Journals >Acta Photonica Sinica
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2022
Volume: 51 Issue 9
25 Article(s)
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Lightweight Real-time Detection Model of Infrared Pedestrian Embedded in Fine-scale
Yinhui ZHANG, Pengcheng ZHANG, Zifen HE, and Sen WANG
The growing number of cars makes accidents more frequent. Due to the poor visibility conditions of drivers at night, the accident rate is higher than during the day. Therefore, various assisted driving technologies to enhance driving safety are widely used to reduce traffic accidents in the nighttime environment, amongThe growing number of cars makes accidents more frequent. Due to the poor visibility conditions of drivers at night, the accident rate is higher than during the day. Therefore, various assisted driving technologies to enhance driving safety are widely used to reduce traffic accidents in the nighttime environment, among which infrared cameras have unique advantages at night. On the one hand, the visible light imaging of general cameras is easily affected by other interference light sources, and the low-quality images obtained in the nighttime environment with insufficient light will make pedestrian detection extremely difficult. The infrared camera technology based on the object's thermal radiation and reflection imaging can achieve barrier-free night vision without being affected by the interference light sources. On the other hand, the decreasing cost of infrared imaging equipment makes its application scenarios more and more common. Aiming at the night driving environment with a high accident rate, a night infrared image pedestrian detection model is proposed, which can detect pedestrians on the road at night in real-time. This research has important value and broad market application prospects in vehicle assisted driving, providing higher security for vehicles and pedestrians.Aiming at the problems of insufficient information such as color and texture of infrared images, low detection accuracy compared with visible light images, a large number of detection model parameters, and dependence on high-performance GPU resources, resulting in slow detection speed and other problems, a multi-scale embedding method fused with fine-scale pedestrian objects was proposed. Detection layer, lightweight real-time detection TIPRD model. First, to obtain more accurate infrared pedestrian location features, a 64×64 fine-scale detection layer is embedded on the original Yolov4-tiny structure to form a multi-detection layer structure, and a CSP module is added to deepen the backbone network to fuse the location features of infrared pedestrians; Secondly, in view of the relatively fixed aspect ratio of the infrared pedestrian target, K-means++ clustering is used to analyze the preset parameters of the a priori frame suitable for the infrared pedestrian target for improvement of the match between the a priori frame and the infrared pedestrian target. Finally, in order to reduce the model parameters, the model is processed through the BN layer channel pruning, and the model before pruning is used as the teacher model. At the same time, the model after pruning is used as the student model. The knowledge distillation algorithm is used instead of fine-tuning to complete the micro-control of TIPRD. While ensuring the detection accuracy, the model parameters are greatly reduced and the model is further lightened.Experiments based on the Yolov4-tiny network model show that using three strategies of fine-scale multi-detection layer embedded in 64×64, adding a CSP module and a priori box clustering can improve the detection accuracy of infrared pedestrian targets by 8.6%. But with the increase of model parameters, the model size increases by 1.4M, and the FPS decreases by 11.4 frames/s. Therefore, Yolo-pedestrian needs to be channel pruned to achieve model lightweight. After pruning the BN layer channel, the model detection accuracy will be reduced to varying degrees. Therefore, this paper uses knowledge distillation instead of fine-tuning to achieve the accuracy recovery of the model after pruning. When the pruning rate is 0.8, the model size is compressed by 20.9M, and the FPS is increased by 8.4 frames/s. The model can maintain the original accuracy after pruning through the knowledge distillation algorithm, achieving a lightweight model. Under the premise of approximating the accuracy of the Yolov4 network model, the size of the TIPRD model is only 1.5% of the Yolov4 model, which is far smaller than other detection models of the same type. Compared with the Yolo-pedestrian model before pruning, the FPS is improved by 9.4 frames/s. At the same time, the TIPRD model also has an extremely fast detection speed of 88.7 frames/s, which meets the requirements of real-time detection.For the assisted driving system with limited computing resources, a lightweight model TIPRD with high accuracy is proposed, which provides a certain reference value for the application of infrared pedestrian detection in the nighttime assisted driving system deployed on the mobile terminal. Firstly, the structure is improved based on the Yolov4-tiny network. The CSP structure is circulated on the original network structure to strengthen the network feature extraction ability, and a detection layer with a size of 64×64 is added. A feature fusion line is added between the new detection layer and the backbone network to fuse the location features of infrared pedestrians and enrich the semantic information of feature maps. And according to the relatively fixed length and width of pedestrian targets, the K-means++ clustering algorithm is used to analyze the preset model parameters of the apriori frame suitable for infrared pedestrian detection, which improves the match between the apriori frame and the pedestrian target; the model accuracy is improved by 8.6 points. Percentage points, verifying the effectiveness of our improvements on the Yolov4-tiny algorithm. Secondly, based on the improved pedestrian detection model, the BN layer channel pruning strategy is used to achieve compression, and the knowledge distillation algorithm is applied to complete the micro-adjustment of the model. On the premise of maintaining accuracy, the deep compression of the model is realised, and the model's size is compressed. At the same time, the test speed reaches 88.7 frames/s, 8.4 frames/s higher than before pruning, which meets the requirements of real-time detections. Finally, the deployment of the TIPRD infrared pedestrian detection model at night on the Jetson Nano (2GB) mobile terminal development platform is realised, and the FPS is increased by 1.7 frames/s, by which the feasibility of running the model in the mobile terminal is further verified and good engineering application value is shown..
Acta Photonica Sinica
- Publication Date: Sep. 25, 2022
- Vol. 51, Issue 9, 0910001 (2022)
Infrared and Visible Image Fusion Algorithm Based on Dynamic Range Compression Enhancement and NSST
Manli WANG, Xiaolong WANG, and Changsen ZHANG
In order to overcomes the problems of fusion image details loss, edge blur, lack of contrast and clarity existing in the existing fusion algorithms, an infrared and visible image fusion algorithm based on dynamic range compression enhancement and the non-subsampled shearlet transform is proposed. Fully retain details aIn order to overcomes the problems of fusion image details loss, edge blur, lack of contrast and clarity existing in the existing fusion algorithms, an infrared and visible image fusion algorithm based on dynamic range compression enhancement and the non-subsampled shearlet transform is proposed. Fully retain details and edge information of infrared and visible images. Firstly, the weak visible image is enhanced by the high dynamic range compression enhancement method, and the visible image with good brightness and contrast is obtained. Secondly, the infrared and visible images are decomposed by the non-subsampled shearlet transform, and the corresponding low-frequency and high-frequency coefficients are obtained. Then, the high-frequency coefficients are reduced by the hard threshold shrinkage to suppress the noise in the high frequency coefficients. The average fusion method based on visual-saliency-map weighting and the fusion method based on large absolute value are used to fuse the low-frequency and high-frequency coefficients respectively. Finally, the fused image is reconstructed by the inverse non-subsampled shearlet transform. In order to evaluate the algorithm objectively, spatial frequency, information entropy, edge intensity, average gradient, noise variance and natural image quality evaluation are used as image quality evaluation indexes. To verify the effectiveness of the proposed algorithm, feasibility verification experiment, parameter analysis experiment and performance comparison experiment were carried out respectively. The feasibility verification experimental results show that the spatial frequency, edge information and average gradient of the fused images are significantly improved compared with the original infrared and visible images, which shows that the proposed algorithm can effectively improve the contrast and clarity of the image, and has a good edge preservation ability. At the same time, the information entropy of the fused images and the original infrared and visible images have little difference, which indicates that the proposed algorithm can protect the details of the original image well. In the parameter analysis experiment, to analyze the influence of the selection of threshold shrinkage proportion coefficient on the quality of the fused image, subjective visual analysis and objective data analysis were carried out on the test results under different parameter combinations, and a group of better threshold shrinkage proportion coefficient was obtained. In the performance verification experiment, the fusion performance of the proposed fusion algorithm and other seven comparison algorithms is compared from subjective and objective aspects. Compared with the other seven algorithms, the proposed algorithm has bright background, high contrast, intact edge details, and better overall visibility of the fused image. It has advantages in spatial frequency, edge information, information entropy and average gradient, among which the advantages of spatial frequency, edge information and average gradient are more prominent, indicating that the proposed algorithm has better performance in texture detail expression, edge detail feature retention and visual clarity. To further verify the efficiency of the algorithm, 10 groups of infrared and visible images with the size of 270×360 were selected, and the average time of each group of images was obtained. The operating efficiency of the proposed algorithm is better than that of the other two comparison algorithms based on the non-subsampled shearlet transform decomposition, but lower than that of DTCWT, WLS-VSM and TE-MST. In order to verify the noise reduction effect of the proposed algorithm, Gaussian noise with variance of 5 and 10 is added to the test image respectively to form the noise test image. Eight algorithms are used to fuse the two groups of noise images respectively. Compared with the other 7 algorithms, the noise variance index of the fused image obtained by the proposed algorithm is the smallest, which indicates that its noise suppression effect is better. Experimental results show that this algorithm can fuse infrared and visible images effectively, which cannot be achieved by a single type of image, and thus improve the image identification reliably. Compared with existing fusion algorithms, this algorithm has certain advantages in detail information retention, contrast enhancement and edge blur suppression..
Acta Photonica Sinica
- Publication Date: Sep. 25, 2022
- Vol. 51, Issue 9, 0910002 (2022)
Fast Hash_LBP Moving Target Detection Algorithm Based on Hamming Distance Constraint in Complex Background
Liya QIU, Weilin CHEN, Fanming LI, Shijian LIU, Xiaoyu WANG, and Linhan LI
Moving target detection algorithm is to detect the changing region in the input image sequence, so as to extract the target from the background. It is very important for subsequent target recognition and tracking. Due to the complex and changeable natural environment, there is a large number of dynamic backgrounds and Moving target detection algorithm is to detect the changing region in the input image sequence, so as to extract the target from the background. It is very important for subsequent target recognition and tracking. Due to the complex and changeable natural environment, there is a large number of dynamic backgrounds and changing lights in complex weather, such as rain, snow, fog, vegetation shaking and water surface fluctuation, which has always been the primary problem of moving target detection in complex scenes. In this paper, when the camera has a fixed angle of view, the background modeling algorithm is adopted. To suppress the problems of dynamic background, slow target absorption and image coding noise, based on ensuring real-time performance, the time-frequency domain and frequency-domain characteristics of the input image are analyzed. Because the dynamic background fluctuates in a certain gray range. Through the texture features processed by LBP, the influence of illumination change can be suppressed. An adaptive threshold moving target detection algorithm based on texture features is proposed. First of all, the algorithm converts the input image sequence into a grayscale image, and uses the perceptual hashing algorithm to calculate the average pixel value in the window of 3×3 to remove the high-frequency details in the image part and improve the calculation speed. Then, the dynamic background and noise are processed preliminarily. The frequency domain of the input image is analyzed, and the maximum frequency of the gray frequency distribution histogram of each pixel is counted as the reference background and compared with each frame to obtain the difference matrix. The standard deviation of all values greater than 10 and less than 100 in the difference matrix is calculated as the adaptive threshold, and the gray value of each frame is corrected. Then the dynamic background and noise are processed a second time. Firstly, the Local Binary Pattern (LBP) is used to process the reference background and each frame of the image after preliminary processing to obtain the LBP value. Then, the hamming distance 3 was selected as the threshold to correct the LBP value of each frame. Finally, the LBP texture feature map of each frame is analyzed from the frequency domain, and the background modeling and foreground extraction are carried out according to the polymorphic frequency distribution. To suppress the influence of illumination change, dynamic background and noise on foreground extraction, this paper proposes an improved Hash_LBP algorithm combined with a perceptual hash algorithm and uses Hamming distance to constrain it. Experiments show that the proposed algorithm can effectively suppress noise, illumination change and dynamic background in a variety of complex scenes such as infrared and visible light, quickly and accurately extract foreground targets, and the algorithm is also effective in dynamic background suppression for ViBe and GMM algorithms..
