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A piezoelectric deformable mirror for phase compensation is designed and fabricated and its piezoelectric properties are ex situ characterized by using the Fizeau interferometer. Moreover, an iterative global optimization algorithm is developed to realize a quick and accurate approach to the target figure. The in situ phase compensation performance of the mirror and its ability to optimize the focusing spot size are verified under the focusing beam mode and with the hard X-ray speckle scanning metrology. The experimental results show that the focal spot size is compressed from 43.4 μm to 12.9 μm after phase compensation. The research here lays a foundation for the realization of rapid phase compensation in the Shanghai Synchrotron Radiation Facility.
PDF Acta Optica Sinica, 2020, 40(9): 0934001
There are important practical applications of high reflective films in the far-ultraviolet band. In order to obtain high reflectance, the far-ultraviolet broadband high reflective films are prepared by depositing an Al films protected by MgF2 films with the high temperature three-step evaporation method, and these samples are annealed. The results show that the far-ultraviolet broadband high reflective films after improved preparation and annealing can possess a reflectance as high as 90% at 121.6 nm, close to the theoretical design value, and meanwhile, the effect of scattering loss is also analyzed. The narrowband reflective filter films are prepared based on the optimized LaF3/MgF2 film structure. The peak reflectance at central wavelength of 122.5 nm is 75% and the full width half maximum is 8 nm, which indicating the expected effect of the theoretical design has been obtained. However, the annealing process damages the film surface to induce the increase of scattering loss and the decrease of film reflectance.
PDF Acta Optica Sinica, 2020, 40(9): 0931001
Atmospheric and oceanic optics
In this study, we discuss the O2-O2 inversion effective cloud fraction of the absorption band observed at 477 nm. First, the O2-O2 slant column densities are retrieved in 460--490 nm using the differential optical absorption spectroscopy (DOAS) algorithm, and the stripes occurring in the O2-O2 slant column densities are corrected. Second, the SCIATRAN radiative transfer model is used to set different nodes with respect to solar zenith angle, viewing zenith angle, relative azimuth angle, surface albedo, surface altitude, cloud fraction, and cloud pressure to establish the lookup table of the O2-O2 slant column densities and reflectance. Subsequently, we invert the aforementioned lookup table to obtain the lookup table of cloud fraction. Finally, we use the O2-O2 slant column densities, continuum reflectance, and related solar geometry information to perform multi-dimensional interpolation for obtaining the effective cloud fraction for the environmental trace gas monitoring instrument (EMI). The EMI results are compared with the OMI (ozone monitoring instrument) cloud fraction to verify the accuracy of the proposed algorithm and both denote that the frequency percentages decrease from low cloud cover to high cloud cover. When the cloud fractions are 0 and 1, their frequency percentages are observed to be considerably high. The OMI and EMI cloud fractions are in good agreement and have a high correlation coefficient (R) of 0.82.
PDF Acta Optica Sinica, 2020, 40(9): 0901001
A method combining the asymptotic radiative transfer (ART) theory and discrete ordinate radiative transfer (DISORT) method is proposed in this study to retrieve the snow spectral albedo. First, based on the assumption that snow grain shape is the second generation Koch fractals, snow grain size results are retrieved using different satellite data and three methods derived from the ART theory. The average results, although different, are all close to 50 μm. Second, assuming the snow grains are spherical, snow spectral albedo is calculated using the DISORT model in the 0.3--5.0-μm region based on the retrieved snow grain size results. Snow black-sky and white-sky spectral albedo are calculated using the ART theory in the 0.3--1.5-μm region. The small difference between the two types of snow spectral albedo calculated through the two radiation transfer models in the 0.3--1.5-μm region reveals that the assumption of different snow grain shapes is reasonable. The method combining the two radiation transfer models can calculate the snow albedo of the solar spectrum. Finally, the snow spectral albedo calculation using DISORT model also considers the light-absorbing impurities such as black carbon. The study area is located in northeast border proximity to Siberia demarks region, where the impurities have a very limited influence on snow spectral albedo. While in the northeast area, which is heavily industrialized, the impurities can markedly decrease snow spectral albedo at visible wavelengths.
