Optical design

Design of automatic out-of-focus correction system for near-space lidar receivers
Li Yingchao, Zhang Qing, Liu Zhuang, Shi Haodong, Fu Qiang
2024, 53(4): 20230645. doi: 10.3788/IRLA20230645
[Abstract](22) [FullText HTML] (5) [PDF 2815KB](6)
  Objective  Lidar is the main way to obtain three-dimensional geographic information within the military, and the data results obtained through this way are also widely used in resource exploration, land use, environmental monitoring and national key construction projects, providing extremely important original information for the national economy, social development and scientific research, and has achieved significant economic benefits, showing good application prospects. The lower temperature of the near space can reach –60 ℃, the optical antenna as the core component of LiDAR, its optical components have strict requirements for temperature changes. Temperature variations can lead to thermal deformation of the element, resulting in problems of defocusing and focal plane translation, which reduces the coupling efficiency. Improving the coupling efficiency can increase the detection rate, and the off-axis reflective optical system can be realized without obstruction, which can improve the energy utilization. However, unlike conventional refractive or coaxial reflection systems, each optical element in the off-axis reflection system does not have rotational symmetry, and its temperature deformation after temperature change is not uniform, so solving the effect of temperature change on the focus of the off-axis parabolic mirror is the key to improve the coupling efficiency of LiDAR.  Methods  In this paper, the LiDAR is modeled, and the temperature field is simulated by the finite element analysis method for the model. The Zernike polynomials are used to fit the surface shape data obtained after the analysis to obtain the shape and position change of the reflector after temperature deformation, and the optical design software is used to obtain the optimal focal point position. Finally, the optical fiber position is adjusted by the focusing device to achieve the effect of focusing.  Results and Discussions  The PV value of the mirror of the designed LiDAR is less than 10/λ. Its optimal focus position curve is obtained by optical design software, and a temperature-adaptive focusing structure is designed according to this curve, through which the RMS radius of the mirror compensated by the focusing structure decreases from 26.495 μm to 15.93 μm, the spot radius is reduced by 39.9%, and the coupling efficiency is improved from 15.8% to 91%. This focusing method does not need to keep track of the changes in the temperature of the reflector and reduces a certain amount of weight and cost compared to focusing with a motor. However, the method requires a certain temperature response time, can not adjust the focus to the best position at the first time after the temperature change. If the temperature changes frequently, the motor should be used to quickly adjust. Due to the working height of the blimp is more fixed, and its ambient temperature does not change much, the method is feasible.  Conclusions  Aiming at the problem that the off-axis parabolic mirrors used in the near-space lidar will deform at low temperatures which leads to a decrease in the efficiency of the reflected light when it is coupled into the optical fiber, the self-focusing technique is investigated, and the auto-compensating lidar assembly is designed to offset the effect of temperature on the system. The deformation of the receiving system under thermal load is analyzed using the finite element method to obtain the discrete deformation data of the mirror surface, and the Zernike polynomials are used to fit the surface shape after the deformation of the mirror surface, and the simulation of the optical design software concludes that the change of the focal length of this LiDAR receiving system after optimization has a linear relationship with the temperature, and the compensation position is determined by the optical design software. A temperature adaptive adjustment mechanism is used to reduce the effect of out-of-focus amount caused by thermal deformation, which improves the coupling efficiency by more than 80%.
Adjustment and testing method of large aperture mirror Bipod support structure
Li Bin, Gao Sisi, Wang Haichao, Chen Jiayi, Chen Xi, Lu Yuting
2024, 53(4): 20230657. doi: 10.3788/IRLA20230657
[Abstract](12) [FullText HTML] (2) [PDF 3610KB](5)
  Objective  In order to effectively deal with the interference of gravity error in the installation and adjustment of large aperture remote sensing camera on the surface, the installation and adjustment mode of large aperture remote sensing camera is gradually changed from the horizontal form of optical axis to the vertical form of optical axis. Correspondingly, the support structure and assembly requirements of the main mirror of the camera are also constantly innovated. In order to meet the needs of mirror support and weight reduction, the Bipod structure is a more common structural form in the form of mirror support of 1 m and above caliber. Different from the traditional frame support form, it has a great change in the optical and mechanical structure bonding and structural position calibration needs. In order to ensure that the assembly positioning accuracy and surface shape change of the mirror under the Bipod support structure meet the requirements of the system index, a high-precision assembly method for the Bipod support structure is proposed.  Methods  In this paper, a method of assembly and adjustment of large-aperture mirror is proposed, which combines multi-objective spatial position conversion and Stewart structure motion inversion. This method can effectively ensure the accuracy of the optical and mechanical structure adhesion points, and realize the high-precision six-dimensional adjustment between the mirror and the main bearing plate only by relying on the mirror's own support structure. By decomposing the error index of the optical system of the remote sensing camera, the requirements of the reflector installation index are determined (Fig.4). Then the laser tracker was used to measure and construct the spatial coordinate system of the mirror and the support pad. The coordinate system restoration principle based on the nominal point was adopted to achieve the normalization of the coordinate system. According to the measured data of the nominal point on the support pad, the position of the support pad was corrected and fixed (Fig.8). Finally, based on the characteristics of the Bipod structure itself, the relationship between the length of the Bipod rod and the position of the mirror is calculated (Eq.14), and the position adjustment between the mirror and the main structure of the camera is realized by adjusting the length of the adjusting rod.  Results and Discussions  The bottom-up error distribution method was adopted to sort out the tolerance of various error sources in the process of optical system installation. Error distribution follows a top-down order, with the sum of the squares of the error coefficients at the lower level equal to the square of the error at the higher level. Finally, the wave aberration RMS caused by the main mirror assembly is 0.025λ, including the influence of the position deviation and surface shape change of the main mirror assembly. The laser tracker was used to measure and locate the position of the support pad and the mirror, and the assembly of the mirror and the support pad was completed. The surface shape variation (RMS) before and after the installation was 0.008λ (Fig.9), which met the requirements of the installation error index. A laser tracker was used to measure the position deviation between the mirror and the bearing plate, and the adjusting length of the Bipod rod was calculated using the Stewart structure motion inversion algorithm. After the final debugging, the setting position error of the mirror was 0.004 1 mm, which met the setting error index requirements.  Conclusions  Based on the setting process of a large aperture reflective remote sensing camera with Bipod structure, the setting error interval of the main mirror in the system is analyzed, and the shape error RMS value of the mirror assembly after setting is determined not to exceed 0.025λ. Based on this standard, the positioning, bonding, adjustment and detection scheme of the mirror Bipod structure is formulated. The practical results show that the method can effectively control the assembly positioning accuracy and surface shape error of the mirror, the positioning accuracy of the support pad can reach 0.029 mm, and the position adjustment accuracy of the mirror and the bearing plate can reach 0.041 mm. The installation results can meet the system imaging requirements of large aperture remote sensing cameras.
Miniaturization analysis and design of the common-optical path dual-wave infrared optical lens
Qi Chen, Jin Yangming, Xie Xiaoyu, Hou Huihui, Li Yongsheng
2024, 53(4): 20230722. doi: 10.3788/IRLA20230722
[Abstract](14) [FullText HTML] (4) [PDF 1934KB](7)
  Objective  This paper aims to realize the miniaturization design of medium and long wavelength common optical path optical systems.  Methods  An optical index allocation model for secondary imaging structures under size constraints based on Gaussian optics and primary aberration theory was established. Lens combinations of high, medium and low relative dispersion materials in the form of "−, +, −" structures are used as the primary mirror structure to correct the secondary spectrum; Meanwhile, field lenses and aspheric surfaces are used to correct spherical aberration to reduce the relay mirror. The light height is set to balance the residual aberration of the primary mirror.  Results and Discussions  Because of the high edge light height and small optical power of the primary mirror, its spherical aberration, chromatic aberration and secondary spectrum are the main sources of aberrations in the system. To correct the secondary spectrum, the "−, +, −" structure can be used. A lens combination of high, medium and low relative dispersion materials is used as the main mirror structure. At this time, the residual aberration of the main mirror is small. Field lenses are used to reduce the light height of the relay lens group and aspherical surfaces are used to correct spherical aberration to balance the residual main mirror Aberration.  Conclusions  Based on the above theory, a common optical path dual-wave infrared small optical system with a medium wave wavelength of 3.7-4.8 μm and a long wave band of 7.7-9.5 μm is designed. The total length of the system is no more than 135 mm, the structure is small and compact, the optical transfer function is close to the diffraction limit, and the operating temperature range covers −40-60 ℃ and is not sensitive to temperature. The analysis and design of a medium and long-wavelength common optical path miniaturized optical system based on the secondary imaging structure optical index distribution model are realized to meet the needs of a medium and long-wavelength common optical path miniaturized optical system.
