In this paper, based on the principle of slant visibility detection, the SBDART (Santa Barbara DISORT Atmospheric Radiative Transfer) model is used to solve the radiation transfer equation, and detailed theoretical simulation and analysis of the sky background radiation are carried out. First, using the aerosol models from SBDART model, the sky background radiation results of different types of aerosol models under sunny conditions are obtained. The distribution trends of solar direct radiation and atmospheric scattering radiation at different heights are discussed. The influences of single scattering radiation albedo and asymmetry factor on the results are analyzed. A calculation method combined with Lidar and SBDART model is presented to obtain the real sky background radiation. The aerosol data obtained by a scanning lidar is taken as the input parameter of SBDART model, and the distribution results of solar direct radiation in the real atmosphere and sky background radiation at different heights are calculated. In order to validate the correctness of the results, the solar direct radiation synchronously measured by the solar photometer is compared and verified. The results show that a consistent change trend can be obtained between them, showing positive correlation with a correlation coefficient of 0.99. Finally, taking blackbody as the target, the influence of sky background radiation on slant visibility is preliminarily discussed. The actual sky background radiation obtained by combining Lidar and SBDART models can provide guarantee for accurate inversion of slant visibility.

An anomalously reflective polarization-independent high-efficiency multilayer slanted grating with broad spectral and angular bandwidths under normal incidence was designed by rigorous coupled wave analysis and using a simulated annealing algorithm. To enhance the efficiency and broaden the bandwidth, a four-layer sandwiched structure with a gradually changing refractive index was proposed. Based on the proposed structure, we designed a polarization-independent anomalously reflective grating structure in the visible band. It exhibits a spectral bandwidth of 130 nm (550--680 nm) with -1st efficiencies higher than 93% and polarization-dependent loss less than 0.2 dB. Moreover, simulation results suggest that the grating structure features an angular bandwidth of 47° (-2°--45°) and large range of fabrication tolerances. Therefore, the proposed multilayer slanted grating will attract great interest for its potential applications in virtual/augmented reality displays and metasurface-based devices, especially for metalenses in the visible range.

A vertical optical coupler with high integration and coupling efficiency is designed based on a subwavelength grating. The proposed vertical coupler shows the coupling efficiency above 97% within the wavelength range of 1.5--1.6 μm. Moreover, the length of the device is only 15 μm, which is one tenth of the size of the traditional vertical coupler based on an adiabatic tapered waveguide and improves the integration of the system. In terms of duty ratio of subwavelength grating, tip size of tapered waveguide and dislocation of waveguide, the designed couplers have large tolerances to process errors, which shows a good application prospect in the field of heterogeneous integration.

To improve the accuracy of the point spread function (PSF) extraction of planar grating imaging systems and develop PSF models, a fitting method of edge spread function based on the Boltzmann function is proposed. A hyperbolic PSF model with the incident angle as the independent variable is constructed, and then the PSF distribution rules of the grating imaging systems are obtained. Finally, the Lucy-Richardson algorithm is employed to restore images with different degrees of blur, and the quality of the restored images is evaluated. A grayscale mean gradient (GMG) improvement of above 60.2% and structural similarity (SSIM) improvement of above 66.5% are obtained. In a comparison with the restoration effects of different fitting methods, the proposed method shows better image restoration for large aberrations than other similar methods. The developed PSF model can also accurately represent the characteristics of grating imaging systems.

The selection of the state of polarization (SOP) of incident light plays a significant role in the performance of optical fiber pressure sensor based on the polarization properties. In order to improve the sensitivity and linearity of the sensor, a simple and fast optimization scheme for optimal incident light polarization state based on Muller matrix is proposed to replace the traditional “blind adjustment” of SOP. In the experiment, with the squeezing devices based on piezoelectric ceramics and polarimeters, the optimal performance is achived with sensitivity of 0.2410 N -1 and linearity of 99.9%, which agree well with the theoretical prediction. The proposed scheme will greatly improve the performance and extend the practical applications of the sensors based on the fiber polarization properties.