Acta Photonica Sinica
- Publication Date: Sep. 25, 2022
- Vol. 51, Issue 9, 0910003 (2022)
Real-time Bad Pixel Detection and Compensation Method for Short-wave Infrared Camera
Xuedong SONG, Yingchao MA, Qi ZHOU, Da LIAN, Luwei YU, and Xiaonan MAO
Point target measurement camera is one of the key parts in celestial navigation technology, which is used to measure star's centroid coordinate to assist celestial navigators such as star trackers to obtain precise positions and attitude information. Point target measurement cameras using short-wave infrared focal Point target measurement camera is one of the key parts in celestial navigation technology, which is used to measure star's centroid coordinate to assist celestial navigators such as star trackers to obtain precise positions and attitude information. Point target measurement cameras using short-wave infrared focal plane detectors have the advantages of low power consumption and high detection rate. Due to factors such as manufacturing materials and production processes, infrared focal plane has bad pixels. The on-orbit infrared camera usually compensates for the bad pixels measured by ground-based test equipment. However, because the infrared camera is inevitably affected by the space irradiation environment and high-energy particles when the infrared camera is on-orbit, new bad pixels will appear, which seriously affects the camera's imaging quality and the accuracy of extraction result of star's centroid coordinate. Therefore, it is meaningful and important to study an accurate and reliable bad pixel detection and compensation algorithm for the on-orbit infrared camera. Nowadays, there are many researches on the detection and compensation of bad pixels. But these methods have some shortcomings. Some methods have a large amount of calculation in the detection process, which lead to the bad real-time performance of the on-orbit infrared camera detecting and compensating bad pixels. Other methods have low detection accuracy, because the detection and compensation results are affected by neighboring bad pixels and different background. This paper analyzes the characteristics of the gray value of bad pixels and proposes a real-time bad pixel detection and compensation method based on gray gradient and sparse vision. The method uses a
Acta Photonica Sinica
- Publication Date: Sep. 25, 2022
- Vol. 51, Issue 9, 0910004 (2022)
Research on Ultrasonic Response Characteristics of Structures Excited by Laser Transient Grating
Dong YAO, Bo GAO, Yingzheng SONG, Qun LI, and Guilong GAO
Ultrasonic Nondestructive Testing (NDT) supports field use and has a strong resolution, which is a key developing method in the field of structural health supervision and testing. In recent years, contact-type technical paths such as patch detection, medium coupling detection, and air coupling detection based on piezoeUltrasonic Nondestructive Testing (NDT) supports field use and has a strong resolution, which is a key developing method in the field of structural health supervision and testing. In recent years, contact-type technical paths such as patch detection, medium coupling detection, and air coupling detection based on piezoelectric sensors have been formed, which are widely applied to many industries. With the integration of high-quality pulsed laser technology and the research on the mechanism of laser-matter interaction, the detection technology of pulsed laser-excited ultrasound has gradually developed. This technology is expected to solve the problems of surface pollution and fixed detection area caused by traditional piezoelectric-excited adhesive sensors and coating coupling agents. The ultrasonic wave excited by pulsed laser includes longitudinal wave, transverse wave and surface wave, and its propagation velocity is related to the density and elastic constant of the material. In the past, the spot source mode and line source mode of a pulsed laser beam, as well as the line source array mode modulated by lens array and fiber bundle with fixed physical structure, limited the flexibility of spatial expansion of pulsed laser, and also restricted the development of structural response characteristics and signal-structure correlation analysis under the new excitation mode. The disadvantages of pulsed laser excitation are: the monochromatic coherence of laser restricts the modulation ability of beam spot, which leads to the limitation of ultrasonic time-frequency mode, meanwhile, the structural damage threshold limits the energy of pulsed laser, which leads to the shortage of ultrasonic signal intensity. In this paper, the spatial expansion of pulsed laser is combined with laser ultrasonic nondestructive testing technology. Then, the structural response of laser transient grating acting on aluminum alloy plate is studied from two aspects: numerical analysis and experimental research by adopting the idea of numerical analysis to reveal the law and experimental research to verify the method. By deploying observation points at different distances and directions from the center of the grid excitation, the peak gain and the decrease of energy density of the grid excitation signal are obtained for the first time, and the direction angle of near-field enhancement is revealed. The structure size of the simulation model is 50 mm×50 mm×5 mm, the number of grids is 189 062, and the number of computing nodes is 32 028. Mesh encryption is carried out near the center of the upper surface of the aluminum plate, and transition treatment is carried out in a certain range to meet the comprehensive requirements of convergence of the loading area and controllable overall calculation scale. In the field of laser processing and laser processing, aiming at the laser absorption process of rough surface, the reflection-absorption comprehensive model and lumped test method are developed to measure the laser absorption rate. In terms of numerical analysis, the influence of surface roughness on absorptivity under the framework of reflection and absorption model was studied. The excitation process of laser transient grating with a 1mm diameter, 1ns pulse width and 6 mJ single pulse input was simulated, and the comparative analysis of point laser source and line laser source with the same energy was carried out. The numerical results show that the peak value of ultrasonic signal under transient grating excitation was 2~5 times that of point source excitation when the observation distance less than or equal to 4 mm, and the surface energy density of the structure was about 1% of that excited by point source and 12.7% of that excited by line source. The principle of the experiment is that the laser beam spot generated by the pulse laser is split and interfered with by the transient grating module, then, a "bright and dark" laser transient grating is formed. The transient laser grating acts on the surface of the aluminum plate, and the ultrasonic wave is excited in the aluminum plate by thermoelastic effect, which causes the longitudinal displacement of the structural surface. The laser interferometer is used to collect the displacement of the structure surface which is 2 mm and 10 mm away from the center of grating action, and the collected signal is displayed by oscilloscope. The device includes pulse laser, transient grating module, laser interferometer, oscilloscope and aluminum alloy plate. In terms of experimental research, the transient grating module was developed, and the laser transient grating ultrasonic experiments were performed on aluminum plate. The experimental results show that the amplitude of ultrasonic was about 1 nm under 60 kHz high pass filtering, the maximum relative deviation of the surface displacement peak was 8.91%, and the deviation of surface acoustic wave velocity was 6.62%, corresponding to the signal delay at 10 mm from the center of the grating. Synthesize the above analysis, the dispersion of laser beam spot excited by laser transient grating reduces the energy density per unit area of the structure surface, and forms ultrasonic enhancement along the grating direction, which lays a good foundation for improving the signal-to-noise ratio and ensuring the safety of the structure..
Acta Photonica Sinica
- Publication Date: Sep. 25, 2022
- Vol. 51, Issue 9, 0914001 (2022)
Ultra-thin Photonic Crystal Mirror for Vertical-cavity Surface-emitting Lasers
Haijing WANG, Jun WANG, Jiachen LI, Chunyang XIAO, Yanxing JIA, Rui MING, Bojie MA, Zhuoliang LIU, Kai LIU, Yiming BAI, Yongqing HUANG, and Xiaomin REN
A new type of photonic crystal mirror was proposed as the P-side mirror of the vertical-cavity surface-emitting laser, and the reflection characteristics of the photonic crystal mirror were studied and analyzed by using the three-dimensional finite-difference time domain method. In this paper, taking 850 nm vertical-caA new type of photonic crystal mirror was proposed as the P-side mirror of the vertical-cavity surface-emitting laser, and the reflection characteristics of the photonic crystal mirror were studied and analyzed by using the three-dimensional finite-difference time domain method. In this paper, taking 850 nm vertical-cavity surface-emitting laser as an example, a photonic crystal mirror suitable for it was designed. To make photonic crystal mirrors meet the requirements of 850 nm vertical-cavity surface-emitting lasers for mirrors, it needs to have high reflectivity and wide bandwidth in the 850 nm band. The structural parameters that need to be optimized in the photonic crystal mirror include a grating layer, a two-dimensional photonic crystal layer, and a spacer layer between the grating layer and the two-dimensional photonic crystal layer. It can be seen that many structural parameters need to be optimized, so the control variable method is used to optimize the parameters. Because our research group has done a lot of research on grating structure, to reduce the amount of calculation, it is necessary to optimize the two-dimensional photonic crystal structure first and then optimize the grating structure. After optimization calculations, the optimal structural parameters of the photonic crystal mirror are that the grating period is 636 nm, the thickness is 162 nm, the duty cycle is 16.35%, the thickness of the spacer layer is 86 nm, and the air hole radius of the two-dimensional photonic crystal structure is 84 nm, the period is 212 nm, and the height is 90 nm. When the photonic crystal mirror has optimal structural parameters, its high reflectivity bandwidth in TE optical mode is 106 nm, and the ratio to the central wavelength is 12.5%. At this time, the reflectivity of the photonic crystal mirror at the center wavelength of 850 nm is greater than 99.5%. However, the reflectivity in the TM optical mode is lower than 80%, so the photonic crystal mirror not only meets the requirements of the vertical-cavity surface-emitting laser for the mirror, but also has a wide polarization selectivity. In terms of the thickness of the mirror, the overall thickness of the photonic crystal mirror is very thin. Its thickness is 338 nm, which is 12.4% of the thickness of conventional P-type distributed Bragg mirrors and 61.7% of the thickness of high-contrast subwavelength grating mirrors. In terms of thermal conductivity of materials, the equivalent thermal conductivity of the photonic crystal mirror is 46% higher than that of the distributed Bragg mirror, so the photonic crystal mirror has good equivalent thermal conductivity. Therefore, considering factors such as the thickness of the mirror and the equivalent thermal conductivity, the photonic crystal mirror is beneficial to heat dissipation and improves the optoelectronic performance of the laser. In addition, the etching depth of the photonic crystal structure of the photonic crystal mirror is relatively small, so it will hardly affect the series resistance and current density. Moreover, the two-dimensional photonic crystal structure has strong optical confinement, which is also beneficial to further reduce the threshold current density of the laser. In addition, the photonic crystal mirror also avoids the stress problem caused by volume shrinkage after oxidation of high-aluminum compounds in high-contrast subwavelength gratings to form low-refractive-index oxides.Therefore, the new photonic crystal mirror proposed in this paper can replace the P-type distributed Bragg mirror of the traditional vertical-cavity surface-emitting laser. At the same time, photonic crystal mirrors are conducive to promoting the rapid development of vertical-cavity surface-emitting lasers in optical communication, optical interconnection, and optical information processing.They also conducive to promoting vertical-cavity surface-emitting lasers to enter new application fields continuously..
