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X-ray intensity correlated interferometry provides a feasible technical way to realize high-resolution pulsar information acquisition. However, the energy resolution of existing X-ray detectors is limited, and the detected X-ray signals have spectrum broadening to some extent, which may result in measurement accuracy decrease. To solve this problem, a method is proposed to correct the spectrum broadening in intensity correlated interferometry based on the scaling relationship between the coherence curves at different energies. The measurement errors before and after modification are analyzed by simulation, and the influence of noise on the correction is discussed. A simulated interferometry experiment is carried out with the visible light. The results show that the measurement values after correction are consistent with the theoretical ones, thus the effectiveness of the method is validated.
PDF Acta Optica Sinica, 2019, 39(10): 1034001
We establish a measurement system based on low atomic number absorption spectrum of fluorescence emission via the beamline station 08U1A of Shanghai Synchrotron Radiation Source, and explore the partial fluorescence yield (PFY) based on absorption method. The feasibility and applicability of this setup is verified on low Z elements in soft condensed matter and semiconductor areas. The research objects of CF4 gas and iodine methylamine lead (CH3NH3PbI3) verify the feasibility of PFY absorption spectroscopy. Then the low limit of sensitivity to sample concentration is determined.
PDF Acta Optica Sinica, 2019, 39(10): 1034002
Optics at surfaces
Due to the influence of the external environment, the satellite''s surface is often irregularly folded, and these folds have some influences on the optical properties. So folds need to be taken into account in the modeling of the optical characteristics of space objects. However, a large number of surface cells in fold surface will lead to a dramatic increase in computational complexity. This paper considers pleats as a kind of ‘material’. A method based on macroscopic optical scattering cross section measurement is proposed to obtain the bidirectional reflectance distribution function (BRDF) data of the fold material. Furthermore, the error back propagation neural network is used to establish BRDF model of fold material, which replaces the complex modeling process of folds and greatly simplifies the calculation. The problem of poor real-time performance is solved under the acceptable accuracy. By combining the experiment with the simulation to compare the BRDF model designed in this paper with the traditional BRDF model, it is verified that the error of the model designed in this paper is much smaller than that of the traditional model.
PDF Acta Optica Sinica, 2019, 39(10): 1024001
According to the principle of improving the light output from scintillators by using photonic crystals (PhCs), we add four PhCs to the 5-μm-thick surface of a lutetium-yttrium oxyorthosilicate (LYSO) scintillator to improve the performance of the scintillator detectors. Four PhC structures are used, i.e., hexagonal air-holes, square air-holes, square cylinders, and square cubic rods. By scanning the period, ratio, and height of PhCs, we acquire four optimized structures using the Rsoft and MATLAB softwares, which is performed under the condition of a 420-nm-center wavelength of LYSO. The simulation results indicate that all the optimized PhC structures exhibit an increased light output and the structure with hexagonal air-holes shows the highest output (up to 17.1945%).
PDF Acta Optica Sinica, 2019, 39(10): 1031001
The exposure of high-nonlinearity chalcogenide glass fibers to ultrashort pulses is an important approach for achieving broadband mid-infrared supercontinuum (SC). The average output power of a generated SC is mainly limited by the laser damage threshold (LDT) of the material. In this work, the laser damage characteristics of the germanium-antimony-sulfur (Ge-Sb-S) chalcogenide glass are studied using a laser with a pulse width of 216 fs, a central wavelength of 1030 nm, and a repetition rate of 1-1000 kHz to elucidate the composition dependence of the LDT of the chalcogenide glass as well as the mechanisms of damage to the material exposed to irradiation pulses with different repetition rates. Results show that the LDT of the Ge-Sb-S glass decreases with increasing repetition rate of the irradiation pulse. The glass with high average bond energy is found to have a high LDT, and its stoichiometric composition exhibits optimal resistance to optical damage. When the repetition rate of laser irradiation pulse is less than 10 kHz, the glass is observed to be mainly damaged by avalanche ionization. In contrast, thermal accumulation is the dominant factor promoting damage when the repetition rate of the irradiation pulse exceeds 10 kHz.
PDF Acta Optica Sinica, 2019, 39(10): 1016001
Organic-inorganic lead halide perovskites are intensively studied for their large potential in low-cost and high-resolution light-emitting devices due to the advantages of solution-processability, good color purity, and facile emission wavelength tunability. We study the perovskite materials containing a mixture of methylammonium (MA +) and ethylammonium cations (EA +), which show the cation-ratio dependent emission wavelength. Introduction of EA + cations increases the photoluminescence quantum yield and photo-stability simultaneously, reduces the density of defect states significantly. Related green-emitting devices show a maximum external quantum efficiency and power efficiency of 7.7% and 25.1 lm·W -1, which are 1.9 and 1.7 times as those of the MAPbBr3-based analogs, respectively, along with improved operational stability.
PDF Acta Optica Sinica, 2019, 39(10): 1016002
To address the problem of the complex light source structure of a directional backlight in naked eye 3D display, a design and fabrication method for a kind of side-glowing plastic optical fiber (POF) array backlight is researched. An arrangement formula of the side-glowing POF array is obtained by analyzing the directional imaging optical path composed of side-glowing POF array and cylindrical micro-lens array. A kind of side-glowing POF array for directional backlight is fabricated and the crosstalk from the directional backlight is measured. The results show that the minimum crosstalk in the transverse regions of -200 mm to 200 mm is less than 1% at the optimal distance of 530 mm in the designed viewing area. Finally, the effects of cylindrical lens aberration, POF array optical fiber dislocation, and increasement of viewing angle on crosstalk degree of POF array directional backlight are analyzed.
