Sulfur dioxide (SO2) retrieval results obtained by the multi-axis differential optical absorption spectroscopy (MAX-DOAS) have weak absorption intensity and are vulnerable to inversion bands and the state of aerosol. An inversion method for the vertical profile and vertical column density of tropospheric SO2 using ground-based MAX-DOAS is investigated. The SO2 optimum inversion band (307-330 nm) is determined by comparing the inversion errors. The differential slant column density is obtained precisely. As the state of aerosols in the atmosphere is an important factor affecting the inversion of trace gases such as SO2, a two-step inversion method is used. The first step is to retrieve the aerosol profile by measuring the differential slant column density of O4 gas. The second step is to input the aerosol profile into the radiation transmission model, and the vertical distribution profile (0-4 km) and vertical column density of the tropospheric SO2 are obtained by the vertical inversion algorithm of trace gas concentration. The 0-100 m inversion results of the SO2 profile are compared to ground point instrument data, and the comparison results demonstrate that the two techniques agree well. The study shows that MAX-DOAS is an effective method for retrieving the vertical distribution and vertical column density of tropospheric SO2.

A method to investigate optical path turbulence based on laser spot distortion and a convolutional neural network (CNN) is proposed. Utilizing the CNN, we evaluated the spot distortion of laser beams resulting from airflow disturbance in space propagation. As a result, details of turbulence on the beam propagation path can be obtained. Experimental results demonstrate a high correlation between the evaluation parameter and the turbulent intensity (wind speed) measured by an anemoscope. The proposed method provides a turbulence analysis with short distance, high speed, and low cost.

Atmospheric correction is an important part of remote sensing quantification. The commonly used atmospheric correction method is the dark target method, which is suitable for application in dense vegetation areas but is less suitable in areas with low coverage of vegetation. In this study, an atmospheric correction method is proposed based on spectral matching. Herein, an invariable target in urban areas is used as the entry point. Further, we develop an atmospheric correction method for the Gao Fen-1 (GF-1) satellite panchromatic and multispectral sensor (PMS) camera. This method uses the 6S radiation transmission model to construct an atmospheric correction parameter lookup table for obtaining the inversion spectra of cement pavements from different images under different atmospheric conditions, and the average measured spectrum of the cement pavement is considered to be the reference spectrum. By the angle matching between the test spectrum and the reference spectrum, the closest spectral curve is estimated, the atmospheric correction parameters are determined, and the image is atmospherically corrected. The experimental results denote that this method works well, and the surface reflectivity obtained based on the inversion is consistent with the typical ground spectral data, which restores the surface to its actual situation and provides a novel atmospheric correction method that can be applied in areas having sparse vegetation.

The cold atomic clock, which uses laser cooling techniques, is expected to obtain more accurate time-frequency reference in the space microgravity environment than on the earth. This paper proposes a scheme for a new type of space cold atomic clock based on in-situ atomic detection. This design realizes the capture, cooling, state selection, microwave interrogation, and quantum state detection of cold atoms in a microwave cavity. Further, this design allows for a shorter clock cycle, higher time duty cycle for microwave interrogation, and a more compact structure than our previous space cold atomic clocks. Using a Boitier a Vieillissement Ameliore (BVA) crystal oscillator as a local oscillator, we analyze and estimate the stability of the cold atomic clock due to the Dick effect and quantum projection noise. In addition, we estimate sources affecting the uncertainty of the cold atomic clock. The results indicate that the proposed space cold atomic clock is expected to reach a stability of 5.9×10 14τ-1/2 and uncertainty of 1×10-16. These results are superior to those of existing cold atomic microwave clocks which use a BVA crystal oscillator as a local oscillator.

Linear gratings are only suitable for the amplitude modulation of images. To overcome this limitation, the anti-counterfeiting technology for phase-modulated latent images is studied based on the frequency-modulated image. The latent image is hidden in an error diffusion-screened image using the phase modulation method. Mutual information is considered to be the evaluation index of the hiding effect. An irregular grating is designed based on the grating extraction principle. Further, the application scope of the amplitude-modulated images is explored based on the concept of irregular grating design. The experimental results show that irregular gratings can clearly extract latent images from the frequency-modulated image and are suitable for the different tonality of the amplitude-modulated image. Thus, the applicability of the anti-counterfeiting technology based on phase-modulated grating for the frequency-modulated screening mode and amplitude-modulated full-tone latent image extraction is verified. The irregular grating obtained by relying on the original manuscript exhibits the property of anti-duplication, and endows the technique with natural anti-counterfeiting capability.

Blue-green LED communication is considered to be an effective means to solve the problem of underwater short-distance high-speed wireless data transmission. However, as LED divergence angle is usually quite large, the geometric loss is great in such communication link, which would reduce the communication distance. Aiming at this problem, a method is proposed to compress the emitter angle of underwater LED array light source with the total internal reflection (TIR) lens in this work. First, the divergence angle of LED array source is compressed from 130°to 7°. Then, a communication transmitter prototype is developed with the source, and a test system is built in a large tank to test the performance of the transmitter. The experimental results show that the communication prototype designed in this paper can support a maximum transmission rate of 23 Mbit/s when the underwater transmission distance is 16.6 m. Compared with the case without TIR lens, the maximum transmission distance increases 9.3 m at the same rate. It shows that the method of secondary light distribution with TIR lens can effectively reduce the transmitter's divergence angle and the link loss of transmission system, enhance the transmission capacity of communication system. It provides a new technical means for improving the transmission performance of underwater LED communication.

