2023 Vol. 44, No. 2

2023, 44(2): 1-6.
Improvement and analysis of optical axis consistency of an electro-optical device
DUAN Hongjian, WANG Jianjun, HU Bo, MENG Haijiang, ZUO Xiaozhou, MA Youheng, SHI Leilei
2023, 44(2): 239-245. doi: 10.5768/JAO202344.0201001
The optical axis consistency index of an electro-optical device with side hanging structure of a sensor is out of tolerance. Through the analysis of the structure of the optical bench, sensor cover and other related parts of the electro-optical device, the flatness of the mating surface of optical bench, sensor cover and the sensor mounting surface of optical bench was optimized, the accuracy was improved, and the wall thickness was appropriately increased. The modal analysis of the optical bench before and after improvement was emphasized. The improved electro-optical device was verified and tested by building a test device, and the test data were analyzed. The test results show that the improvement measures are effective in improving the optical axis consistency of the electro-optical device.
Design of wide-band continuous zoom optical system with 40x large zoom ratio
WANG Meiqin, PAN Haijun, LIU Bin
2023, 44(2): 246-252. doi: 10.5768/JAO202344.0201002
In order to achieve the aiming, identification and tracking of long-distance targets, photoelectric tracking systems often require their load systems to be continuous zoom, and have high requirements of large zoom ratio, wide band and consistency of optical axis. In this paper, the analysis of structural form and initial structural parameter of calculation of the continuous zoom system are introduced in detail, and on the basis of clarifying the relevant technical parameters, zoom ratio of 40, wide-band visible continuous zoom optical system is designed. The continuous zoom optical system is a designed to use spherical mirrors, which are easy to process, and the MTF value of each field of view is above 0.2 at 150 lp/mm, smooth cam curve, no inflection points, and good stray light control which has little impact on the system. Finally, through the location test, the system has good consistency of optical axis, which is less than 0.1mrad. Therefore, the performance indicators of the system are well met the requirements of the use of the equipment, providing reference value for the engineering implementation of the photoelectric tracking system.
Optical simulator design for hardware in loop testing of medium and long range angles-only navigation of spacecraft
DU Ronghua, LIAO Wenhe, ZHANG Xiang
2023, 44(2): 253-261. doi: 10.5768/JAO202344.0201003
An optical simulator was designed in loop testing of medium and long range angles-only navigation of spacecraft, which can verify the performance and reliability of the hardware, software and algorithms of the angles-only navigation vision sensor. Firstly, the system description of the optical simulator was given, including the hardware composition and the star map simulation software design. The commercial and off-the-shelf components were selected as the hardware, which could greatly save research and development costs and shorten the development cycle. Then, the relationship between screen brightness and star magnitude, and the optical principle of the optical simulator were emphatically introduced. Finally, the star map simulation of the optical simulator was tested, and an angles-only navigation model based on Clohessy-Wiltshire (CW) equation was introduced for hardware in loop testing, which verified the optical simulator after geometric calibration could be used for hardware in loop testing of medium and long range angles-only navigation of spacecraft. The relative position accuracy is increased by 25.5%, and the relative speed accuracy is increased by 31.3% compared with that before geometric calibration at the terminal time.
Design and illumination analysis of tiny-spherical microscopic optical system
LIU Xiaoyin, YANG Lei, YANG Tong, SU Xiaoqin, XIE Hongbo
2023, 44(2): 262-267. doi: 10.5768/JAO202344.0201004
With the advantages of non-contact and high accuracy, the machine vision detection technology is gradually replacing manual detection methods and becoming an important part of industrial production detection. As the front end, the optical system is a core part of the machine vision detection technology. A bi-telecentric optical imaging system was designed for the detection of smooth spherical surfaces within a radius of 10 mm. The optimized system was composed of 7 lenses and 1 filter, and 2 aspherical surfaces were used to correct the aberration. The modulation transfer function (MTF) curve was greater than 0.3 at 40 lp·mm−1, which meaned that the system was of good imaging performance. Since the aperture was small and the object to be tested was a specular-like material, the lighting simulation was carried out, and multiple surface light sources were selected to illuminate the object. The simulation results show that the designed bi-telecentric optical system realizes the imaging of surfaces with large curvature, and a matching illumination method is proposed.
