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Issue 9 ， 2023
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Influence of frequency mixing on measurement accuracy of full-field heterodyne dynamic interferometer
Abstract：The full-field heterodyne dynamic interferometer has the advantages of a high measurement accuracy and strong anti-interference ability for surface shape measurement. It is suitable for large-focal length surface shape measurement and the measurement of large-aperture optical elements. However， the performance of the optical components in the interferometer system is not ideal， and there are errors in the assembly of components， which introduce frequency aliasing in the interferometer optical path and affect the measurement accuracy of the interferometer. To analyze the influence of frequency mixing on the measurement accuracy of the full-field heterodyne dynamic interferometer， a theoretical model of the measurement error introduced by frequency mixing is established according to the causes of frequency mixing， and the influence of the frequency mixing degree on the measurement accuracy is analyzed. The results indicate that the measurement error is nonlinearly and positively correlated with the degree of mixing and that the mixing causes a periodic error with the same frequency as the interference fringe on the measured surface shape. In this study， a full-field heterodyne dynamic interferometry experimental system is established to examine the effects of different mixing degrees on the surface measurement accuracy. When the mixing frequency is 0.029， the surface shape measurement error is 0.053λ. When the mixing frequency is 0.120， the surface shape measurement error is 0.110λ. The experimental results are consistent with the simulation results. The research presented in this paper has practical significance for the development of high-precision full-field heterodyne dynamic interferometers.Keywords：interferometry;full-field heterodyne;frequency mixing;measurement error- 126
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Published：2023-05-15 -
Cs/NF3 activation of InGaAs photocathode
Abstract：To improve the photoemission performance of the current InGaAs photocathode and explore a new activation recipe， two activation experiments of Cs/NF3 and Cs/O were utilized to study the effects of different activation recipes on the performance of the InGaAs photocathode. For InGaAs samples of the same photocathode structure， activation experiments， decay experiments， spectral response measurements， and surface element composition analysis were performed to analyze the relationships between the activation recipes and the characteristics of the photocathode from the perspectives of the white light photocurrent， stability， spectral response， and surface element composition. The experimental results for Cs/NF3 and Cs/O activation are as follows. The InGaAs photocathode sample activated by Cs/NF3 is significantly better than that activated by Cs/O with regard to the white-light photocurrent， spectral response， and cutoff wavelength， and the enhancement effect of the spectral response is particularly obvious in the near-infrared range. At 1 064 nm， the spectral response of the sample activated by Cs/NF3 is 4.7 times that of the sample activated by Cs/O. In contrast to the phenomenon that the GaAs photocathode can become more stable through Cs/NF3 activation， the stability of the InGaAs sample activated by Cs/O is significantly better than that of the sample activated by Cs/NF3. In addition， the advantages of the cutoff wavelength and spectral response induced by Cs/NF3 activation are lost after decay.Keywords：InGaAs photocathode;Cs/NF3 activation;Cs/O activation;stability;spectral response- 132
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Published：2023-05-15 -
Design of low temperature infrared off-axis three-mirror collimation system
Abstract：An off-axis three-mirror collimating optical system is designed to test the infrared equipment in a vacuum 100-K cryogenic environment. The system adopts athermal design， and the whole structure adopts SiC material. A C-shaped opening expansion sleeve is used as a flexible structure to compensate for low-temperature deformation at the connection between the mirror body and the support structure. The whole system is partially wrapped in a radiation cooling panel， except for the reflecting mirror. The low-thermal conductivity thermal insulation support structure plays a thermal shielding role in the heat conduction chain to realize the thermal insulation support and rapid refrigeration of the system. In the 100-K low-temperature environment， according to a system simulation analysis， the primary， secondary， and tertiary mirror wavefront error is <λ/50； an analysis of the whole structure indicates that the wavefront errors of the primary， secondary， and tertiary mirrors are ≤λ/30. The wavefront error of each field of view of the system is in the range of λ/14-λ/8 at room temperature and in the range of λ/8-λ/7 at 100 K. According to the Rayleigh criterion， the wavefront is considered to be