A programmable optical fiber delay system is built by using 17 optical switches and 16 optical fibers with different lengths. The system can realize the delay range of 0-65535 ns,step length of 1 ns, maximum data refresh rate of 100 Hz, delay stability of 0.2 ns, jitter delay less than 0.32 ns, and loss less than 31 dB. The delay accuracy, loss, and data refresh rate of the programmable optical fiber delay system are studied. The influences of loss and dispersion on delay measurement are analyzed. A threshold compensation method to reduce delay measurement error is proposed to compensate for the delay measurement error caused by loss and dispersion.

This paper proposes an integrated optoelectronic chip pair that can simultaneously receive and transmit signals by integrating a vertical cavity surface emitting laser (VCSEL) unit in the vertical direction of a PIN photodetector (PIN-PD) unit. The two units can transmit in two bands at the same time. In particular, one terminal can transmit and receive optical signals at the central wavelengths of 805 and 850 nm, respectively; the other terminal can transmit and receive optical signals at the central wavelengths of 850 and 805 nm, respectively. This study mainly introduces and optimizes the structure and performance of one end of the optical signal transmission at the wavelength of 805 nm and theoretically analyzes the electrical isolation and optical decoupling of the VCSEL and PIN-PD units, which further ensures that the structure can work as a transceiver at the same time.

The angular velocity input/output characteristics of a spin-exchange relaxation-free(SERF)atomic spin gyroscope under two different pumping conditions were experimentally tested via an optical fiber Sagnac atomic spin precession closed-loop detection technique based on circularly polarized probe lights. The nonlinearity of the SERF atomic spin gyroscope output was discovered. Based on the basic principle of the SERF atomic spin gyroscope, the nonlinear response model was established and the simulation study was carried out, and the simulation results were in agreement with the experimental results. This indicates that the nonlinearity of the SERF atomic spin gyroscope is determined by the internal interaction of atoms and is closely related to the total electron relaxation rate Rtot.

This study proposes a method for reducing signal crosstalk between LP11a and LP11b spatial modes via digital orthogonal filtering. The experimental results of testing the intensity-modulation direct-detection (IM-DD)-mode-division multiplexing transmission based on mode-selective photonic lanterns demonstrate that the bit error rate of the PAM8 signal can be reduced by at least two orders of magnitude via digital orthogonal filtering and conventional interference cancellation in back-to-back, 500-m few-mode fiber transmission. With respect to signal processing, this method provides a solution for modal crosstalk mitigation in low-cost, short-distance IM-DD-mode-division multiplexing transmission.

This study proposes an optimization method for detecting the low-frequency vibration of a cantilever beam using the 3 dB point of the resonant peak of an excessively tilted fiber grating (Ex-TFG). First, the axial and bending strain characteristics of Ex-TFG are theoretically analyzed and experimentally verified. The spectral response characteristics of the Ex-TFG vibration sensor modulated by light intensity are also theoretically analyzed. The 3 dB point of the Ex-TFG resonance peak is proposed to detect the vibration signal, and subsequently, a corresponding experimental research is conducted. The results are as follows. First, the resonance wavelength of Ex-TFG is blue shifted owing to both axial and bending strains. The axial strain sensitivities are -2.8 and -1.8 pm/με for the TE and TM modes, respectively, whereas the transmission intensity remains unchanged. Bending strain sensitivities based on the light intensity modulation in the curvature range of 0-0.4 m-1 are 2.6 dB/m-1 and 1.2 dB/m-1, respectively. Bending strain sensitivities based on wavelength shift are -3.34 nm/m-1 and -2.53 nm/m-1, respectively. Second, during vibration detection, the vibration acceleration sensitivities of the resonance peaks of Ex-TFG at the 3 dB points are 113.54 and 100.93 mV/g for the TE and TM modes, respectively, which are over two times higher than those (100% point) of the resonance peaks. The signal-to-noise ratios (SNRs) of the TE and TM modes at the 3 dB point are also higher (approximately 10 dB) than those (100% point) of the resonance peaks. In addition, the SNR increases with the increasing cantilever beam thickness. Third, under the same condition, the TE mode exhibits higher vibration response sensitivity than the TM mode; however, the output of the TM mode has better stability than the TE mode.

A mobile solar simulator is a requisite apparatus for conducting the field test prior to launching a coded solar-sensor-equipped satellite, which can provide simulated sunlight signal and sunlight vector signal for the test. This study provides a discussion on the structural configuration and working principle of the mobile solar simulator. Specifically, we present the description and design of the LED array light source and the collimating optical system in the device. For the light source, a power formula is applied to determining the number of LEDs, and the Sparrow criterion is considered to be the basis for analyzing the LED spacing. In particular, the array spacing is determined from the effect of irradiation uniformity. Furthermore, a mathematical model of cylindrical mirror is established based on the principle of an edge ray. The design and simulation of the collimating optical system are performed using the LightTools software. The experimental results denote that the mobile solar simulator has an irradiation area of 10 mm×50 mm, where the irradiance becomes 393 W·m-2 at a working distance of 50 mm. Further, the irradiation nonuniformities at three incident angles of -13°, 0°, 38° are better than ±7.3%, and the exit angle is 0.78°. This solar simulator can complete the encoded solar sensor field test after the installation of a satellite.

To address the difficulty in detecting a dim-small target in single frame image caused by the complex background and polymorphism of the target, a method of rough extraction for threshold segmentation and precise detection for multi-point signal-to-noise ratio (SNR) is proposed. In the rough extraction stage, an improved threshold segmentation algorithm based on robust principal component analysis (RPCA) is proposed. The ratio of the mean value of the neighborhood sparseness to the mean value of the whole sparse image is used for the threshold segmentation, so as to further eliminate the isolated noise and the edge clutter of background cloud. In the precise detection stage, a multi-point constant false alarm detection algorithm based on statistical characteristics is proposed. The SNR of each pixel of candidate points in the neighborhood is obtained, and then the target point is extracted based on the false alarm rate threshold and statistical quantity threshold. The problem of polymorphic features caused by the dispersion of target energy will be overcome. Experimental results show that the detection probability of this algorithm reaches 95.6% under complex background, and the false alarm rate is 56.1% and 47.1% lower than that of single pixel and neighboring pixel based SNR computing methods, respectively.