Monte Carlo method for solving underwater light field
HE Dahua, CHENG Pu, LI Yangyang
2023, 44(2): 268-274. doi: 10.5768/JAO202344.0201005
Underwater light field distribution has a decisive impact on the underwater photoelectronic imaging quality, and the transmission degradation model of underwater images can be obtained through the underwater light field distribution. Giving the optical parameters of water body, the probability model of underwater photon transmission was established. By using the Monte Carlo method to simulate the underwater photon motion, the volume density distribution of the underwater photon scattering points could be obtained and the underwater light field could be solved. Under the condition that the water volume scattering function was spherical symmetry, the underwater light field distribution solved by Monte Carlo method was in good agreement with the results of the theoretical formula, which verified the effectiveness of this method. Finally, the distribution of underwater light field formed by underwater ideal point light source was given. Based on this conclusion, the application range of Monte Carlo method in solving underwater light field can be extended to the case when the water volume scattering function is a general expression.
Digital holographic speckle denoising method based on compound modulation of object light
WANG Chunyang, KONG Yong, LI Huan, WU Hu, ZHANG He, WANG Meng
2023, 44(2): 275-285. doi: 10.5768/JAO202344.0201006
In digital holography, the speckle noise caused by the high coherence of laser source will reduce the imaging quality. In order to overcome this problem, the combination of a quartz depolarizer and an electronically controlled liquid crystal diffuser for digital holography was proposed. By adding a quartz depolarizer to the object light path, the depolarization performance of the depolarizer was used to make the linearly polarized light in the object light path into randomly polarized light, which reduced the degree of coherence with the reference light and realized the suppression of speckle noise. By applying an electric field to the liquid crystal diffuser, the movement of the liquid crystal molecules and the orientation change of the optical axis could be caused. The liquid crystal driving voltage could be adjusted to change the different scattering effects on the incident light, thereby effectively reducing the coherence of the light source. The reconstructed images using the quartz depolarizer and the electronically controlled liquid crystal diffuser were compared and analyzed respectively with the reconstructed images of the combined device. The effectiveness of the proposed method was verified by experiments. The experimental results show that the peak signal to noise ratio of the reconstructed images of two experimental objects under the combined device reaches 16.91 and 18.30, the structural similarity is 0.47 and 0.43, the equivalent number of looks is 42% and 30% higher than the original reconstructed image, and the speckle index is 1.52 and 1.29, respectively. The proposed method can better remove the speckle noises and retain more details of the image.
Infrared simulation imaging of ship wakes and sea surface
HUANG Ziliang, ZHANG Haochun, WANG Qi
2023, 44(2): 286-294. doi: 10.5768/JAO202344.0201007
Ships have an extremely important position in the military field. Their wakes will cause relatively obvious changes in the temperature and height of the sea surface, and have the characteristics of long duration and difficult to eliminate. Therefore, the simulation of ship wakes and infrared radiation image of the sea surface can identify the ship targets more intuitively, which has a strong military demand. Based on the data of atmospheric transmittance to 8 μm~14 μm band in different background environments simulated by MODTRAN software, a ship wake infrared radiation model was established by combining Cox-Munk slope probability distribution model and considering wave occlusion factors to simulate infrared wake images with different background environments and different detection distances. The simulation results show that under the same detection conditions, although the radiation brightness of the ship wakes decreases with the increase of the detection distance, the ship wakes on the sea surface are more easily identified since the shielding effect of the rough sea surface on the infrared radiation is considerably diminished. The atmospheric transmission model has a great influence on the infrared imaging results. In summer, the background radiation energy is substantial and the average occlusion effect of sea surface is modest, so the infrared imaging of ship wakes is clearer.
