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Synchrotron radiation infrared (SRIR) light has the advantages of wide spectral range, small divergence angle, high brightness, and high signal-to-noise ratio. Combined with traditional infrared spectroscopy technology, SRIR microspectroscopy technology is used in infrared spectroscopy microscopy for samples, and the micron-level spatial spectral information of samples can be obtained. By taking an MiTeGen polyimide loop as the sample and using SRIR light from line station BL01B1 of Shanghai synchrotron radiation facility as the light source, we perform the synchrotron radiation infrared three-dimensional (3D) microspectroscopy experiments based on point scanning method. The two-dimensional microspectral information of the MiTeGen polyimide loop under different angles is collected based on point scanning method. The microspectral information in the wavenumber range of 1495--1485 cm -1 is selected, SRIR 3D microscopic reconstruction is performed by using algebraic iteration algorithm for the amide Ⅱ chemical component of the polyimide ring, and a whole 3D reconstruction image is obtained. The research shows that this method can reconstruct the 3D infrared microscopic structure of the sample''s chemical components with a high signal-to-noise ratio.
PDF Acta Optica Sinica, 2020, 40(3): 0334001
Atmospheric and oceanic optics
This study presents a simple and effective tasseled cap transformation based green tide index (TCT-GTI) algorithm by combining the band characteristics of the geostationary ocean color imager (GOCI). By analyzing the identification results of green tide from visual judgement, and comparing them with the monitoring results of two existing remote-sensing algorithms (AFAI and IGAG algorithms), we find that the TCT-GTI algorithm shows relatively high accuracy and reliability. In 2017, the TCT-GTI algorithm is applied to the extraction of green tide information in the Yellow Sea of China using the multi-view GOCI images. We analyze the daily variation characteristics of the green-tide coverage area and study the drift trajectory of the green tide event in 2017. The obtained results show that the green-tide coverage area reaches the maximum at 12:00, which may be influenced by factors such as photosynthesis. This bloom event of green tide in 2017 experiences a drifting trajectory from northwest to the northeast; i.e., it drifts from the offshore waters along Yancheng City in Jiangsu Province, to the east of the South Yellow Sea, then continues to move northeastward, reaches the south bank of the Shandong Peninsula, and gradually disappears.
PDF Acta Optica Sinica, 2020, 40(3): 0301001
The propagation control characteristic of Gaussian optical waves in parity-time (PT)-symmetric Kerr nonlinear planar waveguide with a Gaussian distribution is numerically studied based on the theoretical model of the optical-wave propagation in a PT-symmetric waveguide. The PT-symmetric waveguide requires the refractive-index distribution of the waveguide to have an even symmetry, whereas the gain/loss distribution must be odd. The results demonstrate that in the Gaussian PT-symmetric waveguide, the fundamental-mode Gaussian optical wave can form a wave-shaped beam that has a stable propagation. When the strength of the refractive-index distribution is increased, the oscillation frequency of the wave-shaped beam increases and the oscillation amplitude decreases. When the refractive index becomes larger, the two beams of the first-order Gaussian optical wave in the PT-symmetric waveguide can be restricted at the center of the waveguide, allowing stable propagation of the two beams. The second-order Gaussian optical wave requires a stronger refractive-index distribution for it to be restricted. This work lays a theoretical foundation for the application of PT-symmetric waveguide in all-optical switching control.
PDF Acta Optica Sinica, 2020, 40(3): 0319001
Fourier optics and signal processing
The Fourier transform is commonly used in interferogram analysis. However, owing to the truncation effect, the direct application of Fourier transform for sampled data tends to cause spectral leakage. To address this problem, the apodization function is often adopted. In this paper, we first analyze the performances of different interferogram apodization functions. Then, the effects of the mainlobe width and sidelobe attenuation of the apodization functions on spectral leakage are studied. Finally, we propose an improved triangular window apodization function based on the zero-order Bessel function. It can accelerate the sidelobe attenuation by weighing the triangular window function. Results from extensive experiments show that the improved triangular window apodization function can effectively suppress energy leakage. When compared to the triangular window, this study suggests that the proposed method improves the mean of the peak-to-peak signal-to-noise ratio by 4.9% and the root mean square signal-to-noise ratio by 3.5%. In addition, these results are more accurate than that achieved using the Blackman window. Further, the mainlobe width of the improved triangular window apodization function is 0.043π, which is closed to that of the Hanning window. Therefore, our proposed method proffers a good frequency resolution.
