Current Articles

2024, Volume 53,  Issue 3

Space optics
Advances in ultraviolet polarization detection for space astronomy
Shan Ruiyan, Dong Lianqing, Li Kang, Zhang Muyao, Zheng Guoxian, Zhang Zhuo, Yang Lixin, Shao Junjie
2024, 53(3): 20230547. doi: 10.3788/IRLA20230547
[Abstract](41) [FullText HTML] (9) [PDF 2390KB](23)
  Significance   In the realm of scientific advancement and national development, emerging space-based astronomical detection technologies play a pivotal role. Their high-precision observations afford unique opportunities, deepening our comprehension of the universe and propelling the forefronts of astrophysics and cosmology. These detections not only furnish indispensable data for the validation and development of theoretical models but also instigate the emergence of novel theories in fundamental physics. On the national scale, possessing advanced space-based astronomical detection capabilities not only underscores a nation's prowess in the scientific domain but also provides a crucial platform for nurturing high-caliber research talent. This, in turn, contributes to the nation's competitive edge on the global scientific stage. Therefore, the imperative nature of researching and developing novel space-based astronomical detection technologies is evident.   Progress   This article delivers a comprehensive examination across three dimensions: the advancement of ultraviolet polarization space observations, the current global landscape of ultraviolet polarization payloads in both domestic and international contexts, and the pivotal technologies associated with ultraviolet polarization payloads. Addressing the progress in ultraviolet polarization space observations, the study elucidates the significance of ultraviolet polarization within the domains of solar physics, planetary science, and interstellar matter research. Furthermore, the article provides an overview of the prevailing global scientific research developments in this field. Concerning the development status of ultraviolet polarization payloads both at home and abroad, given the absence of relevant payloads in China, the emphasis is placed on introducing typical international space-based astronomical ultraviolet polarization payloads, elucidating their detection targets, and summarizing their prospective development directions. Regarding the key technologies associated with ultraviolet polarization payloads, the article synthesizes the performance indicators of both existing and planned astronomical ultraviolet polarization payloads. It is evident that contemporary ultraviolet polarization detection primarily hinges on the fusion of polarization and spectroscopic detection, and a singular ultraviolet polarization datum falls short of meeting the demands of astronomical observations. With escalating observational requisites, the necessity for heightened precision in spectral resolution and polarization measurement accuracy is underscored, thereby imposing heightened demands on ultraviolet polarization devices. Furthermore, given the concentration of ultraviolet radiation signals in the far-ultraviolet wavelength range in the cosmos, which exhibits weaker intensity compared to the visible and infrared bands, there exists a stringent requirement for high overall transmittance and detection efficiency of the system. Building upon these considerations, the article furnishes a forward-looking and succinct perspective on specific key technologies and future directions across three focal areas of ultraviolet optical coatings, ultraviolet polarization systems, and ultraviolet detectors.   Conclusions and Prospects   Following an in-depth analysis and synthesis of advancements in ultraviolet polarization space observations both at home and abroad, this review delineates current challenges in ultraviolet polarization detection. These challenges encompass suboptimal optical detection efficiency, subpar polarization measurement accuracy, and the high complexity and cost associated with device development. In response to these challenges, this study puts forth future development directions for space-based ultraviolet polarization detection technology. These directions encompass the exploration of cutting-edge coating technologies, such as Atomic Layer Deposition (ALD), advancements in the high-reflectance performance of multilayer reflective films, the application of emerging dynamic components like electro-optic modulators in the UV spectrum, the development of on-chip ultraviolet-sensitive polarization detectors, the expansion of ultraviolet solid-state detectors into the EUV wavelength range, enhancements in detector sensitivity, and the exploration of innovative ultraviolet detector technologies. This forward-looking perspective is geared towards not only addressing existing challenges but also propelling significant advancements in space-based ultraviolet polarization detection technology.
Optical design
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](39) [FullText HTML] (13) [PDF 2308KB](9)
  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](23) [FullText HTML] (1) [PDF 3728KB](17)
  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.
Lasers & Laser optics
Optical fabrication
Uncertainty error technology for magnetorheological finishing of optical elements
Gao Bo, Fan Bin, Wang Jia, Wu Xiang, Xin Qiang
2024, 53(3): 20230595. doi: 10.3788/IRLA20230595
[Abstract](27) [FullText HTML] (4) [PDF 5283KB](7)
  Objective  As a new optical machining technology, magnetorheological finishing has many advantages, such as high machining certainty, stable convergence efficiency, controllable edge effect, small subsurface damage layer, so it has a wide range of applications in the field of high-precision optical machining. According to the principle of magnetorheological finishing, high-precision machining process requires stable removal function and accurate dwell time distribution of each dwell point. However, the actual machining process is often affected by various kinds of errors, which makes the actual machining results deviate from the ideal results. In the field of high-precision machining, small errors will also have a great impact on the surface accuracy and the errors of each frequency band, and even lead to the non-convergence of the surface errors. With its continuous development, aspherical optics have the advantages of correcting aberrations, improving image quality and reducing system weight, so it has been widely used. However, the surface of aspherical optical components is complicated, and the manufacturing process is more difficult than that of spherical optical components. In the existing research, the machining of aspherical optical elements by magnetorheological finishing is realized through the cyclic process of inspection and machining, but there are problems such as the uncertainty of surface accuracy and the uncontrollable mid-spatial error.  Methods  The processing method based on uncertainty error can increase the certainty of the processing process, optimize the process flow, and effectively suppress the mid-spatial error. Therefore, in order to realize the high-precision machining of optical elements, the uncertainty error method is adopted, and the ideal surface accuracy can be obtained in the actual machining process. The feasibility of the scheme is verified by simulation processing, experimental verification and error compensation on the aspherical surface.  Results and Discussions   The magnetorheological finishing experiment of #B is carried out under uncertainty error. According to the experimental results, the surface error RMS value increases from 15.432 7 nm (Fig.10(a)) to 19.317 nm (Fig.11(g)), and the uncertainty error of surface error RMS value is controlled at 3.884 3 nm after machining compared with the predicted result before machining. The mid-spatial error RMS value increases from 10.262 nm (Fig.10(b)) to 13.282 nm (Fig.11(h)), and the uncertainty error of the mid-spatial error RMS value is controlled at 3.02 nm. The experimental results show that the method based on uncertainty error not only effectively converges the surface error, but also reasonably restrains the mid-spatial error. It provides theoretical support for surface error and mid-spatial error suppression in magnetorheological finishing. This method has important practical value for realizing high-precision magnetorheological machining of optical components.  Conclusions  The uncertainty theory in magnetorheological finishing is analyzed, and the removal function uncertainty error and position uncertainty error are specifically analyzed, and the process flow of magnetorheological finishing under the uncertainty error is summarized. On this basis, two off-axis aspheric mirrors are used to verify the process. The errors in the magnetorheological finishing of off-axis aspherical surface #A are analyzed in detail to determine the uncertainty in the machining of off-axis aspherical surface #B, so as to guide the machining of off-axis aspherical surface #B. The experimental results show that the surface accuracy and mid-spatial error of the off-axis aspherical surface #B meet the engineering requirements. On the basis of uncertainty analysis, the process flow is optimized, and the surface error convergence is achieved and the mid-spatial error is suppressed by means of uncertainty.
Ocean optics
Design and test of a blue-green dual-wavelength oceanic lidar system
Ji Lufeng, Liu Bingyi, Zhu Peizhi, Liu Jintao, Zhang Kailin, Wu Songhua, Tang Junwu
2024, 53(3): 20230597. doi: 10.3788/IRLA20230597
[Abstract](65) [FullText HTML] (7) [PDF 3815KB](10)
  Objective  The optical characteristic parameters of marine water are one of the key points of marine research. Most of the optical characteristics vary with depth, and oceanic lidar is one of the effective technical means to detect water profile information. Based on the requirement of water optical profile detection, a two-wavelength oceanic lidar system with blue and green light is developed. The light source uses blue and green wavelength with small attenuation coefficient in water to obtain a larger detection depth. The system has dual wavelength and polarization detection channels, which can be used to obtain the backscattered echo signal and polarization signal simultaneously, and can realize the continuous detection of nearshore and ocean water. This paper first introduces the design scheme of the lidar system, including transmitting, receiving, acquisition and control subsystem and auxiliary facilities, and then describes the data preprocessing methods, including quality control, peak alignment, background noise removal and deconvolution. The reliability of the lidar system is verified by the detection experiment in the offshore. The results show that in clean ocean waters, the attenuation coefficient of 486 nm lidar is less than 532 nm, which means that 486 nm has better detection performance in open water.  Methods  The lidar system consists of transmitting, receiving, acquisition and control subsystems and auxiliary facilities (Fig.1-2). The transmitting subsystem is used to transmit 486 nm and 532 nm linearly polarized light to detect the target. The receiving subsystem is responsible for receiving and detecting the echo signal of the water target and realizing the photoelectric conversion. The acquisition and control subsystem is responsible for high-speed data acquisition and storage, system integrated control and state monitoring. Auxiliary facilities are used to ensure that the lidar works in a stable working environment. Then the data preprocessing methods are introduced, including quality control (Fig.5), peak position alignment (Fig.6), background noise removal (Fig.7) and deconvolution (Fig.8).  Results and Discussions  In order to verify the reliability of the system, the detection experiment is carried out in the offshore, and the backscattered signal of the water (Fig.10) is obtained, and the attenuation coefficient profile information (Fig.11) is obtained by combining the optical inversion algorithm. The results show that in the test oceanic waters, the laser radar system can obtain the optical characteristic parameter profile of the effective detection depth of about 30 m in the dynamic range of 2 and 3 orders of magnitude at the wavelength of 486 nm and 532 nm respectively (Fig.9). At the same depth, even though green light is more energetic, about 1.5 times that of blue light, compared with 532 nm, the attenuation coefficient of 486 nm lidar is smaller, indicating that 486 nm wavelength has better detection performance (Fig.12). From the general distribution trend of optical characteristic parameters, the attenuation coefficient of lidar obtained by the system increases with the increase of depth.  Conclusions  The blue-green dual-wavelength oceanic lidar system is developed to obtain the backscattered echo signal of blue-green band and the profile information of attenuation coefficient of lidar synchronously in clean oceanic waters, and realize the continuous detection of oceanic water. The results show that the attenuation coefficient of 486 nm lidar is less than 532 nm, which means that 486 nm has better detection performance in open water. The data results verify the reliability of the system and the performance of the blue-green wavelength detection channel, and provide a strong support for the application potential of oceanic lidar in the field of oceanic detection.
