Search Results (83)

This article presents a structured spectral-stability assessment of an nJ-class ultrafast fiber laser generating femtosecond pulses with an approximate pulse duration of 115 fs, based on an ensemble of 61 consecutively acquired optical spectra. The study is motivated by the practical need to extract reliable short-sequence stability information from routine compact-spectrometer exports without requiring a separate pulse-diagnostic instrument at the initial assessment stage. For each spectrum, peak wavelength, centroid wavelength, FWHM bandwidth, integrated spectral area, correlation with the ensemble mean spectrum, and RMS deviation were calculated. Principal component analysis (PCA) was then applied to reduce the full spectral ensemble into a compact diagnostic space and to identify the dominant modes of residual spectral variation. The analyzed spectra yielded a peak wavelength of 775.31 ± 0.19 nm, a FWHM bandwidth of 7.95 ± 0.20 nm, an integrated spectral area of 10.43 ± 0.42 a.u.·nm, and a correlation with the mean spectrum of 0.99957 ± 0.00019, confirming a highly repeatable spectral envelope. PCA showed that PC1 and PC2 explained 66.50% and 12.60% of the variance, respectively, while PC3 contributed only 1.90%, indicating that the measured variability was weak and largely low-dimensional. These results demonstrate that consecutively exported optical spectra can provide a defensible and physically interpretable short-sequence stability assessment of ultrafast femtosecond fiber lasers, offering a practical route for routine monitoring, early-stage diagnostics, and future integration with simultaneous temporal and spectral characterization. Full article

We extended self-similar amplification to a large-mode-area tapered Yb-doped fiber (LMA T-YDF) with longitudinally decreasing nonlinearity. The theoretical analysis and numerical simulation demonstrate that T-YDFs with different nonlinearity profiles can achieve self-similar evolution, which is confirmed by a self-similar amplifier that employs two kinds of T-YDFs. Further experimental study indicates that the T-YDF with a large core diameter at the thin end can achieve self-similar evolution across a wide range of pump powers and generate 51 W average power, 34 fs nearly transform-limited (TL) pulses with 32 dB gain. To the best of our knowledge, this is the first theoretical and experimental demonstration of self-similar amplification in T-YDFs. The high-gain feature of the T-YDF simplifies the laser system and can be used to build a compact all-fiber high-power femtosecond laser source. Full article

Due to the ultra-trace concentrations of perfluoroalkyl compounds (PFCs) existing in environmental aqueous matrices, it is imperative to develop sensitive and high-enrichment-efficiency approaches for the determination of these emerging pollutants. In this study, a nitrogen-doped carbon dot-functionalized hollow fiber membrane (NCDs@HFM) was fabricated and employed in solid-phase microextraction (SPME) mode for the simultaneous identification of eight perfluoroalkyl carboxylates (PFCAs). The NCDs@HFM offers several advantages, including multiple active binding sites, chemical durability, a large specific surface area and environmental compatibility. Owing to these properties, the NCDs@HFM-based SPME demonstrated high extraction efficiency for PFCAs, where enrichment factors for target molecules could reach 35–61 fold under the optimum conditions. This established method was then integrated with liquid chromatography–tandem mass spectrometry (LC-MS/MS) for the qualitative and quantitative analysis of eight representative PFCAs in drinking and environmental water samples. The limits of detection (LODs, S/N = 3) and quantitation (LOQs, S/N = 10) of the method were at the scale of 0.0018–0.015 μg/L and 0.006–0.050 μg/L, respectively. This proposed method exhibited good precision, with RSDs below 13.2% and satisfactory accuracy, with recoveries ranging from 70.6% to 122.5%. The developed method was successfully applied in the identification of eight typical PFCAs in drinking and environmental water samples. This method exhibits several merits, including low cost, high sensitivity, good reliability and reusability, representing a promising alternative for measuring trace levels of PFCAs in aqueous matrices. Full article