Compression and reconstruction of photon counting integral imaging
CUI Yumin, YIN Liju, SUI Liguo, ZHOU Hui, DENG Yulin
2023, 44(2): 295-306. doi: 10.5768/JAO202344.0202001
As an important naked eyes three-dimensional display technology, integral imaging technology completely records the three-dimensional scene information while the huge amount of data puts pressure on the transmission and storage. In order to achieve effective compression and reconstruction of images, according to the characteristics of photon counting integral imaging, a scheme for image compression and reconstruction based on the distributed compressed sensing theory was proposed. In this scheme, images were divided into reference images and non-reference images, which were set with different measurement rates and reconstructed respectively. To ensure the reconstruction quality of non-reference images, a joint reconstruction algorithm was proposed. Firstly, the block measurements were performed on the non-reference images, and the image blocks classification was carried out by considering the correlation with the reference images. Then, combined the measurement information of the reference images, a new measurement vector was constructed to complete the initial image reconstruction. In order to further improve the image reconstruction quality, the image was reconstructed with secondary residual compensation to obtain the final reconstruction results. Finally, a large number of experiments were conducted by setting different measurement rates. The experimental results show that the image reconstruction quality of the proposed algorithm can reach 30 dB when the measurement rate is 0.25, and can reach 35 dB when the measurement rate is 0.4. The performance of the algorithm has certain superiority.
Real-time detection principle and experimental study of low and slow small flying objects
YU Kaiyang, ZHANG Lei, WANG Keyi
2023, 44(2): 307-313. doi: 10.5768/JAO202344.0202002
Birds strike and flight without approval of UAV are two major hidden dangers threatening the safety of flight takeoff and landing, and the above-mentioned birds and UAV are both low and slow small flying objects. In order to protect the safety of the airport clearance area, it is necessary to develop an early warning system with the function of anti-low and slow small flying objects. A real-time detection system for low and slow small flying objects were designed and implemented. Based on the field programmable gate array (FPGA) driven camera arrays, the sky videos with large field of view were collected in real time. Combining the parity shunt algorithm and inter-frame differential algorithm which were suitable for hardware operation to detect moving targets, the system frame rate reached 17 fps@1 024×768 pixel, and the average detection accuracy was 99.69%. Gigabit ethernet, optical fiber and switch were used to transmit the front-end runway video to the back-end tower command center, which supporting 3 km long-distance transmission. Compared with the traditional software-based serial processing, the proposed system has the advantages of high real-time, low power consumption and small size, which is suitable for deployment in practical application scenarios.
Electroluminescence image enhancement technology of half-cut photovoltaic module based on DCGANs
HE Xiang
2023, 44(2): 314-322. doi: 10.5768/JAO202344.0202003
Aiming at the problem of model overfitting caused by insufficient training samples in the automatic identification process of electroluminescence (EL) defects for half-cut photovoltaic modules, the deep convolutional generative adversarial networks (DCGANs) were adopted to generate the half-cut photovoltaic module EL images with controllable attributes. The similarity between the generated EL images and the captured EL images was evaluated by using the multi-scale structural similarity (MS-SSIM) index. The evaluation results show that the MS-SSIM indexes of all types of EL images generated by DCGANs and captured EL images are greater than 0.55, and most of the MS-SSIM values are near 0.7. In the training process of the classification models, the accuracy of the test set increases with the increase of the number of images generated in the training set. When the number of generated images reaches 6 000, the accuracy of the test set reaches 97.92%. The experimental results show that the DCGANs can generate half-cut photovoltaic module EL images with high quality and controllable attributes, which can better solve the problem of model overfitting caused by the lack of training samples.
Weather recognition method based on convolutional neural network and feature fusion
HOU Xiaoming, QIU Yafeng
2023, 44(2): 323-329. doi: 10.5768/JAO202344.0202004
In the field of solar power generation such as solar water heaters and solar cells, the climate factors such as rain, snow and cloudy days will seriously affect the power generation effect, and the work of solar servo system must also consume the energy. Therefore, it is extremely important to quickly judge the current weather conditions and design an adaptive on-off servo system. When the weather is rainy or snowy, the system should be shut down to reduce the energy consumption. In view of the problems of low efficiency, poor accuracy and large amount of calculation of traditional weather recognition methods, a weather classification set with multiple categories on the basis of public weather images was created, and a weather image recognition technology based on convolutional neural network and feature fusion was provided. By using the traditional way to obtain the color, texture and shape of the image as the bottom features of the whole model, it was improved on the basis of the original visual geometry group-16 (VGG16) model, so as to extract the deep features of the image. Finally, the bottom features and deep features were fused and output on Softmax, and the total recognition rate is 94%.