PDF Acta Optica Sinica, 2019, 39(10): 1011001
The existing three-dimensional micro-particle image velocimetry systems obtain the spatial position of tracer particles through scanning or imaging from multiple perspectives, which leads to the complexity of the system and difficulty in making an instantaneous measurement of three-dimensional velocity in micro-scale flow. In this paper, a light field micro-particle image velocimetry technique based on a microlens array is proposed. In this technique, the instantaneous light field information of tracer particles in the micro-scale flow field can be recorded by a single camera in a single photographic exposure. Further, in combination with the point spread function model of the light field microscopic imaging system calculated by wave optics theory, the instantaneous spatial position distribution of tracer particles in the micro-scale flow field can be reconstructed by a deconvolution method. The reconstruction resolution and the spatial position error are analyzed and discussed. Experiments on micro-scale flow field reconstruction are carried out, and the feasibility of the reconstruction method for light field microscopic imaging is verified.
PDF Acta Optica Sinica, 2019, 39(10): 1011002
We establish a source simulation system by using a digital micromirror device (DMD) as a display device and investigate the imaging distortion of an optical system. First, we illustrate the general design of the system and calculate the object-space depth and shape variables caused by the different attitude azimuth of the DMD by using geometric optics. Then, we choose the optimum attitude. Subsequently, we simulate the system by using optical software, establish the imaging distortion measurement template and algorithms, and analyze the installation error of the DMD''s attitude angle. The analysis shows that the imaging distortion is about 1.3% caused by the 0.3° pitch angle or azimuth deviation. Finally, guided by the design and simulation results, the optical path of the experimental system is completed. The results show that the average distortion corresponding to the maximum field of view is 0.385%.
PDF Acta Optica Sinica, 2019, 39(10): 1011003
Atmospheric and oceanic optics
Radiometric calibration is one of the key technologies for quantitative application of optical remote sensing satellite data. This study proposes an on-orbit radiometric calibration approach for a high-resolution optical remote sensing satellite based on sub-pixel targets. The detection reference is the reflected point source, and the ground synchronization measurement is mainly used. Atmospheric transmittance and incident pupil radiance are obtained by simplifying radiometric transfer calculation. The method can also effectively isolate the response of a remote sensing image created by sub-pixel targets from the response produced by sky path radiance, ground-atmospheric coupled radiance, and background radiance, according to multi-energy sub-pixel target setting and on-orbit point spread function detection results. The approach can break the limit of time, locale, and weather conditions and achieve high precision calibration for the optical remote sensing satellite with a full dynamic range in complex environments. Results show that calibration uncertainty for the high-resolution optical remote sensing satellite based on sub-pixel targets is lower than 3.2%. Moreover, the calibration result of the proposed method is different from that of the gray-scale target method by 3%. A small and light sub-pixel target can satisfy the high frequency on-orbit calibration application of the optical sensing satellite.
PDF Acta Optica Sinica, 2019, 39(10): 1001001
It is difficult to detect and identify dim and small targets on the sea surface in complex backgrounds, such as low contrast, shore island background, and foreground occlusion. This study proposes a method based on infrared polarization images to detect dim and small targets on the sea surface. Herein, we propose an improved hue-saturation-intensity (HIS) color space fusion algorithm based on the perceived color of the human visual system, which combines infrared polarization information and infrared intensity information of sea surface image. We design a sea surface region segmentation method based on the infrared polarization image, and the sea surface region is segmented as a candidate target enhancement region. The context-aware saliency algorithm is used to calculate the saliency of the sea surface HSI color space fusion image. The sea surface HSI color space fusion image is corrected using the saliency map to obtain a dim and small target enhanced image. The contrast of target and background and the local signal-to-noise ratio are used to evaluate the features of the fusion enhanced image. Results obtained show that in comparison with the existing methods, the proposed method can enhance dim and small targets and suppress the background interference. The evaluation index of the proposed method is higher than that of other existing methods, which provides support for the detection and identification of surface ship targets.
PDF Acta Optica Sinica, 2019, 39(10): 1001002
An underwater moving target detection algorithm based on water surface characteristic wave is proposed to overcome the shortage of effective detection methods for photoelectric polarization imaging modes. Based on the wind-induced gravity wave model and the water surface characteristic wave model of an underwater moving target, the mixed wave images under different states are simulated and used for the research of the algorithm. The algorithm uses the Radon transform to extract the linear wave characteristic, and average filter and standardization are employed to preprocess images, thereby eliminating the adverse effect of Radon transform on detection. The double-neighborhood adaptive threshold method is employed to extract partial peak points in Radon transform domain. The algorithm employs continuous wavelet transform to extract features and support vector machine to judge the peak points, thereby improving the detection accuracy. The experimental result shows that the algorithm is feasible for characteristic wave detection, which also provides a new way for underwater moving target detection.