To enhance the visual effect of the infrared thermal image of electrical equipment and accurately detect its operating state, an image segmentation method based on a new threshold algorithm is proposed. First, the method performs Fourier filtering on the original image to form an automatic gradient graph. Then, for each specific type of fault distribution, the line model with N adjacent points is fitted to calculate the slope difference to find the optimal threshold for different types of fault regions. Finally, by morphological iterative etching, the target area is separated from the noise spots to obtain a clear segmentation image. This method is suitable for various fault types, which only requires calibration of N and determination of different segmentation cases, with others being processed automatically. The results show that the segmentation accuracy of this method is 85%, and the error rate is 0.0182%. The effectiveness and versatility of the proposed method are verified by using different types of infrared thermal fault images.

A generative adversarial network (GAN) in the semi-supervised learning architecture was used to address the problem of the scarcity of labeled data in X-ray image classification. Initially, we used a softmax layer to replace the output layer of an unsupervised GAN, extending it to a semi-supervised GAN. In addition, we defined additional labels for the GAN-synthesized samples to guide the training process and optimized the network parameters using a semi-supervised training strategy. Then, the discriminator network obtained by the training was used for X-ray image classification. From tested front-view chest X-ray images of six lung diseases, we find that the proposed method substantially enhances the supervised learning with limited labeled data. Further, the proposed method demonstrates superior classification performance compared with other semi-supervised methods.

In view of the complex and changeable morphological structure and scale information of retinal vessels, an U-shaped retinal vessel segmentation algorithm based on the adaptive morphological structure and scale information is proposed. First, the gray image of retina is obtained by synthetically analyzing the three-channel frequency information of the image with two-dimensional K-L (Karhunen-Loeve) transform, and the contrast information between the vessel and the background is enhanced by multi-scale morphological filtering. Then the preprocessed image is trained end-to-end by using the U-shaped segmentation model, and the data is enhanced by local information entropy sampling. The dense deformable convolution structure of the network coding part captures the multi-scale information and shape structure of the image effectively according to the informations of the upper and lower feature layers, and the pyramid-shaped multi-scale dilated convolution at the bottom enlarges the local receptive field. At the same time, introducing deconvolution layer with attention mechanism in decoding phase, which effectively combines the bottom and top feature mappings, can solve the problems of weight dispersion and image texture loss. Finally, the final segmentation result is obtained by using the SoftMax activation function. This approach achieves average accuracies of 97.48% and 96.83% and specificities of 98.83% and 97.75% on the DRIVE (Digital Retinal Images for Vessel Extraction) and STARE (Structured Analysis of the Retina) datasets respectively, which is better than the existing algorithms.

Polarization imaging can be used to characterize anisotropic muscle tissue, but the conventional methods can only give the qualitative structure and morphology of muscle development rather than quantitative comparison. In this paper, a quantitative polarization imaging system is constructed. Two perpendicular polarizers are rotated synchronously to get a set of images, and then the quantitative phase retardance of each zebrafish sarcomere is calculated through image processing. This quantitative polarization image is independent of the brightness of light source, the exposure time, etc., hence can be used to compare the morphology and distribution of zebrafish muscle taken at different time. The results revealed differences in the process of muscle growth between wild-type and muscle-damaged zebrafish.

The diagnosis and treatment of root fractures are difficult issues in dentistry. A three-dimensional swept source optical coherence tomography (OCT) system was constructed. Utilizing cardiovascular endoscopic imaging catheter, we imaged 46 root section samples with root fractures(width ranges from 8 μm to 283 μm). The results show that the system has a diagnostic accuracy as good as 98.3% when the width of root fractures is greater than 30 μm. The diagnostic accuracy is 36.3% for fractures smaller than 30 μm. For the 25 fracture-free samples, the accuracy is 92%. Further analysis suggests optimization of the relative position and angle between the catheter and sample sections, as well as 3D reconstruction of images. These improvements achieve 100% accuracy for above-30 μm fractures and 57.6% accuracy for below-30 μm ones. This experiment suggests a promising future for the application of endoscopic OCT in the diagnosis of dental root fractures.

This study proposes a radial multi-sub-mirror array structure based on the sparse aperture imaging characteristics of a liquid lens. A simplified model of sparse aperture imaging is given, and the analytical expression of the modulation transfer function (MTF) is deduced according to the pupil function of the multi-sub-mirror structure. Subsequently, a concrete distribution of the multi-sub-mirror array structure is given and the structure parameters are simplified using a dimensionless method. The structures of two types of radial multi-sub-mirror arrays are discussed. The calculations of the fill factor, redundancy, MTF, and related characteristic parameters are completed. Moreover, the physical phenomena represented by the parameter scanning are discussed. The structural and imaging characteristics of these two arrays as well as the characteristics of the radial multi-sub-mirror array structure and the composite ring-shaped structure are then compared and analyzed. Results show that: when the average MTF of these two structures and the frequency characteristic values in MTF are similar, the type II structure with larger actual equivalent aperture, actual cut-off frequency, and lower redundancy should be chosen; the type II structure has some advantages in imaging; because the multi-sub-mirror array structure based on a liquid lens adopts dimensionless simplification parameters, the conclusion is generally suitable for arbitrary size of enclosing circle; compared with the composite ring-shaped structure, the radial multi-sub-mirror structure has larger actual cut-off frequency and equivalent aperture when the fill factors are the same.

