In this study, the polarization properties of light scattered by aerosols are investigated in actual atmospheric conditions at different visibilities considering the multiple scattering effects and the Earth's surface reflection. The vector radiation transfer equation is solved using the successive order scattering method. First, the visibilities of all the days in 2019 are evaluated. Representative high- and low-visibility atmospheres are then used as the transmission media. The macroscopic information and the microphysical properties of the aerosols provided by AERONET are used in the numerical simulations of the polarization properties of the scattered light. The coupled-surface reflection model is subsequently used to determine the properties of light scattered by the Earth's surface. The Stokes vectors are obtained as the simulation results, and the polarization degrees of the scattered light are derived. Our simulation results show that the aerosol scattering properties tend to be different in high-visibility weather conditions compared to low-visibility ones. As the solar zenith angle increases in low-visibility weather, the I, Q, and U elements of the first-order downward scattered light vary irregularly. However, these elements increase at all the orders of upward scattered light and at the high-order downward scattered light. Conversely, for high-visibility weather, the values of the I, Q, and U elements of all the scattered light orders that possess similar variation characteristics, increase with the solar zenith angle. The corresponding polarization degrees exhibit similar trends. The results of our study can be useful for the remote sensing of the polarization properties of aerosol scattered light and for determining the microphysical properties of aerosols.

In order to study the optical properties of different modes of marine atmospheric aerosols, coarse and fine modal classification and complex refractive index inversions are carried out and studied based on the Mie scattering theory of spherical particles. This is done using tools such as a visibility meter, automatic weather station, optical particle counter (OPC), and cavity attenuated phase shift (CAPS) single scattering albedo monitor. The inversions are carried out for atmospheric aerosols in the coastal waters of Maoming, Guangdong. The results show that the refractive index of fine-modal aerosol at 530 nm is approximately 1.35 (±0.01)?0.019 (±0.003)i when the relative humidity is greater than 55%, and it is 1.37 (±0.02)?0.020 (±0.003)i when the relative humidity is less than 55%. The refractive index of coarse-modal aerosol is approximately 1.40?0.002 (±0.002)i when the relative humidity is greater than 55%, and it is 1.48 (±0.02)?0.005(±0.002)i when the relative humidity is less than 55%. There is a clear difference in the refractive indexes of aerosol particles in different modals, which has a reference value for studies of the marine aerosol climate effect and the establishment of aerosol models in the Maoming sea area.

The refractive index structure constant Cn2 is an important atmospheric parameter reflecting turbulence intensity. Herein, a new Cn2 inversion method is proposed based on the coherence length r0 and isoplanatic angle θ0 data of the entire atmospheric layer to address the problem related to the fact that many initial input data are needed for inversion and inversion without single type data. Based on the generalized Hufnagel-Valley (HV) turbulence model, the theoretical relationship between r0 and θ0 is deduced. Based on the measured data of r0 and θ0 in Nanshan, Xinjiang, the seven parameters of the generalized HV model are obtained using inverse calculations followed by the determination of the Cn2 profile. These seven parameter values are substituted into the deduced theoretical relationship of r0 and θ0 to calculate the value of θ0 on any single day. The simulation results show that the variation trend of the fitted average Cn2 and single-day Cn2 profiles are in good agreement with the Xianghe model, and the coincidence degree is high. The average value of the daily correlation coefficient between the calculated and the measured θ0 profiles reaches 81.95%, and the highest value is 87%. The results verify the feasibility of the proposed method and provide a reference for the inversion of the Cn2 profile.

In this study, we developed a novel fiber Bragg grating (FBG) sensing network system that flexibly configures the number of sensors according to the priority of the monitored area, thus, improving the bandwidth utilization efficiency and increasing the number of sensors in the priority area. Because of the differences in the degree of spectral overlap of each channel, it is essential to achieve fast classification and accurate demodulation of overlapping spectra. The continuous wavelet transform (CWT)-particle swarm optimization (PSO) algorithm was used to achieve the overlapping spectrum classification and demodulation of the FBG sensing network. First, CWT was used to segment the spectral signals, and the overlapping spectra were classified according to their characteristics. Then, PSO was used to demodulate multiple FBG overlapping spectra. The simulation results show that the proposed method effectively decreases the demodulation time, and the maximum demodulation error is within 10 pm. This study provides an approach for fast and accurate demodulation of overlapping spectra in large-capacity FBG sensing networks.

