This paper presents a method for high-dimensional information modulation and demodulation using a vortex beam for underwater wireless optical communication by identifying the superposition state of orbital angular momentum in the vortex beam. The method and process of identifying the superposition states of two vortex beams are described. The topological charge numbers in the superposition states are obtained based on the light intensity distribution diagram of the superposition states after water channel transmission. Based on the results, it is experimentally demonstrated that the superposition state of orbital angular momentum can achieve the modulation and demodulation of 16-dimensional information under low-topological-charge conditions. Thus, this study provides a feasible scheme for applying vortex beams in underwater wireless optical communication.

The diffuse attenuation coefficient of downwelling irradiance (kd) is one of the most critical optical parameters in oceanic optics. The optical parameters of waters around the Dalian port polluted by dispersed oil (oil in water) were measured, and the underwater light field was simulated using the radiative transfer model (Hydrolight) to analyze the influence of oil in water on kd and to establish a semi-analytical model for kd. The simulation results show that the kd spectra of various water depths increase with increasing oil concentrations. Moreover, kd increases with increasing depth, particularly for the waters containing high concentrations of dispersed oil, and the faster it approaches its steady asymptotic value. In addition, this study also shows that the contributions of the absorption coefficient (a) and backscattering coefficient (bb) to kd of water polluted by dispersed oil differ from those of a and bb to kd of natural water body, so that the parameter values of the kd semi-analytical models of water polluted by dispersed oil and natural water body are significantly different. Therefore, the existing kd semi-analytical model for natural water body is unsuitable for accurate calculations of kd of water polluted by dispersed oil. Hence, a semi-analytical model of kd was developed based on the simulation results of Hydrolight. The accurate downwelling irradiance can be further calculated using kd obtained with this developed semi-analytical model to provide a rapid and reliable solution for the underwater light field simulation of waters polluted by dispersed oil.

Aiming at the problem that the semi-empirical model in the aerosol fine particle retrieval algorithm on the directional polarimetric camera(DPC) platform carried by the GF-5 satellite is not suitable for the estimation of urban surface polarization reflectance. In this paper, based on the empirical orthogonal function method of DPC, the inversion of the optical thickness of aerosol fine particles is carried out. The aerosol radiative contribution is calculated based on Mie scattering, the surface contribution is calculated by the empirical orthogonal function method, and the optical thickness of aerosol fine particles is inverted by using the multi-angle polarization data and the vector radiative transfer equation. The inversion results of this study are consistent with the moderate-resolution imaging spectroradiometer distribution trend of aerosol fine particle optical depth products, quantitatively compared with the measurement results of AERONET Beijing, Xianghe, and Hong Kong stations, the correlation coefficients are 0.97, 0.96, and 0.9, the mean absolute error is 0.08, 0.07, and 0.12, and the root mean square error is 0.12, 0.11, and 0.17, which verifies the high precision and rationality of the algorithm. Finally, the monthly average data of aerosol fine particle optical thickness in some areas of China in 2019 are presented, and the changes of aerosol fine particle optical thickness in Shandong are analyzed. It is found that June is the highest period of the year, with an average value of 0.7. The above results verify the reliability of the algorithm and provide technical support for DPC to effectively monitor the spatiotemporal distribution of aerosols

Polytetrafluoroethylene has broad application prospects and great potential value in the field of photonic crystal substrate materials because of its low dielectric constant and low loss of light transmission at 0.7-2.5 THz. This paper describes the use of femtosecond laser, with a pulse duration of 388 fs and repetition frequency of 100 kHz, to research the preparation of two-dimensional photonic crystal from polytetrafluoroethylene sheet. By fitting the experimental results, the calculated radiation exposure was found to be 840 mJ/cm2 at wavelength of 1040 nm. In addition, it was revealed that laser parameters affected the quality of the two-dimensional photonic crystal and the single-layer micropore processing quality under the multi-pulse tapping path were the best. Further, the effects of laser power, scanning speed, and scanning times on the periodic cylindrical structure were investigated. Desirable preparing results were obtained at an average laser power of 9 W, scanning speed of 100 mm/s, and scanning times of 9. The results of this study can be useful for preparing two-dimensional photonic crystal of polytetrafluoroethylene by ultrafast laser.

Hollow Gaussian beam is a hollow beam class that does not carry orbital angular momentum, its dark spot size is small and has a simple function formula. Hollow Gaussian beam has potential application in the fields of atomic guiding, particle manipulation, and optical communication. Meanwhile, zone plates, also known as Fresnel zone plates, focus light waves through diffraction onto a receiving object. Therefore, this relationship can be applied to design zone plates which generate hollow Gaussian beams. This study demonstrates that the zone plate focal plane complex amplitude distribution function is proportional to the Fourier-Bessel transform of the equivalent pupil function which describes the effects of transmission on waves. The zone plate equivalent pupil functions were calculated to produce the 1st, 3rd, and 6th order hollow Gaussian beams. Subsequently, the structural parameters of these three zone plates were designed. The wavelength and focal length of the zone plates were also changed to determine their influence on the structure parameters and dark spot size. Next, the focal plane light fields of these three modified zone plates were simulated using the Rayleigh-Sommerfeld diffraction integral. The simulation results are consistent with expectations which verifies the reliability and accuracy of the equivalent pupil function method to design Fresnel zone plates.

In the process of production, processing and assembly of grating waveguide display system, due to the systematic error, cumulative error, and artificial assembly error of micro nano processing equipment, the imaging quality of the system will be affected, resulting in double image, blur, and other problems. In order to solve the above problems, this paper designs a one-dimensional pupil expanding grating waveguide, and uses the ray tracing simulation method to analyze the influence of grating waveguide parallelism error, grating period error, and system assembly error on the imaging quality. The imaging quality of the grating waveguide is processed and adjusted experimentally. By controlling the flatness of the waveguide within 0.3' and the grating period tolerance within 0.2 nm, a one-dimensional exit pupil extended grating waveguide is fabricated. After collimation and alignment, the 30 × 12° field of view angle is reached, good augmented reality display effect and clear imaging quality are achieved, which is of guiding significance for practical mass production.

