红外与激光工程

Design of optical polarization system for defect detection on highly reflective surfaces

Peng Xing, Zhai Dede, Shi Feng, Tian Ye, Song Ci, Tie Guipeng, Shen Yongxiang, Qiao Shuo, Shen Xiao, Zhang Wanli, Wang Sheng, Ruan Ningye

2023, 52(6): 20220863. doi: 10.3788/IRLA20220863

[Abstract](347) [FullText HTML] (72) [PDF 3369KB](74)

Objective The defect detection of laser additive manufacturing (AM) has always been a technical problem that restricts its development. Due to the complexity of the defect generation mechanism, the insufficient detection information of the highly reflective workpiece surface, the low precision, the complex detection conditions, and other reasons, it is difficult to achieve high-precision and robust detection of defects. When the defect detection system based on reflective illumination performs detection on the surface of a metal part with high reflectivity, the pixels of the image sensor are usually overexposed due to the strong reflective light, resulting in a large amount of annihilated defect information, and it's difficult to highlight and extract the information of the defect area. Therefore, in view of the engineering problem of the high-precision robust detection and evaluation of surface defects of high reflective metal workpieces manufactured by laser AM, a polarization detection system based on a high reflective suppression effect is designed, which can effectively avoid background clutter interference and improve the defect detection capability in complex detection environment. Methods The system is designed based on Q-type aspheric surface, which has a strong aberration correction ability and simplifies the system structure. The deviation between the seventh surface shape and the best-fitting spherical surface is only 0.371 μm (Fig.2-3). The deviation between the 9th surface shape and the best-fitting spherical surface is only 0.434 μm. The focal length is 50 mm, the number of F is 2, and the working distance is 300 mm. Results and Discussions The simulation results show that the modulation transfer function is better than 0.42 at the Nyquist frequency of 144.93 lp/mm, meeting the requirements of the image quality (Fig.4). The tolerance analysis and 2 000 Monte Carlo analysis results indicate that the tolerance range is reasonable and meets the processing and assembly conditions under the condition of satisfying the image quality of the polarization detection system (Fig.10-11). To verify the suppression effect of the defect polarization detection optical system on the highly reflective light of the detection surface, the experimental device is built based on the designed polarization detection optical system (Fig.12). Based on the constructed polarization detection system, the detection images under different polarization angles are collected and converted from the RGB channel to the HSV channel for threshold determination. Furthermore, based on the Stokes vector method, the defect polarization information in the high-reflection detection image is extracted. The Stokes vector, degree of polarization, and angle of polarization detection image are calculated. The calculated image is fused to achieve the high-reflection suppression reconstruction of the defect detection image, thus achieving efficient and robust high reflection area characterization and analysis. The experimental results show that the fused image has a prominent role in the edge contour of the defect area, and the contrast between the defect area and the adjacent background area has been effectively improved, making the details of the defect clearer and more intuitive (Fig.14). The overall contrast, clarity, and information content of the image have been improved. Besides, to objectively and quantitatively evaluate the quality of the fused image and compare it with the original intensity image, the average gradient (AG), entropy (E), spatial frequency (SF), edge intensity (EI), and standard deviation (SD) are used to evaluate the image, the results are shown (Fig.15). Compared with the original intensity image, the average improvement rates of the average gradient, information entropy, spatial frequency, edge intensity, and standard deviation of the fused image are 163.46%, 20.04%, 163.20%, 123.03%, and 28.41% respectively. Conclusions The results fully illustrate that the polarized image after fusion processing has more abundant information, the image details are clearer, the contrast of the defect area is higher, and the edge contour information of the defect is clearer. The feature extraction and characterization analysis of the surface defects of highly reflective metal workpieces in metal manufacturing are of great significance.

Recent progress and prospect of laser imaging processing technology （invited）

Hu Yihua, Zhao Luda

2023, 52(6): 20230169. doi: 10.3788/IRLA20230169

[Abstract](641) [FullText HTML] (202) [PDF 3162KB](203)