Acta Photonica Sinica
- Publication Date: Sep. 25, 2022
- Vol. 51, Issue 9, 0914002 (2022)
Research on All-solid-state Continuous-wave 228 nm Deep Ultraviolet Laser
Zhibin ZHAO, Cheng CHENG, Quan LI, Dongxin XU, Hao CHEN, Guojun LIU, Zhongliang QIAO, Debo WANG, Quan ZHENG, Yi QU, and Baoxue BO
Deep UV laser has very important applications in sterilization, Raman spectroscopy and material processing. In this study, the 914 nm fundamental frequency optical cascade nonlinear optical frequency conversion was realized by the frequency doubling in the V-shaped cavity and Lens focusing method, extra-cavity quadruplDeep UV laser has very important applications in sterilization, Raman spectroscopy and material processing. In this study, the 914 nm fundamental frequency optical cascade nonlinear optical frequency conversion was realized by the frequency doubling in the V-shaped cavity and Lens focusing method, extra-cavity quadruple frequency structure, 228 nm CW laser was obtained. In general, higher average power and better beam quality were required for the continuous operation laser to achieve the extra-cavity frequency doubling compared with the pulse operation. In order to obtain a higher output performance of 457 nm CW laser, this paper firstly analyzes the influence of the length change of the V-shaped cavity arm on the spot size at different positions in the cavity by theoretical calculation. The effects of different arm lengths of V-cavity on the output performance of 457 nm laser generated by LD end pump Nd:YVO4/LBO was investigated experimentally. Finally, at pump power of 26 W, 457 nm CW laser output of 2.2 W, TEM00, has been achieved. On the basis of frequency doubling, extra-cavity frequency doubling using Type-I phase matching BBO crystal, was performed, and 228 nm CW DUV laser with 6 mW power has been achieved, the laser spot was elliptic and the power stability was 1.8% within one hour. The pump source is an 808 nm fiber-coupled LD with a core diameter of 400 μm and a numerical aperture of 0.22, with maximum CW power of 110 W. The pump light coupling system consists of two plano-convex mirrors with a focal length of 10 mm and a 45° polarizer, and its imaging magnification is close to 1∶1. The parameters of Nd:YVO4 crystal are: Nd3+ atomic doping concentration is 0.1%; 4×4×5 mm3 in size; the left facet was antire-flection coated at 808 nm and 1 064 nm and high reflection coated at 914 nm; the right facet was antireflection coated at 914 nm, 1 064 nm and 1 342 nm wavelengths. The laser crystal was wrapped in a layer of indium foil on the side and secured on a copper heat sink, which is capable of controlling the temperature through circulating water cooling. The output mirror M is a flat concave mirror with a curvature radius of 100 mm. It is coated with 914 nm high reflection film, 457 nm, 1 064 nm and 1 342 nm antireflection film. The high reflection mirror M2 was a flat concave mirror with 200 mm in radius of curvature which was high reflection coated at 457 nm and 914 nm. The V-shape cavity was formed by the left facet of Nd:YVO4 crystal M1 and M and M2, where the angle between the two arms is 5°. The size of LBO frequency doubling crystal is 4×4×15 mm3, both facets of which were antire-flection coated at 457 nm, 914 nm and 1 064 nm. M3 is a 457 nm focusing lens with a focal length of 150 mm and coated with 457 nm antireflective film. The size of BBO frequency doubling crystal is 4×4×8 mm3, and the Type-I phase matching was adopted, the cutting angle is θ=61.4°. The 457 nm laser is output from mirror M and focused through M3 focusing lens, BBO crystal is placed at the focus, and 228 nm CW deep UV laser was obtained by frequency doubling of BBO crystal. The splitting prism M4 was used to separate 457 nm and 228 nm lasers. Considering the thermal focal length generated by Nd:YVO4 crystal, ABCD matrix and stable cavity conditions were used to calculate by Matlab program, and the influence of arm length L1 and L2 on the size of light spot in the cavity was obtained. As can be seen spot size at different positions of the cavity is insensitive to L1 length change, but sensitive to L2 length change. The L2 length of the V-cavity was optimized experimentally to obtain a more appropriate mode matching between the pump light and the fundamental frequency light. The maximum output power of 457 nm CW laser was 2.2 W and the light spot was TEM00 mode when the pump power was 26 W. Finally, 457 nm CW laser with output power of 2.2 W passes through the BBO crystal to produce 228 nm deep UV laser with 6 mW power. The relationship between the output power of 228 nm laser and the injected power of 457 nm laser is shown, and the stability is 1.8%..
Acta Photonica Sinica
- Publication Date: Sep. 25, 2022
- Vol. 51, Issue 9, 0914003 (2022)
210~250 nm Tunable Narrow Linewidth Ti：Sapphire Laser
Meng LI, Xin MENG, Jinming HU, Jingjing CHENG, and Guilin MAO
All-solid Ti:Sapphire tunable laser at 210~250 nm is of great importance in laser processing, laser etching, biomedicine and spectroscopy. Especially, it has good advantages in spectroscopy. In the field of Raman spectroscopy, the excitation efficiency of ultraviolet lasers is much higher than visible light. In the fieAll-solid Ti:Sapphire tunable laser at 210~250 nm is of great importance in laser processing, laser etching, biomedicine and spectroscopy. Especially, it has good advantages in spectroscopy. In the field of Raman spectroscopy, the excitation efficiency of ultraviolet lasers is much higher than visible light. In the field of the K-band of ultraviolet absorption spectroscopy, 210~250 nm is an important band for analyzing the molecular structure of substances. For example, molecules containing two conjugated double bonds can be detected when the strong absorption peaks occur. These require that the tunable band of Ti:Sapphire laser is converted to the ultraviolet band through nonlinear optical frequency doubling technology. Moreover, the ultraviolet tunable lasers should have the advantages of good stability, wide tunable range, good beam quality, high laser signal energy, and good directivity. To meet the above application requirements, the research of all-solid Ti:Sapphire tunable laser at 210~250 nm is demonstrated. Based on the principle of Ti:Sapphire tunable laser, the characteristic of the Ti:Sapphire crystal is learned. Given the rate equation, the interation between Nd:YLF 527 nm pump source and Ti:Sapphire crystal is studied. Besides, it is studied that the principle of frequency doubling output of Ti:Sapphire fundamental frequency from infrared to ultraviolet. Moreover, the design index and overall scheme of the Ti:Sapphire ultraviolet tunable laser is given. The key technology of TEM00 mode Nd:YLF 527 nm laser is studied. Firstly, the basic principle of Nd:YLF higher-power laser based on MOPA technology is studied. Secondly, the schematic of 527 nm pump source laser based on MOPA technology is studied and the pump laser is developed. Finally, the output characteristics of the 527 nm laser is analyzed. The design method of Ti:sapphire fundamental frequency cavity is studied. Based on the method, a fundamental frequency cavity based on the straight cavity structure using grating selection frequency is designed. Furthermore, the techniques of tuning and narrow linewidth compression are researched. Last, the nonlinear frequency doubling technology, phase matching principle, and factors affecting frequency doubling conversion efficiency is deeply researched. Given the theory above, the crystals of second-harmonic generation and crystals of fourth-harmonic generation are selected and designed. A Ti:Sapphire tunable laser system is developed, and the output characteristic parameters are collected and analyzed. System obtains the fundamental frequency laser of 760~1 000 nm, and obtains the maximum fundamental frequency power of 912 mW at 820 nm. The second harmonic generation of 420~500 nm is obtained using LBO crystal, and the maximum power of 380 mW is obtained at 440 nm. Then, the fourth harmonic generation of 210~250 nm is obtained using BBO crystal. Maxi-mum conversion efficiency of fourth harmonic generation is achieved with 33.3% at 230 nm. At 220 nm, the stable output of up to 85 mW is obtained, and its fluctuation does not exceed 1.5% within 120 minutes..
Acta Photonica Sinica
- Publication Date: Sep. 25, 2022
- Vol. 51, Issue 9, 0914004 (2022)
Fabrication Technology of Distributed Feedback Semiconductor Laser Based on Buried Metal Mask
Yu SHANG, Jie FAN, Haizhu WANG, Yonggang ZOU, and Xiaohui MA
With the development of semiconductor laser technology, surface grating semiconductor laser devices’ structure is becoming more complex. An independent micro nano process is often required to fabricate surface grating structures. There are some difficulties in the manufacturing process of ridge surface grating. Due to With the development of semiconductor laser technology, surface grating semiconductor laser devices’ structure is becoming more complex. An independent micro nano process is often required to fabricate surface grating structures. There are some difficulties in the manufacturing process of ridge surface grating. Due to the ridge waveguide and other structures made in advance, the surface of epitaxial wafer presents a state of ups and downs, and the photoresist pattern edge as grating mask is prone to deformation, which destroys the grating morphology and affects the performance of grating. If the process scheme of etching the ridge waveguide first and then grating etching is adopted in the manufacturing process, the first etched ridge waveguide will make the surface of the epitaxial wafer uneven, resulting in uneven photoresist thickness after spin coating and damage the exposure pattern. Moreover, due to the uneven surface of the insulating dielectric layer produced after this process, it is difficult to corrode the insulating layer on the grating when corroding the insulating layer to form the electrode window, which will eventually affect the carrier injection and damage the output power of the semiconductor laser. Therefore, in the fabrication process of surface grating semiconductor laser, the fabrication process of surface grating is usually carried out first. However, this requires additional grating protection to avoid the subsequent device process damaging the grating structure, which will make the device preparation process cumbersome and not conducive to the preparation. In addition, during the subsequent fabrication of ridge waveguide, it is necessary to homogenize the photoresist again to expose the ridge waveguide. During the homogenization, the photoresist on the surface of epitaxial sheet can not be evenly distributed due to the grating structure existing in advance, which will affect the exposure of ridge waveguide and further lead to the residual photoresist in the grating groove during development, which is difficult to remove after subsequent process steps, damage to the quality of the device. In this paper, a fabrication process of surface grating distributed feedback semiconductor laser based on a buried metal mask is proposed. This fabrication process can reduce the influence of device process on the grating structure without additional grating protection process. At the beginning of the process, a Ni-Au metal layer is fabricated on the surface of semiconductor epitaxial wafer to form a hard mask of surface grating. After the process of waveguide and passivation layer, the passivation layer on the surface of the waveguide is removed to form the electrode injection window and expose the buried Ni-Au metal mask. Under the blocking effect of buried Ni-Au metal mask and passivation layer, a surface grating structure is formed on the surface of waveguide by dry etching process. The high-order surface grating distributed feedback semiconductor laser diode with grating period of 10 μm is fabricated by the fabrication process. By stripping out the metal hard mask on the surface of the epitaxial wafer in advance and making the insulating dielectric layer first, and then etching the grating, the residual problem of SiO2 insulating layer is avoided. Due to the solid texture of the metal hard mask, the morphology of the grating and wide strip ridge waveguide structure will be more intact than that of the photoresist mask, which expands the manufacturing process of surface grating distributed feedback semiconductor laser and is also conducive to the improvement of device performance. The experimental results show that, compared with the fabrication process using photoresist as surface grating mask, the fabrication process based on buried metal mask ensures the morphology of the surface grating and the periodic distribution of the refractive index in the surface grating. Therefore, the single longitudinal mode half height full width of the device is reduced from 0.56 nm to 0.23 nm, and an output power of 242 mW is obtained at 1 A. At the same time, the far-field spot of the device is intact and the beam quality is good. This fabrication process improve the morphology of the grating and enhance the spectral characteristics of the device effectively. The new device technology ensures the morphology of the fabricated grating and is conducive to the improvement of the performance of laser devices, it provides more flexible and diverse methods for the fabrication of the same type of devices..
