Considering the broadening of the laser pulse time delay caused by multiple scattering in ocean channels, we investigate the modeling of underwater wireless laser pulse responses using Monte Carlo simulations and Gamma functions. Seawater optical characteristics are analyzed, and a closed-form expression of a multi-Gamma function is proposed to simulate the impulse response in underwater channels. The theoretical analysis and modeling results show that the model uses four Gamma functions to represent laser pulse transmission under water, i.e., four paths with different lengths generated owing to the difference in the scattering intensity. The first three paths are mainly quasi-ballistic light paths with low scattering series and short optical path, whereas the last one presents a high scattering series and multiple scattered light. Simultaneously, the model accuracy and superiority are verified via comparison, and the proposed method is used to simulate the impulse response of laser transmission under different system parameters. The obtained results well describe the laser pulse scattering characteristics and time delay broadening under water.

Herein, the Monte Carlo method of vector radiative transfer is used for comparing the difference between the reflectances of spherical and plane-parallel ice cloud atmospheres. The variations of polarized radiative transfer characteristics of the ice cloud atmospheres with the optical thickness, effective radius, ice water content, relative azimuth angle, surface albedo, and ice crystal model are calculated. These characteristics are obtained at three wavelengths (0.65, 0.85, and 1.55 μm) and a large zenith angle (85°). The results denote that the difference between the ice cloud atmospheric reflectances of two atmospheric modes increases with the increasing zenith angle. This difference can reach a maximum of 55%. For a range of wavelengths, ice crystal models, and relative azimuth angles, the atmospheric reflectance of ice clouds is observed to considerably vary in the spherical atmosphere mode, and the variation in the atmospheric polarization of ice clouds is complex. Further, the atmospheric reflectance and ice cloud polarization are sensitive to the changes in the ice cloud optical properties and surface albedos. The ice cloud atmospheric reflectance increases with the increasing optical thickness, mass concentration of ice water, and surface albedo, whereas the polarization degree decreases. Furthermore, with increasing effective radius, the ice cloud atmospheric reflectance decreases, whereas the polarization degree increases.

We investigate the coherence properties of a cosine-Gaussian-correlated Schell-model (CGSM) pulse scattered from a quasi-homogeneous random medium. Further, we derive an analytic expression to obtain the temporal coherent degree of the scattered field in the far-zone and investigate the influences of the pulse parameters and properties of the medium on the temporal coherent degree of the scattered field. The numerical calculations denote that the cosine order plays an important role in the distribution of temporal coherent degree of the scattered field. Furthermore, we compare the coherent characteristics, similarities, and differences between a CGSM pulse and a Gaussian Schell-model pulse.

The bandwidth and spurious-free dynamic range (SFDR) of the microwave photonic link can be improved by suppressing the nonlinear distortion factor in the microwave photonic link. A linearization scheme based on double-electrode Mach-Zehnder modulator and balanced detection is proposed. This scheme combines phase control, optimized bias voltage, and balanced detection techniques. Theoretical analysis results show that the proposed system can eliminate second-order harmonic distortion(HD2), as well as suppress third-order inter-modulation distortion(IMD3) and fifth-order inter-modulation distortion(IMD5). Simulation analysis carried out on OptiSystem programming environment shows that the second-order inter-modulation distortion and HD2 are completely eliminated, and IMD3 and IMD5 are mainly suppressed under the background noise. Among them,the IMD3 is suppressed by about 45.76 dB, the third-order SFDR reaches 114 dB·Hz 2/3, and the fifth-order SFDR reaches 137.97 dB·Hz 4/5.

In this study, we analyze the focusing properties of the cylindrical vector beams through a subwavelength grating lens with a negative refractive index and the effects of grating material and geometric parameters on the focusing properties. Here, the equivalent negative refraction can be attributed to the subwavelength grating's diffraction order of -1, and the focusing plane-concave lens can be designed using the equivalent optical path method. Further, we use the finite element method in a cylindrical-symmetric coordinate system to simulate and analyze the effects of different material refractive indices, equivalent negative refractive indices, and predetermined focal lengths on the focusing properties. The results denote that the material refractive index significantly affects the energy efficiency of the focal field, the focus size is minimized by an equivalent negative refractive index of -1, and the short predetermined focal lengths yield small focus sizes. The proportion of longitudinal electric-field components in the focal field is the main factor that determines its lateral size. Therefore, the optimum focusing effect can be obtained by selecting an appropriate negative refractive index of the grating and a material refractive index and optimizing the design of the plane-concave lens of the negative refractive grating. This study provides a reference for modulating the focal field of cylindrical vector beams and designing micro/nanostructured devices in related fields.

A wavefront reconstruction error analysis method based on the phase diversity (PD) method for diffraction optical systems is proposed. Based on the wavefront reconstruction model of the diffraction optical system, the physical factors which cause wavefront reconstruction error are analyzed, and the influencing factors are theoretically modeled. Furthermore, the analysis method for wavefront reconstruction error is established. Taking a diffraction optical system as an application example, the influences of various physical factors on the wavefront reconstruction error are analyzed, and the error analysis model and method are verified. The experimental results show that the average deviation between the wavefront reconstruction error obtained by the error analysis model and the actual error is less than 13.5%. The method proposed in this paper can be used as the basis of application of space based diffraction optical systems.