PDF Acta Optica Sinica, 2020, 40(3): 0307001
Optics in computing
In this work, the resolution and measuring range of the Horn-Schunck global optical flow algorithm and Lucas-Kanade local optical flow algorithm in fringe displacement measurement are analyzed. The simulation results show that when the relative error of the Horn-Schunck algorithm and Lucas-Kanade algorithm is less than 2%, and the resolution of Horn-Schunck algorithm and Lucas-Kanade algorithm can reach 10 -13π. Furthermore, the corresponding displacement resolution on image plane is 1.6×10 -12 pixel. This shows that the resolution of the two algorithms is consistent with that of the four-step phase shift method. In the case of noise, the resolution of both algorithms reaches 0.01π, and the displacement resolution of the corresponding image surface is 0.16 pixel. When the relative error is less than 2% and the root-mean-square error is less than 3%, the measurement ranges of Horn-Schunck algorithm and Lucas-Kanade algorithm are 0--17π/100 and 0--52π/100, approximately 0--π/6 and 0--π/2, respectively, and the measurement ranges are less affected by noise.
PDF Acta Optica Sinica, 2020, 40(3): 0320001
In this paper, a method of measuring gas temperature, velocity, and mole fraction of H2O in a high-speed field based on near-infrared absorption spectra of H2O is proposed. The method is applied to a supersonic combustion field and some phenomena and laws in the combustion field are analyzed. The experiment is performed in a scramjet isolation section and extended section based on wavelength modulation spectroscopy using two H2O absorption spectral lines, and synchronous measurements of multiple parameters are realized. After performing the ignition test under combustion, the measured velocity, temperature, pressure, and mole fraction of H2O are consistent with the predicted values when there is no shock wave in the isolation section, and the relative errors are less than 3.0%, 7.2%, 8.2%, and 3.9%, respectively. If the isolation section has a shock wave, the gas velocity, temperature, pressure, and mole fraction of H2O have considerable differences. Additionally, information about the air mass flux has a small difference, but the fluctuation considerably increases. The experimental results show that the sensor based on wavelength modulation spectroscopy has great engineering value.
PDF Acta Optica Sinica, 2020, 40(3): 0330001
Rapid recognition and detection of waterborne pathogens is of considerable significance for determining water quality and ensuring its safety. In this study, the multiwavelength transmission spectra of Escherichia coli, Klebsiella pneumoniae, Staphylococcus aureus, and Salmonella typhimurium are measured. Further, a recognition method of bacterial species in water bodies is proposed based on the principle of similarity, cosine similarity, Pearson''s correlation coefficient, and joint similarity algorithm. It is found that different similarity algorithms have different sensitivities to the spectral difference of different bacteria. The principle of similarity shows the highest recognition rate for Klebsiella pneumoniae, reaching 98.2%; remarkably, the recognition rate of cosine similarity and Pearson''s correlation coefficient for Staphylococcus aureus are 100%. Joint similarity algorithm can realize the complementary advantages of different algorithms and effectively improve the reliability and stability of the recognition results. The recognition rates of joint similarity algorithm for low concentrations of Klebsiella pneumoniae, Staphylococcus aureus, Salmonella typhimurium, and Escherichia coli are 98.2%, 100%, 94.1%, and 91.4%, respectively, whereas the recognition rates for higher concentrations are 100%, 100%, 100%, and 96%, respectively.
