Based on the single particle aerosol hygroscopic growth model, the hygroscopic growth models of the hydrophilic and hydrophobic two-particle agglomerated aerosols are established, respectively. The scattering characteristics of two types of condensation particles at different relative humidity are calculated by using the discrete dipole approximation method. The results show that in the humidity range of 40%--90%, the scattering coefficient of the hydrophilic two-particle condensed aerosol (e.g., NaCl-NaNO3) has two jumps, and the position and increase of the scattering coefficient jump are closely related to the volume ratio of NaCl-NaNO3 particles. After the second deliquescence, the scattering coefficients of hydrophilic agglomerated particles with different volume ratios show an exponential growth trend with the increase of relative humidity. The greater the volume fraction of NaNO3, the larger the increase in scattering coefficient. For hydrophilic and hydrophobic two-particle agglomerated aerosols (e.g., NaCl-Soot) with different volume ratios, the scattering coefficients show an exponential growth trend with the increase of relative humidity. The larger the NaCl volume ratio, the larger the growth rate of the scattering coefficient. But the relative position relationship between hydrophilic and hydrophobic particles after deliquescence has little effect on the scattering coefficient of agglomerated particles. The above results can provide a reliable theoretical basis for further research on the hygroscopic scattering characteristics of multi-particle aggregated aerosols.

A periodic metasurface structure composed of quartz and silicon nano beams was proposed. The periodic gradient change in the phase distribution can be used for obtaining a specific angle deflection of the vertically incident light, and the deflection angle can be adjusted by changing the width and period of the silicon nano beam. Simulation results show that the deflection efficiency exceeds 60%, and the far-field divergence angle reaches about 3° in the deflection angle range of ±45°. The proposed structure is simple and can be combined with a tunable laser and a wavelength division multiplexer to compose a beam steering device and thus the beam angle can be tuned by changing the wavelength, which can be applied to solid-state beam scanning with large angle range.

The optical fiber sensing technology has the advantages of long-distance transmission and distributed detection, therefore having great application potential in the field of cable line condition monitoring. In this paper, a phase-sensitive optical time-domain reflectometry (Φ-OTDR) is used to interrogate partial discharges in the fault diagnosis of power cable joints. By virtue of stochastic features of the Rayleigh scattering coherent signal, the signal variance is chosen as a detection quantity of the weak ultrasonic signal. In the partial discharge experiment, five fiber loop sensors were wrapped around some ultrasonic monitoring points on the cable joint, to prove the distributed locating capability of the Φ-OTDR system. The signal characteristics of the electrical measurement method and fiber-optic acoustic measurement method for local discharge were compared, and the positions of the five fiber loop sensors were calibrated by a piezoelectric sensor, which verified the features of the Φ-OTDR distributed ultrasonic fiber sensors in partial discharge measurements.

An implicit information modulation algorithm based on the highest layer of the Laplacian pyramid of the blue channel is proposed to solve the problems of poor implicit effect and complex algorithm occurring in the existing visible light implicit imaging communication modulation algorithm. The proposed algorithm takes the advantage of the insensitivity of the human visual system to the yellow and blue components, performs the Laplace pyramid decomposition only on the blue channel image, and embeds the implicit information at the highest level. In addition, considering the effect of the texture difference among images on the implicit effect, the image entropy is used as an indicator to measure the texture complexity of images. An image classifier is designed based on the texture complexity and the implicit effect is also analyzed. The simulation results show that the proposed modulation algorithm can achieve an average peak signal-to-noise ratio value of 40.46 dB. Compared with that of the general Laplace pyramid modulation algorithm, the performance is improved by 5 dB, and the implicit effect of the system is effectively improved.

In this paper, we carried out theoretical and experimental research on the structure of a side-polished fiber Mach-Zehnder interferometer (MZI) based on a microtaper. To be specific, after the side-polished depth of the fiber was appropriately controlled, the refractive index (RI) sensing performance of the MZI was much better than that of a traditional fiber MZI without side polishing. Besides, the research results showed that when the side-polished depth reached 41.7 μm, the sensitivity of RI sensing was -117.145 nm/RIU at the RI of about 1.34. Furthermore, we measured the temperature and humidity simultaneously by depositing the hydrophilic graphene oxide (GO) on the surface of the side-polished fiber MZI, obtaining the sensitivity of temperature and relative humidity sensing of 131.77 pm/℃ and -76.1 pm/%RH, respectively. In conclusion, the proposed side-polished fiber MZI has the advantages of high sensitivity, low cost, and simple fabrication, thus displaying a wide range of application prospects in the field of biochemical sensing.