A monolithic all-fiber high-energy chirped pulse amplification (CPA) system with a managed large dispersion is demonstrated. Considering the nonlinearity in the amplification system, two temperature-tuning cascaded chirped fiber Bragg gratings (CFBGs) with a large dispersion of 200 ps/nm are employed as stretchers to stretch the pulse duration to more than 2 ns in the time domain. The main amplifier, with centimeter-level length, a large mode area, and high-gain silicate glass fiber, increases the energy to 293 μJ at 100 kHz. A reflective grating pair with a high density of 1740 lines/mm is used to compress the large-dispersion chirped pulse into a compact structure. Owing to the high-order dispersion pre-compensation by the CFBGs and the large-sized grating with high diffraction efficiency, we achieved a compressed pulse duration of 466 fs with a maximum pulse energy of 250 μJ, corresponding to a compression efficiency of more than 85% and a well-preserved beam quality of M² < 1.3. To the best of our knowledge, this is the highest pulse energy ever reported in a monolithic fiber femtosecond amplifier. Full article

Currently, unreinforced steel fiber-reinforced concrete (USFRC) has not been widely adopted in underground engineering within China. However, extensive research has demonstrated that incorporating steel fibers can effectively enhance the mechanical properties of concrete, such as tensile strength, shear strength, residual flexural tensile strength, and also improve its durability. This study, based on the Qiandong experimental section of Dalian Metro Line 4, aims to investigate the failure modes, bearing capacity, and calculation methods for reinforced concrete (RC) and USFRC lining segment joints under compression-bending loading. The objective is to provide a reference for the application of USFRC lining segments in domestic underground engineering. The main conclusions are as follows: (1) The primary failure mode of RC segment joints is large-area crushing of concrete on the outer curved surface, with tensile crack widths on the inner curved surface less than 0.20 mm. The failure mode of USFRC segment joints is characterized by a 2.50 mm wide tensile crack below the loading point. (2) The bolt strain at failure for RC segment joints is approximately twice that of USFRC joints, with both reaching the yield strength and entering the plastic deformation stage. The bolt stress versus bending moment curve exhibits two distinct growth stages. USFRC can effectively control bolt deformation and stress, thereby enhancing bearing capacity. (3) The joint rotation angle versus bending moment curve follows a bilinear model. Under identical bending moments, the rotation angle of RC segment joints is significantly larger than that of USFRC joints. In the two stages, the rotational stiffness of USFRC joints is 367.13% and 763.82% of that of RC joints, respectively. (4) Bolts do not influence the bearing capacity of the segment joints. Existing calculation models in current design codes can accurately predict the ultimate bearing capacity of both RC and USFRC segment joints, demonstrating high prediction accuracy. Full article

A novel anti-resonant hollow-core fiber (AR-HCF) is proposed. The fiber uses cross-nested circular cladding tubes including single-nested and non-nested tubes in the near-infrared spectral region of 1.3–1.7 μm. All the anti-resonant tubes are used to minimize the confinement loss (CL) of the LP₀₁ core mode. The non-nested tubes are also employed to achieve single-mode performance through strong mode coupling between the cladding mode and the LP₁₁ core mode. The impact of the structural parameters on the CL of the modes is analyzed by using the finite element method (FEM). Optimization results indicate that the CLs of the LP₀₁ mode and the LP₁₁ mode are 0.18 dB/km and 5.88 × 10³ dB/km at 1.55 μm. Consequently, the higher-order mode extinction ratio (HOMER) achieves 3.27 × 10⁴. Additionally, the mode field area of the fiber exceeds 4720 μm² and the corresponding mode field diameter of the LP₀₁ mode is more than 77 μm across the spectral region of 1.3–1.7 μm. In the practical applications, the fabrication tolerance is analyzed. The collapse of anti-resonant tubes within 0–2 μm and positional offsets between 0° and 4° both have minimal impact on fiber performance, thereby ensuring the stability of the system. Compared with other reported fibers, the proposed fiber demonstrates superior performance. Full article

To study the mechanical properties and displacement evolution of rock masses near coal-seam normal faults under mining disturbances; this paper utilizes fiber optic monitoring and distributed strain measurement techniques to achieve the fine monitoring of the entire process of stress–displacement–strain during mining. The experimental design adopts a stepwise mining approach to systematically reproduce the evolution of fault formation; slip; and instability. The results show that the formation of normal faults can be divided into five stages: compressive deformation; initiation; propagation; slip; and stabilization. The strength of the fault plane is significantly influenced by the dip angle. As the dip angle increases from 30° to 70°, the peak strength decreases by 23%, and the failure mode transitions from tensile failure to shear failure. Under mining disturbances, the stress field in the overlying rock shifts from concentration to dispersion, with a stress mutation zone appearing in the fault-adjacent area. During unloading, vertical stress decreases by 45%, followed by a rebound of 10% as mining progresses. The rock layers above the goaf show significant subsidence, with the maximum vertical displacement reaching 150 mm. The displacement between the hanging wall and footwall differs, with the maximum horizontal displacement reaching 78 mm. The force chain distribution evolves from being dominated by compressive stress to a compressive–tensile stress coupling state. The fault zone eventually enters a stress polarization state and tends toward instability. A large non-uniform high-speed zone forms at the fault cutting point in the velocity field, revealing the mechanisms of fault instability and the initiation of dynamic disasters. These experimental results provide a quantitative understanding of the multi-physics coupling evolution characteristics of coal-seam normal faults under mining disturbances. The findings offer theoretical insights into the instability of coal-seam normal faults and the mechanisms behind the initiation of dynamic disasters. Full article