flawless. When λ = 632.8 nm， the modulation transfer function （MTF） of the system at a 50 lp/mm frequency is >0.7 at room temperature； at a low temperature， the MTF is >0.6. Meet the system at 50 lp/mm greater than 0.6 use requirements. The results indicate that the collimating optical system can output parallel light stably in a 100-K vacuum environment to meet the test requirements of low-temperature infrared equipment. In a material level test of fast radiation cooling， the temperature of the SiC mirror blank is stable at 130 K after 18 h. After 30 h， the mirror blank temperature stabilizes at 110 K. The experiment verifies the feasibility of fast radiation cooling of the SiC mirror.Keywords：optical system design;collimation system;off-axis three-mirror system;Infrared;optical mechanical thermal analysis;low temperature- 83
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Published：2023-05-15 -
Model establishment and error correction of FMCW lidar
Abstract：The geometry of frequency-modulated continuous-wave lidar deviates from that of the design model owing to the mechanical machining and assembly of the lidar. In this study， the effect of the sub-coordinate system offset and roll on the coordinate measurement accuracy of the instrument is investigated， and a correction model is developed for the geometric error of the lidar. This model can increase the measurement accuracy of the measurement system without changing the hardware structure of the system. First， a set of lidar coordinate systems is established， and the sources of spatial coordinate measurement errors are analyzed. The geometric error transfer of the measurement coordinates is achieved by applying the transformation matrix between the coordinate systems. Then， the geometric errors of the different coordinate systems are combined， and an explicit expression for the geometric spatial coordinate error of the lidar is established. Based on this， a least-squares optimization objective is established for obtaining the error factors and the corrected coordinates. The obtained error factors can be used as corrections for subsequent coordinate measurements. Finally， this method is used to design a calibration field with a laser tracker as the high-precision measurement instrument and the spherical center of the target sphere as the standard point. A system error correction experiment is performed by employing the laser tracker and lidar to evaluate the target sphere at the same position. The experimental results indicate that the average error of the lidar spatial distance measurement is reduced from 0.044 8% to 0.003 8% and the maximum error value is reduced from 4.17 to 0.30 mm after the correction， thereby confirming the effectiveness of the lidar geometric error calibration and error correction method.Keywords：lidar;geometric error;least square method;error correction- 54
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Published：2023-05-15 -
Fiber Bragg grating based 3D flutter deformation measurement of flapping wing robot
Abstract：Bionic flapping-wing robots have become a global research hotspot because of their considerable potential in military and civilian applications. Measuring and analyzing the flapping deformation characteristics of flapping-wing robots are important for improving their flight performance， and these are challenging research topics in the field of flapping-wing robots. The existing measurement methods include numerical simulation， stereovision camera， and structured light projection measurement. There are problems with these techniques； e.g.， it is difficult to determine the boundary conditions， and visual occlusion occurs. Therefore， a dynamic deformation measurement method of contact flapping wing based on a fiber Bragg grating is proposed in this paper. A fiber Bragg grating flexible sensor based on a polyimide film is designed. The flexible sensor is arranged on the flapping-wing surface in the form of an array to monitor the real-time strain of the surface. The real-time strain data are reconstructed into the real-time three-dimensional （3D） shape of the flapping wing by using a reconstruction algorithm based on curvature. The strain variation of the wing surface in a stable flapping period is monitored. Then， a 3D deformation analysis is performed. The results indicate that the flapping-wing surface strain mainly occurs around the support rod， and the maximum value in the flapping stage is –50.6 με and 98.1 με， respectively. The deformation of the flapping-wing surface mainly occurs at the trailing edge of the wing surface， and the maximum values of flapping down and flapping up are –2.06 and 4.02 mm， respectively. This study provides a new measurement idea and technique for the dynamic deformation measurement of flapping wings. Deformation monitoring under outdoor flight conditions will be performed later to provide a scientific basis for improving the flight performance of ornithopters.Keywords：optical fiber sensing;fiber Bragg grating;flapping wing robot;three-dimensional reconstruction;deformation monitoring- 118
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Published：2023-05-15