In order to solve the problem of low classification accuracy of hyperspectral images in the case of limited training samples, we proposed a tandem three-dimensional(3D)-two-dimensional(2D) convolutional neural network model combining dilated convolution with attention mechanism. First, with the tandem 3D-2D convolutional neural network as the basic structure, the model used 3D convolution to simultaneously extract the spatial-spectral features of hyperspectral images and 2D convolution to further extract high-level spatial semantic information. Then, by introducing dilated convolution to enlarge the receptive field of the convolution kernel, we constructed a multi-scale feature extraction structure for the fusion of multi-scale features. Finally, the attention mechanism was applied to make the network pay attention to important spatial-spectral features and suppress noise and redundant information. Furthermore, we performed a comparative experiment between the proposed algorithm and four deep-learning-based algorithms on two common data sets. The results show that the proposed model achieves the most accurate classification results and effectively improves the classification accuracy under the condition of limited training samples.

In recent years, the application of deep learning in biomedical image processing has received widespread attention. Based on the basic theories of deep learning and medical applications, this paper proposes an improved three-dimensional dual-path brain tumor image segmentation network to improve the detection accuracy of brain tumors in nuclear magnetic resonance imaging sequences. The proposed algorithm is based on 3D-UNet. First, the improved dual-path network unit is used to form the encoder-decoder structure similar to UNet. While retaining the original features, the network unit can also generate new features in texture, shape, and edge of the brain tumor to improve the accuracy of network segmentation. Second, the multi-fiber structure is added to the dual-path network module, which reduces the amount of parameters while ensuring the accuracy of the segmentation. Finally, after the group convolution in each network module, the channel random mixing module is added to solve the problem of accuracy reduction caused by group convolution, and the weighted Tversky loss function is used to replace the Dice loss function to improve the segmentation accuracy of small targets. The average Dice_ET, Dice_WT, and Dice_TC of the proposed model are better than 3D-ESPNet, DeepMedic, DMFNet, and other algorithms. The research results have certain practical significance and application prospects.

In order to solve the problems of subjective, low accuracy and cannot be used for continuous monitoring of existing psychological stress detection methods, a non-contact psychological stress detection method combining heart rate variability (HRV) and facial expression is proposed in this work. The method extracts HRV information from video images by image photoplethysmography technology, and obtains facial expressions by establishing expression recognition model through VGG19 network. HRV and facial expressions are used as feature inputs, and support vector machine is used for training classification to realize the detection of stress state and non-stress state. Experimental results show that the stress classification accuracy of the method can reach 81.4%, which can effectively improve the accuracy of mental stress detection. The method can be applied to psychological testing of ordinary people, athletes, criminals, and other fields.

Aiming at the limitation of the measurement conditions in most of the measurement implementation experiments, a method for measuring the binocular vision motion parameters based on dual cameras without points with the same name is proposed. The initial values of position and pose are calculated by the method, and the initial values are optimized by the global orthogonal iterative algorithm. The measurement method does not need the participation of the same name point in the realization process, but only needs more than 6 target cooperative mark points in the field of view of two cameras, so as to obtain the position and attitude parameters of the validator in the process of motion. The hovering, obstacle avoidance, and landing stage of the spacecraft are tested. The measurement method is compared with the binocular pose measurement method based on the same point, the monocular pose measurement method based on orthogonal iteration and the implied value of total station. The default value obtained by the station instrument is compared. The attitude measurement error is less than 0.5°, and the position measurement error is less than 0.01 m.

