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Near-infrared BRDF study of material surfaces at varying temperatures
Ma Wang-jiehui, Liu Yan-lei, Chen Zhi-ying, Liu Yu-fang
 doi: 10.37188/CO.2019-0256
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The spectral polarized BRDF of a brass surface in the near-infrared region was measured using the absolute measurement method with a device that was designed in-house. The temperature range was 20−800 ℃, and the influence of temperature on the BRDF was analyzed. The results indicate that temperature has an obvious influence on the BRDF of brass. With an increase in temperature, the BRDF was almost constant at first, then increased before finally decreasing. Scanning electron microscope testing, roughness measurement and X-ray diffraction analysis of the brass surface at different temperatures were performed. The test results indicate that the influence of temperature on BRDF can be attributed to variation in surface morphology and chemical composition.
Research on three-dimensional single-molecule localization microscopy imaging based on compressed sensing
ZHANG Sai-wen, LIN Dan-ying, YU Bin, LENG Xiao-ling, ZHANG Guang-fu, TIAN Ye, TAN Wei-shi
 doi: 10.37188/CO.2020-0003
Abstract(1) FullText HTML(5) PDF 2479KB(0)
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  Objective  In order to achieve fast three-dimensional localization of high-density fluorescent molecular images, a three-dimensional compressed sensing model was established and studied using the CVX method, the Orthogonal Matching Pursuit algorithm and a homotopy algorithm. The models’ measurement matrix was then designed.  Method  Firstly, the system’s theory and design were both developed using the three-dimensional point-spread function imaging theory of fluorescence microscopy. Then, the process of fluorescence microscopic imaging was simulated, through which the images generated in the established compressed sensing model were analyzed using the CVX method, Orthogonal Matching Pursuit algorithm and homotopy algorithm. The recall rate, localization accuracy and reconstruction time were each compared. Finally, the simulated biological samples and the collected cells in the laboratory were analyzed using the homotopy algorithm, and thus three-dimensional super-resolution imaging was achieved.  Result  It can be seen from the comparative results that the homotopy algorithm is two orders of magnitude faster than the CVX method when the reconstruction density and localization accuracy have little deviation. The localization accuracy of the homotopy algorithm is twice higher than that of the OMP algorithm.  Conclusion  The homotopy algorithm is of immediate significance for 3D super-resolution fluorescence microscopy imaging, which can save computing time and achieve real-time imaging.
Flat-field calibration method for large diameter survey mirror aperture splicing
LU Shi-tong, ZHANG Tian-yi, ZHANG Xiao-hui
 doi: 10.37188/CO.2019-0252
Abstract(7) FullText HTML(10) PDF 5073KB(0)
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The accurate flat-field calibration of large-diameter space survey telescopes is an important prerequisite for achieving some established scientific goals. At present, it is common practice to provide a uniform flat-field reference through a flat-field screen or a large-diameter integrating sphere, which is used to check the consistency of an image’s plane response. To address issues with the uniformity of flat-field screen illumination and the difficulty of preparing large-size integrating spheres, a flat-field calibration method based on sub-aperture scanning is proposed in this paper, which improves the uniformity of the flat-field reference and the uncertain calibration caused by stray light. First, we complete a sub-aperture flat-field calibration theory analysis, establish a sub-aperture flat-field calibration mathematical model, plan the sub-aperture scanning route and scan aperture size, and perform the initial design of the parameters of the collimation system for calibration. Secondly, we complete the image surface illumination simulation verification experiment. Finally, we set up an experiment to scan the planned sub-apertures, build full-aperture illuminance data, and verify the feasibility of the above-mentioned large-aperture space survey telescope sub-aperture stitching flat-field calibration scheme. The experimental results show that the full-aperture illuminance information can be restored using the full-aperture stitching method to scan the image surface energy of the system and by using the sub-aperture stitching method to compare and contrast the full-aperture image surface illuminance. The superimposed gray value in our experiment was 233.350 and the error was 1%. It is therefore verified that the sub-aperture stitching method can be used for flat-field calibration of large-diameter sky survey telescopes, and has practical value in real-world applications.