Significance Laser imaging refers to an imaging method that emits a specially designed laser signal, receives the laser echo, and processes it to obtain attribute information such as an image of the target. Laser imaging has wide applications in target detection, satellite surveying, smart agriculture, national defense and aerospace, and other fields. It contains a series of signals and information processing processes, including denoising, radiation, geometric correction, point cloud processing of laser echo signals, and subsequent data processing of various imaging tasks (such as laser ranging, laser image reconstruction, target detection, etc.), and have a critical impact on imaging quality and play a crucial role in the application of imaging information. Currently, with the continuous development of imaging systems and imaging hardware, laser imaging processing technology has increasingly high requirements for processing accuracy and speed, and involves a wider range of technical fields. Especially with the rapid development of machine learning technology represented by deep learning, it has achieved better results than traditional technologies in many classic problems, and has also been successfully applied in laser imaging processing technology, providing a new development direction for laser imaging processing. Progress This paper first introduces the characteristics of laser imaging processing technology of typical imaging system (Fig.1). We explained the characteristics of imaging processing technologies under various laser imaging systems, identified the similarities and differences between laser imaging processing technologies under different systems, and conducted a comparative analysis of laser imaging processing technologies under typical imaging systems (Tab.1). In summary, it can be found that although there are differences in the names of signal and information processing contents corresponding to different systems, the common contents of laser imaging signal processing can be summarized into four aspects of signal denoising, radiation correction, geometric correction, and point cloud processing. The common contents of imaging information processing can be summarized into three common processing contents of laser ranging, image reconstruction, and object detection. 　　Based on the summarized common methods of laser imaging signals and information processing technology, we conducted separate studies. In the current research status of laser imaging signal processing technology, we focus on the laser signal denoising, correction and laser point cloud processing technology. In the research of signal denoising, we have conducted research based on wavelet transform, empirical mode decomposition, variational mode decomposition, and hybrid methods. We have also conducted specialized research on the application of deep learning algorithms in laser signal denoising. Representative algorithms are shown (Fig.5). The laser signal correction focuses on two aspects of laser signal radiation and geometric correction. And in point cloud signal processing, we mainly summarized the work on denoising and background removal, and focused on the work based on deep learning. Besides, we have organized and summarized the research on laser information processing for laser ranging, image reconstruction and target detection information processing technology. In the section of laser image reconstruction, we conducted research on three aspects of stereo matching, point cloud data stitching, and laser reflection tomography reconstruction. In object detection, the traditional method and deep-learning based method were elaborated, and classic point cloud object detection algorithms based on deep learning algorithms were studied (Fig.9-10). 　　Based on the classification of laser imaging processing technology in this paper, we finally analyzed the current challenges and future development directions of laser imaging processing technologies, and summarized the current development of laser imaging technology and future laser imaging processing technology examples. It is hoped that it can provide some reference for the research related to laser imaging. Conclusions and Prospects Laser imaging has always been a hot topic in the field of optical imaging and signal processing. In the past 20 years, laser imaging signal and information processing technology has made great progress. In the previous studies, deep learning has been deeply applied to laser imaging processing. Through the powerful representation learning ability of deep learning, great improvements have been made in laser imaging processing quality, precision, robustness and other aspects. In the future research on different signal and information processing tasks, the standardization of large-scale data sets for imaging tasks and more robust deep neural network processing paradigm will be the further development direction of the research. It should be noted that laser imaging processing technology is not limited to the contents in this paper. There are many other signal and information processing technologies not involved in this paper, which worth further study and exploration by researchers.

Fiber laser from interdisciplinary perspective: review and prospect (invited)

Zhou Pu, Jiang Min, Wu Hanshuo, Deng Yu, Chang Hongxiang, Huang Liangjin, Wu Jian, Xu Jiangming, Wang Xiaolin, Leng Jinyong

2023, 52(6): 20230334. doi: 10.3788/IRLA20230334

[Abstract](423) [FullText HTML] (108) [PDF 2742KB](144)