A novel six-probe measurement system is proposed for reconstructing deep hole axes. This system is based on the two-point method in three dimensions. The probe can obtain the center position of the measured section and radius of the circle at each step. This novel approach has the following advantages. 1) The effects of the straightness error of the sensor motion device on the axis measurement and fitting results can be eliminated. 2) The approach has remarkable universality. The holes with different apertures can be measured by adjusting the relative position between displacement sensors. 3) Probe movement and data acquisition can be realized automatically which reduces the amount of labor and improves measurement efficiency. 4) The process of measurement is simple; the hole axis can be fitted using only a single scanning measurement, after which related parameters can be evaluated. These advantages were demonstrated by theoretical analyses and simulations. Further, experiments showed that this novel system can provide high lateral resolution.

Power insulators are widely used in the electric power field owing to their high mechanical strength, crack resistive surface, and slow aging speed. A terahertz wave reflection propagation model with three layers of power insulators is constructed to study the air gap between layers of power insulators. An air gap defect sample of rubber-epoxy resin plate was fabricated by simulating the air gap between layers of power insulator. The waveform analysis of the samples containing the interlayer air gap defects reveals that the thickness of the interlayer air gap defects is 100 μm. Moreover, the thickness of the interlayer air gap is calculated. The accuracy of THz detection of the interlayer air gap thickness is 95.61%, as verified by a digital display thousand meter. The energy integral imaging was performed on the defect samples and the maximum between-class variance (Ostu) binarization method was used to extract the air gap defects. To verify the recognition accuracy of THz detection, the optical coordinate measuring machine (CMM) 3D was used to reconstruct the bonding defects and extract the defect area, and the detection accuracy is 95.29%. This study provides a new method for detecting interlayer air gap defects in power insulators.

In pulsed laser-ranging systems, the amplitude and pulse width of the echo pulse typically vary owing to the electrooptical/photoelectric conversion process and the changes in the target position or reflectivity, which results in significant errors when the fixed threshold method is used directly. Although an automatic gain control (AGC) system can minimize the error, most AGCs measure the peak value through the peak holding circuit, which causes problems such as peak holding attenuation, instability, and slow response speed. Therefore, a 250-MHz-high-speed analog-to-digital conversion module is used in this study to obtain the peak value of the echo pulse and achieve peak retention and zeroing in the field programmable logic gate array. Compared with the AGC system with a peak-holding circuit, the system has a simplified circuit, no attenuation of voltage peak, and a faster response. The experimental results show that the designed laser ranging system can effectively control the amplitude of the echo pulse within a narrow range (0.76?1.44 V) to reduce measurement errors.

In this study, to improve the microstructure of the cladding layer and strengthen the surface properties of 42CrMo, the mechanism of the influence of temperature on the microstructure and microhardness of the cladding layer under different cladding powers were investigated using numerical simulation and cladding tests based on the multipass cladding of H13-TiC composite powder on 42CrMo substrate. In addition, based on the temperature variability of material properties, a laser cladding model is established, and the mechanism of the influence of power on the surface temperature and residual stress fields of the cladding layer is analyzed. Furthermore, the microstructure and element distribution of the H13-TiC composite cladding layer were analyzed by scanning electron microscope, and the relationship between the morphology distribution of TiC particles and microhardness was studied. The results show that when the laser power increases from 2000 W to 2850 W, the peak temperature of the third track increases by 34.4% and reaches 3471.53 ℃ at 2850 W. The residual stress on the track surface reveals the distribution law of large in the middle and small at both ends. The number of TiC fine particles in the cladding layer gradually increases with an increase in temperature. When the power was 2850 W, the TiC fine particles were uniformly dispersed, increasing microhardness of 33%?50% outside the TiC aggregation area. Here, the microstructure and microhardness of the cladding layer are improved by analyzing the variation law of the temperature and residual stress fields between tracks during power increase, thereby providing a theoretical basis for the application of laser cladding technology in pick material 42CrMo.