A fast calculation method for deflection angles of dual fast steering mirrors for beam pointing control
Huang Zefan, Li Yanwei, Xie Hongbo, Yang Rui, Gu Jiarong, Xie Xinwang
2024, 53(3): 20230582. doi: 10.3788/IRLA20230582
[Abstract](52) [FullText HTML] (15) [PDF 2308KB](19)
  Objective  Internal temperature drift, mechanical structure deformation, thermal effects of optical components, and other factors lead to misalignment of the emitted laser beam, causing drift of the spot on the target surface (Fig.1) and affecting laser applications. In order to suppress this phenomenon, it is necessary to establish a beam pointing control system. Currently, there are primarily two approaches of reflective approach using fast steering mirrors and transmission approach using rotating prisms. Control system with single fast steering mirror can correct the angle deviation of beam and the deflection angle is easy to calculate. However, it cannot address the coupling misalignment between the position and angle of the beam. Control system with dual fast steering mirror can simultaneously correct the position and angle deviation of the beam. However, it introduces coupling issues and makes it difficult to find suitable deflection angles. Currently, there have been numerous theoretical analyses on the reverse angle problem of prism rotation in the transmission approach, while the coupling theory of the dual fast steering mirrors remains unclear, and the linear fitting of mirror deflection angle and spot position offset lacks theoretical support. Therefore, it is necessary to explore in depth the inherent relationship between beam pointing and the deflection angles of dual fast steering mirrors and find a rational method to calculate the deflection angles.  Methods  The dual fast steering mirror beam pointing control system consists of two branched optical paths (Fig.2). Using ray tracing, a theoretical model of this system has been established (Tab.1). This model reflects the mathematical relationship between the beams and the deflection angles of the dual fast steering mirrors. Based on this model, a simulation environment can be created. By analyzing this theoretical model, it is observed that the complex coupling relationship between the deflection angles of the fast steering mirrors and the spot position deviation can be approximated as a linear relationship under small deflection angles. A fast calculation method for the deflection angles of the dual fast steering mirrors is proposed. Firstly, a data set of spot position deviations and deflection angles is collected using an iterative convergence strategy (Fig.5). Then, a shallow neural network is trained based on the historical data accumulated from iterative convergence strategy (Fig.8). Finally, the trained neural network is used to quickly determine the output deflection angles of the dual fast steering mirrors based on the input detector's spot position deviation. The data collection and neural network training processes of the iterative convergence strategy can be performed offline, without increasing the time required for beam pointing control.  Results and Discussions   The experimental results in the simulation environment demonstrate that the proposed iterative convergence strategy effectively solves the deflection angles for beam pointing control (Fig.6-7), with an average iteration step of 9.09. The fast calculation method based on shallow neural networks establishes a direct mapping between spot position deviation and deflection angles, and the result can be obtained after a single computation. The experimental results in the simulation environment show that compared to the beam state before control, the position deviation in the X and Z directions is reduced by 99.32% and 99.46% respectively, the angle deviation is reduced by 99.07% and 98.98% with the average comprehensive deviation being reduced by 99.16% (Tab.2). This method effectively suppresses the original beam misalignment. The shallow neural network only requires one-step solving process, eliminating the need for multiple iterations and greatly improving the calculation speed.  Conclusions  After the derivation and analysis of the theoretical model of the dual fast steering mirror beam pointing control system, the coupling phenomenon is explained, and it is demonstrated that there exists an approximate linear relationship between the deflection angle of the fast steering mirrors and the spot displacement under small angle conditions. The simulation experimental results show that with the proposed fast calculation method the deflection angles of dual fast steering mirrors for beam pointing control can be solved quickly and effectively. In engineering applications, a simulation model can be constructed based on the actual optical path parameters and form a simulation dataset. Subsequently, iterative convergence control can be performed in the actual optical path to form a real dataset. Integrated learning, transfer learning, and model fusion can be performed based on both datasets to reduce the requirement for a large training dataset for shallow neural networks.
Design and optimization of a single-core axis in a ground-based photoelectric imaging system
Zhu Hanwang, Xue Xiangyao, Shao Mingzhen, Zhang Wenbao, Li Shang, Wang Xiushuo, Wang Guangyi, Yang Xinyu
2024, 53(3): 20230629. doi: 10.3788/IRLA20230629
[Abstract](31) [FullText HTML] (2) [PDF 3728KB](25)
  Objective  The study aims at addressing a critical need in ground-based optoelectronic imaging, which is enhancing the surface form accuracy of primary mirrors under extreme conditions such as large pitch angles and significant temperature variations. This aim is vital as it directly impacts on the quality of optical imaging, an increasingly important factor in various applications ranging from scientific research to defense. Recognizing the limitations of traditional support structures in these challenging environments, a novel monolithic shaft support structure was developed in this paper. This new design was targeted to significantly improve the stability and thermal adaptability of primary mirrors, ensuring their performance in demanding conditions. The study involved rigorous theoretical analysis using Castigliano's second theorem and practical optimization using advanced techniques like the multi-island genetic algorithm. These methods were integral to balancing structural stability with precise surface form accuracy, setting a new benchmark in the field. In essence, this research sought to revolutionize the design and functionality of support structures for medium-caliber primary mirrors in ground-based optoelectronic systems, enhancing their reliability and performance in extreme environments. This advancement was not just an improvement but a necessary step to meet the growing demands for high-quality optical imaging in diverse and challenging conditions.   Methods  A novel single-core-axis support structure was proposed to enhance the mirror's stability and adaptability to thermal expansion. The study utilized Castigliano's second theorem for an in-depth analysis of the impact of the single-core-axis stress size chain parameters on the mirror surface errors. Further, an integration of Isight platform and a multi-island genetic algorithm was employed for optimizing the structural parameters. This approach allowed for a fine-tuned balance between structural stability and surface accuracy.   Results and Discussions  In this study, the fabricated support structure was integrated into the optical system, achieving a primary mirror surface precision with an RMS of 4.4 nm and a PV of 56.28 nm, primarily affected by manufacturing errors and gravitational load. The mirror was positioned horizontally along the optical axis to induce maximal surface deformation (Fig.12). Surface accuracy assessments at room temperatures of 20 ℃ and 40 ℃ revealed RMS values of 15.81 nm and 19.23 nm, and PV values of 83.17 nm and 91.98 nm, respectively. The 20 ℃ temperature variation introduced a form error RMS of 3.42 nm and a PV of 8.81 nm (Fig.13). Extrapolating from these results, under an extended temperature range (−40 ℃ to +40 ℃), the estimated RMS and PV errors are approximately 10.3 nm and 26.5 nm, respectively, well within acceptable limits for optical imaging systems.　　These findings, validated through interferometric analysis (Fig.13), demonstrate the design's capability to maintain mirror surface accuracy under varied temperature conditions, confirming its suitability for diverse environmental applications.   Conclusions  This research addressed the design of support structures for medium-caliber ground-based optoelectronic imaging equipment in extreme temperature differential environments. The monolithic shaft support structure adopted significantly reduced thermal strain and maintained rigid body displacement within acceptable limits. Key structural parameters were analyzed using Castigliano's second theorem, and a multi-island genetic algorithm was employed for multi-objective optimization of structural components. Simulation and physical experiments validated that the primary mirror's surface form error adhered to optical imaging requirements even under significant temperature variations (ΔT=80 ℃). Notably, the optimization of RMS and PV values improved by 59.99% and 23.2%, respectively, with a 21.96% enhancement in rigid body displacement. Future work will focus on extending the application of the monolithic shaft structure to larger aperture mirrors and broader temperature ranges, further optimizing and validating its optical and structural stability. This study provides essential insights for the development of ground-based optoelectronic imaging systems.