　　Since the turn of the century, China has been a research hub for fiber lasers. The National University of Defense Technology's research into fiber lasers began during the "11th Five-Year Plan" period and has lasted approximately 15 years, yielding a number of peer-reviewed research outputs. The optical engineering field underpins the majority of fiber laser research at the institution. Optical engineering is one of the university's dominating fields, with good results in recent discipline review, providing a high-level scientific research platform and talent team for fiber laser research. On the other hand, fiber laser development benefits from the advantages of reasonably complete subject categories as well as helpful exploration and practice in interdisciplinary aspects. From an interdisciplinary standpoint, this paper sorts out several important breakthroughs in the interdiscipline of fiber laser and electronics, materials, control, intelligence, nano, and other disciplines in the university, and analyzes the opportunities faced by interdisciplinary scientific research and interdisciplinary construction from four perspectives: the evolution of scientific research paradigm, subject driving, application demand traction, and the inception of interdisciplinary scientific research and interdisciplinary construction. Significance & Progress The National University of Defense Technology's main fiber laser research is based on the discipline of optical engineering; Research in the fiber laser began during the "11th Five-Year Plan" period, has been about 15 years, and has achieved a series of peer-recognized research results. During the "11th Five-Year Plan" period, the university concentrated on scientific research in the fields of fiber laser coherent synthesis and supercontinuum fiber light source, and officially began related work in the fiber laser discipline, achieving research achievements represented by kilowatt fiber laser coherent synthesis system and high-power near-infrared supercontinuum light source. During the "12th Five-Year Plan" period, the university focused on high-power fiber lasers, gradually expanding its research into high-power fiber lasers, fiber passive devices, and so on, and achieved innovative results in cascade pumping high-power fiber lasers, special wavelength fiber lasers, high-brightness laser bunders, and high-power ultrafine lasers. Since the "13th Five-Year Plan," the research focus has shifted to the development of laser fiber materials and software, as well as the development of laser full machines to form a complete chain. As a representative of the corporation, we have obtained independent intellectual property software, virtual simulation courses, various types of laser fiber, high power and high beam quality single frequency/narrow linewidth/broadband fiber laser, cascade pump/semiconductor direct pump high power and high beam quality fiber laser, high power visible light/near infrared/mid-infrared supercontinuum light source, thousand-beam laser phase control/high power fiber coherent synthesis system. 　　This study examines the prospects for cross-disciplinary research and construction from four perspectives: the growth of scientific research paradigms, subject driving, application demand pulling, and science-education integration. The university's fiber laser approach has produced a number of notable research results, with input from other disciplines playing an essential role. The in-depth analysis, however, reveals that relevant research is primarily "reference" and "inspiration" between disciplines, such as the development of fiber simulation software mentioned in the introduction and ultra-long-term stable high-performance pulsed fiber laser, etc., which is more the result of personnel familiar with "optical engineering," "software engineering," and "nanoscience" working together to promote. In reality, there aren't many cross-disciplinary construction and cross-scientific research items. There are basically no examples like Logan Wright who have made significant contributions to various disciplines such as nonlinear fiber optics and artificial intelligence at the moment. The scientific foundation of relevant researchers must be strengthened further. However, with the rapid advancement of a new round of scientific and technological revolution, industrial change, and continuous innovation of education and teaching methods, it provides new opportunities for cross-scientific research and cross-disciplinary construction, as well as a broad space for the development of the fiber laser direction, and the fiber laser direction will continue to produce more innovative results. Conclusions and Prospects After approximately 15 years of development, the university's research of fiber laser has accomplished a number of significant outcomes that are strongly tied to the support of the optical engineering discipline and the deep cross-integration of other associated disciplines. The continuous human science and technology development in this century is continuous comprehensive development, from comprehensive to more comprehensive, and this integration tendency is reflected in scientific research, discipline construction, personnel training, and other aspects. Although the development of fiber laser has experienced many problems and challenges, the research of fiber laser will continue to create more novel outcomes with the continued advancement of interdisciplinary construction and cross-scientific research.

Research progress in levitated optomechanical sensing technology (invited)

Zhang Haoming, Xiong Wei, Han Xiang, Chen Xinlin, Kuang Tengfang, Peng Miao, Yuan Jie, Tan Zhongqi, Xiao Guangzong, Luo Hui

2023, 52(6): 20230193. doi: 10.3788/IRLA20230193

[Abstract](339) [FullText HTML] (91) [PDF 5072KB](77)

Significance With the rapid development of laser technology in the last century, microscopic optomechanical effects have gradually been discovered by researchers. In 1971, Arthur Ashkin in Bell Laboratory discovered the acceleration and trapping of particles by radiation pressure, and first proposed the concept of "optical potential wells", also known as "optical trap". In 1976, Ashkin achieved optical levitation of a fused quartz sphere in ultrahigh vacuum and pointed out its feasibility of high-precision sensing in low-damping environments. In 1986, Ashkin constructed an optical gradient potential trap using tightly focused beams to capture particles, which announced the birth of optical tweezers and raised a new era of levitated optomechanical sensing technology. Thanks to the pioneering work of Ashkin, and with the development of vacuum technology, levitated optomechanical sensing technology emerged. The technology has great characteristics of non-contact, high sensitivity, and feasible integration. Compared to previous quantum sensing based on the cold atom interference or nuclear magnetic resonance, this new technology involves larger particles with much more uniform atoms, which allows intuitive observation of particle morphology. Meanwhile, levitated optomechanical sensing technology enables ultra-high sensitive detection at room temperature without the need of the complex cryogenic environment. Therefore, the levitated optomechanical system can be considered as an "ideal platform" for precise measurements, where its accuracy is gradually approaching the standard quantum limits. The technology has also played significant roles in many cutting-edge fields including microscopic thermodynamics, dark-matter explorations, and macroscopic quantum state preparations. Progress Firstly, we describe the basic theory of the levitated optomechanical sensing. Tested physical quantity can be measured by sensing the motion parameters of the optical-trapped particles. Relevant key sensing technologies contain the loading of the particles, the enhancement of the optical forces, the displacement detections, the calibration of the voltage coefficient and the feedback cooling. These specific technologies are remarkably developed in recent years. For instance, feedback cooling has achieved occupation numbers below 1, which opens the door to quantum ground-state at room temperature. During the last decades, levitated optomechanical sensing is widely used in the measurements of the basic physical quantities, such as the extremely weak forces, the accelerations, the microscopic mass, the residual electrical quantities, and ultra-small torques. We have listed the typical applications of levitated optomechanical sensing. It can realize a force sensitivity of ~10⁻²¹ N$ /\sqrt{\mathrm{H}\mathrm{z}} $ and acceleration sensitivity of ~100 ng$ /\sqrt{\mathrm{H}\mathrm{z}} $. It also has achieved microscopic mass resolutions of 10⁻¹² gram and an electric intensity sensitivity of 1 μV/(cm·$\sqrt{\mathrm{H}\mathrm{z}} $). When the particles are optically driven to high-speed rotation, accurate torque measurements can be achieved with a sensitivity of ~10⁻²⁹N·m$ /\sqrt{\mathrm{H}\mathrm{z}} $. Conclusions and Prospects The trends of the technology are summarized and relevant suggestions are given. With the progress of its engineering, levitated optomechanical sensing is moving towards practical applications. The current levitated optomechanical sensing is developed in two routes of high-precision and integration. The former orients towards the demand for basic research, mainly using spatial optical components and pursuing lower noise floors. The latter orients towards practical applications using integrated optics and micro-nano processing. In the next step, we need to pay more attention to effective combination of levitated optomechanical sensing technology and other disciplines, and continue to strengthen the engineering practical research. We hope to achieve technical breakthroughs and practical applications of relevant sensors such as the light force accelerometers and the optomechanical gyroscopes.