A few-mode ring fiber laser based on an all-fiber mode multiplexer/demultiplexer (MUX/DEMUX) was designed, which achieved switchable lasing output in LP01, L P11, LP21, and hybrid modes. Experimental results show that the output lasing mode of the ring fiber laser can be switched between the three lowest-order linearly polarized (LP) modes and hybrid modes by employing a low mode-crosstalk all-fiber mode MUX/DEMUX and a simple 4 × 1 optical switch. Thresholds of 40, 60, and 80 mW and slope efficiencies of 1.2%, 0.82%, and 0.56% were obtained for the LP01, LP11, and LP21 mode operations, respectively. Numerical simulation and parameter optimization of the designed few-mode ring fiber laser structure adequately resolved the wavelength shift problem, with a line width of 3 dB less than 0.032 nm being realized.

The polarization states of the optical signals transmitted through optical fibers change randomly because of the influence of the external environment. Therefore, a real-time polarization compensation module must be inserted into the optical fiber communication systems, such as the polarization multiplexing communication system and the polarization encoding fiber quantum key distribution(QKD) system, to compensate the polarization changes that can be attributed to the random birefringence effect in optical fibers. Thus, the stable operation of the corresponding communication system can be ensured. In this study, an experimental system that can compensate for the variation of the polarization states in real time is proposed according to the polarization compensation system based on wavelength-division multiplexing in optical fibers. In this system, two reference lights with conjugated polarization states are prepared and an integrated polarization detector is used for detecting the changing polarization states in real time. This system can be applied to the optical fiber communication systems. The experimental results show that when this system is applied to a QKD system, continuous and stable operation can be achieved at a transmission distance of 5 km for more than 8 h with a quantum bit error rate of 1.96%.

In this study, a fiber curvature sensor was proposed based on a rough side-polished single-mode fiber with a sensitizing technology. In addition, the transmittance of sensitive area in case of the fiber curvature sensor was accurately measured using the pulse self-reference demodulation technique. The experimental results obtained based on the cantilever beam denote the linear response coefficient of the sensor sensitive area is 0.593 and that the noise amplitude in the measurement system is 5.9×10 -5. The curvature resolution of the system can become 9.95×10 -5 m -1 when the cantilever beam displacement is converted to curvature. Furthermore, the curvature sensor has a simple structure and a wide response bandwidth and can be used in a time-division multiplexing system.

Aiming at the problem of multiple and overlapping faculae of the birefringence sun sensor, a high-precision facula center extraction method based on ellipse fitting is proposed. After preprocessing for multiple facula images, different regions of interest are segmented. Then, the overlapping faculae are quickly distinguished and segmented by detecting the shape features of facula in the regions of interest. Finally, the central coordinates of each facula are extracted by ellipse fitting method. The simulation results show that the method can quickly distinguish overlapping faculae, calculate the number and radius of faculae, and extract the sub-pixel central coordinates of the circular and ellipse faculae. This method has no limitation on the size and quantity of faculae, and it can get satisfactory results for incomplete faculae.

Dual-spectral computed tomography (DSCT) is used to reconstruct material- and energy-selective images to distinguish scanned object's materials and thus has numerous applications in medical and industrial fields. The image reconstruction method is vital in DSCT imaging, among which extended algebraic reconstruction technique (E-ART) can reconstruct basis material images with high quality. However, slow convergence limits its application. Herein, an accelerated extended algebraic reconstruction algorithm (AE-ART) was proposed. The main idea of the proposed algorithm is to increase the angle between the projection curves of high and low energies or to reduce the conditional number of coefficient matrix of those two projection curves by adding weights to the basis functions. Simulated experiments of dental phantom were conducted and the results indicated that the convergence rate of AE-ART was 30% higher than that of E-ART.

