedded FBGs from different adhesive lap-joint specimens and successfully reflected the severity of changes in the full spectral shapes during the course of tensile failure. Moreover, the trends in these PD outputs corroborate with the V value previously proposed to describe the qualitative change in FBG spectral shape. Full article

In the field of fiber-optic sensing, effectively reducing the noise of sensing spectra and achieving a high signal-to-noise ratio (SNR) has consistently been a focal point of research. This study proposes a deep-learning-based denoising method for fiber-optic sensors, which involves pre-processing the sensor spectrum into a 2D image and training with a cycle-consistent generative adversarial network (Cycle-GAN) model. The pre-trained algorithm demonstrates the ability to effectively denoise various spectrum types and noise profiles. This study evaluates the denoising performance of simulated spectra obtained from four different types of fiber-optic sensors: fiber Fabry–Perot interferometer (FPI), regular fiber Bragg grating (FBG), chirped FBG, and FBG pair. Compared to traditional denoising algorithms such as wavelet transform (WT) and empirical mode decomposition (EMD), the proposed method achieves an SNR improvement of up to $13.71 \mathrm{dB}$, an RMSE that is up to three times smaller, and a minimum correlation coefficient ($R^2$) of no less than 99.70% with the original high-SNR signals. Additionally, the proposed algorithm was tested for multimode noise reduction, demonstrating an excellent linearity in temperature response with a $R^2$ of 99.95% for its linear fitting and $99.74\%$ for the temperature response obtained from single-mode fiber sensors. The proposed denoising approach effectively reduces the impact of various noises from the sensing system, enhancing the practicality of fiber-optic sensing, especially for specialized fiber applications in research and industrial domains. Full article

We demonstrate a low latency delay of a radio frequency (RF)–linear frequency-modulated (LFM) pulse by modulating it onto optical carriers from a Kerr comb and sending the signal through a concatenation of off-the-shelf linearly chirped fiber Bragg gratings (LC-FBGs) and chirped-and-sampled FBG (CS-FBG). We characterize the frequency response and latency of the LC-FBG and CS-FBG. Then, experimentally, the LFM pulse performance is characterized by measuring the peak sidelobe level (PSL) at the output of the tunable delay system. The experiment, performed with an LFM pulse of 1 GHz bandwidth at a 10 GHz center frequency, shows a PSL better than 34.4 dB, attesting to the high quality of the buffer RF transfer function. Thus, the proposed optical memory buffer architecture, utilizing compact devices based on a Kerr comb and FBGs, offers several benefits for delaying LFM pulses, including (i) a larger tunable delay range, (ii) low latency, (iii) wide bandwidth, and (iv) high PSL. Full article

The temperature and strain fields monitoring during the preparation process of buoyancy materials, as well as the health status after molding, are important for mastering the mechanical properties of buoyancy materials and ensuring the safety of operators and equipment. This paper proposes a short and high-density femtosecond fiber Bragg grating (fs-FBG) array based on different temperature coefficients fibers. By optimizing the parameters of femtosecond laser point-by-point writing technology, high-performance fs-FBG arrays with millimeter level gating length and millimeter level spatial resolution were prepared on two types of fibers. These were successfully embedded in buoyancy materials to achieve in-situ online monitoring of the curing process and after molding. The experimental results show that the fs-FBG array sensor has good anti-chirp performance and achieves online monitoring of millimeter-level spatial resolution. Intelligent buoyancy materials can provide real-time feedback on the health status of equipment in harsh underwater environments. The system can achieve temperature monitoring with an accuracy of 0.56 °C and deformation monitoring with sub-millimeter accuracy; the error is in the order of micrometers, which is of great significance in the field of deep-sea exploration. Full article