Image processing
Optimization method of PSO-PID control for interferometric closed-loop fiber optic gyroscope
Liu Shangbo, Dan Zesheng, Lian Baowang, Xu Jintao, Cao Hui
2024, 53(3): 20230626. doi: 10.3788/IRLA20230626
[Abstract](22) [FullText HTML] (4) [PDF 2486KB](13)
  Objective   PSO-PID control optimization algorithm based on the interferometric closed-loop fiber optic gyroscope has been widely used in military and civil fields, such as aerospace, defense equipment, navigation survey, vehicle inertial navigation system and other industrial systems. These applications are developing in the direction of lightness, low power consumption, long life, high reliability, no self-locking and mass production. PSO-PID controller can improve the dynamic response of fiber optic gyroscope and effectively track the angular rate input of fiber optic gyroscope. Fiber optic gyroscope is based on Sagnac effect in closed optical path, so its bandwidth is much larger than that of traditional gyroscope. In digital closed-loop fiber optic gyroscope, the response speed of optical path is very fast, and the system bandwidth is mainly determined by the detection circuit. Choosing a suitable digital controller is helpful to improve the dynamic performance of fiber optic gyroscope.  Methods   The system block diagram of fiber optic gyroscope (Fig.1) is established, and the ICFOG closed-loop system is equivalent to a mathematical model (Fig.2) by analyzing the working principle of fiber optic gyroscope, and finally the closed-loop discrete control system is deduced. On this basis, a new PSO-PID compound controller is designed (Fig.3), and the optimization algorithm steps of PID controller of standard PSO are analyzed (Fig.4). The controller can adjust parameters ${K_p}$,${K_i}$ and ${K_d}$ online during operation (Fig.15). At the same time, by comparing with the PID parameter tuning method of BP neural network (Fig.5), fuzzy PID parameter tuning method (Fig.6) and PID control method, the advantages of PSO-PID control are illustrated by comparing the angular rate input tracking speed of fiber optic gyro (Fig.12) and the angular rate input tracking error of fiber optic gyro (Fig.13).  Results and Discussions  Using PSO-PID control method, it is found that the fitness value changes rapidly. When the number of iterations is 15, the fitness value can reach the optimal solution, and the optimal solution is 21.892 5. At the same time, the tracking time of FOG angular rate input is 1.2 s. Compared with BP-PID, PID, and F-PID control methods, the tracking speed is increased by 1.91, 3.5 and 1.75 times respectively. After the PSO-PID control method, the tracking error is $4.7 \times {10^4}$ m, which is smaller than other control methods. Compared with F-PID, BP-PID and PID control methods, its control accuracy is improved by 45.27%, 46.03% and 66.30% respectively. According to the comparison of dynamic performance of different control methods (Tab.1), it is known that PSO-PID controller can achieve the control goal quickly and has a small tracking error.  Conclusions   Based on the mathematical model of fiber optic gyro, this paper puts forward an optimization scheme of fiber optic gyro digital controller. The traditional digital controller is improved, and the PSO-PID controller is proposed and simulated. Compared with many control methods, the simulation results show that PSO-PID controller can shorten the adjustment time and reduce overshoot, thus effectively improving the dynamic performance of fiber optic gyroscope on the premise of ensuring stability, and has important engineering significance and practical value. To apply this optimization scheme to engineering practice, more external factors and more detailed control parameter analysis need to be considered, which will be the focus of later research.
Image highlight removal method based on parallel multi-axis self-attention
Li Pengyue, Xu Xinying, Tang Yandong, Zhang Zhaoxia, Han Xiaoxia, Yue Haifeng
2024, 53(3): 20230538. doi: 10.3788/IRLA20230538
[Abstract](26) [FullText HTML] (3) [PDF 7138KB](10)
  Objective  Highlights are manifested as high bright spots on the surface of glossy materials under the action of light. The highlights of the image can obscure background information with different degrees. The ambiguity of the image highlight layer model and the large dynamic range of highlights enable highlight removal to be still a challenging visual task. The purely local methods tend to result in artifacts in the highlight areas of the image, and the purely global methods tend to produce color distortion in highlight-free areas of the image. To address the issues caused by the imbalance of local and global features in image highlight removal and the ambiguity of highlight layer modeling, we propose a threshold fusion U-shaped deep network based on parallel multi-axis self-attention mechanism for image highlight removal.  Methods  Our method avoids the ambiguity of highlight layer modeling by implicit modeling. It uses the U-shaped network structure to combine the contextual information with the low-level information to estimate the highlight-free image, and introduces a threshold fusion structure between the encoder and decoder of the U-shape structure to further enhance the feature representation capability of the network. The U-shaped network uses the contraction convolution strategy to extract the contextual semantic information faster. It gradually recovers the low-layer information of the image by expanding, and connects the features of the various stages of the contraction path in the corresponding stages of the expansion path. The threshold mechanism between the encoder and decoder is used to adjust the information flow in each channel of the encoder, which allows the encoder to extract features related to highlights as much as possible at channel level. The threshold structure first performs high- and low-frequency decoupling and feature extraction for the input features, then fuses the two types of features by pixel-wise multiplication, and finally uses the residual pattern to learn the low-level features complementary. In addition, the parallel multi-axis self-attention mechanism is used as the unit structure of the U-shaped network to balance the learning of local and global features, which eliminates the distortion and artifacts of the recovered highlight-free images caused by the imbalance extraction of local and global features. The local self-attention calculates local interactions within a small P*P window to form local attention. After the correlation calculation of the small window, the window image is mapped to an output image with the same dimension as the input image by the inverse operation of the window segmentation operation. Similarly, the global self-attention divides the input features into G*G grids with larger receptive fields. Each grid is a cell for calculating correlation, which has an adaptive size of the window space. The larger receptive field window of calculating correlation facilitates the extraction of global semantic information. For the loss function, the squared loss and the mean absolute error loss are the widely used loss functions in the image restoration field. The squared penalty magnifies the difference between large and small errors. It usually results in excessively smooth restored images. Therefore, the mean absolute error loss is used as the loss function to train our network.  Results and Discussions  Qualitative experiments on real highlight images show that our method can remove highlights from images more effectively, and other compared methods usually cannot remove highlights accurately and efficiently. They are prone to produce artifacts and distortion in highlight-free areas of the image. Quantitative experiments on real-world highlight image datasets show that our method outperforms five other typical image highlight removal methods in both PSNR and SSIM metrics. The PSNR values are higher than those of the second-best method by 4.10 dB, 7.09 dB, and 6.58 dB on the datasets of SD1, RD, and SHIQ, respectively. The SSIM values of our method also outperform those of the second-best method with gains of 4%, 9%, and 3% on three datasets. In addition, we also conduct ablation studies for the network structure, and the experiment verifies the effectiveness of the threshold fusion module and the parallel multi-axis self-attention module; The threshold fusion module can increase the PSNR by 0.68 dB and the SSIM by 1%, and the multi-axis self-attention module can increase the average PSNR value by 0.55 dB and the SSIM by 1%. It can also be seen from the visual results of each ablation experimental model that with the gradual optimization of the network structure, the results of image highlight removal are visually improved. The outputs of the pure convolution-based deep network models of MI and M2 have more highlight residuals and produce distortion in the highlight-free areas of the image. The models of M3, M4 and M5 combining CNN with the self-attention module visually achieve better results.  Conclusions  The experimental results show that good visual results for highlight removal on both public natural and textual image datasets are achieved with our method, which outperforms other methods in terms of quantitative evaluation metrics.
Fast calculation of radiative heat transfer coefficient between diffuse and non-diffuse surfaces
Li Fubing, You Qi, Leng Junmin, Yang Linhao
2024, 53(3): 20230611. doi: 10.3788/IRLA20230611
[Abstract](24) [FullText HTML] (2) [PDF 2327KB](19)
  Objective  Radiative heat transfer is one of the three basic modes of heat transfer and has an important impact on the study of the temperature distribution and infrared radiation characteristics of the outer surface of a space target. For solving the radiation heat transfer between incomplete gray-diffuse surfaces system (both diffuse surfaces and non-diffuse surfaces in the model), there is usually a lack of corresponding analytical solutions. The Monte-Carlo method has the advantage of good calculation accuracy, but it has the disadvantage of long calculation time. In order to solve this problem, this paper proposes a representation of the ray reflection energy of the diffuse surface and a calculation method for the radiative transfer coefficient of the incomplete diffuse surfaces system based on the diffuse reflection characteristics of the diffuse surface. This reduces multiple ray tracing before the ray energy threshold is reached and improves the calculation speed.  Methods  A method for expressing the reflected energy of diffuse surfaces is proposed. When the Monte-Carlo method is used to conduct ray tracing, if a ray hits a diffuse surface, the reflection energy of the ray is defined as the diffuse emission beam set in the upper half space of the surface using the diffuse reflection characteristics of the diffuse surface (Fig.2(b)). In addition, modifications were made to the Monte-Carlo tracking process. First, the radiative transfer coefficient of the diffuse surface was calculated using the Monte-Carlo method. Then, when calculating the radiative transfer coefficient of diffuse surfaces, if the light emitted from the surface intersects with the diffuse surface, the radiative transfer coefficient of the other surfaces is multiplied by the reflected energy, and the reflected energy absorbed by the other surfaces is calculated to end the ray tracing process, avoiding subsequent multiple ray tracing and achieving fast calculation of diffuse surfaces to improve the computational efficiency of the entire system (Fig.5).  Results and Discussions   Using the cube model and assuming that the No.1 and No.2 surfaces are diffuse and the rest of the surfaces are specular (Fig.4(a)), the radiative transfer coefficient from each surfaces to the other surfaces (including itself) are calculated using the Monte-Carlo method and the fast algorithm proposed in this paper, and the results of the calculations are shown (Tab.1-2). It can be seen that both of them have the same accuracy, but as for the computation time, the new method is more efficient due to the significant reduction of the number of ray tracing times in Monte-Carlo (Fig.6). In addition, the 13-facet L-type unenclosed cavity model is used as proof to show that the method is also applicable in complex models. Finally, taking the cube model as an example, the advantages of the fast method compared with the Monte-Carlo method are analyzed from the theoretical point of view. For the emitted beam on a non-diffuse surface, the average tracing times of its rays are much smaller than that of the Monte-Carlo method, and the higher the reflectivity of the surface element, the more significant the computational advantages are. For example, if the energy threshold is 0.001, the number of diffuse reflective surfaces in the model is 2. When the surface reflectance is 0.4, the calculation time of the non-diffuse reflective surfaces in the fast calculation method is 0.307 times that of the Monte-Carlo method. When the reflectance of the surface is increased to 0.8, the calculation time of the non-diffuse reflective surface is only 0.081 of that of the Monte-Carlo method, which is more than ten times higher.  Conclusions  Aiming at the problem of the traditional Monte-Carlo method in calculating the radiative transfer coefficient between diffuse surfaces and diffuse surfaces for a long time, a fast calculation method is proposed. Firstly, the realization principle of the method and the difference with Monte-Carlo method are introduced, and then the cube model and L-shape unenclosed cavity model are used to compare the calculation results and calculation time of the fast method to Monte-Carlo method, which illustrates the advantages of fast method over Monte-Carlo method in terms of the calculation efficiency, and then the coefficient affecting the calculation advantages of the method are illustrated through the theoretical analysis, and finally the outlook on the next step of how to improve the calculation time of the diffuse surface elements is proposed.
