Laser range-gated scan-type imaging technology has the advantages of lower cost and multiple scattering reduction. Synchronous operation between imaging scan and laser-beam scan is the technical difficulty in the scanning imaging technology. Herein, a scan-type range-gated LiDAR system based on spatial light modulator (SLM) is designed to temporally and spatially shield backscattered light while ensuring the reliable receipt of reflected light from the targes. Measurements are performed in light scattering environments with different attenuation lengths to realize the 2D and 3D imaging implementations. Experimental results show that the synchronization between light-beam scan and range-gated imaging scan can be effectively achieved in this system. Moreover, the proposed system is applicable to visible-range extension of optical imaging in a light scattering environment.

To grasp the influences of sea and land winds on the atmospheric turbulence characteristics in an offshore complex terrain, an atmospheric turbulence field experiment was implemented at Yangmeikeng Environmental Ecology Center in Shenzhen city. The data involved in atmospheric refractive index structure constant, atmospheric sound virtual temperature, wind profile are obtained by the micro-thermal meter, ultrasonic anemometer and Doppler lidar. By analyzing the effects from sea and land winds on the power spectrum, isotropy, and energy dissipation rate of turbulence, we find that the turbulence in sea winds develops more fully than in land winds. In the sea winds, the frequency range of isotropic coefficients close or equal to 1 spans 0.05-50 Hz, and the speed spectral power law is near -5/3. In land winds, the atmospheric turbulence only shows an isotropy at very small proportion of time at 0.8-10 Hz. The speed spectral power law seriously deviates from -5/3, tends to a high value, and possesses an average value of -1.3. Under the conditions of sea and land winds, the relationship between turbulence energy dissipation rate and turbulence intensity is linear. The fitting slopes of data from the micro-thermal meter are 1.1 and 0.76, and those from the ultrasonic anemometer are 0.73 and 0.28, respectively. The conclusion here can provide some reference for the study on laser transmission in a marine environment.

In this study, a two-channel thermal dissociation cavity ring-down spectroscopy (TD-CRDS) system based on cavity ring-down spectroscopy technology was established for the rapid and simultaneous measurement of ambient nitrogen dioxide (NO2) and organic nitrates (ON). NO2 was directly measured by its absorption at 406 nm, while ON was indirectly measured by the NO2 product of its thermal decomposition at 450 ℃. The ON conversion efficiency reached 99% at this temperature. The effective absorption cross-section of NO2 was obtained by the convolution of the high-resolution absorption cross-section of NO2 using a laser with a central wavelength of 406.02 nm. The laser spectrum was 5.74×10 -19 cm 2·molecule -1. A stability test of the heating device revealed an optimal flow rate of 1 L/min. The simultaneous measurement of the ambient NO2 by the NO2 and ON cavities exhibited good consistency, with the correlation coefficient R2=0.99. After optimization, the detection limit of the system reached up to 2.42×10 9 molecule·cm -3 (standard deviation: 3σ; temporal resolution: 1 s). Comparing the TD-CRDS system with the long-path differential optical absorption spectroscopy (LP-DOAS) in terms of NO2 measurement, good consistency was obtained (R2=0.93), indicating that the system has high measurement accuracy. The TD-CRDS system was applied to field observation, and the time series of NO2 and ON concentration were obtained.

Herein, an ultraviolet dual-wavelength lidar was developed for the detection of atmospheric aerosols in the troposphere. The Nd∶YAG laser emitting beams of 355 and 266 nm at a frequency of 10 Hz were used as a light source by the dual-wavelength lidar. This dual-wavelength lidar achieved fine separation and extraction of the Mie scattering signal at ultraviolet wavelengths. Furthermore, it was not affected by the solar background light when 266-nm signal was used to measure the optical characteristics of the aerosol. The signal-to-noise ratios (SNRs) of the actual detection were compared with the simulation results at two wavelengths. Findings indicate that the SNR of 355-nm signal was lower during daytime detection; however, the SNR of 266-nm signal was lower during night-time detection. These results were consistent with theoretical calculations. The detection height of 266-nm signal can reach 2 km during daytime and that of 355-nm signal can reach 5 km during night-time upon analyzing the data of SNR of 10. Atmospheric observations were conducted using the proposed dual-wavelength lidar. The ozone concentration, aerosol characteristics, and extinction coefficient were studied and analyzed on hazy and sunny days. Moreover, the influence of ozone concentration on the inversion extinction coefficient and Angstrom index was analyzed. Results show that a greater ozone concentration corresponds to a larger inversion error of the extinction coefficient. The extinction coefficient of the aerosol in hazy weather was larger than that in sunny weather.