We develop a sparse tensor constrained reconstruction (STCR) algorithm which utilizes the nonlocal similarity prior information and divides the computed tomography (CT) image into a series of patch groups. The multidimensional low-rank decomposition method for tensors is used, and the prior information is introduced in the low dose computed tomography (LDCT) reconstruction to establish an object function. The object function is optimized by alternating iteration of the CT reconstruction image update step and the patch group sparse coding step in the iterative process. The performance of the STCR algorithm is verified through experiments based on simulation data and clinical data. Preliminary experimental results show that, compared to the classical reconstruction methods, the proposed method can produce better images in terms of structure preservation and noise suppression.

The geometric calibration errors of deflectometry are the principle factors that limit the precision of low-order surface measurements. In this study, the relationship between the alignment error of geometric calibration and low-order surface measurement error of a flat mirror is analyzed. Further, the sensitivity equations and weight factors that describe the relationship between the alignment error and surface measurement error are introduced and verified using simulations and experiments. The results denote that the alignment errors will introduce tilt, defocus, astigmatism, and coma aberrations to the surface measurement results. Furthermore, the surface measurement errors are observed to be proportional to the alignment errors. This study aids in achieving a system configuration of deflectometry that can improve the accuracy of the low-order surface measurements and provides a theoretical guide for assessing and analyzing the surface measurement errors associated with the deflectometry measurements.

An articulated laser sensor is a new kind of trans-scale and non-contact 3D coordinate metrological instrument. In order to achieve precise measurement, the system parameters should be calibrated with high-accuracy, especially the spatial pose of laser beam. A calibration method for the spatial pose of laser beam based on the combination of planar target and spherical target is proposed in this paper. The 3D coordinate of laser spots can be obtained by establishing matrix relation between the pixel coordinate system and the world coordinate system. Then the spatial pose of laser beam can be determined by linear fitting, and the spatial poses of rotation axes can be obtained by minimum-zone circle fitting. The experimental results show that the maximum distance measuring error is about 0.05 mm in the measuring range of 1 m, and the novel calibration method is precise and effective.

A stroboscopic online phase measurement profilometry (PMP) for high-speed rotating objects is proposed in this paper. In order to realize the online three-dimensional surface shape detection of a high-speed rotating workpiece, the traditional linear sinusoidal and traditional gray-coded gratings are replaced by circular sinusoidal and high-density binary-coded gratings, respectively. Further, the special stroboscopic synchronization control unit is designed to simultaneously trigger the projection system to project the phase shift grating and image acquisition unit to obtain the image synchronously, thereby obtaining the phase shift deformation stripes of the object at the same “frozen” position. Therefore, the static PMP phase shift algorithm can be used to reconstruct the three-dimensional surface shape information of the object. The experimental results demonstrate the feasibility and practicability of the proposed method. This method can effectively prevent the machining error caused by multiple clamping inconsistencies when the workpiece is detected offline, improve the workpiece detection and machining efficiencies, and be used to reconstruct the three-dimensional surface shape of other high-speed rotating objects.

Taking LYTRO camera as an example, a fast multi-target ranging method based on microlens array (MLA) light field camera is proposed. The process of the method is divided into three parts. In the first part, the paper acquire the re-focusing sequence images of the data by calculating the point spread function, restoring the color of the original data and conducting the super resolution. Then the pretreatment process of the target is completed. In the second part, a direct ranging method is proposed by using the principle of triangle and patch method. In the third part, the existing relative ranging method is optimized by super-resolution processing, which greatly improves the accuracy of the original algorithm. At the same time, the correctness of the algorithm is verified by the improved Laplace operator. Finally, a fast multi-target ranging algorithm for MLA light field camera is obtained by combining the two algorithms. Experiments results show that, for small number of objects to be measured, the direct ranging method is with high accuracy and fast speed, and for large number of objects to be measured, the combination of direct ranging method and relative ranging method can not only ensure the accuracy, but also greatly improve the timeliness. The proposed method can provide important data reference and more accurate evaluation basis for depth acquisition and three-dimensional reconstruction of light field camera.

A method for discriminating the appearance quality of soybeans based on the low-rank sparse (LRS) representation frame of multimodal lexicon features in the visible spectrogram is presented to accurately determine the soybean quality level. Firstly, multi-scale spatial gradient features and YCbCr color space features of the visible spectrogram of soybeans are extracted and regarded as visual vocabularies. The Kernel K-means clustering algorithm is used to form the local distribution cluster center of visual vocabularies in kernel space,thereby generating a vision lexicon. Secondly,the LRS representation method is used to couple the two type of features, thereby eliminating the effect of redundant information in high-dimensional heterogeneous modal dictionary descriptors. Finally, the LRS representation coupling multi-modal dictionary features are classified according to the metric between samples in the high-dimensional coupling space. The proposed method makes full use of multi-modal and multi-scale spatial gradient features and YCbCr color space features to describe the semantic feature attribution of appearance quality of soybeans. The experimental results show that the recognition accuracies of training set and prediction set are 92.7% and 80.1% respectively, and the discrimination accuracy of the proposed method is better than that of single-visual-mode based vision lexicon feature representation method.

In view of the noise interference to video images and the complexity of background change, the traditional Census transform eigenvalue dependence on the central pixel is improved, and the Census template is established to maintain the robustness of the Census transform to light changes. A new background modeling method is established by combining the improved Census transform eigenvalue, image pixel value, update frequency, latest update time and dynamic index. The background texture difference is adaptively selected and fused with multiple features to update the background model. According to the dynamic index, the background change complexity is established, and different update rules are established to improve the stability of the model for light mutation and complex scene processing. After testing multiple sets of standard video sequences, the detection accuracy of this algorithm is better than that of other algorithms, which effectively improves the influence of light mutation on foreground target extraction, increases the robustness to light mutations and complex scenes, and reduces the false foreground caused by holes and pixel shift of the moving target.