Photoelectric measurement
Method for measuring laser damage threshold of optical thin film elements based on quantitative damage evaluation
Xin Lei, Yang Zhongming, Meng Jun, Liu Zhaojun
2024, 53(3): 20230614. doi: 10.3788/IRLA20230614
[Abstract](27) [FullText HTML] (2) [PDF 2616KB](7)
  Objective  Optical thin film components play a critical role in high-power lasers, and their ability to withstand laser-induced damage is crucial for the overall performance of the laser systems. Accurate measurement of the laser-induced damage threshold (LIDT) of thin film components is of great significance for improving the lifetime and output efficiency of lasers. However, the traditional test method for laser LIDT, based on the scheme outlined in GB/T 16601, involves evaluating the threshold value through damage probability, which requires numerous repetitive experiments and is both cumbersome and time-consuming. Moreover, the evaluation based on whether the damage occurs on the film surface introduces certain errors. The stability and reliability of the damage are influenced by factors such as laser stability, environmental disturbances, coating processes, and internal defects, which cannot be eliminated and affect the measurement results and accuracy. Additionally, probability statistics require testing many samples, resulting in high capital costs. Therefore, it is necessary to develop a fast and efficient method for measuring LIDT.  Methods  In this paper a novel method of quantitative evaluation of laser-induced damage degree (QELDD) is presented, for quantitatively assessing the degree of laser-induced damage in thin film components. The method involves analyzing the quantification of laser-induced damage at different energy densities and evaluating the LID threshold (LIDT) through damage trend fitting. To accurately quantify the laser-damaged area, super-resolution white-light interferometric measurement is employed, which ensures nanoscale measurement accuracy. Simulation results demonstrate that the proposed method allows for the three-dimensional reconstruction of nanoscale damage defects with a reconstruction volume error of less than 0.01%. Experimental samples, including laser resonator mirrors and window plates, were measured using this method, without repeating the laser damage experiments. The results were found to be consistent with those obtained using the S-on-1 method, with a deviation not exceeding 0.5 J/cm2. The standard deviations of the measurement results were 0.361 J/cm2 and 0.064 J/cm2, respectively.  Results and Discussions  In the simulation results of the laser damage structure model on the surface of the test element, the optimization algorithm achieved a relative error of less than 0.01% for the three-dimensional measurement results (Fig.7). And the reconstruction deviation value is in nanometers order (Fig.8). In the experiment, samples of two laser components are used. The measurement results for the laser resonator mirror are shown (Fig.10, Tab.1). The standard deviation of multiple measurement results is 0.361 J/cm2, and the difference from the S-on-1 result is less than 0.5 J/cm2. Similarly, the measurement results for the window slice are shown (Fig.11, Tab.2). The standard deviation of the multiple measurement results is 0.064 J/cm2, and the difference from the S-on-1 result is less than 0.3 J/cm2. The proposed QELDD method is based on single irradiation results of a single sample with different energy densities, eliminating the need for repetitive testing on multiple samples. This ensures good stability and accuracy while maintaining efficiency.  Conclusions  In this paper, a new laser damage threshold measurement method for optical films based on quantitative evaluation of the damage degree is proposed. The laser damage area is characterized and quantified with high precision using image super-resolution white light microscopic interferometry. The structural characteristics of the laser damage are also summarized. Based on the quantitative parameters of the laser damage degree, we fit and calculate the laser damage threshold. In the experimental setup, a 1 064 nm laser damage system is utilized, and the laser resonator mirror and window plate are selected as samples for testing. The standard deviations of the two sample results are 0.361 J/cm2 and 0.064 J/cm2, respectively. When compared with the results obtained using the S-on-1 method, the deviation of the measured results does not exceed 0.5 J/cm2, indicating good stability and accuracy of the proposed method. During the quantification and characterization of the laser damage area, the QELDD method effectively distinguishes between valid and invalid damage points based on the damage characteristics, thereby eliminating the influence of invalid damage on the result fitting, and improving measurement efficiency. This method introduces a new approach to LIDT measurement, facilitates further research on the laser damage mechanism of optical components, and provides a theoretical basis for enhancing the manufacturing and coating processes of optical components.
Infrared technology and application
Research on packaging technology for 40 K dual-band long-wave detectors
Wang Xiaokun, Chen Junlin, Luo Shaobo, Zeng Zhijiang, Li Xue
2024, 53(3): 20230654. doi: 10.3788/IRLA20230654
[Abstract](59) [FullText HTML] (11) [PDF 2390KB](35)
  Objective   Cryogenic optical technology is a crucial support technology for weak target and multispectral infrared detection. In order to achieve precise temperature control and prevent contamination in the cryogenic optical system, it is common to integrate the cryogenic optics with the detectors inside a cryocooler.  Methods   A specific hyperspectral camera requires the integration of a 320×64 quantum well detector and a 320×64 type II superlattice, co-planarly assembled with dual-band micro-filters to create a long-wave dual-band detection dewar assembly. The required operating temperature for the detector is 40 K, and it is achieved using a pulse tube cryocooler.The dewar adopts a windowless design and is integrated with the cryogenic optical system cryocooler using flexible bellows for hermetic sealing and precise alignment adjustments.  Results and Discussions   Addressing the challenges of three-dimensional assembly of the dual-band detector at 40 K, low-stress assembly of the detector and filters, and efficient heat transfer between the cryocooler and detector, this study investigates the three-dimensional assembly of the detector (Fig.4-6), a heat layer structure for efficient heat transfer at 40 K with low-stress integration with the detector (Fig.7), low-stress filter support (Fig.15), and the coupling between the dewar and the cryocooler (Fig.12). Innovative approaches such as a three-point Z-axis adjustment assembly method, an Al2O3 carrier composite molybdenum substrate for the detector, a molybdenum support structure for the integrated dual-band filters, and a coupling method with stress isolation for the cryocooler and detector are proposed.  Conclusions  Ultimately, this research achieves a detector flatness better than ±2.06 µm (RMS) at 40 K (Fig.6), low-temperature stress of the detector less than 22.06 MPa (Fig.8), low-temperature deformation of the dual-band filter membrane less than 8.55 µm, and a temperature gradient of 2.6 K (Fig.14) between the detector and the cryocooler. The dewar assembly with a 40 K long-wave dual-band infrared detector has been verified through 2000 hours of continuous operation and 300-on/off cycles, with no significant change in component performance before and after testing, meeting the requirements for engineering applications (Fig.16).
Mid-infrared up-conversion imaging based on chirp polarization crystals
Han Zhaoqizhi, Ge Zheng, Wang Xiaohua, Zhou Zhiyuan, Shi Baosen
2024, 53(3): 20230585. doi: 10.3788/IRLA20230585
[Abstract](73) [FullText HTML] (6) [PDF 2089KB](35)
  Objective  The mid-infrared band (2.5-25 μm) has important applications in the field of spectroscopy and imaging. Spectral migration technique up-converts mid-infrared signal light to visible/near-infrared light through a non-linear frequency process, which is then detected using high-performance detectors based on wide-band gap materials such as silicon. Compared to schemes directly using traditional semiconductor detectors, this technique has the advantages of fast response and room temperature operation. Bulk crystals have large aperture to realize array detection. In particular, chirped polarized crystals have obvious advantages in imaging acceptance bandwidth and field of view due to their large phase-matching bandwidth. Previous up-conversion imaging theory, however, didn't consider the nonlinear process of signal light in the crystal to affect the propagation. Therefore, there is some deviation between the theoretical analysis and the up-conversion imaging results under the weak signal light condition. Based on the basic imaging principle, a simple physical model of the up-conversion imaging process is presented by solving the coupled wave equation using finite difference method and considering the effect of nonlinear process on the optical propagation. On this basis, a theoretical derivation for up-conversion imaging under coherent/incoherent radiation illumination conditions based on chirped polarized crystals is provided.  Methods  A mid-infrared up-conversion detection imaging system based on a chirped polarized crystal is built (Fig.3). The target object is illuminated by the thermal radiation of an electric soldering iron, then the visible light in the signal is filtered out by a band pass filter (BP1). A strong 1 080 nm pump light is directed into the crystal through a dichroic mirror (DM) along with the signal beam. Through a 4f system, the up-conversion results of the target are imaged on the EMC CD. A chirped polarized lithium niobate (CPLN) crystal with a period interval of 0.01 μm and a period range of 21.6-23.4 μm is used in the experiment. The length of the crystals is 40 mm and the cross section size is 2 mm×3 mm. The temperature of CPLN crystal is controlled by a home-made temperature controller, whose fluctuation is ±0.002 ℃.  Results and Discussions  By using a mature spectrometer to measure the spectrum after up-conversion and combining with the law of conservation of energy, the accepted spectrum of the up-conversion process in the corresponding mid-infrared band can be obtained (Fig.4). The corresponding mid-infrared acceptance range is 2 915-3 512 nm, and its full-width of half-max (FWHM) is 597 nm. Due to the low transmittance of the DM at wavelengths greater than 3 400 nm, the actual conversion bandwidth is larger than the direct measurement results, which is in agreement with the numerical calculation results (Fig.2). In contrast, the wavelength acceptance bandwidth of single-period polarized crystals is only on the order of nanometers. In the up-conversion imaging results (Fig.5), the largest one-dimensional size of the target is 3.62 cm, corresponding to 125 mm propagation distance, thus the full angle of the field of view is 16.59°, which is slightly smaller than the numerical calculation result in Fig.2. Under the condition of weak signal light, the background of pattern directly imaged by mid-infrared light through the mercury cadmium telluride thermal imager is full of white noise, making it difficult to identify the target contour information, while the pattern obtained by the up-conversion method with the same power of light has clean background and high signal-to-noise ratio (SNR), which also can realize high SNR of the single photon level imaging (Fig.6-7). In addition, applications of up-conversion imaging under coherent/incoherent radiation illumination conditions are reported. The optical edge enhancement imaging is realized for the objects illuminated by mid-infrared coherent light (Fig.8). Real-time video frame rate imaging of incoherent illuminated objects is realized, and its temperature characteristics can be analyzed (Fig.9).  Conclusions  In the experiment, chirped polarized crystal is used to realize the up-conversion imaging detection of the mid-infrared receiving bandwidth of 597 nm and the field angle of view of 16.59°. By comparing with traditional mercury cadmium telluride mid-infrared detector, the up-conversion imaging technique has obvious advantages in improving the signal-to-noise ratio and sensitivity of imaging, and the low-light imaging of mid-infrared is realized by using the photon flux of 1.05×105 Hz. The paper further shows the application of the up-conversion imaging system to the objects illuminated by correlation light and incoherent light. This work has conducted a comprehensive study on the up-conversion based infrared imaging system, which will provide a basis for the design of various application scenarios and improve the system design.
Research progress and development trends of antimonide-based superlattice infrared photodetectors
Zhang Jie, Huang Min, Dang Xiaoling, Liu Yixin, Chen Yingchao, Chen Jianxin
2024, 53(3): 20230153. doi: 10.3788/IRLA20230153
[Abstract](66) [FullText HTML] (12) [PDF 3184KB](41)
  Significance   Infrared focal plane arrays (FPAs) are indispensable core components in many fields such as aerospace remote sensing, deep space exploration, national defense and security, resource exploration, and industrial control. In recent years, the antimonide-based superlattice is drawing the research interests from all over the world. It has become the prominent candidate to achieve infrared detectors with high-uniform large arrays, extended detection wavelengths to long wave and very long wave, two-color detection and so forth, due to its excellent uniformity, low dark current, relatively high quantum efficiency as well as the tunable detection wavelengths which almost covers the full infrared wavebands from near-infrared (NIR) to very long wave infrared (VLWIR). The basic technical principles of the antimonide-based superlattice for infrared detection, the several development stages and key results, as well as the development trends of the Type-II superlattice infrared focal plane arrays, are sequentially introduced and discussed. As antimonide-based superlattice evolves towards higher pixel density, larger specifications, higher operating temperature, longer detecting wavelength, two-color (multi-color), avalanche devices, it is depicted that the antimonide-based superlattice will always play an important role in many fields especially for infrared sensing and imaging.   Progress  The development process of antimonide-based superlattice focal plane detectors is divided into three stages. The first stage spans from 1980s to the very beginning of 21st century. This stage includes the proposal of the concept of superlattice infrared detection technology, theoretical calculation and analysis of the performance of superlattice detectors, epitaxial growth of superlattice materials (first MBE growth by HRL), and some preliminary research on basic optoelectronic properties. The research results of this stage demonstrate the decent capabilities of superlattice materials for infrared detection. The second stage spans from the very beginning of 21st century to year about 2010. This stage mainly focuses on breakthroughs in key technologies for the preparation of high-performance focal plane devices. Particularly, the advanced heterostructures are studied and prepared to suppress the dark current of superlattice long-wavelength detectors. And the etching and sidewall passivation technologies of superlattice materials are explored to prepare superlattice FPA devices. Through these technical breakthroughs, FPAs with 1024 pixel×1024 pixel (Tab.1) and detecting wavelength longer than 10 μm are achieved. The third stage starts from about 2010 and until now. This stage is mainly about the improvement of superlattice focal plane preparation capabilities and the realization of engineering applications, and government becomes an important strength which quickly and efficiently promotes the developments of superlattice technologies. Under the support of related government agencies, Western countries with more technological accumulation make breakthroughs in key technologies such as superlattice structure design, material growth, and chip preparation processes. The VISTA project dominated by American government is a typical case with successful results and deep influence. FPAs with millions of pixels (up to 6 K×4 K), pixel pitches of less than 10 micrometers (e.g. ~5 μm), operating temperature as high as ~180 K are reported. Such superlattice FPAs have already been used in super transport aircrafts, the International Space Station (Fig.7), hyperspectral equipment and so forth.   Conclusions and Prospects   Since the idea of InAs/GaSb superlattice infrared detector was first proposed, it has been over 30 years during which domestic and foreign researchers have successively obtained a series of infrared detectors with large array, high temperature operation, long wave/multi-color detection, through structural design optimization and preparation technology improvement. Antimonide-based superlattice FPAs show advantages such as high uniformity, high stability, and high preparation controllability, and are widely used in aerospace fields such as infrared remote sensing and imaging. Now, to fabricate detectors with higher performance, higher requirements for superlattice materials are put forward. Essentially, materials with longer minor carrier lifetime, higher quantum efficiency and novel structure are explored. Based on the deeper studies, superlattice infrared focal planes are practically developing towards higher pixel density, larger specifications, higher operating temperature, longer detecting wavelength, two-color (multi-color) detection, avalanche devices, etc.