Design method of beam shaping system for double free-form surfaces based on Virtual Surface Iteration method
Zhu Quanjin, Ma Haotong, Chen Bingxu, Xing Yingqi, Lin Junjie, Tan Yi
2024, 53(2): 20230587. doi: 10.3788/IRLA20230587
[Abstract](104) [FullText HTML] (20) [PDF 3567KB](23)
  Objective  The double free-form optical beam shaping system can adjust the spatial intensity distribution of the beam without altering its phase distribution. Still, it requires solving for the shape distribution of the double free-form optical surfaces by setting a virtual plane. This study reveals that applying the traditional single virtual surface method to design beam shaping systems with compact structures (short distance between double free-form optical elements) and large beam amplification (the ratio of the cutoff radius of the outgoing beam to the incident beam) ratios have some drawbacks, such as significant errors in solving for the double free-form optical surfaces and reduced shaping effectiveness, which generally includes the energy efficiency of the whole system for the beam and the irradiance uniformity of the beam after shaping.  Methods  This paper presents a method for designing a free-form optical beam shaping system based on a virtual surface iteration strategy and the concept of misalignment is proposed to evaluate the difference between the obtained second free-form surface and the virtual surface. The first step involves the creation of a virtual plane at the vertex of the second free-form surface and the virtual surface serves as the target surface of the beam exiting from the first free-form surface. Subsequently, all the discrete points on the first free-form surface can be obtained by using the virtual surface and Snell's law, in this case, all discrete points on the second free-form surface can be obtained by using Snell's law given the outgoing beam of the first free-form surface and the target surface of the whole beam shaping system. Finally, an iterative process updates the virtual surface to approximate the true shape of the second free-form surface.  Results and Discussions   The quantitative analysis examines the influence of the beam amplification ratio β and the axial distance D between the two optical elements on the misalignment between the virtual surface and the actual surface. A negative correlation is observed between β and misalignment, while a positive correlation exists between D and misalignment. Importantly, it is noticeable that with an increase in iterations, the value of misalignment rapidly approaches 0, thereby verifying the effectiveness of the Virtual Surface Iteration method. Two distinct beam shaping systems have been designed: a transmitted double free-form surfaces system and an off-axis two-mirror system. Simulation results demonstrate that both systems achieve over 95% irradiance uniformity (Tab.1) and more than 99% energy efficiency (Tab.1). Furthermore, employing the Single Virtual Surface method relatively enhances irradiance uniformity by 2.93% and energy efficiency by 8.930%.  Conclusions  This paper presents a design method for the beam shaping system of the double free-form surface based on a virtual surface iteration strategy. The proposed method employs ray tracing to calculate discrete points on the free-form surface. It utilizes the virtual surface iteration strategy to minimize the misalignment between the virtual and real surfaces. This approach ensures that the virtual surface continuously approaches the actual shape of the second free-form surface, thereby enhancing the coupling between the free-form surfaces in the beam shaping system. Additionally, this study analyzes the correlation between misalignment and parameters of the beam shaping systems, concluding that the misalignment is positively associated with both the beam amplification ratio and the compactness of the spatial structure of the system. Subsequently, simulation software is employed to design and simulate a coaxial transmission beam shaping system as well as an off-axis double-mirror beam shaping system. These simulations yield outgoing beams with ideal irradiance uniformity and energy efficiency. Compared to virtual plane methods, our proposed approach significantly improves shaping effects, thus validating its effectiveness in laser processing, medical treatment, optical information processing, and other fields requiring laser beam shaping systems.
Tolerance desensitization method based on principal component analysis and nodal aberration theory
Guan Zihan, Wang Min, Li Xiaotong
2024, 53(2): 20230590. doi: 10.3788/IRLA20230590
[Abstract](120) [FullText HTML] (33) [PDF 2130KB](33)
  Objective  Optical systems with low tolerance sensitivity have good machinability and high manufacturing yield, reducing processing and adjustment costs. To achieve this, it is necessary to evaluate and optimize the system's tolerance sensitivity in its design, so it is necessary to study related desensitization methods. Traditional desensitization methods include using global optimization algorithms, establishing multiple structures or a combination of both to conduct extensive searches in the solution space, which requires a large amount of computational resources and has low optimization efficiency. Besides, the method by controlling the system structure and ray tracing parameters (such as surface curvature, ray deflection angle, etc.) or optimizing specific aberration distributions to obtain tolerance-insensitive structures lacks the analysis of introduced aberrations, and there is still a certain blindness in the setting of evaluation functions, which also affects the optimization efficiency. Therefore, in order to achieve higher optimization efficiency, this paper proposes a tolerance desensitization method based on the analysis and control of introduced aberrations, and provides corresponding operation counts.  Methods  Zernike polynomials are used to quantify aberrations. Based on this, linear algebra theory and Monte Carlo analysis are used to find the aberration change rule of the system after introducing perturbations. The main introduced aberrations are then determined through the aberration field and eigenvalue distribution after dimension reduction (Fig.7, Fig.11). Asymmetric perturbations and axial perturbations that may occur during the system manufacturing process are modeled. The introduced aberrations caused by the perturbations are described based on the node aberration theory, and the key surfaces are determined through statistical analysis (Fig.8, Fig.12). According to the correspondence between Zernike terms and wave aberrations, the aberration space is transformed, and a corresponding evaluation function is proposed. Based on the previous analysis, the weights and application surfaces of each term of the evaluation function are determined, and then it is included in the optimization process to suppress the generation of new aberrations. The analysis and optimization ideas of this method are shown (Fig.3).  Results and Discussions   This method has been applied to the design of the F#11 optical system (Structure 1) and the NA0.5 optical system (Structure 2). After optimization, the expected machining performance has been significantly improved. Taking the MTF performance at the specified spatial frequency on the axis with a 98% confidence level as an example, the performance of the two systems after optimization has increased by about 68% (Fig.9) and 20% (Fig.13) respectively. Compared with the optimization using the TOLR operation number in Zemax software, the optimization time of Structure 1 has been reduced from 7 hours to 36 minutes, and tolerance desensitization has been successfully achieved in the optimization of Structure 2.  Conclusions  A method for reducing tolerance sensitivity based on the analysis and suppression of introduced aberrations is proposed. The obtained Zernike coefficient matrix is processed by the method of principal component analysis, and the dimensionality reduction of the aberration space is realized according to the obtained eigenvalues and their corresponding eigenvectors. After analyzing the dimensionally reduced aberration space, the main introduced aberration items after perturbation are clarified. The types of aberrations caused by asymmetric perturbations and axial perturbations in the optical system are analyzed, and the quantitative expression of the introduced aberration items is obtained based on the node aberration theory. According to the correspondence between Zernike terms and primary aberrations, the expression of the evaluation function M is derived. The evaluation function is applied to two design examples, and the optimization results show that this method has higher optimization efficiency compared to existing methods, and it has a tolerance desensitization effect on optical systems with different complexities and different introduced aberration characteristics.
Area array TDI space camera for GEO target detection
He Lin, Deng Wudong, Song Liguo, Zhang Xuguo, Huang Yeping, Liu Yuchen, Lei Wenping
2023, 52(9): 20230022. doi: 10.3788/IRLA20230022
[Abstract](130) [FullText HTML] (29) [PDF 1954KB](35)
  Objective  Space target detection is the basis of space situation awareness, and space-based space target observation can be independent of the geographical location of ground-based observation stations, with unique advantages. For geosynchronous orbit (GEO) targets, the current space-based system mainly uses optical observation means. In order to improve the detection sensitivity of distant and dim targets, it is usually necessary to increase the aperture of the visible light camera to increase the light collecting capacity. As a result, the weight and volume of the camera will be significantly increased, and the resource occupation and development cost of the satellite platform will be greatly increased. Therefore, it is necessary to study a new camera observation method, which can not only achieve higher sensitivity detection, but also meet the requirements of small and light payload for satellites.   Methods  In order to meet the requirements of observation GEO target from LEO, the time delay integration (TDI) push-scanning image technology of linear array CCD usually used in the earth observation camera is applied to the area array CCD, and the time delay integration (TDI) imaging mode of the area array camera is designed (Fig.2). According to the orbit characteristics of low-orbit zero inclination satellite, the GEO target speed with different inclination angles is analyzed (Tab.1), and the CCD charge transfer speed is designed to match the target speed. The point target signal is enhanced through charge accumulation, so as to improve the detection sensitivity of the camera. The principle of TDI imaging with area array is described, the calculation model of camera signal to noise ratio is derived, the main imaging parameters such as target motion, integration time, detection sensitivity and signal to noise ratio are calculated, and the influence on point target detection imaging is analyzed. In TDI mode, the east-west relative velocity of the target is compensated to zero (Fig.3), the north-south vertical image movement and the maximum integration time of the typical inclination target are calculated (Tab.2). It can be concluded that the longer the integration time is, the greater the north-south vertical image movement is. The larger the target dip angle is, the greater the north-south velocity component is, and the greater the image movement is. The maximum integration time allowed by the camera is 3.2 s when observing a target at an inclination of 15° under extreme conditions (Tab.2). Finally, the design parameters of the camera are determined. The angular resolution of the camera is 15", the aperture is 160 mm, the integration time is 3 s, and the detection sensitivity is 15 Mv (Tab.3).  Results and Discussions   A principle prototype is developed. The star simulator is used as the dim point target in the darkroom. The principle prototype is installed on a high-precision turntable to image the point target. The turntable is used to simulate the relative speed of the camera and the target. The turntable speed is adjusted to match the charge transfer speed of the camera. The camera works in the area array TDI mode to image the target and collect images. The point target image signal-to-noise ratio formula is used to calculate the camera SNR, and then complete camera sensitivity and signal-to-noise ratio index tests. Different TDI stages are adjusted to complete the camera response linearity test under different integration times. The test results show that the camera has 96 TDI stages, the sensitivity is better than 15 magnitude, the signal-to-noise ratio is greater than 5 (Fig.4), and the linearity response is good (Fig.5), which verifies the correctness of the camera design indicators.   Conclusions  A visible light camera based on the area array TDI imaging mode is studied for observing GEO targets from low-orbit zero inclination satellites. Compared with the natural rendezvous and array staring imaging modes, area array TDI mode can increase exposure time through time delay integration without increasing the camera optical aperture or adding gimbal, so as to improve the camera detection sensitivity. On the one hand, it improves the camera detection sensitivity when the camera aperture is unchanged; On the other hand, the camera adopts the area array output mode, which can ensure the observation arc length requirements and facilitate the ground system to determine the target orbit. It provides a new technical approach for realizing higher sensitivity detection of GEO target from LEO.