Adhesive joints possess a number of advantages over traditional joining methods and are widely used in composite structures. Conventional non-destructive examination techniques do not readily reveal joint degradation before the formation of explicit defects. Embedded fiber Bragg grating (FBG) sensors and the resistance of carbon nanotube (CNT)-doped conductive joints have been proposed to monitor the structural integrity of adhesive joints. Both techniques will be employed and compared in the current work to monitor damage development in adhesive joints under tensile and cyclic fatigue loading. Most of the previous works took measurements under an applied load, which by itself will affect the monitoring signals without the presence of any damage. Moreover, most FBG works primarily relied on the peak shifting phenomenon for sensing. Degradation of adhesive and inter-facial defects will lead to non-uniform strain that may chirp the FBG spectrum, causing complications in the peak shifting measurement. In view of the above shortfalls, measurements are made at some low and fixed loads to preclude any unwanted effect due to the applied load. The whole FBG spectrum, instead of a single peak, will be used, and a quantitative parameter to describe spectrum changes is proposed for monitoring purposes. The extent of damage is revealed by a fluorescent penetrant and correlated with the monitoring signals. With these refined techniques, we hope to shed some light on the relative merits and limitations of the two techniques. Full article

This paper presents the development, analysis, and application of chirped fiber Bragg gratings (CFBGs) for dynamic and static measurements of beams of different materials in the single-cantilever configuration. In this case, the beams were numerically analyzed using the finite-element method (FEM) for the assessment of the natural frequencies and vibration modes of the beam for the dynamic analysis of the structural element. Furthermore, the static numerical analysis was performed using a load at the free end of the beam, where the maximum strain and its distribution along the beam were analyzed, especially in the region at which the FBG was positioned. The experimental evaluation of the proposed CFBG sensor was performed in static conditions for forces from 0 to 50 N (in 10 N steps) applied at the free end of the beam, whereas the dynamic evaluation was performed by means of positioning an unbalanced motor at the end of the beam, which was excited at 16 Hz, 65 Hz, 100 Hz, and 131 Hz. The results showed the feasibility of the proposed device for the simultaneous assessment of the force and strain distribution along the CFBG region using the wavelength shift and the full-width at half-maximum (FWHM), respectively. In these cases, the determination coefficients of the spectral features as a function of the force and strain distribution were higher than 0.99 in all analyzed cases, where a potential resolution of 0.25 N was obtained on the force assessment. In the dynamic tests, the frequency spectrum of the sensor responses indicated a frequency peak at the excited frequency in all analyzed cases. Therefore, the proposed sensor device is a suitable option to extend the performance of sensors for structural health assessment, since it is possible to simultaneously measure different parameters in dynamic and static conditions using only one sensor device, which, due to its multiplexing capabilities, can be integrated with additional optical fiber sensors for the complete shape reconstruction with millimeter-range spatial resolution. Full article

With the development of fiber optical technologies, fiber Bragg grating (FBG) sensors are frequently utilized in structural health monitoring due to their considerable advantages, including fast response, electrical passivity, corrosion resistance, multi-point sensing capability and low-cost production, as well as high accuracy and resolution over a long period. These characteristics allow FBG to be a proper alternative sensing element for displacement measurements. In this article, the recent sensing advances and principles of detection of FBG-based displacement sensors are illustrated. Specifically, the latest FBG-based displacement technologies are examined from three principles of detection, i.e., wavelength, intensity and phase signal demodulation. Regarding wavelength detection methods, the problem related to the cross-sensitivity can significantly be reduced depending on the new type of cantilever–FBG-based sensing developed. Vice versa, only the packaging method of FBG prestressed between two fixed ends can still avoid the chirp phenomenon in the reflection spectrum. Moreover, to attenuate the influence of temperature variations on the accuracy of FBG displacement sensors, specific temperature self-compensation structures were successfully designed according to the concepts of phase signal demodulation. In future investigations, different elastic structures and gratings manufactured through special fibers and new methodologies for temperature compensation will still highly refine the efficiency of FBG-based displacement sensors. Full article