Nondestructive grading test of rice seed activity using near infrared super-continuum laser spectrum
JIN Wen-ling, CAO Nai-liang, ZHU Ming-dong, CHEN Wei, ZHANG Pei-guang, ZHAO Qing-lei, LIANG Jing-qiu, LV Jin-gaung, KAN Rui-feng, YU Ying-Hong
 doi: 10.37188/CO.2020-0027
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In view of the urgent need for seed selection technology in agriculture and for grading detection of the vigor of peeled rice seeds, we propose a new method of detecting the vigor of rice seeds based on near-infrared super-continuous laser spectrum to overcome the significant issues in pre-existing universal brown rice detection technology. Firstly, we designed a near-infrared absorption spectroscopy system that detects seed viability and measures the NIR spectra of husked rice seeds that were picked over three different years. The results showed that the activity gradient of the rice seeds was correlated with the characteristic absorption peak of their NIR absorption spectrum. Then, the spectrum of seed was optimized with a pretreatment algorithm of normalization, second derivative correction and orthogonal signal correction. Finally, a principal component analysis (PCA) model was established to reduce the dimension of the spectrum and determine the optimal number of principal components. A partial least squares discriminant analysis model (PIS-DA) was established. The analysis showed that the transmission absorption spectrum detection system designed in this paper combined with the PLS-DA discrimination model could classify rice seeds of different vigor with an accuracy of 94.44% and 95.92%. After screening, the germination rate of rice seeds could reach 97.17%. The results showed that it was feasible to achieve non-destructive classification of rice seed activity using near-infrared spectroscopy with high accuracy.
Optical coherence tomography: principles and recent developments
LU Dong-xiao, FANG Wen-hui, LI Yu-yao, LI Jin-hua, WANG Xiao-jun
 doi: 10.37188/CO.2020-0037
Abstract(3) FullText HTML(1) PDF 3754KB(1)
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Optical Coherence Tomography (OCT) is a new imaging technique that uses interference in low coherent light by measuring the delay and magnitude of backscattered or reflected signals from the sample. OCT technology can provide real-time structural information with one-dimensional depth and two- and three-dimensional tomography at micron-scale resolution. Besides its high spatial resolution, OCT imaging is beneficial for its non-contact and non-invasive methodology. The system is also easy to operate and relatively portable. OCT technology is mainly applied in the biomedical imaging field for diagnoses, making up for the shortcomings of the low penetration depth in confocal microscopes and the low resolution in ultrasonic imaging. At present, OCT technology has been used as the clinical standard for the diagnosis of retinal diseases, and the combination of OCT technology and endoscope technology has become an important tool for the clinical diagnosis of cardiovascular and gastrointestinal diseases. It also provides references for early cancer diagnosis, surgical guidance and postoperative rehabilitation of musculoskeletal diseases. To broaden the application of OCT technology and improve its medical detection capabilities, researchers are committed to increasing the penetration depth of OCT imaging in biological tissue, improving the system's resolution and signal-to-noise ratio, and optimizing its overall performance. This review introduces the principle and classification of OCT systems, their applications and their recent progress in various biomedical fields.
Application of emerging transition metal dichalcogenides in ultrafast lasers
SUN Jun-jie, CHEN Fei, HE Yang, CONG Chun-xiao, QU Jia-yi, JI Yan-hui, BAO He
 doi: 10.37188/CO.2019-0241
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Ultrafast laser technology is one of the most active research frontiers in lasers, physics and information science. It is widely applied in industrial processing, biomedicine, lidar and other fields. Because of their unique physical structure and excellent photoelectric properties, two-dimensional materials have a wide operating band, controllable modulation depth and short recovery time when they are employed as saturable absorbers in ultrafast lasers. Among them, transition metal dichalcogenides have become a focus of research because their band-gap is continuously adjustable. In this paper, we introduce the characteristics of transition metal dichalcogenides and the fabrication methods of saturable absorber devices. The research progress of ultrafast lasers based on emerging transition metal dichalcogenides is reviewed, and the development trend is highlighted.
Variable image distance bending using an elliptical bending mechanism with a constant cross-section mirror
ZHOU Bo-wen, WANG Nan, ZHU Wan-qian, XUE Song, TIAN Ying-zhong
 doi: 10.37188/CO.2019-0250
Abstract(205) FullText HTML(123) PDF 1783KB(7)
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In this paper, the surface shape error of latest elliptical bending mechanism with a constant cross-section mirror is studied when the object distance is fixed (when the position of the mirror in the light path is invariable) and the image distance is adjusted drastically (when the position of the sample is changed). Based on theoretical analysis and the finite element analysis, theoretical slope error is calculated when it is caused by bending a mirror with a width equal to an elliptical cylinder with a different shape (different image distances at the reflection point). Then, a bending experiment of a prototype of the elliptical bending mechanism is conducted. Experimental results and analysis indicate that the slope error between the bending mirror and an ideal ellipsis will increase with a decrease in image distance, and the slope error of the mirror will increase more quickly as the image distance is shortened. When the initial slope error of the reflecting mirror is 0.397 μrad, the slope error of the bent mirror in the whole range of image distance (21.5~3.8 m) is 0.402~0.560 μrad and the repeatable accuracy is 0.051 urad, which meets the design requirements of the beamline of the Shanghai Synchrotron Radiation Facility(SSRF).It is proven that in elliptically bending mirrors, continuous adjustment of the image distance from the focusing mirror can be achieved by using a bending mechanism with constant cross-section mirror.