PDF Acta Optica Sinica, 2019, 39(10): 1001003
Aerosol optical depths (AODs) are calculated for the Xi''an region based on the observation data of a sky radiometer (POM-02) and Beer-Lambert-Bouguer law. These AODs are used as true values to compare with three inversion algorithms'' results of AODs in the MODIS C061 product of NASA Terra satellite, and the accuracies and applicability of those three inversion algorithms for the Xi''an region are discussed. Furthermore, we statistically analyze the spatial distributions and seasonal variation characteristics of AODs for the Xi''an region and its surrounding region using MODIS aerosol products. The results indicate the following: there is the best relationship between the AODs inverted from the MODIS DT&DB product and POM-02 data among those three MODIS inversion algorithms of Terra satellite, and its correlation coefficient is approximately 0.92. Thus, the MODIS DT&DB product is the most appropriate product for studying climate change and air pollution in the Xi''an region. Dust storms and human activities are the main sources of aerosols in the Guanzhong Plain. The AODs are greater in the eastern part and less in the western part of Guanzhong Plain, and the high-value centers are mainly distributed in the regions such as Xi''an, Xianyang, and Weinan. The AODs in the Xi''an and Xianyang regions reach their maximum and minimums in the spring and autumn, respectively. However, the AODs from other regions in the Guanzhong Plain show apparently seasonal variation characteristics with greater values in spring and summer and lower values in autumn and winter.
PDF Acta Optica Sinica, 2019, 39(10): 1001004
An underwater turbulence recognition method based on the detection of vortex interference stripe displacement is proposed. The transmission and interference characteristics of the Laguerre-Gaussian beam in underwater turbulence are simulated using the random phase screen method. The displacement characteristics of the stripes caused by the interference between vortex beams with different topological charges and Gauss beams in free space after turbulence are studied and experimentally analyzed. Experimental and theoretical results show that in a strong turbulent environment, the larger the topological charge, the better the transmission ability of the vortex beams propagating in the turbulence; under the same turbulence intensity, the larger the topological charge, the smaller the displacement of the position of interference stripes. The displacement of interference stripes increases with increasing turbulence intensity of vortex beams with similar topological charges. This indicates that the method for detecting the displacement of vortex interference stripes can effectively detect turbulence intensity and ship wake.
PDF Acta Optica Sinica, 2019, 39(10): 1001005
In view of the fact that other harmonic components besides the second harmonic still contain concentration information,three concentration inversion methods based on 2nd-/4th-harmonics, 2nd-/4th-/6th-harmonics, and 2nd-/4th-/6th-/8th-harmonics, as well as their corresponding expressions, are presented while the optimal value of the modulation depth for each inversion method is evaluated. The main forms of noise interference in gas concentration detection systems are also analyzed and the usefulness of multiple harmonic analysis for noise suppression is proved theoretically. Finally, an experimental verification scheme is given for reference. Our simulation''s results indicate that the above methods can reduce concentration inversion errors by 31.38%, 42.03%, and 47.45%, respectively, as compared with the 2nd-harmonic-based method.
PDF Acta Optica Sinica, 2019, 39(10): 1030001
In this study, a detection and correction method for interferogram spikes is proposed based on filtering and predefined threshold detection to satisfy the requirement of real-time spike detection and correction. We present the algorithm hardware implementation based on a field-programmable gate array. In case of less hardware resources, the detection and correction algorithm achieves high detection accuracy, especially for the spikes of the central fringe of interferogram. The algorithm proposed in this study can be used for real-time on-board interferometric data processing.
PDF Acta Optica Sinica, 2019, 39(10): 1030002
We investigate the terahertz characteristic absorption spectra of homocysteine. First, we use the density functional theory to analyze the vibration and rotation modes of homocysteine and ensure that the theoretical absorption peak is within the testing range of the terahertz spectroscopy system. Subsequently, the characteristic absorption spectra of homocysteine with different concentration are measured based on the terahertz time-domain spectroscopy system, which can be accurately performed using the linear fitting equation. These results exhibit considerable significance for the accurate and rapid diagnosis of homocysteine-related diseases in clinical medicine.
PDF Acta Optica Sinica, 2019, 39(10): 1030003
The sugar content and firmness of red globe grapes are important indicators for evaluating their quality. This study explores nondestructive detection methods and best prediction models for determining the sugar content and firmness of red globe grapes based on hyperspectral imaging technology. The hyperspectral images of 213 samples, in the wavelength range of 400-1000 nm, are collected in three placement orientations (horizontal, fruit stalk-side down, and fruit stalk-side up). The optimal orientation for spectral imaging is compared and analyzed, and subsequently the spectrum is preprocessed in the optimal orientation. Several preprocessing methods, i.e., genetic algorithm (GA), successive projections algorithm (SPA), competitive adaptive reweighed sampling (CARS) algorithm, and uninformative variable elimination algorithm (UVE), are applied to the images to extract characteristic wavelengths from the original spectra. Using chemometrics methods, combined with either partial least squares regression (PLSR), least squares support vector machine (LSSVM), and random forest (RF) analysis based on full spectra and characteristic wavelengths, several protocols are established to mathematically predict the sugar content and firmness of red globe grapes from the images. Results show that the sugar and firmness model based on RF performs the best. The optimal model for predicting sugar content proves to be RF optimized by GA (GA-RF), with corrected-set correlation coefficient (Rc) and predicted-set correlation coefficient (Rp) values of 0.969 and 0.928, respectively, and corrected-set root-mean-square error (RMSEC) and predicted-set root-mean-square error (RMSEP) values of 0.266 and 0.254, respectively. The optimal model for predicting firmness proves to be RF optimized by moving-average method and SPA (MA-SPA-RF), with Rc and Rp values of 0.961 and 0.932, respectively, and RMSEC and RMSEP values of 2.119 and 1.634, respectively. These results prove the sugar content and firmness of red globe grapes can be nondestructively predicted via hyperspectral imaging.