The static Allan analysis of variance cannot effectively analyze and identify the random error of a laser gyroscope under static conditions. Further, it cannot provide an accurate basis for the random error compensation of a laser gyroscope under dynamic conditions. Therefore, in this study, we propose a time-frame dynamic Allan analysis of variance method to conduct dynamic Allan analysis of variance and identify the random error terms via piecewise modeling. The grey GM (1, 1) prediction model is established to identify the random error associated with the parameters that have to be predicted. However, the problem of large fluctuation can be observed with respect to the data from the traditional GM (1,1) prediction model is not complete; therefore, we introduce wavelet filters to smoothen the original data and the residual error correction model to improve the GM (1,1) prediction model. The test results denote that the prediction accuracy of the improved GM (1, 1) prediction model is higher than that of the traditional GM (1,1) prediction model for the random error coefficients of laser gyro under the same working condition.

Analytical mode matching is an efficient method to calculate the diffraction of subwavelength grating. For stacked subwavelength gratings, the tangential component of an electromagnetic field is continuous at the boundary of adjacent different dielectric layers, and the contribution of each waveguide mode of the electromagnetic field in each layer to the electromagnetic field in the layer is equal to that of the tangential component of the electromagnetic field in the adjacent layer. The modes between adjacent layers are matched. Thus, the analytical mode matching can be applied to stacked subwavelength dielectric gratings. The diffraction efficiencies and the electromagnetic field distributions calculated using the analytical mode matching and the rigorous coupled wave analysis method are compared in this study. Comparison results show that the proposed analytical mode matching is an effective method for calculating the diffraction of stacked subwavelength dielectric gratings owing to its unique advantage in convergence and calculation speed and it offers a profound understanding of the diffraction effects of stacked subwavelength gratings on electromagnetic waves. The proposed method combined with optimization methods can be used to design new stacked subwavelength gratings and metasurfaces.

In this study, the diffraction efficiency analysis of a rectangular groove grating and a triangular groove grating was performed using the vector diffraction theory. Accordingly, the diffraction efficiency of the triangular groove grating was found to be about one time higher than that of the rectangular groove grating. Based on a new five-axis ultra-precision single-point diamond lathe cutting technique, the diamond tool-head movement errors, the movement spacing standard deviation range and the machining deviation of grating line position are determined and the effect of tool-head wear on grating diffraction efficiency was also analyzed. On this basis, a convex blazed grating with curvature radius of 70 mm, groove density of 60 line/mm, and diameter of 52 mm was developed. And the peak relative diffraction efficiency is larger than 80% and the average relative diffraction efficiency is larger than 60% in the 1000-2500 nm spectral region. The actual hyperspectral data obtained during the flight shows that the shortwave infrared imaging spectrometer developed using the proposed convex grating meets the requirements of geological remote-sensing technology.

Herein, we designed a novel D-type symmetric two-core photonic crystal fiber surface plasmon resonance refractive index sensor. A significant dual resonance peak phenomenon occurred in the visible and near-infrared bands when the surface plasmon resonance effect generated at the interface between two-core and metal-sensing layer in this structure is combined with different metal-sensing layers. The mutual independence of the dual resonance peaks was analyzed using the finite element method. Moreover, the effects of the spacing and diameter of air holes, the thickness of metal-sensing layers, and the radius and spacing of nanometer columns in the structure on the dual peaks were examined. Results demonstrate that after the structural parameters are optimized, double peak resonance facilitates the good sensing performance of the sensor. In the refractive index range of 1.32-1.43, the respective average sensitivities corresponding to the double resonance peaks are up to 6209.09 nm/RIU and 8390.91 nm/RIU, and the figures of merit are greater than 19.64 RIU -1and 27.06 RIU -1. These results provide a theoretical reference for the design of a photonic crystal fiber surface plasmon biosensor.