PDF Acta Optica Sinica, 2020, 40(9): 0901002
To rapidly detect quinolones in a water environment, a method combining three-dimensional (3D) fluorescence spectrometry and bilinear least squares/residual bilinearization (BLLS/RBL) is proposed to detect flumequine (FLU), enrofloxacin (ENR), and levofloxacin (LVFX) in water. This method not only accurately analyzes three antibiotics having serious spectral overlap, but also obtains more reliable quantitative prediction results than parallel factor (PARAFAC) method. The average recoveries of FLU, ENR, and LVFX predicted by BLLS/RBL are 98.46%, 99.10%, and 101.69%, respectively. Moreover, the respective root mean square errors are 4.33, 0.33, and 0.26 μg·L -1; the sensitivities are 2.8 × 10 3, 3.5 × 10 4, and 5.5 × 10 4; and the limits of detection are 0.72, 0.06, and 0.03 μg·L -1. Results show that the 3D fluorescence spectrum combined with BLLS/RBL provides a reliable method for the detection of quinolones in water.
PDF Acta Optica Sinica, 2020, 40(9): 0930001
Based on the two-dimensional distribution remote measurement system of air pollutants, this study examined the two-dimensional distribution remote measurement of pollutants by the passive imaging differential optical absorption spectroscopy technology combined with the image optimization algorithms. The concentration information of pollutants and the visual images were accurately matched using a dual-channel system that combines the ultraviolet spectra with the visual images. For the problems of the data processing of pollutants under complex backgrounds, particularly the problem that of data missing under conditions of dense smoke and obstacles, the observed values were screened and processed. Subsequently, the smoke plume diffusion model was combined with the curved cubic spline interpolation algorithm to optimize the concentration information in the entire area, and thus to quickly obtain the two-dimensional distribution image information of pollutants with high spatial resolution.
PDF Acta Optica Sinica, 2020, 40(9): 0930002
Fiber optics and optical communications
To enhance the physical-layer security of optical fiber communication systems, a novel anti-interception communication system based on an optical encoding/decoding technology is proposed in this study. Owing to the relatively small capacity of the traditional address codes, the address codes used by the legitimate users are vulnerable to the brute-force searching attacks by eavesdroppers. Once the user’s address code is obtained, all the information transmitted in the fiber channel will be stolen by the eavesdropper. To address this problem, a new type of 2D-wavelength-hopping/time-spreading (WH/TS) code is first constructed, and the implementation of the reconfigurable encoder/decoder is described. Then, the novel scheme of anti-interception communication system is designed, and a wiretap channel model is established. Finally, the transmission and security performances of the anti-interception communication system are studied using the VPI transmission Maker Optical Systems simulation software. The simulation results show that a high-speed and long-distance anti-interception communication system can be achieved based on the proposed optical code-division multiple-access coding/decoding technology.
PDF Acta Optica Sinica, 2020, 40(9): 0906001
Optical design and fabrication
In this study, we proposed a method to design an optical system for conducting a specific type of aircraft cabin thermal load tests. Thus, a solar radiation simulator was developed. Herein, a single lamp was designed; subsequently, the spatial structure of the lamp was studied. A reasonable installation location for the lamp was ascertained using the pre-adjustment method. Further, the optical system was processed and installed according to the design scheme. Some relative tests were also performed. The results prove that the calculation error associated with the irradiance distribution is 12.0%, demonstrating the feasibility of the proposed method. Furthermore, the deviation between the thermal effects obtained based on the calculation and design value is -3.1%, proving that the system achieves the expected requirements. The temporal instability associated with the irradiance is ±0.52%, and the system exhibits favorable stability. This study is expected to have a considerable reference value with respect to the design of a solar radiation simulator.