In this paper, we design a new type of terahertz orbital angular momentum (OAM) optical fiber with excellent transmission characteristics over a wide frequency range. Within the 0.8–1.8 THz frequency band, it shows stable support for transmission of the fifth-order OAM mode. Its dispersion control effect is excellent; it maintains the confinement loss of most modes at the extremely low level of 10⁻¹⁰ dB/m; its maximum dispersion is only 5.57 ps/THz/cm; and its effective mode field area is greater than 1.11 × 10⁻⁷ m². These characteristics jointly endow this optical fiber with broad application prospects and significant research value in the field of terahertz communication. With the continuous advancement of technology in this field, this optical fiber is expected to become a key component when building efficient, reliable, and large-capacity communication systems. Full article

The growing popularity of Personal Light Electric Vehicles (PLEVs), such as e-scooters, has revolutionized urban mobility by offering compact, cost-effective, and environmentally friendly transportation solutions. However, safety concerns, including inadequate infrastructure, poor protective measures, and high accident rates, remain critical challenges. This paper presents the design and development of an innovative self-balancing microvehicle under the H2020 LEONARDO project, which aims to address these challenges through advanced engineering and user-centric design. The vehicle combines features of monowheels and e-scooters, integrating cutting-edge technologies to enhance safety, stability, and usability. The design adheres to European regulations, including Germany’s eKFV standards, and incorporates user preferences identified through representative online surveys of 1500 PLEV users. These preferences include improved handling on uneven surfaces, enhanced signaling capabilities, and reduced instability during maneuvers. The prototype features a lightweight composite structure reinforced with carbon fibers, a high-torque motorized front wheel, and multiple speed modes tailored to different conditions, such as travel in pedestrian areas, use by novice riders, and advanced users. Braking tests demonstrate deceleration values of up to 3.5 m/s², comparable to PLEV market standards and exceeding regulatory minimums, while smooth acceleration ramps ensure rider stability and safety. Additional features, such as identification plates and weight-dependent motor control, enhance compliance with local traffic rules and prevent misuse. The vehicle’s design also addresses common safety concerns, such as curb navigation and signaling, by incorporating large-diameter wheels, increased ground clearance, and electrically operated direction indicators. Future upgrades include the addition of a second rear wheel for enhanced stability, skateboard-like rear axle modifications for improved maneuverability, and hybrid supercapacitors to minimize fire risks and extend battery life. With its focus on safety, regulatory compliance, and rider-friendly innovations, this microvehicle represents a significant advancement in promoting safe and sustainable urban mobility. Full article

Multiband transmission is, nowadays, being implemented worldwide to increase the optical transport network capacity, mainly because it uses the already-installed single-mode fiber (SMF). The G.654E SMF, due to its attributes (e.g., low-loss, and large-effective area in comparison with the standard G.652 SMF), can also increase network capacity and can also be used for multiband (MB) transmission. Nevertheless, in MB transmission, power mode coupling arises when bands with wavelengths below the cut-off wavelength are used, inducing multipath interference (MPI). This work investigates the impact of the MPI, due to mode coupling from G.654E SMF, in the transmission reach of a C+L+S band transmission system. Our results indicate that for the S-band scenario, the band below the wavelength cut-off, an approximately 25% reach decrease is observed when the MPI/span increases to −26 dB/span, considering quadrature phase-shift keying (QPSK) signals with a 64 GBaud symbol rate. We also concluded that if the L-band were not above the wavelength cut-off, it would be much more affected than the S-band, with an approximately 52% reach decrease due to MPI impact. Full article