Research progress of ultra-narrow-linewidth Brillouin fiber laser (invited)

Chen Mo, Wang Jianfei, Lu Yang, Hu Xiaoyang, Chen Wei, Meng Zhou

2023, 52(6): 20230131. doi: 10.3788/IRLA20230131

[Abstract](242) [FullText HTML] (58) [PDF 4452KB](89)

Significance Ultra-narrow-linewidth (~ kHz) lasers have attracted much research interest because of their wide applications in optical communications, fiber sensors, and so on. The linewidth of lasers affects the performance of the systems, such as the communication length, the minimum detection signal, and the measurement accuracy. Brillouin fiber lasers (BFLs), based on stimulated Brillouin scattering (SBS) in fibers, present Hz-scale ultra-narrow linewidth due to their intrinsic linewidth-narrowing effect. With its development in the past several decades, the compact Brillouin/erbium fiber laser (BEFL) becomes the frontier of the research on BFLs. Unlike the erbium-doped fiber lasers or laser diodes, the compact BEFL presents Hz-scale linewidth without complicated feedback loop or extremely precise isolation from the temperature and vibration variations. Besides the advantage of ultra-narrow linewidth, the BEFL simultaneously presents very stable central frequency and stable fast tuning. These outstanding performances make the compact BEFL a very ideal laser source for many applications, especially for phase-generation-carrier (PGC) interferometric fiber sensors. Progress The development of narrow-linewidth BFLs went through three stages, i.e. the early-days BFLs, the traditional BEFLs, and the compact BEFLs. The BFLs were introduced according to the three development stages. In the early days, the BFL was based on SBS in a single-mode fiber resonator (Fig.1-4). The Brillouin pump (BP) was injected in the cavity and generates SBS in the single-mode fibers. The BFL needed high pump threshold or critical pump coupled resonator due to the small Brillouin coefficient. The Brillouin/erbium fiber laser (BEFL) was then proposed to overcome the need of a pump couple resonator by introducing an erbium-doped fiber amplifier in the resonator (Fig.5). The BEFL presents low threshold and high output power (Fig.6(b)), but it needed over 100-m single-mode fibers as the Brillouin gain medium. Long cavity causes mode hopping easily. Many studies were carried out to establish single-longitudinal BEFL, such as multi-resonance-cavity BEFL (Fig.7), short-cavity BEFL based on high-nonlinearity special fibers (Fig.8), single-mode BEFL based on Brillouin pump preamplification (Fig.9). Short-cavity, low-threshold BEFLs were desirable. Until 2012, the compact BEFL was proposed based on a length of erbium-doped fiber (EDF) providing both the Brillouin gain and linear gain (Fig.10). It presented short cavity and low threshold. In 2013, an all-polarization-maintained ultra-short-ring-cavity compact BEFL was reported (Fig.11). The mechanism, characteristics, and applications of the compact BEFL were studied in the following 10 years. A series of progress has been achieved on the studies of compact BEFL. This kind of fiber laser showed 3-Hz ultra-narrow linewdith (Fig.16), stable central frequency, and stable fast tuning (Fig.26). The phase noise of the BEFL is lower than the state-of-the-art commercial laser diodes (Fig.18). The outstanding performance of the compact BEFL leads to many important potential applications, such as in high-accuracy interferometric fiber sensors (Fig.27-28) and Brillouin distributed fiber sensors (Fig.29-30). Conclusions and Prospects The optical communication and sensing systems are in great need of high-performance ultra-narrow-linewidth lasers. The BFLs, based on SBS in fibers, present Hz-scale ultra-narrow linewidth. The BFLs have already been developed to the stage of the compact BEFLs, which present ultra-narrow linewidth, stable central frequency, and stable fast tuning simultaneously. Compared with the state-of-the-art narrow-linewidth external-cavity laser diodes, the compact BEFL presents even lower phase noise. The applications in interferometric fiber sensors and distributed fiber sensors are validated for the compact BEFL. The advantages of the applications of the compact BEFLs are verified. The compact BEFL has a fully independent intellectual property, it has great significance for the localization of producing many important optoelectric information systems. The research aims to provide some reference for the study and applications of narrow-linewidth lasers in the future. It is expected that the compact BEFL will be modularized so that it could be widely used in more applications.