Aiming at the problem of short performance of automotive Al-Mg-Si alloy laser cold metal transition (CMT) composite weld, the weld grain size was refined to improve the mechanical properties of the weld from the perspective of Ti alloying. After Ti micro-allying, the Ti(Al1-xSix)3 precipitates gradually appeared and acted as heterogeneous nucleation particles, which enhanced the nucleation rate. Moreover, the volume fraction of Ti(Al1-xSix)3 increased (1.7%?4.2%) with the addition of thicker Ti foils (40 μm?80 μm), which significantly improved the α-Al nucleation rate and effectively refined the weld grain. Consequently, the addition of 80 μm Ti foils resulted in an average gran size of only 21 μm. The grain refinement via Ti micro-allying substantially improved the microhardness and tensile strength. Following the addition of 80 μm Ti foils, the microhardness and tensile strength reached 61 HV and 225 MPa, respectively, which were 7.0% and 8.2% higher than the values obtained without the addition of Ti foils, respectively.

In order to meet the connection requirements of metal and polymer in industrial lightweight, the laser connection experiment of titanium alloy TC4 and carbon fiber reinforced polymer PBTCF30 is carried out using a semiconductor continuous laser. By performing texture pretreatment on the surface of titanium alloy, the joint can be mechanically riveted to improve its strength. The joint shear strength and pretreatment time are taken as the responses, and the mathematical model is established using the response surface method according to the relationship between the two responses and the relevant process parameters. The interactive effects of the texture-scanning diameter, texture-scanning spacing, texture-scanning times, laser connection power, and laser connection speed on the joint shear strength and pretreatment time are analyzed. Finally, the optimal solution is obtained using the accelerated particle swarm optimization (APSO) method. The results show that the three parameters that have the greatest influence on the joint strength are: scanning spacing-scanning times, scanning times-connection speed, and connection speed-connection power. The validation of the model and the optimization results show that the predicted value of the model is consistent with the experimental value, and the reliability of the proposed model is confirmed.

This study investigates the impact of powder flow field convergence characteristics on layer height under different carrier gas flow rates and powder feeding amounts. A gas-solid two-phase flow model was developed using Fluent software and used to simulate the convergence characteristics of the powder flow field under different carrier gas flow rates and powder feeding amounts. The model was validated through experimental testing. The results show that as the carrier gas flow rate increases, the dispersion of the powder flow field at the nozzle outlet also increases and the powder convergence concentration decreases. In addition, as the powder feeding amount increases, the powder concentration field at the nozzle outlet and convergence concentration increase, while the convergence position remains unchanged. The model was found to be accurate as the simulation results were consistent with the experimental results.

To realize the continuous pumping source output of high-power semiconductor lasers, we design a stacked structure of multi-tube semiconductor lasers based on a microchannel heat sink packaging. Based on the simulation using finite element analysis software, this structure can expand the heat transfer channel of a single-tube semiconductor laser by assisting heat sink and cylindrical fins inside the microchannel. Additionally, the heat conduction effect is enhanced compared with the traditional groove microchannel. Furthermore, the oblique fin structure is optimized to control the water flow rate and promote the mixed flow effect, further improving the heat dissipation performance of the microchannel and power fitting on multiple single tubes under its packaging. The theoretical maximum output power can reach 128.75 W, which can realize the pumping of multi-tube semiconductor lasers in continuous working mode and meet their heat dissipation needs under the premise of low cooling power consumption required for microchannel heat sinking.