A circular quadrature amplitude modulation based on probability-geometric hybrid shaping (PS-GS-Cir-MQAM) vector terahertz (THz) signal transmission scheme is proposed to improve the transmission performance of a radio-over-fiber (ROF) transmission system. The transmission performance of the 130 GHz band PS-GS-Cir-MQAM signal in an RoF system with an intensity modulator (IM) and in-phase/quadrature (I/Q) modulator cascaded is analyzed via co-simulation in Matlab and VPI, and the signal transmission performance is compared and analyzed. The improvement effect of different shaping methods on the error code performance is evaluated. The results show that the error performance of the proposed PS-GS-Cir-MQAM THz signal is better than those of the other three shaping formats. Among them, the PS-GS-Cir-32QAM THz signal shows the most significant improvement in error performance. Compared with the other three shaping formats, the signal power gain increases by 0.88, 1.4, and 2.48 dB,respectively. The proposed hybrid shaping-based vector THz signal is advantageous in terms of transmission; specifically, it overcomes the nonlinear effects of optical devices and optical fibers.

The relationship between wavelength demand and network physical connectivity is studied to solve the wavelength resource shortage problems in dynamic optical satellite network, and a method based on a time-space conflict map is proposed to analyze the characteristics of wavelength resources. The method for randomly generating network topology connections is utilized to discretize the dynamic topology of an optical satellite network into a space conflict map and a time conflict map, which express the physical connectivity and effective service window of the optical satellite network, respectively. Based on the time-space conflict map theory, the path selection probability of space conflict avoidance and the effective window service probability of time storage are established and multiplied to obtain the conflict probability in time-space and calculate the number of wavelengths required in the network. The results show that wavelength requirements are closely related to the network physical connectivity, maximum number of link hops, and number of transponders. More wavelength resources should be allocated to an optical satellite network with a larger business overlap factor.

Spectrum fragmentation has become a primary factor that affects spectrum utilization in elastic optical networks. To reduce the spectrum fragmentation rate, effective measurement of the spectrum fragmentation is necessary. Thus, in this study, a time-frequency fragmentation-aware model is developed based on a detailed analysis of the generation mechanism and probability of fragmentation resources, and a spectrum allocation algorithm is proposed to select the spectrum with the least degree of fragmentation for connection requests. Simulation results show that compared with the contrast algorithm, the proposed algorithm can reduce the service blocking rate and improve spectrum utilization while effectively reducing the spectrum fragmentation rate.

In the servomechanism control of laser communication optical transceivers, the equivalent compound control technology based on speed and acceleration lag compensation greatly improves the tracking as well as aiming accuracy of the optical transceiver while having little impact on the system stability. First, this study establishes the two-way tracking and aiming model for the satellite laser communication terminal, and thereafter analyzes the source of working disturbance on the optical terminal leading to solve the equivalent sinusoidal signal of motion and disturbance. Based on the double-loop closed-loop control of speed and position, the study of equivalent compound control technology is carried out. Through the variance-based sensitivity analysis of control parameters, the optimal design of speed and acceleration lag compensation parameters is completed. Finally, a motion simulation experiment is performed indoors for simulation analysis and experimental verification. The results show that after adopting the equivalent compound control, the tracking accuracy is 56.8 μrad in the state of double-end motion and the accuracy error is reduced by 80.06%. This suggests that the equivalent compound control technology can greatly improve the dynamic tracking accuracy of rough tracking of the optical transceiver.

The minimum frequency resolution plays a critical role in ensuring the capacity of visible light-positioning systems. Lower resolutions can support more available addresses. In this study, the primary factors influencing visible light imaging, such as the object distance and exposure time of the image sensor, were investigated to expand the system capacity. Next, a quantitative analysis of the minimum frequency resolution was conducted based on the above factors. Finally, the minimum frequency resolutions required to accurately achieve light-emitting diode identity (LED-ID) at different object distances were determined using the mainstream LED-ID recognition algorithm based on the visible light-positioning system, characterized by the imaging region size and the number of bright stripes. The experimental results reveal that when the object distances are 50, 100, and 200 cm, the minimum frequency resolutions are 60, 100, and 200 Hz, respectively. With an increase in the object distance, the image detection ability decreases, and the minimum frequency resolution increases. When the LED-ID recognition rate reaches at least 95%, an appropriate frequency interval can be selected to efficiently allocate available addresses. When the object distance is 250 cm, the system capacity increases by 120%. This study provides a valuable reference for selecting frequency intervals in practical application scenarios.

Space division multiplexing is one of the main technologies to greatly improve the data transmission capacity of a single optical fiber. Few-mode erbium-doped fiber amplifier is essential to compensate for the transmission loss in long distance mode division multiplexing transmission systems. Therefore, obtaining equalization gain in all modes supported by few-mode erbium-doped fiber is vitally important, and the high differential modal gain will reduce the transmission performance of the system. In this study, 18 μm/124 μm few-mode erbium-doped fiber was fabricated by modified chemical vapor deposition technique, and a two-mode erbium-doped fiber amplifier based on the fiber was demonstrated experimentally. The gain is above 19.4 dB for LP01 and LP11a modes and its differential modal gain is lower than 0.66 dB between 1535 nm and 1560 nm when the LP11b mode is pumped.

This study develops and demonstrates a sensor based on a strongly coupled seven-core fiber (SCF) supermode Bragg grating for multiparameter measurements. Two Bragg resonance notches are observed in the transmission spectrum, corresponding to HE11-like and HE12-like supermodes of the SCF. Because the SCF is spliced between two standard single-mode fibers with central alignments at both ends, the transmission spectrum of the device also comprises an interferometric profile owing to the Mach-Zehnder interference of the supermodes. The responses of the device to temperature and strain are experimentally characterized. The obtained temperature and strain sensitivities of the supermode Bragg grating notches are 9.56 pm/℃, 9.55 pm/℃ and 0.64 pm/με, 0.584 pm/με, respectively. The obtained temperature and strain sensitivities of interferometric dips are 11.8 pm/℃ and -0.925 pm/με, respectively. This device can be potentially used to measure the temperature and strain simultaneously.