PDF Acta Optica Sinica, 2019, 39(10): 1030004
Fiber optics and optical communications
Considering the cat-eye effect based retro-modulating laser communication as the application background, a large-field cat-eye echo power distribution model is established based on the Collins diffraction formula by transforming the angular misalignment of the cat-eye lens into a line offset. The effects of the defocusing amount, incident angle, cat-eye diameter, and focal length on the echo power distribution and original return characteristics of the cat-eye echo beam are analyzed and verified with experiments. The results show that the larger the incident angle is,the smaller the echo spot area is,and the stronger the diffraction is. At the same incident angle, the larger the aperture of the cat-eye is, the smaller the focal length is, and the smaller the echo spot affected by the incident angle is. The divergence angle of the positive defocus echo spot at the same defocusing amount is smaller than that of the negative defocus. The center shift of the cat-eye echo varies linearly with both the incident angle and the defocusing amount, and there is a positive defocusing amount that causes the echo beam to return to the original path.
PDF Acta Optica Sinica, 2019, 39(10): 1006001
To address the problem of difficulty in distortion specttum demodulation of fiber Bragg grating (FBG) based sensing networks,we propose a wavelength demodulation technique based on estimation using a distribution algorithm (EDA). We construct a theoretical function of distortion spectrum based on the super Gaussian function and transform the wavelength detection problem of the distorted FBG sensing network into a function optimization problem. The proposed method is used to demodulate the distortion spectrum of a FBG sensing network through an experiment. The results denote that EDA can not only maintain an average detection accuracy within 1 pm even when the spectrum of FBG is distorted but also quantitatively estimate the distortion degree of FBG. When compared with the traditional peak detection methods, the proposed method can effectively identify the Bragg wavelength from a distortion spectrum. The proposed method provides a novel method to extend the service life and enhance the reliability of an FBG sensor network.
PDF Acta Optica Sinica, 2019, 39(10): 1006002
To meet constraints on size, weight, and power consumption in small satellite platforms and explore alternative solutions for radio frequency and laser-based communication in medium- and short-distance inter-satellite links, this study explores the feasibility of visible-light communication (VLC) based medium- and short-distance inter-satellite communication in multi-satellite networks. Using the satellite tool kit (STK), the binary star formation flying configuration is constructed, background-light noise introduced by the sun and stars is quantitatively analyzed, and a medium- and short-range inter-satellite VLC link model is constructed. Then, to improve the link capacity and reliability, the VLC link is further developed into a single-input multi-output VLC (SIMO-VLC) system. The influences of different diversity merging algorithms on the link’s performance are evaluated by numerical simulation. Simulation results show that the stray light power introduced by solar radiation is below 10 μW throughout most of the time period, and the background light power introduced from the ground is reduced by 75%. In order to achieve a data transmission rate of 112.5 Mbit/s and a bit error rate of 1×10 -6 within 20 km, the light power required by the system is at least 4.55 W. Compared with the single-input single-output system, the SIMO-based diversity detection scheme can achieve better performance, and the required transmission power and communication distance are effectively improved with the increase of the receiving branch, which can provide a reference for the design of VLC-based inter-satellite links and extending their outdoor application scenarios.
PDF Acta Optica Sinica, 2019, 39(10): 1006003
Mode division multiplexing is becoming a potential approach to overcome the capacity crunch of a single-mode fiber transmission system. In the future mode- and wavelength-division multiplexing system, different spatial modes and the wavelength gain difference will affect the transmission capacity and speed. In this study, to realize the modal gain equalization and wavelength gain flatness, a 44.5 km few-mode fiber with ultra-low loss is pumped by the 1480 nm pump light with LP11 mode,and the distributed Raman amplification through the few-mode fiber is realized. The few-mode erbium doped fiber is pumped by the residual pump light, and the lumped few-mode erbium doped fiber amplification is realized. A remotely pumped few-mode fiber amplifier is experimentally demonstrated. The average equivalent on-off gain is greater than 15 dB, and a differential modal gain approximately 2 dB in the wavelength range of 1560-1600 nm is achieved.
PDF Acta Optica Sinica, 2019, 39(10): 1006004
This study proposes an online monitoring technology for oil and gas pipeline leakage based on a high-fidelity fiber-optic distributed acoustic sensor (HiFi-DAS) and introduces the measurement principle and technical advantages of DAS. The DAS can successfully detect and locate the leakage in a gas pipeline in a simulated field experiment. Considering the complex field environment and strong background noise, the wavelet denoising algorithm is adopted to further improve the detection sensitivity of leakage events, and gas leakage under pressure of 0.05 MPa is accurately detected in field experiments. Experimental results show that the proposed DAS system has superior performance in the field of oil and gas pipeline safety monitoring.
PDF Acta Optica Sinica, 2019, 39(10): 1006005
This study presents a dynamic strain sensor system based on the fiber ring laser containing a semiconductor optical amplifier. In this sensing system, the semiconductor optical amplifier based fiber ring laser in combination with a fiber Bragg grating is used as a wavelength selecting component of the fiber laser to detect the external dynamic strain signal. A non-tunable fiber Fabry-Pérot filter is deployed outside the laser cavity as an intensity demodulator, and the function of separating and outputting the reflected signals of multiple fiber gratings can be realized by configuring the fiber bandpass filter. Experimental results show that the proposed sensing system has a good response to the dynamic strain signal at a frequency of up to 200 kHz, and it has the ability to demodulate the MHz frequency range signal. The feasibility of multiplex demodulation is also verified. The system has a simple structure and low cost, and can be applied to the detection of dynamic strain signals in structural health monitoring.