This study proposes a new algorithm for designing diffractive optical elements to solve the issues resulting from the fact that geometric mapping theory does not involve the diffraction effect, which results in an inability to obtain a precise output beam and a serious ringing effect of the output beam. The proposed algorithm sets a parameter for reflecting the diffraction effect. The optimal value of this parameter is determined using an iterative method. The initial phase is modified using the the improved Gerchberg-Saxton (GS) algorithm, and the required phase distribution of the diffractive optical elements is finally obtained. When compared with the traditional GS algorithm, the proposed algorithm can effectively suppress the speckle and ringing effect and obtain an output beam with improved uniformity.

In this paper, we propose a novel algorithm for compensating the common phase noise in coherent optical orthogonal frequency division multiplexing system. The proposed algorithm adopts the common phase noise blind estimation method which is based on the two-dimensional projection histogram,and a pilot-aided phase noise estimation method is combined, thereby realizing the effect compensation for the common phase noise. First, the common phase noise of frequency domain symbols caused by few comb-type pilots can be estimated initially and compensated. Second, by using the two-dimensional projection histogram, the constellation of received signals can be mapped in two-dimensional digital images, and the fine compensation for the common phase noise can be achieved. To verify the performance of our scheme, we build a 20 Gbit/s simulation system based on the commercial OptiSystem and MATLAB over 50 km standard single mode fiber transmission. The corresponding results show that, the proposed algorithm can solve the problem that the bias with period of π/2 exists in the projection histogram blind estimation algorithm. Compared to the traditional pilot-aided scheme, our scheme uses only fewer numbers of pilots and achieves better bit error ratio performance.

A recognition method for multiclass intrusion events based on zero-crossing rate feature extraction is proposed; in this approach, the intrusion signal is processed by segments, and the zero-crossing rate of each segment is used as the input feature vector for the pattern classifier. The support vector machine (SVM) classification and recognition algorithm is adopted to classify and train numerous intrusion data and save the model parameters. In unknown intrusion events, the feature vector is extracted and fed into the trained SVM model to realize high-efficiency and high-accuracy pattern recognition. A Michelson interferometer-based fiber perimeter security system is developed and a 2-km-long fiber optic cable is installed in the outdoor fence for experimental verification; 120 groups are used with a total of 600 experiments being performed under five different cases: shearing cable, climbing fence, swaying fence, tapping cable, and no intrusion. Experimental results confirm that the proposed method can quickly and accurately identify the tested types of common event signals; the average recognition rate reaches 97% and the response time is up to 0.1 s.

Based on a interferometric optical fiber hydrophone, a fiber Bragg grating (FBG) hydrophone towed array with a diameter of 20 mm is realized. This single-fiber hydrophone array without splices is realized by using the multi-wavelength FBG continuous writing technique. The average acoustic pressure sensitivity of a hydrophone unit is -143.9 dB, and the fluctuation of sensitivity is limited within 3 dB. To suppress the crosstalk in the FBG hydrophone array, weak-reflectivity FBGs are utilized,reducing the crosstalk to 53 dB. Moreover, polarization-induced fading is eliminated by three-channel polarization diversity receiving. A 32-element FBG hydrophone towed array with a diameter of 20 mm is fabricated.

In this study, a low-complexity dual-polarization carrier phase recovery algorithm has been proposed based on a simplified extended Kalman filter (EKF),which is called XY-EKF algorithm. The signal component required by the Kalman filter is reduced by estimating and compensating for the phase offset between two polarizations. Further, the phase recovery of two polarizations is performed only using the real part (imaginary part) of the X polarization state and the imaginary part (real part) of the Y polarization state, thereby reducing the complexity of the algorithm while maintaining the estimation accuracy. The XY-EKF algorithm is applied to the dual-polarization 16QAM coherent optical transmission system with transmission rate of 224 Gbit/s. The simulation results denote that XY-EKF algorithm exhibits a performance similar to that exhibited by classical EKF algorithm when the products of the laser linewidth and symbol duration are 10 -5 and 10 -4 with a 1/3 reduction in complexity. Furthermore, when the BER is 3.8×10 -3, the linewidth tolerance of the XY-EKF algorithm is 1 MHz higher than that of the EKF algorithm with an identical optical signal-to-noise ratio.

Herein, using a spectral broadening evolution testing setup, the relationship between the polarization-extinction ratio (PER) of the photonic crystal fiber (PCF) output light and spectral broadening evolution is observed. By measuring, comparing, and analyzing the lifetimes of the three tapered PCFs, we find that the tapered PCFs experience four-photon absorption processes at the pump light wavelength. The reason for the spectral broadening performance degradation of the tapered PCF is that the multiphoton absorption in the fiber core results in the color-center defect. A light source with low photon energy is proposed to reduce the multiphoton absorption. The experiment verifies that using the pump light with wavelength of 1040 nm can effectively prolong the spectral broadening lifetime of the tapered PCF.

