Search Results (27)

Full control of the light field at the tip of the fiber holds the possibility of producing structured illumination patterns such as LG-beams or vector light fields, which have important applications in different fields such as imaging and quantum technologies. In this work, we show how, by measuring the transmission matrix (TM) and shaping the input of a few-mode fiber, we are able to produce cylindrical vector beams at the fiber output. We use singular value decomposition (SVD) to analyze the TM and use the singular vectors as the basis for beam shaping. We demonstrate the method in three different commercially available fibers supporting 6, 12 and 16 modes each. Full article

The shear behavior of beams cast with steel fiber reinforced concrete and provided with stirrups is a complex phenomenon that depends on various factors. In the present research effort, a hybrid support vector regression model combined with a particle swarm optimization algorithm is provided, to explore the relationship between the material and dimensional characteristics of a concrete beam and its shear strength. A database with diverse material properties associated with the shear strength of a steel fiber reinforced concrete beam was established from numerous reliable published research articles and was utilized for the development and evaluation of the model. The obtained results from the hybrid support vector regression model were then validated through the results of the artificial neural network and convolutional neural network models combined with the particle swarm optimization algorithm. In conclusion, the adopted hybrid support vector regression approach was proven to be a successful engineering technique that can be used in structural and construction engineering problems. Full article

In response to the urgent demands of the Internet of Things for precise indoor target positioning and information interaction, this paper proposes a multi-band analog radio-over-fiber mobile fronthaul system. The objective is to obtain the target’s location in indoor environments while integrating remote beamforming capabilities to achieve wireless access to the targets. Vector signals centered at 3, 4, 5, and 6 GHz for indoor positioning and centered at 30 GHz for wireless access are generated centrally in the distributed unit (DU) and fiber-distributed to the active antenna unit (AAU) in the multi-band analog radio-over-fiber mobile fronthaul system. Target positioning is achieved by radiating electromagnetic waves indoors through four omnidirectional antennas in conjunction with a pre-trained neural network, while high-speed wireless communication is realized through a phased array antenna (PAA) comprising four antenna elements. Remote beamforming for the PAA is implemented through the integration of an optical true time delay pool in the multi-band analog radio-over-fiber mobile fronthaul system. This integration decouples the weight control of beamforming from the AAU, enabling centralized control of beam direction at the DU and thereby reducing the complexity and cost of the AAU. Simulation results show that the average accuracy of localization classification can reach 86.92%, and six discrete beam directions are achieved via the optical true time delay pool. In the optical transmission layer, when the received optical power is 10 dBm, the error vector magnitudes (EVMs) of vector signals in all frequency bands remain below 3%. In the wireless transmission layer, two beam directions were selected for verification. Once the beam is aligned with the target device at maximum gain and the received signal is properly processed, the EVM of millimeter-wave vector signals remains below 11%. Full article

CVBs (cylindrical vector beams) are widely used in optical imaging, optical trapping, material processing, etc. In this study, based on mode-selective couplers and passive mode-locking fiber technology, a cylindrical vector fiber amplifier with an MOPA (master oscillator power amplifier) structure was developed. In the experiment, the pre-amp stage reached 19.87 mW output power and a CVB output with a mode purity greater than 97%. The measured beam quality factor was M² = 2.1. The CVB output power obtained by the first-amp stage was 152.4 mW, and the mode purity was greater than 92%. The measured beam quality factor was M² = 1.99. The internal inhomogeneity and external effects of the isotropic LMA (large-mode-area) fiber led to a decrease in beam quality and mode purity. After amplification, the gain of the fundamental mode was higher and the power was greater, resulting in a greatly reduced mode purity. This CVB fiber amplifier yielded important research value in expanding the applications of high-power fiber lasers. Full article