The improvement in reconstruction quality has always been the research focus of ghost imaging. A joint bilateral filter is embedded in the projected Landweber iterative algorithm, which is used for reconstruction in ghost imaging. This method can effectively remove the noises in the intermediate results of the projected Landweber iterative algorithm using the joint bilateral filter, and the reconstruction quality of the projected Landweber iterative algorithm is thus improved. The numerical simulation and the experimental results show that the proposed algorithm can effectively reconstruct the intensity image of the target. The proposed algorithm has higher reconstruction quality, if compared with the common ghost imaging reconstruction algorithms. At the same time, the influences of background noise and number of measurements on reconstruction quality are analyzed, and it is proved that the reduction of background noise and the increase of number of measurements improve the reconstruction quality of the proposed algorithm.

In this study, we propose the liquid crystal lens hill climbing autofocus algorithm. The liquid crystal lens is an electronically controlled focusing lens, which exhibits good lens performance. In a traditional glass lens imaging system, the hill climbing autofocus function requires the voice coil motor to continuously move the lens for obtaining the optimal clear image plane position. The autofocus function of the proposed algorithm is obtained by varying the voltage. Thus, the entire large-scale focusing process can be realized without any mechanical movement, miniaturizing the focusing system. The experimental results show that the liquid crystal lens exhibits good focusing performance and high imaging resolution.

To overcome current hardware limitations related to baseline distance and focal length, this paper proposes a method for improving the ranging accuracy of binocular systems using super-resolution imaging based on binocular zoom. The zoom system achieves super-resolution reconstruction, improving the imaging resolution of the binocular system, and consequently, improving the ranging capability and decreasing the relative ranging error, without altering the remaining hardware components. Experimental results reveal that the binocular zoom ranging system has a smaller relative ranging error at the same distance; moreover, the binocular zoom ranging system has a greater range while keeping the relative ranging error constant. The binocular stereo vision system with synchronous zoom can improve the imaging and ranging resolutions of three-dimensional scenes and provides a novel and effective way of building a small long-distance binocular ranging system.

A diffractive telescope is limited by its diffraction efficiency, and its imaging quality is easily affected by non-imaging order diffracted light. To resolve this problem, this study proposes a block-matching and 3D collaborative filtering image restoration algorithm based on adaptive noise estimation. First, the noise variance of a blurred image is estimated using principal component analysis, which is then combined with the known point-spread function, and the clear image is restored using the proposed algorithm. The proposed algorithm is tested in an imaging system built from diffraction telescopes. Results of numerical simulation and measurement experiment show that the proposed algorithm can improve the modulation degree of restored images by 3.58 times compared to that of raw images and can effectively improve the details of restored images and facilitate the faint-object imaging. Therefore, the proposed algorithm provides an effective way for high-contrast imaging of faint objects using diffraction telescope systems.

A high-precision coherent laser ranging method based on Kalman filtering is proposed using a single-linear frequency modulated continuous wave (S-LFMCW) laser as the transmitting signal . By measuring the beat frequency between the echo signal and the local oscillator signal, the frequency difference is first obtained. The distance and speed of the target are then calculated with this frequency difference signal through analog-to-digital conversion and digital signal processing. Finally, the Kalman filter algorithm is used to fuse the measured data, and the target distance estimation is obtained with high accuracy. The experimental results show that the proposed method is suitable for large-scale dynamic coherent laser ranging lidar systems. The effective ranging range of the system is 8 m to 2.7 km, and the ranging accuracy is within 0.1 m.

To overcome the shortcomings of infrared thermography, a novel method of metal fatigue damage assessment based on the characteristics of surface infrared polarized thermal imaging is proposed in this paper. First, the theoretical basis of polarized thermal imaging for metal fatigue damage assessment is provided. Then, the change rule of surface micromorphology in the process of fatigue damage is experimentally explained. Next, the target image is segmented and analyzed using the Tsalis entropy algorithm. Finally, the feature selection is performed using principal component analysis and a nonlinear prediction model based on back propagation (BP) neural network is constructed. Experimental results show that during the process of metal fatigue damage, the results of model training, verification, and test have a good correlation with the actual experimental data, and the average prediction error of tension and tension fatigue damage for the specific test target Q235 plate is less than 20%.