Temperature control of extinction tube for the 2.5-meter large-field and high-resolution telescope
Pan Cong, Ye Yu, Gu Bozhong, Shuai Yulin
2023, 52(9): 20230024. doi: 10.3788/IRLA20230024
[Abstract](101) [FullText HTML] (18) [PDF 1931KB](26)
  Objective  The extinction tube of the 2.5-meter large-field and high-resolution telescope receives thermal radiation from the main focal point, heats the air in the lens barrel, generates random turbulence, reduces the astronomical seeing, and affects the imaging quality of the telescope. In order to solve this problem and meet the temperature control index requirements of temperature control, the Smith-Active Disturbance Rejection Controller (ADRC-Smith) is designed.  Methods  The designed ADRC-Smith controller uses the automatic disturbance rejection control combined with the Smith predictor method, which adds a Smith estimation module to the ESO signal input of the linear ADRC, uses the Smith predictor to cancel the time delay term in the closed-loop characteristic equation, and uses the automatic disturbance rejection controller to improve the response speed and robustness of the temperature control system to achieve accurate control of the extinction cylinder wall temperature. Firstly, the mechanism modeling and model parameter identification of the extinction cylinder temperature control system are carried out (Fig.3), the model of the extinction cylinder temperature control system is established, and the structure (Fig.5) as well as the parameter adjustment method of the ADRC-Smith controller are given. Secondly, according to the system model, the temperature control system of the matting cylinder is simulated when the model is accurate and the model is out of alignment to analyze the feasibility of the controller (Fig.6-7). Finally, the temperature control system of the matting cylinder at the main focus is set up for carrying out stability and reliability test to verify the practicality of the controller (Fig.1-2).  Results and Discussions  The experiment results of the stability test show that the ADRC-Smith controller can quickly track the ambient temperature, control the temperature of the extinction cylinder wall within 2 ℃ of the ambient temperature, and the corresponding response time and settling time are about 59 s and 173 s respectively, and the following error is about 0.14 ℃ (Fig.9, Tab.1). After that, the reliability test of the controller is carried out, and the reliability of the controller is further verified by introducing heat source interference to cause large fluctuations in the ambient temperature (Fig.10). The results show that the ADRC-Smith controller can improve the performance of the extinction tube temperature control system of the 2.5-meter large-field and high-resolution telescope.  Conclusions  Aiming at the temperature control research of the 2.5-meter large-field and high-resolution telescope extinction tube, the control goal is to control the temperature of its outer wall within 2 ℃ of the ambient temperature. The temperature control system of the extinction cylinder was designed and built. The temperature control system model was established through mechanism analysis and model parameter identification. According to the characteristics of the controlled object, the ADRC-Smith controller is designed by combining Smith predictor and active disturbance rejection controller. The controller has no system deviation, which can overcome the large lag of the system, has fewer parameters and can be easy to get the parameters. On the basis of the simulation results which verify the feasibility of the controller, the stability and reliability test are carried out respectively, and it is verified that the controller can be applied to the extinction cylinder temperature control system of 2.5-meter large-field and high-resolution telescope, and the system performance can be improved. The research results can also provide guidance for the design of the temperature control system of its primary mirror and reflecting diaphragm.
Optical performance of linear Fresnel condenser under different aiming strategies
Wang Chenglong, Yan Bolong, Xu Mao, Ma Jun
2023, 52(9): 20230259. doi: 10.3788/IRLA20230259
[Abstract](88) [FullText HTML] (18) [PDF 2466KB](21)
  Objective  The linear Fresnel solar concentrator is one of the technologies of concentrator solar thermal power generation (CSP). Because the primary mirror are discrete flat mirrors and installed near the ground, the system has the advantages of strong wind resistance, which is especially suitable for large-scale construction in the northwest region of China with excellent solar energy resources but high wind speed. The existing research results show that the solar irradiation on the surface of the collector tube is very uneven in linear Fresnel concentrator system due to the different aiming strategies and the reflection profile of the secondary mirror. In addition, the medium in the tube is difficult to quickly divert the heat from the surface of the collector tube, the collector tube is easy to bend and deform, resulting in the breaking of the outer glass tube of the vacuum tube and the increasing heat loss of the system. The uniformity of heat flux density distribution on the absorber tube surface has a significant impact on the optical and thermal performance, as well as safe operation of linear Fresnel concentrating solar systems. Therefore, it is necessary to study the method for improving the uniform distribution of energy flux density to improve system efficiency and system security.   Methods  A linear Fresnel concentrator model was built using Tonatiuh optical simulation software, a cylindrical reflector was selected as the research object, and the relationship between the focal length (radius of the reflector) and the position of the aiming point on the aiming plane and the width of the spot was studied. Based on the fact that the width of the spot is not greater than the width of the secondary mirror opening, the focal length of the mirror and the distance from the center of the aiming point are determined. According to the different distribution of aiming points, five different aiming strategies are proposed, and the specific data are shown (Tab.2). Based on this, the optical efficiency and uniformity of heat flux distribution of the linear Fresnel concentrator under different aiming strategies were explored.   Results and Discussions   When the height difference between the edge and the bottom of the mirror is 2.85 mm, the spot width is the smallest （Fig.3）, and it is far less than the aperture width of the secondary mirror opening. The aiming point is more than 0.04 mm off center, the spot width exceeds the aperture width of the secondary mirror (Fig.5). Using the aiming strategy of uniform distribution of aiming points, the concentrator will maintain high optical efficiency while improving the uniformity of energy flow distribution on the surface of the collector tube (Fig.7). With the optimized aiming strategy, the optical efficiency of the linear Fresnel concentrator can reach 87.4% (Fig.8), the standard deviation of energy flux density on the surface of the collector tube is reduced from 45.3% to 30.7% (Fig.8), and the energy flux density at the top of the collector tube is increased by 10.2% (Fig.9).   Conclusions  The uniformity of energy flux distribution on the surface of the collector tube of linear Fresnel concentrators can be improved by adopting different aiming strategies. By rationally distributing aiming points evenly on both sides of the center, the best spot uniformity on the surface of the collector tube and the best optical efficiency of the system can be achieved. When the aiming strategy is applied to the preheating of the empty pipe of the linear Fresnel concentrators, the bending deformation of the collector tube is much smaller than the existing aiming strategy under the variable duty ratio tracking mode. The research results can provide theoretical support for the optimal design of linear Fresnel concentrator heat collection system.