In the practical application of space laser communication, an alignment error will affect the coupling efficiency of single-mode fibers, which must be corrected accurately. Under ideal conditions, the plane wave-single-mode fiber coupling efficiency and Gaussian light-single-mode fiber coupling efficiency models with no alignment errors or other influencing factors were first analyzed and compared. The effects of the coupling efficiency of a single-mode fiber with two alignment errors, lateral and longitudinal fiber offsets were studied. A method was proposed to improve the coupling efficiency of single-mode fibers based on MPLC technology. The change in the coupling efficiency before and after converting the plane wave into Gaussian light and correcting the alignment error using multi-plane light conversion (MPLC) was simulated numerically. The simulation results showed that the plane wave was converted to a beam that is similar to Gaussian light by MPLC, and the coupling efficiency was 18.54% higher than the plane wave coupled directly to a single-mode fiber. The coupling efficiency was improved to more than 99% after correcting the transverse and longitudinal offsets by MPLC. This method breaks through the limitation of mismatch between spatial light and single-mode optical mode and can effectively correct the alignment deviation error. The method also has theoretical significance for improving the coupling efficiency of spatial light to single-mode optical fibers.

With the information field continuously evolving, B5G and 6G have been introduced, and the development of digital communication has entered a stage of faster updates. To adapt to higher-speed communication and more diverse channels, different modulation formats have been proposed and applied in different environments. The purpose of changing modulation format in different time and channels is to maximize the channel utilization. However, the modulation format changed at the transmitter is unknown to the receiver, which does not facilitate the connection between different communication subsystems, thus affecting the construction of a large-scale air-space-ground-sea integrated communication network. Therefore, modulation format recognition may be key to solving this problem. Because artificial intelligence (AI) has great advantages in signal processing and classification based on feature extraction, modulation format recognition based on feature extraction combined with AI classification algorithm has great research and practical value. This study introduces several methods based on feature extraction and AI and analyzes, discusses, and summarizes their application in the communication field.

Owing to the capacity of processing linearly frequency-modulated signals with GHz-bandwidths in the optical domain, microwave photonic receivers with de-chirp processing are key to realizing radars with high resolution. In this paper, general theoretical models for microwave photonic receivers with de-chirp processing are proposed, based on which the gains for eletrical signal in the de-chirp processing are derived. Moreover, the noise and dynamic range of the receiver are analyzed using the proposed models. The results reveal that the gain for noise in the microwave photonics-based de-chirping receiver with one output channel is twice that for the signal. Additionally, the impact of key parameters on the performances of a typical receiver is investigated via numerical simulations. We employs an electrical pre-amplifier with a gain of 20 dB in a photonics-based de-chirping receiver. This helps us obtain an improvement of 10.8 dB on the sensitivity with a dynamic range degradation of less than 2 dB when the half wave voltage of the electro-optical modulator decreases from 6 V to 1.5 V. However, if the half wave voltage is less than 3 V, an electrical pre-amplifier with a gain lower than 30 dB is recommended to avoid major deterioration of the dynamic range.

An all-dielectric metamaterial with a simple structure and can excite toroidal dipole resonance, and that exhibits a high quality factor (Q) and high sensitivity (FOM) value is proposed. Based on the current density and electric field distribution of toroidal dipole resonance, the intrinsic physical mechanism that the metamaterial can excite the toroidal dipole is analyzed. Numerical simulation shows that the Q and FOM values of the all-dielectric metamaterial can reach more than 14000 and approximately 672.7 nm/RIU, respectively. Based on the harmonic oscillator coupling model and the electric field distribution at the resonant wavelength, the influence mechanism of spacing between the rings and thickness of the detector on the Q value and the resonant wavelength is also analyzed. This study can provide a theoretical basis for the design and preparation of high-quality toroidal dipole resonance metamaterial sensors for use in biological and chemical detection.

The cloud camera adopts the threshold method to detect cloud distribution information in the observation target area. In the cloud judgment algorithm, the accuracy of the radiance will affect the final inversion result. An information model between the response value of the pixel and the radiance was established. Improvement measures are proposed for the situation where the output response values of each pixel of the cloud camera are unequal when the incident light is uniform. First, according to the information model, the reasons for the different response values of the detector were analyzed, and the relative radiometric calibration model was established. It is difficult for the light emitted by the large-diameter integrating sphere radiation source to cover all the fields of view simultaneously due to the large field of the cloud detection camera. Therefore, a subfield of view measurement method was designed. The relative radiometric calibration of the cloud camera was performed using the calibration model and the subfield of view measurement method. The experimental results indicate that the nonuniformity of the imaging results of the integrating sphere is approximately 6% before correction. After the relative radiometric calibration correction, the nonuniformity is less than 1.5% under a typical signal-noise-ratio (SNR) and approximately 3.25% under a low SNR. The results of the calibration meet the requirements of the design of the cloud camera.

Liquid crystal displays (LCDs) are used for projection in phase measuring deflection (PMD) surface shape detection. The multilayer transparent structure of the LCD refracts the light path, generating positional deviation errors. The error requires point-to-point correction to improve the final measurement accuracy. In this study, an equivalent transparent layer was modeled based on the LCD multilayer transparent structure, and a corresponding PMD model was established for analysis and compensation for the transparent layer. The standard spherical mirror and flat mirror were detected using a compensation algorithm to PMD. The root-mean-square (RMS) error of the surface shape decreases by 20%-40% compared with that before compensation. The error caused by the refraction effect of the LCD transparent layer in a single-screen PMD can be compensated using the proposed method. The RMS error is decreased, and the detection result is closer to the actual surface error after compensation than before compensation.

The spectral reconstruction theory of the reconstructive spectrometer is applied to the Fourier transform spectrometer. The method takes advantage of the high spectral resolution of the reconstructive spectrometer and the inherent high incident optical throughput of the Fourier transform spectrometer. Verification experiments were performed in the wavelength range of 520-530 nm with a simple structured experimental setup of a spatial heterodyne Fourier transform spectrometer. We used the collected pattern images of input beams with different single wavelengths for spectral calibration experiments. The spectrometer can realize unique one-to-one mapping between the patterns and wavelengths required by the spectral reconstruction theory. Spectral reconstruction using the spectral calibration patterns realizes a spectral resolution of 0.10 nm, a considerable improvement from ~5.65 nm obtained using the Fourier transform spectrometer principle. Finally, an additional pattern image of the beam with a 525 nm wavelength was used in the spectral reconstruction experiment. The reconstructed spectrum contains reconstruction errors, and the full width at half maximum (FWHM) of the spectral signal peak is ~0.30 nm. The correlation analysis of the pattern images shows that spectral reconstruction is affected by noise during pattern image collection and the high similarity of the pattern images of the incident beams of adjacent wavelengths. Nevertheless, the reconstructed spectrum reveals the spectral information of the incident light. Moreover, the smaller FWHM spectral signal peak than that obtained by the Fourier transform spectrometer verifies the feasibility and the advantage of high spectral resolution of applying the spectral reconstruction theory to the Fourier transform spectrometer.