Light intensity and spatial coherence characteristics of laser coherent detection in a turbulent atmosphere
REN Jian-ying, SUN Hua-yan, ZHAO Yan-zhong, ZHANG Lai-xian
 doi: 10.37188/CO.2019-0194
Abstract(581) FullText HTML(535) PDF 2547KB(9)
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In this paper, the cross-spectral density function of target reflected light in laser detection is obtained by using generalized Huygens-Fresnel principle and Goodman target scattering theory. On the basis of above, the expression of intensity distribution and spatial coherence length of target reflected light is derived. The influence of different light source and target reflected light parameters on the intensity distribution and coherent length of the target reflected light is simulated by using the expressions obtained in this paper under turbulent atmospheric conditions. The results show that the coherence length of the light source has little effect on the normalized light intensity distribution; the coherence length of the received light is smaller with a larger beam waist radius and reflected light radius, and the coherence length increases at a slower rate as the transmission distance increases. In the process of weak turbulent atmospheric transmission, the influence of light source parameters on the received light is much stronger. The larger the beam waist radius, the smaller the received light intensity and coherence length value. During strong turbulent atmospheric transmission, the influence of atmospheric turbulence on the received light is dominant.
Method of enhancing the quality of in-line holographic images for micro-milling tool
CHENG Ya-ya, YU Hua-dong, YU Zhan-jiang, XU Jin-kai, ZHANG Xiang-hui
 doi: 10.37188/CO.2019-0217
Abstract(510) FullText HTML(286) PDF 4472KB(11)
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When tool setting with digital in-line holography, the zero-order image and defocused twin-image can form strong and complex background noise, which gets superimposed on the real image and seriously reduces the quality of the reconstructed image. To improve quality of interferential images in digital in-line holography, a holographic image enhancement method using an improved self-snake model is proposed. The improved self-snake model selects a diffusion intensity according to the gradient of the initial image. The experimental results show that the improved self-snake model can avoid the appearance of jagged edges and “pseudo-contours” caused by large gradient background noise during the diffusion process. This improvement outweighs the shortcomings of the self-snake model in holographic imaging. In addition, compared with the phase retrieval and multi-plane reproduction approaches, the improved self-snake model filtering method proposed in this paper not only has better suppression for interferential images but also can enhance the edge of the tool, which is conducive to the actualization of tool-setting using on digital in-line holography.
Optical system design and simulation of a wide-area fundus camera
CHEN Wei-lin, CHANG Jun, ZHAO Xue-hui, JIN Hui
 doi: 10.37188/CO.2020-0066
Abstract(96) FullText HTML(52) PDF 1321KB(11)
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A wide-area fundus camera used for screening the retinae of infants was designed. In this paper, the design methods of the device’s illuminating and imaging systems were investigated. Based on James Polans’ wide-field human eye model and a set of ophthalmic anatomy data, an infant eye model was established. Then, a tapered fiber scheme was proposed for wide area illumination. Finally, the design method of a wide-area fundus camera imaging system, including the contact lens and relay lens, is introduced. The design example shows that the Field Of View (FOV) of the wide-area fundus camera can reach 130 °, and the object resolution of the fundus can reach 10 μm. The design results meet the national standards YY0634-2008 for fundus imaging equipment and meet the requirements for infant retina screening.
An automatic focusing method of a telescope objective lens based on the defocusing estimation of a circular edge response
LUO Qi-jun, GE Bao-zhen
 doi: 10.37188/CO.2019-0247
Abstract(215) FullText HTML(184) PDF 2247KB(15)
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A new defocusing estimation algorithm based on the response curve of a circular edge is proposed, through which the calibration of some focusing parameters and the automatic focusing of the telescope objective lens are achieved. The relationship between the gray-scale response of the edge of the circular pattern and the defocusing radius is established. Using the double threshold mean filtering of the defocusing radius around the circle, the accurate defocus radius of the blurred image is calculated and the influence of motion blur and noise is reduced. According to the linear relationship between defocus radius and focus distance, the broken line fitting method is used to obtain the focus distance. Then, by using multiple object and focus image distances, the parameters of the focusing model of the ranging method are optimized and the automatic focusing of the imaging system is achieved. Through simulation and experimentation, the feasibility and robustness of the defocusing estimation algorithm are verified. The images taken by the calibrated autofocus imaging system are clear, have a physical resolution that reaches half of their theoretical value, and have a resolvable line width better than 0.354 mm when the shooting distance is between 43 m and 52 m.