PDF Acta Optica Sinica, 2020, 40(9): 0922001
To improve flux uniformity on a planar receiver, we propose an improved design method for a parabolic dish concentrator via the optimization of the rearrangement of each mirror unit. A new non-imaging dish concentrator is also designed. In this study, a flux homogenization optimization model is established for the target area of the planar receiver and the dish concentrator is optimized by the motion accumulation ray-tracing method combined with the genetic algorithm. Moreover, the focusing flux distribution on the optimized concentrator and parabolic dish concentrator is demonstrated, and the local concentration ratio, non-uniformity factor, peak concentration ratio, and interception efficiency on the planar receiver are investigated. Finally, the potential applications of the new concentrator are discussed and the effect of the optimized dish concentrator on the flux homogenization of the planar metal coil receiver is verified. Results show that the non-imaging dish concentrator has the best effect on flux homogenization. It can significantly reduce the non-uniformity factor from 3.62--4.22 to 0.18--0.25 and the peak concentration ratio from 24737--37245 to 1722--2055. This work provides an advanced solution for the flux homogenization of planar solar receivers and an innovative idea to improve the existing parabolic dish concentrators.
PDF Acta Optica Sinica, 2020, 40(9): 0922002
In this study, we propose a local stereo matching algorithm based on guidance images and an adaptive support region. First,the guidance images can be obtained by preprocessing the rectified input images. During the matching cost calculation stage, we propose a gradient calculation method, which combines the gradient information of the guidance and input images to calculate the gradients along the x and y directions, respectively, and subsequently integrates the absolute difference (AD) and the Census transform to develop a matching cost calculation function. Further, we use a guided filter based on the adaptive support region during the cost aggregation stage. During the disparity refinement stage, a multi-step refinement method is proposed based on the adaptive support region and then the final disparity map is obtained. The experimental results prove that after disparity refinement, the average error (Avgerr) and root-mean-square error (RMSE) are reduced by 43.7% and 38% respectively for all the regions and by 33.7% and 30.9% for the non-occluded regions. The proposed algorithm exhibits improved robustness and can be used to obtain high precision disparity results.
PDF Acta Optica Sinica, 2020, 40(9): 0915001
The stereo matching algorithm based on convolutional neural network improves the accuracy considerably; however, most algorithms still cannot meet the real-time requirements. In this study, we propose a real-time stereo matching algorithm with hierarchical refinement, which initializes the disparity map at a low-resolution level and gradually restores the spatial resolution of the disparity map. The proposed algorithm uses a lightweight backbone network for extracting the multi-scale features and simultaneously, the features are inversely fused to achieve an improved robustness without significantly affecting its real-time performance. Furthermore, we propose a multi-branch fusion module to progressively refine the disparity map. After the different modes in different regions are automatically clustered, the residuals of the disparity map are predicted. Subsequently, the final results are combined based on the cluster weights to ensure that the regions exhibiting different characteristics can be effectively processed. Based on the KITTI test dataset, the operating rate obtained using the proposed algorithm is 20 frame/s, and the error rate is reduced by approximately 30% when compared with the DispNetC algorithm. These exhibit a comparable operating efficiency.
PDF Acta Optica Sinica, 2020, 40(9): 0915002
Tracking algorithms implemented in Siamese networks utilize an offline training network to extract features from a target object for matching and tracking. The offline-trained deep features are less efficient for distinguishing targets with arbitrary forms from the background. Therefore, we proposed a tracking algorithm for a Siamese network based on target-aware feature selection. First, the cropped template and detection frames were sent to a feature extraction network based on ResNet50 to extract the shallow, middle and deep features of the target and search regions. Second, in the target-aware module, a regression loss function was formulated for target-aware features and an importance scale for each convolution kernel was obtained based on backpropagated gradients. Then, the convolution kernels with large importance scales were activated to select target-aware features. Finally, the selected features were inputted into the SiamRPN module for target-background classification and the bounding box regression was applied to obtain an accurate bounding box of the target. Results of experiments on OTB2015 and VOT2018 datasets confirm that the proposed algorithm can achieve robust tracking of the target.