Point cloud registration algorithm based on 3D shape context features
ZHOU Zixiang, HUANG Dandan, LIU Zhi
2023, 44(2): 330-336. doi: 10.5768/JAO202344.0202005
Aiming at the problems of large amount of point cloud data, long registration time and low registration accuracy in the registration process of point cloud, a point cloud registration algorithm based on intrinsic shape signatures (ISS) and 3D shape context (3DSC) was proposed. Firstly, the down-sampling of point cloud was carried out by using a voxel grid filter. Then, the ISS algorithm was adopted to extract the feature points, which were described by 3DSC, and the rough matching was performed according to the improved random sample consensus (RANSAC) algorithm. Finally, the improved iterative closest point (ICP) algorithm was utilized to accurately match the point cloud. The experimental results show that compared with 3D normal distribution transformation (NDT) algorithm based on ISS+3DSC and ICP algorithm based on sample consensus initial aligment (SAC-IA), the proposed algorithm has higher registration accuracy and efficiency, and also has a better matching effect on point clouds with large data volume.
Image deblurring based on multiple local residual connection attention network
CHEN Qingjiang, WANG Qiaoying
2023, 44(2): 337-344. doi: 10.5768/JAO202344.0202006
Aiming at the problem that existing image deblurring algorithms based on convolutional neural network are not clear in the restoration of image texture details, an image deblurring algorithm based on multiple local residual connection attention network was proposed. Firstly, a convolutional layer was used to extract the shallow features. Secondly, a new multiple local residual connection attention module based on residual connection and parallel attentional mechanism was designed to eliminate the image blur and extract the context information. Moreover, a pairwise connection module based on dilated convolution was adopted to restore details. Finally, a convolutional layer was used to reconstruct the clear images. The experimental results show that the peak signal to noise ratio (PSNR) and structure similarity (SSIM) on GoPro data set are 31.83 dB and 0.927 5, respectively. Both qualitative and quantitative results show that the proposed method can effectively restore the texture details of blurred images, and the network performance is better than that of the comparison method.
Extraction method of gear interference image foreground region based on adaptive threshold
ZHAO Yichao, WANG Xian, JIAO Mingxing
2023, 44(2): 345-353. doi: 10.5768/JAO202344.0202007
Gray method of tooth surface objects is one of the important methods to extract the gear interference image foreground region in laser phase-shifting interferometry. In view of the problem of limited measurement efficiency and limited accuracy caused by the manual threshold setting and neglect of different image edge features, a foreground region extraction method of gear interference image based on adaptive threshold was proposed. Firstly, the morphological characteristics of gear tooth surface and difference of each edge vertex were analyzed, and the image was divided into regions. Then, according to the changing rule of edge gray scale, the mask results were obtained by selecting qualified pixels through neighborhood window, so as to realize the foreground region extraction. Finally, the segmentation results of four groups of algorithms and traditional methods were compared according to five kinds of image evaluation indexes. The results show that the matching accuracy between the algorithm and reference results is improved by about 3.5%~4.5% based on the automatic image processing, the probabilistic rand index (PRI) is improved by about 3%~4%, the variation of information (VOI) is improved by 15%~25%, the global consistency error (GCE) is reduced by 2.5%~3.5%, and the final phase information accuracy is increased by 9 μm~15 μm. The results meet the requirements of accuracy, and the method can be widely used in foreground extraction of gear interference image.
Real-time splicing method of image data based on embedded panoramic video stream
WANG Jiaoying, LI Liangfu, HE Xi, HUANG Xiying, WANG Jie, ZHOU Guoliang, CAO Yuju, ZHANG Xuhui
2023, 44(2): 354-364. doi: 10.5768/JAO202344.0202008
Aiming at the influence of airborne panoramic video stream, a panoramic real-time splicing method based on embedded mode was proposed. The speeded up robust features (SURF) points in the image were extracted to generate the feature descriptors. The matching degree was determined by calculating the Euclidean distance between two feature points. After affine transformation, the Poisson transform was used to realize the fusion and smoothing between images. The above processes were executed concurrently on the target equipment, and customized optimization was carried out according to the characteristics of each process to realize the panoramic real-time splicing. After test, the proposed method realizes the seamless effect at the splicing joint, and the splicing speed reaches 30 Hz, which can meet the requirements of real-time display.