PDF Acta Optica Sinica, 2019, 39(10): 1006006
We present a hydrogen sulfide (H2S) gas sensor based on a no-core-multimode-no-core (NMN) fiber structure. The sensor is fabricated by using two no-core fibers (NCF) which are spliced at both the ends of a multimode fiber (MMF), and the NMN structure is constructed. Different high-order modes can be excited when light travels from a single-mode fiber (SMF) to NCF. When light enters MMF, the high-order modes and fundamental mode transmit in the cladding and core of MMF, respectively, resulting in phase difference and mode interference. Simultaneously, the transmission characteristics of NCF and MMF having different lengths are optimized. The outside surface of NCF is coated with a thin titanium dioxide film by the dip-coating method; thus, a rapid response to H2S gas can be achieved when the film absorbs the gas. With an increase in the H2S concentration, the interference spectra denote a red shift. A sensitivity of 7.36 pm/10 -6 and good linear relation are obtained in the H2S volume fraction range of 0-3×10 -5. In addition, titanium dioxide exhibits good selectivity to H2S, and the response and recovery time of the sensor are approximately 50 s and 65 s. The sensor has advantages of simple structure, high sensitivity, and easy manufacturability, and it can be applied in the field of safety monitoring of the low-concentration H2S gas.
PDF Acta Optica Sinica, 2019, 39(10): 1006007
A novel double-trench-assisted fan-segmented cladding fiber (SCF) is proposed and researched. Compared with traditional fan-SCF and single-trench-assisted fan-SCF, the proposed fiber structure has a larger mode area and better high-order mode (HOM) suppression. Numerical investigations show that, when the bending radius is 20 cm, the effective mode area of the fundamental mode (FM) reaches up to 1096 μm 2 and the loss ratio between HOM and the FM is greater than 100 at a wavelength of 1.55 μm. In addition, the proposed fiber structure is insensitive to the bending orientation and its properties remain stable with a bending orientation ranging from -180° to 180°.
PDF Acta Optica Sinica, 2019, 39(10): 1006008
A downward-illuminated 100 W LED was studied herein, and its temperature and thermal stress distributions were calculated under three conditions: pure thermal conduction, thermal radiation and thermal conduction coupling, and convection, thermal radiation, and thermal conduction coupling. The following factors were evaluated: temperatures of the characteristic points on the lens’ external surface; the effects of air convection in the closed cavity of lens and surface radiation on the lens’ temperature distribution; the effect of surface emissivity on the central temperature of the lens; the effect of the thermal expansion coefficient on the maximum thermal stress. Results show that thermal convection produces a negligible rise in temperature (less than 1%), whereas surface radiation results in a 13.3% temperature rise at the lens’ center. The temperature of the lens’ center varies approximately linearly with the light source’s emissivity and the lens surface area, whereas the maximum thermal stress varies linearly with the thermal expansion coefficient. The maximum thermal stress is concentrated at the lens’ corners, whereas the maximum total deformation displacement is concentrated at its center. Therefore, when designing high-power LED lenses, a low-emissivity coating should be considered to reduce the lens’ temperature while still satisfying the optical requirements. To reduce thermal stress and deformation, a material with a small thermal expansion coefficient should be used and the lens’ corners should be avoided.
PDF Acta Optica Sinica, 2019, 39(10): 1023001
Optical design and fabrication
We design an uncooled dual-band infrared optical system whose operating wavelengths include 3-5-μm mid-wave infrared and 8-11-μm long-wave infrared wavelengths. The effective focal length of this system is 21 mm, and the F-number is 2.6. The system comprises four lenses fabricated with germanium and chalcogenide glass (Ge10As40Se50), and the fourth surface is a Q-type asphere. The system is optimized to achieve athermal effect. The design result shows excellent image quality in the temperature range of -40-60 ℃; additionally, the modulation transfer function is close to the diffraction limit. We design an optical system with an even asphere and the same initial parameters and structure. By comparing, we find that the Q-type asphere has more advantages in optical system design: its stronger ability to correct aberration and smaller deviation from the best-fit sphere are conducive to improving fabrication accuracy and efficiency.
PDF Acta Optica Sinica, 2019, 39(10): 1022001
In order to make full use of the information extracted from the middle layer and prevent information from losing excessively, a new image fusion method based on a convolutional auto-encoder and a residual block is proposed, which is composed of an encoder, a fusion layer, and a decoder. First, the residual network is introduced into the encoder, the infrared and visible images are fed into the encoder, and the convolution layer and residual block are used to obtain the feature map of the image. Then, the obtained feature map is fused by using an improved fusion strategy based on L1-norm similarity, which is integrated into a feature map containing the salient features of the source image. Finally, the loss function is redesigned and the decoder is used to reconstruct the fused image. The experimental results show that compared with other fusion methods, the method effectively extracts and preserves the deep information of the source image, which makes the fusion result have certain advantages in both subjective and objective evaluation.