It is the truth that many large-scale engineering structures have large deformation while they are still functional. Based on the bending loss characteristics of an optical fiber, we present a large range and linear sensor with simple construction for displacement measurement. The linear relationship between the measured displacement and the bending loss of an optical fiber is proved theoretically, and its expression is derived. A series of calibration experiments and performance tests are conducted. The experimental results show that the sensor is characterized by a wide measurement range of 0--200 mm, a sensitivity of 0.1668 dB·mm -1, a minimum displacement resolution of 0.06 mm, a repeatability error of 0.26%, and a hysteresis error of 2.22%. The designed sensor can be used for the crack detection and the continuous monitoring for large-scale engineering structures and has a promising application prospect.

In this study, we intend to determine the coupling properties of the fundamental mode HE11 with respect to the high-order core vector modes (TE01, TM01, and HE21) in case of the mechanically induced microbend long-period fiber gratings (MLPGs). Therefore, the effects of various parameters, including the grating period, microbend amplitude, and coupling coefficient, on the coupling of the vector modes in the MLPGs were analyzed with respect to the step-index and inverse-parabolic-index fibers. The results denote that the coupling coefficient plays a critical role during the mode coupling process and that the strength of vector mode coupling can be effectively tuned by applying pressure to vary the microbend amplitude of the fibers. The MLPGs of the inverse-parabolic-index fiber can convert the high-order vector modes (TE01, TM01, and HE21) at specific wavelengths, and the resonant wavelength obtained using the high-order mode can be tuned. The MLPGs possess potential application value with respect to vector mode multiplexing, orbital angular momentum generation, and multiplexing.

Herein, two closed prestressed high-strength concrete (PHC) pipe piles were tested under static pressure at the construction field to explore the applicability of fiber Bragg grating (FBG) sensing technology and elucidate the necessity of temperature compensation. FBG temperature sensor and low-temperature-sensitive FBG strain sensor were symmetrically installed on both sides of the PHC pipe pile using the grooving method. The temperature change and strain state of the pile under static pressure were measured using FBG demodulator. The test result indicates that during the process of pile sinking, the temperature variation of the soil around the pile is 2.62 ℃. Further, the accuracy of the test results of the low-temperature-sensitive FBG strain sensor will be affected if no temperature compensation measures are undertaken. In this test, all low-temperature-sensitive FBG strain sensors survived, facilitating dynamic and real-time monitoring of the change law of pile-tip resistance, pile-axial force, side-friction resistance, and unit-side friction resistance under static pressure. Moreover, the sensor possessed the function of temperature compensation, which can comply with field test requirements, providing a new method for the application of FBG sensing technology in geotechnical engineering field tests.

In this study, we propose a turbulent flow-velocity measurement system based on the optical fiber Fabry-Perot (F-P) sensing technology, which can be used to monitor the high-speed airflow velocity at hydrojunctions. The proposed system comprises an airflow sensor holder and a polarized low-coherence interferometry demodulation section. The flow-velocity is calculated using two F-P sensors, which can be used to measure the pressure and temperature variation of the airflow field. Further, the interference signal obtained using a low-cost linear CCD can be accurately demodulated by tracing the peak position of a single fringe pattern and using the seven-step phase shifting method. The two F-P sensors are calibrated in the temperature range of 20-40 ℃ and the pressure range of 98-110 kPa. The pressure absolute error of the sensor used to measure the stagnation pressure is less than 0.018 kPa, while that with respect to the temperature of the sensor used to measure the airflow field temperature is less than 0.08 ℃. The proposed system is used to measure the airflow velocities ranging from 9.3 to 93 m/s, and the absolute error of the flow-velocity is less than 0.49 m/s. The experimental results indicate that the proposed system can be employed to measure the flow-velocity of high-speed turbulence with a low cost and possesses a wide application prospect.