PDF Acta Optica Sinica, 2020, 40(3): 0330002
Fiber optics and optical communications
In this paper, an enhanced optical spatial modulation (EOSM) system with a variable number of activated lasers is proposed to solve the problems of low transmission rate and laser utilization in the traditional optical spatial modulation system. The spatial domain mapping is increased by activating index combinations of one or two separate lasers each time. The characteristics of pulse position modulation (PPM) are specifically utilized to distinguish the various types of mapping. In this work, a detailed discussion of the mapping rules of the spatial domain and the signal domain is given. The theoretical upper bound of the bit error rate (BER) of the EOSM system for the weak turbulence channel is derived by using the joint bound technique. Furthermore, the performance of the proposed system is compared with that of three existing optical spatial modulation systems. The results show that the transmission rate of the EOSM system is greater than those of the spatial pulse position modulation (SPPM) and spatial pulse amplitude modulation (SPAM) systems when the number of lasers and the modulation order are fixed. Considering a transmission rate of 6 bit·s -1 and a modulation order of 4, the BER of EOSM system is similar to that of the SPPM system, but it is significantly better than those of the SPAM system and the generalized spatial pulse position modulation (GSPPM) system. When the BER is 10 -3, the signal-to-noise ratio of EOSM system is improved by 4.5 dB and 1.2 dB compared with those of the SPAM and GSPPM systems, respectively. The computational complexity of the EOSM system is 17.78% and 2.6% higher than those of the SPAM and GSPPM systems, respectively, and 70.2% lower than that of the SPPM system. Moreover, the EOSM system can effectively improve the utilization of the laser and greatly reduce the construction cost of the system.
PDF Acta Optica Sinica, 2020, 40(3): 0306001
To address the problems of low efficiency and lack of ability of single indicating cluster target of a laser-guided weapon indicator, a large-angle laser deflection control system used for multi-target indication is designed based on the beam deflection control and beam-angle amplification techniques, and a liquid crystal-phased array and multiplex volume holographic grating are used as controls. The selection rules of the parameters affecting the deflection characteristics of the liquid crystal optical phased array and the design method of the large angle beam deflection system are analyzed in this study. Additionally, the indication process of the laser multi-target indication system is simulated and the error source and correction method are simulated and theoretically analyzed. Results indicate that the proposed system can rapidly achieve quasi-continuum and programmable deflection control of the laser beam in two-dimensional space with a maximum light beam deflection angle of 15.63°. Furthermore, the proposed error correction method can effectively improve the indicated precision of the laser and reduce the error angle by about 50%. Overall, the proposed laser multi-target indication system can be used to realize quasi-continuum and the precise quasi-instantaneous indication of multiple targets in two-dimensional space, thus offering theoretical support for the design of a laser multi-target indicator.
PDF Acta Optica Sinica, 2020, 40(3): 0323001
In this study, we propose a multi-scale kernel correlation filter algorithm for visual tracking based on the fusion of adaptive features to promote the robustness of visual tracking in complex scenarios and tackle the tracking failure problems that can be attributed to illumination variation, target deformation, scale variation, occlusion, etc. First, two kernel correlation filters are separately trained using two different features. Then, the peak side-lobe ratio of the responses and the correlation filter response consistency of two consequent frames are considered to be the weight factors for feature fusion. Meanwhile, an adaptive strategy is adopted to fuse two responses for estimating the position. Next, multi-scale image patches are sampled to construct a scale pyramid based on the estimated position center, and the Bayesian method is employed to estimate the optimal scale of the target. Finally, the tracking model is updated according to the confidence of the tracking result to prevent the deterioration of the model. 51 video sequences are selected for conducting tracking evaluation, and the visual tracking algorithms that exhibited excellent performances in recent years are compared with our proposed algorithm. The experimental results demonstrate that the proposed algorithm effectively reduces the interferences, including the illumination variation, target deformation, scale variation, and occlusion. High tracking accuracy and success rate can be achieved using the aforementioned sequences, and the overall performance of our algorithm is observed to be better than those of the comparison algorithms.
PDF Acta Optica Sinica, 2020, 40(3): 0315001
In the complex tracking environments, such as those with target occlusion and illumination change, the existing correlation filtering tracking algorithm can not stably track the target for long periods in real time. A long-term tracking algorithm based on model update and fast re-detection is proposed. The proposed algorithm initially builds a long-term target tracking framework based on the existing correlation filtering tracking algorithm with target location and the scale change and subsequently proposes a model monitoring updating mechanism, which enter into updating or re-detection link according to the correlation energy values of maximum response and average peak response. Then, the bit dimension of the extracted features can be reduced to 512 using the re-detection method based on extracting descriptor features to accelerate the re-detection rate. The algorithm presented in this study selects 20 representative sequences from OTB-100 dataset for testing. The average success rate, average accuracy, and average rate are observed to be 0.706, 0.805, and 48.5 frame/s, respectively. Furthermore, the average accuracy of the self-collected datasets reaches 87.65%, satisfying the accuracy and real-time demands of long-term tracking in complex situations, including scale change and occlusion.