In this study, surface-metallized CNT (Ni-Cu-CNT)/Cu composite coatings were deposited on a Cu substrate by using the laser-assisted low-pressure cold spraying technique. The microstructure and wear resistance of the composite coatings were studied. The results show that under laser irradiation, significant plastic deformation occurs in the Ni-Cu-CNT reinforcement phase and Cu bonding owing to thermal softening. Ni-Cu-CNT particles are embedded into the plastic-deformed Cu particles to form a continuous, dense, and well-bonded composite coating. The thickness of the composite coating can reach 2609 μm, and the CNTs are uniformly distributed in the composite coating while maintaining its structural integrity. The composite coating with Ni-Cu-CNTs shows excellent wear resistance, the average friction coefficient reduces to 0.385, and the volume wear rate reduces to 1.49×10-4 mm3/(N·m). Moreover, the Ni-Cu-CNT reinforcing phase in the composite coating reduces the wear of the coating by bearing the load of the grinding ball on the coating surface. In addition, Ni-Cu-CNTs separate from the coating to form a lubricating layer on the coating surface during the wear process, reducing the friction coefficient of the coating, and, thus, improving the wear resistance of the composite coating.

In this study, three types of nitinol (NiTi) lattice structures [namely, body-centered cubic structure with Z-axis reinforcement pillars (BCCZ), face-centered cubic structure with Z-axis reinforcement pillars (FCCZ), and face-body-centered cubic structure of shaft reinforced pillars (FBCCZ)] having high stiffness were prepared by applying selective laser melting. The effects of the structure type and the porosity defect on the fatigue property were investigated. The experimental results show that the FBCCZ exhibits the best fatigue performance, reaching 107 loading cycles at the maximum compressive stress of 25 MPa. The BCCZ and FCCZ exhibit poor fatigue performance, reaching 107 loading cycles at the maximum compressive stress of approximately 15 MPa. The analysis results show that when the cyclic loading stress is large, the structure type has a significant effect on the fatigue property of NiTi alloys. The smaller the bending deformation and local stress concentration of the structure, the better is the fatigue property. Therefore, the best fatigue property is exhibited by the FBCCZ, followed by the FCCZ, and the BCCZ exhibits the worst. When the cyclic loading stress is small, the influence of micro defects on the fatigue property becomes significant than that of the structure type. The difference between the fatigue properties of the FCCZ and BCCZ gradually decreases as the cyclic loading stress decreases. The porosity defects in the sample promote the initiation of fatigue cracks and degrade the fatigue performance during cyclic loading.

The residual temperature field of a neodymium glass laser amplifier slab after thermal recovery significantly affects beam depolarization and wavefront distortion. When asymmetry occurs during the cooling process, it causes an asymmetrical distribution of the residual temperature field, which in turn affects the repeatability of the high-power laser beam between shots. This study conducted a simulation of the residual temperature field distribution characteristics derived from weak and strong cooling applied to the cladding sides of an N41-type neodymium glass slab. Changes to the residual temperature field caused by asymmetric cooling on the cladding sides were then investigated, where the degree of change is related to the strength of cooling on the cladding sides of the neodymium glass slab. Heat dissipation on the light-passing surfaces and cladding interfaces of the neodymium glass slab was further analyzed when weak cooling transformed to strong cooling.

Chalcogenide glass is a critical material for thermal imaging systems. Although chalcogenide glass is suitable for molding, the processing characteristics of this brittle material have not yet been thoroughly analyzed. To evaluate the micro-deformation mechanism of chalcogenide glass, a nanoindenter was used to perform indentation, variable-load scratch, and constant-load scratch tests. The experimental data reveal that the discontinuous stepped (pop-in) phenomenon occurs during indentation, indicating that chalcogenide glass undergoes elastic-plastic transformation. Furthermore, the elastic modulus and hardness of chalcogenide glass samples were determined. Upon observing sample morphology after indentation, it is determined that material accumulates at the sample edges. This causes the elastic modulus and hardness calculated using the Oliver-Pharr method to be higher than the experimental values. Therefore a semi-elliptical model should be used to correct the deviations. The depth-length curve for scratch depth was obtained by performing a variable-load scratch experiment. Furthermore, the removal methods of chalcogenide glass during each stage were analyzed. The critical load of brittle-plastic transition is 92.3 mN. Analysis of the scratch test data under a constant load reveals that the critical load of brittle-plastic transition ranges between 50 and 120 mN. This demonstrates the accuracy of the critical load determined using the scratch test under a variable load.