Infrared thermal imaging detection and defect classification of honeycomb sandwich structure defects
Tang Qingju, Gu Zhuoyan, Bu Hongru, Xu Guipeng
2024, 53(3): 20230631. doi: 10.3788/IRLA20230631
[Abstract](39) [FullText HTML] (7) [PDF 2949KB](23)
  Objective  In order to realize the accurate classification of GFRP/NOMEX honeycomb sandwich structure defect types, an infrared thermal imaging detection system was built to collect heat maps of defects and healthy areas, and a GFRP/NOMEX honeycomb sandwich structure defect classification model was constructed by using convolutional neural network and transfer learning technology to realize quantitative detection of defect categories.  Methods  The experimental study of pulse infrared thermal imaging detection was carried out on the specimen. The training data set was constructed using the data obtained from the experiment, and the fine-tuned convolutional neural network model after transfer learning was trained to realize the quantitative detection of defect categories. Firstly, GFRP/NOMEX honeycomb sandwich structure specimens with delamination, debonding, water accumulation and glue plugging defects were prefabricated, and a pulsed infrared thermal wave detection system was built. The FLIR A655SC infrared thermal imager was used to collect the surface temperature distribution field of the specimens under pulse excitation. Secondly, the defects in the heat map are cut into 90 pixel×90 pixel, and the data are expanded by rotating 90°, 180°, 270°, horizontal flipping, vertical flipping and adding Gaussian noise operations. The pre-trained VGG16, MobileNetV2, ResNet50, InceptionV3, and DenseNet201 convolutional neural network models use transfer learning technology to fine-tune the back-layer structure of the network. Finally, the constructed data set is randomly divided into training set, verification set and test set, and the network is trained. The value $ \varphi $ and Accuracy are used as evaluation indexes to evaluate the generalization ability and classification effect of the model.  Results and Discussions  The VGG16, MobileNetV2, ResNet50 network, InceptionV3 and DenseNet201 network fine-tuned models based on transfer learning technology are trained (Fig.9). The VGG-16-1 network model has the fastest convergence speed, the network is stable, and the training process has no large fluctuations. At the same time, the confusion matrix is used to describe the classification results of the test set data by the six networks (Fig.10). It can be seen that the six models can realize the classification task of five categories of defects prefabricated by GFRP/NOMEX honeycomb sandwich structure. The values of $ \varphi $ and Accuracy are shown (Tab.4). The classification Accuracy of VGG16 and ResNet50 fine-tuned models reaches 99.94%, 99.10% and 98.95% respectively, and the scores of five categories of $ \varphi $ are all higher than 96%. Compared with the two fine-tuning models of VGG16 network, the Accuracy and value of VGG-16-1 are higher than those of VGG-16-2. VGG-16-1 has only one misjudgment for the 1 612 defect data of the test set, and the network convergence speed is fast and stable, achieving a better classification effect. Although the overall score of ResNet50 is not as good as VGG-16-1, its network training speed is fast and can also achieve better classification effect.  Conclusions  The data set is constructed by using the real infrared images collected by the infrared thermal imager detection test, and the data is expanded for small samples. Based on the transfer learning technology, the network model structures of VGG16, MobileNetV2, ResNet50, InceptionV3 and DenseNet201 are fine-tuned, and the stability and convergence speed of the training process are compared and analyzed. Besides, the performance of the network was evaluated using a test set that did not participate in the training. The results show that by fine-tuning the transfer learning operation of the pre-trained classical convolutional neural network model, different types of defects of GFRP/NOMEX honeycomb sandwich structure can be well classified, and the quantitative detection of defect categories can be accurately realized.
Optical devices
Plasmon-enhanced ZnO-based nanowire heterojunction array photodetector
Wu Hui, Peng Jialong, Jiang Jinbao, Li Hansheng, Xu Wei, Guo Chucai, Zhang Jianfa, Zhu Zhihong
2024, 53(3): 20240006. doi: 10.3788/IRLA20240006
[Abstract](32) [FullText HTML] (8) [PDF 1908KB](10)
  Objective  Low-dimensional ZnO-based photodetectors have high responsivity and high photon absorption ability. However, the limited absorption range and reduced carrier lifetime of ZnO constrain its potential applications in optoelectronics. This study presents a novel plasmon-enhanced photodetector with zinc oxide (ZnO) nanowires-zinc selenide (ZnSe) heterojunction arrays.  Methods  One-dimensional (1D) ZnO nanowires with ZnSe shell heterostructures were synthesized on FTO substrate using low-temperature hydrothermal and chemical vapor deposition methods. Subsequently, silver nanoparticles were uniformly deposited on the heterojunction through capillary self-assembly, resulting in a plasmon-enhanced heterojunction array photodetector (Fig.1). The built-in electric field within the heterostructure accelerates the effective separation of photogenerated electrons and holes, thereby promoting the charge carrier transport characteristics of the optoelectronic device. Leveraging the surface plasmon resonance effect, noble metal nanoparticles exhibit excellent localized field enhancement, effectively enhancing the material's light absorption. Material morphology, structure, and chemical composition were characterized through scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray spectroscopy (XRD), and Raman spectroscopy analyses. The photovoltaic characteristics of the detector were systematically analyzed using standard techniques such as the chopped light voltammetry method, optoelectronic transient response measurement, and time-current curves.  Results and Discussions  Under visible light irradiation, the photoresponsivity of the Ag nanoparticle-enhanced ZnO/ZnSe heterojunction nanowire array photodetector far exceeds that of the ZnO/ZnSe heterojunction nanowire array photodetector and the ZnO nanowire array photodetector, reaching a maximum of 2.8 mA/W (Fig.3). At a bias voltage of 0.8 V and under visible light irradiation at 100 mW/cm2, the responsivity of the Ag nanoparticle-enhanced ZnO/ZnSe heterojunction nanowire array photodetector is approximately 52 times higher than that of the pure ZnO nanowire array photodetector, with a responsivity of 1.71 mA/W (Fig.4). This indicates that the formation of the heterostructure, coupled with Ag nanoparticle modification, indeed promotes the separation of photogenerated electron-hole pairs in the photodetector, enhancing the material's photodetection ability. Compared to the pure ZnO nanowire array photodetector, the ZnO/ZnSe heterojunction nanowire array photodetector exhibits significantly enhanced absorption in the range of 400 nm to 600 nm. Furthermore, the Ag nanoparticle-enhanced ZnO/ZnSe heterojunction nanowire array photodetector demonstrates an approximately 5-fold increase in absorbance within the wavelength range of 400 nm to 800 nm (Fig.5). This indicates that the formation of the heterostructure broadens the device's spectral response range. The presence of Ag nanoparticles not only extends the detector's spectral response range but also greatly enhances the intensity of light absorption. The optoelectronic transient testing system reveals that the average rise time of 1D ZnO nanowires is reduced by approximately 32% after modification (τrise=1.812 ms). The average delay time (τdelay) has decreased from the original 3.357 ms to 1.803 ms, representing a reduction of about 54%. This substantial improvement in response speed is illustrated (Fig.7). The current curve of the photodetector over time was tested, and the plasmon-enhanced ZnO/ZnSe heterojunction array photodetector maintained 48% of its photocurrent density after 10 hours of illumination, indicating favorable operational longevity. Compared to the initially fabricated device, the photodetector maintained approximately 52% of its photocurrent density after prolonged exposure to air (15 months), highlighting its robust long-term stability (Fig.8).   Conclusions  Compared with the detector based on pure ZnO nanowire array, the detector has excellent photoelectronic performance. Under visible light, the photodetector demonstrates responsivity of 1.7 mA/W and average rise/delay times of 1.812 ms (1.803 ms), exhibiting exceptional stability over 10 hours of continuous testing. This provides a cost-effective, scalable approach for developing high-performance photodetectors, with promising applications in wearable devices, optical communication systems, environmental sensors, etc.
Optical communication and sensing
Design and transmission characteristics of high-order orbital angular momentum transmission fiber (inside back cover paper)
Zhao Lijuan, Wu Yujing, Xu Zhiniu
2024, 53(3): 20240007. doi: 10.3788/IRLA20240007
[Abstract](22) [FullText HTML] (3) [PDF 7755KB](9)
  Objective  Orbital angular momentum based multiplexing is a special form of space division multiplexing, different OAM modes are orthogonal to each other, based on which different modes can carry different information, and multiple OAM beams with different topological loads can be used as carriers for information transmission, which can greatly improve the channel capacity of the communication system without the need for additional bandwidth. Compared with the transmission of OAM in free space, the transmission of OAM modes in optical fibers can effectively avoid the interference of external factors, and ordinary optical fibers are unable to meet the requirements for the transmission of OAM modes. Photonic crystal optical fibers, as a kind of special optical fibers with high structural designability, offer the possibility of realizing the transmission of OAM modes. In order to achieve high quality transmission of more OAM modes, it is necessary to design photonic crystal optical fibers with suitable structures that can support the transmission of OAM modes.  Methods  In this paper, a novel photonic crystal fiber structure based on a positive hexagonal arrangement of air holes is proposed. The fiber introduces rectangular air holes with high air filling rate and high refractive index materials to fill the ring transmission region, which can effectively improve the refractive index difference between the ring transmission region and the cladding, and the hexagonal arrangement of the air holes is conducive to the improvement of the effective refractive index difference between the modes. Structure optimization and optimal structure verification take into account the number of OAMs that the fiber can support as well as the effective refractive index difference between modes and mode purity. The optimal fiber structure is obtained through structural optimization, and the performance of the fiber is analyzed using finite element analysis.  Results and Discussions  The results of the finite element method analyses show that the optimal optical fiber structure is optimized to support 142 OAM modes in the commonly used S+C+L+U band, with the topological charge ordering up to 36. Moreover, the proposed fiber has good transmission characteristics. The confinement loss is below 10-9 for all eigenmodes, which is at least one order of magnitude lower than the typical photonic crystal fibers; The maximum effective mode field area can reach 206.18 μm2 and the minimum nonlinearity coefficient is as low as 0.397 W−1∙km−1; Flat dispersion and minimum dispersion variation are as low as 1.457 8 ps/(nm∙km); And purity are 93.4%-96.8% for all eigenmodes. Based on the effect of manufacturing errors on the performance of the optical fiber, it can be seen that the optical fiber does not require high manufacturing accuracy.  Conclusions  In order to achieve high-quality transmission of more number of OAM modes, this paper proposes a design method of photonic crystal fiber based on hexagonal structure by combining the two ways of rectangular air holes and the filling of annular transmission region with high refractive index materials. The introduction of rectangular air holes and high refractive index materials increases the refractive index difference between the ring transmission region and the cladding, which in turn facilitates the stable transmission of a larger number of OAM modes. The performance analysis of the optical fiber by Comsol Multiphysics finite element analysis software shows that the photonic crystal fiber can not only support a larger number of OAM modes, but also has the characteristics of low confinement loss, low nonlinear coefficient, flat dispersion change, and high mode purity, which is valuable for high-capacity optical fiber communication.