PDF Acta Optica Sinica, 2020, 40(3): 0315002
To solve the problem related to spatial regularization weight, which is independent of the target content, and model degradation during tracking in correlation filter, a correlation filter tracking algorithm based on temporal awareness and adaptive spatial regularization is proposed herein. First, we extract the gray, color name, and histogram of oriented gradient (HOG) features to improve the aptitude of the model. Second, the initial spatial regularization weight with the target information is obtained using an image saliency detection algorithm. Subsequently, we add an adaptive spatial regularization term to the objective function to alleviate the influence of the boundary effect on correlation filter. Finally, a temporal awareness term is introduced to make the correlation filter learn the information between adjacent frames and reduce the overfitting risk of the model when handling inaccurate samples. In this paper, we evaluate the performance of the proposed algorithm on the public OTB-2013 and OTB-2015 datasets. The results show that the algorithm in this paper has good robustness in a variety of complex scenes, better than other comparison algorithms in tracking success rate and distance accuracy, and the speed reaches 24.2 frame·s -1, which can meet the real-time requirements.
PDF Acta Optica Sinica, 2020, 40(3): 0315003
Lasers and laser optics
Mode hopping causes varying degrees of degradation in the coherence of single-wavelength fiber ring lasers, which seriously affects the performance of optical systems. In this work, a method for coherence collapse suppression based on a matched interferometer is designed, and the suppression mechanism is analyzed. A space-light matched interferometer (SLMI) with adjustable arm difference and an all-fiber matched interferometer (AFMI) are constructed separately, and the suppression effect is verified experimentally. Experimental results show that when the equivalent arm difference of the SLMI continuously approaches the theoretical value, multi-mode-matching interference can be monitored. This is equivalent to single-longitudinal-mode laser interference, thereby successfully suppressing coherence collapse. The AFMI is compact and easy to incorporate into a fiber coherence detection system. It is found that the AFMI can effectively suppress the coherent collapse and the sharp increase of phase noise spectrum level introduced by random mode hopping; this helps to reduce the false alarm probability of the system. As a passive method, matched interferometers play a significant role in suppressing or evading coherence collapse caused by mode instability.
PDF Acta Optica Sinica, 2020, 40(3): 0314001
Using the split-step Fourier method to solve the steady state traveling wave equation of tapered semiconductor laser amplifier, the optical and thermal characteristics of two kinds of laser amplifiers (with linear and nonlinear angle-opening structures) were analyzed by numerical simulation. By comparing the input current-output power curve, input power-output power curve and the number of filaments of the two laser amplifiers, the formation mechanism of filaments in the amplifiers was studied, and the reason of the different optical field distribution in the two laser amplifiers was explained. Results show that the nonlinear angle-opening structure can not only make the gain distribution more consistent with the optical field distribution, but also reduce the coupling between the reflected light and incident light at the edge of the waveguide. Furthermore, the tapered laser amplifier with nonlinear angle-opening structures has a higher optical-optical conversion efficiency and a more stable mode output.
PDF Acta Optica Sinica, 2020, 40(3): 0314002
A tunable, linearly polarized Yb-doped fiber laser, based on a 45°-tilted fiber grating and a fiber Bragg grating with a linear cavity, is proposed. The use of the tilted fiber grating as a linear polarizer provides an output laser with a central wavelength of 1065.90 nm, a 3 dB bandwidth of 0.03 nm, and a polarization extinction ratio higher than 35 dB. The polarization extinction ratio remains stable for 4 h. The temperature characteristics of the beam reflected by the fiber Bragg grating make the central wavelength continuously tunable in the range of 1065.92--1066.87 nm; during the tuning operation, the output polarization extinction ratio is maintained at ≥30 dB.
PDF Acta Optica Sinica, 2020, 40(3): 0314003
The Raman enhancement factors of single molecule and multiple molecules in slot-waveguide coupling structure are analyzed and calculated theoretically by photon Green’s function in waveguide modes combined with the quantum optical form of molecules. The average enhancement factor, the Purcell factor and the percentage of waveguide collection are used as the main performance parameters to compare the difference between the independent slot-waveguide and the composite slot-waveguide. The results show that the composite slot-waveguide achieves a considerably higher Raman enhancement factor (two to three orders of magnitude higher than that of the slot-waveguide), which is mainly because of the combined increase of the electric field, the Purcell factor, the light-matter interaction volume, and the Raman signal collection efficiency.