Aiming at the problem of distortion of the panoramic image collected by the conical catadioptric panoramic imaging system, a panoramic image expansion method of inner surface of cylindrical objects under the conical bidirectional projection model is proposed. Forward and reverse projection models with distortion parameters are established, first, the pixels of the panoramic image of cylindrical inner wall are reversely mapped to the expanded image, and then forward the missing pixels in the expanded image to the original panoramic image, through the panoramic image and the expanded image corresponding to the coordinate mapping relationship of the pixels, the expansion of the panoramic image of cylindrical inner wall is realized. The experimental results show that the tangential and radial distortion of the panoramic image of cylindrical inner wall are corrected better, the positioning accuracy of the bidirectional projection model of the cone surface is sub-pixel level, comparing with the approximate expansion method of concentric rings, the information entropy and the average gradient are both improved to different extents, it can be effectively applied to the expansion of panoramic images of cylindrical inner wall.

When the iterative closest point (ICP) is used for model registration of the aircraft skin surface, problems such as dislocation and local minimum value will appear since the surface is flat and smooth and has few features. For this reason, a registration method of skin point clouds based on contour constraints was proposed. Firstly, a new description method of Cκ feature points for a three-dimensional contour was defined, and the initial Cκ feature points were clustered and filtered based on the distance constraint, realizing the accurate feature description of skin point clouds. Secondly, on the basis of the similarity constraint of fast point feature histogram based on distance (FPFH-d) features, the corresponding point pairs of point clouds and model feature points were found to achieve the initial registration of the skin contour. Finally, according to the ICP algorithm, the contour constraints of Cκ feature description were fused for the precise registration of the skin. Furthermore, the speed and accuracy of the new algorithm were tested by the point clouds from the Stanford public database. In comparison with the fast point feature histograms-sample consensus initial alignment (FPFH-SAC-IA), the initial registration accuracy of the proposed algorithm is improved by 48.47% and 77.29%, respectively, and the speed is increased by 70.35% and 97.08%, respectively, which proves the universality and effectiveness of the extraction algorithm of Cκ feature points. In addition, based on the proposed algorithm, we conduct experiments to verify the measurement data of aircraft skin and the results demonstrate that the registration accuracy reaches 100% and the global error is better than 3.5 mm in the range of 12 m 3. In conclusion, the method proposed in this paper can effectively solve the problems of dislocation and local minimum value during skin registration.

High precision polarization-scanner (POSP) can obtain high precision multispectral polarization-radiation information of the target by using the simultaneous polarization-measurement technology of partial aperture and partial amplitude. Its measurement accuracy is one of the key indexes affecting in-orbit application of load. After the instrument is developed, polarization and radiometric calibration and measurement accuracy evaluation are completed under laboratory conditions. To test laboratory calibration results, we carry out the natural target detection under the ground of the verification experiment. The sky is scanned along the main plane of the sun with POSP in a clear day. The data of sky radiance and polarization degree obtained are compared with the data collected by CE318N sun-sky polarization radiometer at the same time, and the factors affecting the data of the two instruments are discussed. The experimental results show that the two instruments have a good consistency, with the consistency deviation of radiance and polarization less than 4% and 0.005, respectively. The accuracy of POSP laboratory calibration and the detection ability under natural targets are verified, which can provide a basis for the subsequent processing and application of space-borne data.

To achieve high accuracy with less computational time in the phase shifting interferometry, a series of two-step random phase shifting algorithms based on fast least-squares method are proposed. First, the phases extracted by the sum and difference normalization algorithm with double pre-filtering, the sum and difference normalization algorithm with single pre-filtering, and Gram-Schmidt orthonormalization method are used as the initial values for iteration. Then, the two phase shifted interferograms without filtering are used in the calculation with the least-squares method, and the final phase is obtained. In order to save time, only limited pixels are chosen to take part in the iteration. The comparison indicates that the two-step phase shifting algorithm based on the sum and difference normalization algorithm with single pre-filtering and fast least-squares method has the best comprehensive performance, and it can obtain high accuracy with less time.