The contour intersection method is a novel method for inverting the complex refractive index of small particles. Currently, there are no reports on the use of this method for the inversion of the complex refractive index of nanoparticles. In this study, Au nanospheres are considered as the research object, and the feasibility and reliability of the contour intersection method in the inversion of the complex refractive index of nanoparticles are investigated. The Mie theory and dielectric function size correction model are used to calculate the relationship between the light scattering and absorption characteristics of Au nanospheres and the complex refractive index. The complex refractive index of the particles is obtained by a combination of the contour intersection method, Mie theory, and dielectric function size correction model. The backscattering efficiency constraint method is proposed for determining the unique solution among multiple valid solutions for the contour intersection method. The effects of the complex refractive index step size, particle size, and measurement error on the inversion results are quantitatively analyzed. Finally, the inversion accuracy of this method is compared with that of the traditional iterative method. The results show that if the scattering light efficiency and light absorption efficiency of Au nanospheres are known, the accurate complex refractive index can be obtained using the contour intersection method; when the measurement error is less than 5%, the accuracy of the inversion results can be ensured; under the same conditions, the inversion results of the contour intersection method are better than those of the iterative method. This simple inversion method is reliable for the measurement of the complex refractive index of Au nanospheres.

Upconverting particle (UCP)-assisted near-infrared (NIR) photopolymerization (UCAP) is a novel photopolymerization method, in which the upconversion luminescence efficiency of UCPs is the key factor that affects the photopolymerization efficiency. In this study, a series of NaYbF4∶Tm3+ microcrystals are synthesized by an ethylenediamine tetraacetic acid (EDTA)-assisted hydrothermal method, and the effects of adding sodium fluoride (NaF) and EDTA, the doping ratio of the sensitizer, and the hydrothermal temperature on the morphology and upconversion luminescence intensity of UCPs are investigated. Furthermore, the effects of UCPs on near-infrared photopolymerization are studied. The results indicate that the addition of the fluorine source promotes the phase transition and increases the aspect ratio of the crystals, and that a moderate amount of the fluorine source can enhance the upconversion luminescence of the product. In addition, decreasing the doped amount of the sensitizer, elevating the temperature, and decreasing the amount of ligand could each enhance the fluorescence intensity of UCPs to some extent. Moreover, increasing the intensity of NIR light could enhance the fluorescence intensity and the contribution of the UV emission band of the particles, resulting in a faster polymerization rate as well as a higher functional group conversion rate. Finally, the UCPs synthesized under optimal conditions could be successfully applied in direct 3D printing of NIR light-curable inks, demonstrating their application potential in 3D printing.

Quantum impedance Lorentz oscillator (QILO) is a newly established model for quantizing Lorentz oscillators based on Bohr-Somerfeld theory and quantum mechanical selection rules. Based on this model, the second harmonic characteristics of Li2SnTeO6, CsClO3, and Na2Nb4O11 optical crystals were analyzed and numerically simulated, and a method for estimating the second-order nonlinear optical coefficients of crystals was proposed. First, we calculated the effective quantum numbers before and after the atomic transition according to the peak frequency and full width at half maximum of the optical crystals. Then, considering the second nonlinear effective parameter of QILO, we inferred and determined the second-order polarizability of the three crystals as a function of wavelength. As a result, the second harmonic generation coefficients of the three crystals at 532 nm were 0.17 pm/V, 0.69 pm/V, and 1.17 pm/V, respectively, which agree well with those from the first principle. The results show that the second-order electric susceptibility based on QILO model is helpful to analyze and improve the efficiency of sum frequency, difference frequency, and frequency multiplication of materials, and the method is simple, calculation time is less, and calculation efficiency is high.