The fine-grained characteristics of blood vessels are difficult to obtain, and the details of the blood vessels are obscured when the current mainstream methods of retinal vascular segmentation are employed. This paper proposes an improved U-Net model algorithm to address these problems. The convolution layer of quadratic-cycle residual difference was used to replace the original convolutional layer in the upper and lower sampling of U-Net to improve the utilization rate of the features. A multichannel attention model was introduced in the decoding part to improve the segmentation effect of small blood vessels with low contrast. Results show that the accuracies of the algorithm in DRIVE (Digital Retinal Images for Vessel Extraction) and STARE (Structured Analysis of the Retina) databases are 96.89% and 97.96%, the sensitivities are 80.28% and 82.27%, and the AUC performances are 98.41% and 98.65%, respectively. All these parameters are higher than those of existing advanced algorithms. The proposed algorithm can effectively improve the segmentation accuracy of fine blood vessels in fundus images.

Because of the strong scattering effect of scattering media, the traditional optical imaging systems cannot be used to image and track objects behind these media. To address this problem, this paper proposes a method for tracking and reconstructing the moving objects behind a scattering medium. Based on the theory of optical memory effect of scattering media, the point spread function of a scattering imaging system is solved using prior information. The axial scaling relationship of this point spread function is used to determine the axial displacement and the cross-correlation of the reconstructed moving objects is used to determine the lateral offset. An experimental system based on the proposed method also achieve the fast reconstruction and motion tracking of an object behind the scattering medium. The proposed method can track object displacement by collecting only continuous speckle images of the displaced object. Thus, this approach does not require specific devices but provides excellent results, which is helpful in the application of imaging moving objects through scattering media in the field of biomedicine.

Time delay signature (TDS) of an external cavity and bandwidth are two important parameters that affect the applications of chaotic lasers. In this work, the output laser from a semiconductor laser with phase-modulated optical feedback was injected into another semiconductor laser with optoelectronic feedback. The two lasers thus formed an optically injected semiconductor laser subject to optoelectronic feedback, i.e., a master-slave laser system, which can be used to reduce the TDS of a chaotic laser and increase its bandwidth. The effects of external light injection coefficient, feedback intensity, and pumping factor on the TDS of the chaotic laser output from the system were numerically studied, and the influence of the TDS of the chaotic laser output from the master laser on the TDS of the chaotic laser output from the slave laser was investigated. Moreover, the bandwidth of the chaotic laser output was investigated based on the effective suppression of TDS. Results show that this scheme can effectively reduce the TDS of the chaotic laser, and increasing the external light injection coefficient, feedback intensity, and pumping factor can widen the bandwidth of the chaotic laser.

Aiming at addressing the problem in detecting the internal defects of large thickness materials, the locations and sizes of internal defects are detected through the diffraction effect of defects on laser ultrasonic. First, the Comsol software is used to study the interaction process between laser ultrasonic and internal defects and we establish the calculation model for received laser ultrasonic signals. Using the laser ultrasonic testing system, an experiment on detecting the locations and sizes of defects in a 45-steel block are conducted. The experimental results show that the relative position and size errors in defect detection are within 5% and 10%, respectively. This experiment proves the feasibility of detecting the locations and sizes of internal defects using the laser ultrasonic diffraction bulk wave method. Furthermore, it achieves a non-contact detection via laser excitation and reception.