PDF Acta Optica Sinica, 2019, 39(10): 1015001
Placing a certain number of coded points is generally required in the current industrial photogrammetry system. However, many industrial products are not suitable for the arrangement of coded points. This study proposes a novel method of industrial photogrammetry without using coded points. Our method only requires a projection device to project the speckle texture, a scale bar to recover scale, and a multi-angle camera to capture various images of the measured object. A “coarse-to-fine” two-step reconstruction strategy is devised to solve the multi-view geometry with high precision. Moreover, a rotation-free digital image correlation (RFDIC) method is proposed for high-accuracy point-matching between images with large rotational angles. The experimental results verify that the error in measuring the length of scale bar by the RFDIC method is below 0.01 mm/m and the error of the RFDIC method compared with that of the latest commercial point cloud construction method for three dimensional measurement system can reach approximately 0.055 mm, which satisfies the precision requirements of industrial photogrammetry.
PDF Acta Optica Sinica, 2019, 39(10): 1015002
Aiming at the problem of the complexity of the existing global calibration method for multi-camera system with non-overlapping fields, an accurate calibration method with non-overlapping fields of view (FOV) cameras based on spatial constraints is proposed. Firstly, spliced small targets distributed in the field of respective camera are fixed together to make up a large calibration planar. A large-field camera is used to measure the spliced small targets, and the obtained relationship between the calibration plates is used as the correlated parameter between cameras. Then, small targets in the fields of view are obtained by non-overlapping FOV cameras, the reprojection error functions of cameras can be obtained based on spatial constraints, and the transformation matrix between non-overlapping FOV cameras is nonlinearly optimized by the Levenberg-Marquardt algorithm. The experimental results show that the reprojection errors of the global calibration method are 0.33 mm and 0.57 mm for the X-axes and Y-axes, respectively. It achieves global calibration of non-overlapping FOV cameras with high precision and stability.
PDF Acta Optica Sinica, 2019, 39(10): 1015003
Lasers and laser optics
Herein, we propose a high-resolution C-band tunable fiber laser based on an echelle grating and a digital micromirror device (DMD). By employing the wavelength tuning performance of DMD and the high-resolution characteristics of an echelle grating, we design a cross-dispersion structure based optical alignment system, realizing the high-resolution wavelength tuning capability. Experimental results show that the laser demonstrates flexible tuning in 1542-1558-nm region by remotely loading holograms onto the DMD. The wavelength tuning resolution is approximately 0.036 nm, and the 3-dB linewidth of the output signal is less than 0.02 nm. The side-mode suppression ratio exceeds 40 dB, the center wavelength drift is less than 0.013 nm, and the power fluctuation is less than 0.07 dB within 1 h at room temperature.
PDF Acta Optica Sinica, 2019, 39(10): 1014001
Since the material properties of metal additive components differ from those produced by traditional manufacturing processes, the original ultrasonic nondestructive testing methods are no longer applicable. To study the ultrasonic nondestructive testing characteristics of metal additive components and improve the ultrasonic nondestructive testing accuracy, TA15 alloy specimens are prepared by laser melting deposition with different laser powers and overlap rates. The sensitivity and sound velocity of the longitudinal wave, which are excited by contact ultrasonic testing with a phased array ultrasonic equipment, are compared and analyzed. Results show that ultrasonic sensitivity is considerably affected by laser power. Ultrasonic velocity is affected by the forming direction and process parameters. Forming direction is a more important parameter. The material and process parameters of the reference block should be the same as those of the parts to be tested. In addition, the sound velocity in each detection direction of the parts should be calibrated.
PDF Acta Optica Sinica, 2019, 39(10): 1014002
Vision, color, and visual optics
A spectral-reconfiguration solution based on nonvisual and visual effects is proposed to achieve healthy lighting. A four-channel spectrally reconfigurable calculation model that directly solves the duty cycle of pulse-width modulation according to the circadian-action factor, chromaticity coordinates, and illuminance is established. A remotely controllable and spectrally reconfigurable light-emitting-diode source is designed for experimental verification. The tunability of the circadian-action factor and color-rendering index is discussed, providing theoretical guidance for the selection of relevant parameters. Experimental results show that the relative error between the measured and theoretical values is small. Furthermore, the calculation model can guide spectral reconfiguration on the same chromaticity coordinate, achieving regulation of the circadian-action factor and color-rendering index. Therefore, the computational model can conduct spectral reconfiguration well. This study provides a useful reference for the design of healthy lighting sources for the next generation.
PDF Acta Optica Sinica, 2019, 39(10): 1033001
By using the method of constant stimuli, binocular-disparity based and motion-parallax based cylinders with various depths were produced in three-dimensional space as experimental stimuli. Observers were asked to walk in the three-dimensional space, and depth perception data at different viewing distances were obtained by using binocular-disparity cue, motion-parallax cue, and both binocular-disparity and motion-parallax cues separately. The experimental results show: at different viewing distances, binocular-disparity and motion-parallax cues have different influences on depth perception. In the far-distance viewing condition, the weight of motion parallax cue in depth perception is relatively larger than that in the near-distance viewing condition; in the near-distance viewing condition, when binocular-disparity and motion-parallax cues are combined, there is no promotive effect in depth perception and the effect of motion parallax cue is very small.
PDF Acta Optica Sinica, 2019, 39(10): 1033002
In order to measure the grating constants of the plain rainbow holographic materials quickly and conveniently, the spectrophotometers (loop light illumination) and scanning equipments commonly used in color measurement fields are used to collect the spectral energy and image information of different plain rainbow holographic materials by fixing the measuring point and rotating the plain rainbow holographic materials in the range of 0°-90°. It is found that with the change of rotation angle, the spectral energy and image color information of the plain rainbow holographic materials collected in the range of 0°-45° and 45°-90° show a periodic and symmetrical change. From the grating equation, it is found that there are two effective grating constants in the side length and diagonal directions of the holographic master materials. The experimental results are in good consistency with those obtained by high amplification ratio microscopes. The measured results can provide a theoretical basis for the examination of the microstructures of light pillar holographic materials and plain rainbow holographic materials.