We propose a method for increasing the viewing angle during the computational reconstruction of integral imaging to solve the problem of narrow viewing angle during integral imaging. Initially, the adjacent elemental image units are aligned and stitched for enlarging the elemental image region corresponding to each microlens based on the integral imaging principle and similarity of the adjacent elemental images. When compared with the traditional method, the proposed method can accurately integrate elemental images by using the corresponding microlens when the viewing direction considerably deviates from the optical axis and exceeds a certain range of traditional perspective. However, because of the addition of image registration and stitching of adjacent elemental images during computational reconstruction, the calculation time increases.

To effectively estimate the transmittance of the hazy images and improve the darkness of the fog removal image, an image dehazing algorithm is proposed based on convolutional neural network and dynamic ambient light. Firstly, a transmittance estimation network is designed based on convolutional neural network. Then, an image library containing paired real hazy images and transmittance images is constructed. And randomly block sampling is performed to obtain the paired hazy patches and transmittance patches which are used as training sets for training the transmittance estimation network. After that, the trained network is used to estimate the transmittance of hazy images and then smooth the acquired transmittance. At the same time, considering the problem of uneven illumination of images, dynamic ambient light is used to replace global atmospheric light. Finally, the smooth filtered transmittance and dynamic ambient light are used to restore the images. Experimental results show that the algorithm can not only effectively restore the images, but also significantly improve the brightness and saturation of the restored images.

This study proposes a novel fusion method that combines a pulse-coupled neural network (PCNN) and guided filtering to solve the issues of lacking details and virtual shadows in infrared and visible images. First, to eliminate the additional noise caused by the multi-impulse of a pixel in firing time matrix T, the traditional PCNN model is improved by simplifying its structure and adding restraint items into the pulse generating unit. Second, using original images as input, guided filtering is utilized to improve T with more edge details and salient information. Finally, based on the modified T, the weight fusion rule is adopted to obtain the fusion image. Following the firing mechanism analysis of the PCNN model, a new parameter setting method combining constraints is proposed to reduce the model's parameter setting complexity. Experimental results show that the proposed method provides efficient, satisfactory, and well-detailed fusion results, and obvious virtual shadows scarcely appear in the fusion image. Additionally, the cross entropy and space frequency indexes of the results are superior to those of other current fusion methods.

To improve the reconstruction accuracy of the focused morphology recovery technique and find the optimal focused morphology recovery algorithm, a simulation technology of image defocus is proposed in this paper, which is used to assess the reconstruction accuracy of the focused morphology recovery algorithm. A three-dimensional model is conducted and a texture image is selected. Then, the uniformly-spaced point sampling on the model surface according to the image resolution is performed and organized point clouds are generated. The acquired data obtained through point sampling is processed and the texture is mapped. At last, a set of sequence simulation images which contain the ideal image or images with noises are generated. The sequence images are used to verify the focused morphology recovery algorithm in experiments. The reconstructed depth data and the actual depth data are compared to accurately evaluate the performance of the proposed algorithm. The research results demonstrate that the image defocus simulation can assess the performance of the focused morphology recovery algorithm effectively and provide further support to find more algorithms with higher stability and accuracy.

The full-range complex frequency domain optical coherence tomography based on sinusoidal phase modulation has the advantages of good effect on suppression of the mirror image and high velocity sensitivity with the highest signal-to-noise ratio (SNR) near the zero-path difference position. It has been widely utilized in the field of Doppler imaging. However, this technology is not suitable for the sample with high-speed motion affected by phase demodulation. In this paper, we present a novel approach, i.e., the complex frequency domain Doppler optical coherence tomography, based on phase difference resolved technology. After obtaining the interference tomography signal by Fourier transform of two-dimensional interference spectrum signal, and using the differential phase of the adjacent tomography signal to reconstruct the complex tomography signal, we obtain the full-range tomography images and Doppler images after sinusoidal phase demodulation. It is shown from the simulation and experimental results that this technique can reduce the broadening of the signal spectrum due to high-speed motion, so that more accurate Doppler phase shift and larger velocity detection range can be obtained.

This study proposes an improved underwater polarization-difference imaging model to overcome the problem of distortion that occurs in image processing based on conventional underwater polarization-difference imaging model. By this method, the inversion process and correction parameters are introduced for the correction of target forward-scatter intensity,thereby avoiding the image distortion caused by the global parameters’ direct inversion and the negative local gray values associated with the traditional underwater polarization-difference imaging model. Using active linear-polarization illumination and underwater-target imaging detection, we obtain the polarization image data with an FD-1665 polarization imager. Using objective image-quality evaluation parameters, we compare multiple sets of experimentally processed images. Results show that the proposed model effectively improves the image-contrast value by more than 40% in comparison with those obtained by the conventional model, indicating the effectiveness of the improved polarization difference imaging model.

The segmented primary mirror is employed to construct the next-generation ground- and space-based large telescopes. Because of many disturbances, the segmented mirrors within the segmented primary mirror easily deviate from the desired position, leading to rigid misalignment. In particular, the piston error of segmented mirror along the direction of local optical axis and the tip/tilt error seriously affect the imaging quality. The statistical properties of the misaligned discontinuous wavefront caused by piston and tip/tilt errors and the centroid of the image field of the misaligned segmented optical system are analytically evaluated from a statistical perspective. Further, the Strehl ratio is calculated based on the Maréchal approximation by combining the statistical properties from the random discontinuous wavefront. The results of this study can be used to improve the understanding of the performance of the segmented imaging system.