Aiming at the problems of low accuracy and high false alarm rate caused by artificial targets in the process of optical remote sensing image docked ship detection. This paper proposes a new method based on edge line gradient features and aggregation channel features for docked ship detection. The multi-structural and multiscale element morphological filters are used to realize the division of sea and land. According to the rectangular shape characteristics of the port in remote sensing images, the edge gradient tangent angle and the port concave and convex features are defined to locate the port,obtaining collection of port region of interest. The aggregation channel features of ships will be extracted and used to train the classifier for the docked ships by AdaBoost algorithm. The trained classifier is used to confirm the real ships in the port. Compared with traditional HOG feature and Haar feature, the proposed algorithm has better detection effect, and its precision and recall rate are greatly improved.

The feature pyramid network (FPN) adopts the method of upsampling and addition when fusing different scale feature maps. However, the spatial stratification information of the upsampled feature map is seriously lost, so that direct addition will inevitably make certain errors. At the same time, the deep feature information of the FPN structure is poorly forward-transferred, and its auxiliary effect to the shallower layer basically disappears. This paper uses the advantages of Long Short-Term Memory (LSTM) network in processing context information to improve the FPN structure. A top-down memory chain is established between feature layers of different depths, and a multi-gate structure is constructed to filter and fuse the information on the memory chain to generate a higher semantic feature map with stronger representation ability. Finally, the improved FPN structure is added to the SSD (Single Shot MultiBox Detector) algorithm framework, and a new feature fusion network, MSSD (Memory SSD), is proposed and verified on the Pascal VOC 2007 data set. Experiments show that the improved algorithm has achieved better test results, and it has certain advantages compared with the current advanced detection algorithms.

Aiming at the problems of memory waste and low efficiency in three-dimensional (3D) object recognition algorithm based on original point pair feature (PPF), a 3D object recognition algorithm based on enhanced point pair feature (EPPF) is proposed. By multiplying the fourth component of the original PPF with a sign function, a more distinguishing PPF is obtained, which eliminates the ambiguity of the original PPF. Considering the self-occlusion of the 3D model of the target to be identified, the large numbers of redundant point pairs existing in the target 3D model hash table are eliminated by means of the viewpoint visibility constraint between the point pairs, which reduces the memory overhead and improves the accuracy and efficiency of the 3D object recognition algorithm. The experimental results on the open dataset and the actual collected dataset show that the proposed 3D object recognition algorithm can improve recognition accuracy and recognition efficiency.

The design of multi-band metamaterial perfect absorbers is of great significance in the field of multi-color optics. In this study, a multi-band metal-insulator-metal metamaterial absorber is designed. Its surface electromagnetic response unit is a three-loop nested metal ring array. The finite-difference time-domain method is used to calculate the absorption spectra and electromagnetic field density distributions of the unit. The results show three absorption peaks at 1.44, 2.28, and 3.25 μm with maximal absorptivity of 98.5%, 99.6%, and 99.9%, respectively. The physical mechanism for these peaks is ascribed to the excitation of magnetic polaritons. The influence of the structural geometric parameters on the resonance is systematically analyzed. By altering the height and outer diameter of metal rings, different resonance wavelengths can be independently tuned. The absorption spectra of the structure are very robust to the polarization angle of incident light. In addition, the refractive index infrared sensing performance of the metamaterial absorber is examined. It is found that the structure exhibits excellent performance for refractive index sensing. The maximum figure of merit can reach 8.3 RIU -1(RIU is the refractivity unit), and the corresponding sensitivity reaches 1.08 μm·RIU -1. The proposed absorber can be applied to the sensing field and provide new insights for the design of other metamaterials.

A range of Sr3P4O13:Ce3+,Tb3+ phosphors were prepared via a high temperature solid-state reaction method, and their phase compositions, particle morphologies, and luminescence properties were systematically studied by the X-ray diffractometry, scanning electron microscopy, and fluorescence spectroscopy. The results show that there is an overlap at 300-400 nm between the emission spectra of Sr3P4O13:Ce3+ and excitation spectra of Sr3P4O13:Tb3+. Under near-ultraviolet excitation (290 nm), the phosphors emit blue light (approximately 300-420 nm) for Ce3+and yellow or green light (approximately 480-500 and 530-560 nm) for Tb3+. When the mole fraction of Ce3+is 0.08 and the mole fraction of Tb3+ increases from 0.01 to 0.09, the 4f→5d electronic transition of Ce3+ transfers energy to the 5D3 and5D4 levels of Tb3+, decreasing the luminescence intensity of Ce3+ and gradually increasing the luminescence intensity of Tb3+. This proves that Ce3+→Tb3+ energy transfer occurs in the Sr3P4O13 host. Therefore, the Ce3+→Tb3+ energy transfer mechanism is studied, and it is found that when the mole fraction of Tb3+ is 0.09, the energy transfer efficiency reaches its highest value (86.46%). The corresponding color coordinates of Sr2.61P4O13:0.24Ce3+,0.15Tb3+ are in the green region; thus, Sr3P4O13∶Ce3+,Tb3+ co-doped phosphors are expected to be promising green fluorescent materials.