Integrated optics
Integrated spectral detection based on lensless speckle image coding
Zhou Tianbiao, Huang Siyuan, Wen Long, Chen Qin
2024, 53(3): 20240010. doi: 10.3788/IRLA20240010
[Abstract](47) [FullText HTML] (6) [PDF 2798KB](19)
  Objective  In recent years, on-chip spectroscopy has garnered considerable interests across multiple applications, primarily owing to its exceptional integration capabilities. Benefiting from mature image sensors with millions of pixels, wavelength-dependent image coding technology has emerged as a promising integrated spectroscopy method. However, achieving accurate decoding of spectral information in the images typically requires objective lenses and larger working distances, thereby increasing both the complexity of the optical system and the overall size of the inspection system. This limits its use in portable platforms such as mobiles. The aim of this work is to develop a lens-free image encoding method for on-chip spectroscopy and to address how to extract accurate spectral information from encoded images with high correlation.  Methods  A lens-free speckle image encoding method was developed for on-chip spectroscopy, where a PVP film embedded with Au nanorods was intimately integrated on an image sensor with a mm3 scale (Fig.1). The speckle of each signal wavelength was recorded to construct a responsive matrix for image encoding in advance. Although the image correlation is high in such a compact configuration (Fig.3), a convolutional neural network algorithm was applied to decode the image-spectrum relation by classifying the images with multilayer perceptron to extract high-level features (Fig.2). Once the calibration of image encoding is finished, the real-time spectral reconstruction takes only 1 s.  Results and Discussions  Spectral information was encoded in the speckle patterns generated by the Au nanorods. Conventional compressive sensing algorithm failed to reconstruct the original spectra due to the large image correlation in such a compact configuration (Fig.4). A convolution neutral network algorithm was developed to extract spectral features from low-contrast images and demonstrated accurate spectral reconstruction. For monochromatic light at 610 nm, 630 nm, 650 nm, 670 nm, and 690 nm, the deviation of the peak wavelength is consistently less than 1 nm in all cases (Fig.4(a)). Similar results were observed in the cases of single-layer frosted glass, double-layer frosted glass and triple-layer annealed gold particles, which shows the robustness of this method. The transmission spectra of three bandpass filters with varying filter ranges, as well as the emission spectra of a single LED were presented (Fig.5). And in every instance, the system predicts a wavelength peak deviation of less than 1 nm, with the spectral shape also demonstrating good agreement, indicating the applicability of the technology.  Conclusions  The work shows the possibility to develop an advanced image decoding algorithm based on convolution neutral network to compensate the limited hardware for on-chip spectroscopy based on image encoding. Such a ultracompact configuration together with decent spectroscopy performance enables potential applications in on-site inspection and distributed sensor network.
Invited paper
Research progress on polarimetric imaging technology in complex environments based on deep learning (invited)
Hu Haofeng, Huang Yizhao, Zhu Zhen, Ma Qianwen, Zhai Jingsheng, Li Xiaobo
2024, 53(3): 20240057. doi: 10.3788/IRLA20240057
[Abstract](263) [FullText HTML] (22) [PDF 3058KB](146)
  Significance  Polarization information, as one of the fundamental physical characteristics of light waves, can provide information about the intrinsic properties of the target. Polarimetric imaging technology digitizes the polarization information of the measured target field through digital processing. This approach effectively reduces the interference from the light propagation environment, thereby improving the imaging quality of the target and enhancing perception of its characteristics. In complex environments, polarimetric imaging has significant advantages. However, in complex environments such as scattering and low illumination, the degradation mechanism of polarized images exhibits nonlinear characteristics, leading to high complexity in polarized information interpretation methods. Deep learning methods possess powerful feature extraction and learning capabilities, enabling the recovery of polarized information by learning the mapping rules hidden in the large-scale collected data. This approach is particularly suitable for complex signal processing problems like polarimetric imaging, which involves multiple dimensions and interrelated signals.  Progress   First, the basic theory of the polarimetric imaging is introduced, including the principles of polarimetric imaging and a macroscopic description of polarimetric imaging issues in complex environments. Next, the general workflow of deep learning polarization imaging technology in complex environments is introduced. Based on deep learning, polarimetric imaging technology in complex environments uses the multi-dimensional polarimetric parameters collected by the polarimetric imaging system as input data. It leverages the nonlinear feature-fitting capabilities of neural networks to obtain image restoration results. Essentially, this approach transforms the nonlinear inverse problem of polarimetric imaging restoration in complex environments into a pseudo-forward problem, avoiding the challenges associated with solving nonlinear inverse problem algorithms. The representative developments of research in deep learning polarimetric imaging technology in response to scattering and noise, two of the most representative complex imaging environments, have been elaborated. From the inception of research in this field, the developmental trajectory of the field has been systematically outlined. In the early stages, polarimetric imaging technology in complex environments based on deep learning primarily relied on supervised training. Due to the challenges in collecting real-world data, researchers explored solutions using unsupervised, self-supervised, transfer learning, and simulation algorithms. Researchers also delved into the incorporation of prior knowledge and physical models into networks, leading to training approaches embedded with physical models or guided by prior knowledge. Overall, these representative works have made significant contributions to addressing the difficulties in constructing large-scale datasets, enhancing the generalization performance of networks, and exploring the interpretability of the networks. To better illustrate the connections and distinctions among research works, and to streamline the developmental process in this field for reader convenience, a summary has been compiled in the form of a table. The table provides task types, training methods, and characteristics of representative works for easy reference.  Conclusions and Prospects   With the rapid development of deep learning, polarimetric imaging technology in complex environments has achieved remarkable research progress. Existing studies indicate that, due to the multiple parameters and inherent correlations in polarized information, this multi-dimensional and interrelated signal processing problem is well-suited for the application of deep learning. The combination of deep learning and polarimetric imaging technology enables further improvement in optical imaging quality, meeting the imaging demands of complex environments and demonstrating more prominent advantages. The generalization ability, interpretability, and parameter lightweighting of deep learning technology remain areas that require further in-depth research. There is a continued need for refinement in multimodal fusion strategies, exploration of the underlying principles of network polarimetric parameter image restoration, and the design of network structures tailored for polarized multidimensional data to enhance real-time performance. Further efforts are essential to consolidate the feasibility of deep learning models in polarimetric imaging within complex environments, to enhance the adaptability of models to changes in complex environmental conditions, and to make them more universally applicable across different scenarios.
Atmospheric optical turbulence prediction method for satellite-ground laser communication (invited)
Guo Yingchi, Li Lang, Li Chen, Gao Chunqing, Fu Shiyao
2024, 53(3): 20230729. doi: 10.3788/IRLA20230729
[Abstract](70) [FullText HTML] (5) [PDF 3377KB](38)
  Significance  The prediction of atmospheric turbulence has great significance both in science and engineering, which provides key parameters and references for domains like astronomical observation, site selection, satellite-ground laser communication, and remote sensing. Especially in satellite-ground laser communication, predicting key parameters of atmospheric turbulence can schedule satellite-ground data transmission links in advance, and pre-deploy adaptive optical schemes to compensate turbulence effects, so as to establish effective communication links and suppress the performance degradation of data transmission. Therefore, atmospheric turbulence prediction is crucial and become an important issue, which needs to be addressed for most of laser scenarios in atmosphere.  Progress   This review consists of three sections. In the first section, firstly, the widely used meso-scale numerical prediction scheme to forecast atmospheric turbulence is introduced in detail. This scheme is accomplished by turbulence parameterization schemes, which establishes the relationship between the turbulence characteristics and the conventional meteorological parameters output from mesoscale meteorological model. Mesoscale meteorological model has been well developed, the most representative models include Meso-Nh(Non-hydrostatic mesoscale atmospheric model), MM5(Mesoscale Model 5), WRF(Weather Research & Forecasting Model) and Polar WRF. Many achievements have been made in turbulence parameterization schemes, including Hufmagel model, Tatarski model. Then, the relevant work of using mesoscale numerical prediction method to forecast atmospheric turbulence in typical regions is reviewed.  The second section presents recent advances regarding deep learning in atmospheric turbulence prediction, and discusses its advantages and limitations. This section first introduces the research achievements of deep learning in meteorological forecasting, and then introduces the research advances of deep learning in atmospheric turbulence forecasting. Based on a large amount of data, deep learning scheme can establish a relationship between the input data and the target label without any prior formula. In atmospheric turbulence prediction, deep learning is used to establish the relationship between meteorological parameters and atmospheric turbulence parameters, but the prediction accuracy is also limited by the accuracy of meteorological parameters.  In the third section, a short-time atmospheric coherence length prediction method called TsVMD-AR is introduced. TsVMD-AR model uses VMD (variational mode decomposition) algorithm and AR (autoregression) algorithm to forecast the short-term atmospheric coherence length. This scheme reduces the interference and coupling between the multi-scale feature information in the dataset, makes the complex internal features of the dataset easier to obtain. The results show that the established TsVMD-AR model is obviously superior to other models and is suitable for daily atmospheric turbulence prediction.  Prospects   We hope this review will provide more valuable information for people who is working in scenarios of laser applications in atmosphere turbulence, and inspire more wonderful ideas towards abilities of more accurate and faster turbulence grasp.