Review of backscattering problems in optical gyros (invited)

Tan Zhongqi, Ji Hongteng, Mao Yuanhao, Wu Geng, Jiang Xiaowei, Guan Shiyu, Chen Dingbo, Quan Yuchuan

2023, 52(6): 20230181. doi: 10.3788/IRLA20230181

[Abstract](342) [FullText HTML] (109) [PDF 4507KB](102)

Significance Optical gyro, as an indispensable part of the modern inertial navigation technology, has been widely used in aerospace, military equipment and even civil field. It is based on Sagnac effect, which induces the optical difference between two signals propagating in opposite directions within the optical path rotation. So far, optical gyro has developed from traditional laser gyro to fiber optic gyro and integrated optical gyro, which are smaller and consumes less power. In terms of working principle, optical gyro could be divided into two categories, active optical gyro and passive optical gyro. The former is essentially a laser, using the frequency beat of the opposite propagating light to derive the external rotation signal, while the latter is based on the phase or frequency difference generated by the outside laser to measure the rotation. However, both of them are necessary when considering the backscattering problems in gyro during the process of improving the working accuracy. Progress In order to better understand the backscattering mechanism in optical gyro, it could start with self-consistent equation under semi-classical theory, followed by analyzing the coupling mode of opposite propagating light wave. And the practical research of backscattering mechanism in optical gyro mainly focuses on lock-in effect and noise characteristics under macro conditions, taking corresponding methods to suppress them. For laser gyros, the lock-in effect caused by backscattering mechanism is usually solved by frequency bias. With the development of optical fiber, fiber optic gyroscope is invented. According to different working principles, it can be divided into interference fiber optic gyroscope, resonant fiber optic gyroscope and Brillouin fiber optic gyroscope. Their manifestation of backscattering is also slightly different. For the fiber optic gyroscope, it is based on the phase difference generated by the interference of the opposite propagating light wave. Therefore, the noise of backscattering disturbs the actual phase measurement. The main solution is to use the wide spectrum light source or phase modulation. For resonator fiber optic gyroscope, the noise caused by backscattering can be divided into two aspects. One is the light intensity of the scattered light wave itself, and the other is the mutual interference between the scattered light and the signal. The former will lead to the nonlinearity of the gyroscope output, while the latter results in the bias noise of the frequency measurement. The common solution is to carry out frequency modulation or phase modulation on the system, which suppresses the effect of backscattering noise. For Brillouin fiber optic gyro, it is an active resonant gyro in essence, like laser gyro. Therefore, frequency bias and phase modulation are often used to overcome the influence of lock-in effect. At present, integrated optical gyro with more fine structure and smaller volume is also a valuable research direction. However, it is identical to fiber optic gyros except for the waveguide resonator or micro resonator. Therefore, there is no great difference with fiber optic gyros in analysis methods for backscattering problems. It is worth noting that the Brillouin gyros on chip, which benefit from the reduction of stimulated Brillouin scattering threshold, can spontaneously generate Stokes light with a large frequency difference, so as to fundamentally solve the problem caused by the lock-in effect. Conclusions and Prospects Backscattering in optical gyro is a common phenomenon, which will bring in the inevitable noise or lock-in effect leading to the deterioration of the gyro performance. At present, the common research focuses on using different modulations to suppress the adverse effects of backscattering, with the purpose to improve the performance of gyro. However, there are still some unsolved problems. One is the precision measurement of backscattering in the optical gyro, and the other is to minimize the magnitude of backscattering in the optical gyro through reasonable design and processing. Both of them are beneficial to better understand the backscattering mechanism of optical gyro at the micro level, reduce the adverse effects of backscattering fundamentally, and improve the actual working efficiency of optical gyro. In addition, it is also worth thinking about how to cleverly use the characteristics of backscattering, remove its adverse aspects, and promote its application in precision measurement.