Influence of optical axis pointing error on optical pose measurement accuracy
HU Xiaoli, TANG Minggang, CAI Wenze, ZHANG Yulun, ZHANG Siqi, ZHANG Sanxi
2023, 44(2): 365-370. doi: 10.5768/JAO202344.0203001
To verify the influence of optical axis pointing error on optical attitude measurement accuracy and provide a theoretical basis for subsequent real-time equipment to realize attitude measurement, based on the central axis method, the algorithm steps were split, and the influence of optical axis pointing error was traced. It was concluded that the influence of pointing error on attitude processing results could be analyzed from two aspects. At the same time, the direct influence of the intersection algorithm in the model and the indirect influence of the dynamic reference outside the model were deduced and analyzed. Through the combination of simulation calculation and measured data, the conclusion that the pose angle error is no more than 0.1° in typical medium and long-term optical attitude measurement in the pointing error of 200" was obtained. It lays a theoretical foundation for the reliability analysis of the existing attitude measurement and the capability expansion of the subsequent range equipment.
Line structured light calibration system based on planar complementary target
ZHAI Yusheng, YANG Ben, ZHANG Zhifeng, WANG Rui, XI Haiqi, LI Shangshang, GENG Lijie, ZHANG Ruiliang, KUANG Cuifang
2023, 44(2): 371-379. doi: 10.5768/JAO202344.0203002
To improve the calibration efficiency and accuracy of line structured light sensor, a plane checkerboard and concentric circle complementary line structured light calibration system was designed integrating with self-backlighting and pose-adjustable functions. Based on the geometric relationship between the real projection position of the concentric circle center and the center of projection ellipse, the nonlinear optimized compensation model for eccentric error was established, which precisely determined the eccentric error compensation position of the circle center under the perspective projection. Compared with the traditional calibration method, the proposed method reduced the re-projection error by 84.7% and effectively solved the problem of high-precision calibration for eccentric error compensation of circular markers. By combining the plane passing through the optical center and the center line of the light bar with the target plane in the camera coordinate system, the coordinate information of the spatial intersection lines was obtained for many times to increase the feature points, and the problem of lower calibration precision of plane fitting due to the fewer feature points was solved by using the least square method to fit the optic plane equation. In complex environments, the average error of the outer diameter of the large-size grinding wheel is 0.005 1 mm measured by repeated experiments, and the results show that the designed calibration system has certain precision, simplicity, and practicability.
Error analysis of precision scanning system for measuring reflectivity of large-aperture optical elements
ZHANG Biao, ZHANG Jinyu, JI Xiao, DUAN Yuanyuan, WU Lei, LI Gaoping, YU Dongyu, YIN Wanhong
2023, 44(2): 380-385. doi: 10.5768/JAO202344.0203003
In order to measure the reflectivity of large-aperture and high-reflectivity optical elements in high energy laser transmission system, a precision measurement system for two-dimensional scanning of large-aperture optical elements was designed. The structure and working principle of the system were introduced, the factors affecting the measurement accuracy of the system were analyzed, and the influence of systematic error of the scanning system on the measurement accuracy was theoretically analyzed. The results show that the measurement error is 10−6 magnitude when the horizontal deviation is 0.29 mm perpendicular to the beam propagation direction. When the variation of cavity length is small, the rotation axis deviation can be compensated and the system can be fine adjusted by adjusting the ring-down cavity mirror. By fitting the data of light intensity and time, the corresponding first-order exponential function fitting curve was obtained, and the ring-down time as well as the reflectivity was obtained by calculation. Through contrast analysis, this kind of error analysis method can more effectively measure the reflectivity of cavity mirrors and can reduce the error brought by the experimental data itself.