The objective of the secondary optical design of a collimating illumination system is to increase the outgoing light intensity and improve flux utilization. In this study, the influence of the source parameters on the illumination performance of pure-refraction-type and total-internal-reflection-type collimating illumination systems are investigated. The expressions for the relationship between the system's outgoing light intensity, flux utilization, source luminous exitance, and light distribution curve index are derived theoretically and verified by experiments. The results show that the system's outgoing light intensity is proportional to the source luminous exitance. For the total-internal-reflection-type collimating illumination system, a small light distribution curve index leads to a higher light flux utilization, larger outgoing light intensity, and wider lighting range compared to those of the pure-refraction-type collimating illumination system. In contrast, a large light distribution curve index leads to a larger outgoing light intensity and better spot slope in the pure-refraction-type collimating illumination system. This conclusion can provide a theoretical basis for matching and optimizing the source parameters and light distribution element structure in the secondary optical design of collimating illumination systems.

Currently, commercial underwater lenses are mainly those that have small field angles and large optical lengths. Based on the initial structures of six glass lenses and the characteristics of underwater imaging lenses, this study designs a 5-megapixel ultra-short focal underwater wide-angle monitoring lens using an ultra-short focal length scheme. Its full field of view is 95°, the focal length is only 3.1 mm, the total optical length of the system is 32.8 mm, and the rear working distance is 5.3 mm. The lens is composed of a waterproof window and a lens group consisting of four spherical and two aspherical lenses (including a filter and an aperture stop). When the lens operates underwater, the modulation transfer function is greater than 0.3 at a frequency of 250 lp/mm in the entire field of view, and the optical performance of the central field-of-view system is close to the diffraction limit. With an IMX675 sensor, the lens achieves a good imaging effect. An analysis of temperature and tolerance shows that the lens can adapt to changes in water temperature and meet the requirements of practical production and assembly.

To realize the broadband imaging of large aperture diffractive optical systems, we analyze the problem of increasing relay lens diameter due to the increased bandwidth of traditional diffractive lenses based on the Schupmann structure optical path model. We propose using a harmonic diffractive lens as the primary mirror to construct a large aperture broadband diffractive optical system. A large aperture optical system with an aperture of 10 m and a spectrum covering 400?900 nm is designed, and the aperture of the relay lens is reduced by 2.4 m compared with the traditional design. To verify the design method, an imaging optical system with an aperture of 80 mm and a spectrum of 400?900 nm is designed, and imaging experiments are carried out on the system. By checking the resolution target image with no chromatic aberration, the broadband imaging design method based on a harmonic diffractive lens as the primary mirror is verified, which provides an idea for designing a large aperture diffractive optical system.

A dual-band optical system with a long back working distance and limited far zoom is proposed and designed for visible light (486?656 nm) and near-infrared (900?1700 nm) to satisfy the urgent demand for multiband fluorescence imaging in biomedicine imaging research. A zoom structure suitable for the dual-band system is adopted to address technical problems such as the broad range of chromatic aberration variation and limited selection of component optical power caused by the dual-band long-back-working distance zoom system. The initial optical powers of four groups of zoom structures of the system are calculated. The feasibility of the initial structure of the zoom scheme is verified using the ideal paraxial plane. The independent aberration design is performed for each group element of the system. In the common optical path part, dual-band aberration is optimized. The rear group uses a splitter prism to separate the two bands. Different rear fixed groups are designed for dual bands to correct the residual aberration of the system, and dual-band imaging under the long back working distance is realized. The system exhibits good tolerance characteristics, the zoom curve is smooth without an inflection point, the image plane is stable during the zooming process, and the imaging quality is excellent.

Based on the principle of electrowetting liquid lenses, an aspherical double-liquid lens model with a square cavity structure is designed using the COMSOL simulation software, and the accuracy of the MATLAB tool for fitting the interfacial shape of the model is analyzed. The specific device structure is designed and processed, and the surface change of the aspheric double-liquid lens is investigated. The experimental results of the aspheric surface are obtained via image processing and surface fitting analysis, thus verifying the feasibility of the aspheric liquid lens with a square cavity structure.

Owing to the unique properties of quantum mechanics, the quantum key agreement has unconditional security in theory. In this paper, a quantum key agreement protocol is designed. The four-particle cluster states are used as quantum sources. Two communication parties conduct joint Bell measurements respectively, and code through the controlled NOT gate and Hadamard gate to achieve shared secret. Here, weak measurement and quantum measurement reversal methods are used to deal with decoherence during transmission. The proposed key agreement protocol not only has the ability to respond to various participant and external attacks but also has higher communication efficiency.