Interferometric test of coaxial folded mirrors for visible/near-infrared imaging systems (invited)

Xiong Yupeng, Lu Wenwen, Huang Cheng, Chen Fulei, Chen Shanyong

2023, 52(6): 20230175. doi: 10.3788/IRLA20230175

[Abstract](272) [FullText HTML] (71) [PDF 5602KB](49)

Objective Photoelectric imaging system serves as the “eye” of all kinds of equipment, which plays an indispensable role in scene detection and target recognition. To acquire more abundant target information, one of the development directions is multi-band fusion detection. However, the existing multi-band imaging system mostly adopts the discrete structure, with large system volume architecture, high manufacturing cost, and lack of spatial consistency due to parallax between the discrete systems. The challenges pose difficulties in image fusion and other back-end processing. Multi-band common aperture, also a common configuration, is generally used to split the front optical path with optical components, and subsequently respond to the detection requirements of different bands through the discrete rear optical path. To address these issues, coaxial folded mirrors for visible/near-infrared imaging systems are designed in this paper. Methods To guarantee the surface accuracy and relative orientation accuracy for multiple mirrors, an interferometric null test with a computer-generated hologram (CGH) is proposed (Fig.5). Diamond turning technology is applied to machining the mirrors. In this approach, two CGHs are designed for the null test of the monolithic primary/tertiary mirrors and the monolithic secondary/fourth mirrors (Fig.6, Fig.9). Ghost image of disturbance orders of diffraction is effectively separated by properly choosing the power carrier and the axial position of the CGH. A single CGH is capable of simultaneously measuring both the surface error and the relative orientation error of multiple mirrors (Fig.8). The result of the interferometric null test shows multiple mirrors are measured with nearly null fringes, indicating high accuracy in terms of surface form and orientation. Moreover, no ghost disturbance is observed. Results and Discussions The optical components undergo the diamond turning process, and the mirror blank is shared among the primary mirror and the three additional mirrors, allowing for simultaneous processing (Fig.12). After processing, a CGH is used to conduct zero compensation measurements on both mirrors (Fig.13). The measured surface shape error is shown (Fig.14), and the primary mirror and the three mirrors demonstrate a combined surface shaper error of PV 0.87λ, RMS 0.12λ; Interference diagram reveals that the ghost image stripes only exist outside the main mirror and the three mirror stripes, and they do not form interference. The primary mirror and the three mirrors reach a near-zero fringe state at the same time, indicating a high level of surface shape accuracy and mutual pose accuracy (reaching the sub-wavelength level), which meets the imaging requirements of the system. Conclusions The study proposes an interferometric null test with a CGH for the coaxial folded mirrors in visible/near-infrared imaging systems. The method involves the creation of multiple holographic regions with different functions on the same CGH substrate, which allows for the generation of the aspheric wavefronts of different shapes after the diffraction of the incident test wavefront. Consequently, the zero position of different mirror shapes can be tested at the same time. Following ultra-precision machining based on CGH compensation measurement, the mirror shape accuracy and pose accuracy attain a sub-wavelength level, which realizes direct assembly without additional assembly and adjustment for optimal imaging performance. Similarly, by positioning reference processing, multiple similar systems are nested coaxially, which enables multi-band coaxial imaging from visible light to near-infrared. Such capability holds obvious advantages for unmanned platform target detection and fast image fusion processing.

Design, simulation and implementation of direct LD pumped high-brightness fiber laser (invited)

Wang Xiaolin, Wang Peng, Wu Hanshuo, Ye Yun, Zeng Lingfa, Yang Baolai, Xi Xiaoming, Zhang Hanwei, Shi Chen, Xi Fengjie, Wang Zefeng, Han Kai, Zhou Pu, Xu Xiaojun, Chen Jinbao

2023, 52(6): 20230242. doi: 10.3788/IRLA20230242

[Abstract](1077) [FullText HTML] (90) [PDF 16977KB](136)