Influence of field of view angle and positioning error on spectral radiance measurement
HE Sijie, DAI Caihong, CHENG Qiutong, WU Zhifeng, LI Ling, WANG Yanfei
2023, 44(2): 386-391. doi: 10.5768/JAO202344.0203004
In order to reduce the inconsistency of spectral radiance caused by the aiming area differences between different spectroradiometers, the influence of field of view (FOV) angle and positioning on measurement accuracy was discussed. By establishing the spectral radiance measurement model of sunlight whiteboard and tungsten halogen lamp whiteboard, the differences between the average radiance and central radiance in the field of view of different geometric optical paths were analyzed. Numerical simulation shows that the correction factor of sunlight whiteboard path is only related to the size of FOV angle, while that of tungsten halogen lamp whiteboard path decreases with the increases of the measurement distance. When the distance is 600 mm, the correction factor of 8° and 14° FOV angle changes to 0.993 5 and 0.980 2, respectively. Finally, the experiment verified the influence of the angle error and displacement error on spectral radiance when using the tungsten halogen lamp whiteboard system. The results show that the correction factor presents asymmetry to the horizontal angle error and displacement error, and the difference between the two sides is as high as 2%. Therefore, the measurement results can be corrected according to the FOV angle and geometric positioning, which helps to improve the uncertainty level of spectral radiance measurement.
Research on temperature control and self-tuning for 30℃~420℃ blackbody
HU Tieli, WANG Honghong, LI Siwei, CAO Feng, HU Xinyi, FAN Zhe, YANG Yuxin, GUO Jian, YOU Yue, YANG Ke, LI Hui, YU Yang
2023, 44(2): 392-397. doi: 10.5768/JAO202344.0203005
As the standard infrared light source, it is required that 30℃~420℃ blackbody can quickly heat up to the setting temperature, and keep the temperature stable. For the characteristics of large difference and large lag of heating and cooling power, the self-tuning method of impulse response based on the bang-bang control was used to obtain the parameters of blackbody temperature rise overshoot and maximum heating and cooling rate. Using the compound intelligent temperature control strategy, the temperature of 30℃~420℃ blackbody could rise speedily in the first stage. When approaching the setting temperature, the blackbody gradually reached and stabilized at the setting temperature. The experimental results show that 30℃~420℃ blackbody rises to the target temperature without overshoot, and the temperature stability is ±0.03 ℃/min, which reached the international level of similar products.
Speckle autocorrelation resolution measurement and correction
LUO Shiqi, LIU Jingzhou, CHEN Xuanhui, XIE Xiangsheng
2023, 44(2): 398-405. doi: 10.5768/JAO202344.0203006
Speckle correlation is the basis of speckle-based optical measurements and imaging recovery technologies, which determines the resolution of the opitcal system. At present, the theoretical description of speckle size (granularity or resolution) is not accurate enough and lacks of experimental verification. The influencing factors of speckle pattern autocorrelation size were explored, compared it with the same numerical aperture objective, and the scattering lens properties of thin scattering medium were revealed. Through the multiple sets of measurements for speckle autocorrelation and lens focusing size, the results show that the apodization function will affect the resolution, and the Abbe criterion needs to be corrected according to the specific optical path, which has a certain reference value for speckle-based measurement and imaging technology.
Measurement for optical properties of gun flame and smoke
LUO Yanling, WANG Xicheng, WANG Lyu, GAO Meng, WANG Man, CHENG Hongliang
2023, 44(2): 406-411. doi: 10.5768/JAO202344.0203007
For requirements of detecting, positioning and warning of sniper in sniping operation, the research of optical properties for gun flame and smoke was developed. According to the documentary data for gun flame and smoke and performance parameters of optical measuring equipment, a detailed field test scheme of optical properties for gun flame and smoke was designed. The optical properties for gun flame and smoke of a 5.8 mm and 7.62 mm caliber ballistic rifle were tested in the field. The test results show that the radiation of gun flame and smoke is mainly concentrated in 2 μm~5 μm, the duration was approximately 6 ms, the maximum radiation energy appears around 2 ms, and the radiation intensity of gun flame and smoke of 5.8 mm ballistic rifle is higher than 7.62 mm in both medium waves and long waves. The measurement results of optical properties for gun flame and smoke provide an important basis for the detection of gun flame and smoke and the improvement of weapon propellant.