Influence of proximity focusing structure and electric field distribution on electron trajectory in the EBCMOS
WANG Wei, LI Ye, CHEN Wei-jun, SONG De, WANG Xin
 doi: 10.37188/CO.2020-0063
Abstract(22) FullText HTML(19) PDF 4214KB(5)
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In order to obtain high-resolution Electron Bombarded CMOS (EBCMOS) imaging devices, we study the effect of electric field distribution on the electron trajectory in proximity focusing EBCMOS devices. Three different electric field distributions are obtained by designing different EBCOMS structure, namely, the nonparallel, partially parallel, and parallel equipotential surfaces between the photocathode and the Back Thinned CMOS (BSB-CMOS). The electron trajectories in each case are simulated according to electromagnetism theory and monte carlo simulation method. The results indicate that, when the BSB-CMOS is bombarded by photoelectrons, the scattering diameter can be reduced to 30 μm under the condition that the surface of the electron multiplying layer is covered with 30 nm ultra-thin heavily doping layer and the voltage between electrodes is maintained at 4000 V while the distance between photocathode and BSB-CMOS is 1 mm. This structure is helpful to realize electrons focusing and achieve EBCMOS with high resolution. Then, the influence of the distance and voltage between the photocathode and BSB-CMOS on scattering diameter is studied. The results indicate that the electric field strength increases with the decrease of proximity distance and the increase of the acceleration voltage. This work will provide theoretical guidance for improving the resolution characteristics of EBCMOS imaging devices.
Fiber-reinforced silicon carbide and its applications in optical mirrors
ZHANG Wei, ZHANG Ge, GUO Cong-hui, FAN Tian-yang, XU Chuan-xiang
 doi: 10.37188/CO.2020-0052
Abstract(87) FullText HTML(47) PDF 1305KB(5)
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Fiber-reinforced silicon carbide composites with excellent mechanical and thermal properties are widely used in aerospace, nuclear energy, automobile, chemical industry and many other fields, especially in optical mirrors. This paper introduces the characteristics of fiber-reinforced silicon carbide composites. The advantages and disadvantages of different preparation processes of fiber-reinforced silicon carbide composites are compared. The protective effects of different interface layers on fibers and composites are expounded. The application progress of fiber-reinforced silicon carbide composites in the field of optical mirrors at home and abroad is summarized. Finally, the research direction to be carried out for realizating large-scale application of fiber-reinforced silicon carbide mirror blanks is analyzed.
Analysis of the corneal surface and peripheral defocus after orthokeratology
LIU Bao-kai, LIU Yong-ji, XIE Pei-ying, GUO Xi, GU Jian-da, YU Hao
 doi: 10.37188/CO.2019-0248
Abstract(213) FullText HTML(123) PDF 912KB(2)
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A new method of corneal shape analysis method is proposed. It not only eliminates the influence of corneal thickness on the shape of cornea after orthokeratology, but also reflects the asymmetry of a cornea. A reference surface is introduced into the analysis of the height data of the anterior surface of the cornea to eliminate the influence of corneal thickness. On the basis of above, anterior surface of the cornea is divided into the optical zone, transition zone and peripheral zone. The results show that the optical zone diameter is 1.9±0.27 mm, and the curvature radius is 8.32±0.38 mm; the transition zone diameter is 6.56±0.38 mm, and the curvature radius is 7.48±0.55 mm; the curvature radius of the peripheral zone is 10.49±1.83 mm. After orthokeratology, the horizontal refraction of the transition zone is lower than its vertical refraction. The refraction of the nasal side is greater than that of the temporal side and the refraction of the upper side is greater than that of the lower side. A semi-customized eye model is established based on the obtained parameters and the results show that its peripheral defocus is myopic after orthokeratology and its defocus is asymmetrical in each direction, which is consistent with clinical observations.
Optimal design of a 2.7 m standard spherical inspection mirror support
GAO Jing-jing, JIAO Chang-jun, HUANG Shen, ZHANG Zhen, BI Yong
 doi: 10.37188/CO.2019-0225
Abstract(172) FullText HTML(93) PDF 5376KB(5)
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Aiming at the problem that the decrease of inspection accuracy caused by an increase in the diameter of a spherical inspection mirror, the weight support parameters of the 2.7-meter standard spherical mirror are optimized and the structural design of its adjustment frame and support system is designed. Firstly, the 54-point equal-force support ring is optimized for the mirror body using a finite element and genetic algorithm. In order to increase the rigidity of the mirror body when increasing the nesting hole, the bottom support force and the side support force of each ring are optimized separately and the influence of support radius and support force error on the support deformation is statistically analyzed. Finally, based on the analysis results, a structural frame of the standard inspection mirror and a support system are designed. The analysis results show that after optimization of the 54-point support position of the standard spherical mirror, the bottom support force and the side support force of each ring and under the condition that the spherical mirror support deformation is less than 1/115λ(λ=632.8 nm), the bottom support position is disturbed by ±2 mm, the side support position is disturbed by ±0.6 mm, and the support force disturbance is ±3 N, the support deformation is less than 1/70λ, which shows little deterioration. This meets the requirements of standard spherical mirror support.