PDF Acta Optica Sinica, 2020, 40(3): 0313001
In this study, the extinction characteristics of the soot agglomerated particles have been analyzed to study their influence on the quantum satellite communication performance. Furthermore, the relations among the transmission distance, particle number concentration, link attenuation, amplitude damping channel capacity, entanglement fidelity, and channel bit error rate are established and simulated by combining the particle mass concentration conversion formula. According to the simulation results, the concentration of the soot agglomerated particles increases from 2.22×10 12 to 1.11×10 13 m -3 and link attenuation increases from 0.40 to 2.47 dB when the transmission distance is 2 km. Additionally, the amplitude damping channel capacity and entanglement fidelity exhibit varying degrees of decline, whereas the channel bit error rate increases from 0.0056 to 0.0067. The soot agglomerated particles considerably influence the quantum satellite communication performance. Therefore, the quantum communication system parameters should be appropriately adjusted to the soot environment in case of quantum satellite communication, which may cause different influences, to improve the stability during the information transmission process.
PDF Acta Optica Sinica, 2020, 40(3): 0327001
This paper proposes a scheme for high-efficiency collection of a single emitter using a tapered hollow waveguide probe. The collection efficiency of the single-emitter fluorescence by the tapered hollow waveguide probe is numerically simulated. When the single emitter is radially polarized and the probe is designed with the optimized geometrical dimensions, the maximum collection efficiency reaches 25.3%, which is generally higher than that of the traditional method using a lens with large numerical aperture. The average collection efficiency of the probe can reaches 21.7% for the single emitter with polarization in different directions, and the optimized working distance is 0.75 μm. Furthermore, the collection efficiency and working distance of the probe for the single-emitter fluorescence are not sensitive to the wavelength of the light emitted by the single emitter, so the probe can be used for high-efficiency detection of all kinds of particles with different wavelengths and luminous particles with very broad optical spectra. The probe has a micron-sized diameter and is easily combined with other micro/nano structures. Therefore, the probe can efficiently detect many types of emitters, such as a single atom, single molecule, quantum dots, diamond color center, and other particles. The probe is also expected to detect chemical and biological micro-luminescent bodies.
PDF Acta Optica Sinica, 2020, 40(3): 0327002
The remote sensing image semantic segmentation in rural areas is the basis for urban and rural planning, vegetation and agricultural land detection. Segmentation of a high-resolution remote sensing image of rural areas is difficult because of the complex image information. Herein, we designed a complete symmetric network structure that includes a pooled index and a convolution used to fuse semantic information and image features. The Bottleneck layer is constructed using 1×1 convolution and employed to extract the details and reduce the parameter quantity, deepen the filter depth to build an end-to-end semantic segmentation network, and improve the activation function to further enhance network performance. The experimental results show that the accuracies of the proposed method and the classical semantic segmentation networks U-Net and SegNet are 98.4%, 80.3%, and 98.1%, respectively on the CCF dataset. Thus, the proposed method achieves better performance than the other two methods.
PDF Acta Optica Sinica, 2020, 40(3): 0310001
We propose a deep model for pattern classification of computed tomography (CT) images of lung tissues based on the improved deep resiual netwk (ResNet). To address the problem of lack of availability training data, we adopt a transfer learning method to reduce the requiement of a neural network model for large data, thereby decreasing overfitting. The transfer learning strategy uses massively available unlabeled lung CT data as the pre-training data. We perform unsupervised representation learning by maximizing the deep mutual information and matching the prior distribution. The results of contrast experiments show that the improved ResNet achieves improved classification accuracy, the effectiveness of utilizing the unlabeled lung CT data for transfer learning and the classification performance of the network model is improved.
PDF Acta Optica Sinica, 2020, 40(3): 0310002
The measurement of linearly-polarized Stokes vector can be extended to the measurement of full Stokes vector (FSV) by placing a phase retarder, such as a waveplate, in front of the division-of-focal-plane (DoFP) polarization camera. In this study, we propose two optimization strategies to estimate FSV by matrix-decomposition and pseudo-inverse models for DoFP-camera-based FSV polarimeters. According to different FSV estimation models, these two strategies can minimize the measurement variance under the condition of two light-intensity acquisition, thereby optimizing the measurement precision. In particular, by using the inherent measurement redundancy of the DoFP camera, we also propose a self-calibrated analytical solution of the phase retardance of waveplate. This self-calibrated algorithm can replace the step of separate calibration of the phase retardance of waveplate in practical application and can provide technical support for real-time polarization measurement in dynamic environments.