To solve the problems of high missed-detection rate and low recall rate existed in the YOLOv3 algorithm for detecting traffic lights, a traffic light detection method based on the optimized YOLOv3 algorithm is proposed. First, the K-means algorithm is used to cluster the data. By combining the clustering results with the statistical results of traffic light labels, the number and the width-height ratios of the prior boxes are determined. Then, the network structure is simplified according to the size characteristics of traffic lights. The 8× downsampling information and the 16× downsampling information are fused with high-level semantic information, and the object feature detection layer is established on two scales. Meanwhile, to avoid the disappearance problem of traffic light features with the deepening of the network, two sets of convolution layers are reduced before two object-detection layers, and thus the feature extraction steps are simplified. Finally, in the loss function, Gaussian distribution characteristics are used to evaluate the accuracy of the boundary box to improve the precision of traffic light detection. The experimental results reveal that the detection speed of the optimized YOLOv3 algorithm can reach 30 frames/s and the average precision is 9 percent higher than that of the original network, which effectively completes the detection of traffic lights.

Herein, Chlorella pyrenoidosa was taken as the subject, and photosynthetic fluorescence parameters of algae were used as toxicity evaluation indexes to study the response law of multiple photosynthetic fluorescence parameters under Cu 2+ toxicity. The results are as follows: the inhibition effect of F0, Fm, Fv, Fv/Fm, Yield, rP, JVPⅡ, α and Ek on Cu 2+ toxicity is significant, and yield, rP, α and Fv/Fm could be used as the evaluation indexes for Cu 2+ toxicity analysis at 24 h; the inhibitory effect of Yield, rP, α and Fv/Fm has a good dose-effect relationship with Cu 2+ toxicity. The fitting correlation coefficient R2 of the logistic function is 0.9989, 0.9992, 0.9991 and 0.9977, and the resulting EC50-24h is 61.05, 66.31, 69.41 and 99.61 μmol·L -1, respectively. The parameters Yield, rP and α have higher fitting correlation coefficients than the conventional parameter Fv/Fm, and their EC50-24h values decrease by 38.7%, 33.45% and 30.3%, respectively. The experimental results could provide reference for the detection of biological toxicity based on photosynthetic inhibition effect of algae.

In this work, based on the theory of electromagnetically induced transparency, optical bistability behavior in a diamond tin-vacancy color center system was theoretically investigated. It was found that a change in system parameters, such as the detuning of probe and coupled fields, intensities of the coupled fields, and cooperation parameters, can significantly change the quantum coherence characteristics of the system. This can be used to control the threshold of optical bistability of a solid system. In addition, the mutual transformation of optical bistability and optical multistability can be realized by properly adjusting the intensity of the coupled fields.

In the fabrication of cylindrical single-crystal silicon mirrors, there are many disadvantages, including low removal efficiency, poor figuring accuracy, and difficult surface-quality controllability. Herein, ion-beam figuring (IBF) with a titled incident beam was proposed to optimize the process technology of fabricating cylindrical single-crystal silicon mirrors. First, the theoretical model of the tilted incident IBF removal process was established to determine the relationship between removal efficiency and incident angle. Second, when the ion beam was incident obliquely, the mechanism of the microcosmic morphology was also analyzed to determine the relationship between surface quality and incident angle. Finally, the removal efficiency and surface quality were comprehensively optimized to obtain the optimal incident angle for the tilted incident IBF process, and an innovative process technology based on tilted incident IBF was proposed. A convex cylindrical single-crystal silicon mirror with dimensions of 100 mm×100 mm was tested using the tilted incident IBF process. The total machining time was reduced by 53.7% using the titled incident incident IBF process compared with that using the vertical incident IBF process. Experimental results indicate that the tilted incident IBF process can not only improve the machining efficiency but also obtain high accuracy and quality surface.

In this study, a GaN microdisk is fabricated on a sapphire substrate by hydride vapor phase epitaxy (HVPE). The microdisk has a flat surface and a regular hexagonal shape with a diameter and height of approximately 27 μm and 15 μm, respectively. Photoluminescence (PL) experimental results show that the optical resonance modes of the microdisk are different in the vertical and horizontal orientations, with the latter supporting the whispering gallery mode (WGM) oscillation. When the microdisk is irradiated by a high-energy pulsed laser at room temperature with an excitation light power larger than 7.8 μW, a multimode laser signal near 374 nm wavelength is obtained in the PL spectra, in which the WGM laser is dominant, and the quality factor can reach 3742. Finally, using the COMSOL simulation software, a light field simulation is performed on the resonant cavity to analyze its optical mode characteristics.