A roller monitoring method based on a distributed fiber-optic acoustic sensor (DAS) is investigated to identify roller faults in a belt conveyor. Based on the principle of using a DAS to detect the fault vibration characteristic signal of an idler, a belt conveyor test system with a belt length of 30 m is constructed to simulate different fault conditions, such as idler jamming, no bearing, and fracture for experimental data collection. By analyzing the collected sound signals in the time and frequency domains, the characteristic quantities under different faults are extracted to identify the faults in the idler. The experimental results show that the DAS can detect the fault vibration characteristics of the belt conveyor idler and has application prospect in the fault monitoring of industrial belt conveyor idler.

A three-filter-response switchable microwave photonic filters (MPF) based on micro-ring resonator (MRR) and fiber Bragg grating (FBG), using phase-intensity modulation, is proposed and experimentally demonstrated. By tuning the relative wavelengths of FBG reflectance spectrum, MRR notch, and optical carrier, the filter response of MPF can be switched between bandpass, flat-top bandpass, and high-pass. The bandpass, flat-top bandpass, and high-pass responses have bandwidth tuning ranges of 5.56-7.68 GHz, 6.23-11.92 GHz, and 5.83-10.86 GHz, respectively. Insertion loss of MRR under three responses is less than 10 dB. The switchable responses enable the proposed MPF to maintain higher flexibility, with a broad application such as frequency measurement and spurious suppression scenarios.

Selective laser melting (SLM), the main additive manufacturing process, has been extensively used in manufacturing. The scanning strategy is one of the key parameters that affects the forming performance. In this study, two of the most popular scanning pathways were selected as the research objects to show their effect on thermodynamic laws. First, a thermodynamic coupling simulation model conforming to the characteristics of the SLM process was established, and a secondary development was performed based on finite element software to realize the simulation development of different paths. Then, a set of process parameters was used as a simulation example. The results show that the weld pool geometry and thermal stress of the striped scanning path are bigger than those of the chessboard pattern. Finally, several experimental datasets, including molten pool morphology and residual stress tests, confirmed that the simulation results were accurate. This study shows how the scanning trajectory affected the thermodynamics of SLM forming and provided an effective process simulation technique for engineering practice, which is convenient for SLM forming quality prediction analysis.

To improve the surface performance of 20Cr13 stainless steel parts, the laser cladding technology was used for producing 15-5PH alloy coating on the surface of 20Cr13 stainless steel substrate so that its surface strengthening can be achieved. The geometric morphology, microstructure, phase, friction and wear properties, and microhardness of the cladding layer under the better process parameters obtained by signal-to-noise ratio analysis were analyzed by using a digital microscope, optical microscope, X-ray diffractometer, scanning electron microscope, energy dispersive spectroscopy, micro-hardness tester, and friction and wear tester. The results show that positive relationship between melt width, melt height, melt depth, and laser specific energy of the melt channel, the metallurgical combination between the 15-5PH cladding layer and 20Cr13 stainless steel is good without defects like cracks and pores. The structure of the coating is composed of equiaxed and columnar crystals, and hard phases such as granular NbC, NiCx, and ε-Cu are precipitated in the coating. The microhardness and friction and wear performance results show that the microhardness of the coating is about 2.4 times of that of the substrate. At the same time, the wear resistance of the coating is significantly improved compared with that of the substrate, and its wear forms are adhesive wear and abrasive wear.

This study investigated the preparation of high-strength Al-Mg-Sc-Zr alloy samples by laser melting deposition technology. The effects of energy density and powder feeding rate modifications on the density, microstructural evolution, and mechanical properties of the deposited samples were investigated using metallographic, scanning electron microscopes, microhardness, and tensile properties at room temperature. Results showed that under a constant powder feeding rate, the deposited samples' densification behavior and density gradually increased with an increase in energy density. This trend became more significant with an increase in the powder feeding rate. Hence, when the powder feeding rate was set to 5.5 g/min, the density of 50-150 J/mm2 samples specifically increased from 97.88% to 99.47%. Nevertheless, the sample with the best comprehensive mechanical properties was deposited under optimized technological conditions, namely an energy density of 100 J/mm2 and a powder feeding rate of 2.5 g/min. Its density, yield strength, tensile strength, elongation, and microhardness were 99.51%, 268 MPa, 450 MPa, 18.4%, and 120.18 HV0.2, respectively.

In this paper, a three-temperature heat transfer model was developed based on the temperature transfer process in the femtosecond laser scanning processing of the face gear material 18Cr2Ni4WA, and a multi-pulse energy accumulation model in the scanning process was established. The simulation analyzed the changes of electron lattice temperature when changing the laser energy density on the ablated material, the changes of maximum temperature of electron, lattice and material surface under multi-pulse loading, and the changes of loading energy when changing the scanning speed and scanning interval, and concluded that with the increase of energy density, the maximum temperature of electron increased from 37000 K to 44000 K, and the final equilibrium temperature increased from 17000 K to 22000 K. Under multi-pulse loading, with the increase of energy density, the maximum temperature of electrons also increases to a certain extent, and the equilibrium temperature of the highest temperature of the material surface also increases, from 2600 K to 3250 K. With the increase of scanning speed and scanning interval, the accumulated energy of multi-pulse has a certain decrease, and the energy distribution scale is increasing. The experiments analyzed the effects of different energy densities, scanning speeds, and scanning intervals on the femtosecond laser ablated surface gear material, and the roughness analysis of the ablated morphology was carried out, and it was concluded that the quality of the ablated morphology was better when the energy density was 4.34 J/cm2, the scanning speed was 300 mm/s, and the scanning interval was 18 μm. These provide a research basis for improving the surface morphology quality of the surface gear materials processed by femtosecond laser scanning.