Athermalization design for annular aperture folding imaging system based on light-digital combination (cover paper·invited)
Ma Dechao, Piao Mingxu, Xie Yafeng, Zhao Yuanming, Niu Qun, Zhang Chengran, Wang Zhe, Zhang Bo
2024, 53(3): 20240013. doi: 10.3788/IRLA20240013
[Abstract](121) [FullText HTML] (10) [PDF 4660KB](28)
  Objective  With the advancement and development of science and technology, the demand for miniaturized optical systems is becoming increasingly significant. The total length of the optical system can be reduced by folding the optical path in the annular aperture folding imaging system. However, the annular aperture folding imaging system only uses a piece of base material, so the annular aperture folding imaging system cannot achieve high-quality imaging in a wide temperature range. In order to reduce the impact of temperature on the imaging quality and simplify the optical system structure, the wavefront encoding method is introduced to design a light-digital combined annular aperture folding imaging system.  Methods  The design principle of the annular aperture folding imaging system based on light-digital is studied. The relationship between the obscuration ratio and the phase mask parameters is studied to achieve defocus consistency (Fig.5). In the image decoding part, the image restoration effect is analyzed. The synthetic PSF model is studied through simulated annealing algorithm (Fig.2). High-quality imaging in a wide temperature range is achieved through a light-digital combination method.  Results and Discussions  A long-wave infrared annular aperture folding imaging system is designed (Fig.3). The focal length is 70 mm, the system aperture is 98 mm, the full field of view is 8°, and the total length is 25 mm. The synthesized PSF is constructed by simulated annealing algorithm. When over a wide temperature range, the high-quality image restoration is achieved through a single filter. Although the PSNR of the restored image dropped by 3.572 3 dB at the design temperature, the PSNR of the restored image at −40 ℃ also increased from 19.417 3 dB to 24.461 5 dB, which increased by 5.044 2 dB. The PSNR of the restored image at 60 ℃ also increased from 19.751 9 dB to 24.460 9 dB, which increased by 4.709 0 dB. This method outperforms traditional PSF image restoration at the design temperature. Image artifacts and blur are significantly reduced by this method.  Conclusions  Athermalization of infrared annular aperture folding imaging system is achieved. The light-digital combination method is introduced into the annular aperture folding imaging system. The annular cubic phase mask is introduced into the annular aperture folding imaging system, and the restored image is achieved through image restoration. The relationship between central obscuration and phase mask parameters is studied. The conclusion that increasing central obscuration will reduce PSF consistency is studied. The synthesized PSF is constructed by simulated annealing algorithm. When over a wide temperature range, high-quality image restoration is achieved through a single filter. In order to verify the effectiveness of this theoretical model, an annular aperture folding imaging system based on light-digital combination is designed. The total length is 25 mm, the focal length is 70 mm, the system aperture is 98 mm, and the full field of view is 8°. High-quality image restoration is achieved by synthesizing PSF when the temperature is between −40 ℃ and 60 ℃. Although the PSNR of the restored image dropped by 3.572 3 dB at the design temperature, the PSNR of the restored image at −40 ℃ also increased from 19.417 3 dB to 24.461 5 dB, which increased by 5.044 2 dB. The PSNR of the restored image at 60 ℃ also increased from 19.751 9 dB to 24.460 9 dB, which increased by 4.709 0 dB. Compared with PSF image restoration at design temperature, this method significantly reduces image artifacts. The study not only simplifies the infrared imaging optical system, but also uses a light-digital combination method to overcome the temperature limitations of the annular aperture folding imaging system. A new idea is provided for the miniaturization of infrared systems across a wide temperature range.
Newsletter
Application of 632 nm FMCW lidar for simultaneous velocity and distance measurement in humid environment
Zhang Xinyu, Jiang Lili, Song Ran, Zhang Zhijun, Li Bingbing, Su Juan, Wu Qi
2024, 53(3): 20240093. doi: 10.3788/IRLA20240093
[Abstract](30) [FullText HTML] (6) [PDF 1090KB](16)
  Objective  In high sea conditions and complex environments, for achieving safe landing of unmanned aerial vehicles (UAV) on unmanned surface vehicles (USV), it is necessary to accurately measure the distance and speed simultaneously between them and provide real-time feedback to the control system. However, commonly used GPS navigation and vision-based navigation approaches often suffer from insufficient dynamic positioning. These technologies could only measure distance but not the relative speed between UAV and USV. Frequency modulated continuous wave (FMCW) lidar, which could simultaneously measure both velocity and distance, has great potential for application in autonomous landing between UAV and USV during high sea conditions.For different applications, FMCW lidar could utilize different frequency modulation schemes on the optical carrier. Commonly used frequency modulation scheme includes the triangular, the sawtooth, and the sinusoidal waveforms. It is an economical and convenient approach to measure velocity and distance simultaneously using a triangular waveform modulated FMCW lidar. Considering eye safety, FMCW lidar normally deploys lasers with wavelengths longer than 1550 nm. However, the light absorption in the moisture above air/sea surface is too large for the infrared wavelength range.  Methods  In this paper, the lidar system employing a 632 nm distributed Bragg reflector (DBR) semiconductor laser which operating the water vapor transmission wavelength band and an FMCW technology was proposed and experimentally demonstrated. The symmetric triangular waveform modulation was achieved by directly modulating the laser injection current. The schematic diagram of the system is shown in Fig.1(a), and the experimental setup of the system is shown in Fig.1(b). The modulated laser output, with continuous frequency tuning, was further divided into two beams by a beam splitter (BS). Of which, one beam is used as the reference light, and another beam is used as the detection beam and incident on the measurement target. The detection light, which is reflected (or scattered) by the target, is collected and mixed with the reference light by a photo-detector (PD) for coherent heterodyne detection. The beating signal that carries information about the distance and velocity of the target is recorded by a digital storage oscilloscope (DSO). By performing operations such as fast Fourier transform (FFT), the beat frequencies of the up-sweep and down-sweep bands could be obtained, and then the distance and velocity information of the target could be calculated. The experimental results show that the modulation bandwidth of the system is 12.5 GHz without mode hopping using the internal modulation scheme of direct current injection, and the modulation period is 5 kHz(0.2 ms).  Results and Discussions  The measurement accuracy of distance of this FMCW system was tested by moving the target with 5 cm a step, ranging from 10 cm to 130 cm. The measured distances of the target were compared to the reference distances, as shown in Fig.2(a). The results demonstrate a strong correlation between the measured distance and the reference distances, with a linear fitting curve slope of 1.00121, R-squared value of 1, and a maximum relative standard deviation (RSD) of 0.3. The RSD is defined by the following formula, where S is the standard deviation (also denoted as SD) and $ \bar{x} $ is the mean value.                $RSD=\dfrac{S}{\bar{x}}\times 100\mathrm{\%}=\dfrac{\sqrt{{\displaystyle\sum_{i=1}^{n}{\left({x}_{i}-\bar{x}\right)}^{2}}/({n-1})}}{\bar{x}}\times 100\mathrm{\%} $  Further reducing the step size for movement to test distance resolution, experimental verification showed that the system's distance resolution is 1.5 cm. The accuracy of velocity measurement of the FMCW system was verified by measuring the linear speed of a scattering point on the standard rotating disc with high-precision control of the rotational frequency. A comparative experiment was conducted with a continuous wave (CW) system using the same laser operated under continuous wave. The results of the speed measurement are shown in Fig. 2(b). The linear fitting results show that within the velocity range of 10 cm/s to 125 cm/s, the FMCW system has a linear fitting curve slop of 0.99991 and an R-squared value of 0.99999 when compared to the reference velocity. The measurement resolution is 0.5 cm/s with RSD of 0.6%. On the other hand, The CW system has a fitting curve slope of 1.00214, an R-squared value of 0.99999, and a RSD of 1.2%, which is higher than that of the FMCW system. The experimental verification demonstrates that the FMCW system not only achieves synchronous measurement of target velocity and distance, but also provides a higher speed measurement accuracy than the continuous wave (CW) laser speed measurement system.  Conclusions  When UAV is performing precise landing on USV in high sea conditions, it is important to measure both the velocity and distance between them simultaneously. In response to this requirement, this paper proposed and experimentally demonstrated a lidar system based on the 632 nm laser which was frequency-modulated by continuous wave (FMCW) for simultaneous measurement of both velocity and distance. The 632 nm semiconductor laser was modulated by a directly injected triangular-wave current. The modulated light was incident on the moving target. The beat frequency signal generated by the interference of the scattered light from the target and the reference light was demodulated to extract information about the velocity and distance. The experimental results show that the FMCW lidar system has a measured distance range of 10 cm to 130 cm, with a resolution of 1.5 cm and a relative standard deviation (RSD) of 1.5%. The measured speed range was from 10 cm/s to 125 cm/s, with a resolution of 0.5 cm/s and a relative standard deviation (RSD) of 0.6%.
Correction of pressure effect in calibrating nitrate concentration of seawater
Zhang Naixin, Zhu Xingyue, Shan Baoyi, Xu Jian, Wu Qi
2024, 53(3): 20240095. doi: 10.3788/IRLA20240095
[Abstract](21) [FullText HTML] (4) [PDF 1147KB](6)
  Objective  Optical nitrate sensors have advantages for in-situ exploration and the potential for long-term observation in deep-sea environment. However, measurement of optical nitrate is influenced by substrates in seawater, especially bromide ions (Br-), within the relevant spectral range, causing a spectral shift in the ultraviolet (UV) intensity spectrum. In recent years, the pressure coefficient of the UV absorption spectrum of bromine under seawater pressure of 2 000 meters has been studied and experimentally confirmed, that the bromide in seawater affects the UV absorption spectrum. Considering that the seabed minerals are primarily distributed undersea at depths ranging from 1000 to 6000 meters, achieving accurate in-situ detection of nitrate concentration becomes crucial for the assessment of impact of seabed mining on the marine eco-system, establishment of an early warning system for the marine mining environment. Currently, products of nitrate sensors are limited to the submerged depths of approximately 2000 meters. No nitrate sensor product is available beyond this depth. This paper reports a calibration method for nitrate measurements within the pressure range of 0-50 MPa (0-5000 meters), aiming to improve the accuracy of nitrate measurements in deep-sea environments.  Methods  A system capable of measuring the UV spectrum of seawater under deep-sea pressure is constructed in this work. The light emitted from a deuterium lamp is transmitted through a fiber-optic beam splitter, dividing it into two paths. One path passes through a fiber-optic attenuator, while the other path goes through a pressure vessel. The two signals are then combined at an optical switch and selectively transmitted through it. Finally, the data processing module performs data acquisition and calculation. To simulate the deep-sea environment, the pressure vessel is connected to a weight manometer. The UV absorption spectra at different pressure are measured by controlling the external pressure. A continuous flow analyser was used to calibrate the nitrate concentration in the seawater samples collected from Aoshan Bay. Different levels of nitrate (0-50 μmol/L) were added to the Aoshan Bay seawater, and these seawater samples with different nitrate concentrations were measured by the measurement system.  Results and Discussions  The measurement results revealed a decrease in the absorbance of seawater samples with an increase in pressure. To investigate the pressure-induced changes in different substrates in seawater, identical pressure tests were conducted for nitrate solution (50 μmol/L), Aoshan Bay seawater, and sodium bromide solution (840 μmol/L). The absorbance results obtained are depicted in Fig.2(a). Notably, the absorbance of seawater and bromide under pressure exhibited a similar trend, whereas the absorbance of nitrate remained largely unaffected by pressure. Subsequently, pressure correction of seawater UV absorption spectra was conducted at pressures ranging from 0 to 50 MPa using two algorithms for spectral pre-processing, including standard normal variate transform (SNV) and multiplicative scatter correction (MSC), and regression prediction with the partial least squares regression (PLS) algorithm. The results are presented in Fig. 2(b). It is evident that the R2 is 0.991, MAE is 1.980 μmol/L, MBE is −0.042 μmol/L, and root mean square error (RMSE) is 2.505 without using any pressure correction. The R2 is 0.997, MAE is 1.294 μmol/L, MBE is 0.037 μmol/L, and RMSE is 1.620 using the MSC-PLS algorithm. The R2 is 0.989, MAE is 2.308 μmol/L, MBE is 0.098 μmol/L, and RMSE is 3.085 μmol/L using the SNV-PLS algorithm. Therefore, with the utilization of the MSC-PLS pressure correction algorithm, the prediction results are superior to those without using any pressure correction. This suggests that the pressure correction algorithm improves measurement accuracy. The MSC-PLS algorithm has the highest R2 and the smallest error range, indicating its superior pressure correction and data prediction capabilities.  Conclusions  The primary objective of this study is to enhance the accuracy of optical nitrate measurements in the deep-sea environment by addressing the influence of substrates such as bromide on UV absorption spectra. A system capable of measuring the UV spectrum of seawater under deep-sea pressure is constructed, utilizing a deuterium lamp, fiber-optic components, and a pressure vessel. The experimental results demonstrate variations in UV absorption spectra between 200-240 nm under different pressure conditions at the same nitrate concentration. The SNV and MSC algorithms are employed for pressure correction, and MSC-PLS algorithm exhibits superiority in predicting nitrate concentrations under the pressure range of 0-100 MPa (R2 of 0.997). Therefore, the proposed method offers potential applications in mining exploration and environmental monitoring.