PDF Acta Optica Sinica, 2020, 40(9): 0915003
Three-dimensional (3D) display has gained wide attention for its natural and intuitive performance. Herein, we proposed a finger sleeve haptic interaction method based on a suspended 3D display system. The fingertip coordinates were extracted by Leap Motion and a binocular camera was used to assist the coordinate transformation between light field display area and interaction area. Then, the position relation between the hand and display area was examined and the signal was transmitted to vibrate the finger sleeve, thus the haptic interaction was finally realized. The results show that the proposed method can be used to complete the coordinate transformation between light field display area and interaction area with a transformation precision of within 5 mm. Furthermore, when the user interacts with a 3D display system, the effective and real-time haptic interaction enhances their real sensory experiences.
PDF Acta Optica Sinica, 2020, 40(9): 0915004
To improve the precision and robustness of 3D object detection based on stereo vision, a novel 3D object detection algorithm based on iterative self-training is proposed. To acquire the precise object point clouds for 3D object detection task, a disparity estimation algorithm based on iterative self-training is first proposed, which is capable of improving the disparity accuracy of object region by increasing the supervised signal in object region iteratively and introducing a selective optimization strategy. Then a self-adaptive feature fusion mechanism is proposed in network architecture, which adaptively fuses the features from multimodal information to obtain the precise and robust object detection results. Compared with the recent and popular algorithms based on vision system, the proposed 3D object detection algorithm achieves a great improvement in precision.
PDF Acta Optica Sinica, 2020, 40(9): 0915005
Lasers and laser optics
To study electromagnetic pulse (EMP) in the target chamber of nanosecond laser inertial confinement facility, this study uses the independently developed EMPIC-2D calculation software to perform the numerical simulation. The escaping electrons generated by the interaction of high-intensity laser pulses with solid target were considered as the input parameter and the electromagnetic pulse intensity in the chamber were considered as the as output. Results show that EMP frequency mostly distributes between 0 MHz and 2 GHz. As the width of the ejection waveform widens, the high-frequency nanosecond signal component decreases, while the low-frequency component slightly changes. Peak values of electromagnetic field (close to the electron emission point) also decrease with the extension of ejection time (1--10 ns). Compared with the simulation results of picosecond laser pulses, nanosecond simulation results have lower electromagnetic intensity. Its low frequency compositions are similar to picosecond results; however, high frequency compositions higher than 1 GHz are observed to greatly decrease.
PDF Acta Optica Sinica, 2020, 40(9): 0914001
We have designed and constructed a synchronized fiber laser system in the all-optical and passive way, which enables to deliver dual-color ultrafast pulses with long-term stability and high timing precision. This synchronization system consisting of two Er- and Yb-doped fiber lasers mode-locked by nonlinear amplifying loop mirrors adopts a master-slave configuration. The injection pulse induces a non-reciprocal nonlinear phase shift within the laser resonator cavity, and thus an adaptively synchronous mode-locking output is realized. After optimizing the experimental parameters including pulse energy of master injection and filter bandwidth within laser resonator cavity, we achieved a 26 mm tolerance range of cavity-length mismatch. The pulse synchronization system here favors advantages such as simple structure, plug and play, good stability, and polarization-maintaining output.
PDF Acta Optica Sinica, 2020, 40(9): 0936001
We study the properties of dark solitons of the nonlinear Schr?dinger equation with (2n+1)-th order nonlinearity. We give the uniform analytical expression for a static dark soliton and find that the width of the static dark soliton decreases with the increase of the nonlinear power index, and its depth remains unchanged. The evolution behavior of the moving gray soliton is studied, and the general expression of the wave function of the moving gray soliton as a function of space and time is given. It is found that if we give the speed of a moving gray soliton, the density and phase shift decrease as the nonlinear power index increases. The energy of the moving gray soliton decreases with the increase of its speed for a given nonlinear power index. Finally, the numerical simulation is given to verify the analytical results.
PDF Acta Optica Sinica, 2020, 40(9): 0927001
A measurement and verification method based on Michelson interferometers is proposed for investigating the influence of medium parameters on the temporal coherence of stimulated Brillouin scattering (SBS). The physical mechanism behind the influence of temperature on the temporal coherence of SBS is presented theoretically, and the influence of water temperature on the temporal coherence of SBS is measured and analyzed experimentally. The research results indicate that the temporal coherence of SBS is directly related to water temperature. As the temperature decreases, the temporal coherence of SBS deteriorates. When the temperature is fixed but the optical path difference of coherent light increases to a certain value, the corresponding coherent pulse waveform of SBS is no longer quasi-Gaussian.