A wavefront analysis method of pinhole point-diffraction based on waveguide theory is proposed in this paper. In this method, the pinhole is treated as a circular waveguide and the modes in the waveguide are calculated by analytical method to acquire the electric field distribution at the back surface of the pinhole. Next, diffraction wavefront in the far field is derived by the vector diffraction theory. Finally, the pinhole transmittance, the intensity and the phase of the diffraction wavefront are analyzed in detail. Results show that the boundary conditions of the waveguide cause the rotation asymmetry of the electric field components at the back surface of the pinhole, and then introduce the astigmatism aberration into the diffraction wavefront. In order to make the transmittance larger than zero, the pinhole diameter must be greater than 0.6λ to ensure that the modes in the pinhole meet the waveguide transmission conditions. The difference of the amplitude distribution between two electric field components in diffraction wavefront leads to rotational asymmetry of the intensity of the diffraction wavefront, while the difference of the phase distribution makes the diffraction wavefront elliptically polarized. Simulation results obtained in this method provide vital reference for the design of the pinhole structure in the point-diffraction interferometer.

Polarization detection tests are carried out on China's self-developed Scanning Polarimeter (POSP), mainly including on-ground tests and in-flight tests. Among them, the on-ground tests are to scan the sky to obtain the data of polarization degree and radiation luminance of the sky, while the in-flight tests are to scan the earth's surface to obtain the data of polarization degree and radiation luminance of the earth's surface. In order to compare and analyze the data of polarization degree and radiation luminance obtained by POSP, a three-beam simultaneous polarization camera is equipped for polarization detection during polarization detection tests. The results show that the data of polarization degree and radiation luminance obtained by the two polarization instruments have good consistency, and the validity of the data obtained by POSP in polarization detection tests is preliminarily verified, which can provide an effective basis for the inversion of atmospheric aerosol parameters in the later stage.

The active ranging method cannot be applied to high-altitude inclined-angle large-area aerial imaging because of the limits of operating range and impulse frequency of the laser range finder. Aiming at the problem, the location algorithm based on multiple stereo imaging measurements is developed for the same target region. According to platform position and attitude information measured by airborne position and orientation system (POS) and outer and inner gimbal angel of aerial camera, a mapping relation between the geographical position of target area and image pixels is built. The ellipsoidal earth model is used to solve the initial geographical positions of the points in target area, and Kalman filter is applied to autoregressive prediction. The influence of measurement error on location accuracy is analyzed with Monte Carlo method. The simulation results show target location accuracy is less than 20 m and 10 m after stereo imaging for 40 times and 180 times, respectively. The validity of the target location algorithm is verified by flight tests in which the aircraft flies at altitude of 9800 m and target inclined angle is 78°. Target location accuracy through stereo imaging for 40 times is less than 35 m, which meets the requirement of engineering applications.

This study proposes a new technique for surface measurement by using fringe-based optical flow. The principle of measuring surface shape by optical flow method is introduced, and the changes of projected fringes are analyzed from the perspective of optical flow. The theoretical relationship among the optical flow, height distribution, and phase distribution of the measured surface is established under parallel projection. A numerical simulation conducted with an established spherical crown geometric model shows that the optical flow method can be used to directly calculate the height of the measured object. Practical measurement of the object and comparison of the measured results with those of the phase shift method reveal that the optical flow method can accurately restore the object's phase; in addition, it demonstrates good robustness to the noise emanating from the void area of the measured object. Unlike traditional surface shape measurement techniques, the optical flow method only needs two frames to accurately restore height or phase distributions. Because the optical flow method itself contains the time factor and only requires two images to directly obtain surface shape distribution, it is more suitable for dynamic measurement than the phase shift method.

The tunable delay of the silicon nitride micro-ring can be indirectly obtained by measuring the phase characteristics with the high-frequency resolution optical vector-network analysis method. However, the delay-spectrum-measurement stability of the silicon nitride micro-ring is seriously affected by some factors such as phase noise, laser-carrier-frequency fluctuation, and signal-to-noise ratio of the measurement system. This study experimentally analyzes these effects on the delay spectrum measurement of the silicon nitride micro-ring. By optimizing the measurement system and data processing method, a high-resolution measurement is achieved with delay and extinction ratio resolutions of approximately 10 ps and 0.04 dB, respectively. This work provides important reference value for the silicon nitride micro-ring measurement and paves the way for its applications in microwave photonic beamforming systems.

A current node correction method is proposed herein to solve the nonlinear variation of optical frequency during continuous frequency modulation of semiconductor laser sources. First, the proposed method establishes the relationship between the laser drive current nodes and the position of the minimum points of the actual beat signal; then, the current node parameters are compensated according to the position deviation between these actual minimum points and the ideal points. After several iterations, the laser frequency modulation is nonlinearly corrected. A fiber-optic frequency-modulated continuous wave laser interferometric ranging system is realized, and the linearization output of the distributed feedback semiconductor laser is obtained via the proposed current node correction method to perform a ranging experiment. The results show that the proposed method is a simple and effective calibration method, and a standard deviation below 11 μm and a linearity of 0.03% within the measurement range of 800 mm are obtained. The proposed method can be widely used for frequency-modulated continuous wave interference.