Transverse mode control of laser intracavity oscillation is crucial for generating high-purity cylindrical vector beams (CVBs). We utilized the mode conversion and mode selection properties of two-mode long-period fiber gratings (TM-LPFGs) and two-mode fiber Bragg gratings (TM-FBGs) to achieve intracavity hybrid-mode oscillations of LP₀₁ and LP₁₁ from an all-few-mode fiber laser. A mode-locked pulse output with a repetition rate of 12.46 MHz and a signal-to-noise ratio of 53 dB was achieved with a semiconductor saturable absorber mirror (SESAM) for mode-locking, at a wavelength of 1550.32 nm. The 30 dB spectrum bandwidth of the mode-locked pulse was 0.13 nm. Furthermore, a high-purity CVB containing radially polarized and azimuthally polarized LP₁₁ modes was generated. The purity of the obtained CVB was greater than 99%. The high-purity CVB pulses have great potential for applications in optical tweezers, high-speed mode-division multiplexing communication, and more. Full article

Macro-synthetic fibers are increasingly used in concrete as secondary reinforcement to control temperature and shrinkage cracks, improving durability by limiting crack widths. However, their impact on the shear strength of structural elements remains underexplored, particularly when used in combination with traditional steel reinforcement. To address this knowledge gap, this study developed and calibrated a non-linear numerical model to simulate the shear response of macro-synthetic fiber-reinforced concrete (PFRC) elements, using finite element software VecTor2. The model was calibrated with experimental data from PFRC panels subjected to pure shear loading, incorporating a custom concrete tension-softening model to capture the contribution of fibers. Validation against a broad range of PFRC beam experiments from the literature demonstrated the model’s accuracy, achieving an average predicted-to-experimental shear strength ratio of 0.99 (COV = 5.5%). Additionally, the model successfully replicated key response characteristics such as deformation patterns, crack propagation, and residual strength. The proposed modeling approach provides valuable insights into the interaction between fiber volume and transverse reinforcement. It also serves as a powerful tool for future numerical studies, addressing the existing data gap on PFRC behavior and exploring the synergistic effects of macro-synthetic fibers and steel reinforcement on shear strength. Full article

This paper presents a polarizing beam splitter (PBS) based on a hexagonal lattice silver-filled photonic crystal fiber (PCF) with two silver wires, which possesses advantages such as a short splitting length, high extinction ratio (ER), and an ultra-wide bandwidth in commonly used communication bands. Utilizing the full-vector finite element method (FV-FEM), thorough investigations were conducted on lasers within the wavelength range of 1.1 to 1.9 μm. The PBS demonstrates a working bandwidth of 725 nm (1.14 to 1.865 μm) under an ultra-short splitting length of 55.3 μm, with an ER exceeding 20 dB, covering all bands of O + E + S + C + L + U optical communication, and achieving a maximum ER of 74.65 dB, where the surface plasmon resonance (SPR) effect of silver metal plays a significant role. It not only features an ultra-short splitting length and an ultra-wide splitting bandwidth but also exhibits excellent manufacturing tolerances and anti-interference capabilities. This polarizing beam splitter represents a promising candidate in communication and may find various applications in optical communication. Full article

This paper presents a novel pentagonal structure dual-core photonic crystal fiber polarizing beam splitter (PS-DC-PCF PBS) filled with a nematic liquid crystal (NLC) in the central hole. Unlike previous designs with symmetric arrangements, the upper and lower halves of the structure have different air hole arrangements. The upper half consists of air holes arranged in a regular quadrilateral pattern, while the lower half features a regular hexagonal arrangement of air holes. By filling the central hole with birefringent liquid crystal, the birefringence of the structure is enhanced, reducing the coupling lengths along the x polarization and y polarization directions. The polarization properties, coupling characteristics, and the influence of different structural parameters on the extinction ratio of the polarizing beam splitter are analyzed using the full-vector finite element method. Simulation results demonstrate that the designed PS-DC-PCF PBS achieves a maximum extinction ratio (ER) of 72.94 dB with a splitting length of only 61.9 μm and a wide operating bandwidth of 423 nm (1.324–1.747 μm), covering most of the O, E, S, C, L, and U communication bands. It exhibits not only ultra-short splitting lengths and ultra-wide splitting bandwidth but also good manufacturing tolerances and anti-interference capabilities. The designed PS-DC-PCF PBS could provide crucial device support for future all-optical communication systems and has potential applications in fiber optic communication or fiber laser systems. Full article