Aiming at the photoelectric performance measurement for GaInP/InGaAs/Ge triple-junction solar cells used in aerospace, we introduce the solar simulator method and the high altitude balloon method. The measurement principle of the solar simulator method and the traceability chain of key values are described. Based on the dual light source steady state solar simulator and the spectral mismatch analysis, the photoelectric performances of the GaInP/InGaAs/Ge triple-junction solar cells are measured, and the key parameters such as short circuit current, open circuit voltage, maximum power and their corresponding temperature coefficients are obtained. In addition, we introduce the high altitude balloon calibration method. The GaInP/InGaAs/Ge triple-junction space solar cells are carried by the Helium balloon to arrive at the altitude higher than 35 km and their photoelectric performances under high altitude natural sunlight are measured. Simultaneously, their current-voltage characteristic data and the real-time temperature data are collected. After temperature correction, the data collected under high altitude natural sunlight is compared with that by the solar simulator method. The results show that the maximum relative deviations in the short circuit current, open circuit voltage and maximum power are 2.61%, 2.13% and 1.63%, respectively. It means that there is a good consistency between these results of the two measurement methods.

To secure the centroiding accuracy of star image spots under the interference of sun straylight noise, we proposed a new centroiding method featuring the subtraction of background threshold and straylight slope noise in this paper. First, the gray noise of sun straylight was modeled as a slope within the window of a star image spot. For the new centroiding method, the error analysis equation of centroiding was deduced regarding the parameter estimation errors of the straylight slope model, especially the function relation concerning the window size. Then, the least-square parameter estimation formulas for the straylight slope model were given based on the window edge pixels. Finally, simulation tests were conducted through three centroiding methods, namely, the traditional gray weighted centroiding method (GWCM), the threshold subtracted GWCM, and the new method. The testing results show that the centroiding accuracy of the proposed method is two times and fifteen times higher than that of the two traditional methods. In conclusion, the method proposed in this paper is an effective centroiding method for star image spots under the interference of sun straylight noise and has a certain value in engineering applications.

To study the optical properties of multilayer AlGaN films with high Al content and calculate the relevant optical parameters, we fitted the transmission spectra of two samples based on the method of the multilayer-film transfer matrix. According to the mechanism of the AlGaN materials absorbing the incident light on different wavelengths, considering the out-of-band weak absorption of the materials, we established an absorption coefficient model in the all-band range (200--800 nm). At the same time, the root mean square (RMS) parameter of surface roughness was introduced to characterize the effect of the surface roughness of the materials on the transmission spectra. Furthermore, the transmission spectra of two Al0.65Ga0.35N samples with different structural parameters were fitted by the proposed model, and the fitting results were in good agreement with the experimental results. The absorption coefficient of Al0.65Ga0.35N material in the all-band range provided reliable experimental data for the study of the response spectra of solar-blind UV detectors. Additionally, the parameters such as film thickness, refractive index, and surface roughness of the Al0.65Ga0.35N materials were also obtained.

We report the first continuous-wave (CW) all-fiber gas Raman laser source. An all-fiber gas cavity with a length of 50 m and filled with high-pressure hydrogen was formed by fusion-splicing solid-core single-mode fiber and photonic band gap hollow-core fiber. A high-power CW 1540 nm fiber amplifier was used as the pump source, and efficient CW 1693 nm Stokes laser was obtained by pure rotational stimulated Raman scattering of hydrogen. Furthermore, by adding a high-reflection fiber Bragg grating with a center wavelength of 1540 nm at the output end of the all-fiber gas cell, the Raman threshold is reduced by 38.2%, the maximum output Stokes power is 2.15 W, and the Raman conversion efficiency inside the gas cell is 72.2%, while the optical-to-optical conversion efficiency in terms of the total pump power is 31.7% due to the relatively high splicing loss. This work provides a feasible way for compact, high-efficiency, and high-power 1.7 μm fiber laser.

A tunable dual-color synchronized picosecond pulse generation technique based on supercontinuum filtering is experimentally studied. The output pulse of an all-polarization-maintaining Er-doped fiber laser is divided into two channels, one of which is coupled to the highly nonlinear fiber, and the supercontinuum covering the emission band of Yb-doped fiber is obtained. With the combination of a narrow-band tunable filter with an all-polarization-maintaining Yb-doped amplifier, a tunable laser output with average power of 70 mW, pulse duration of 4.0 ps, and central wavelength of 1025--1055 nm is achieved. The other output of the Er-doped fiber laser passes through the narrow-band filter and the Er-doped fiber amplifier to generate a laser output with average power of 200 mW, pulse duration of 4.2 ps, and central wavelength of 1580 nm. The above tunable two-color picosecond pulse based on supercontinuum filtering possesses good synchronization characteristics and can be used as pump light and Stokes light for coherent anti-Stokes Raman scattering. It is found that the fluctuations of pulse amplitude and average power are smaller and the relative intensity noise is lower when the spectral components at flat positions of supercontinuum are selected as the seed light of the Yb-doped amplifier.