TDLAS detection of propylene with complex spectral features
ZHONG Li, SONG Di, JIAO Yue, LI Han, LI Guo-lin, JI Wen-hai
 doi: 10.37188/CO.2019-0203
Abstract(2118) FullText HTML(475) PDF 6833KB(16)
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To satisfy the need for propylene measurement in the olefin production process, Tunable Diode Laser Absorption Spectroscopy (TDLAS) was studied to improve analytical performance. In this paper, a numerical simulation approach is proposed using absorbance from a spectral database to obtain the optimized design parameters, which is independent of spectral features. In the simulation, the effect of a wider linewidth laser on the absorbance profile was considered. Through the comparison of simulation results and experimental collection, the TDLAS-based propylene analysis apparatus was developed correspondingly. It has a 1 628.5 nm center wavelength broad-tuning DFB laser. A differential method was utilized in demodulated spectral acquisition to eliminate bias voltage. The multivariate linear regression model was employed to reduce the strong spectral interference from the background components in the analysis. Based on the simulated field test, the max relative error is 0.55% in the 0~1% range for the step test. For the long-term test, the standard deviation (1σ) is 9.3×10−6 for 0.2% propylene concentration. The best standard deviation is 1.33×10−6 at 221.9 s of integration time through Allen variance analysis. In the anti-interference test, the max error of 19.17×10−6 is demonstrated for 0.2% propylene concentration while methane and ethylene concentrations vary. The disadvantages of traditional methods such as the Gas Chromatogram (GC) and soft measurement methods are overcome by modulated absorption spectroscopy. The TDLAS system for heavy hydrocarbon detection with complex spectral features was demonstrated to have distinct advantages in precision, stability and interference suppression through multivariate regression modeling.
Influence of turbulent atmosphere on the effect of coherent beam combining
SONG Ji-kun, LI Yuan-yang, CHE Dong-bo, GUO Jin, WANG Ting-feng, LI Zhi-lai
 doi: 10.37188/CO.2019-0197
Abstract(1156) FullText HTML(835) PDF 4001KB(9)
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Coherent beam combining is a promising technology for achieving a high-power laser beam with good beam quality. However, turbulent atmosphere is one of the key factors that restrict its application and development. This paper focuses on the influence of atmospheric Greenwood frequency on the correction effect of the coherent combination system based on Stochastic Parallel Gradient Descent (SPGD) algorithm. At first, the influence of different turbulence intensities on the correction effect of coherent combination systems is analyzed by numerical simulation under static atmospheric conditions. Then, a set of rotating phase screens that meet Kolmogorov’s statistical law are generated by numerical calculation to simulate the turbulent atmosphere and study the correction effect of coherent combination system at different atmospheric Greenwood frequencies. Finally, an experimental platform is established to demonstrate the coherent combination effect of two laser beams. The simulated and experimental results show that when the system's control algorithm iteration frequency (350 Hz) is constant, the disturbance of turbulent atmosphere to the phase and light intensity of laser beams will increase with atmospheric Greenwood frequency, making the effect of coherent combination worse.
Algorithmic study of total petroleum hydrocarbons in contaminated soil by three-dimensional excitation-emission matrix fluorescence spectroscopy
GU Yan-hong, ZUO Zhao-lu, ZHANG Zhen-zhen, SHI Chao-yi, GAO Xian-he, LU Jun
 doi: 10.37188/CO.2019-0216
Abstract(873) FullText HTML(326) PDF 3396KB(10)
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Assessment of Total Petroleum Hydrocarbons (TPHs) from contaminated soils demands reliable measurement to analyze the types and contents of mixed petroleum hydrocarbons in soils. Three-dimensional excitation-emission matrix (3DEEM) fluorescence spectroscopy has been demonstrated as a powerful technology for rapidly analyzing mixed petroleum hydrocarbons by identifying its abundant spectral information. However, detection precision in soil still demands improvement. This study investigates the correction methods of 3DEEM fluorescence spectra to correct the complicated matrix and scattering effects of soils. To improve the analytical accuracy, parallel factor analysis (PARAFAC) and the Alternating Trilinear Decomposition method (ATLD) were used to qualitatively and quantitatively analyze mixed petroleum contaminated soils. The methods were used to assess three commonly observed petroleum hydrocarbons: machine oil, lubricating oil, and diesel oil. Compared with the results of PARAFAC, the average recoveries of ATLD increased from 85% to 95%, implying that ATLD can effectively distinguish between similar fluorescence spectra and is more effective in the detection of the components and total content of petroleum in soil. This work can have applications of risk assessment and remediation techniques.