Measurement method of helicopter blade root torsion angle based on perspective transformation
CUI Shuaihua, YU Lei, ZHU Junqing, YAO Tian, XIONG Bangshu, OU Qiaofeng
2023, 44(2): 412-419. doi: 10.5768/JAO202344.0203008
In the existing methods, images of markers are often taken from bottom to top. When the blade is twisted greatly, there is an obvious angle between the principal optic axis of the camera and the normal vector of a marker plane, resulting in perspective distortion of the captured images, which leads to relatively large measurement errors. In order to solve the problem raised above, a method of measuring the torsion angle of helicopter blade root based on the perspective transformation was proposed. Firstly, the target was vertically fixed under the blades, so that the camera was directly facing the propeller hub to capture images. Secondly, the four vertex coordinates of the blades in the target images of azimuth to be measured and positive azimuth were calculated when operating at the low-speed leveling state, and the perspective transformation matrix of the azimuth to be measured was solved. Moreover, the perspective transformation matrix was used to correct the distorted image of the azimuth to be measured under the high-speed motion state of the blades. Finally, the coordinates of the circle center in the reference image and the corrected image were obtained, and then the torsion angle of azimuth to be measured was calculated. The experimental results show that the measurement error of each azimuth is less than 0.2°. This method not only has high measurement accuracy, but also has the advantages of multi-azimuth dynamic measurement, which has a good application prospect in the field of motion parameters measurement of helicopter blade root.
Automatic detection method of forest diseases and insect pests based on spectral images
ZHOU Xiaoli, ZHOU Lijun, YI Lita, LIU Yu
2023, 44(2): 420-426. doi: 10.5768/JAO202344.0203009
The natural forest areas in China are widely distributed and the terrain is complex. Relying on the traditional patrol detection method of forest rangers to prevent and control forest diseases and insect pests is inefficient, so it is difficult to find early forest diseases and insect pests in time, which may miss the best time for prevention and control. In view of this problem, a deep learning network based on multispectral image detection of forest diseases and insect pests was designed, and a set of detection software was developed. Through the UAV hanging flight experiment, the built deep learning network was used to complete the detection of infected areas in forest areas, and the detection results were analyzed.
Multi-color layer laser scanning and auto-focus projection technology
YANG Fan, MU Da, HOU Maosheng, YANG Jialiang, WANG Wenbo, LIU Yang
2023, 44(2): 427-436. doi: 10.5768/JAO202344.0207001
In order to break through the restriction of application scene caused by the limitation of single wavelength of conventional laser projector and realize the layer projection of different parts, different materials and different assembly processes, a multi-color layer laser scanning and auto-focus projection system was designed and established. Two wavelengths of laser were adopted as the light source, and according to the different positions of dichromatic mirror, the two laser scanning and auto-focus projection schemes of multi-color co-optical axis and separate optical paths were proposed, and the corresponding mathematical models of optical system were deduced. By adjusting the distance between mirror groups through the auto-focus function of the system, the minimum light spots of different colors could be focused on the projectors at different distances. Two kinds of projection optical systems were simulated by ZEMAX optical design software, and the reliability and projection effect of the two systems were compared and analyzed. The experimental results show that on the projection plane at 3 m, the spot diameter of various wavelengths of the co-optical axis system is within 0.8 mm and the spot size is uniform, which can achieve the scanning and auto-focus projection function of multi-color layers.
Characteristics of cavity tuning of half-external cavity Nd:YAG and Nd:YVO4 microchip solid-state lasers
CHEN Weiguang, DENG Yong, ZHANG Shulian
2023, 44(2): 437-443. doi: 10.5768/JAO202344.0207002
Microchip solid-state laser is an important light source for precision measuring instruments because of its small size and long life. The plano-plano and half-external cavity Nd:YAG and Nd:YVO4 microchip lasers were constructed. The length of the laser resonator was changed by controlling the expansion and contraction of the piezoelectric ceramics. At the same time, the longitudinal mode and wavelength were observed by using F-P scanning interferometer and wavemeter. The cavity tuning characteristics of the two microchip solid-state lasers were studied, including the relationship between the cavity length and the optical power, the optical power and the variation rules of single and double longitudinal modes in the process of laser longitudinal mode sweeping through the laser output bandwidth. The experimental results show that the single and double longitudinal modes appear alternately during the process of cavity tuning, and the cavity length and pump current jointly affect the output mode and optical power of the laser.