Development of emission optical system for laser wireless power transmission
Meng Xiangxiang, Shang Han, Xin Mingrui, Wang Xudong, Qiu Mingjie
2023, 52(9): 20230115. doi: 10.3788/IRLA20230115
[Abstract](74) [FullText HTML] (24) [PDF 7330KB](35)
  Objective  The emission lens of the laser wireless power transmission system is mostly a collimated lens, which is designed using the optical fiber collimation principle and the non-focus mode of the optical design software. The end face of the optical fiber is placed at the focal plane of the lens, and the beam on and off the axis is emitted externally in the form of parallel beam. Because the end face of the optical fiber has a object height, there is a geometric divergence angle between the on-axis beam and the off-axis beam. For the collimated lens with the image plane at infinity, the off-axis beam and the on-axis beam present a staggered superposition state on the illuminated surface at a relatively close distance. Even if the illuminance distribution of the rectangular fiber core is uniform, the light spot on the receiving surface of the power transmission still presents a Gaussian distribution that gradually weakens from the center to the periphery, and the light spot boundary is not clear, which reduces the power transmission efficiency of the laser wireless power transmission system. In order to improve the optical power transmission efficiency of the laser wireless power transmission system and avoid the blurring of the light spot boundary and the poor illumination uniformity at the receiving surface at a distance of hundreds of meters caused by the use of a collimated lens, the development of a focusing transmission optical system based on the conjugate imaging principle was carried out.   Methods  Firstly, the design principles of collimation method and conjugate imaging method are analyzed theoretically. Then, aiming at the 808 nm semiconductor laser light source output by optical fiber, a transmitting optical system with a focal length of 550 mm and an aperture of 260 mm is designed using conjugate imaging method (Fig.2). Focusing design is realized through the movement of optical fiber end face. The movement of optical fiber end face under different focusing distances is analyzed (Fig.4). Compared with the design results of collimation method after focusing, the wave aberration at 200 m-1 km is smaller (Fig.5). Lighttools software is used to simulate and compare the illumination spot before and after focusing.   Results and Discussions  The simulation results show that by adding a focusing mechanism to the transmission optical system designed based on the conjugate imaging principle, clear light spot boundaries can be obtained at different distances (Fig.6). The structure of the laser emission optical system is designed. The focusing structure rotates 360° and the end face of the optical fiber moves 2 mm, which meets the requirement of 1.19 mm of total end face movement of the optical fiber in the range of 200 m-1 km. The laser emission optical system is processed. The test optical path is built with ZYGO interferometer, standard lens and plane reflector. The wave aberration RMS of the laser emission optical system when focusing to infinity is 0.092λ(λ= 632.8 nm) (Fig.9). The results show that the laser wireless power transmission system can obtain a clearer and more uniform illumination spot by using the focusing emission optical system designed based on the conjugate imaging principle.   Conclusions  A focusing laser emission optical system is developed, which can be used for laser wireless power transmission at different distances. Through theoretical analysis of the collimation method and the conjugate imaging principle, the design method of the laser emission lens is determined in the case of non-infinite distance. The optical system design is carried out. The relationship between the focusing movement and the energy transfer distance is analyzed. The changes of the light spot before and after focusing at different distances are simulated and compared. Finally, the equipment development is completed, and the optical performance test is carried out to meet the design requirements.
Image rotation compensation mechanism of large field of view space camera and its optimization design
Liu Shudi, Tian Haiying, Shao Jianbing
2023, 52(7): 20220878. doi: 10.3788/IRLA20220878
[Abstract](113) [FullText HTML] (30) [PDF 4757KB](30)
  Objective  Space-based target detection is the main way to observe space debris. In recent years, with the gradual increase of space debris, it is difficult for small field of view space cameras to meet the observation needs, and the use of large field of view space cameras is increasing. During the observation of space debris, due to the orbital motion of the satellite itself and the motion of the two-dimensional turntable, the image rotation will occur in the imaging of the large field of view space camera, especially when observing dim targets, the camera's exposure time will increase, and the generated image rotation will also increase. It seriously affects the accuracy of recognition and reduces the efficiency of large field of view space camera. Therefore, image rotation compensation must be carried out for large field of view space camera.  Methods  In order to determine the performance index of image rotation compensation, the imaging coordinate system of the system is established (Fig.1), and the image rotation of the system is calculated by the homogeneous coordinate change method. According to the performance index of image rotation compensation, a new type of image rotation compensation mechanism based on the inner and outer rings of symmetrical right straight circular flexure hinge is proposed (Fig.3). Then, the flexibility and accuracy formula of the flexible element of the image rotation compensation mechanism is deduced according to the second theorem of Cassegrain, and the relationship between the flexibility and the structure size is analyzed. Then, the image rotation compensation structure is optimized by genetic algorithm. Finally, the static and modal analysis of the image rotation compensation mechanism is carried out by simulation (Fig.11, 12, 14), and it is verified by experiments.  Results and Discussions   By analyzing and calculating the ± 2′ image rotation of a large field of view space camera, an image rotation compensation mechanism composed of eight completely symmetrical flexible elements is designed for the image rotation change. By analyzing it, the relationship between the flexibility and accuracy of the flexible element and the size of the flexible element is obtained (Fig.7-8). Through genetic algorithm, the final design size of the flexible element is t=0.5 mm, r=5.5 mm, w=18 mm, l=9 mm. The simulation analysis results show that the maximum displacement component in the plane is 77.5 μm. The error with theoretical calculation model is 1.79%, far less than 5%, which meets the design requirements of the system; The maximum stress is 65 MPa, which is far less than the allowable stress of 330 MPa, which meets the design requirements of the system. The image rotation compensation mechanism has high stability and safety. Through experimental verification, the experimental value and theoretical error of the image rotation compensation mechanism are also less than 5%, and the image rotation compensation mechanism has good linearity in the working range (Fig.14). The results of modal analysis (Tab.3) show that all modes of the system meet the design requirements.  Conclusions  For the image rotation generated by the large field of view space camera during imaging, the image rotation angle generated by the camera is calculated by the homogeneous coordinate transformation method to be ± 2′, and then a set of image rotation compensation mechanism based on the flexible element is designed by this technical index, the mathematical model of the image rotation compensation mechanism is established, and the flexibility matrix and precision matrix of the flexible element of the image rotation compensation mechanism are derived; Then, according to the derived formula and the stress and fundamental frequency of the system, the image rotation compensation mechanism is optimized by genetic algorithm. Finally, the image rotation compensation mechanism is determined to be composed of the inner and outer rings connected by 8 straight beam fillet flexible elements. When the total force is 115 N, the camera is compensated with ± 2′ image rotation, and the maximum displacement of the inner ring is 77.5 μm. Then the first six natural frequencies of the system are verified by the finite element simulation, which meet the system design requirements, and the system is verified by experiments. According to the experimental results, the system has good linearity, and the error between the experimental results and the simulation results is less than 5%, which verifies the reliability of the system.
Low spatio-temporal frequency wavefront aberration correction technology of solar telescope
Zhao Simin, Gu Naiting, Huang Linhai, Xiao Yawei, Zhang Lanqiang, Cheng Yuntao, Du Zongzheng
2023, 52(7): 20220887. doi: 10.3788/IRLA20220887
[Abstract](166) [FullText HTML] (54) [PDF 5321KB](25)
  Objective  Solar telescopes are important equipment for conducting solar physics research and predicting space weather. During operation, large aperture solar telescope systems are affected by factors such as optical and mechanical structural deformation caused by solar radiation, gravitational deflection in different directions, wind-borne optical structural deformation, and environmental temperature changes, resulting in wavefront aberrations, leading to significant degradation in the imaging quality of the solar telescope system, and restricting the resolution of solar atmospheric imaging. Adaptive optical systems are the main means of correcting low spatio-temporal frequency aberrations during the operation of solar telescopes, but their correction of low-order aberrations wastes a large amount of travel and sacrifices their ability to correct high-order aberrations. Therefore, it is necessary to correct the low spatio-temporal frequency aberrations during the operation of the solar telescope without increasing the complexity of the solar telescope system.  Methods  A simulation system and an experimental system have been established for the 60 cm POST solar telescope system. The sensitivity matrix of the displacement of the secondary mirror rigid body is calculated, and the low spatio-temporal frequency aberration is introduced using a deformable mirror to simulate low-order aberrations. The aberration of the optical system's field of view on the axis is observed using a Hartmann camera. The displacement of the secondary mirror rigid body required for correcting the aberration is calculated using the sensitivity matrix method. Finally, the introduced low spatio-temporal frequency aberration is corrected by adjusting the position of the secondary mirror rigid body. The results of the system fine assembly are shown (Fig.4).   Results and Discussions  The low spatio-temporal frequency aberrations for simulated solar telescope systems are corrected, the ability of secondary mirror rigid body displacement is quantitatively analyzed to correct different types of low-order aberrations, and the principles for correcting low spatio-temporal frequency aberrations are provided. The simulation results are verified through experiments, where the RMS value of the aberration after correction for the position mismatch error of the secondary mirror pair is lower than 9% of the original value (Fig.9), the RMS value of the aberration after correction for the non-mismatch error is lower than 40% of the original value (Fig.10), and the RMS value of the aberration after correction for the multi-source mixing error is lower than 15% of the original value (Fig.11).  Conclusions  A wavefront correction algorithm and implementation system for specific scenes have been constructed with adaptive optics. The real-time wavefront correction has been completed using a hexapod driven secondary mirror. The studies of correction for position mismatch error, non-mismatch error, and multi-source mixed error have been conducted, and multiple sets of experiments have been conducted. Without increasing the complexity of the optical system, the low spatio-temporal frequency aberration of the system has been reduced, and the imaging resolution of the solar telescope has been improved. The secondary mirror rigid body displacement correction method can reduce the low spatio-temporal frequency aberration during the operation of solar telescope systems without adding optical components, and has good development prospects and application value.