PDF Acta Optica Sinica, 2019, 39(10): 1033003
Since the traditional single image dehazing algorithm is susceptible to the prior knowledge constraint of hazy image and color distortion, this paper proposes a multi-scale convolutional neural network (CNN) single image dehazing method based on deep learning, which realizes image dehazing by learning the mapping relationship between hazy image and atmospheric transmission. According to the hazy image forming mechanism of atmospheric scattering model, an end-to-end multi-scale full CNN model is designed. The shallow layer features of hazy image are extracted by convolution layer operation, and then the deep features are extracted by multi-scale convolution kernel in parallel. Then the shallow layer features and deep features are fused by jump connection. Finally, the non-linear regression method is used to obtain the corresponding transmission features of the hazy image. According to the atmospheric scattering model, the haze-free image is restored. The model is trained by using hazy image data sets. The experimental results show that the proposed method can achieve good dehazing effect in the experiments of synthesizing hazy images and real natural hazy images. The proposed method is superior to other contrast algorithms in subjective and objective evaluations.
PDF Acta Optica Sinica, 2019, 39(10): 1010001
Herein, a method for generating tunable nonparaxial accelerating beams is proposed theoretically by spectral phase modulation and verified experimentally. The mathematical model of the relationship between spectral phase and beam propagation trajectory is established based on the stationary phase approximation and the principle of optical caustics. Theoretical simulations and experimental results show that the proposed method overcomes the limitations of conventional paraxial approximation, and nonparaxial accelerating beams are generated. Such accelerating beams with flexible and tunable trajectories have potential applications in the areas of optical particle manipulation, particle transport and guidance, and super-resolution imaging.
PDF Acta Optica Sinica, 2019, 39(10): 1026001
Diffraction and gratings
Herein, a mid-infrared absorber based on the hexagonal boron nitride material is designed. The absorber is an one-dimensional grating comprising a truncated pyramid-type unit structure whose absorption mechanism is based on magnetic polaritons and Fabry-Perot cavity resonance effect. The influences of the structural parameters, working wavelength, and incident angle of the absorber on absorption performance are analyzed by using the finite element algorithm. Results show that,under optimized structural parameters, the absorption of the absorber can reach 80% or more in the range of 5.6-14.5 μm when the incident angle range is 0° -75°. The absorber designed herein is expected to be applied to the sensing and stealth aspects at the mid-infrared band.
PDF Acta Optica Sinica, 2019, 39(10): 1005001
In this work, the frequency domain and time domain response analysis model of ultrashort pulses via chirped volume Bragg grating (CVBG) is constructed based on the matrix method. Aiming at the bandwidth requirement of CVBG for the hundred-femtosecond (fs) fiber chirped pulse amplification (FCPA) system, we systematically study the influence of the diffraction bandwidth on the pulse stretching and compression effect of CVBG and the impulse response characteristics of the broadband CVBG to incident pulse with different chirp parameters. The results show that the diffraction bandwidth of CVBG increases linearly with the increasing chirp rate and thickness. When the diffraction bandwidth of CVBG is smaller than that of the incident pulse, the shearing of the spectral components will cause distortion of the stretched pulse and broadening of the compressed pulse with respect to the incident pulse. To achieve stretching-compression reciprocity of the one-hundred-femtosecond pulse, it is necessary to ensure the diffraction bandwidth of CVBG no less than 60 nm. As designed, a broadband CVBG with 40 mm thickness is stretched first and then compressed, and linear chirp pulses with a spectral width of 16.64 nm are obtained. All of the output pulses are infinitely close to the Fourier transform constrained (FTL) pulse and the diffraction efficiency is as high as 84%, which provides a theoretical reference for the implementation of the fs CVBG pulse compressor.
PDF Acta Optica Sinica, 2019, 39(10): 1005002
We propose an improved design solution of large-aperture photon sieve. All the pinholes in one Fresnel ring are regarded as a whole, and the diffraction field formula is deduced after approximation and transformation based on the nonparaxial far-field pinhole diffraction model. The main work of this paper focuses on the internal condition of pinhole ring diffraction model. According to calculation, the diffraction field calculated by the pinhole ring model is basically consistent with that calculated by the traditional far-field pinhole model, the large photon sieve designed by the new method focuses well on the focal plane, and the computational efficiency is greatly improved. This method could benefit the flexibility of design and fabricating the larger aperture photon sieve.
PDF Acta Optica Sinica, 2019, 39(10): 1005003
Remote sensing and sensors
The existing hyperspectral image generative adversarial network(GAN) classification algorithm cannot fully extract spectral and spatial-spectral features, which leads to the degradation of hyperspectral image classification accuracy. To resolve this issue, this study proposes a hyperspectral image classification algorithm based on a two-channel GAN. Improved one- and two-dimensional GAN classification frameworks are used to extract complete spectral and spatial-spectral features, respectively. Those features are nonlinearly fused to form a more comprehensive spatial-spectral features for classification. The experiments on two commonly used hyperspectral image datasets show that the proposed algorithm achieves the best classification accuracy; further, the results verify the effectiveness and advantages of the proposed algorithm.