Design and analysis of stress-free clamping of mirrors used in free-electron laser beamlines
ZHAO Chen-hang, LU Qi-peng, SONG Yuan, GONG Xue-peng, WANG Yi, XU Bin-hao
 doi: 10.37188/CO.2019-0131
Abstract(872) FullText HTML(280) PDF 4524KB(3)
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The reflector is an important optical element in free-electron laser beamlines. Deformation error caused by gravity can seriously affect the image quality of a beamline. To reduce deformation error, a gravity compensation scheme based on the Bessel point theory is proposed and a stress-free clamping device is designed. Taking a 440 mm × 50 mm × 50 mm mirror as an example, the analysis results indicate that the deformation error in the bottom surface of a mirror clamped with the traditional support method is 1.647 μrad. Adopting the newly designed device proposed in this paper, the results of a finite element analysis showed that the deformation error reduced to 0.085 7 μrad, which is better than the engineering index of 0.1 μrad. To prevent the mirror from moving when switching modes, a small clamping force of no more than 2 N can be added to the mirror, at which point the surface error of the mirror becomes 0.093 9 μrad. Additionally, a dynamic analysis of the device is also carried out, which indicates that the device mutes the low natural frequency, which means that resonance will not occur during operation. Therefore, this scheme satisfies our requirements for the beamline.
Modification of Soluble Solids Content sorting line based on light source transmitting and receiving integrated probe
LIU Yan-de, RAO Yu, SUN Xu-dong, XIAO Huai-chun, JIANG Xiao-gang, XU Hai, LI Xiong, XU Jia, WANG Guan-tian
 doi: 10.37188/CO.2019-0165
Abstract(901) FullText HTML(950) PDF 7626KB(4)
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Traditional quality sorting methods have been unable to meet people's increasing demands for fruit flavour and quality. Producers must therefore develop their traditional quality sorting methods to achieve sugar content sorting and ensure favourable flavour and quality. To address this, the near-infrared reflection spectra of navel oranges were collected separately through two different detection methods. The spectral energy of their ring transmission and diffuse reflection had to be stronger than that of the multi-point transmission and diffuse reflections. The positions of their peaks and troughs had to be approximately the same. The near-infrared diffuse reflectance spectra were preprocessed using baseline correction, multivariate scattering correction, first and second derivatives to reduce the influence of stray light and noise. A Partial Least Squares (PLS) model for the sugar content information that was collected through the two different reflection detection methods was established for their comparison and analysis. The experimental results show that the baseline correction preprocessing method produced the best results between the two methods. Its predicted correlation coefficient of sugar under ring transmission and diffuse reflection detection was 0.81 and its root mean square error was 0.46° Brix. The estimated correlation coefficient of the sugar content model using the multi-point transmission and diffuse reflection detection method was 0.76 and its root mean square error was 0.53° Brix. This research shows that it is feasible to use PLS modeling and near-infrared diffuse reflectance spectrum to upgrade the sugar content sorting methodology used on production lines.
Fan-shaped Mid-infrared Chiral Metamaterials Based on Indium Tin Oxide and Their Circular Dichroism
ZHU Ye-xin, LI Ya-nan, SHI Wei-jie, ZHANG Wen-tao, YAN Chang-chun
 doi: 10.37188/CO.2019-0190
Abstract(558) FullText HTML(343) PDF 1224KB(17)
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A mid-infrared chiral metamaterial was designed to overcome the problems of large volume and high cost of traditional mid-infrared laser polarization state controls, The fan-shaped chiral structure material made of Indium Tin Oxide (ITO) was designed and its Circular Dichroism (CD) characteristics in the mid-infrared band were studied. The CD variation characteristics of the structure are analyzed by changing the filling material, the thickness, their size, their number, and their material. The simulation results show that when the filling material is silicon and the number of fan blade has six, the strongest CD signal is 0.052 and is obtained near the 5.3 μm wavelength by selecting the appropriate fan thickness and size. Moreover the structure composed of ITO exhibits great broadband circular dichroism compared to that with silver and gold, which provides a new concept for the design of broadband polarization-state control devices in the mid-infrared band.
Research progress of high-precision surface metrology of a K-B mirror
ZHANG Shuai, HOU Xi
 doi: 10.37188/CO.2019-0231
Abstract(294) FullText HTML(161) PDF 9566KB(13)
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The advanced light source represented by the new generation of the diffraction limit synchrotron radiation source and the full-coherent X-ray free-electron laser has become an indispensable research tool in many fields. The continuous development of advanced light sources drives the rapid progress of ultra-precision optical manufacturing. The surface precision of a K-B mirror, a key focusing optical element in advanced light sources, is an important factor, which should be less than tens of nano radians. However, high precision K-B mirror surface metrology still has great technical challenges and is now a research hotspot in the scientific community. This paper introduces typical K-B mirror surface metrology, including reflection profile measuring technology such as the Long Trace Profiler (LTP), the Nanometer Optical component Measuring (NOM), and stitching interference metrology. Current K-B mirror surface shape technologies are summarized and the upcoming research progress is prospected.