Acta Photonica Sinica
- Publication Date: Sep. 25, 2022
- Vol. 51, Issue 9, 0914005 (2022)
Effect of Mg2+ Doping on Luminescence Properties of the Up-conversion Luminescent Material Na2Zn3Si2O8：Er3+
Biyinue·jilili Su, Lidan·aieken Pa, Qingling Wang, Hanguli·alimu Re, Jing Gao, and Eken·sidike Ai
In recent years, up-conversion luminescent materials have received increasing attention because of their excellent luminescence properties in various fields such as biology, chemistry, medicine, and lighting. According to research reports, studies on up-conversion luminescent materials have mainly focused on fluorides,In recent years, up-conversion luminescent materials have received increasing attention because of their excellent luminescence properties in various fields such as biology, chemistry, medicine, and lighting. According to research reports, studies on up-conversion luminescent materials have mainly focused on fluorides, oxides and other substrate materials that usually exhibit relatively poor physical and chemical stability, so the selection of substrate materials with closely matched lattice and dopant ions is the key to achieving high efficiency in up-conversion luminescence. In this paper, Na2Zn3Si2O8: x%Er3+, y%Mg2+ orange-red phosphor was successfully prepared by the high-temperature solid-phase method using Na2Zn3Si2O8, which is a silicate with stable physical and chemical properties, as the main matrix and Er3+ ions with rich energy level structure as the activator. The effects of the alkali metal ion Mg2+ on the up-conversion luminescence of Na2Zn3Si2O8: x%Er3+ phosphors were analyzed by a series of characterization tests, including crystal structure, X-ray Diffraction (XRD) and Rietveld refinement, up-conversion emission spectra, Fourier Transform Infrared Spectra (FT-IR), variable temperature emission spectra, monochromatic analysis, Chromaticity Coordinates (CIE), Color Temperature (CCT) and Color Purity (Cp). Na2Zn3Si2O8 belongs to a trigonal structure with space group P1(1) and cell parameters of a=0.512 4 nm, b=0.883 nm, c=1.350 4 nm, α=72.59°, β=101.76°, γ=89.11°, v=0.568 8 nm3, and can provide a good crystal field environment for the luminescent center (Er3+). The XRD and Rietveld refinement results show that the doped ions Er3+ and Mg2+ successfully replace the lattice positions of Zn2+ ions with similar ionic radii under the synthesis conditions of 950℃/180 min. The doping of alkali metal ions Mg2+ does not cause the formation of spurious peaks or secondary phases, which further proves that the synthesized phosphor is a pure phase structure. The refinement factor further proves that the synthesized phosphor is a pure phase structure. The characteristic green and red emission of Er3+ ions in the Na2Zn3Si2O8: x%Er3+, y%Mg2+ series phosphors can be seen in the upconversion emission spectra under 980 nm excitation, and the concentration burst phenomenon occurs at the doping concentration >3%. The concentration burst between adjacent Er3+ ions is caused by electric quadruple-electric quadruple (q-q) interaction; the doping of Mg2+ ions enhances the characteristic emission peak of Na2Zn3Si2O8:3%Er3+ at 661 nm by a factor of 16. The weakening of the absorption peaks in the Fourier Transform Infrared Spectra (FT-IR) spectra can prove that the Na2Zn3Si2O8:Er3+ and Mg2+ series phosphors are pure phase structures and the reason for the enhanced luminescence. To further demonstrate the thermal stability of the synthesized phosphors, the emission spectra were analyzed in the range of 25℃ ~250℃. It is found that when the detection temperature reaches 150℃, the integrated intensity of the emission peaks of Na2Zn3Si2O8: Er3+ and Mg2+ phosphors at 661 nm is 73.68% of that at room temperature (25 ℃), and the peak intensity of the emission peaks is 66.63% of that at room temperature (25 ℃). This indicates that the phosphor has good thermal stability of luminescence. Finally, the monochromaticity (Sgr) and CIE coordinates of the green and red emission of Na2Zn3Si2O8: Er3+, Mg2+ series phosphors were analyzed. It is found that the orange-red emission (Sgr≈-1) of the phosphor at different temperatures is consistent with the CIE coordinates, and the CIE coordinates are close to the standard red emission coordinates (0.67, 0.33), and the color purity reaches 71.45%. The optimized Na2Zn3Si2O8: Er3+, Mg2+ series phosphors have good luminescence efficiency, good luminescence thermal stability and color temperature, which are useful for the exploration of high-efficiency orange red up-conversion luminescent materials and solid-state lighting..
Acta Photonica Sinica
- Publication Date: Sep. 25, 2022
- Vol. 51, Issue 9, 0916001 (2022)
SERS Detection of Furfural in Oil Based on CNTs-FAgNPs Substrate
Yinhui YU, Wenjiang ZHU, Sumin WU, and Qian ZHOU
As we all know safe and reliable operation of electrical equipment is the first line of defense to avoid major accidents in power systems. Electrical equipment will produce some aging characteristics due to electrical and thermal aging in the operation process, such as furfural, acetone, methanol, formic acid, acetic aAs we all know safe and reliable operation of electrical equipment is the first line of defense to avoid major accidents in power systems. Electrical equipment will produce some aging characteristics due to electrical and thermal aging in the operation process, such as furfural, acetone, methanol, formic acid, acetic acid, etc. The study shows that furfural can be used to evaluate the aging life of transformers accurately. At present, the detection of furfural dissolved in oil mainly adopts high performance liquid chromatography and ultraviolet spectrophotometry. However, oil-paper insulation degradation products are diverse and complex, and these traditional methods are only suitable for laboratory operations. Raman spectroscopy enables in situ detection of furfural dissolved in oil without the need for extraction. In 2015, the Osaka Laser Technology Research Center in Japan realised the first in-situ Raman spectroscopic detection of furfural standard samples in transformer oil, with a minimum concentration of 104 mg/L, which is far from the standard for oil-paper insulation aging. Surface Enhanced Raman Spectroscopy (SERS) can effectively improve the detection sensitivity of the features to be tested, and it is very effective to introduce this technology to evaluate the aging state of electrical equipment. To the aging of the transformer oil in many features high sensitivity tests of furfural, the researchers developed the gold and silver core-shell structure used as SERS substrate. They found that the furfural characteristic peak intensity corresponding to the relative standard deviation of only 5.8%, the stability of the core-shell structure of basement is higher. Still, the gold nanoparticles were almost were silver shell coated leading to the sensitivity of detection can't play to the largest, the detection limit could only reach 20 mg/L, and the antioxidant capacity was weak. To solve the problems of SERS substrate such as low sensitivity, easy oxidation and poor homogeneity, some scientific researchers introduced carbon nanomaterials into the preparation of SERS substrate. Based on their large specific surface area, strong adsorption capacity and oxidation resistance, when combined with precious metal materials with strong electromagnetic enhancement characteristics, they were used to prepare of SERS substrate. It is expected that more metal nanoparticles will be adsorbed on the surface of the carbon material, forming more "hot spots" and generating stronger Raman signals. SERS has the potential to improve the in situ detection limit of dissolved furfural in oil. In this paper, the chemical reduction method was adopted, AgNO3 was used as the precursor, ascorbic acid was used as the reducing agent, the crystal morphology was controlled by PVP, and the rough flower-like silver nanoparticles were generated by controlling the stirring rate. The functionalised CNTs were mixed with flower-like silver nanoparticles to prepare carbon nanotube-modified flower-like silver nanoparticles (CNT-FAgNPs). The key point of this technology is that after acidification treatment, carbon nanotubes adsorb more hydroxyl and carboxyl groups on their surface, which is convenient for better combination with FAgNPs. The CNT-FAgNPs were adsorbed on the surface of gold-coated (Indium Tin Oxide, ITO) glass as the SERS substrate to achieve high-sensitivity in situ detection of furfural dissolved in oil with different concentrations. Studies have shown that the structural gap of the rough three-dimensional nanostructures formed by the combination of nano silver flowers with electromagnetic enhancement effect and CNTs can effectively enhance the Raman signal, while the large specific surface area and strong adsorption capacity of CNTs promote the formation of more "hot spots". The lower limit of detection for furfural in transformer oil is up to 7 mg/L, and the relative standard deviation of characteristic peaks at different substrate positions is 3.01%, which proves that the prepared substrate has good sensitivity and consistency, showing great potential for in-situ detection of trace aging characteristics in transformer oil..
Acta Photonica Sinica
- Publication Date: Sep. 25, 2022
- Vol. 51, Issue 9, 0930001 (2022)
[in Chinese]
Zhanshan Wang, Huasong Liu, and Jinlong Zhang
Acta Photonica Sinica
- Publication Date: Sep. 25, 2022
- Vol. 51, Issue 9, 0951600 (2022)
Fiber Optics and Optical Communications
Generation of Visible to Near-infrared Supercontinuum in Highly-nonlinear Photonic Crystal Fiber
Haihu YU, Jianwen WU, Yue MA, Xiaotao YANG, and Yu ZHENG
Supercontinuum refers to the phenomenon that the spectrum of a high power pulse transmitted in a nonlinear medium is broadened by a variety of nonlinear effects. The generation of supercontinuum can greatly broaden the spectrum of optical signals, usually to the range of tens to hundreds of nanometers. In addition, supSupercontinuum refers to the phenomenon that the spectrum of a high power pulse transmitted in a nonlinear medium is broadened by a variety of nonlinear effects. The generation of supercontinuum can greatly broaden the spectrum of optical signals, usually to the range of tens to hundreds of nanometers. In addition, supercontinuum light has the advantages of super brightness, wide band, high stability and high spatial coherence. Therefore, it has great application value in many fields, such as optical frequency calculation, optical communication, optical coherence tomography and biomedical science. In 1976, the supercontinuum in fiber was observed for the first time in nanosecond pulses generated by dye lasers, but the band coverage of the supercontinuum is narrow and the pump power required is high. The photonic crystal fiber made up for these deficiencies. Photonic crystal fiber, also known as microstructured fiber, is a special fiber whose cross section has two-dimensional periodic refractive index variation and extends indefinitely along the fiber axis. Photonic crystal fibers exhibit many special optical properties due to the variation of refractive index contrast between core and cladding. Photonic crystal fiber plays an important role in optical fiber communication, optical fiber sensing, meteorology, medical imaging and supercontinuum generation due to its flexible structure and special optical properties. Photonic crystal fiber can obtain the position of zero dispersion point at the desired wavelength by adjusting the structure, thus generating supercontinuum spectrum based on various nonlinear effects. For the generation of supercontinuum in photonic crystal fibers, the excitation conditions and the optimization and adjustment of its flatness and spectrum width have always been the focus of research. Especially in engineering applications, supercontinuum has many specific requirements. Although some progress has been made in the study of visible to near-infrared supercontinuum in photonic crystal fibers, the reported spectrum broadening is still limited and the required fiber length is relatively long. In order to improve the spectral width and make it have higher application value, in this work, based on the simulation calculation to explore the influence of optical fiber air hole structure size on dispersion, the optimized optical fiber structure geometric parameters are obtained. After independently designing the optical fiber, a kind of solid-core photonic crystal fiber with high nonlinearity is obtained by using the stack method. The full vector finite element method was used to simulate the photonic crystal fiber, and the zero dispersion point of the photonic crystal fiber was obtained at 880 nm. At the pump wavelength, the photonic crystal fiber has a nonlinear coefficient of 33.67 km-1?W-1 and an effective mode field area of 4.72 μm2. Through the establishment of a supercontinuum experimental device, a 1 030 nm, 150 fs linear polarization ultrafast fiber source is coupled into the photonic crystal fiber, and the coupling efficiency is 52.7% at low power. The generation process of supercontinuum from visible to near infrared region is studied under different pump power and different optical fiber length. It can be seen from the analysis that when the average pumping power increases in 0.5 m long photonic crystal fiber, the output spectrum broadening increases accordingly. At the maximum average pump power of 1 320 mW, a supercontinuum spectrum with a broadening range from 475 nm to 1 870 nm is obtained, and the flatness of the spectrum is improved compared with that at low pump power. By studying the effect of fiber length on the supercontinuum spectrum, it can be found that the supercontinuum is further broadened and flatness is improved with the increase of fiber length under the condition of constant average pumping power. Finally, the broadband supercontinuum output from 450 nm to 1 900 nm was achieved in the 1.5 m long photonic crystal fiber, and the spectrum has good flatness and coherence. Such broadband light sources have potential applications in optical coherence tomography, spectroscopy, communications, early cancer detection and food quality control..