Significance Direct LD pumped high power fiber lasers have the advantages of low cost, high conversion efficiency, and good beam quality, finding applications as diverse as industrial processing, medical treatment and fundamental research, etc. The brightness, which is determined by the output power and beam quality, is a crucial parameter of fiber lasers that affects the application effectiveness. However, the power scaling of high-brightness fiber lasers is mainly constrained by the nonlinear effects and the transverse mode instability (TMI). It is noteworthy that IPG Photonics announced the 10 kW and 20 kW single-mode fiber laser in 2009 and 2013, respectively, but no domestic fiber laser products with > 6 kW output power and beam quality factor M²<2.0 are commercially available until now (Mar.2023). There are multiple reasons for the huge gap between the domestic high-brightness fiber laser industry and its foreign counterparts. In addition to the late start time and the less advanced material processing technology, the lack of theoretical guidance and original theoretical-based solutions make it difficult to overcome the technical bottleneck for power scaling of high-brightness fiber lasers, and the lack of fiber laser simulation software also slowed down the process from fundamental research and laboratory demonstration to industrial products. Therefore, it is of significant necessity to develop fiber laser software to aid the fiber laser design and accelerate the process from simulation results to industrial products. Progress To overcome the aforementioned difficulties, researchers from National University of Defense Technology have conducted fundamental theoretical research and developed fiber laser simulation software SeeFiberLaser with independent intellectual property rights. The software can simulate the generation, amplification and transmission of fiber lasers with different time domain characteristics and effects such as amplified spontaneous radiation, stimulated Raman scattering (SRS), stimulated Brillouin scattering and transverse mode competition can be considered in the simulation. The simulation results can output data of the power, spectrum, spot pattern, and time domain as required, which greatly helps the study of fiber laser theory, engineering design and scientific research. With the aim of suppressing SRS and TMI, systematic solutions are proposed, including exploiting the backward pump scheme, optimizing the pump wavelength, employing spindle-shaped fiber design, etc., which are proven effective to improve the performance of fiber lasers through theoretical simulation. Then, industrial fiber oscillators are simulated and optimized based on the SeeFiberLaser software by studying the effects of the active fiber length, operating wavelength, the reflectivity of the output coupling fiber Bragg grating, and the pump wavelength. Furthermore, high-brightness fiber amplifiers that are capable of delivering 8-10 kW output power are simulated and optimized based on the SeeFiberLaser software by considering the active fiber's length and pump absorption coefficient, the backward pump power as well as the core diameter and length of the quartz block head. Furthermore, experimental studies are carried out to verify the effectiveness of the above-mentioned theoretical solutions, including optimizing the pump wavelength as well as employing backward pumping for improved TMI threshold, and exploiting spindle-shaped fiber for SRS and TMI mitigation. Moreover, a 6 kW oscillating-amplifying high-brightness fiber laser based on pump wavelength optimization, 7 kW backward pumped high-brightness fiber laser, and 10 kW fiber laser based on fiber with a small core-to-cladding ratio are experimentally demonstrated, fully proving the functionality and great potential of the proposed solutions. Last but not least, the technical schemes of higher brightness fiber laser are prospected, which include employing integrated multifunctional passive devices on a single piece of passive fiber, adopting ytterbium-doped and energy transfer integrated fiber, and exploiting gain-resonator integrated design scheme, facilitating better good beam quality and improved stability. Conclusions and Prospects Power scaling of high-brightness fiber laser is a complex yet challenging work, which requires comprehensive investigation and optimization. This paper analyzes the impact of various factors on the laser in the fiber laser design process and proposes methods, such as variable core diameter fiber, optimized pump wavelength, etc., to improve the performance of fiber lasers. Noteworthily, the SeeFiberLaser software is developed to bridge the fundamental research outcomes of fiber laser technology and industrial products, which were proven quite effective in high-power fiber laser optimization for both industrial and research use. In addition, 6-10 kW high-brightness fiber lasers have been experimentally demonstrated based on the proposed optimization solutions. Looking forward, higher-brightness robust fiber laser could be expected by developing integrated multifunctional passive devices, ytterbium-doped and energy transfer integrated fiber design, and gain-resonator integrated design scheme.

Optimization of quantum-enhanced receiving method for weak optical signal (invited)

Dong Chen, Guo Chang, Wu Tianyi, Ran Yang, Dang Kezheng, Li Fuquan, Zhou Zichao

2023, 52(6): 20230189. doi: 10.3788/IRLA20230189

[Abstract](170) [FullText HTML] (47) [PDF 2068KB](44)