A femtosecond laser-inscribed fine-core long-period grating with low temperature sensitivity
MING Xin-yu, GUO Qi, XUE Zhao-kang, PAN Xue-peng, CHEN Chao, YU Yong-sen
 doi: 10.37188/CO.2020-0015
Abstract(169) FullText HTML(118) PDF 5819KB(10)
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In order to reduce crosstalk caused by temperature during refractive index and strain testing, the temperature, refractive index and strain response characteristics of fine-core long-period fiber gratings were studied. A long-period fiber grating with a period of 50 μm was successfully prepared on a single-mode fiber with a core diameter of 6 μm using the femtosecond laser direct writing method. The results show that long-period fiber gratings processed with low laser energy in fine-core fibers have lower temperature sensitivity, and maintain a larger extinction ratio and better spectral quality. The loss peak of this fine-core long-period fiber grating drifts only 1.7 nm in the 20 ~ 700 °C temperature range. The grating is also highly responsive to changes in the refractive index. when ambient refractive index is in the range of 1.4065 ~ 1.4265, its sensitivity reaches 882.51 nm/RIU, and its strain sensitivity is −2.2 pm/με. This fine-core long-period fiber grating can better reduce crosstalk caused by temperature in the refractive index and strain tests.
Overview of 2D grating displacement measurement technology
LIU Zhao-Wu, YIN Yun-Fei, Jirigalantu, YU Hong-Zhu, WANG Wei, LI Xiao-Tian, Bao He, LI Wen-Hao, HAO Qun
 doi: 10.37188/CO.2019-0237
Abstract(363) FullText HTML(238) PDF 1989KB(9)
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Ultra-precision displacement measurement technology is not only the basis for precision machining, but also plays a decisive role in the chip manufacturing industry that is rapidly developing such that it is following Moore's Law. The grating displacement measurement system based on the grating pitch is widely used in multi-degree-of-freedom displacement measurement. Compared with the laser displacement measurement system, the grating displacement measurement system greatly reduces the environmental requirements for humidity, temperature and pressure. In this paper, the development status of the optical structure of displacement sensing systems based on two-dimensional gratings from recent years is introduced. The principles of zero-difference and heterodyne grating interferometrys are also introduced. The optical structure based on a single-block two-dimensional grating is reviewed. The development history of the optical structure in single-block two-dimensional grating to coupling designs of multi-block two-dimensional gratings is summarized, the advantages and disadvantages of several types of two-dimensional grating displacement measurement systems are compared and analyzed, and then the development trend of two-dimensional grating displacement measurement system is speculated. Finally, the engineering process of the two-dimensional grating displacement measurement system is summarized.
Recent advances in high-power continuous-wave ytterbium-doped fiber lasers
DANG Wen-jia, LI Zhe, LI Yu-ting, LU Na, ZHANG Lei, TIAN Xiao, YANG Hui-hui
 doi: 10.37188/CO.2019-0208
Abstract(278) FullText HTML(165) PDF 4430KB(6)
Abstract:
High power continuous-wave ytterbium-doped fiber lasers have unique advantages such as high electro-optical efficiency, excellent beam quality and good thermal management. For these reasons, these fiber lasers are widely used in industrial processing, national defense and military, and scientific research. However, their non-linear and thermal effects at high-power conditions limit the further improvement of their output power. In this paper, the formation mechanism and corresponding suppression methods of stimulated raman scattering and thermally induced mode instability are analyzed. We hope that these analyses can provide some reference for the design and integration of high-power fiber laser systems. The research results for overcoming these limited factors introduced since 2015 are then discussed in detail. This paper is concluded by predicting the development prospects of high-power continuous-wave ytterbium-doped fiber lasers.
The effects of metallic contacts on the lasing characteristics of organic thin films
HAO Ya-ru, DENG Zhao-qi
 doi: 10.37188/CO.2020-0007
Abstract(258) FullText HTML(97) PDF 1252KB(8)
Abstract:
Optical loss caused by metallic contacts are thought to be a major obstacle to the achievement of organic laser diodes. We find that multi-channel emissions and Surface Plasmons (SPs) by designing a proper distributed feedback structure can allow successful lasing in organic thin films in the presence of contacting electrodes and even show better lasing performance when compared to metal-free cases. In this paper, a lower threshold (0.026 mJ/pulse) laser emission is achieved with the Ag metal electrode on the grating structure with a period of 740 nm. Since there is no increase in device thickness, the electrical properties are not reduced when the optical properties are improved.