Modern Applied Optics
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Magnetic compound fluid polishing of concave optical glass with small curvature
Abstract：To realize efficient ultra-precision polishing of concave spherical optical glass with small curvature， a hemispherical head polishing device based on magnetic compound fluid （MCF） with a high apparent viscosity and stable distribution is proposed. First， the magnetic-field distribution of three different magnetic sources （axial magnetized cylindrical permanent magnet with a planar iron board and concave iron board installed above it） is simulated with the use of ANSOFT Maxwell. It is found that the addition of the concave iron board can strengthen the corner effect， and the magnetic field can be more concentrated. The dimensions of the magnetic sources and magnet eccentricity are optimized according to the simulation results. Second， by observing and comparing the behavior of the MCF slurry with different compositions and magnet eccentricities on the polishing head， the composition of the MCF slurry and magnet eccentricity are determined. Finally， a polishing experiment is performed on the concave spherical K9 glass workpiece with a curvature radius of 15.4 mm and center depth of 2.24 mm using the optimized polishing solution and self-made polishing device. Following 90 min of polishing， the root-mean-square （RMS） of the workpiece surface is reduced from 0.719 μm to 11.7 nm， and the surface roughness （Ra） is reduced from 0.552 μm to 9.656 nm. It is verified that the designed polishing device can achieve efficient nanoscale polishing of concave spherical workpieces with small curvature.Keywords：ultra-precision fabrication;magnetic compound fluid;concave spherical surface with small curvature;corner effect;magnetic field optimization;behavior of polishing fluid- 139
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Published：2023-05-15 -
Design and verification of dual-cell cesium optically pumped magnetometer
Abstract：Regarding the wide application of geomagnetic field measurement in magnetic prospecting， navigation， and positioning， the measurement range of the single-cell self-oscillating optically pumped magnetometer （OPM） is limited by the phase-shift circuit， and the existing dual-cell OPM abroad exhibits a low integration degree and poor portability. In this study， the integration of the dual-cell OPM is improved to break the blockade and improve the research level of the domestic OPM. Through the derivation and demonstration of the principle of the OPM， a highly integrated structure design of the dual-cell cesium OPM is presented. A VCSEL is used as the pump light source， and a knife-edge right-angle prism mirror is used as the spectroscope， while other components are arranged symmetrically along the optical axis. A dual-pump light structure with left- and right-handed circularly polarized light and a dual-cell arrangement is used， which achieves precise phase-shift compensation and avoids the need for a phase-shift circuit. The geomagnetic field in the experimental environment is 37 586.79 nT， as measured by a prototype， and the magnitude of the triaxial component is obtained via the scanning method. The measurement range of the OPM prototype is 25 700-77 000 nT， and the sensitivity is approximately 20 pT- 46
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Published：2023-05-15 -
Motion control of multi-channel double crystal monochromator based on parallelism error compensation
Abstract：This paper proposes a control strategy to address the issue of crystal parallelism errors that affect the performance of the new multi-channel double-crystal monochromator （DCM） system. The control strategy integrates iterative learning control （ILC） and model predictive control （MPC） to reduce random position errors of the slave motor and eliminate repetitive mechanical installation errors， respectively. The parallelism errors between the master and slave crystals are converted into the repetitive reference motion trajectory of the slave motor， and the MPC is used to improve the slave motor position tracking within a single rotation cycle. Meanwhile， ILC iterations are applied during rotation cycles to eliminate repetitive mechanical installation errors. The proposed control strategy is verified using a single-axis motor motion experimental platform. Experimental results demonstrate that the motor position tracking error reaches 1.44″–reduction by 99.64%， 98.52%， 98.26%， and 73.33% as compared to the errors of PID， MPC， DOB+MPC， and ILC+PID combination controller strategies， respectively. The proposed control strategy effectively compensates for crystal parallelism errors and improves parallel alignment accuracy， providing practical application value for the performance enhancement of the new multi-channel DCM system.Keywords：motion control;iterative learning control;model prediction control;multi-channel double crystal monochromator- 56
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Published：2023-05-15 -
Mounting optimization on large aperture KDP crystal to minimize assembling deformation