Mid-infrared tunable laser is widely used in several fields, such as medical, military, and environmental protection. Herein, a tunable parametric laser with a central wavelength of 2.6 μm is generated by pumping a KTP crystal based optical parametric oscillator (OPO). The OPO employs an extra-cavity single resonance structure. Type-II phase matching is used to obtain a maximum nonlinear coefficient. Experimental results demonstrate that the wavelength of the oscillator can be adjusted in the range of 2.4-2.8 μm. The maximum pulse energy is 12.6 mJ with a pumping energy of 155 mJ, whereas the conversion efficiency is 8.1%. The beam quality is measured to be five-fold of the diffraction limit.

We investigate visible to near-infrared supercontinuum generated by random fiber laser structure. Half-opened cavity random fiber laser is established, using germanium-doped fiber with several kilo-meters to provide random distributed feedback and Raman gain and ytterbium-doped fiber to supply active gain. Supercontinuum covering 600-1700 nm with 20 dB bandwidth of more than 660 nm is achieved. It shows that random fiber laser can be a novel supercontinuum source, which can be utilized to many applications where high robustness and low cost are required.

For a stereo matching method based on deep learning, network architecture is critical to ensuring the algorithm's accuracy; efficiency is also an important factor to consider in practical applications. A stereo matching method with a sparse cost volume in the disparity dimension is proposed herein. The three-dimensional sparse cost volume is created by shifting right-view features with a large step to substantially reduce the memory and computational resources in a three-dimensional convolution module. The matching cost is nonlinearly sampled via multiclass output in the disparity dimension, and the model is trained by merging two types of loss functions, such that the proposed method's accuracy is improved without any notable reduction in efficiency. The proposed algorithm reduces running time by about 40% while improving accuracy compared with the benchmark algorithm on the KITTI test dataset.

To improve the robustness of object tracking in complex situations, a new algorithm based on adaptive feature updating is proposed. First, hierarchical deep and hand-crafted features are simultaneously extracted from the object, and multiple fusion feature experts are constructed through multi-feature fusion by using different linear combination methods. Second, the credibility score of each expert is computed and the highest score is selected as the tracking feature of the current frame. A position correlation filter is then constructed to predict the frame's target position. Finally, the reliability of the tracking result is detected. When this reliability is found to be lower than a certain threshold, the fusion feature updating mechanism is initiated, and the temporal and semantic informations are added to the re-track, which reduces the error accumulation of the model. The proposed algorithm is tested on OTB-2013 and OTB-2015 datasets, and the obtained results are compared with those of 9 recently developed popular algorithms. Our proposed algorithm demonstrates a higher success rate and better robustness in complex situations, such as fast motion, background clutter, motion blur, and deformation, than existing algorithms.

A method that combines facial dividing feature line and point heatmap regression is proposed to address the problem of low accuracy of face landmark detection caused by different facial feature types and scales in the cases of large posture changes and occlusion. A deep learning model based on two-stage stacked hourglass network is designed to realize feature analysis and landmark location. Based on the proposed method, the detection algorithm is developed, and the proposed method is compared with other methods by experiments based on several common image datasets. The experimental results show that the proposed method can adapt to the applications of large posture changes and face partial occlusion. Compared with other methods, the proposed method has less detection error and higher accuracy in face landmark detection.

Herein, Sr5MgLa2-x-y(BO3)6∶xBi 3+, yM (M = Eu 3+, Y 3+) (0≤x, y ≤ 1) phosphors are synthesized using the high-temperature solid phase method. Further, scanning electron microscopy and X-ray powder diffractometer are utilized to characterize the morphology and structure of the samples, and an ultraviolet-visible spectrophotometer and a fluorescence spectroscope are utilized for characterizing the luminescent properties of the samples. The results denote that the samples are pure phase in terms of the doping concentration. Sr5MgLa2-x(BO3)6∶xBi 3+exhibits a single peak blue emission with a crest value of 431 nm under an excitation of 339 nm. This emission can be attributed to the transition of Bi 3+from 3P1 to 1S0, and the quenching concentration is x=0.24. The Sr5MgLa1.76-y(BO3)6∶0.24Bi 3+, yY 3+ emission peak intensity intensifies with an increase in the Y 3+ concentration, and the excitation peaks denote a red shift. With the excitation of an ultraviolet light, the Sr5MgLa2-x-y(BO3)6∶xBi 3+, yEu 3+phosphor causes blue emission, which originates from Bi 3+, and red emission, which originates from Eu 3+. Further, there is an energy transfer process from Bi 3+ to Eu 3+, and the energy transfer efficiency can be calculated based on the fluorescence decay curve. Finally, an adjustable blue to red emission can be obtained by changing the doping amount of Bi 3+ and Eu 3+ in the Sr5MgL a2-x-y(BO3)6∶xBi 3+, yEu 3+ phosphors or by changing the excitation wavelength.