This study focuses on the prediction of concrete cover separation (CCS) in reinforced concrete beams strengthened by fiber-reinforced polymer (FRP) in flexure. First, machine learning models were constructed based on linear regression, support vector regression, BP neural networks, decision trees, random forests, and XGBoost algorithms. Secondly, the most suitable model for predicting CCS was identified based on the evaluation metrics and compared with the codes and the researcher’s model. Finally, a parametric study based on SHapley Additive exPlanations (SHAP) was carried out, and the following conclusions were obtained: XGBoost is best-suited for the prediction of CCS and codes, and researchers’ model accuracy needs to be improved and suffers from over or conservative estimation. The contributions of the concrete to the shear force and the yield strength of the reinforcement are the most important parameters for the CCS, where the shear force at the onset of CCS is approximately proportional to the contribution of the concrete to the shear force and approximately inversely proportional to the yield strength of the reinforcement. Full article

Spin–orbit coupling and nonparaxiality effects during the propagation of vortex vector light beams in a cylindrical graded-index waveguide are investigated by solving the full three-component field Maxwell’s equations. Symmetry-breaking effects for left- and right-handed circularly polarized vortex light beams propagating in a rotationally symmetric graded-index optical fiber are considered. The mode-group delay in a graded-index fiber due to spin–orbit interaction is demonstrated. A scheme for observing the temporal spin Hall effect is proposed. It is shown that the relative delay times between vortex pulses of opposite circular polarizations of the order of 10 ps/km can be observed in graded-index fibers for high-order topological charges. Full article

Because of the special absorption peak, pulsed lasers at 1.7 μm have been rapidly developed in medical treatment, biological imaging and so on. Introducing the cylindrical vector beam (CVB) may further promote these special applications due to its unique intensity, phase and polarization characteristics. Herein, we have experimentally demonstrated the generation of wavelength-tunable pulsed CVBs at 1.7 μm based on a thulium-doped all-fiber laser. A bandpass filter with a wide bandwidth combined with nonlinear polarization rotation technology is used to obtain pulsed laser emission at 1.7 μm. By taking advantage of a home-made Lyot filter and mode selective coupler (MSC), pulsed CVBs can be obtained with a wavelength tuning range of 66 nm (1720–1786 nm). The development of wavelength-tunable pulsed CVBs at the 1.7 μm waveband has significant potential applications in deep bioimaging and laser processing. Full article

In this paper, a novel microwave photonic filter (MPF) based on a single longitudinal mode Brillouin laser achieved by parity time (PT) symmetry mode selection is proposed, and its unparalleled ultra-narrow bandwidth as low as to sub-kHz together with simple and agile tuning performance is experimentally verified. The Brillouin fiber laser ring resonator is cascaded with a PT symmetric system to achieve this MPF. Wherein, the Brillouin laser resonator is excited by a 5 km single mode fiber to generate Brillouin gain, and the PT symmetric system is configured with Polarization Beam Splitter (PBS) and polarization controller (PC) to achieve PT symmetry. Thanks to the significant enhancement of the gain difference between the main mode and the edge mode when the polarization state PT symmetry system breaks, a single mode oscillating Brillouin laser is generated. Through the selective amplification of sideband modulated signals by ultra-narrow linewidth Brillouin single mode laser gain, the MPF with ultra-narrow single passband performance is obtained. By simply tuning the central wavelength of the stimulated Brillouin scattering (SBS) pumped laser to adjust the Brillouin oscillation frequency, the gain position of the Brillouin laser can be shifted, thereby achieving flexible tunability. The experimental results indicate that the MPF proposed in this paper achieves a single pass band narrow to 72 Hz and the side mode rejection ratio of more than 18 dB, with a center frequency tuning range of 0–20 GHz in the testing range of vector network analysis, which means that the MPF possesses ultra high spectral resolution and enormous potential application value in the domain of ultra fine microwave spectrum filtering such as radar imaging and electronic countermeasures. Full article