Abstract：In the high-power laser facility， control of the surface deformation of the large-aperture KDP crystal is the key factor to reduce the frequency-conversion efficiency. To improve the assembling quality of the KDP crystal， a point-supporting process method is proposed for minimizing the assembly deformation. First， a genetic algorithm is used to optimize the support points and their distribution scheme. Second， the finite-element method is used to optimize the assembling preload. Finally， mounting optimization design process experiments are conducted to evaluate the surface deformation and the frequency-doubling conversion efficiency. The experimental results indicate that the proposed method is effective for minimizing the assembling deformation of the KDP crystal； the measured PV value is 6.51 μm， and the measured conversion efficiency of second-harmonic generation reaches 72.6% with excellent assembling repeatability. This result significantly improves the frequency-doubling efficiency and the quality of the far-field spot and has been widely used and promoted in engineering.Keywords：laser facility;KDP crystal;assembly deformation;point-supporting;frequency conversion efficiency- 43
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Published：2023-05-15
Micro/Nano Technology and Fine Mechanics
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Multi-lane line detection and tracking network based on spatial semantics segmentation
Abstract：Target detection networks based on deep learning has some problems in the field of lane line recognition， such as unclear lane differences， low recognition accuracy， a high false detection rate， and a high missed detection rate. To solve the aforementioned problems， a lightweight lane detection and tracking network， SCNNLane， based on spatial instance segmentation， was proposed. In the coding part， the VGG16 network and the spatial convolution neural network （SCNN） were applied to improve the ability of the network structure to learn spatial relationships， which solved the problems of blurring and discontinuity in lane prediction. Simultaneously， based on LaneNet， two branch tasks after encoding the output were coupled to improve poor foreground and background recognition and indistinguishability between lanes. Finally， the method was compared with five other semantic segmentation-based lane-line algorithms by using the TuSimple dataset. Experimental results show that the accuracy of this algorithm is 97.12%， and the false detection rate and missed detection rate are reduced by 44.87% and 12.7% respectivel， as compared with LaneNet， thus meeting the demand of real-time lane line detection.Keywords：machine vision;semantic segmentation;lane line detection tracking;lightweight convolution neural network;spatial convolution neural network- 53
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Published：2023-05-15 -
Road traffic sign recognition algorithm based on improved YOLOv4
Abstract：To address the low recognition accuracy resulting from multiple scale changes in the traffic signs of complex scenes， an improved YOLOv4 algorithm is proposed. First， an attention-driven scale-aware feature extraction module is designed， and the range of receptive fields in each layer is widened to obtain more fine-grained multi-scale features by constructing a hierarchical connection mode similar to the residual structure； this is followed by the generation of a pair of attention maps with directional-aware and position-sensitive characteristics under the attention drive so that the network can focus on key areas with more discrimination. Following this， a feature-aligned pyramid convolution feature fusion module is constructed， and the feature offset between adjacent scale feature maps is obtained via convolution for feature alignment. Finally， the network adaptively learns the optimal feature fusion mode through pyramid convolution and constructs a feature pyramid to identify traffic signs with different scales. Experimental results indicate that the recognition accuracy for the TT100K dataset is improved by 5.4% compared with that of the original YOLOv4 algorithm， which is superior to other recognition algorithms， and the FPS reaches 33.17. Thus， the proposed algorithm satisfies the requirements of accuracy and real-time performance for road traffic sign recognition.Keywords：computer vision;traffic sign recognition;attention mechanism;pyramid convolution;YOLOV4- 65
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Published：2023-05-15 -
Application of SENet generative adversarial network in image semantics description