Design of optical polarization system for defect detection on highly reflective surfaces
Peng Xing, Zhai Dede, Shi Feng, Tian Ye, Song Ci, Tie Guipeng, Shen Yongxiang, Qiao Shuo, Shen Xiao, Zhang Wanli, Wang Sheng, Ruan Ningye
2023, 52(6): 20220863. doi: 10.3788/IRLA20220863
[Abstract](347) [FullText HTML] (72) [PDF 3369KB](74)
  Objective   The defect detection of laser additive manufacturing (AM) has always been a technical problem that restricts its development. Due to the complexity of the defect generation mechanism, the insufficient detection information of the highly reflective workpiece surface, the low precision, the complex detection conditions, and other reasons, it is difficult to achieve high-precision and robust detection of defects. When the defect detection system based on reflective illumination performs detection on the surface of a metal part with high reflectivity, the pixels of the image sensor are usually overexposed due to the strong reflective light, resulting in a large amount of annihilated defect information, and it's difficult to highlight and extract the information of the defect area. Therefore, in view of the engineering problem of the high-precision robust detection and evaluation of surface defects of high reflective metal workpieces manufactured by laser AM, a polarization detection system based on a high reflective suppression effect is designed, which can effectively avoid background clutter interference and improve the defect detection capability in complex detection environment.   Methods  The system is designed based on Q-type aspheric surface, which has a strong aberration correction ability and simplifies the system structure. The deviation between the seventh surface shape and the best-fitting spherical surface is only 0.371 μm (Fig.2-3). The deviation between the 9th surface shape and the best-fitting spherical surface is only 0.434 μm. The focal length is 50 mm, the number of F is 2, and the working distance is 300 mm.   Results and Discussions   The simulation results show that the modulation transfer function is better than 0.42 at the Nyquist frequency of 144.93 lp/mm, meeting the requirements of the image quality (Fig.4). The tolerance analysis and 2 000 Monte Carlo analysis results indicate that the tolerance range is reasonable and meets the processing and assembly conditions under the condition of satisfying the image quality of the polarization detection system (Fig.10-11). To verify the suppression effect of the defect polarization detection optical system on the highly reflective light of the detection surface, the experimental device is built based on the designed polarization detection optical system (Fig.12). Based on the constructed polarization detection system, the detection images under different polarization angles are collected and converted from the RGB channel to the HSV channel for threshold determination. Furthermore, based on the Stokes vector method, the defect polarization information in the high-reflection detection image is extracted. The Stokes vector, degree of polarization, and angle of polarization detection image are calculated. The calculated image is fused to achieve the high-reflection suppression reconstruction of the defect detection image, thus achieving efficient and robust high reflection area characterization and analysis. The experimental results show that the fused image has a prominent role in the edge contour of the defect area, and the contrast between the defect area and the adjacent background area has been effectively improved, making the details of the defect clearer and more intuitive (Fig.14). The overall contrast, clarity, and information content of the image have been improved. Besides, to objectively and quantitatively evaluate the quality of the fused image and compare it with the original intensity image, the average gradient (AG), entropy (E), spatial frequency (SF), edge intensity (EI), and standard deviation (SD) are used to evaluate the image, the results are shown (Fig.15). Compared with the original intensity image, the average improvement rates of the average gradient, information entropy, spatial frequency, edge intensity, and standard deviation of the fused image are 163.46%, 20.04%, 163.20%, 123.03%, and 28.41% respectively.  Conclusions   The results fully illustrate that the polarized image after fusion processing has more abundant information, the image details are clearer, the contrast of the defect area is higher, and the edge contour information of the defect is clearer. The feature extraction and characterization analysis of the surface defects of highly reflective metal workpieces in metal manufacturing are of great significance.
Relative illuminance improvement method of monocentric reflective mobile phone lens
Li Ruolan, Wang Yang, Xu Qianzhi, Zhang Lei, Fu Yuegang
2023, 52(5): 20220763. doi: 10.3788/IRLA20220763
[Abstract](119) [FullText HTML] (36) [PDF 2673KB](27)
  Objective   Enlarging the field of view of an optical system while maintaining good imaging quality is a difficult problem in modern optical design. The large field of view and high resolution of optical lenses are mutually restricted, and it is generally difficult to realize them at the same time. It requires complex structure design, expensive manufacturing, and large volume. Each surface of the monocentric lens is monocentric, and the curved imaging plane is also monocentric with each surface. The special structure enables it to achieve a large field of view and high resolution. It also has the advantages of simple structure, small size, and light weight. It is widely used in aerial remote sensing, security monitoring, photography, videography and so on, and may be first applied in miniaturized mobile phone lenses in the future. However, because the monocentric lens sets a conventional stop in the center to block the light beam of the off-axis field of view, when the field of view is larger, more light will be blocked, which causes greater vignetting, reduces the uniformity of illumination of the imaging plane, and affects the imaging quality. In order to improve the relative illuminance of the monocentric lens, a monocentric reflective mobile phone lens that uses a total reflection surface to control the light beam is designed.  Methods   A monocentric reflective mobile phone lens structure is designed in this paper. The initial structure is obtained by calculating the optical path of two reflective monocentric lenses (Fig.4). The optimized structure consists of a meniscus lens and a hemispherical lens, which are glued together using a low-refractive-index cement (Fig.5(a)). The spot for different fields of view of monocentric reflective lenses using conventional stop and virtual stop are simulated (Fig.6, Fig.8). Under different stop conditions, the relative illuminance curves of monocentric reflective lens are drawn (Fig.9).  Results and Discussions   The designed monocentric reflective mobile phone lens has a focal length of 2.7 mm, a maximum field of view of ±50°, a system F number of 1.8, a total length of 2.7 mm, and a maximum RMS radius of no more than 0.8 μm (Fig.5(b)). Under the conditions of conventional stop and virtual stop, the spot illuminance simulation of monocentric reflective lens is carried out. From the illumination diagrams of different fields of view, it can be seen that under the condition of conventional stop, the shape of the spot becomes ellipse when the field of view is 30°, and the minor axis of the ellipse is smaller when the field of view is 50° (Fig.6). Under the condition of virtual stop, the spot is circular in the 30° field of view, and the spot in the 50° field of view is rounder than the spot with the conventional stop. The relative illuminance curves of the mobile phone lens under the two kinds of stops are drawn, and the results show that the relative illuminance of the monocentric lens using the virtual stop is above 0.85, and the relative illuminance of the monocentric lens using the conventional stop is above 0.64 (Fig.9).  Conclusions   A monocentric reflective mobile phone lens is designed with a total reflection surface to restrict the light. Based on the establishment conditions of the virtual stop and the requirements of the mobile phone lens, an initial structure of the mobile phone lens based on the monocentric reflective lens is calculated. The focal length of the optimized system is 2.7 mm, the maximum field of view is ±50°, the system F# is 1.8, and the total length is 2.7 mm. The illuminance analysis results show that the relative illuminance of the mobile phone lens using the conventional stop gradually decreases with the increase of the field of view, and it is only 0.64 in the 50° field of view. However, the relative illuminance of a mobile phone lens with a virtual stop remains constant at 0° to 28° and is above 0.85 in the 50° field of view. The illuminance uniformity of the full field of view of the monocentric reflective lens using the virtual stop has been significantly improved, which can effectively improve the imaging performance of the system.