PDF Acta Optica Sinica, 2019, 39(10): 1028002
A multichannel thermal infrared radiometer, called CE312, was used to study the infrared characteristics of the Dunhuang radiometric correction field. Site surface radiance and the downwelling atmospheric radiance were obtained by measuring the target site and the infrared standard plate. The multichannel temperature and emissivity separation algorithm was used to calculate the site channel emissivity and temperature. Finally, the optimal offset method was employed to obtain the site emissivity spectrum. The same target area was measured using a 102F Fourier transform infrared spectrometer. The results separated by the iterative spectral smooth temperature and emissivity algorithm were then compared with those separated by the multichannel temperature and emissivity separation algorithm. The comparison results show that the maximum deviation of the channel emissivity obtained by the two methods is within 0.011, and the site temperature deviation is within 0.104 K, indicating that the usage of the multichannel thermal infrared radiometer can separate the site temperature and emissivity in addition to obtaining high-precision thermal infrared site parameters. This test provides a reference for the automated observational absolute radiometric calibration of the satellite remote sensing thermal infrared band based on the Dunhuang radiometric correction field.
PDF Acta Optica Sinica, 2019, 39(10): 1028003
Resultsshow that the uncertainty of the calibration approach is less than 3.6%. The difference in calibration coefficient between the WDTMLR method and the reflectance-based method is less than 5.5%. Moreover, the difference between the reflectivity and radiance of ground object obtained by the satellite sensor and that of field measured value is less than 6%, which demonstrate the effectiveness of on-orbit radiometric calibration. The WDTMLR method can satisfy with the application requirements for high-precision on-orbit calibration in complex environmental conditions.
PDF Acta Optica Sinica, 2019, 39(10): 1028004
Pulsed laser frequency locking method based on molecular absorption is proposed to meet the requirement of Rayleigh high spectral resolution lidar for the frequency stability of pulsed laser emission unit. We construct a pulse laser frequency locking system based on the principle of iodine molecule absorption spectrum, using the GHz magnitude peak holding circuit, proportional integration differential (PID) control algorithm and temperature control system with accuracy of ±0.02 K. First, the accurate temperature-controlled iodine molecular absorption pool is measured by using BBO (β-BaB2O4) crystal frequency-multiplied 532 nm continuous laser, and the absorption spectra of its 1109 line at different temperatures are obtained, so as to determine the frequency discrimination curve. Second, using the fitting equation of frequency discrimination curve, the quantitative relationship between the change of pulse laser energy and the frequency shift and the measurement sensitivity are obtained. Finally, the PID control algorithm is used to compare the difference between the set value of frequency and frequency shift, and the difference is fed back to the seed laser in the form of voltage. The frequency shift of the pulse laser is compensated by changing the frequency of the seed laser, and then the dynamic frequency locking of the pulse laser is realized. The experimental results show that the frequency shift of the pulsed laser is less than ±2.2 MHz within 25 min, which can make Rayleigh high spectral resolution lidar achieve wind measurement error less than ±0.6 m/s and temperature measurement error less than ±0.5 K.
PDF Acta Optica Sinica, 2019, 39(10): 1028005
Medical optics and biotechnology
We propose a percentile half threshold pursuit algorithm (PHTPA) for applying optical molecular imaging modality to bioluminescence tomography (BLT). The BLT reconstruction problem is modeled as an L1/2 regularization problem that can be solved by combining the subspace pursuit (SP) and percentile threshold methods based on the iterative half threshold algorithm (HTA). Several simulations are run on the digital mouse model to evaluate the validity and astringency of PHTPA. The simulation results demonstrate that PHTPA produces more accurate reconstruction results in different source settings when compared with the original HTA and iterative reweighted algorithms.
PDF Acta Optica Sinica, 2019, 39(10): 1017001
Instrumentation, measurement and metrology
S-transform profilometry is a three-dimensional shape reconstruction method based on a lossless and reversible time-frequency technology. This method, a multiresolution technique, can reconstruct the three-dimensional shape of the tested object using the phase information demodulated from a single-shot fringe pattern. Herein, we analyze the factors that may affect the accuracy of S-transform profilometry. Piecewise-mean and curve-fitting methods are proposed to eliminate the background intensity of the fringe. In addition, adjusting factors are introduced into the S-transform kernel function to improve the time-frequency resolution. Simulation and experimental results verify that the proposed method exhibits high accuracy of three-dimensional shape reconstruction because of accurate S-transform coefficients.
PDF Acta Optica Sinica, 2019, 39(10): 1012001
Aiming at the problems of low precision and slow speed of the traditional reconstruction algorithms, we propose a regularization priori based fast all variation algebraic iteration (ARTTV) algorithm to improve the reconstruction precision of the symmetric and asymmetric flames. Further, to improve the reconstruction speed, we establish an extreme learning machine neural network based on the “ARTTV-particle swarm algorithm kernel”, which exhibits approximately the same reconstruction ability as that of the iterative algorithm. The construction speed of the proposed algorithm is approximately 300 times that of the iterative algorithm.
PDF Acta Optica Sinica, 2019, 39(10): 1012002
This paper reviews the research progress of polarizer coating for laser application, mainly related to the study on spectral performance, laser induced damage threshold, and coating stress. The work done by researchers at Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences to achieve the requirements of large size polarizer coating for Inertial Confinement Fusion (ICF) laser application is introduced. The research progress on coating material design, coating design, and coating preparation are also introduced. Large size polarizer coating has been developed and successfully applied to large-scale laser facilities, including SG series high-power laser and ultra-short laser devices.
PDF Acta Optica Sinica, 2019, 39(10): 1000001