Cooled detectors have the advantages of high sensitivity, fast response, and long detection distance as compared with uncooled detectors. Therefore, cooled detectors are widely used in infrared optical systems. To effectively suppress stray light, the cold stop of cooled detectors must be at the real exit pupil position of optical systems. In this study, a method for designing cooled off-axis reflective optical systems is proposed. In particular, an unobscured design is achieved by using an offset aperture stop and a biased input field. The initial configuration of cooled off-axis reflective optical systems is directly obtained based on vector aberration theory. A freeform-surface off-axis three-mirror optical system is designed for a long-wave infrared cooled detector that meets the cold stop matching condition. The F number of the designed optical system is 2.5, the focal length is 300 mm, and the field of view is 3°×5°. Freeform surfaces are used to correct the aberrations of the designed optical system and ensure that the designed system has a good imaging quality. The mirrors are free from tilts and decenters, making the designed optical system to be easily aligned.

This study proposes an optimization method to design the main baffle for suppressing the influence of stray light on the imaging performance of a space camera and satisfying the stringent requirements of small weight and high stability. Further, the position distribution of the vanes of the baffle is analyzed based on the graphical method. A multi-objective optimization model of the baffle is established with regard to minimum structural compliance and mass as well as the constraint of the fundamental frequency. The optimized dimension design of the baffle can be obtained. The mass of the baffle is 0.24 kg, and the lightweight rate is 38.6%. In addition, the dynamic vibration test of the baffle and stray light analysis of the system are conducted. The results denote that the relative error between the finite element analysis results and experimental data is lower than 15% and that the baffle exhibits good stray light suppression performance. This indicates that the performance parameters of the designed baffle satisfy the requirements; furthermore, the feasibility of the optimization method is verified.

In the testing of convex hyperboloid surfaces, the required auxiliary mirror is too large to manufacture. A new method of testing is proposed herein to solve this problem. Based on the Hindle test, the correcting lens and the spherical mirror produce a ray that has no aberration and is incident to the test surface. The distance between auxiliary mirror and hyperboloid mirror is short. This reduces the aperture of the auxiliary mirror, while the effective aperture does not reduce. The formula is derived based on the third-order aberration theory. A test optical system is designed, wherein the aperture of the mirror being tested is 800 mm, the vertex radius is 1800 mm, and the conic constant is -2.25. The peak-valley value of the residual aberration is 0.0003λ (λ=632.8 nm), and the root mean square is 0.0001λ. The analysis shows that this method can be used to test large aperture and large relative aperture convex hyperboloid surfaces with a small auxiliary mirror and short length.

By employing auxiliary waveguides between the exit ends of arrayed waveguides, we experimentally demonstrate a SiON-Based cyclic arrayed waveguide grating router (AWGR) with 7 input and 7 output channels (7×7) and 400 GHz channel gap, to improve the loss uniformity. The field distribution at the image plane of the output free propagation region is adjusted by the auxiliary waveguides, in the other words, a flat-top shape can be obtained at the image plane by optimizing the parameters. The measured loss non-uniformity of the AWGR with auxiliary waveguides improves from 2.09 dB to 0.76 dB for the central input channel and from 1.99 dB to 0.88 dB for the edge input channel compared to the conventional AWGR. The minimum insertion loss of the central channel increases from 2.99 dB to 3.82 dB, and that of the edge channel increases from 4.83 dB to 5.46 dB. The crosstalk of all the channels is approximately 18 dB.

This study proposes a photonic crystal nanobeam (PCN) side-coupling aperture chirped photonic crystal nanobeam cavity (PCNC) structure based on the Fano resonance principle. The Fano resonance can be realized by the destructive interference between the wide continuous state produced by the PCN structure and narrow discrete state generated by the PCNC structure. The generation mechanism of Fano resonance in the structure is qualitatively analyzed via the coupled mode theory, and the structure is simulated by using the finite-difference time-domain method. After quantitatively studying the influences of the structural parameters on the refractive-index sensing characteristics of the system, the structural parameters are optimized. Results show that the optimized figure of merit is as high as 5.1×10 3, providing an effective theoretical reference and technical guidance for the design of integrated photonic-crystal waveguide sensors.

We develop a reflective-mirror-based optical tracking device actuated by the electrowetting effect. It comprises a transparent rectangular cavity, rubber ring, and mirror. When a voltage is applied to the conductive liquid on both sides of the cavity, the conductive liquid on both sides’ surges in the middle. Consequently, the height of the intermediate liquid varies and the mirror forms a tilt angle, thereby changing the deflection direction of the incident light beam, and thus realizing optical tracking. Experimental results that the beam navigation angle can reach 0°-9° with a fast response time (approximately 90 ms). The proposed device has potential applications in space optical communication and radar detection.

In this study, we construct a periodic array of a single layer graphene ribbon based on the surface electric current boundary condition. Further, the periodic array of graphene ribbon is investigated using a finite-difference time-domain simulation and the coupled-mode theory. The results show that the graphene array exhibits a typical surface plasma double-resonance effect in the mid-infrared band. Its periodic array structure can enhance the resonance effect of graphene surface plasma local field. The resonant wavelengths, electrical field energy, attenuation rate, and lifetime of the resonance mode of graphene surface plasma are analyzed. The simulation results demonstrate that the resonance wavelength can be tuned from 16.81 to 11.13 μm when the graphene Fermi energy is increased from 0.4 to 0.8 eV and that the plasma lifetime can be regulated from 21 to 223 fs when the graphene carrier mobility is increased from 0.1 to 1.0 m 2·(V·s) -1. Furthermore, the quality factor increases nonlinearly from 0.14 to 0.19 when the refractive index of the superstrate is increased from 1 to 2. The research methods and conclusions of this study are useful for designing graphene plasma devices.