Acta Photonica Sinica
- Publication Date: Sep. 25, 2022
- Vol. 51, Issue 9, 0906001 (2022)
Precision Tracking System Based on Non-dominated Sorting Genetic Algorithm II Intelligent Parameter Search
Xu WANG, Liang ZHANG, Chengxiang TU, Tingting WANG, and Jianyu WANG
Because the signal light is very weak and the communication link is easily interfered, space optical communication and space quantum communication require extremely high precision for the tracking system of the terminal payload. The accuracy of the fine tracking subsystem determines the tracking accuracy of the entire Because the signal light is very weak and the communication link is easily interfered, space optical communication and space quantum communication require extremely high precision for the tracking system of the terminal payload. The accuracy of the fine tracking subsystem determines the tracking accuracy of the entire terminal system. Therefore, in the fine tracking stage, the system must meet the requirements of high precision and large bandwidth. However, the accuracy of traditional fine tracking systems is easily affected by external disturbances. To achieve higher tracking accuracy and stronger interference suppression capability, based on the traditional precise tracking system of typical optical communication terminals, a design method of an additional integrated module is proposed, and this module is cascaded after the PID controller of the traditional control system. Based on performance indicators such as control bandwidth, interference suppression capability, and stability, the non-dominated sorting genetic algorithm II is used to obtain the global optimal controller parameters, and a precise tracking system with intelligent parameter search is realized, which can achieve the tracking accuracy of sub-micro radian scale. Based on the measured angular interference data of a typical optical communication satellite terminal in orbit, the simulation compares the new system and the traditional system. The results show that on the basis of maintaining the stability of the closed-loop system, the new system can increase the error suppression bandwidth by 33.7% and improve the interference suppression ability of the full frequency band by 19.5%, of which the interference error suppression performance within 10 Hz is improved by more than 95%. For the four frequency bands of 0~1 Hz, 1~10 Hz, 10~50 Hz, and 50~100 Hz, the accuracy of the new system is improved by 99.5%, 95.7%, 71.3%, and 29.9% respectively compared with the traditional system. A physical verification system is built in the laboratory environment, and it is verified that the tracking accuracy and interference suppression performance of the system are greatly improved compared with the traditional system, especially below 10 Hz, the improvement rate is more than 20 times. When the interference frequency is 5 Hz and 10 Hz, the interference rejection ratio of the system reaches -53.57 dB and -46.31 dB, respectively. The experimental results and simulation results are consistent with a good fit. The system can be used in space optical communication scenarios with longer distances and higher precision requirements, which is of great significance to the development of the future space optical communication field..
Acta Photonica Sinica
- Publication Date: Sep. 25, 2022
- Vol. 51, Issue 9, 0906002 (2022)
Multi-aperture High Resolution Imaging Technology Based on Plastic Imaging Fiber Array
Xiaorui DU, Huangrong XU, Wenlong LI, Zhengquan HE, and Depeng KONG
Imaging fiber plays an important role in medicine, industry, aerospace and other fields because of its excellent flexibility, especially in the application of optical fiber endoscope in medicine. Optical fiber image transmission system is usually composed of imaging objective, imaging fiber and image sensor.At present,Imaging fiber plays an important role in medicine, industry, aerospace and other fields because of its excellent flexibility, especially in the application of optical fiber endoscope in medicine. Optical fiber image transmission system is usually composed of imaging objective, imaging fiber and image sensor.At present, the number of pixels in cameras can reach millions or even tens of millions, but the number of pixels in optical fibers is usually only a few hundred thousand.Therefore, the resolution of the system is limited by the resolution of the imaging fiber itself, and the imaging resolution of the whole system basically depends on the number of pixels that the imaging fiber can transmit. At present, the imaging fiber bundles on the market have either high resolution but small total cross-sectional area, or large cross sectional size but fiber diameter up to ten microns. This phenomenon results in insufficient pixels and small image area of high resolution image fiber, while large cross section can not reach high resolution due to technological limitations. To solve the problems, this paper proposes a multi-aperture high-resolution imaging technology based on imaging fiber array, which uses the imaging fiber array and image Mosaic technology to break through the bottleneck of improving pixel number. The number of pixels in the system can be increased by using high resolution and small cross section imaging fiber arrays. Combined with the characteristics of overlapping imaging of microlens array, the problem of information loss caused by direct imaging of imaging fiber array can be solved and the integrity of optical fiber array imaging can be realized.This method is expected to increase the number of pixels in optical fiber image transmission system to millions of order of magnitude and improve the resolution of the system. The imaging fiber is designed to be arranged 6×8, and the microlens array is designed based on the imaging fiber array. There are two groups of aspherical lenses made of PMMA material, and the imaging fiber array and the two groups of microlens arrays have uniform positions. Add a telecentric objective lens in front of the microlens array as the main lens of the image transmission system to solve the problem of complete overlap of adjacent subgraphs caused by direct imaging of the microlens array. The focal length of the lens is 10.1 mm, the aperture coefficient is 6.3, and the field Angle is 88°. The simulation results show that both the main lens and the microlens array can meet the performance requirements of the imaging fiber, and the object information can be successfully transmitted to the imaging fiber. The modulation transfer function value of the system can reach more than 0.5 at 50 lp/mm, without weakening the quality of the primary image, and meet the resolution requirements of the imaging fiber. Experimental results show that the system contains 400 000 effective pixels and the system resolution is 40 lp/mm.The image is clear and complete, which proves that the design of the imaging system has a good feasibility, and has an important practical reference significance for improving the resolution of the optical fiber image transmission system..
Acta Photonica Sinica
- Publication Date: Sep. 25, 2022
- Vol. 51, Issue 9, 0906003 (2022)
Back-reflection Noise Analysis of the Resonant Micro-optical Gyroscope with Different Modulation Techniques
Yi LIN, Yiqun WANG, and Huilian MA
The resonant micro-optical gyroscope has advantages in miniaturization and integration compared with other gyroscopes. The back-reflection noise is one of the main optical noises restricting the sensitivity of the resonant micro-optical gyroscope which mainly comes from the coupling points between the waveguide ring reThe resonant micro-optical gyroscope has advantages in miniaturization and integration compared with other gyroscopes. The back-reflection noise is one of the main optical noises restricting the sensitivity of the resonant micro-optical gyroscope which mainly comes from the coupling points between the waveguide ring resonator and the tail fiber. The model of the back-reflection noise of the resonant micro-optical gyroscope based on the reflection-type waveguide ring resonator is established. The influences of the intensity item and the interference item of the back-reflection noise under the reciprocal system and the nonreciprocal system are analyzed, respectively. When the resonant micro-optical gyroscope system is reciprocal, the intensity item of the back-reflection noise has the same impact on the frequency deviation of the clockwise and the counter clockwise lightwave which counteracts, so it has no impact on measuring the rotation rate. When the system is nonreciprocal, the intensity item of the back-reflection noise introduces the noise of the magnitude of 10°/s. The interference item of the back-reflection noise introduces the noise of the magnitude of 257°/s and 261°/s respectively when the system is reciprocal and nonreciprocal. The suppression effects of the back-reflection noise in the resonant micro-optical gyroscope with different modulation techniques are compared. The separation modulation technique can suppress the intensity item of the back-reflection noise when using different modulation frequency and the interference item of the back-reflection noise can be suppressed below the shot-noise limited sensitivity when the carrier suppression reaches 120 dB which is achievable using four phase modulators. The reciprocal modulation technique can improve the reciprocity of the resonant micro-optical gyroscope and can suppress the residual intensity modulation noise of the phase modulator and the frequency noise of the laser effectively. When using the reciprocal modulation technique, the interference item of the back-reflection noise can be suppressed by carrier suppression but the intensity item of the back-reflection noise can not be suppressed which brings the noise of the magnitude of 10°/s according to the simulation result. So it is necessary to add the optical switch or the pulse modulator in the resonant micro-optical gyroscope system to suppress the intensity item when using the reciprocal modulation technique. The optical switch or the pulse modulator can separate the clockwise and the counter clockwise lightwave in time and avoid the energy coupling between the signal light and the back-reflection light which is equivalent to reducing the back-reflection coefficient. In theory, the intensity item and the interference item of the back-reflection noise can be suppressed totally but the suppression effect is limited by the channel crosstalk of the optical switch or the pulse modulator. According to the simulation result, the intensity item of the back-reflection noise can be suppressed below the shot-noise limited sensitivity when the crosstalk of the optical switch or the pulse modulator is 45 dB. To suppress the interference item of the back-reflection noise below the shot-noise limited sensitivity the crosstalk of the optical switch or the pulse modulator should be better than 115 dB which is difficult to achieve. The above analyses provide the theoretical basis for the establishment of the resonant micro-optical gyroscope system. The resonant micro-optical gyroscopes using the separation modulation technique and the reciprocal modulation technique are established, respectively. The outputs of the two system are tested in 1 800 s. The test results show that the gyro output is stable under the separation modulation system because the intensity item and the interference item of the back-reflection noise are both suppressed. The noise of the magnitude of 10°/s is introduced in the system using the reciprocal modulation technique because the intensity item of the back-reflection noise is not suppressed which is coincident with the simulation result..