A novel nested structure of hollow-core anti-resonant optical fiber is proposed to achieve low loss, large effective mode area, and wide transmission band simultaneously in the near-infrared range of 1200–2200 nm. It is composed of six elliptical cladding tubes nested with six large circular cladding tubes, and six small circular cladding tubes are introduced in the gap of the elliptical tubes. The transmission characteristics of the hollow-core anti-resonant optical fiber are numerically investigated using the full-vector finite element method. The effects of structural parameters such as the cladding tube thickness and the tube diameters on the fiber transmission characteristics are analyzed in detail. The results indicate that within the wavelength range of 1200–2200 nm, the confinement loss remains below 0.017 dB/km, and the minimum confinement loss can be as low as 1.2 × 10⁻⁴ dB/km at 1500 nm. The effective mode area remains as large as ~1142.5 μm². It should be noted that in the wide wavelength range of 1000 nm, the dispersion exhibits excellent characteristics ranging from 0.7 to 1.4 ps/(nm·km). Our fiber can find potential applications in ultra-long-distance and ultra-high-power transmission systems with a wide operating wavelength band. Full article

We report a novel coherent nanosecond pulse combination approach using four-channel non-polarization-maintaining large-mode-area (LMA) Ytterbium-doped (Yb-doped) fiber amplifiers. The stochastic parallel gradient descent (SPGD) and frequency dithering algorithm are introduced to stabilize the synchronization in polarizations and phases among all the channels. The system delivers an average power of ~250 W and a pulse duration of 4 ns with a combination efficiency of around 87% when the repetition rate of a single pulse is limited to 1 MHz, the polarization extinction ratio (PER) at 30 μm core diameter and 250 μm cladding diameter remains around 96%. Full article

Efficient ice protection systems are essential to ensure the operability and reliability of aircraft. In recent years, electromechanical resonant ice protection systems have emerged as a promising low-power alternative to current solutions. These systems can operate in two primary resonant modes: flexural and extensional. While extensional modes enable effective de-icing over large surface areas, their performance can be compromised by interference from flexural modes, particularly in thin, ice-covered substrates where natural mode coupling occurs. This study presents a strategy based on material selection for making the Young’s modulus-to-density ratio uniform. The final objective of this paper is to establish the design rules for a composite leading edge de-icing system. For this purpose, an incremental approach will be used on profiles with different radii of curvature: plate or beam (infinite radius), circular profile (constant radius), NACA profile (variable radius). For beam and plate structures, the paper shows that this coupling can be mitigated by selecting materials with a Young’s modulus-to-density ratio comparable to that of ice. For curved structures, the curvature-induced effect is another source of parasitic flexion, which cannot be controlled solely by material selection and requires careful thickness optimization. This study presents analytical and numerical approaches to investigate the origin of this effect and a design methodology to minimize parasitic flexion in curved structures. The methodology is applied to the design optimization of a glass fiber NACA 0024 airfoil leading edge, the performance of which is subsequently evaluated through icing wind tunnel testing. Full article

In this work, we have proposed and optimized a large mode area segmented cladding fiber (SCF) with an unconventional low-index segment cladding. The low-refractive-index cladding proposed in this paper consists of three parts. There three geometric parameters chosen as design variables were the length and width of the first part and the leg length of the isosceles trapezoid in the second part. To investigate the properties of the proposed SCF, numerical modeling based on the finite element method (FEM) was performed. A high leakage loss ratio (>9000) between the high-order modes (HOMs) and the fundamental mode was achieved at a wavelength of 1310 nm, which is significantly higher than that of conventional SCFs. Additionally, the mode area of the proposed fiber reaches 890 µm² at a core radius of 20 µm. The bending effects on the SCF were also studied. When the bending radius is greater than 0.3 m, the mode area greater than 880 µm² and remains stable, and the leakage loss of the least high-order mode (LP11h) exceeds 30 dB/m. The new fiber design demonstrates significant potential for high-power fiber lasers Full article

The minimum refractive index of the aluminophosphosilicate (APS) core in optical fibers has been determined for a wide range of phosphorous and aluminum concentrations. It was found that the APS core refractive index became higher by ~0.0005–0.0012 as compared to that in optical fiber preform. The analysis of the measured data has shown that at least 0.2 mol.% of Al₂O₃ and P₂O₅ remain in their ordinary form near their equimolar concentrations and do not form an AlPO₄ join (the effect observed for all concentrations of AlPO₄ join from 5 to 25 mol.%). Full article