LD side pumped Er3+,Yb3+∶glass waveguide passively Q-switched laser was reported. By adhesive-free bonding techniques, Co-doped borosilicate glass with a thickness of 0.1 mm was bonded on four sides of the core (atom fraction 1% Er3+,21% Yb3+∶glass) of waveguide. The aim was to block the formation pathway of amplified spontaneous emission (ASE) and improve the output efficiency of laser. In order to improve the pump uniformity and output beam quality of laser, K9 borosilicate optical glass was bonded on both sides of waveguide as the transmission layer of pump. In free-running mode, laser output was obtained with the maximum pulse energy of 34.7 mJ and the slope efficiency of 10.6%. In passively Q-switched mode, a pulse laser was achieved with wavelength of 1.535 μm, single pulse energy of 2.16 mJ, pulse width of 4.7 ns, peak power of 459 kW, and beam quality factor M2=1.53. Experimental results demonstrate that the bonding of Co2+∶glass on the four sides of Er3+,Yb3+∶glass is an effective method to inhibit ASE effect and improve the output pulse energy of laser.

We reported a high energy, high beam quality LD side-pumped narrow nanosecond pulses Nd∶YAGlaser amplifier. The amplification system consists of a nanosecond electro-optic Q-switched oscillator and a two-stage side-pumped Nd∶YAG rod amplifier. The oscillation stage uses Nd∶YVO4 crystal as the gain medium to reduce thermally induced birefringence and reduce intra-cavity losses. The amplification stage adopts a two-stage amplification structure to improve the magnification. Finally, when the pulse repetition frequency is 10 Hz, 1064 nm laser output with maximum single pulse energy of 377 mJ, pulse width of 5.9 ns, and average beam quality factor of 1.86 is obtained.

It is well known that different combinations of laser parameters can produce the same color on the surface of stainless steel. Therefore, a master oscillator power amplifier (MOPA) fiber laser with a wavelength of 1064 nm and maximum output power of 20 W was selected for experiments in this study to explore the qualitative relationship between parameters and color. The color and visible spectrum of color samples were determined and analyzed using a spectrophotometer. The oxide film thickness and surface morphology of the color samples were measured using a scanning electron microscope. The results show that the laser scanning speed, laser repetition rate, and laser power are proportional to change simultaneously, indicating that similar color and oxide films can be formed on stainless steel surfaces. This study is valuable in guiding the practical application of laser color marking.

Based on graphene (RGO)/polylactic acid (PLA) and RGO/Fe3O4/PLA composite absorbent wires, a three-layer pyramid absorbing body was printed using fused deposition forming technology. Through CST simulations and experiments, the influence of combining and distributing the absorbing agents (horizontal and stereo gradient distributions) on the absorption performance of the pyramid was investigated, and the absorbing mechanism was revealed. The results show that, for the homogeneous absorber, the absorption performance of the two-component absorber is better and improves with increasing graphene content. For the gradient distribution absorber (the pyramid height is 16 mm and the bottom dimension is 10 mm × 10 mm), when the stereo gradient distribution is adopted (the three-layer wave absorber graphene is added in amounts of 3%, 5%, and 7% in mass fraction), the strongest wave absorption effect can be achieved. Reflection losses of 6.1-18 GHz are lower than -10 dB, and the effective wave absorption bandwidth can exceed 11.9 GHz. The maximum absorption intensity is -45.8 dB at 17.2 GHz. Compared with the combination of absorbing agents, the distribution mode has a more significant impact on the absorbing ability of the pyramid. The wave absorber with stereo gradient distribution, on the one hand, improves impedance matching characteristics to ensure effective absorption bandwidth; on the other hand, increases multiple scattering, reflection, and spherical diffraction losses to improve absorption intensity.

To ensure high quality of virtual images in augmented reality head-up display (AR-HUD) systems, the size of the AR-HUD should be reduced as much as possible to achieve a longer distance and a larger field of view. Using an off-axis dual-reflection system with a free-form surface, a virtual image display optical path with a virtual image distance of 10 m and a field of view angle of 10°×5° is designed. Furthermore, the Eyebox (aperture diaphragm offset range) is divided into the moving range of a driver's field of view. The system has nine structures, and multiple structures can be used for the optimization simulation. The light spot from each field of view falls within the Airy disk. The modulation transfer function of the image is close to the diffraction limit, and both the distortion and dynamic aberration values are less than the industry standard values. Once the imaging requirements are satisfied, the dust-proof film of the head-up display (HUD) system is drawn, and the light damage simulation is performed; based on the results, sunlight can be prevented from entering the eyes. Finally, the AR-HUD shell is drawn, and the measured volume is 10 L. The display effect is simulated through a user interface (UI) image to verify the correctness and feasibility of the design.

Broadband hyperspectral cameras can comprehensively record the spectral information of a target, which is currently a significant research direction in studies on hyperspectral cameras. However, the broadband is bound to cause problems with excessive chromatic aberration and secondary spectra of the system, affecting the image quality. Therefore, based on an analysis method called the Buchdahl vector dispersion model, an optical system of broadband hyperspectral camera based on linear variable filter is proposed in this study. An image side telecentric transmitted optical system with a focal length of 100 mm, F-number of 5, field of view of 14.2°, and spectral range of 400-1000 nm was designed. The hyperspectral camera based on this system can capture images with a spatial resolution of 21.5 m, spectral resolution of 10 nm, and swath width of 125 km at an altitude of 500 km. The image quality evaluation and tolerance analysis show that the system has an excellent image quality and satisfies fabrication and alignment requirements. The modulation transfer function test results show that the system can satisfy actual application requirements.

Since the magnitudes of the Goos-H?nchen (GH) shifts in multilayered photonic crystals are generally small, it is desirable to find the alternative configurations to achieve the large GH shift. In this work, we investigated the GH shift of the reflected wave in the structure with a metal layer, a dielectric material, and the quasiperiodic photonic crystal by the transfer matrix method, where the quasiperiodic photonic crystal is composed of a dielectric material and monolayer graphene arranged in a Fibonacci sequence. It is found that the GH shift can be enhanced up to 7330 times of the incident wavelength at the specified operating wavelength 2 μm due to the excitation of surface plasmon polaritons of metal. In addition, we discussed the influence of the optical parameters of monolayer graphene, and the thickness of the dielectric material on the GH shift, and confirmed that changing these parameters could achieve the control of GH shift.

In this study, we numerically investigate surface plasmonic triple Fano resonances and optical sensing in the near-infrared band using a hybrid metasurface consisting of concentric C3-hole and circular-ring-aperture unit cells. The results reveal that by changing the symmetry breaking of the C3 unit cells, we can not only induce a tunable multi Fano resonance effect but also enable self-reference optical sensing. In addition, a radiation monitoring sensing capability, which depends on the depth of the Fano dips, can be realized by varying the inner radius of the circular-ring-aperture unit cells. Our results provide a new perspective for the design of compact and tunable Fano resonance photonic devices and enable the incorporation of periodic subwavelength metal nanostructures into relevant biosensing and optical communication applications.