Focused plenoptic cameras have played an increasingly important role in fields like structure from motion (SFM) and scene reconstruction. However, the traditional SFM algorithms cannot be directly applied to focused plenoptic cameras due to the special structures of the cameras. In order to solve this problem, we proposed a complete equivalent multi-camera model of focused plenoptic cameras. On this basis, we employed the SFM algorithms of the traditional multi-camera to give the algorithm examples suitable for focused plenoptic cameras with regard to pose estimation and point cloud triangulation. Finally, the experimental results of simulations and real scene reconstruction verify the correctness of the equivalent multi-camera model and the SFM algorithms, further indicating that the SFM problem of focused plenoptic cameras can be equivalent to that of multi-camera.

It is very important for military defense and space exploration to automatically perceive the category and working state of space targets from the observation images. In order to realize the automatic and accurate perception of spatial target image information, a data-driven spatial target image information perception technology is proposed. The proposed technology is based on a deep convolutional neural network, using massive simulation data and a small amount of real data to train the neural network. The trained neural network can directly perceive information such as load and working status of targets from a spatial target image. Taking two spatial target image information perception tasks as examples, the technical practicability is tested. In the task of space target load recognition, the proposed technology can perform load recognition on unknown space target images with different degrees of blur and different noise levels. The results show that for different space target loads, the average recognition accuracy of the proposed technology exceeds 80%, and the detection speed can reach 50 frame/s. In the task of spatial target state perception, the model combination method is used to build the expert system of spatial target working state perception. Taking the space object image as an example, this paper realizes the perception of the working state information of the space object, and verifies the effectiveness of the data-driven space object image information sensing technology.

In order to study the influence of micropore statistical characteristics of Angel type planar lobster optical devices on the focused imaging performance, a set of point-to-point vacuum X-ray beam test devices was developed in this work. The micro focal spot X-ray beam was used to carry out the focused imaging test on the developed optical device. The knife slit system was used to conduct two-dimensional scanning test on the planar lobster eye optical device. Different degrees of tilt-type process defects were found in different areas of the optical device. The effect of tilt process defects on focused imaging was simulated based on the Monte Carlo software. Experimental results show that the plane lobster eye optical devices used in the experiment can converge the incident X-ray beam into a clear cross image at a focal length of 3650 mm, the maximum full width at half maximum of the focal spot is about 4.63 mm, the corresponding largest angular resolution is about 4.36', and the unevenness of the point spread function is about 33.7%. Simulation results show that tilt-type process defects will decrease the intensity of the central bright spot, the crosshairs are diffuse and discontinuous, the secondary focal spot is distorted, and the image quality is deteriorated.

In this letter, the dispersion of the MgF2 wedge cavity is simulated using the finite element method and the dispersion effect of radius, wedge angle value, and wedge angle position on the full cavity in the communication band is studied. Further, the dispersion curve is used to simulate the frequency and time domains of solitons using the Lugiato-Lefever and thermal migration equations by employing the blue to red detuning process. The impacts of scanning speed, quality factor, and pump power on solitons are evaluated. Combined with previous experiments and assumptions, some important parameters of the MgF2 wedge cavity for the soliton generation are discussed. The data results are useful for fabricating the MgF2 wedge cavity with low anomalous dispersion and generating soliton combs in this cavity.