Owing to its excellent properties, cubic aluminum nitride (c-AlN) is an ideal material for optoelectronic devices such as light-emitting diodes and laser diodes. In this study, the c-AlN/TiN/Si(100) heterostructure is deposited using the laser molecular beam epitaxy technique, and its microstructure and optical properties are investigated. The experimental results show that the AlN film and the TiN buffer layer both show a (200) preferred orientation of cubic rock-salt structure. The interfaces between the c-AlN film, TiN buffer layer, and Si substrate are clear; a second phase is not observed. However, there are certain defects at the interface owing to mismatched stress. The photoluminescence spectrum of c-AlN film exhibits three emission peaks at approximately 376, 520, and 750 nm. The emission peak at 376 nm is related to nitrogen vacancy (VN) and oxygen impurity (ON). The emission peak at 520 nm is related to the recombination of Al vacancies (VAl) and ON. The emission peak at 750 nm can be attributed to the radiation recombination between VAl and the valence band. The electroluminescence emission peak of c-AlN film is approximately at 580 nm, and it is attributed to a deep-level defect emission.

Autofocus technology is a crucial part in the detection of thin film transistor liquid crystal display (TFT-LCD). Because the fast and accurate autofocus is required, an autofocus microscope system is adopted to detect TFT-LCD. The digital grating is projected onto the surface of the measured object. The reflected light is split into two paths using a beam-splitting prism. One is imaged onto the area-array charge-coupled device (CCD) directly, whereas the other is split into two paths using another beam-splitting prism again. A reflecting mirror is used to couple two images of the digital grating onto a line-array CCD. The defocusing direction and defocusing amount are judged by comparing the sharpness of two grating images on the line-array CCD. In addition, a micro-displacement platform is used to move the objective lens and achieve the microscope autofocus. Thus, this study analyzes the digital grating, which affects autofocus, and uses a variable-period digital grating for focusing.

In the present study, a novel Lieb lattice with five points (hereinafter referred to as the Lieb-5 lattice) in the minimum periodic unit is used as a platform. The sites of Lieb-5 lattices are classified into two categories according to their spatial positions, namely, the center and edge lattices. Using the beam-propagation method, we investigate the effects of different intensities of two sets of lattices on out-of-phase octupole-beam propagation. The ratio of the intensities of the two lattice categories is 2∶3; further, according to simulation results for either an on-site or off-site incident, when the intensity of the center lattice is less than that of the edge lattice, as in this case, an eight-peak shape with strong localization is maintained during beam propagation. Otherwise, the beam barely maintains the shape of the eight peaks in the propagation process. The energy between the incident lattices is periodically coupled with the increase of propagation distance, presenting weak localization, because the geometric frustration of the Lieb-5 lattice caused by its unique topological structure inhibits the discrete diffraction of light beams. If the binding property of the lattices, diffraction of the beam, and interaction of the out-of-phase beam stay in balance, the eight-peak structure can be maintained; otherwise, the beam cannot always maintain the eight-peak structure.

This paper presents a novel grating spectrometer design that combines a convexly cylindrical mirror with a concavely cylindrical varied line spacing grating to achieve an extreme resolving power of 33000-70000 in the photon energy range of 0.6-1.5 keV for a Gaussian light source with effective meridional size of 30 μm (root mean square) at an upstream initial object distance of 10 m. This type of spectrometer can be used to diagnose the radiation spectrum of a soft X-ray free electron laser and properly measure the spectral distribution and the detailed structure of the self-amplified spontaneous emission mode due to its extremely high resolving power. The overall optical aberrations of the system are well analyzed and compensated, providing an excellent flat field at the detector domain throughout the entire spectral range. A machine learning support vector machine algorithm is introduced into explore the optimal system to deliver the desired resolving performance. In addition, to enhance the spectral intensity and detection efficiency simultaneously, a sagittal confinement focusing mirror is introduced into the optical path of the spectrometer to reduce the astigmatism in spectral imaging.

Slit spatial filtering technology can be used to significantly ease or even solve problems related to plasma pinhole closure and large scale in current high-power lasers for fusion, which has broad application prospects in fusion laser facilities. According to the diffraction theory of aberrations, this paper numerically analyzes the aberration effect on efficiency of a three-lens slit spatial filter. Two important parameters (near-field contrast of output beam and intensity on inner edge of filter slit) are considered as evaluation parameters of aberration effect in a given filtering system, and the permitted percentages to be changed are given. Different types of aberrations with different magnitudes are introduced into the filtering system, and the tolerance of each aberration is obtained according to the change of parameters. The introduced filtering system is compared with conventional pinhole filters. The results demonstrate that all types of aberrations on the lens may introduce various inhomogeneous phase changes in the wave-front of a light beam and degrade beam quality. In addition, dispersion or deformation caused by aberrations may change the intensity irradiated on the inner edge of slit apertures, which is reduced continuously with increased aberrations. The most noteworthy aberrations relative to the three-lens slit spatial filter are the spherical aberration and field curvature of the first cylindrical mirror.

The moiré effect, which can expand attractive application in the stereoscopic anti-counterfeiting technology and high-precision measurement of the micro-lens array, will occur when the planar micro-lens array is overlapped with the matching micro-pattern array. The relationship between the magnification factor and the direction of the moiré pattern and the vectors of the micro-pattern array layer and the planar micro-lens array layer is obtained based on the principle of the vernier moiré effect. Consequently, the magnification of the moiré pattern relative to the micro-pattern array is found to be approximately equal to the ratio of the periodic vector of the micro-lens array to the difference of periodic vectors of the planar micro-lens array and the micro-pattern array. The formula can predict the position and size of the micro-pattern mapping. For verification, the square aperture planar micro-lens array with an aperture side length of 0.3 mm and a period of 0.315 mm is applied to the micro patterns with different periods and angles. The experimental results of the location and size of the moiré patterns are consistent with the theoretical predictions.