Scale factor compensation technology of laser gyro with total reflection prism
HUANG Yueruihan, MA Jiajun, ZHANG Zipeng, LIU Jianning, JIANG Junbiao
2023, 44(2): 444-449. doi: 10.5768/JAO202344.0207003
In view of the phenomenon that the scale factors of the laser gyro with total reflection prism changes periodically with temperature, the influence of temperature changes on the ring laser area under the condition of frequency stabilization was studied by matrix optical method. The relationship between the scale factors of the laser gyro with total reflection prism and the frequency stabilization voltage was obtained, and it was pointed out that the beam offset was the cause of the periodic change of scale factors with temperature. According to the relationship between the scale factors and the frequency stabilization voltage, the scale factor compensation model of the laser gyro with total reflection prism was established, and the nonlinearity of the scale factors before and after compensation was compared through experiments. The results show that the nonlinearity of the scale factors is increased by more than one order of magnitude by using the proposed model to compensate the scale factors of the laser gyro with total reflection prism. The research has certain reference value for improving the performance of laser gyro with total reflection prism.
Calibration method for high-power and wide-band laser beam divergence angle
DUAN Yuanyuan, JI Xiao, YIN Wanhong, YU Dongyu, ZHANG Jinyu, ZHANG Biao, SONG Yibing, LI Gaoping
2023, 44(2): 450-455. doi: 10.5768/JAO202344.0207004
In the evaluation and analysis of high-power laser system, the beam quality of laser is the decisive factor of the system beam quality and also an important index of acceptance and appraisal of laser, among which the beam divergence angle is an important parameter to identify the laser beam quality. The range of laser wavelength tested in this system is relatively wide, generally between 0.532 μm~10.6 μm, and no suitable detector can cover the whole band. Therefore, a new method was adopted to solve the measurement problem of wide-band beam divergence angle. The method of combining charge coupled device (CCD) imaging and scanning slit was used to measure the wide-band beam divergence angle. The CCD method was used to measure the laser beam divergence angle in visible and near-infrared band (0.532 μm~1.2 μm), and the scanning slit method was used to measure the laser beam divergence angle in mid-infrared band (1.2 μm~10.6 μm). The combination of the two methods can accurately measure the laser beam divergence angle in different wavebands.
Switchable soliton molecules and noise-like pulses in hybrid mode-locking fiber laser
ZHAO Ming, WANG Tianshu
2023, 44(2): 456-461. doi: 10.5768/JAO202344.0208001
The evolution between different types of pulses is the embodiment of the passive mode-locking fiber laser rich dynamics. A hybrid mode-locking fiber laser with multiple pulse switching was reported. When the pump power was 400 mW, the mutual switch of pulses between soliton molecules, harmonic mode-locking and soliton clusters were realized. Then, when the pump power was increased to 600 mW, the noise-like pulse output was realized, and the corresponding output power and single pulse energy were 15.2 mW and 0.86 nJ, respectively. By adjusting the polarization controller, the central wavelength of noise-like pulse could be tuned from 1 895 nm to 1 930 nm. The designed laser has the advantages of mode-locking switching, wavelength tuning and automatic start.
High-sensitivity refractive index sensor based on FMF-CLF-FMF optical fiber structure
ZHANG Jing, LI Yongqian
2023, 44(2): 462-468. doi: 10.5768/JAO202344.0208002
Optical fiber refractive index sensor is widely used for monitoring in various complex environments. A high-sensitivity refractive index sensor based on the structure of few-mode fiber (FMF)-coreless fiber (CLF)-FMF was designed, and the experimental verification was carried out. The sensor consisted of a thinned section of CLF fused between two small sections of FMF to form a Mach-Zehnder interferometer (MZI) for measuring external refractive index, and the fiber Bragg grating (FBG) was used for temperature compensation. The resonance trough of interference spectrum generated by MZI structure was affected by both refractive index and temperature, while FBG was only affected by temperature. The sensitivity matrix was constructed by using the refractive index and temperature sensitivity coefficients of MZI and FBG to realize the simultaneous measurement of refractive index and temperature. Experimental results show that the refractive index sensitivity of MZI is 345.66 nm/RIU, and the temperature sensitivity is 0.0134 nm/℃. Meanwhile, the temperature sensitivity of FBG is 0.0104 nm/℃.