In this study, the characteristics of two-dimensional dispersive dielectric photonic crystals are studied via a numerical approach that combines the finite-element method (FEM) with Newton's iterative method. For a dispersive photonic crystal whose permittivity is dependent on the frequency, the band structure problem is formulated as a nonlinear eigenvalue problem. In this study, first, a discrete variational formulation is derived from Maxwell's equations based on the FEM. Thereafter, by selecting approximate solutions as the initial values for the iteration, this nonlinear problem is solved for different values of the wave vector k based on Newton's iterative method. Consequently, the band structure of dispersive dielectric photonic crystals is obtained numerically. Several dispersive photonic crystals in the transverse electric (TE) and transverse magnetic (TM) modes are investigated. The numerical results reveal that the proposed method is effective for dispersive photonic crystals.

During the formation of a three-dimensional (3D) point cloud of parts using the monocular grating projection system, the reflections of the surfaces and boundaries of parts cause the mutation of part point clouds, making errors in the subsequent reconstruction and measurement process. In this paper, a phase compensation method is proposed. An image-processing algorithm is designed by combining the contour information of the object and the increasing characteristics of the phase-level graph to compensate for the continuous phase graph and reduce the interference caused by the reflections on the surfaces and boundaries of the parts. The experimental results show that phase compensation can reduce the number of point cloud mutations on the surface and boundary of parts. In addition, phase compensation can improve the reconstruction effect of the 3D point cloud in the case of reflections on the surfaces and boundaries of parts.

Pipeline transportation safety is critical for guaranteeing the national economy and people's life, and pipeline leakage monitoring is significant for achieving pipeline transportation safety. This paper proposes a multi-dimensional spatial data fusion algorithm for pipeline leakage monitoring based on a distributed optical fiber vibration sensing (DVS) system. The optical fiber sensing cable was fixed on the side of a pipeline, and the pipeline leakage signals were picked up by the DVS system. These pipeline leakage signals were averaged in the space-time domain based on the time window and spatial resolution, respectively. Finally, a suitable threshold was set to complete the pipeline leakage monitoring alarm. Single-point and multipoint pipeline leakages are tested in the experiment. The signal-to-noise ratio of the single-point pipeline leakage signal increases by 4.5 dB, maximum single-point pipeline leakage alarm rate increases by 19.53%, and maximum multipoint pipeline leakage alarm rate increases by 2.29%. Notably, real-time monitoring and alarms for pipeline leakage under a pressure of 0.2 MPa are achieved.

It is worth noting that determining the shape parameters of a fruit growth period, monitoring fruit development, and solving the problem of fruit characteristic size extraction for quality grading are essential in agricultural fields. However, traditional measurement methods are prone to damage fruit surface morphology, and the noncontact measurement method based on two-dimensional image feature processing has limitations in measuring various morphological parameters (e.g., size and volume). To solve this problem, this paper proposes an adaptive co-opposite-direction slicing noncontact measurement method for the orange shape parameters. First, we use an active structured light three-dimensional camera to obtain the orange point cloud. Consequently, we calculate the height and diameter of the orange point cloud using the search bounding box method. Furthermore, to improve the slice utilization rate, we introduce adaptive conditions to examine the relationship between the change rate of adjacent slices' vertical area and the threshold value to automatically update the slices. After that, the polygon area is updated again to calculate the slice volume. Finally, the complete volume of the orange is calculated using the summation method, and the final calculated value is determined by the mean of the volume obtained through co-opposite-direction calculation. Based on the verification of the orange simulation model and two groups of real orange data sets, the results of regression analysis between the calculated and real values show that the coefficient of determination of each index is greater than 0.95, and the average time is no more than 8.354 s. Compared with other methods, the measurement errors of height, diameter, and volume of the orange simulation model are reduced by 3.5%, 0.9%, 0.7%, and 3.6%.

An optical fiber magnetic field sensor has the technical advantages of strong anti-interference ability, miniaturization, and low cost. An optical fiber three-dimensional magnetic field sensor based on a magneto-optical crystal is proposed, in order to achieve the measurement of the space magnetic field vector. Moreover, a three-dimensional fiber optic vector of a magnetic field sensor probe has been designed, constructed, and experimentally demonstrated. The non-orthogonal error of the three-dimensional fiber optic vector of a magnetic field sensor is analyzed, and through the accurate measurement of the angle between the three sensing units of the optical fiber three-dimensional magnetic field sensor based on magneto-optical crystal, the system's three-axis non-orthogonal error is calibrated and compensated. The experimental test device uses a pair of energized coils to build a one-dimensional magnetic field, and calibrate the three-dimensional optical fiber with the three-dimensional magnetic field sensor system. The three-axis calibration accuracy is 0.19°, 0.26°, and 0.22°, respectively. The experimental results show that the three-dimensional magnetic field sensor based on magneto-optical crystal fiber can measure the magnetic field vector based on the resolution of 0.2 μT magnetic field intensity and 0.5° angle resolution.

The spatial resolution of traditional Brillouin optical time-domain analysis (BOTDA) sensor is often unable to achieve the spatial resolution within 1 m due to the influence of 10 ns acoustic phonon lifetime in optical fibers. The differential pulse pair (DPP) technique can avoid this limitation and obtain higher spatial resolution. However, the traditional DPP technique has problems, such as long measurement time, high difficulty in synchronization, and low signal-to-noise ratio. In this paper, we propose a coded DPP-BOTDA system based on the Brillouin gain-loss effect. the pump pulse light at Stokes frequency and anti-Stokes frequency is synchronously injected into the optical fiber, and the Brillouin gain loss effect of the scattered light is used to differentiate on the optical path to solve the synchronization problem between signals, and the measurement time is half of the traditional DPP method. The influence of the gain characteristics of an erbium-doped fiber amplifier in the sensing system on the decoding results of pulse sequences is analyzed, and the coding gain under the condition of uneven gain is calculated theoretically. The experimental results show that the system can obtain a spatial resolution of 50 cm with a signal-to-noise ratio improvement of 3 dB compared with the traditional single-pulse DPP-BOTDA system.