The one-dimensional orderly nano-corrugation of magnetic quadrilayer thin film was fabricated by means of interference lithography (IL) and magnetron sputtering techniques. Scanning probe microscope (SPM) was used to characterize morphologies of the samples. The magneto-optical Kerr effect (MOKE) and optical parameters measurement were performed with a homemade MOKE system and ellipsometry (ELLIP-A), respectively. The experimental results show that the magneto-optical Kerr signal of the nanostructure is significantly enhanced, the Kerr peak is connected with the stripe width and the thickness of intermediate HfO2 layer, and the magneto-optical properties of the corrugated magnetic thin film can be tuned by the thickness of the intermediate HfO2 layer. Furthermore, the enhancement of transverse magneto-optical Kerr effect is observed. The theoretical calculations show that the magneto-optical Kerr enhancement of the system can be manipulated significantly by the coupling between plasmon resonance and cavity effect.

The vector optical field with complex spatial structure plays an important role in the manipulation of the optical fields. As one of the eigen laser fields in the elliptic coordinate system, Ince-Gaussian (IG) field has richer spatial degrees of freedom than Laguerre-Gaussian beam and Hermite-Gaussian beam, and it is one of the fundamental fields to construct a complex vector optical field. This work is based on the theory of spatial superposition of the orthogonal polarized even and odd IG modes. IG vector optical fields with different orders and spatial structures are generated by a spatial light modulator which modulate the even and odd modes separately. Experimental results are studied and compared with the simulated results. The results demonstrate the feasibility of generating IG vector optical fields by dual modulation and parallel measurement.

Based on the conclusion that azimuthally polarized vector beam with topological nuclei has radial component after tight focusing, the tight focusing formula of the azimuthally polarized vortex beam is amended. We afresh study the tight focusing properties of the azimuthally polarized higher-order Laguerre-Gauss vortex beams through a diffractive optical element and a high numerical aperture lens. The results show that a new three-dimensional (3D) triple optical chain, composed of a 3D main optical chain and two symmetrical 3D paraxial secondary optical chains, is obtained along optical axis near the focal plane. The effects of the radial mode number of associated Laguerre polynomial and the interception ratio of incident beam, the structure of the diffractive optical element, and the numerical aperture of the focusing lens on the triple optical chain are analyzed in detail. Results show that the change of radial mode number will destroy the structure of triple optical chain. By adjusting the interception ratio and the structure of the diffractive optical element, the triple optical chain with higher symmetry can be obtained again, and the high degree of freedom control of the triple optical chain can be achieved.

In order to improve the quality of the Bessel beam generated by a deformable mirror and explore the factors that influence its quality, the influences of the ring number of the deformable mirror actuator and the incident angle of the beam on the beam quality are studied. When generating a conical phase with 10-μm amplitude, the root mean square value of the residual error decreases from 70 nm to 26 nm while the ring number increases from 3 to 5. The distribution of axial light intensity of the generated Bessel beam is closer to the ideal distribution. This result indicates that the increasing ring number of the deformable mirror actuator can effectively improve Bessel beam quality. In addition, Bessel beams are generated by beams with incident angles of 15°, 30°, and 45°. The experimental results show that the Bessel beam quality is good and unaffected by the incident angle. This paper provides recommendations and references for generating high-quality Bessel beams by using deformable mirrors, and these recommendations are expected to be beneficial for the applications of Bessel beams.

Being limited by the accuracy of the atomic clock process and radio signal measurement, it’s difficult to achieve high-precision time synchronization between Roland C stations. Based on the quantum entangled microwave signal and cavity electro-opto-mechanical system, the more accurate synchronized time difference information of Roland C main and auxiliary stations can be obtained by converting the microwave quantum signal into the optical frequency domain for coincidence detection. Through theoretical analysis and simulation, the conditions of microwave and optical conversion in the cavity, as well as the effect of cavity dissipation on the fidelity are obtained. The optimal phase sensitivity detection can be realized by controlling the driving field parameters of the cavity, and the precision level can reach the picosecond level. Compared with the original synchronization method, this scheme can effectively improve the time measurement accuracy, without using expensive atomic clock and measuring the pulse arrival time.

In order to improve the calibration frequency of optical satellite remote sensors, a high-frequency absolute radiometric calibration method based on the surface hyperspectral bidirectional reflectance distribution function (BRDF) model is proposed, and the long time series absolute radiometric calibration of Suomi-national polar-orbiting partnership spacecraft (Suomi-NPP) visible infrared imaging radiometer suite (VIIRS) is realized. The principle of the absolute radiometric calibration method based on the surface hyperspectral BRDF model is introduced. In April 2018 and August 2018, the surface directional reflectance of the Dunhuang test site was measured by the hyperspectral BRDF manual measurement system, and the hyperspectral BRDF model parameters of the Dunhuang site were inversed based on the semi-empirical kernel-driven model. Based on the BRDF models, the apparent reflectance of VIIRS M1-M11 bands in the whole year of 2018 was calculated, and compared with the satellite-observed ones. The results show that the effective calibration number of Suomi-NPP VIIRS is 51 in 2018, the relative deviation of model-calculated apparent reflectance and satellite-observed ones of the M1-M7 bands is less than 3.23%. The proposed method can effectively improve the calibration frequency of satellite and track the change of radiometric characteristics of the load in time.

The problem of distance structure difference between the word vectors and visual prototypes of remote-sensing scene classification seriously influences the performance of the zero-shot scene classification. Herein, a fusion method based on analytical dictionary learning is proposed to exploit the consistency among the different kinds of word vectors for the performance improvement of the zero-shot scene classification. Firstly, the common sparse coefficients of different kinds of word vectors of scene classification are extracted by analytical dictionary learning method and acted as the fused word vector. Secondly, the visual prototypes are embedded into and structure-aligned with the fused word vector by analytical dictionary learning method similarly, to reduce the distance structure inconsistency. Finally, the prototypes of the unseen classes in the image feature space are obtained via joint optimization, and the nearest neighbor classifier is used to complete the classification of remote-sensing scenes from the unseen classes. Quantitative and qualitative experiments are also conducted on three remote-sensing scene datasets with the fusion of various word vectors. The experimental results show that the fused word vector is more structure-consistent with the prototypes in the image feature space, and the zero-shot classification accuracies of the remote-sensing scenes can be significantly improved.