Acta Photonica Sinica
- Publication Date: Sep. 25, 2022
- Vol. 51, Issue 9, 0906004 (2022)
Three-wavelength Dynamic Demodulation Technique for Multi-cavity Fiber Fabry-Perot Sensor
Qianyu REN, Pinggang JIA, Jiang QIAN, Jun WANG, Wenyi LIU, and Jijun XIONG
Fiber-optic Fabry-Pérot sensors have a wide range of applications, including aerospace, large-scale construction, oil collection, and many other fields. In many cases, dynamic parameters, such as dynamic pressure, vibration, acoustics, and ultrasonics are required to be measured. In order to measure these parameteFiber-optic Fabry-Pérot sensors have a wide range of applications, including aerospace, large-scale construction, oil collection, and many other fields. In many cases, dynamic parameters, such as dynamic pressure, vibration, acoustics, and ultrasonics are required to be measured. In order to measure these parameters, a variety of fiber-optic Fabry-Pérot sensors are produced. In some fields, the multi-cavity fiber-optic Fabry-Pérot sensor is inevitable for some advantages. For example, in the field of aerospace engine testing, dynamic pressure is a key parameter that often needs to be measured, and the micro-electro-mechanical system external Fabry-Pérot interferometer pressure sensors with multiple Fabry-Pérot cavities are often designed for aerospace engine pressure measurement due to their consistency and airtightness. Moreover, multi-cavity Fabry-Pérot sensors are good candidates for multi-parameter measurements. The different Fabry-Pérot cavities with different lengths are used to measure different parameters to achieve multi-parameter measurement. Therefore, multi-cavity Fabry-Pérot sensors are becoming increasingly important in engineering applications. However, extracting dynamic signals in multi-cavity Fabry-Pérot sensors is a challenge. In this paper, an improved passive three-wavelength phase demodulation technology based on a broadband light source is proposed for dynamic interrogation of the shortest cavity in a multi-cavity Fabry-Pérot sensor. According to the principle of low coherence interference, when the optical path difference introduced by the Fabry-Pérot interferometer is less than the coherent length received by the photodetectors, interference occurs. In contrast, when the optical path difference introduced by the Fabry-Pérot interferometers is longer than five times the coherence length, the interference phenomenon becomes insignificant and it can be considered that the interference disappears. Therefore, a flat-top amplified spontaneous emission light source and three broadband fiber filters were used to ensure the interference only occurs in the short cavity. The quadrature signals are obtained by three filtered optical signals with arbitrary cavity length using an improved phase calibration algorithm. The established signal calibration algorithm allows the demodulation technology for arbitrary short cavity lengths and arbitrary central wavelength. The demodulation technology can work without the direct-current voltages, so the demodulation system can reduce the fiber-optic disturbance noise. The arctangent algorithm is established to extract vibration signals by the quadrature signals. Compared with the previous phase calibration algorithm, the phase calibration algorithm proposed in the paper is more concise. The experimental system was consisted of a reflective bracket, a light source, a multi-cavity Fabry-Pérot interferometer, a fiber-optic coupler, three fiber filters, three photodiodes, an analog-to-digital conversion and a personal computer. The light from the light source passed through the fiber-optic coupler to the multi-cavity Fabry-Pérot interferometer. A multi-cavity Fabry-Pérot interferometer consists of a gradient-index lens and a 300-μm-thick double-polished quartz glass fixed on a piezoelectric transducer. The light reflected from the interferometer passed through the coupler and through the filters to the photodiodes. Three interferometric signals at each center wavelength were obtained using three photodiodes. The voltage signals were collected by analog-to-digital conversion and transmitted to a personal computer. The feasibility of the demodulation algorithm was verified by simulations and experiments. The experimental results show that the vibration signals with a frequency of 1 kHz and peak-to-peak amplitude of 2.6 μm is successfully extracted with different Fabry-Pérot cavity length, which proves that the three-wavelength demodulation algorithm can be used for optical fiber multi-cavity Fabry-Pérot sensor with arbitrary short cavity length. The demodulation speed is 500 kHz and the demodulation resolution is 0.25 nm. The demodulation technology makes it possible to extract dynamic signals in a multi-cavity Fabry-Pérot sensor. If the spectrometer is used at the same time, the dynamic signal measured by the short cavity and the static signal measured by the long cavity can be interrogated at the same time. This demodulation technology has the advantages of a compact system, low cost, fast speed and high robustness, illustrating its bright potential for multi-cavity Fabry-Pérot sensors..
Acta Photonica Sinica
- Publication Date: Sep. 25, 2022
- Vol. 51, Issue 9, 0906005 (2022)
Reducing Peak-to-average Power Ratio of Coherent Optical Orthogonal Frequency Division Multiplexing System by DCT Cascading ICF Algorithm
Nan ZHANG, Zhengrong TONG, Weihua ZHANG, and Ye LIU
Optical Orthogonal Frequency Division Multiplexing (OOFDM) technology is a new optical transmission technology. It is the product of the combination of Orthogonal Frequency Division Multiplexing (OFDM) technology and optical fiber communication technology, and has all the advantages of these two technologies. In recentOptical Orthogonal Frequency Division Multiplexing (OOFDM) technology is a new optical transmission technology. It is the product of the combination of Orthogonal Frequency Division Multiplexing (OFDM) technology and optical fiber communication technology, and has all the advantages of these two technologies. In recent years, OOFDM has become one of the research hotspots in the field of optical communication due to its unique advantages, especially the Coherent Optical Orthogonal Frequency Division Multiplexing (CO-OFDM) system. The CO-OFDM system has the advantages of high spectral efficiency, receiving sensitivity, and robustness, which make it become a research hotspot for realizing high-capacity, high-speed and long-distance optical fiber communication. The CO-OFDM signal is superimposed by modulated subcarrier signals. When the phases of the multiple subcarrier signals are the same, the superimposed signal power can be much greater than the average power, which can result in high Peak-to-average Power Ratio (PAPR). High PAPR not only causes nonlinear distortion when the system passes through optical amplifiers, DAC, ADC and other devices, but also leads to a decrease Bit Error Rate (BER).To solve the PAPR problem of CO-OFDM in OOFDM, the characteristics of related algorithms are studied. The clipping algorithm is the nonlinear processing of the signal, it will cause degradation performance such as high BER, low transmission rate, short transmission distance. An improved ICF algorithm is studied, which can improve the impact of the clipping algorithm on the system BER because of the iterative process. DCT transform is an orthogonal transform, which has a good effect of decorrelation and concentration the energy of the signal. DCT can change the correlation of the input sequence and further reduce the PAPR of the signal. A new structure of the DCT cascade improved clipping algorithm is proposed.From the time domain waveform diagram, the peak value of the original signal is 4.485, and the peak value of the DCT-ICF algorithm is 3.273. Compared with the original signal, the proposed algorithm effectively reduces the peak value of the time domain waveform by 1.212. From the perspective of the Complementary Cumulative Distribution Function (CCDF), which measures the PAPR distribution of the signal. When the CCDF is 10-4, the PAPR0 value of the original signal is 11.48 dB, the DCT algorithm is 8.022 dB, the ICF (CR=4, iter=4) algorithm is 6.93 dB, and the DCT-ICF (CR=4, iter=4) scheme is 6.795 dB. Compared with the original algorithm, the optimized amplitude of the DCT algorithm is 3.458 dB, the ICF (CR=4, iter=4) algorithm is 4.55 dB, and the proposed algorithm is 4.685 dB. When the BER is 10-3, the Optical Signal to Noise Ratio (OSNR) of the DCT-ICF (CR=3, iter=4) algorithm is 20.76 dB, DCT-ICF (CR=4, iter=4) algorithm is 20.96 dB, and DCT-ICF (CR=5, iter=4) algorithm is 21.47 dB, it can achieve long-distance transmission. And the BER can increase with the increase of the SMF transmission distance. The proposed algorithm has good BER performance. In addition to that, the total amount of computational amount of the proposed algorithm is 19 970.In this paper, the DCT algorithm and clipping algorithm are introduced in detail, and their principles are analyzed. According to their characteristics, the DCT-ICF algorithm is proposed. In terms of Power Spectral Density (PSD), the ICF algorithm improves the nonlinear distortion of the clipping algorithm in the system, and then improves the system performance. From the time domain waveform, the proposed algorithm reduces the signal peak significantly. According to the CCDF simulation curve, the proposed algorithm can effectively suppress the PAPR. From the transformation curve of BER with OSNR and the transformation curve of BER with SMF length, it can be seen that the proposed algorithm can maintain good performance during the transmission process. In consideration of the computational complexity, the proposed algorithm has a lower computational effort. In conclusion, the algorithm is feasible after considering various factors..
Acta Photonica Sinica
- Publication Date: Sep. 25, 2022
- Vol. 51, Issue 9, 0906006 (2022)
Design and Analysis of D-shaped Surface Plasmon Resonance Fiber Biosensor for Liquid Analytes
Qingmin LIU, Shanglin HOU, and Jingli LEI
Surface Plasmon Resonance (SPR) is a prominent optical phenomenon that arises as the extent of energy transferring from photons to surface plasmon waves under appropriate conditions. In the past few years, this optical effect, owing to its high sensitivity, real-time detection, and anti-interference has already been exSurface Plasmon Resonance (SPR) is a prominent optical phenomenon that arises as the extent of energy transferring from photons to surface plasmon waves under appropriate conditions. In the past few years, this optical effect, owing to its high sensitivity, real-time detection, and anti-interference has already been extensively investigated and applied in medical treatment, environment monitoring, biomedical sensing and so on. Based on the principle of SPR, a novel D-shaped gold surface plasmon resonance photonic crystal fiber with one open-ring is proposed for detecting low refractive index materials has been investigated in detail. The proposed photonic crystal fiber of the simulation model is composed of three layers of air holes. The radii of air holes in the first-layer and third-layer are r1 and r3, respectively. While the second-layer air ring consists of air holes with two different radii, r2 and rs. The refractive index of air is fixed at nair = 1 and the radius of the cladding is R. A thin gold film with thickness tg is deposited on the inner surface of the micro-opening analyte channel on the upper side, the radius and the central location of the channel are rs and 2.5×Λ-1.25×rs, respectively. The fiber material is fused silica and the RI is determined by the Sellmeier equation, the relative dielectric constant of gold can be demonstrated by the Drude-Lorentz model. This paper uses the finite element method and sets the boundary conditions of the perfect matching layer for simulation. In order to investigate how the sensing performance of the proposed PCF-SPR sensor is affected by the parameters of the optical fiber, the effect of various parameters of the fiber such as air radii (r1, r2, r3, rs), air hole spacing (Λ) and the gold film (tg) on the SPR loss spectrum have been studied separately. The simulation results show that the confinement loss decreases as r1 increases. This can be attributed to the fact that more energy is confined to the core when r1increases, which affects the coupling between the core and plasmonic modes. At the same time, the confinement loss also decreases with the increase of r2, and the corresponding blue shift occurs with the resonance peaks moving toward a shorter wavelength over the process. The reason is that the increase of r2 will increase the refractive index difference between the plasmonic mode and core mode, which will affect the coupling between them. Therefore, with the increase of r2, the shorter wavelength can excite the plasmonic mode, resulting in the phenomenon of wavelength blue shift in the loss spectrum. Since the air holes of the third layer are located at the outermost part of the fiber, the change of r3 has little impact on the confinement loss, which can greatly reduce fabrication difficulty of the sensor. The pitches between the air holes are also an important factor in confinement loss, the change of Λ will influence the refractive index of core mode and plasmonic mode, which in turn affects the phase matching condition and energy coupling between them. The thickness of gold film plays a vital role in the sensing performance. If the gold film is too thick, the electric field can not penetrate the gold film, which will reduce the sensitivity of the proposed sensor. While if the gold film is too thin, the plasmonic wave will be strongly suppressed due to radiation damping. Therefore, the thickness of gold film can significantly affect the coupling between the core mode and the plasmonic mode. After optimizing the various parameters affecting the sensing performance of the sensor, we analyse the analytes with different refractive indices. Simulation results show that the sensor operates in the near-infrared and mid-infrared region with the wavelength range of 2 020~3 036 nm in the refractive index range of the analyte of 1.18~1.30. When the refractive index of the analyte is in the range of 1.23 to 1.30, the sensor operates in the band of 2 135~3 036 nm, and the average value of spectral sensitivity is up to 11 650 nm/RIU. When the refractive index of the analyte is between 1.29 and 1.30, the sensor operates in the mid-infrared band of 2 648~3 036 nm, and the maximum spectral sensitivity and resolution are 38 800 nm/RIU and 2.37×10-6 RIU, respectively. The proposed sensor shows great significance in detecting low refractive indexes in near- and mid-infrared waveband, and has potential applications in biomedical sensing, water environment and humidity detection and so on..