As an important passive optical component, long-period fiber gratings (LPFGs) have all the advantages of optical fiber sensors, such as anti-electromagnetic interference, corrosion resistance, high sensitivity, small volume, and compatible with fiber system, which have wide applications in the field of optical fiber sensors and optical communications. The paper summarizes the principle of mode coupling, the methods of theoretical analysis, fabrication techniques, and applications of the LPFGs in the field of optical fiber sensors and optical communications. The fabrication techniques include the laser inscribing techniques (UV laser, carbon dioxide laser, and femtosecond laser writing techniques) and non-laser techniques (arc discharge, mechanical micro-bending, cladding etching, fusion tapering, ion implantation, and acoustic wave modulation techniques). For the sensing application of the LPFGs, the characteristics of gratings on the temperature, strain, bending, torsion, and surrounding refractive index have been summarized. And their applications as all fiber filter, mode convertor, polarizer, and mode coupler have also been discussed for the applications in optical communication system. The paper is written in a tutorial style. We aim to provide a general reference for students, academics who are going to join in this field.

Perovskite solar cells have been rapidly developed in recent years. As of 2022, solar cells based on perovskites have achieved a photoconversion efficiency of 25.7%, showing great potential in the field of photovoltaic devices. Despite their high conversion efficiency, the thermal and humidity stability of perovskite solar cells are still considered major barriers to their development. Metal ion doping has proven to be one of the most effective ways to improve the optoelectrical performance and stability of perovskite solar cells. Among the ions introduced, transition metals have been favored by researchers because of their unique properties such as multivalency. In this paper, the latest advancements of doped perovskite photovoltaic devices with transition metal ions are briefly reviewed, and the methods and strategies of doping against the electron transport layer, perovskite active layer, hole transport layer, and metal electrode layer are summarized. The law and mechanism of such methods used to optimize the structure, photoelectric performance, and stability of perovskite photovoltaic devices are discussed.

Q-switching is the main way for thulium-doped fiber lasers to generate nanosecond pulses. First, the application status of active Q-switching, passive Q-switching, and gain switching in thulium-doped fiber oscillators is introduced, and the advantages and disadvantages of the three technologies are compared and analyzed. Second, the typical research results and technical bottlenecks of nanosecond thulium-doped fiber amplifiers with narrow pulse width, high average power, and high pulse energy are introduced, and the optimization measures are analyzed from three aspects: thermal management, nonlinear effect suppression, and amplified spontaneous emission suppression. Finally, the technology development trend of nanosecond thulium-doped fiber oscillator and amplifier is prospected.

Laser cladding has the advantages of compact microstructure, good bonding between coating and substrate, and small dilution rate and deformation. Co-based alloy is widely used in laser cladding because of its high hardness, good wear resistance, high temperature resistance, and corrosion resistance. In this paper, the research status of laser cladding preparation of cobalt-based alloy coatings at home and abroad is analyzed, and the influence of main process parameters such as laser power, scanning speed, and powder feeding rate on the quality and performance of coating cladding is discussed. The related researches on additives such as hard phase ceramic powder, rare earth and solid lubricant additives, and other auxiliary processes to improve the performance of Co-based alloys are summarized. Finally, the insufficiency and development trend of laser cladding cobalt-based alloys are summarized and prospected.

Frequency-locking technology has greatly promoted the development of cavity ring-down spectroscopy in the field of spectral absorption. Therefore, cavity ring-down spectroscopy has gained advantages such as higher precision, better sensitivity, and increased stability. Based on the basic principles of classical cavity ring-down spectroscopy and frequency-locking technology, this paper reviews the current research status of frequency-locking technology in the field of absorption spectroscopy and its applications pertaining to atmospheric trace gases. This paper also lists several frequency-locking technologies that are widely used in the field of cavity ring-down spectroscopy. Finally, the application prospect of frequency-locking technology is determined combining the current development trend of frequency-locking technology and cavity ring-down spectroscopy technology.

When solar-blind wireless ultraviolet light system is used to communicate, if multiple links are active at once, the active links will overlap with effective scatters in space, resulting in mutual interference and a reduction in communication performance. This study develops a method for simulating the wireless ultraviolet non-line-of-sight communication scattering channel and validates the model's accuracy by comparing it to a common interference model. And this model is used to simulate the co-planar interference, non-co-planar interference, and the link interference model under non-co-planar and height difference interference in wireless ultraviolet communication. The results show that the main factors affecting the bit error rate of the channel are the location of the interference terminal and the size of the effective scatterer, in the case of non-co-planar and height difference interference, the inter-link interference can be reduced by adjusting the transmitter elevation angle, and the performance of the communication system can be improved.

The convolutional neural network (CNN) has been widely used in various chemometric tasks in the past few years. However, learning long-range correlations from spectra using the CNN remains challenging, because most CNN architectures utilized in previous studies are quite shallow to avoid overfitting. In this paper, we present an atrous convolutional network (ACPnet) for learning long-range spectral correlation in quantitative spectrometric analysis. Paralleled convolution branches with different atrous rates are assembled to determine the best trade-off between short-range and long-range information. Three data sets, viz. tablets (Raman), soil (NIR), and wines (NMR), are evaluated to demonstrate the versatility of the proposed network. The overall results indicate that the ACPnet achieves better regression accuracies for all three data sets than those of partial least squares regression (PLS), least square support vector machine (LS-SVM), a regular CNN, and an atrous CNN in a cascaded pattern (ACCnet). Furthermore, the features extracted by the ACPnet are fed into different regressors to evaluate the proposed network as a supervised feature extractor. The results of the extraction–regression model show that ACPnet gives better feature-extraction performance than that of a conventional CNN on the three data sets.

Laser-induced breakdown spectroscopy (LIBS) can potentially be employed for remote-sensing in situ detection, which is a crucial technique for deep-space exploration for identifying the composition and content of material elements. The exploration of element composition and mineral distribution characteristics on the surface of Mars is the premise of studying the geological evolution and genesis of Mars. Before launching the Tianwen-1 mission, Mars-simulated exploration experiments were conducted on 15 categories of mineral samples using the Mars surface composition detector (MarSCoDe), and 1920 spectral datasets were collected. This study adopted an efficient classification model to verify the detection performance of the instrument using the artificial fish swarm algorithm (AFSA)-optimized support vector machine (SVM) (AFSA-SVM), which classifies 32 minerals, including igneous and sedimentary rocks and metal minerals. First, the principal component analysis (PCA) was adopted to reduce the dimension of the original spectral data, and the data was trained in AFSA-SVM. Second, AFSA optimized the parameters of SVM and achieved 99.56% mineral recognition accuracy. Finally, AFSA-SVM was compared with other algorithms, including the random forest (RF) algorithm, backpropagation artificial neural network (BPANN), and K proximity (KNN) algorithm. Their accuracy values are 95.60%, 95.80%, and 90.17%, respectively. The results show that the AFSA-SVM algorithm has advantages in assisting LIBS in identifying mineral targets.