In the long-term operation of distributed feedback laser, there exists a problem of wavenumber drift of the output laser center; this affects the measurement accuracy of gas concentration by fixed-point wavelength modulation spectroscopy. In order to solve this problem, a calibration-free fixed-point wavelength modulation spectroscopy based on the wavenumber drift-correction algorithm is proposed for the calibration-free measurement of gas concentration. The absorption spectrum at 4958.97 cm-1 is selected to measure the concentration of CO2 gas for validating the accuracy of this method. The experimental results show that the proposed correction algorithm overcomes the adverse effect of wavenumber drift of the output laser center on the measurement, and effectively improves the accuracy of gas-concentration measurement.

In order to solve such problems as difficulty in directly reading the spectral imaging of Rowland grating and the complex imaging algorithm for flat-field concave gratings, we design a spectrophotometer based on principle of the holographic concave grating for biochemical detection of specific protein analyzers. This structure is based on the Rowland grating, and uses two lenses as the transition elements to transform the curved surface imaging of Rowland grating to the flat-field spectrum. Meanwhile, we simulate and optimize the structure with the optical design software Zemax. The optimization results show that the spectrophotometer whose working band is 320-800 nm with the highest resolution of 0.5 nm and the overall resolution of 1.7 nm, can be directly read by the photodiode array and can achieve plane spectrum. The structural characteristics meet the measurement requirements of specific protein analyzers. The experimental results of the prototype are consistent with the simulation results, and the repeatability, accuracy, stability, and linearity of the instrument are all up to the industry standards.

The absorption-emission spectra of CdSxSe1-x/ZnS quantum dots (QD) dispersed in the aqueous solution and their photoluminescence (PL) intensities varying with time are measured using an ultraviolet-visible-near infrared spectrophotometer and steady state/transient PL spectrometer, respectively. The variation in the PL lifetime with the QD size, x, and temperature is obtained. The measured PL lifetime is mainly decided by the interband-direct transition of QD, and the indirect transition of defect states is the second factor. Empirical formulas are presented for PL-peaking wavelength and PL lifetime depending on the QD size and x. The results demonstrate that the PL lifetime increases as the increasing particle size and decreases as the increasing proportion of the S component. However, it is insensitive to temperature. The measured PL lifetime is approximately 2.51-3.22 μs for the QDs with the size of 4.06-9.22 nm when the x is 9.45-0.366 and the temperature is 15-55 ℃.

SiO2 sol is prepared using the sol-gel process with tetraethyl orthosilicate as the precursor, and the porous SiO2 antireflective coating is fabricated using the dip coating method. Further, the surface of the SiO2 coating is covered using a methyltriethoxysilane (MTES) prepolymer. The objective of the experiment is to use the hydrophobic MTES prepolymer film as a cover of the surface of the SiO2 coating and to modify the surface of the coating. Ultimately, we intend to improve the environmental stability of the porous SiO2 coating. Thus, an surface-modified composite coating exhibiting a peak transmission of 99.67%, refractive index of 1.231, and water contact angle of 123.6° is obtained. Furthermore, the peak transmission of the coating becomes as high as 99.09% and its stability considerably improves after being maintained at a relative humidity of approximately 95% for 475 days. It can also be observed that the modified coating surface is smooth and that the laser-induced damaged threshold is 24.5 J/cm2.

The interaction process between a nanosecond hyper-Gaussian laser pulse and a fullerene C60 molecular medium is studied by numerically solving the paraxial wave and particle rate equations through the hyper-Gaussian laser pulse Crank-Nicholson difference method. The evolution of the strong hyper-Gaussian laser pulse with different orders is simulated. Optical power limiting behavior caused by reverse saturable absorption is observed when the strong hyper-Gaussian pulse is propagating in the C60 molecular medium. The spatiotemporal shape of the hyper-Gaussian pulse can be obviously reshaped during propagation, and incident pulse with flat-topped time envelope distribution and central symmetry turns into the pulse with tip laser pulse distribution and asymmetry gradually. The pulse width decreases by an order of magnitude owing to the strong reverse saturable absorption of C60. Thus, the higher the order of the hyper-Gaussian pulse is, the more the reverse saturable absorption is, and the narrower the width of the output pulse is.

In cone-beam computed tomography (CT) systems, due to the difficult placing of the rear collimator, uncorrected scatter signals can cause a deviation of the measured values from the true ones, reducing the image contrast and signal-to-noise ratio and even producing artifacts. However, a beam stop array (BSA) can be used to effectively estimate the scatter signal distribution after the beam passing through the sample. In this study, the BSA correction method is applied for a cone-beam micro-CT system; the scatter distribution is obtained by placing a correction plate between the X-ray source and the sample. First, the basic principle of BSA-based scatter correction is introduced and the specific experimental devices and steps are given. Then, several samples are analyzed with an independently developed cone-beam micro-CT system. Finally, the quality of the obtained images is evaluated based on digital radiography projections, reconstruction slices, three-dimensional reconstruction images, and so on. Results show that the BSA correction method can effectively reduce the scatter artifacts in cone-beam micro-CT systems and improve the image quality, which verifies the feasibility of applying BSA correction method to cone-beam micro-CT systems. Given the low energy of the X-ray source and the large influence of the focus drift, beam-hardening correction and focus-drift correction are also included, further improving the image quality.