Surface deformation of a large-aperture mirror is affected by the mechanical stress produced on its support points in the process of system adjustment. Moreover, in the process of testing, a change in temperature will also lead to a change in surface deformation. Correct determination of the direction and magnitude of the stress plays a decisive role in solving the wavefront change caused by the stress change in a large-aperture optical system. A new approach for correctly determining the direction and magnitude of the stress, which combines a simulated annealing algorithm and integrated analysis technology to establish the linear relationship between the surface deformation of the mirror and the stress on its support points, is proposed in this paper. Through the simulation, accurate recognition of stress location, direction, and magnitude can be realized, with a stress magnitude recognition accuracy of more than 75%.

To improve the accuracy of stereo matching based on binocular vision applied to weak texture scenes, this study proposes a 3D reconstruction algorithm based on feature extraction using an attention mechanism. The proposed model uses convolutional neural network (CNN) to train feature representation of left and right images and calculates the matching cost of stereo matching. First, during the CNN feature extraction stage, attention mechanism module and channel attention mechanism module are summed to obtain the connection of each pixel in the feature image, enabling the network to capture the context information better and reconstruct weak texture areas more accurately in the reconstruction process. Second, we integrate the semantic coding loss in our neural network. The final loss function is defined as the weighted sum of the semantic coding loss and the reconstruction loss, which can effectively improve the reconstruction accuracy of a region with weak texture. We use KITTI and Sceneflow datasets to validate the algorithm. Experimental results show that the proposed method yields good improvements in accuracy, particularly in areas with weak textures.

Accurate calibration can develop the role of a focused plenoptic camera in the fields like scene reconstruction and non-contact measurement. One of the keys to improve the calibration precision is the accurate feature extraction algorithm. In order to improve the accuracy and efficiency of feature detection, we present a checkerboard corner detection algorithm based on the raw images. First, a robust corner detection operator is used to detect the checkerboard corners in the raw images, and the corresponding relationship between the 2D corners and the 3D plenoptic disc features is used to screen the detected results. Then, the sub-pixel optimization is carried out using the image consistency. The simulated corner detection and calibration experiments are carried out, and the distance measurement experiment is also carried out based on the reconstructed corners obtained by the R29 focused plenoptic camera. The experimental results show that the accuracy of the proposed corner detection algorithm is higher than those of the existing algorithms, and the calibration algorithm based on the proposed corner detection algorithm can achieve more accurate results.

In this study, a natural optical fiber illumination system is proposed based on a lightguide plate concentrator to maximize the utilization of solar energy. Further, the energy in the invisible band is converted into electricity using the spectral module for driving the LED array in the compensation system. Thus, a stable lighting energy output can be obtained under various weather conditions. A simulation software is used for light tracing, and the obtained results denote that approximately 60% of the total luminous flux in case of an indoor area of 100 m 2 and lighting for 12 h can be supplied by solar energy. Subsequently, a lightguide plate concentrator system is developed and tested via concentration experiments for developing a natural optical fiber illumination system to further verify the feasibility and accuracy of the proposed system. When the number of light collection modules increase from 10 to 100, the optical efficiency of the concentrators decreases from 83.6% to 65.1%. The variation trend of the output illuminance of the optical fiber is consistent with the input irradiance of the solar during the biaxial tracking test for optical fiber illumination system at outdoor, indicating the good light gathering performance of the proposed system.

The diffraction efficiency of the traditional single-layer diffractive optical elements (SLDOEs) decreases drastically when the wavelength deviates from the center wavelength, causing image blurring when the elements are used in a wide-waveband infrared system. In this study, we analyze the influence of the field of view on the point spread function (PSF) model of the system containing diffractive optical elements. In addition, we propose an optical-digital joint design method for reducing the influence of the field of view. In this method, the PSF is constant with respect to the field of view by assigning optimization weights to different diffractive orders. Further, the center wavelength of the SLDOE is designed in the medium waveband. The PSF model is used for reconstructing the blurred image in the long waveband to achieve dual-waveband imaging. A dual-waveband infrared refractive-diffractive system is designed using this method. The MTF with respect to the medium waveband at 17 lp/mm is 0.7. The reconstructed image in the long waveband is subjectively clear, and several image quality assessment function values are improved. Results show that the proposed design method enables the application of SLDOEs in dual-waveband infrared systems.