Cylindrical vector beams (CVBs) are the product of polarization modulation of optical fields, and possess both unique focusing characteristics and excellent properties applicable to machining, imaging, communication and other fields. Mode selection couplers comprise a promising new method to realize the long-term stable output of cylindrical vector beam all-fiber lasers. Mode selection couplers have the advantages of a simple structure, high mode conversion efficiency and high mode purity. However, the production process of conventional asymmetric mode selection couplers is more complicated. Therefore, in this paper, a symmetric two-mode coupler with a 1 μm band is designed and fabricated using the finite element method, beam propagation method and fused pull-cone method, and a set of all-fiber passive mode-locked lasers based on symmetric dual-mode couplers are constructed. Finally, we obtain cylindrical vector beam outputs with central wavelengths of 1038.97 nm/1067.72 nm, a repetition rate of 8.78 MHz, pulse durations of 660 ps/656 ps, maximum average powers of 5.25 mW/5.2 mW, and the high mode purity of TM (transverse magnetic)₀₁ mode and TE (transverse electric)₀₁ mode is 97.18% and 97.07%, respectively. Full article

The numerical modeling of structures is a widely preferable approach to investigate the structural behavior of RC beams since it delivers inexpensive predictions for confirming the required goals concurrently with reducing casting, testing time, and effort. Shear-strengthening of reinforced concrete (RC) beams using externally bonded (EB) fiber-reinforced polymers (FRPs) has attracted much attention due to the fact that the FRP strengthening technique has the ability to alter the distribution of stresses between the structural elements and increase the load-carrying capacity. A significant number of experimental studies have been carried out to test the monotonic behavior of FRP shear-strengthened RC beams. Conversely, limited numerical research has been performed to investigate such performance. The VecTor2 software is developed based on the modified compression field theory (MCFT) and is directed to examine the monotonic behavior of retrofitted specimens using fiber-reinforced polymer (FRP) composites. To the authors’ knowledge, the behavior of FRP shear-strengthened beams has not been explored in the literature using the MCFT modeling approach. The main objective of this study is to detect the software’s capability of predicting the experimental outcomes of FRP shear-strengthened RC beams. This research study is carried out in two stages. Initially, the numerical study involves the development of an accurate finite element model to simulate the control specimens. The quality of this model is assessed by comparing the numerical results with the experimental outcomes. In the second phase of the numerical study, the control beam model is modified to accommodate the presence of external FRP composites. The accuracy of this model is again measured by comparing its predictions with the experimental measurements. The goal of these phases is to ensure that the numerical model captures the actual behavior of the tested beams. Additionally, two distinctive modeling approaches are investigated to represent the behavior of FRP composites. The accuracy of the numerical models is verified through comparisons of numerical predictions to experimental results in terms of ultimate loading capacity, load–deflection relationships, and failure modes. It can be stated that the validated numerical model provides alternate means for evaluating the monotonic behavior of both strengthened and non-strengthened RC beams. The predicted results compare very well with the test results of the control specimens when discrete truss elements are employed for the FRP composites. Furthermore, the numerical model provides useful information on the crack patterns and failure modes. Full article

Estimating shear strength is a crucial aspect of beam design. The goal of this research is to develop a shear strength calculation technique for ultra-high performance fiber reinforced concrete (UHPFRC) beams. To begin, a shear test database of 200 UHPFRC beam specimens is established. Then, random forest (RF) is used to evaluate the importance of influence factors for the shear strength of UHPFRC beams. Subsequently, three machine learning (ML)-based models, including artificial neural network (ANN), support vector regression (SVR), and eXtreme-gradient boosting (XGBoost), are proposed to compute shear strength. Results demonstrate that the area of longitudinal reinforcement has the greatest influence on the shear capacity of UHPFRC beams, and ten parameters with high importance (e.g., the area of longitudinal reinforcement, the stirrup strength, the cross-section area, the shear span ratio, fiber volume fraction, etc.) are selected as input parameters. The models of ANN, SVR, and XGBoost have close accuracy, and their R² are 0.8825, 0.9016, and 0.8839, respectively, which are much larger than those of existing theoretical models. In addition, the average ratios of prediction values of ANN, SVR, and XGBoost models to experimental results are 1.08, 1.02, and 1.10, respectively; the coefficients of variation are 0.28, 0.21, and 0.28, respectively. The SVR model has the best accuracy and reliability. The accuracy and reliability of ML-based models are much better than those of existing models for calculating the shear strength of UHPFRC beams. Full article