Lasers & Laser optics
Threshold and incubation effect of femtosecond laser ablation of YAG crystals
Shang Tao, Deng Guoliang, Wang Jun, Wu Jie, Cai Rui, Chen Rubo, Xu Yunlong
2024, 53(3): 20230583. doi: 10.3788/IRLA20230583
[Abstract](47) [FullText HTML] (9) [PDF 1635KB](17)
  Objective   The incubation effect plays an important role in the process of ultrafast laser ablation and processing. This article investigates the threshold and incubation effect of femtosecond laser ablation of YAG crystals with a wavelength of 1 030 nm. Under the shot of a single pulse, the YAG surface undergoes "mild" ablation, with a smaller ablation aperture and depth; When subjected to multiple pulses, due to the influence of incubation, the ablation threshold significantly decreases with the increase of pulse count and eventually converges to a stable value. This article uses three incubation models to compare and study their fitting effects on the ablation threshold of YAG crystals. Through experiments and fitting, it was found that the ablation threshold under single pulse shot is $ {{F}}_{\mathrm{t}\mathrm{h},1} $=(12.27±3.56) J/cm2; Under the shot of multiple pulses, the saturation incubation threshold is $ {{F}}_{\mathrm{t}\mathrm{h},\mathrm{\infty }} $=(1.82±0.37) J/cm2. This study provides a reference for the control of parameters such as energy and pulse number in femtosecond laser precision machining.  Methods  Femtosecond laser adopts external control mode, achieving precise control of the number of pulses through high-precision function/arbitrary waveform signal generator (DG1000Z) and high-precision electronic shutter. The control of laser power is achieved through the combination of a half-wave plate and a polarizer. The laser beam is focused on the surface of the sample using a microscope objective (NA=0.28) with 10×, and the sample is fixed on a high-precision stage. The surface morphology changes and damage processes can be observed online using a CCD camera. After the YAG crystal was irradiated by laser, the ablation morphology of the sample was observed by optical microscope (KEYENCE VH-Z500) and atomic force microscopy (Dimension ICON), and the diameters of different ablation pits were measured. The ablation threshold was obtained by drawing D2–ln${E}_{{\rm{in}}}$curve for calculating ${E}_{{\rm{th}}}$(N) and $ \mathrm{\omega } $.  Results and Discussions  The incubation coefficient S of Model I and the incubation coefficient k of Model III fitted in this article are similar to other transparent materials reported in the literature, such as quartz and sapphire. The incubation model results obtained are also basically consistent with those in the literature. The incubation coefficient of Model II is about twice that of other materials in the literature, but the correlation coefficient is significantly higher, which fits well with the incubation effect of YAG crystals under multiple pulses. Under low-energy flow density laser irradiation, the material undergoes "mild" ablation. When the energy flow density is high, the material undergoes rapid evaporation and removal. Under the shot of multiple pulses, the incubation effect plays an important role in the ablation process. In dielectric and semiconductor materials, defects can be inherent in the material or caused by external conditions, such as laser induced color centers. As the number of laser irradiation increases, the defect density increases with the number of laser irradiation pulses until it reaches saturation. At the same time, the accumulation of defects will lead to an increase in effective absorption coefficient and a decrease in surface ablation threshold until saturation is reached.  Conclusions  We investigate the ablation threshold and incubation effect of YAG crystals under the shot of 1 030 nm femtosecond laser, and experimentally obtains the single pulse ablation threshold. We studied the variation of ablation threshold under multiple pulses and combined three incubation models to investigate the incubation phenomenon under multiple pulses. Among them, Model I is suitable for single pulse ablation threshold fitting, Model II has the best fitting accuracy under multiple pulses, and Model III has the best overall fitting. This study provides parameters and operating conditions for ultra-fast femtosecond laser processing of YAG, providing reference for surface microstructure processing and parameter control of smaller structure.
Frequency stabilization method of optical phase-locked loop He-Ne laser based on acousto-optic modulator
Liu Yusen, Wang Jianbo, Yin Cong, Han Shaokun, Bi Wenwen, Yu Qiuye, Zou Jinpeng
2024, 53(3): 20240003. doi: 10.3788/IRLA20240003
[Abstract](40) [FullText HTML] (7) [PDF 2431KB](11)
  Objective  With the rapid development of the aerospace and microelectronics industries, the demand for ultra precision measurement is also increasing. He-Ne lasers are widely used in mechanical and ultra precision measurement fields due to their excellent coherence and other characteristics. Among them, the thermally stabilized He-Ne laser is suitable as a wavelength scale laser for laser interferometry due to its high frequency stability, good beam quality, and low cost. However, traditional thermally stabilized lasers have poor frequency stability and reproducibility, which cannot further meet the requirements of high-precision laser interferometry for frequency stability and accuracy. This article reports a frequency biased locking system for thermally stable He-Ne laser based on a combination of an acousto-optic modulator and an optical phase-locked loop. This system combines the high-frequency response characteristics of an acousto-optic modulator with the high sensitivity characteristics of an optical phase-locked loop, enabling fast and accurate frequency locking of a thermally stable He-Ne laser.  Methods  This article reports an optical phase-locked loop bias locking system based on an acousto-optic modulator. An iodine stabilized frequency laser is chosen as master laser, and a thermally stabilized He-Ne laser as the slave laser. The beam of the slave laser is modulated by an acousto-optic modulator and locked onto the master laser. The reference signal for frequency offset locking is a 30 MHz signal generated by a signal generator. Data are collected using a frequency counter. The locking result is shown (Fig.9).  Results and Discussions  In the experiment, a highly stable He-Ne laser based on intracavity saturation absorption stabilization was used as the wavelength reference source for thermal stabilization laser locking. Through beat frequency measurement with iodine stabilized laser wavelength reference, the results show that the 1 s wavelength stability of the iodine stabilized laser is $ 1.3\times {10}^{-11} $, reaching $ 4.1\times {10}^{-13} $ in 1 000 s, reproducibility better than $ 1.0\times {10}^{-11} $. The frequency jitter of the laser beat frequency after the system is locked is shown (Fig.10). As a comparison, the figure shows the drift of the beat frequency under free operation. In the experiment, a frequency counter was used to count the beat frequency signal for 30 min in the open-loop state of the optical phase-locked loop. Then, the reference frequency was set to 30 MHz to lock the thermal stabilized frequency laser to the iodine stabilized frequency laser, and the beat frequency after the loop locking was continued to be counted for 180 min. The beat frequency was locked at a bias frequency of 30 MHz, with a fluctuation range below 0.2 Hz. We have achieved high stability frequency locking of thermally stabilized lasers compared to iodine stabilized lasers. The relative Allen variance of the frequency offset of the optical phase-locked loop is shown (Fig.11). Among them, the relative Allen variance of the integration time of 1 s and 1000 s is $ 3.3\times {10}^{-9} $ and$ 1.4\times {10}^{-12} $ respectively.  Conclusions  This article introduces a high stability laser frequency stabilization method based on the combination of an acousto-optic modulator and an optical phase-locked loop. An experiment was conducted using a self-developed optical phase-locked loop system to lock the bias of a thermally stable all cavity He-Ne laser to an iodine stable frequency laser. The signal-to-noise ratio of the beat frequency signal was increased to over 40 dB (Fig.9) through a beat frequency signal detection unit based on an acousto-optic modulator. A digital frequency discriminator and PI control circuit were used to feedback control the acousto-optic modulator, achieving closed-loop control of the optical phase-locked loop. The frequency stability of the thermally stable He-Ne laser is significantly improved, enabling it to meet the requirements for laser frequency stability and accuracy in the fields such as ultra precision interferometry and ultra sensitive spectral detection.
Tunable high-energy mode-locking fiber laser based on PbS quantum dots
Chen Guangwei, Zhao Yue, Hu Guoqing, Qin Ying, Jia Kailin, Chen Li, Li Huiyu, He Jingwen, Zhou Zhehai
2024, 53(3): 20230632. doi: 10.3788/IRLA20230632
[Abstract](26) [FullText HTML] (5) [PDF 1701KB](12)
  Objective  With excellent photoelectric properties, low-dimensional nanomaterials show great promise for applications, such as optical switch, optical communication and industrial materials processing, particularly in combination with ultrashort pulse fiber laser. Passively mode-locking fiber laser is one of the important ways to generate ultrashort pulses. And carbon nanotube, graphene, black phosphorus and so on, have been used in lasing ultrashort pulses. However, low damage threshold is the fatal drawback of low dimensional nanomaterials, resulting in the inability to be applied to high-energy fiber lasers. Functional modified or customized nanomaterials have been studied in generating high-energy pulses. But these fiber lasers have mono-functional and no tunable features. Therefore, it is necessary to establish high-energy and flexible tunable fiber laser to meet the needs of different usage. For this purpose, a PbS quantum dots based passively mode-locking fiber is designed in this paper, where a piece of graded index multimode fiber is added to cavity, acting as a tunable filter and dispersion compensator.   Methods  A tunable high-energy passively mode-locking fiber laser is built in this paper. PbS quantum dots is chosen as the high-damage threshold saturable absorber. A 24 cm-long graded index with slightly stretching is performed to implement a tunable filter (Fig.1-2). Because of the principal modes in multi-mode fiber, the group delay resulting from them is in linear relationship with the length of multi-mode fiber. And they are polarization dependent. Therefore, the total cavity dispersion can be controlled by just adjusting the polarization states of fiber lasers, leading to the operation of cavity switchable.   Results and Discussions  By properly adjusting the polarization controller, a stretched pulsed fiber laser is established under the pump current of 245 mA (Fig.4). The center wavelength is located at 1 568.6 nm, with 3 dB bandwidth of 11.4 nm, and the full width at half maximum (FWHM) is about 361 fs with a fundamental repetition frequency of 9.55 MHz. Rising the pump to 296 mA and adjusting the polarization state, the operation of fiber laser switches to high-energy region (Fig.5). The center wavelength red-shifts to 1 569.5 nm, with a 3 dB bandwidth of 9.3 nm. The FWHM of pulse is about 20 ns with a near-rectangle waveform, where the leading and trailing edges of pulse are asymmetry resulting from three-order dispersion. When the polarization state is adjusted, the group delay resulting from principal mode accumulates, and the bandwidth of filter narrows down (Fig.2). At the same time, the total cavity dispersion increases to negative dispersion zone. With the pump current increasing, the high-energy dissipative soliton resonance pulse occurs. The FWHM can be in the range from 7.7 ns to 23 ns, and the maximum energy in cavity reaches to 34.8 nJ, indicating the damage threshold is more than60 mJ/cm2. It's worth noting that the output spectrum shows asymmetrical sideband in two side of spectrum, which includes dip-type sideband and Kelly sideband. The appearance of dip-type sideband shows local asynchronous dispersive wave and soliton interactions in the cavity.   Conclusions  The proposed fiber laser can be flexible transited between stretched pulse and high-energy dissipative soliton resonance pulses using a bandwidth-tunable filter based on multi-mode fiber. The micro-stretched multi-mode fiber not only has the function of filter, but also plays a vital role in compensating dispersion in ultrashort fiber laser. The versatile fiber laser provides diverse solutions for intelligent lasers.