In order to meet the needs of attitude correction of the optical fiber sensor array, an all-fiber optical sensor combined with FBG and CFBG-FP is proposed to monitor its attitude angle. The FBG sensor for pitch and roll angles detection employs a geometric model solution to shift the center wavelength of the three fiber Bragg gratings connected to it through the mass sphere, allowing for pitch and roll angle detection. Based on the magnetostrictive effect, the CFBG-FP sensor for heading angle detection employs changes of the cavity length of the chirped grating Fabry-Pérot cavity connected to its surface. The method of interpolation data processing and derivation of the attitude angle matrix is used to realize heading angle detection. The experimental results indicate that the FBG sensor for the pitch and roll angles detection has an effective detection range of 180 degrees and an angle measurement error of less than 1.5 degrees. Meanwhile, in the region beyond 10 degrees in the geomagnetic north-south direction, the angle measurement error of the CFBG-FP sensor for the heading angle detection is less than 1.3 degrees. Moreover, the impact of temperature in the system is minimized, demonstrating the validity of this combined fiber optical approach in the attitude angle measurements. Full article

Adhesive joints in composite structures are subject to degradation by elevated temperature and moisture. Moisture absorption leads to swelling, plasticization, weakening of the interface, interfacial defects/cracking and reduction in strength. Moisture and material degradation before the formation of defects are not readily revealed by conventional non-destructive examination techniques. Embedded fiber Bragg grating (FBG) sensors can reflect the swelling strain in adhesive joints and offer an economical alternative for on-line monitoring of moisture absorption under hygrothermal aging. Most of the available works relied on the peak shifting phenomenon for sensing. Degradation of adhesive and interfacial defects will lead to non-uniform strain that may chirp the FBG spectrum, causing complications in the peak shifting measurement. It is reasoned that the full spectral responses may be more revealing regarding the joint’s integrity. Studies on this aspect are still lacking. In this work, single-lap joint composite specimens with embedded FBGs are soaked in 60 °C water for 30 days. Spectrum evolution during this period and subsequent tensile and fatigue failure has been studied to shed some light on the possible use of the full spectral response to monitor the development of hygrothermal degradation. Full article

This review discusses recent achievements on grating fabrications in polymer optical fibers doped with photosensitive materials. First, different photosensitive dopants in polymer optical fibers (POFs) are summarized, and their refractive index change mechanisms are discussed. Then, several different doping methods to fabricate the photosensitive POFs are presented. Following that, the principles of gratings, including standard fiber Bragg gratings (FBGs), tilted fiber Bragg gratings (TFBGs), chirped fiber Bragg gratings (CFBGs), phase-shifted fiber Bragg gratings (PSFBGs), and long period fiber gratings (LPFGs), are reported. Finally, fabrications of different gratings based on photosensitive POFs in the last 20 years are reported. We present our article clearly and logically, so that it will be helpful for researchers to explore a broad perspective on this proposed topic. Overall, the content provides a comprehensive overview of photosensitive POF fabrications and grating inscriptions in photosensitive POFs, including previous breakthroughs and recent advancements. Full article

Dispersion is one of the main factors that limit the development of optical fiber communication systems regarding data rate and long distance transmission of the signal. This is because of increases in dispersion with the increase in data rate and distance, resulting in signal degradation. In this work, we propose an optimal dispersion compensated optical fiber system, which is designed on the basis of Q-factor, eye height, and bit error rate. The system operates at a bit rate of 40 Gbps and a distance of 100 km. According to the optimization scheme, the system is simulated using the modulation format Non Return to Zero (NRZ) with uniform and Linear Chirped Apodized Fiber Bragg Grating (LCAFBG) as dispersion compensator. After deciding the Fiber Bragg Grating (FBG) structure, other key parameters are simulated to meet the requirements. The simulation results show that using NRZ modulation format with a LCAFBG Tanh profile gives better performance. Full article