Abstract：An SENet-based method for image semantics description of generative adversarial networks is proposed to address the inaccurate description of utterances and inadequate involvement of emotional colors in image semantics descriptions. The method first adds a channel attention mechanism to the feature extraction stage of the generator model so that the network can completely extract features from salient regions of the image and input the extracted image features into the encoder. Second， a sentiment corpus is added to the original text corpus， and a word vector is generated through natural language processing. This word vector is then combined with the encoded image features and input to the decoder， and a sentiment description statement is generated to match the content depicted in the image through continuous adversarial training. The proposed method is compared with existing methods through simulation experiments， and it is found to improve the BLEU metric by approximately 15% compared with the SentiCap method； improvements in other related metrics are also noted. In self-comparison experiments， the method exhibits an improvement of approximately 3% in the CIDEr metric. Thus， the proposed network can better extract image features， resulting in more accurate statements describing images and richer emotional colors.Keywords：image semantics description;generator model;feature extraction;adversarial training;SENet networks- 42
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Published：2023-05-15 -
High-resolution optical satellite panchromatic and multispectral image geometric positioning consistency correction based on high frequency correction attitude
Abstract：There are small geometric positioning errors between panchromatic data and multispectral data when optical satellites， which have high attitude measurement accuracy， high attitude stability， and detectors using mechanical staggered stitching， are affected by slight high-frequency attitude errors. In this paper， a high-resolution optical satellite panchromatic and multispectral image geometric positioning consistency correction method based on the high-frequency corrected attitude is proposed to address this problem， and the proposed method is validated by in-orbit satellite data. First， a rigorous geometric model is realized according to the principle of the push-broom satellite. Second， the time-sharing imaging characteristics between mechanical staggered stitching detectors are used to obtain the homonymous image point data by combining geometric positioning constraints and the matching method of the pyramid image search strategy， and the homonymous image point data are used to obtain the high-frequency attitude data in the satellite imaging process. Finally， the obtained high-frequency attitude data are used in the sensor geometry correction processing of multispectral images to obtain multispectral image data corrected by the high-frequency corrected attitude. The results indicate that the proposed method effectively eliminates the small geometric positioning error between panchromatic and multispectral data caused by the slight high-frequency attitude error， so that the geometrically corrected multispectral and panchromatic data have high-precision geometric positioning consistency. The proposed method can improve the relative geometric positioning error in the row direction between the panchromatic and the multispectral data to better than 0.15 pixel of the multispectral image， which lays a solid foundation to produce high-precision image fusion products for high-resolution optical satellites with high attitude measurement accuracy， high attitude stability， and mechanical staggered stitching detectors.Keywords：high-resolution optical satellite;mechanical staggered stitching;slight high-frequency attitude errors;geometric positioning consistency- 63
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Published：2023-05-15 -
Spectral weighted sparse unmixing of hyperspectral images based on framelet transform
Abstract：Hyperspectral sparse unmixing methods have attracted considerable attention， and most current sparse unmixing methods are implemented in the spatial domain； however， the hyperspectral data used by these methods complicate feature extraction owing to scattered information， redundancy， and noisy spatial signals. To improve the robustness and sparsity of the unmixing results of hyperspectral images， a spectral-weighted sparse unmixing method of hyperspectral images based on the framelet transform （SFSU） is proposed. First， we introduce the theoretical knowledge of hyperspectral sparse unmixing and the framelet transform. Following this， we develop a hyperspectral image unmixing model based on the framelet transform using this theory. In this model， a spectral-weighted sparse regularization term is added to construct the SFSU. Finally， to solve the SFSU model， an alternating direction method of multipliers is presented. According to the experimental results， the signal-to-reconstruction error ratio is found to increase by 12.4%-1 045%， and the probability of success （Ps） remains within 16% error. The proposed model demonstrates better anti-noise and sparse performance compared with other related sparse unmixing methods and yields better unmixing results.Keywords：hyper spectral remote sensing;framelet transform;sectral weighted;sparse unmixing;alternating direction method of multipliers（ADMM）- 49
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Published：2023-05-15
Information Sciences
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