Temperature field and stress field of LD end-pumped Yb:YAG crystal
Li Xinyang, Li Long, Ren Jiaxin, He Zhenglong, Shi Pu, Ning Jianghao, Zhang Chunling
2024, 53(3): 20230683. doi: 10.3788/IRLA20230683
[Abstract](36) [FullText HTML] (6) [PDF 3421KB](15)
  Objective  Laser is a device that can emit high energy radiation, and its excellent characteristics make it to be widely used and paid attention to by researchers in various fields. Its most important part is the laser crystal. During the operation of the laser, the crystal will generate a lot of heat inside the crystal because of the crystal thermal effect such as quantum deficit, and cause the gradient distribution of stress and crystal deformation, which seriously affects the beam quality of the laser. Therefore, studying the thermal effect of crystals is an important step to design and manufacture lasers with high beam quality. In this paper, through the simulation of the working state of the laser, the monitoring of the temperature field and stress field inside the crystal is tested, so as to provide data and theoretical support for better laser design.  Methods  In order to simulate the working state of the laser and monitor the temperature field and stress field, the model conditions are simplified based on the actual situation. The temperature distribution in the crystal is analyzed by the Poisson equation in the thermodynamic theory, and the stress distribution in the incident direction is analyzed by the mechanical theory. Then the geometric model is established by finite element analysis, and the theoretical model close to the working state of the crystal is obtained by combining them. The boundary conditions around the crystal are analyzed and the physical field constraints are given to the model. Finally, the control variable method is used to analyze the variables in the system.  Results and Discussions   The initial conditions are as follows. The model size is 3 mm×3 mm×4 mm, doping concentration is 5.0 at.%, absorption coefficient of 940 nm pump light is 5.6 cm−1, thermal conductivity is 0.13 W·cm−1·K−1, Gauss order is 1, cooling temperature T0 is 291 K. The coefficient of thermal expansion is 7.8×10−6 K−1. When the laser pump power is 50 W and the spot radius of the pump surface is 400 μm, the maximum temperature rise of the pump end face is 59.2 K (Fig.3), the maximum stress on the system is 2.380×108 N/m2 (Fig.8), and the maximum deformation is 6.456 7×10−4 mm (Fig.9). When only temperature gradient is considered, the relationship between thermal focal length and power is obtained (Fig.10). If other conditions remain unchanged, when the Gauss order is 1, 2, and 3 respectively, the corresponding maximum temperature rise of the pump end face is 59.18 K, 75.24 K, and 83.99 K (Fig.5). If other conditions remain unchanged, when the half-diameter of the pump light spot is 300 μm, 350 μm, 400 μm, 450 μm, and 500 μm respectively, the corresponding maximum temperature rise of the pump end face is 105.21 K, 77.30 K, 59.18 K, 46.76 K, and 37.88 K (Fig.6). If other conditions remain unchanged, when the pump power is 50 W, 60 W and 70 W respectively, the maximum temperature rise of the reaction is 59.18 K, 71.02 K and 82.85 K (Fig.7).  Conclusions  The light energy distribution is a function of power, spot radius and Gaussian order, and the properties reflected by power, spot radius and Gaussian order are the properties of light energy distribution, and it can be seen that the light energy density is positively correlated with the temperature field and stress field. In addition, the temperature field, stress field and thermal stress variables are also positively correlated. There are many factors affecting thermal focal length, but the most important one is temperature gradient distribution.
Design and application of all-day portable outdoor infrared detection aerosol lidar system
Zhuang Peng, Xie Chenbo, Kang Baorong, Liu Jianming, Xia Xiaowei
2024, 53(3): 20230636. doi: 10.3788/IRLA20230636
[Abstract](37) [FullText HTML] (6) [PDF 2739KB](16)
  Objective  Air pollution control has higher requirements for environmental monitoring equipment. As an active remote sensing instrument, lidar is currently a powerful tool for monitoring the three-dimensional distribution characteristics of atmospheric aerosols in the troposphere. Lidar can compensate for the insufficient spatial distribution rate of existing ground monitoring data, mainly using various monitoring modes such as horizontal, vertical, navigation, and networking. Among them, horizontal scanning can effectively monitor horizontal visibility, dynamically extract the location and transmission path of pollution sources. Moreover, vertical monitoring can analyze the spatiotemporal changes in aerosol vertical diffusion, sedimentation transport, optical characteristics, boundary layer changes and their cloud information, as well as sand and dust monitoring. This article introduces a self-developed infrared lidar that can detect aerosol distribution, visibility, and boundary layer height in real-time under environmental pollution and multiple sudden weather conditions. It has unique advantages in analyzing the diffusion and sedimentation trends of polluted air masses. In the context of increasing observation needs, it can provide more scientific and effective data support for environmental management decisions and meteorological services.  Methods  The detection principle is based on the Mie scattering lidar equation. The schematic diagram and appearance of the aerosol lidar product structure are shown (Fig.2). The system mainly consists of three parts of laser emission unit, optical receiving unit, and data acquisition and processing unit. The main technical specifications are shown (Tab.1). The fundamental frequency 1 064 nm linearly polarized laser of Nd: YAG laser is used as the detection light source, and the telescope uses an aspherical lens as the main mirror. The aspherical lens has the advantages of small aberration and short focal length, which can reduce the volume of the receiving module. The subsequent optical and detection units are composed of fiber coupling devices and small core diameter multimode fibers, which can effectively control the telescope's receiving field of view. The optical channel measures the scattering signal generated by the interaction between 1 064 nm outgoing laser and atmospheric particles. By using the measurement data of the channel and combining with the above inversion method, the distribution characteristics of optical parameters of tropospheric aerosols and clouds can be obtained.  Results and Discussions  This radar can be effectively applied in fields such as obtaining the spatiotemporal distribution characteristics of atmospheric aerosol pollution, monitoring horizontal visibility, and monitoring vertical boundary layers. The specific application conclusions are as follows.  1) Infrared detection lidar horizontal scanning monitoring can timely detect pollution sources in the area. During the monitoring period, a total of 10 main pollution source areas were discovered (Fig.3), distributed in the southeast and northeast sides of the radar points. Based on the comprehensive analysis of particulate matter concentration data from national urban environmental air quality monitoring stations within the scanning area, the pollution source spreads southwestward under the influence of northeast winds, resulting in a significant increase in particulate matter and carbon monoxide concentrations at downwind stations simultaneously.  2) Two infrared detection lidars were observed at the same location as the visibility meter, and the results showed that the correlation coefficient of visibility between the two lidars was 0.98, with a relative error of 7.76% (Fig.10-11). The trend of change was consistent, and the instrument performance was relatively stable. The relative error in comparison and analysis with the visibility meter data is less than 20%, which is consistent with the standard data. The daily variation of visibility shows a unimodal distribution, with higher daytime visibility compared to nighttime. From the 26th to the 27th, visibility decreases. Combined with environmental air quality analysis, an increase in particulate matter concentration is the main factor affecting visibility.  3) The infrared lidar and sounding balloon simultaneously inverted the boundary layer height, and the results showed that the absolute deviation between the lidar boundary layer height and the sounding balloon was 200 m, which can accurately invert the boundary layer height. During the monitoring period, the height of the boundary layer inverted by the lidar showed a diurnal trend, with an average height of 1.2 km (Fig.12). However, during the nighttime period, the height of the boundary layer was significantly higher than that of the sounding inversion, which was affected by residual pollution at night and was not a stable boundary layer height at night.  Conclusions  Infrared detection aerosol lidar has the characteristics of strong atmospheric penetration, little influence by sky background light, and sensitivity to large particles, and has achieved good results in the spatial and temporal distribution characteristics of atmospheric aerosol pollution, horizontal visibility monitoring, and vertical boundary layer monitoring. Horizontal scanning can obtain a map of particulate matter distribution in a large area, find pollution sources in time, and evaluate the impact of pollution sources by combining data such as wind speed, wind direction, particulate matter and carbon monoxide concentration near the ground. The system can accurately capture the distribution and transmission of atmospheric aerosols in real time, and accurately invert information such as horizontal visibility and boundary layer height, and has a wide range of application scenarios in the field of atmospheric parameter monitoring.
Thermal effect of laser diode end pump square Tm:YAG composite crystal
Ren Jiaxin, Li Long, Li Xinyang, Yang Hengxin, Ji Yuxiao, Zhang Chunling
2024, 53(3): 20230717. doi: 10.3788/IRLA20230717
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  Objective  The mid-infrared 2 μm solid-state laser has unique characteristics in terms of output wavelength: it is in the atmospheric window, the absorption band of water and the human eye safety zone. Based on these special properties, it can be widely used in laser imaging radar, doppler coherent wind radar, differential absorption radar and other laser sources used to measure the concentration and temperature changes of the earth's atmosphere. In addition, the 2 μm band laser can also be used as a pump source for optical parametric oscillators to achieve longer wavelength infrared laser output. With the deepening of research on 2 μm solid-state lasers, thermal effect has become a major problem limiting laser output power and beam quality improvement, so it has attracted much attention. During the operation of solid-state lasers, the heat generation is due to quantum deficit, energy conversion between the lower laser level and the ground state, and laser quenching, which leads to uneven temperature distribution inside the laser crystal, resulting in thermal lensing effect. However, by using bonding technology to bond YAG crystals with Tm:YAG crystals to form composite crystals as the working material of lasers, the influence of thermal effect can be effectively reduced. Lasers using composite crystals have advantages such as high reliability, peak power and excellent spot quality  Methods  In this paper, the thermal effect of laser crystals is reduced by introducing two composite crystal models, namely single-ended bond and double-ended bond. By analyzing the working characteristics of continuous LD end-pumped square composite Tm:YAG crystal, a heat source heat model of laser crystal (Square Tm:YAG crystal model and its heat sink experimental device structure schematic diagram as shown in Fig.2) is constructed. Considering the boundary conditions of thermal convection between the crystal rod end face and air and the surrounding constant temperature, the finite element analysis method is used. The thermal effect of laser diode end pump square composite Tm:YAG crystal is studied.  Results and Discussions   The thermal model of laser crystal is established which is more suitable for the actual working conditions. The temperature field, thermal stress field and end shape variables of the composite crystal are numerically calculated by finite element analysis. The effects of single/double end bonding, undoped crystal length and gain crystal length on the internal temperature field and end shape variables of the square composite crystal are discussed. The bonded crystals can significantly improve the thermal effect of crystal end faces.  Conclusions  The thermal model of laser crystal is established which is more suitable for the actual working conditions. When the pump power of the laser diode is 30 W, the radius of the pump spot is 400 μm, the thickness of YAG crystal is 1 mm, and the thickness of gain crystal is 1.5 mm, the maximum internal temperature rise of the square single-ended and double-ended Tm:YAG composite crystal are 81.2 ℃ and 77.9 ℃ respectively. The maximum internal stresses are 146 MPa and 104 MPa respectively. The thermal shape variables are 0.468 μm and 0.172 μm. It can be seen that the composite crystal can effectively alleviate the temperature rise of the crystal and the thermal deformation of the crystal end face, and the double-end bonding has a better effect on reducing the thermal effect of the crystal. When the thickness of the gain crystal is more than 2.6 mm, the influence of the two bonding methods on the maximum temperature rise inside the composite crystal is basically the same.