A method to quickly detect defects in chemical vapor deposition (CVD)-prepared graphene is proposed. After transferring the graphene prepared by CVD to the target substrate, a graphene-gold substrate is prepared for surface plasmon polariton (SPP) imaging. Since SPP imaging is highly sensitive to the change of refractive index on the interface, it is used for detection of the graphene edge. Furthermore, as the surface defects of the graphene change the distribution of SPP fields, the SPP field distribution transfers to the far-field due to SPP leakage radiation effect, which is quickly imaged by charge-coupled device. Herein, the morphologies of graphene edge and surface, defects, and impurities are detected. This method is an improvement on previous low sensitivity, low speed as well as damage detection of traditional detection method, and it achieves high-sensitivity, high-speed, and nondestructive detection for graphene.

Continuous-variable quantum-key distribution based on the local-oscillator approach has been encountered with new security issues due to reference pulses transmitted over insecure quantum channels. Herein, a attack method for eavesdropping and tampering with phases of reference pulses is proposed, allowing the phase-compensation error in the receiver to be increased and secure-key rate of practical systems to be decreased. The secure-key rate of a practical system under phase attack of reference pulses is analyzed using the phase-compensation noise model. Moreover, a method for detecting phase attack is proposed, wherein the phase-compensation-noise variances are monitored. Simulation results show that the secure-key rate estimated by training signals is consistent with its theoretical value, and phase attack can be detected by monitoring phase-compensation-noise variances of the training signals and reference pulses.

A system modeling method based on solar vector is proposed to improve the system modeling and pointing accuracy of convex mirrors which are used as reflective light sources. The fitting coefficient (R2) of 0.999 or higher obtained by linearity fitting of experimental data ensures the reliability of our experimental data. The model is solved by the least-squares method,and inherent geometric errors of the system are obtained. Finally, the correctness of the calibrated model is verified by using the reverse model to solve the target value and comparing the centroid of the sun image. The experimental results show that the angle of the target encoder is essentially the same as that of the actual measurement. To summarize, the standard deviation of pitch angle error is 0.0043°, standard deviation of azimuth angle error is 0.00299°, error range is kept within 0.04°, difference of image centroid pixel is approximately 2 pixel, angular resolution error of pixel is approximately 0.036°, and comprehensive pointing accuracy of the system is better than 0.1°. Therefore, the validity and feasibility of this method are verified; this lays the foundation for the on-orbit absolute radiometric calibration of multi-spatial resolution optical remote-sensing satellite sensors with high accuracy, high frequency, and operational and full dynamic range.

A novel detection framework is proposed based on a multiscale convolutional neural network (MSCNN) to overcome low precision and insufficient generalization ability associated with existing object detection methods for multiscale objects with complex scenes. First, an essence feature pyramid network is constructed to enhance the extraction ability of multiscale features. Then, the focal classification loss is introduced as classification loss function to enhance the learning capability of the MSCNN over complex samples. The proposed method achieves a mean average precision(mAP) of 0.960 over the challenging NWPU VHR-10 dataset. In comparison with the RetinaNet detection method, the mAP of the proposed MSCNN on small- and medium-scale objects increases by 1.5% and 1.9%, respectively. The proposed method is found to be accurate and robust for multiscale objects.

An accurate modeling and verification method is proposed to improve the calculation precision of space-based optical scattering characteristics of space objects. Further, a mathematical model is established for space-based optical scattering characteristics using the bidirectional reflectance distribution function via finite element analysis and vector coordinate transformation based on the object background radiation, surface material properties, geometric dimension, and orbit elements. Subsequently, this model is experimentally verified in a low-temperature and vacuum environment using the measurement platform of the optical scattering characteristics; the modeling data agrees well with the measured data, and the mean square errors are lower than 9.57%, demonstrating the validity of the proposed method.

Soil respiration is the main process of carbon and nitrogen circulation between the earth and atmosphere. Real-time in-situ measurement of gas emissions from different ecological soil environments is a powerful tool for studying the dynamic processes of atmospheric-greenhouse-gas formation, transfer, and consumption, which provides the key scientific basis for revealing the main processes of carbon and nitrogen ecosystem cycling and environmental evolution. In this paper, we describe the development of a laser-spectroscopy system based on a room-temperature, continuous-wave, quantum-cascade laser (RT-CW-QCL), a long-path optical-absorption cell, and a direct-absorption spectroscopic detection technology for the high-sensitivity and high-precision analysis of CO and N2O gas-exchange processes between soil samples under different ecological environments and ambient air. The results show that soil samples from four different ecological environments (i.e., reed, pond, organic, and grassland soils) exhibit visibly different CO and N2O release and absorption processes.