Lightweight and high-sensitive optical camera technology for faint space target detection
Jiao Jianchao, Wang Chao, Yu Yue, Guan Chenhui, Hou Mingyang, Zhang Wenyu
2023, 52(5): 20220709. doi: 10.3788/IRLA20220709
[Abstract](119) [FullText HTML] (19) [PDF 1697KB](50)
  Objective   The space-based space target detection system based on the micro-satellite has the advantages of large-scale high-frequency observation and low cost, so it has developed rapidly. At present, foreign space-based space target detection systems have adopted a large number of micro-satellite platforms, such as Canadian MOST satellite, Sapphire satellite, NEOSSat satellite, STARE satellite, etc., with a payload weight of tens of kilograms, the weight of the whole satellite is about 100 kg, and the faint target detection ability can usually reach more than 13 magnitude. The article has designed and developed a compact, large field of view (FOV) and high-sensitivity optical camera for space target detection, which has a detection magnitude of more than 13 Mv, a detection FOV of 8°×8°, and a weight of 5 kg. It can be widely deployed on micro-satellite platforms or hosted on large satellites. It can give full play to the advantage of cluster detection, realize the wide-range, high-sensitivity, and high-frequency detection of faint space targets such as space debris and asteroids, and provides high real-time data support for space debris collision warning and asteroid research.  Methods   A compact, large FOV and high-sensitivity optical camera is built in this paper. Aiming at the application requirements of lightweight, small size, large field of view and high sensitivity of the detection camera, the paper comprehensively optimizes the optics, structure, electronics and stray light suppression to achieve the best detection capability. In terms of optics, a large field of view and small F-number optical lens has been designed and realized. During the optimization process, the lens size is strictly controlled, and the low-density glass material is optimized. On the premise of ensuring the optical performance, the lightweight and miniaturization is realized, and the optical system field of view is 8°×8°, the optical system length is 280 mm and optical energy concentration is 90% (Fig.6). In terms of structure, an integrated long lens tube structure is designed and realized to ensure that the lightweight lens has high structural stiffness and dimensional stability, and the lens tube weight is less than 1.8 kg. In electronics, the combination of high-sensitivity detector and high-integration low noise circuit is used to achieve good noise suppression and lightweight. In the aspect of stray light suppression, the extinction structure is designed at the key part of the lens barrel, the light shield with chamfer is designed, and the ultra-black coating is sprayed inside to achieve good stray light suppression effect.  Results and Discussions  The lightweight and highly sensitive optical camera has been developed and integrated (Fig.7), with a total weight of 4.9 kg. Through the ground observation test, it is verified that the detection capability of the camera is 13.2 Mv.   Conclusions  With the increasing frequency of space activities, the space situational awareness is crucial for the safe development and utilization of space. Space situational awareness based on micro-satellite is an efficient and cost-effective approach. Aiming at the application requirements of space target detection on micro-satellite platform, a lightweight and highly sensitive optical camera for faint target detection was designed and developed, with a weight of 5 kg and a detection FOV of 8°×8°. It was verified by ground tests that the detection ability was better than 13 Mv. The camera can be widely deployed on micro-satellite, and can detect faint space targets with high sensitivity and high frequency, providing high real-time data support for space debris research and collision warning.
Analysis of point cloud accuracy and beam pointing of array beam through prism scanning
Yang Feng, Shi Zhendong, Jiang Yong, Leng Jie, Wang Yalan, Chen Dezhang, Xu Lin, Song Zhao, Xu Shiyue, Jia Kai, Gao Jianbo, Bai Yang, Zhou Shouhuan
2023, 52(5): 20220689. doi: 10.3788/IRLA20220689
[Abstract](150) [FullText HTML] (46) [PDF 2082KB](28)
  Objective   The prism scanning system is used to achieve optical imaging with both large field of view and high resolution by adjusting the beam direction or optic axis. It is widely used in optical reconnaissance, laser communication, lidar, etc. In airborne laser imaging lidar, the prism scanning system, as a transmission scanning structure, has high optical utilization, effectively reduces the volume of the system, and has the advantages of low power consumption, high precision and good stability. In array imaging lidar, high energy efficiency, high resolution, and broad field detection are all achieved by means of array beam illumination and Risley-prism scanning. However, when sub-beams are obliquely incident on the prism, the rotational symmetry of the traces of ray propagation is broken, the beam deflection of the sub-beams through the prism is different, and the regular beam array produces shape distortion, resulting in beam pointing error and affecting the position accuracy of the point cloud. Therefore, it is necessary to analyze the rules of beam array distortion to improve the accuracy of the point cloud.  Methods   The conical scanning mode that combines the array beam and prism is broken down into multi-beam parallel scanning with numerous incident angles, and the propagation characteristics of the array beam are thoroughly described by the propagation characteristics of all sub-beams (Fig.2). The three-dimensional vector optical approach is used to establish the laser transmission process of the array beam through a Risley-prism (Fig.3), and the relationship between the pointing variation of the sub-beam and the scanning angle of the prism is obtained (Fig.4). The association between beam pointing variability and point cloud data quality is demonstrated by the numerical simulation of imaging process with prism scanning by flight experiment of airborne lidar (Fig.6-7).  Results and Discussions   When the array beam is orthographically and obliquely incident into the prisms with different angles, the beam steering of the prism to each sub-beam is different at various scanning angles (Fig.5(a)). The spatial shape distortion analysis of the array beam is based on the spatial angle difference between the outgoing sub-beam and the central sub-beam. When the prism rotates one cycle, the spatial shape distortion of the array beam is shown (Fig.6(b)). The quality of point cloud data affected by the array beam distortion is evaluated by using plane fitting RMS value as the quantitative index of point cloud position accuracy (Fig.7). Simulation results of ground scanning imaging process of prism in airborne lidar indicate that the plane error RMS is approximately 5 cm at a navigation height of 0.5 km (Fig.8(a)), which varies linearly with navigation height, and slopes at a rate of around 0.1 m/km in prism scanning system with beam array (3×3) (Fig.8(b)) and the accuracy of point cloud plane decreases with the increase of array beam scale (Fig.9) and sub-beam angular separation (Fig.10).  Conclusions   The combination of array beam illumination and prism scanning improves the energy utilization, spatial resolution and detection field of view of airborne lidar system. However, the shape distortion of array beam leads to beam pointing error and affects the accuracy of point cloud position. The array beam incidence prism includes orthographic and oblique incidence. The oblique sub-beam destroys the rotational symmetry of the beam propagating in the prism, and the beam steering ability is different at different scanning positions. Furthermore, the larger the oblique angle is, the stronger the steering ability of the prism to the beam is. Given that the above two work together, the time-varying array beam is emitted when the regular array beam is incident. The relationship between beam pointing error and spatial position error is obtained by using the three-dimensional vector optics method. The increase of the incident angle of the sub-beam and the altitude will lead to the dispersion of the point cloud and the decrease of the data quality. The law of beam array distortion during prism scanning lays a foundation for the correction of subsequent airborne flight test data, especially for the improvement of position accuracy of medium and long-distance airborne lidar. In addition, it provides a reference for the design of array beam combined with multi-prism scanning system.