Herein, the substrate comprising carbon nanotubes and silver nanoparticles (CNTs/Ag1) was first prepared using the one-step method, and then CNTs/Ag1 was decorated by the additional Ag nanoparticle sol (Ag2) to constitute the final composite structural substrate (CNTs/Ag1/Ag2). The corresponding electric field distributions and the enhancement factors were calculated using the three-dimensional finite difference time domain (3D-FDTD) algorithm. Experimental results of Raman test show that the detection limit is 10 -15 mol/L for rhodamine 6G (R6G). Quantitative detection of R6G with a low-concentration of 10 -15-10 -10 mol/L is realized using the normalized ratio k. The relative standard deviation (RSD) value of k is as low as half of the RSD of Raman intensity. Moreover, quantitative detection of crystal violet (CV) solution with a concentration of 10 -9-10 -6 mol/L is achieved. The linearity-fitting degrees of all k values of two probe molecules are larger than 99% at different concentrations.

Based on the Richards-Wolf vector diffraction integral theory and the conclusion that an angularly polarized vortex beam has a radial component after strong focusing with a high numerical aperture, the 4pi focusing characteristics of an angularly polarized Laguerre-Gaussian beam modulated by a diffraction optical element (DOE) are studied. The simulation results show that the flat-top optical field can be obtained in the focal plane by selecting the appropriate intercept ratio, numerical aperture, and outer ring structure of DOE. On this basis, by changing any one of these three parameters, the structure of the flat-topped optical field will be transformed into an optical needle one. By changing any two of these three parameters, a good dark channel structure can be obtained, and all the dark channel structures obtained are basically same. The conversion among these special focal field structures has a potential application in the field of optical micro-manipulation.

Fiberreinforced plastics have excellent properties such as strong corrosion resistance, good dielectric properties, and good thermal properties. They are widely used in aerospace, railway and other related industries. In this paper, the pulse-width modulated terahertz pulse excitation source is used as the input light source, and the measured optical parameters of materials in the terahertz band are fitted and optimized using the Debye model, which are used as the material input parameters in the finite-difference time-domain method. A terahertz pulse detection model for the bonding defects of fiber reinforced plastics is established. The finite-difference time-domain method is used to numerically analyze the propagation characteristics of terahertz waves in the bonding defects of fiber reinforced plastics. The propagation process of terahertz waves in the bonding defect structure and the causes of higher-order echoes are analyzed from the two aspects of time domain waveform and B-Scan imaging. We diagnose and analyze the debonding and delamination defects in fiber reinforced plastics. The thickness of delamination defects in 1 mm thick fiber reinforced plastics can reach 20 μm. The number of layers for layered defect detection with a thickness of 80 μm reaches two layers for fiber reinforced plastics. The proposed method can provide a theoretical support for the non-destructive detection and evaluation of bonding defects in fiber reinforced plastics based on terahertz pulse imaging.

Tunable diode laser absorption spectroscopy (TDLAS) is often used for gas concentration detection. However, the change of ambient temperature affects the absorption line strength, absorption line type, and gas molecular number density and thus results in measurement errors. We designed a high precision temperature control box to control the ambient temperature where the gas cell is located. First, the effects of parameters such as the shape of the temperature control box, thermoelectric cooler position, and airflow vector on the temperature distribution inside the box were simulated using the CFD simulation software. Second, with the help of the simulation results, we optimized the design and processing of the temperature control box. Finally, we completed the fabrication of the temperature control box which can provide a uniform and stable temperature environment for the gas cell. The temperature adjustment range of this box is 32-50 ℃, the control accuracy can reach 0.01 ℃, and a long-term stability is realized. Temperature stability was verified by the CO2 gas concentration detection experiment. Therefore, using CFD simulation to optimize the parameters of the temperature control device, one can obtain a stable and uniform temperature control system, reduce the influence of ambient temperature on the measurement results, and effectively improve the accuracy and stability of gas concentration measurements.