Based on the aperture coupled method, a metal-insulator-metal voltage tunable filter is proposed. The filter consists of two asymmetric rectangular ring resonance cavities and a rectangular waveguide, in which the asymmetric rectangular ring cavity is filled with organic electro-optic material 4-dimethylamino-N-methyl-4-stilbazolium tosylate. Finite element numerical simulation method is used to calculate and analyze the transmission spectrum, resonance wavelength distribution curve, and magnetic field intensity distribution of the asymmetric rectangular ring cavity waveguide structure. The results show that the filter has smooth transmission spectra, wide passband bandwidth with transmittance as high as 97%, and wide stopband bandwidth with transmittance as low as 0.01%. The characteristics of the filter can be adjusted not only by changing the structural parameters, but also by applying a control voltage. The adjustability of the filter is higher, and the proposed optimized structure has a wider stopband. Therefore, this type of filter can be well applied in integrated optics.

In this study, a composite metamaterial structure comprising a gold-nanostructure top layer, an intermediate dielectric layer, and a metal base layer was proposed. An I-shaped cell array comprising three oval nanodisks is the top layer of the metamaterial nanostructure, silicon dioxide is the intermediate dielectric layer, and a gold film is the metal base layer. Herein, the absorption characteristics, electric field distribution, and refractive index sensitivity characteristics of the structure were studied via the finite element method. Thus, three absorption peaks can be observed, and the corresponding absorptions are observed to be 91.06%, 99.63%, and 97.26%. In addition, the influence of the structural parameters and surrounding media on the absorption and response characteristics with respect to the changes in the refractive index was studied. The maximum sensitivity is 425 nm/RIU (RIU is the refractive index unit), and the figure of merit is 14. This study would provide theoretical guidance for developing perfect absorbers by considering the surface plasmon metamaterial structure as a refractive index sensor.

Theoretical design is very critical for the experimental fabrication of a high-efficiency solar photothermal conversion film structure. However, most softwares are usually limited to the optimization of solar absorption under the normal incident condition, rather than focusing on the criteria of solar photothermal conversion efficiency (PCE). In view of the above problems, the transfer matrix method and the genetic optimization algorithm are used to directly optimize the photothermal conversion efficiency of a multilayered metal/dielectric film structure by changing the thickness of each layer of the film. The effects of the film layer number, solar illuminance, and ambient temperature on tungsten-alumina-based (W/Al2O3) multilayered film structures are studied. The results show that the optimal number of film layers is 6 for solar concentration ratio of 1 and 8 for that of 100. These results have important guiding significance in the experimental fabrication of high-efficiency solar photothermal conversion films.

To develop Al-based thin-film optical elements for vacuum ultraviolet and extreme violet bands, the stress characteristics of Al-based thin films and their optimization methods were investigated in this study. Five Al-Si composite films with different Si contents (0, 8.97%, 16.49%, 28.46%, and 45.73%) were prepared by co-sputtering, and their stresses were measured using a real-time stress measurement device. The crystallinity of the films was characterized by X-ray diffraction. The obtained results reveal that the stress in Al film is compressive. With the increase of Si-doping content in Al, the compressive stress decreases and the crystallinity of the Al film and the grain size in Al (111) crystal orientation also decrease, indicating that the crystallization of Al is suppressed. Upon increasing the mass fraction of Si from 18.63% to 31.57%, the compressive stress in the Al film changes to tensile stress, and the tensile stress increases with further increase of Si mass fraction. This study provides technical support for the preparation of Al-based filters and monolayer and multilayer film elements, which will have important applications in the fields of extreme ultraviolet lithography, synchrotron radiation, and astronomical observation.