Stray light analysis and suppression of broad-band spectral imaging system
Ji Kaiyi, Xing Yujie, Niu Xinshang, He Chunling, Dun Xiong, Cheng Xinbin
2023, 52(5): 20220645. doi: 10.3788/IRLA20220645
[Abstract](166) [FullText HTML] (50) [PDF 4015KB](45)
  Objective  The greatest advantage of hyperspectral imaging technology over traditional detection technology is that it can record both spatial information and "fingerprint" spectral information of the observed target, with higher spectral resolution, higher detection capability, and can effectively identify and classify the target to be measured. With the development of hyperspectral imaging technology, the technology related to its core instrument, the Imaging Spectrometer, is also becoming more and more mature. Early developed Imaging Spectrometer are limited by the performance of spectroscopic devices, the band coverage is usually narrow. In order to achieve higher spectral coverage capabilities, multiple Imaging Spectrometer with completed stitching is usually needed, its volume and weight is large. Therefore, the study of a single broad-band spectral imaging system (0.4-1.7 μm) has important research significance. Grating has become the mainstream beam splitting element for broad-band spectral imaging systems because of its high dispersion capability and high environmental stability. The grating-based broad-band spectral imaging system suffers from the problem of crosstalk between multi-order diffraction spectra, which introduces serious stray light, which has a great impact on the performance of the optical system. Therefore, it is particularly important to carry out stray light analysis and suppression schemes for broad-band spectral imaging systems.  Methods  Starting from a typical Schwarzschild structured grating-type broad-band spectral imaging system (Fig.1), the stray light of the optical machine system was analysed using the Monte Carlo non-sequential ray tracing method, Tracepro software was chosen to implement the simulation analysis of the broad-band spectral imaging system (Fig.3). The system stray light was evaluated based on the simulation analysis results (Fig.5), the theoretical calculation of the system stray light was carried out (Tab.3), and the theoretical calculation matched the simulation results. Secondly, based on the stray light path of the system obtained from the simulation, the source of multi-order stray light of the broad-band spectral imaging system was analysed in depth. A scheme of adding filters is proposed to suppress multi-level diffracted stray light. Two types of filters are designed: a sub-area filter (Fig.8) and a linear gradient bandpass filter (Fig.11), respectively.  Results and Discussions   The stray light coefficient is significantly reduced after the addition of the sub-area filter, and the long-wave stray light is better suppressed with a maximum stray light coefficient of 0.011, but in the short-wave band, the stray light is still larger with a maximum stray light coefficient of 0.0826 (Fig.10). Therefore, the stray light in the long wavelength band can only be suppressed by using the sub-area filter, but the stray light in the short wavelength band is not effectively suppressed, which cannot meet the requirement of suppressing multi-order diffracted stray light in the broad-band imaging system. With the addition of a linearly graduated bandpass filter, stray light is not only suppressed at long wavelengths, but also at short wavelengths, and the stray light coefficient is reduced to the order of 10−4 (Fig.14). Therefore, the use of a linear gradient bandpass filter for the suppression of stray light in a broad-band spectral imaging system can meet the requirements of the system.  Conclusions  This paper analyzes the stray light problem of Schwarzschild structured planar grating type broad-band spectral imaging system. The analysis results show that there are two main types of stray light in broad-band spectral imaging system: one is the long wavelength multi-order diffracted light in the optical path multiple reflections and diffraction and short wavelength spectral channel overlap; the other is the short wavelength multi-order diffraction, adding linear gradient bandpass filter can effectively suppress the system multi-order diffracted stray light, which meets the requirements of broad-band spectral imaging system. The work of this paper provides a theoretical basis for the design of grating type broad-band spectral imaging system, and can be used for high-performance broad-band spectral imaging system.
Topology optimization method for key structures of electro-optic systems of loitering munition
Liu Hu, Zhu Lei, Wu Yan, Gao Yu, Yan Weiliang, Cui Kai
2023, 52(5): 20220767. doi: 10.3788/IRLA20220767
[Abstract](137) [FullText HTML] (26) [PDF 6365KB](31)
  Objective   The electro-optic system is an important component of loitering munition. With the rapid development of loitering munition in recent years, the electro-optic systems of loitering munition are expected to be more compact, light, and with more excellent performance. Therefore, advanced design methods are urgently needed to solve the above problems in the electro-optic system design. In order to meet the multiple requirements of loitering munition, an effective design method for key structures of electro-optic systems based on topology optimization was proposed.  Methods   The advanced topology optimization method was employed to raise the design standard of electro-optic systems. Specifically, the stiffness under acceleration conditions in X, Y, Z directions and the first modal frequency were all considered as the optimization design constraints, and mass minimization of the design structure of electro-optic systems of loitering munition was taken as the optimization objective function in the proposed topology optimization method. The variable density method was employed to establish the topology optimization model. The optimized topology layout of the electro-optic systems structure can be obtained by solving the topology optimization model with the help of Altair HyperWorks software, then it would be used to reconstruct the geometric models based on production technology with the software of Unigraphics NX. In the next step, the reconstruction model would be analyzed to confirm if the optimized design could meet all the requirements.  Results and Discussions   As the key structure of electro-optic systems of the loitering munition, the optical bench was analyzed and optimized to improve the design performance using the proposed method. In this typical example, the topology optimized result of optical bench was obtained (Fig.5) and reconstructed (Fig.7). Then the modal analysis (Fig.8) and overload analysis (Fig.9) were executed to verify the performance of optimized reconstructed optical bench. The result showed that the mass of optical bench of electro-optic systems was reduced by 22.4% with stiffness under acceleration conditions in X, Y, Z directions and the first modal frequency maintaining equivalent performances (Tab.1). In the completed example, the optimization method has greatly improved the lightweight level of electro-optic systems of loitering munition, and it offers great help in meeting the requirements of loitering munition system. In especial, the optimized design of optical bench has been produced and successfully applied to the electro-optic systems. Obviously, the proposed method can also be extended to the design of other parts to improve the overall design level.  Conclusions   In this study, an effective design method for key structures of electro-optic systems based on topology optimization is proposed. The mass, stiffness and the first modal frequency of the key structure are all considered in the optimization design method to ensure design effectiveness. As a typical example, the optical bench was analyzed, optimized, reconstructed and checked successively during the whole design flow and the optimized design of optical bench has also been produced and verified in the physical test. Compared to experiential design, the topology optimized optical bench has a significant advantage in weight and stiffness. The result of the example showed that the proposed topology optimization method can effectively benefit the lightweight design of the electro-optic systems of loitering munition. Therefore, the topology optimization method for key structures of electro-optic systems of loitering munition has great appliance and good popularization value.
Design and test of precision secondary mirror adjustment mechanism for space optical remote sensor
Wu Yongjian, Yang Dawei, Sun Xin, Liu Yong, Hu Yongli
2023, 52(4): 20220635. doi: 10.3788/IRLA20220635
[Abstract](189) [FullText HTML] (43) [PDF 1945KB](48)
  Objective   Vertical assembly and adjustment is one of the key technologies of long focal length and large aperture space camera. In order to overcome the inconsistency between the on-orbit and the ground caused by gravity change, material deformation and other factors, the secondary mirror adjustment has become one of the key technologies to correct the defocus of the optical remote sensor and the relative position change of the primary mirror and the secondary mirror. Precision secondary mirror adjustment technology has been widely used in high-resolution space optical remote sensors. For example, Stewart platform 6-DOF parallel mechanism, which has been successfully applied in Hubble telescope and reconnaissance camera, has the advantages of high accuracy, high bearing capacity and high rigidity, and has the ability to precisely adjust the secondary mirror components in the optical system. Many theoretical analysis and engineering research have been done on the 6-DOF adjustment mechanism in China, but the 6-DOF adjustment mechanism also has the disadvantages of complex structure and control system, high cost and relatively large weight. Therefore, it is necessary to develop a single-degree-of-freedom secondary mirror adjustment mechanism with high accuracy, high integration and high reliability to solve the inconsistency between heaven and earth faced by the current high-resolution space optical remote sensor.  Methods   In order to meet the secondary mirror adjustment requirements of a high-resolution camera, a high-precision and high-stability secondary mirror adjustment mechanism combining precision linear transmission, flexible transmission and flexible support technology is built in this paper (Fig.1). The linear transmission device （Fig.4） adopts the redundancy design of one main and one standby, and has precision position telemetry capability. The flexible transmission hinge with transmission ratio of 10∶1 is used for motion transmission. Compared with the traditional gear reducer, it has the advantages of small impact, no wear, stable transmission, and high reliability. At the same time, the flexible hinge has the advantages of high-precision transmission in the range of small displacement. The secondary mirror uses bipod flexible support to design unloading force thermal deformation, and ensures its flexibility along the optical axis direction (focusing direction) through three pairs of 120° flexible guide hinges.  Results and Discussions   This set of precision adjustment mechanism has carried out adjustment precision test after completing the mechanical environment assessment. The test is carried out according to 0.088° rotation of step motor (corresponding theoretical step distance of secondary mirror 8.858 μm). The initial position of the secondary mirror is zero. The secondary mirror is drived to complete the whole adjustment cycle of "zero position→positive limit position→zero position→negative limit position→zero position". The adjustment accuracy test results after the mechanical environment assessment of the optical and mechanical products of the adjustment mechanism show that the mechanism has the ability to achieve high-precision adjustment in a large range (Fig.8-11), and meets the requirements of the on-orbit application of space optical remote sensor.  Conclusions   In this paper, a set of high-precision secondary mirror adjustment mechanism is designed by combining the flexible support, precision linear drive and flexible hinge transmission technology of the second mirror. This paper first introduces the optical and mechanical structure, working principle and transmission link of the mechanism, then describes the design of ultra-light secondary mirror assembly, high-precision linear actuation and high-precision focusing transmission, and finally introduces the adjustment accuracy test after the vibration test. The test results show that the measured adjustment stroke of the set of precision adjustment mechanism is more than ±120 μm, the axial adjustment step precision is 0.18 μm, the maximum translation error of the secondary mirror within the adjustment stroke is 1.3 μm, and the maximum tilt error is 1.9″. It has the characteristics of wide adjustment range and high adjustment accuracy, and meets the requirements of the precision secondary mirror adjustment of the space optical remote sensor.
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