PDF Acta Optica Sinica, 2020, 40(9): 0929001
In this study, we propose retinal vessel segmentation based on fuzzy C-means (FCM) clustering in accordance with the local line structural constraints. The pixel features are extracted via multi-scale match filter and B-COSFIRE filter of the pre-processed image, where the contrast between the vessel and the background is enhanced. Thus, retinal vessel segmentation can be realized using the FCM clustering algorithm according to the local line structural constraints. Finally, the isolated noise points are eliminated via the post-processing operation. The experiment is performed using the DRIVE database. The average accuracy, sensitivity, and specificity are 94.21%, 67.21%, and 98.2%, respectively. When compared with the traditional feature-space-based FCM algorithm, the proposed method exhibits better continuity with respect to the segmented retinal vessels and is more sensitive to the small blood vessels.
PDF Acta Optica Sinica, 2020, 40(9): 0910001
In this paper, a magnification method based on deep learning was used for the study of astronomical images. An effective astronomical image magnification method was proposed based on the structural characteristics of the new vacuum solar telescope (NVST) images. First, the Binning technology was used to down-sample the data to obtain the corresponding low-resolution images. Second, an improved residual dense network was used to fully extract and utilize the multi-level features of low-resolution images, and thus to enable the reconstruction of high-resolution solar images. Finally, the residual, correlation, and power spectrum analysis methods were used to quantitatively evaluate the reconstruction errors in solar images. The experimental results show that compared with the conventional interpolation method, the proposed method can finely magnify the small-scale structures and effectively improve the signal-to-noise ratio in solar images.
PDF Acta Optica Sinica, 2020, 40(9): 0910002
Remote sensing and sensors
In this study, a satellite calibration spectrometer (SCS), as a standard calibration load on HY-1C, was used for calibrating the remaining instruments on the same satellite platform. Further, the cross-calibration method was used to evaluate and verify the accuracy of on-board calibration by comparing the radiances and reflectances obtained via SCS and moderate-resolution imaging spectroradiometer (MODIS) with respect to the same ground area. By processing the calibration data four times on different dates, the maximum relative deviation of the radiance is observed to become less than 3%, which meets the calibration requirement with respect to the remaining loads on the HY-1C platform.
PDF Acta Optica Sinica, 2020, 40(9): 0928001
Based on the solar diffuser of the satellite calibration spectrometer (SCS), we establish an absolute radiometric calibration model. We can obtain the absolute radiometric calibration coefficient of SCS by processing the on-board diffuser calibration data obtained on December 12, 2018. The radiometric calibration coefficient of SCS is cross-validated thrice using TERRA MODIS, which is used as the reference load. In order to improve the accuracy of spectral matching and to eliminate the difference in spectra setting , the measured equivalent radiance of SCS is interpolated to obtain the spectral radiance, and subsequently the obtained results are integrated with the spectral response function of TERRA MODIS to obtain the predicted equivalent radiance. The comparison results show that the predicted equivalent radiance of each band of MODIS is in consistency with the measured equivalent radiance and the maximum relative deviation is 2.78%, which indicates that the SCS radiometric calibration coefficient is reliable and has a high calibration accuracy. It is verified that the parameters such as bidirectional reflection distribution function (BRDF) and transmittance have relatively high test precision.