With the continuous development of medicine, optics, chemistry, communication and other fields, various micro total analysis system, lab-on-a-chip, micro electro mechanical systems and high-precision micro-nano devices began to appear and are gradually used. Most of these systems or structures are realized by preparing three-dimensional micro-nano connected structures in transparent materials by femtosecond laser. Therefore, the main technologies of femtosecond laser preparation of three-dimensional micro-nano structures are introduced, the main applications of micro-nano connectedstructures are summarized, the existing problems of current femtosecond laser preparation of three-dimensional micro-nano connected structures are put forward, and the technology is prospected.

Semiconductor lasers can be applied as ideal laser sources toward the development of laser and sensing technologies. For laser radar measurement, optical pumping, and fiber coupling, semiconductor lasers must have high power and high beam quality. The design of high-power semiconductor lasers promotes the easy production of multi-mode beams in the lateral direction, which decreases the lateral beam quality. Hence, the limitations induced by the lateral mode and the enhancement of the lateral beam quality of high-power semiconductor lasers have become important research topics. This paper focuses on three topics: the tapered laser, the method for the improvement of the lateral beam quality of a broad-area semiconductor laser, and the package structure of a semiconductor laser. Furthermore, the results and progress of domestic and international research on the control of the lateral beam quality are reviewed.

Seismic, infrasonic, and hydroacoustic monitorings are three types of remote waveform monitoring techniques stipulated in the Comprehensive Nuclear Test Ban Treaty (CTBT). Currently, the seismic, infrasonic, and hydroacoustic stations of the CTBT International Monitoring System (IMS) have traditional electrical sensing equipments. The distributed optical fiber-sensing technique is a new sensing method for determining variations in the vibration and strain at each point along the optic fiber based on the optical effect, and it has broad application prospects in the CTBT. The principle and development of the distributed fiber-sensing technique show that it can be directly applied to the detection system for obtaining vibration data during seismic, infrasonic, and hydroacoustic monitorings and applied to existing optical fiber communication facilities for obtaining supplementary information to the monitoring data. The detection system based on the distributed fiber-sensing technique exhibits distributed measurement, high sensitivity, and good stability. This study provides a new approach to improving seismic, infrasonic, hydroacoustic, and other detection equipments. However, the characteristics of the distributed optical fiber-sensing system, such as highly dense channels, high sampling rate, and a large amount of data, pose new challenges to the real-time processing of monitoring data. In addition, the impact of the distributed fiber-sensing technique on IMS monitoring capability should be further investigated.

Metalenses are new advanced planar optical devices based on the metamaterials. They can control the amplitude, phase, and polarization of incident lights with high free degrees to satisfy application requirements. Furthermore, they can perform various functions through different structural designs, such as diffraction-limited focusing and aberration correction. In this paper, we summarize the basic principles, applications, and progress of metalenses. Based on the excitation principle, we classify metalenses into plasmonic and dielectric metalenses. Based on the function, we classify them into tunable, aberration cancellation, and broadband achromatic metalenses. We also summarize the parameter data, advantages and disadvantages, and commercialization process of relevant research. Moreover, we discuss the problems and challenges faced by metalenses and recommend future research directions. The main purpose of this paper is to clarify metalenses and provide potential inspiration for designing high-performance metalenses.

The transmission capacity of optical wireless communication systems has approached the Shannon limit, and the emergence of orbital angular momentum (OAM), as a lateral new space dimension resource, can exponentially improve the capacity and spectral efficiency of optical wireless communication networks. Adopting technologies of OAM-related can provide a potential solution for the realization of ultra-high speed and high capacity cross-scene optical communication in the future. On the basis of comparing and summarizing the relevant research results of OAM at domestic and overseas, this paper introduces the research work of OAM technology in the field of optical wireless communication in Xi'an University of Technology, which is mainly divided into the space generation, propagation characteristics, separation detection and application of OAM beams. Finally, the future development trend and prospect of OAM technology in the field of optical wireless communication are foreseen, which provides new thoughts and reference values for the subsequent researchers to explore in this field.