PDF Acta Optica Sinica, 2020, 40(3): 0326001
Diffraction and gratings
Herein, a sub-wavelength gold (Au) grating is sputtered onto an indium tin oxide electrode of a classical liquid crystal on silicon (LCoS). After it is mounted on a thin liquid-crystal cell with a bottom aluminum electrode, a composite resonant waveguide structure, namely GLCoS, is created. In contrast to LCoS that operates based on liquid-crystal propagation effects, in the GLCoS, the surface plasmon of the upper electrode is resonantly coupled with a TM-Fabry-Pérot (TM-FP) resonator in the grating slits to induce a phase modulation of zero-order reflection. The aluminum electrode acts as a reflective backing plate and contributes the waveguide with the Au grating and thin liquid-crystal cell, which enhances the resonance coupling. While acting as an optical device to control the wavefront, the GLCoS also operates as an electronic control device, applying voltage to change the refractive index of the liquid crystal cell; it can control the dielectric condition of the open cavity FP at the boundary to achieve an active 0--2π phase modulation. The experimental results show that the proposed GLCoS structure can be used in phase-spatial light modulators with the 1-μm-level pixel, and it has prospects for application in holographic video displays with high spatial bandwidth product.
PDF Acta Optica Sinica, 2020, 40(3): 0305001
Remote sensing and sensors
The classical moving surface filtering algorithm demonstrates a significant effect of filtering on various terrains. However, the gross errors still exist when the lowest point of the grid is used as the ground point in the moving surface grid algorithm. This study proposes a confidence interval test method to select the best initial seed points by using the residual, mean square error, and confidence probability as the reference values. The grid overlap method is used to solve the problem of the fracture layer between adjacent grids. Moreover, the hierarchical clustering adaptive threshold determination method is used to determine the elevation difference threshold. For the condition where special grids with insufficient special seed points cannot fit the surface, planes are established and thresholds are set to determine whether the plane can meet the condition. This study compares the improved moving surface filtering algorithm with the classical moving surface filtering algorithm by qualitative and quantitative experiments. The findings demonstrate that the improved moving surface algorithm can obtain a better filtering effect than the classical moving surface algorithm.
PDF Acta Optica Sinica, 2020, 40(3): 0328001
Instrumentation, measurement and metrology
The digital image correlation (DIC) method is an important non-contact displacement and strain measurement method. To accurately measure inhomogeneous strain, least absolute deviation fitting of displacements is introduced into the DIC method. As least absolute deviation fitting cannot be solved by analytical methods, a hybrid particle swarm optimization algorithm based on simulated annealing is used. By conducting numerical experiments on the formation of fictitious shear band and the similar material fault slip experiment, the strain measurement results of least square fitting and least absolute deviation fitting for inhomogeneous strain measurements are compared. Results show that the inhomogeneous strain measurement accuracy for least absolute deviation fitting is better than that for least square fitting, while these two methods exhibit the same accuracy for the homogeneous strain measurement.
PDF Acta Optica Sinica, 2020, 40(3): 0312001
The key in the quality evaluation of the speckle is to construct model which can describe the relation between speckle pattern feature parameters and measurement error of digital image correlation method. Till date, no theoretical analysis model describing the relation between the speckle pattern and its power spectrum has been reported. To address this issue and considering the perspective of stochastic process analysis, the relations between the auto-correlation function of the binary speckle and parameters of speckle duty, speckle radius, and gray value are investigated herein. Furthermore, the theoretical analytical form of the binary speckle power spectrum is obtained according to the Wiener-Khintchine theorem. Finally, the theoretical analysis results are verified by numerical experiments. It is observed that the theoretically derived results are consistent with the numerical experiment results on the main lobe of the power spectrum and on several side-lobes nearby. Considering the power spectrum, the maximum value of the main spectrum is consistent with the experimental results. This model can be applied to subsequent speckle error analysis studies.
PDF Acta Optica Sinica, 2020, 40(3): 0312002