Non-symmetrical design of a compact, lightweight HMD optical system
HUANG Song-chao, FENG Yun-peng, CHENG Hao-bo
 doi: 10.37188/CO.2019-0193
Abstract(273) FullText HTML(121) PDF 2664KB(21)
Abstract:
In non-symmetric optical systems, the field of view is narrow, the diameter of their exit pupil is narrow, their optical structure is complicated, their cost of manufacturing is high, and assembly adjustment is troublesome. To address these problems, free-form mirror is applied in the system. The design requirements and working principle of the dual mirror non-symmetrical optical system are firstly discussed. Then, the off-axis structure control method of the three-mirror non-symmetric optical system is analyzed. Finally, the XY polynomial free-form mirror is used to fold the optical path, eliminating the obstruction, enlarging the field of view, correcting the off-axis aberration, and a non-symmetrical optical system is designed suitable for Helmet-Mounted Display(HMD). The designed dual mirror non-symmetrical optical system has a field of view of 60° × 30° and a pupil diameter of 8 mm. At a cutoff frequency of 52 lp/mm, the full field of view Modulation Transfer Function(MTF) value is greater than 0.25 and system distortion is less than 5%. This monocular system’s weight is about 190 g. The design results show that the non-symmetrical optical system has an improved field of view and image quality, it is compact and lightweight, and can be applied to a HMD.
Generation of a 49-GHz, high-repetition-rate, all-polarization-maintaining, frequency-locked multicarrier
WANG Chao, XIAO Yong-chuan, LIN Shu-qing, YU Cai-bin, QU Peng-fei, LI Ru-zhang, SUN Li-jun
 doi: 10.37188/CO.2019-0191
Abstract(224) FullText HTML(106) PDF 2277KB(5)
Abstract:
Frequency-locked multicarrier with high repetition rate is an ideal tool for microwave channelization and optical communications. To meet the needs of those applications, we propose a multicarrier laser with a repetition frequency of 49 GHz. The I/Q Modulator (IQM) works at the Single-Frequency Shifting (SSB) state by carefully optimizing the Radio Frequencies (RFs) and their three bias points, resulting in a signal-to-noise ratio of 27.5 dB. The Recirculating Frequency Shifter (RFS) architecture is employed to generate an optical comb with high flatness. By optimizing the power of RFs for the balance of gain and loss of intracavity, we successfully generate 28 frequency-locked subcarriers with flatnesses lower than 3 dB and Tone-to-Noise Ratios (TNR) larger than 29 dB. Meanwhile, an Fabry-Perot (FP) etalon is used to increase the repetition-rate, resulting in 14 frequency-locked subcarriers with flatnesses lower than 2.7 dB, TNR larger than 19 dB, average powers of more than 9 dBm and carrier spacings at 49 GHz. By applying all-polarization-maintaining components and integrated technology, the system shows one-push and long-term running properties. The standard deviation of power jitter of the multi-carrier frequency comb through the half hour is only 0.5%, which shows that this scheme has great potential applications in channel communications and microwave channelization.
Design of a freeform curved prism imaging spectrometer based on an anastigmatism
ZHANG Jia-lun, ZHENG Yu-quan, LIN Chao, JI Zhen-hua
 doi: 10.37188/CO.2019-0049
Abstract(288) FullText HTML(172) PDF 1481KB(15)
Abstract:
In this paper, an algorithm for calculating the initial structure of the Offner freeform curved prism imaging spectrometer with secondary mirror external reflection is designed. The ray tracing method is used to obtain the formula for the propagation of light on the optical surfaces of an Offner spectrometer with secondary mirror external reflection. The formula can determine the structural parameters of the optical component. The off-axis beam astigmatism theory commonly used in the analysis of off-axis systems is used to analyze image quality, and a reasonable threshold is set to judge the structure algorithm. The initial structure that meets the design requirements is obtained through iterative optimization in Matlab and the initial structure is optimized in Zemax. To verify the performance of the proposed algorithm, the initial structure of a free-form prism spectrometer with a spectral range of 380~780 nm, a numerical aperture of 0.15 and a spectral resolution of 6 nm is designed. After being optimized in Zemax, the system reached the design index and the spectral line bending and color distortion are both less than 0.1 pixels. The algorithm proposed in this paper can quickly calculate the initial structure such that the requirements are satisfied, and can simplify its complexity.
Optical system design and stray light suppression of catadioptric space camera
LÜ BO, FENG Rui, KOU Wei, LIU Wei-qi
 doi: 10.37188/CO.2019-0036
Abstract(223) FullText HTML(108) PDF 2441KB(11)
Abstract:
A lens group is used as an aberration correction group to solve the limited field of view angle and low imaging contrast at a large field of view in the coaxial two-mirror optical system. The lens group adopts reasonable optical power and pitch, it expands the field of view of two-reflection mirrors and improves imaging quality in the camera's full field of view. Taking an engineering application as an example, we design and develop an optical system with a 750 mm focal length, a field of view of 2ω=3.45, an average transfer function better than 0.2 at 108 lp/mm, and an optimized design for its secondary mirror hood that suppresses stray light without a main mirror barrel hood. Simulation stray light was optimized by using TracePro software. The results show that the stray light Point Source Transmittance (PST) in the non-imaging field of view ranges from 10−3 to 10−6. The system meets the requirements for traditional ground target detection and imaging, the feasibility of a compact large-field refracting optical stray light suppression structure is verified, and a certain reference for the design and optimization of commercial coaxial refracting optical systems is provided.