In this study, the design scheme for a prism-based spatial heterodyne spectrometry is presented. This technology is implemented by replacing the gratings in a traditional spatial heterodyne spectrometer with dispersion prisms and plane mirrors. Axial optical path analysis is employed to derive the matching relationship between the component parameters. A theoretical simulation of the interferogram is performed using simulation software. The influences of the refractive index and vertex angle of the dispersive prism on the spectral resolution are analyzed. The experimental platform of the prism-based spatial heterodyne spectrometry is developed to provide a preliminary verification of its feasibility at wavelengths of 635 nm and 650 nm. The experimental results show that the spectral resolution is 1.07 nm when the dispersion prism is set to BK7 glass (refractive index of 1.51452 at wavelengths of 650 nm) and the vertex angle is 30°; the simulation results show that the light energy utilization rate is 94.43%. The experimental results are consistent with the theoretical results, thereby demonstrating the feasibility and scientificity of this design scheme and providing new concepts for the study of the spatial heterodyne spectrometry. In addition, the spectral resolution can be further improved by selecting a higher-refractive-index dispersion prism and increasing the vertex angle.

High-precision radiometric calibration of hyperspectral infrared sounders is crucial for its quantitative application. In this study, the relative deviation obtained by radiance calibration of infrared hyperspectral atmospheric sounder (HIRAS) on FY-3D satellite is evaluated based on infrared atmospheric sounding interferometer (IASI) on European meteorological satellites MetOp-A and MetOp-B and Y-3D satellite through simultaneous nadir overpass. According to the observation data of strictly spatial and temporal matching of both instruments, the uniform scenes are selected by using the higher spatial resolution data of medium resolution spectral imager (MERSI-II) on the same platform of FY-3D. The spectral resolution of IASI data is converted to the same spectral resolution of HIRAS through forward-inverse Fourier transform method before inter-comparison. Because the matching samples which satisfy the collocation criteria are distributed in the North and South Arctic regions with low target temperatures, the inter-comparison results are evaluated by mean and standard deviation of HIRAS-IASI brightness temperature (BT). The results show that the comparison results between HIRAS and MetOp-A/B are similar, the consistency of North Arctic regions with high temperature is better than that of South Arctic regions. HIRAS agrees well with IASI at the long-wave IR (LWIR) and middle-wave IR (MWIR) in low temperature environment with less than 1 K mean deviation and below 0.5 K in most channels. There are no apparent scene-dependent features for individual spectral channels, the standard deviation is less than 2 K, and varies with the spectral channels, slightly larger at the position where the absorption line is sharp. The spectral BT of HIRAS is lower than that of IASI with <1.5 K mean deviation in most channels at short-wave IR (SWIR), the deviation shows scene-dependent features, and the standard deviation decreases as the target temperature increases. The long-term trend of BT difference (from April to December, 2018) analysis shows that long-term consistency of HIRAS is stable, the mean deviation at SWIR is slightly larger in the lower target temperature conditions.

With the development of image recognition technology, the demand for face recognition systems with an iris recognition filter is constantly increasing. With large incident angles, recognition failure often occurs owing to a large spectral offset; therefore, a bandpass filter with good low-angle effect is required. Herein, a bandpass filter with large angle and small spectral offset is designed based on the spectral splitting principle. By adjusting the hydrogen gas flow, the refraction index of Si-H is improved while maintaining the low absorption, enhancing the angle effect of the iris recognition filter. Based on available literature on the deposition of thin films, the problem of film removal due to its loose internal structure is solved herein by adopting a special film system structure; testing results show that this structure meets the technical requirements of iris recognition filters.

Herein, to improve the reflectivity of open-type retro-reflectors at 808-nm wavelength, the impact of glass micro-beads' refractive index on the retro-reflection is theoretically analyzed. The optical simulation software TracePro is used to simulate and analyze the performances of three retro-reflector's structural units (glass micro-beads) with different refractive indexes (1.93, 2.00, and 2.20). The theoretical reflectivity of the conventional open-type retro-reflector's structural unit on the ordinary substrate is only 23.6% when the light is perpendicularly incident, however its theoretical maximum reflectivity is enhanced by 3 times, reaching 98.1%, when a high-reflectivity substrate is used. A high-reflectivity optical film having a reflectivity of 99.3% is designed and fabricated at 808-nm wavelength, and a retro-reflector on the substrate coated with such high-reflectivity optical film is prepared. Retro-reflectors using these two types of substrates are experimentally evaluated, and the results demonstrate that the glass micro-beads with the refractive index of 1.93 on the high-reflectivity substrate provide the best retro-reflectivity. At vertical incidence, the retro-reflectivity value of the retro-reflector on the high-reflectivity substrate reaches 46.2%.

In this study, we propose a spherical crystal based on X-ray fluorescence imaging system with high spectral resolution and full field of view (FOV). Further, the spatial resolution, FOV, spectral bandwidth, and photon collection efficiency are analyzed. Next, we present the design of the fluorescence imaging system for the Kα emission of a metal (V-Zn) with median atomic number according to the theoretical analysis, and the analytic theory and a custom-written Monte Carlo ray-tracing code are used in calculation and simulation. Our theoretical analysis and simulation demonstrate that the X-ray fluorescence imaging technique exhibits a high spatial resolution (6.5 mm), and high spectral resolution (<16.5 eV@4.6-9 keV).