Acta Photonica Sinica
- Publication Date: Sep. 25, 2022
- Vol. 51, Issue 9, 0906007 (2022)
Instrumentation, Measurement and Metrology
Signal Processing for Height Measurement of High-aspect-ratio Structure Based on Low-coherence Interferometry
Yifeng SUN, Zhishan GAO, Xiaoxin FAN, Jiale ZHANG, Jianqiu MA, and Qun YUAN
Low-coherence scanning interferometry is an effective method used for measuring characteristic parameters of microstructures with the advantage of non-destruction. However, a large range of vertical scanning is required for the sampling points at the bottom of the high-aspect-ratio structure, which causes the offset ofLow-coherence scanning interferometry is an effective method used for measuring characteristic parameters of microstructures with the advantage of non-destruction. However, a large range of vertical scanning is required for the sampling points at the bottom of the high-aspect-ratio structure, which causes the offset of light intensity and reduces the contrast of coherence signals. Besides, the aliasing coherence signals induced by the complex diffraction effect appear at the step edge and two main envelopes can be extracted. All these phenomena obstruct the locating of the coherence peak and then mislead the height measurement. Algorithmic processing seems to be an effective method to solve the above problems including suppressing the component of offset and demodulating the aliasing coherence signals. Due to the central shielding effect of Mirau-type objectives, the effective signals returned from the sampling points at the bottom of the high-aspect-ratio structure can be further reduced. Combined with the consideration of the magnification in the measurement of the structure with narrow linewidth, Linnik-type objectives are selected in the interference system. In this paper, the complete ensemble empirical mode decomposition with adaptive noise is used to decompose the original signals into a series of intrinsic mode functions. This algorithm can decompose the original signals into different frequency components without prior knowledge. By extracting the intrinsic mode functions with high-frequency and then replacing the original signals, the offset with low-frequency is filtered out and the contrast of coherence signals is further improved. The complex diffraction effect at the edge of the high-aspect-ratio structure results in the coherence signals containing two sets of the envelopes, among which the abnormal envelope is induced by the additional coherence signals. These additional coherence signals correspond to the interference fringes appearing near the upper edges and extending horizontally into the air. However, their contrast is even higher than that of the corresponding interference fringes appearing at the lower surface of the structure. Thus, the abnormal envelope corresponding to the upper surface has a higher amplitude, which obstructs the coherence peak position of the effective envelope. A more complicated condition is that the sampling points far away from the step edge has normal coherence signals containing the single envelope. Therefore, the received coherence signals in the full field of view are mixed up and not applicable to one processing method. What is necessary is that the current position of each sampling point must be determined. The envelope corresponding to the correct surface of the step structure can be used for coherence peak locating and height measurement. For the amplitude of the coherence signals, the shielding effect of the high-aspect-ratio structure can lead to a small result, relatively. This can be considered as the intensity information which helps to distinguish the lower surface of the structure. Besides, there have been obvious discrepancies between the positions calculated by the centroid method for the envelopes extracted from the coherence signals. This is another way to distinguish the different surfaces and can be regarded as contrast information. In this paper, binarization processing is realized by combining the contrast and intensity information extracted from the coherence signals. The purpose of combining these two kinds of information is to further highlight the discrepancy between the upper and lower surfaces. After identifying the upper and lower surfaces of the high-aspect-ratio structure, the envelope corresponding to the current position of the sampling point is selected as the effective envelope for locating the coherence peak and height measurement. A high-aspect-ratio groove with a depth of 101.77 μm and a line width of 10.97 μm is selected to be the measurement sample. The structural parameters of this sample have been certified by the China Metrology Institute and a test report has been issued. Using the algorithm proposed in this paper to conduct ten repeatability measurements, the mean of the height measurement results can be calculated as 101.093 μm and the relative error is 0.67%. These two parameters demonstrate the accuracy of the algorithm. Besides, the standard deviation is 0.316 μm which illustrates the robustness and stability of the algorithm. Due to the general trend toward miniaturization, this non-destructive metrology offers significant advantages in height measurement of high-aspect-ratio structures, without introducing any physical upgrade of the instrument..
Acta Photonica Sinica
- Publication Date: Sep. 25, 2022
- Vol. 51, Issue 9, 0912001 (2022)
Range Prediction Deviation Real-time Correction Algorithm for Space Debris Laser Ranging
Jian GAO, Zhipeng LIANG, Xingwei HAN, Xue DONG, Ning AN, Guanyu WEN, and Chengzhi LIU
In practical space debris laser ranging systems, the performance of operation depends on the accuracy of range predictions. However, widely used Two-Line-Element (TLE) predictions usually present too large range prediction deviation, which can not satisfy laser ranging demands, or requires a long time of so-called rangIn practical space debris laser ranging systems, the performance of operation depends on the accuracy of range predictions. However, widely used Two-Line-Element (TLE) predictions usually present too large range prediction deviation, which can not satisfy laser ranging demands, or requires a long time of so-called range searching. To improve the performance of space debris laser ranging operations, we further investigated the mechanism of range prediction deviation. With the help of orbital relative motion theory and ground to space observation geometry, we constructed a mathematical model for range prediction deviation of orbital objects on near-circular orbit. The predicted and actual positions on orbit were considered as adjacent objects. The orbital relative motion of adjacent objects was described by Clohessy-Wiltshire (C-W) equation, which models the relative motion, as well as their derivatives to time. Combining the relative motion vector with its time derivative made the six-dimensional state of relative motion (the state). Given the C-W equation as state transition, the state at any time was determined from the state at initial time, a.k.a. the initial state. On the other hand, the ground to space observation geometry provided linear mapping from the six-dimensional state of relative motion to two-dimensional observational angular offsets. In this manner, the Equation-of-Motion (EOM) and Equation-of-Observation (EOO) were formed for the initial state. We adopted a classical Kalman filter to solve for the initial state of relative motion from observations. Once estimated, the initial state was propagated to observation time and mapped into range prediction deviation. Thus, the optimal estimation of the current range prediction deviation was achieved in real-time. The testing scenario was setup to assess the real-time correction algorithm. The scenario contains nine Low-Earth-Orbit (LEO) satellites representing different orbit types. The simulated time span contains 20 days, from Dec. 1st to Dec. 20th in 2021. To simulate TLE-based space debris laser ranging, the nominal prediction trajectory data was generated with TLE, while the reference trajectory was generated by official Consolidated Prediction Format (CPF) data. Comparing prediction and reference trajectory formed real deviation data for each satellite pass. Simulated observational angular offset data was generated and fed to the correction algorithm as input, while the real deviation data was used as a reference, to assess the output. Each simulated pass started when elevation rises above 10 degrees in the scenario. The correction algorithm was iterated once per second, receiving angular offset data and outputting range correction data. Uncertainty of angular observation was assumed 2 or 5 arc-seconds in separate test cases. The correction algorithm began to run at the start of pass, and finished while the correction residue of range deviation was less than 100 m. The simulated passes were set to end with elevation below 10 degrees. Various criteria were adopted to assess the correction algorithm, including maximum range deviation, correction time, and percentage in the whole pass. The correction algorithm was found to be effective. For selected satellites, the max range deviations were about 500 m. In the test case with 2 arc-seconds observation uncertainty, the algorithm took less than 1.3 minutes in average to finish correction. In other words, the correction algorithm took no more than 15% of the whole pass time to make corrections, bringing down range prediction deviations from around 500 m to less than 100 m. The study demonstrates the correction algorithm's effectiveness in meeting laser-ranging demands. Further development and application in space debris operation is recommended..
Acta Photonica Sinica
- Publication Date: Sep. 25, 2022
- Vol. 51, Issue 9, 0912002 (2022)
An Event-based Method for Extracting Star Points from High Dynamic Star Sensors
Sikang ZENG, Rujin ZHAO, Yuebo MA, Zifa ZHU, Yuping TANG, and Zijian ZHU
Star sensor is a device that relies on measuring the pointing of the navigation star in the spacecraft coordinate system to determine the attitude of the spacecraft, and is the highest precision attitude sensor, whose accuracy can reach the angular second level, with the advantages of no drift, long working life, etc. Star sensor is a device that relies on measuring the pointing of the navigation star in the spacecraft coordinate system to determine the attitude of the spacecraft, and is the highest precision attitude sensor, whose accuracy can reach the angular second level, with the advantages of no drift, long working life, etc. It is a fundamental and critical device for the survival and performance improvement of spacecraft. It is of strategic importance in space applications such as remote sensing to the earth, deep space exploration and high precision detection. Most of the current star sensors use traditional optical sensors. Conventional star sensors use frame-based imaging modes, such as global exposure and roll-up exposure, which generate a frame of star image within a fixed exposure time (usually 100 ms). However, under the high dynamic conditions of large spacecraft maneuvers, this imaging mode produces motion blur and "star point trailing" effect, which, together with the noise effect, significantly reduces the signal-to-noise ratio of the star map, resulting in lower accuracy or even failure of star point extraction, and severely limits the dynamic performance of the star sensor. Star point center of mass extraction is a crucial part of the star sensor, which is directly related to the final attitude measurement accuracy of the star sensor. The sensitive navigation stars are long-range targets, and their orientations in the celestial coordinate system are high-precision astronomical observations with milliarcsecond accuracy and almost constant, so the final attitude accuracy of the star sensor depends on the extraction accuracy of the star point masses in the digital images. Therefore, the accuracy of the final solution of the star sensor depends mainly on the extraction accuracy of the star point center in the digital image. In the traditional star sensor, the average gray value of the whole image is used as the background noise gray value, and then the background noise is subtracted from the star point region gray value to obtain the corrected star point, and finally the exact center of the star point is calculated by the gray scale center method. In this paper, we propose an event-based star point extraction method for high-dynamics star point sensors to address the problem that the accuracy of star point center extraction decreases or even fails under high dynamics (≥3
Acta Photonica Sinica
- Publication Date: Sep. 25, 2022
- Vol. 51, Issue 9, 0912003 (2022)
Optical Design and Fabrication
Design of a Solar-blind Ultraviolet Imaging System for Corona Detection
Suhao CHEN, Bo LÜ, Weiqi LIU, Rui FENG, and Zhonglun WEI
During real-time monitoring of high voltage corona discharge, the solar radiation intensity is much higher than the radiation generated by corona discharges, which will interfere with the detection process. However, due to the absorption of the earth's ozone layer, the radiation intensity of the sun's radiationDuring real-time monitoring of high voltage corona discharge, the solar radiation intensity is much higher than the radiation generated by corona discharges, which will interfere with the detection process. However, due to the absorption of the earth's ozone layer, the radiation intensity of the sun's radiation to the ground becomes very weak in the solar-blind ultraviolet band of 240~280 nm. Observing high-voltage wires in this band can reduce the interference of the environment, thereby improving the imaging contrast and reducing the false detection rate. In order to expand the detection range and improve the detection accuracy of the solar-blind UV imaging system, the optical system needs to have a higher resolution while ensuring a larger field of view. In this paper, a solar-blind ultraviolet optical system with a large field of view, large relative aperture and high resolution is designed. We have formulated the design index according to user needs, and selected an anti-telephoto lens as the initial structure according to the design index requirements. During optimization, we first replaced the lens material with fused silica and calcium fluoride materials commonly used in the ultraviolet band. According to the principle of chromatic aberration correction, the positive lens is replaced by calcium fluoride with a large Abbe number, and the negative lens is replaced by fused silica with a small Abbe number. The refractive index of these two materials is low and the Abbe number difference is small in the solar-blind ultraviolet band, which is not suitable for chromatic aberration correction, and the typical achromatic structure doublet lenses can not transmit ultraviolet radiation, so we added two sets of double separation structures to correct chromatic aberration. Due to the high requirements for the surface roughness of the lens in the ultraviolet band, the limitation of materials and apertures, and the consideration of technological difficulty, no aspherical lens was introduced in this design. The final design uses 12 standard spherical lenses, with a total optical length of 90 mm, a full field of view of 56°, and a relative aperture of 1/2. In the full field of view, the distortion is less than 2.2%, the relative illumination is greater than 70%, the Modulation Transfer Function (MTF) of the system is greater than 0.65 at the spatial frequency of 110 lp/mm, and the optical system has good imaging quality. After that, we analyzed the thermal defocusing caused by the thermal deformation of the lens and lens barrel materials and the thermally induced refractive index change of the optical material in the temperature range of -20~60 ℃. The results show that the MTF of the system decreases significantly when the ambient temperature changes. The thermal defocus amount of the system has an approximate linear relationship with the temperature change, so we can use the passive mechanical compensation method to correct the thermal difference of the system. We use ABS plastic with high coefficient of linear expansion as the compensation lens barrel to compensate for thermal defocusing. The MTF of the compensated system is greater than 0.4 in the working temperature range, realizing athermal design. Finally, through reasonable tolerance allocation, the MTF of the tangential and sagittal directions of the system after processing and adjustment decreases relatively uniformly under the influence of tolerances. Under the probability of 80%, the MTF at the Nyquist sampling frequency of the system is greater than 0.35, and the system still maintains a high imaging quality, which meets the actual requirements of corona detection..