PDF Acta Optica Sinica, 2020, 40(9): 0928002
In this study, a novel method for splicing LiDAR temperatures was proposed to solve the problem of low LiDAR detection heights when fog-haze conditions were encountered. Accordingly, a typical fog-haze case was selected as the research sample. High-resolution weather research and forecasting (WRF) model temperatures were specifically used to splice LiDAR temperatures. The splicing method focused on key technologies, including a fitting region selection technique, a coordinate height layer analysis method, a correction method between model data and LiDAR data, an optimal splicing region selection method, and an evaluation method for splicing results. The maximum height of splicing data was extended to approximately 20 km, including the entire troposphere and the lower-middle stratosphere. This was especially larger than the original height of the LiDAR data (2 km). According to a series of detailed quality assessments, the splicing data were very reliable, with a perfect match trend between the splicing profile and the standard profile and a maximum error of less than 1.5%. There was a better fit between LiDAR data and model data in the optimal splicing region. The advantages of both model data and LiDAR data were fully exploited in the proposed splicing method. Based on this, the data with a larger detection layer and high-quality temperature profile was reconstructed. Moreover, the proposed splicing method was also suitable for other complex weather conditions.
PDF Acta Optica Sinica, 2020, 40(9): 0928003
Instrumentation, measurement and metrology
A high-efficiency optical method was developed to solve the problems of measurement inefficiency and instability in the helix of a traveling wave tube (TWT). This method offers several improvements over existing methods. First, a device for helix placement and adjustment based on double standard bar rolling was chosen to make the rotation angle of the parts controllable and the placement straightness within 0.01 mm. Second, a linear viewfinder was used to effectively increase the stability of the automatic edge finding process by avoiding burr and overturn interference; the repeatability of the measurement results was better than 0.8 μm. Next, a port inner diameter measurement setup based on a 45° rectangular prism was developed to obtain a distinct contrast imaging effect, and the helix’s outer surface roughness was measured using a laser confocal microscope; the measurement results were in good agreement with those in process law. Finally, high efficiency and flexibility was achieved by applying a segmented array programming strategy and intersection distance algorithm for the detection of the stepping pitch helix; the detection time for a single helix reduced to one-third that of the existing method. These results are of great significance for improving the qualification rate of helical TWT products and realizing their batch production.
PDF Acta Optica Sinica, 2020, 40(9): 0912001
A wave plate is commonly used for polarization measurement and polarization calibration of a large solar telescope. In this study, we developed a measurement system based on the dual-beam polarization analyzer configuration to measure the phase retardance and azimuth angle of the wave plate, and derived the corresponding mathematical model. The influence of azimuth error of the polarizer in the single beam measurement system was overcome by fitting the azimuth of the polarizer. At the same time, we analyzed the retardance range of the measuring plate, and accurately measured the wave plate used in polarization calibration and polarization measurement. Finally, we analyzed the main error sources of the system, including the power noise of the light source, position error of the rotating stage, and nonlinear response of the detector. Subsequently, we corrected the detector''s nonlinear response after calibration. The measurement errors of phase retardance and azimuth angle are within 0.02° for a quarter-wave plate and a wave plate with phase retardances of 127°. The retardance and azimuth errors are less than 0.05° for the wave plates with phase retardances of 27°--145° and 215°--333°.
PDF Acta Optica Sinica, 2020, 40(9): 0912002
Atomic and molecular physics
This work mainly presents the system development and clock transition spectroscopy detection of a transportable 87Sr optical lattice clock. The optical clock system uses a miniaturized physical system with a size of 120 cm×50 cm×60 cm, which connects the modularized sub-optical system through fibers. After subsequent to cooling with a first stage 461 nm laser and a second stage 689 nm laser, a cold atomic cloud with an atomic number of 1.02×10 6 and an atomic temperature of 5.45 μK is obtained. The lattice light with a magic-wavelength is used to load 87Sr in one-dimensional optical lattice with a lifetime of 434 ms, and an atomic temperature in lattice of 4.63 μK. The atoms are detected using an ultra-narrow linewidth 698 nm clock laser to obtain the clock transition spectrum with distinguishable sidebands, the degenerate spectrum with narrow linewidth, the spin-polarized spectrum, and the Rabi-flopping curve. The spin-polarized spectrum with linewidth of 11.79 Hz is obtained under the condition of clock laser interrogating, which is fairly close to the theoretical value of Fourier limit linewidth and can be as the frequency reference for the future optical clock closed-loop.
PDF Acta Optica Sinica, 2020, 40(9): 0902001