There are differences in the geometric positioning accuracy of the domestic mainstream optical satellites with 2 m spatial resolution in different areas. For this reason, based on a rational polynomial coefficients (RPC) model and block adjustment, we proposed a method to test the geometric positioning accuracy of multiple satellites through the same control benchmark. Specifically, taking the Guyuan flat area in Hebei Province as the control area, we evaluated and tested the geometric positioning accuracy of high-resolution single-scene and stereo images from the GF1 series satellites (GF1, GF1-B, GF1-C, and GF1-D), the Ziyuan-3 series satellites (ZY3-1 and ZY3-2), and the Tianhui-1 satellite (TH-1). The research results are shown as follows: (1) Without considering the ground control points, the plane errors of single-scene images from the GF1 series satellites and TH-1 are mostly better than 4.2 m and about 6.36 m, respectively, while the accuracy of stereo images from ZY3-1 is high, with the plane and elevation errors being respectively 11.29 m and 3.5 m or so. (2) Considering the ground control points, all the plane errors of single-scene images from the GF1 series satellites are better than 13.3 m, and those from ZY3-1, ZY3-2, and TH-1 are better than 5.46 m, while for the stereo images of ZY3-1 and ZY3-2, the plane error is about 4.01 m and 4.29 m, and the elevation error is about 1.71 m and 1.61 m, respectively. It follows that the proposed method is reasonable and feasible to evaluate the geometric positioning accuracy of several high-resolution domestic optical satellites.

To meet the requirements of high sensitivity, resolution, and reliability in pressure measurements, in this paper, we proposed a high-precision optical fiber pressure sensor with a diaphragm-type Fabry-Perot (F-P) cavity based on the demodulation principle of frequency-modulated continuous wave (FMCW) laser interference. Firstly, a pressure measurement model based on this principle was deduced. Secondly, a pressure measuring device was developed and subsequently the sealed cavity formed by the SUS631 stainless steel diaphragm was continuously inflated through an air pump to collect the characteristic curve of the central deformation of the SUS631 stainless steel diaphragm (which is the amount of change in the F-P cavity length) with the air pressure when the air pressure changed in the range of 0--600 kPa. Furthermore, the cavity change of F-P cavity was measured at 25 ℃ and different pressures and the causes inducing the drift of the cavity change were discussed. Finally, the resolution of pressure measurements without the system error was acquired. The results show that the FMCW pressure sensor has a sensitivity of 286.55 nm/kPa and a resolution of 0.287 nm/Pa and its random measurement error displays a normal distribution. The above analyses verify that the proposed FMCW optical fiber pressure sensor can perform measurements with high sensitivity, resolution, and stability.

The technology of underwater polarization difference imaging can significantly suppress the backscattered light, the key factor affecting the quality of underwater imaging. Therefore, it is an effective method to obtain clear images in the underwater scattering environment. Traditional polarization difference methods are based on the polarization images at two orthogonal polarization directions and have obvious effects on suppressing the backscattered light. However, the corresponding degree of modulation freedom is low, which limits the further improvement of imaging quality. To solve this problem, we proposed an improved method of underwater polarization difference imaging in this paper. After the difference of polarization images in two optimal polarization directions and the introduction of the weight coefficient for the difference item, this method could realize the underwater polarization difference imaging with three degrees of freedom. The experimental results show that compared with the traditional polarization difference imaging, the proposed method can better restrain the backscattered light, highlight the signal light of the objects, and finally achieve the underwater clear imaging with higher quality.

Head-mounted display (HMD) is the main technology to realize virtual reality (VR), however to restore the color temperature of visual virtual reality is still a problem to be solved. In order to eliminate the perceived difference in HMD interference sources, we analyze the relationship between color temperature and spectral area ratio on the basis of colorimetry and develop an optical filter film to compensate for color temperature preference under gender differences. Combined with the Matlab engineering calculation software, the least square method-Gauss Newton iterative method is used to perform curve fitting of the discrete point function of spectral data, and the conversion relationship between color temperature of an OLED light source and spectral area ratio is derived inversely. By analyzing the optical properties of materials and combining the optical film theory, we analyze the sensitivity of the film layer with the Essential Macleod thin film design software. By reducing errors and optimize the film structure, we achieve the preparation of color temperature filter films. The thin films are prepared by the method of electron beam heating double gun co-evaporation. The crystal control and the light control are monitored to accurately control the deposition thickness of the thin layer and the sensitive layer and thus the experimental errors are reduced. The spectra, surface roughness and environment of the two film systems are tested and the results show that the proposed filter can used to improve the color temperature of an OLED light source to 4876 K and 9333 K, to solve the problem of color temperature preference under the gender difference in HMD, and to restore the VR visual color.