Acta Photonica Sinica
- Publication Date: Sep. 25, 2022
- Vol. 51, Issue 9, 0922001 (2022)
Design and Optimization of Image Slicer in Coherent Dispersive Spectrometer
Yidong WANG, Ruyi WEI, Zhengmao XIE, Kai ZHANG, and Shasha CHEN
Image slicer is an important optical device in astronomical observation spectrometer. It can effectively improve the resolution and energy transmittance of the instrument. The image slicer can divide the circular image spot into strips and arrange the strips in a straight line, so that all the image spots can pass throImage slicer is an important optical device in astronomical observation spectrometer. It can effectively improve the resolution and energy transmittance of the instrument. The image slicer can divide the circular image spot into strips and arrange the strips in a straight line, so that all the image spots can pass through the spectrometer slit. Image slicers are commonly used in astronomical observation spectrometers to help instruments achieve high spectral resolution with medium apertures. Image slicers can be divided into 4 categories according to their working principles. Among them, Bowen-Walraven type is the most widely used image slicer type. Coherent dispersive spectroscopy is a technique that combines an interferometer and an intermediate resolution spectrometer. It measures the phase change of the interference fringes of the stellar spectral lines after the Doppler frequency shift, and calculates the radial velocity change of the star and the mass of the planet. Since the phase difference has a certain amplification factor relative to the wavelength offset, when the spectral resolution is the same, the radial velocity detection accuracy of the coherent dispersion technique can be greatly improved compared with the traditional echelle grating method.This paper is based on the coherent dispersive spectrometer used to detect exoplanets by the radial velocity method. The radial velocity detection accuracy is expected to be less than 1 m/s, and the detection target is K/M dwarf stars. The structure of the coherent dispersion spectrometer consists of collimating mirror, Sagnac interferometer, imaging mirror group, image slicer, relay mirror group, slit, dispersion grating and CCD. The working spectral range of the spectrometer is 660~900 nm, the system transmittance at the center wavelength is about 0.4, and the spectral resolution is 0.03 nm. In order to meet the requirements of energy utilization and spectral resolution, the system needs to use the image slicer to realize the target surface multiplexing of the CCD and the reasonable matching of the numerical aperture. Therefore, setting a reasonable number of segmented images and the F number of the imaging lens group to achieve a good segmentation effect is of great significance to the improvement of system performance.In order to reduce the influence of imaging defects on the system, two design schemes of the image slicer are modeled and calculated in this paper. This paper also studies the relationship between the thickness of the reflective cavity and the incident angle and the defocusing and object point repetition, and deduces the general design formula of the thickness of the optical reflective cavity, which provides an important reference for the design of the image slicer. In addition, for the coherent dispersive spectrometer system used for exoplanet detection, this paper simulates the defocus and object point repetition under different F numbers and segmentation numbers. By analyzing the simulation results, the following conclusions are obtained: 1) With the increase of the F number and the number of divisions, the defocus amount increases significantly, and the defocus phenomenon becomes more obvious. 2) The phenomenon of object point repetition appears in all simulation results, which is determined by the design principle and cannot be avoided. 3) The design results of the two design schemes are relatively similar. Since the optical path in the Bowen?Walraven type design is propagated through the glass medium, the defocus amount is larger than that of the simplified type. The ratio of the diffuse spot diameter to the image spot diameter is the same for both methods. Based on the comprehensive simulation effect, and considering the requirements of the coherent dispersion spectrometer system, it can be considered that the imaging defects are relatively balanced and the energy loss is less when the star image is divided into 4 under the condition of F/24, which is a relatively suitable solution. In addition, since the defocus amount of the simplified type is smaller, and only the flat mirror needs to be processed, the cost is lower, so the simplified design scheme can be adopted.The work of this paper plays an important role in achieving the expected performance of the instrument, and provides a reference and application reference for other high-resolution spectrometers to determine system parameters. At the same time, the work of this paper provides a general design idea for Bowen?Walraven and simplified image slicer design, which is instructive for optimizing the design process of image slicer..
Acta Photonica Sinica
- Publication Date: Sep. 25, 2022
- Vol. 51, Issue 9, 0922002 (2022)
Optical Device
Broadband Terahertz Metamaterial Absorber Based on Patterned Graphene
Yue FENG, Hai LIU, Cong CHEN, Peng GAO, Hao LUO, Ziyan REN, and Yujia QIAO
The frequency of terahertz radiation is 0.1~10 THz, which is between microwave and infrared. In terms of energy, it is between electrons and photons. Terahertz, like X-rays and sound waves, can penetrate the surface of objects for imaging. In addition, the terahertz frequency is very high, so its spatial resolution is The frequency of terahertz radiation is 0.1~10 THz, which is between microwave and infrared. In terms of energy, it is between electrons and photons. Terahertz, like X-rays and sound waves, can penetrate the surface of objects for imaging. In addition, the terahertz frequency is very high, so its spatial resolution is also very high; it also has very high temporal resolution because of its short pulse (picosecond order). Terahertz radiation has been widely used in safety inspection because different chemicals can absorb terahertz radiation at different frequencies to different degrees, showing unique frequency characteristics. In addition, it shows unique advantages and broad application prospects in many fields such as radar, communication and nondestructive testing. It can be predicted that terahertz technology will be one of the major emerging fields of science and technology in the 21st century. However, there are relatively few functional devices in the terahertz band at present. The fundamental reason is that most natural materials can only exhibit weak electromagnetic response when interacting with Terahertz waves, which limits the further development of terahertz technology. The absorption of terahertz waves, especially the complete absorption, has great potential application value in electromagnetic stealth, thermal sensing and thermal imaging. Therefore, the search for an absorbing material that can perfectly absorb the terahertz band has become a major research topic in the direction of materials science in various countries.Traditional absorbing materials are mostly designed based on the principle of Salisbury absorption screen. Its typical weakness is that it is very thick and bulky. With the increasing demands on the performance of absorbing materials in the fields of communication and stealth, traditional absorbing materials can no longer meet the needs of civil and military applications. Therefore, the development of more lightweight and miniaturized new wave absorbing devices has become an urgent task at present.With the discovery and research of metamaterials, it provides an effective way to realize terahertz functional devices, especially terahertz absorption devices. Metamaterials are artificial electromagnetic structures typically made of subwavelength metals on dielectric or semiconductor substrates. Compared with traditional materials, metamaterials have some special properties, such as changing the normal properties of light or electromagnetic waves, and such effects can not be achieved by traditional materials. The research shows that using the exotic electromagnetic properties of metamaterials can not only improve the performance of antennas and microwave devices, develop new equipment, but also provide a new technical means for the development of new absorbing materials.Most of the metamaterial absorbers proposed so far are composed of precious metals such as gold and silver. Once the size is determined, it is difficult to adjust the resonance frequency and absorption intensity. At present, the research on terahertz metamaterials at home and abroad is more focused on the application of metamaterials in the realization of tunable functional devices in the terahertz band, especially the realization of tunable terahertz functional devices. Therefore, the development of tunable metamaterial absorbers will be of great significance for the application of metamaterials. Fortunately, the medium on which processing is relied also plays a role in the electromagnetic properties of metamaterials. Studies have shown that the control of the optical properties of metamaterials can be achieved very effectively by combining metamaterials with media with tunable optical properties such as graphene, liquid crystal and phase change materials. Among them, graphene, as a band-free semiconductor, has attracted much attention in the field of materials science in recent years due to its unique electrical and optical properties. Graphene also has important applications in the research of optically tunable metamaterials. Through chemical or electrostatic doping, the carrier concentration and Fermi level of graphene can be changed, which is very wide on the terahertz frequency range, effectively change the position of the resonance peak and can be used to implement the infrared to the terahertz frequency range adjustable perfect absorber, polarizer, filters and other optical components.Based on the selected topic background and the research status, this paper aims at the research of tunable dielectric metamaterials and the design and optimization of broadband terahertz metamaterial absorber structures, mainly on the optical properties and tunability of graphene in metasurfaces. The main research content is to take graphene two-dimensional planar metasurface as the research object, design a patterned graphene broadband metamaterial absorber model, and conduct simulation analysis through FDTD solutions optical simulation software. The modulation of graphene on the optical properties of metamaterials and the control of the amplitude, polarization and propagation of terahertz waves by metamaterial are studied based on the FDTD method and principle of surface plasmon resonance, and the structural parameters are optimized by progressive simulation. The absorber adopts a classical sandwich structure containing a patterned single-layer graphene metasurface, a dielectric layer, and a metal backplane. The structure of the graphene pattern consists of graphene resonators of different sizes, ensuring a high absorption rate while broadening the absorption bandwidth. The results show that when
Acta Photonica Sinica
- Publication Date: Sep. 25, 2022
- Vol. 51, Issue 9, 0923001 (2022)
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