Although the polyolefin film porosity is known to affect the material's mechanical properties, insulation, and penetration, few accurate methods are available for its nondestructive detection. In this study, the effective permittivity is used as a bridge for establishing four equivalent models between the equivalent refractive index and porosity of materials. By practically testing the terahertz time-domain spectrum of the battery separator and microporous filter membrane, the refractive index, permittivity, porosity, and other material properties are quantitatively obtained. The results show that for a biphasic medium comprising the polyolefin material and air, the porosity and permittivity are highly correlated. The average relative error is approximately 2.53% between the porosities measured using the improved effective medium model and conventional gas displacement method; therefore, terahertz-spectroscopy-based porosity measurements are feasible for polyolefin films and should become a supplementary film-detection method.

Isocarbophos is a highly effective broad-spectrum pesticide that is often used in vegetable pest control. However, it is highly toxic and produces residues that pose a severe risk to human health. Therefore, this paper proposes an approach based on metamaterial terahertz technology for detecting isocarbophos at low mass concentrations in vegetables. According to the split-ring resonator principle, a high-performance metamaterial is designed with a quality factor (Q) of 65. By testing the terahertz spectra of different mass concentrations of isocarbophos in cabbage leaves containing metamaterials, an exponential fitting model between the frequency shift of the resonance peak and the mass concentration is established. The determination coefficient R2 is 0.99765, and the lowest mass concentration detected in the experiment is 0.00087 mg/L. Compared with traditional methods, metamaterial-based terahertz technology has a lower detection limit and simplifies the pretreatment process. The proposed method determines low mass concentrations of isocarbophos in vegetables rapidly and effectively.

Imidazole and its isomer pyrazole in the terahertz band have unique spectral features that can be used in their identification. To further investigate the formation mechanism of terahertz spectral features, the molecular structures of imidazole and pyrazole are modeled and optimized in this study based on the density functional theory. The vibration characteristics corresponding to different characteristic absorption peaks are determined considering the potential energy distribution. Moreover,the weak intermolecular interactions are qualitatively and quantitatively analyzed via graphical analysis of interaction region indicator, energy decomposition analysis based on force field and topology analysis of electron density. The results show that imidazole and pyrazole have different molecular vibration characteristics owing to differences in the dispersion interactions and quantity and strength of the hydrogen bonds, resulting in significant differences in the frequencies of their characteristic absorption peaks in the range of 0.4-2.4 THz. Additionally, the formation mechanism of the characteristic absorption peaks is related to the differences in the vibration modes and weak intermolecular interactions. Therefore, this study provides an important reference for identifying nitrogen-containing heterocyclic pesticide raw materials and acquiring microstructural information.

Compared with other denoising algorithms, wavelet denoising is preferable. The wavelet function and decomposition level greatly influence the quality of denoising; however, determining the wavelet function and decomposition level is challenging in actual X-fluorescence spectral denoising. This paper proposes a wavelet algorithm based on Russian roulette optimization for X-ray spectral denoising to address this problem. The summation of the coefficients of determination of the quantitative models (Cr, Mn, Co, Ni, Cu, Zn, As, and Pb for soil samples) R2 is considered as the optimization objective. The Russian roulette optimization strategy updates the wavelet function and decomposition level. Subsequently, after the number of iterations is selected, the optimal wavelet function and decomposition level of each soil sample spectrum are selected. The approach is validated on the X-ray fluorescence spectra of 55 certified reference soil samples. The R2 value of all eight heavy metals is higher after optimization, and the sum of the R2 values of the quantitative models of the eight elements increases from 7.8383 to 7.8704. This technique can be used as an alternative for wavelet denoising applied in rapid elemental measurements.

Considering the significant influence of thermal stress on the coating quality during the in situ fabrication of new iron particle-reinforced alumina coating by laser melting, we investigated the thermal stress of a single-pass composite alumina coating on the surface of titanium alloy in this study. The representative volume element method was used to simulate and calculate the thermodynamic parameters of the new coating. The heat source model of the laser-induced thermal reaction heat was established using a combination of the raw-dead cell method and internal raw-dead heat source. The thermal stress distribution pattern of the coating components at the end of the cladding under different combinations of process parameters was calculated and analyzed. The results indicate that the thermal stresses are primarily concentrated in the coating and its bonding surface with the substrate and the tensile stresses on the coating along the melting direction are the main causes of transverse cracks in the coating. Owing to the laser-induced thermal reaction, the coating cracks increase with laser power and laser scanning speed. At 600 W laser power, the coating has the lowest number of cracks when the scanning speed is 2 mm/s. Moreover, at a scanning speed of 5 mm/s, the coating has the lowest residual stresses when the laser power is 300 W.

Phosphor-converted white LEDs have the advantages of high energy efficiency, low cost, and long life. They are widely used in lighting area. Improving the luminous efficiency of phosphor-converted white LEDs has always been a research hotspot. In order to design and optimize the high-performance packaging of white LEDs, combination of simulations and experiments are used to research and analyze LED chip packaging. We use a special bracket and dual-chip package to increase luminous efficiency. On this basis, the phosphor coating process is improved, the phosphor excitation efficiency is improved, and the overall LED luminous efficiency is increased by about 6%. This study also investigates the change in LED luminous efficiency when the distance between the remote phosphor and chip changes.

The detection and classification of color deficiency is frequently required in occupational physical examination, computer-aided recognition systems for color deficiency, and basic research on color vision mechanisms. Twenty-two color deficient subjects (eight deuteranopes, two protanopes, six deuteranomalous trichromats, and six protanomalous trichromats) were tested using D-15, FM 100-Hue, and Neitz anomaloscope tests. By comparing the test results of D-15 and FM 100-Hue with those of the anomaloscope, the limitations of hue tests in detecting and classifying color deficiency were revealed. This study provides a reference for detecting and classifying color deficiency in various applications.