In the field of geology, terahertz time-domain spectroscopy, with the characteristics of high signal-to-noise ratio, wide frequency bandwidth, and low incident wave photon energy as well as the advantages of simple instrument operation and fast testing speed, is used to characterize the composition, structure, and other parameters of rocks and minerals. In particular, the optical properties of rocks and minerals, water content in minerals, characterization of filler minerals, and modulation of minerals to terahertz waves are assessed. This provides a new analytical method for studying mineral formation conditions, mineralization, and mineral applications, thereby expanding the application scope of terahertz spectroscopy.

The large amount of unstructured data generated by the Internet of Things and cloud computing has recently increased the demand for data computing power and energy efficiency. Referencing the information processing method of the biological brain with neuron and synapse as basic units, neuromorphic computing can simulate the biological nervous system from the aspects of interconnection architecture and information processing mode, and realize ultra-low power processing of real-time information, which have become the forefront of the development of computing technology in the big data era. The processing of computational data in the optical domain makes photonic neuromorphic computing research important owing to its high application potential. On the one hand, photonic neuromorphic computing can take advantage of high-speed transmission, low power consumption, and high parallelism of photons. On the other hand, it can also prevent photoelectric and electro-optic conversion, thus, reducing additional time and power consumption. In recent years, phase-change materials (PCM), as a kind of optical material with high refractive index contrast and non-volatile property, whose refractive rate can be continuously adjusted under the driving of optical, electrical, and thermal excitations, have provided a feasible solution for non-volatile photonic neuromorphic computing and have become the current research hotspot. In this paper, we first introduce the basic principle and implementation method of photonic neuromorphic computing. Subsequently, we discuss the principle of utilizing phase-change materials in photonic neuromorphic computing. According to the unique characteristics of phase-change materials selected in different implementation schemes, two kinds of phase-change materials and different applications of optical synapse devices and integrated arrays are then summarized. Finally, we prospect the development of photonic neuromorphic computing techniques based on phase-change materials.

Heavy-metal pollution of groundwater is a severe threat to the environmental safety and human health. Owing to their high sensitivity and good selectivity, fluorescence probe detection methods have been commonly used to determine the concentration of heavy metals. Because of the large size and high cost of the light source and optical system, the current heavy-metal fluorescence detection devices cannot meet the demands of in situ detection. In this study, we develop an in situ fluorescence detection device for the analysis of heavy metals in groundwater samples. The optical detector of the device uses ultraviolet (UV) light-emitting diodes (LEDs) as the light source and exhibits a confocal optical path design with a fluorescence collection angle range and an outer diameter of up to 102° and 60 mm, respectively, to achieve miniaturization and decrease cost. A commercial benchtop fluorescence spectrophotometer was used as a reference device. The devices were equipped with identical Hg2+ ion fluorescent probes, and comparative performance tests were conducted. The experimental results demonstrate that the as-developed fluorescence detection device exhibits good stability and a remarkable linear response, with a correlation coefficient (R2) and detection limit of 0.989 and 1.47×10-9, respectively. Furthermore, the performance parameters of the fluorescence detection device are comparable to those of the benchtop fluorescence spectrophotometer.

The rapid and accurate detection of the oil-paper insulation aging state has attracted considerable attention. In this study, classification of the original Raman spectral aging state of oil-paper insulation is performed without feature extraction. First, the aging state of the insulation paper is divided into 10 categories according to the measured polymerization degree. Raman spectroscopy is performed on the oil-paper insulation samples in each aging state. Finally, 169 groups of Raman spectra are classified by the K-nearest neighbour(KNN) algorithm and integrated enhanced KNN algorithm. The results indicate that the KNN algorithm after integration enhancement has a stronger recognition ability for the original Raman spectrum, its discriminant accuracy is 98.32%, and it has better stability. It is proved that the discriminant model based on the integrated enhanced KNN algorithm accurately discriminates the original Raman spectra of oil-paper insulation. The proposed model simplifies the diagnosis of the aging state of transformer oil-paper insulation using Raman spectra and is of considerable significance for research on this topic.