　　Under the framework of classical theory, the performance of coherence detection is limited by the standard quantum limit (SQL) corresponding to the shot noise. However, the quantum-enhanced receiving technology can break the SQL and approach the Helstrom limit by introducing the displacement operation and converting the classical measurement into the measurement of photon number states. Unambiguous state discrimination (USD) is one of the commonly used discrimination strategies for quantum-enhanced reception. However, due to the limited energy of weak signals, the traditional USD quantum-enhanced receiving method has a high error rate of weak signal recognition. A hybrid measurement scheme for quadrature phase-shift-keying (QPSK) coherent states is developed. The scheme firstly converts the discrimination of QPSK coherent states into the distinction of BPSK coherent states by homodyne detector (HD), and then realizes the unambiguous discrimination of coherent states by BPSK quantum-enhanced receiving measurement. The simulation results show that the hybrid scheme is superior to the classical measurement scheme in the average photon number between 3.2 and 11.3, and has a larger signal range than the traditional QPSK quantum-enhanced receiving scheme. Objective Coherent states are a critical carrier in optic communication and quantum information processing due to their intrinsic resilience to the loss of coherence. Unambiguous state discrimination (USD), which aims to realize the error-free discrimination of coherent states by outputting "no result" for the finite ambiguous results, is especially essential in quantum key distribution and quantum digital signatures. Recently, Becerra first experimentally demonstrated a generalized quantum measurement for USD of four non-orthogonal coherent states with a displacement operator and single-photon detector (SPD). As the unambiguously correct probability is still significantly low, Ref. introduces an adaptive feedback strategy and presents an adaptive generalized measurement scheme for USD of QPSK coherent states. However, the adaptive measurement scheme still needs to be improved to realize better performance. Thus, it is crucial to develop new schemes that can unambiguously discriminate coherent states with performance surpassing the ideal heterodyne strategy. This paper presents a new hybrid scheme that unites a homodyne detector (HD) and a quantum measurement scheme for USD of QPSK coherent states. Methods To realize the unambiguous state discrimination, this paper presents a new hybrid measurement scheme based on these (Fig.1). The scheme consists of two successive measurements toward the coherent states. The first measurement is conducted by a homodyne detector, which can exclude half of the four possible states of the QPSK coherent states. The result of the first measurement gives a feed-forward to the second measurement. And the second measurement is conducted by a quantum measurement scheme and finally discriminates the signal states. The received QPSK coherent state $\left|{\alpha }_{m}\right\rangle=\left|\alpha \right|{{\rm{e}}}^{i\left(m-1/2\right)\pi /2}, {m}=\mathrm{1,2},\mathrm{3,4}$ is first divided by a beam splitter (BS) with transmittance $ T $ and reflectivity $ R $. To simplify the calculation, we use $ {t}^{2}=T $ and $ {r}^{2}=R $ to denote the transmittance and reflectivity, respectively. The transmitted part $\left|{\alpha }_{Tm}\right\rangle=\left|t\alpha \right|{{\rm{e}}}^{i\left(m-1/2\right)\pi /2}$ and reflected part $\left|{\alpha }_{Rm}\right\rangle=\left|r\alpha \right|{{\rm{e}}}^{i\left(m-1/2\right)\pi /2}$ of the received signal state are respectively output to the quantum measurement stage and HD stage. We can change the partitional ratio $ {R}_{Q,H}={t}^{2}/\left({r}^{2}+{t}^{2}\right)\approx {t}^{2} $ by selecting the appropriate beam splitter. Results and Discussions The $ M=1+2 $ hybrid scheme can realize a lower error ratio than the heterodyne strategy when achieving the same correct unambiguous results probability for coherent states with mean photon number $ 4.2\leqslant {\left|\alpha \right|}^{2}\leqslant 12.6 $(Fig.8). And $ M=4 $ quantum measurement scheme can only beat the heterodyne strategy for coherent state with mean photon number $ {\left|\alpha \right|}^{2}\leqslant7.41 $ while the $ M=1+3 $ hybrid scheme can realize it with mean photon number $ 3.2\leqslant {\left|\alpha \right|}^{2}\leqslant 11.3 $(Fig.10). Furthermore, the hybrid scheme has a lower error ratio for coherent state with a mean photon number more than 6.7 compared with the quantum measurement scheme. This phenomenon notes that the hybrid scheme has a more excellent application range than the quantum scheme. Conclusions A hybrid measurement scheme with an adaptive feedback strategy is proposed to unambiguously discriminate the QPSK coherent states, which converts the discrimination of four coherent states to two coherent states by HD and conducts the final discrimination by quantum measurement scheme. Here, we fully consider the non-ideal factors in the practical implementations, such as detection efficiency and dark count rate of detectors, visibility and transmittance of displacements, and develop the model of the hybrid measurement scheme. The simulation results clearly show that the hybrid scheme with $ M=1+2 $ can beat the heterodyne strategy for coherent states with $ 4.2 \leqslant {\left|\alpha \right|}^{2} \leqslant 12.6 $. Furthermore, compared with the quantum measurement scheme, the hybrid scheme with $ M=1+3 $ has a higher probability of correct unambiguous results and a lower error ratio for coherent states with $ 6.7\leqslant {\left|\alpha \right|}^{2} $. The hybrid scheme uses HD with less energy to convert a complex four-state discrimination problem to a simple two-state discrimination problem, which improves the probability of obtaining an unambiguous conclusion. However, this scheme is also limited by the HD and quantum measurements and can only achieve better performance within a specific range.

Mid-infrared BaGa₄Se₇ optical parametric oscillator with high conversion efficiency (invited)

Bian Jintian, Kong Hui, Ye Qing, Yao Jiyong, Lv Guorui, Xu Haiping, Zhou Quan, Wen Kaihua

2023, 52(6): 20230178. doi: 10.3788/IRLA20230178

[Abstract](200) [FullText HTML] (67) [PDF 2085KB](35)

　　The LiB₃O₅(LBO) was inserted into the branch of the L-shaped BaGa₄Se₇(BGSe) optical parametric oscillator (OPO) to improve the conversion efficiency for the first time. When the pump laser energy is 115 mJ (1.06 μm), the idler light (3.5 μm) energy was 16.18 mJ, corresponding to the conversion efficiency of 14.06%, and the slope efficiency was 18.4%, which was the highest conversion efficiency of BGSe OPO pumped by 1 μm laser. The signal, idler, and pump wave waveform in BGSe L OPO cavity with and without LBO crystals was simulated, and the output waveform of idler light was given. Compared with traditional OPO cavities, L-type OPO cavities (with frequency doubling crystals) suppress the inverse conversion under high-energy pumping conditions, achieving higher idle frequency light conversion efficiency. Objective The mid-infrared (IR) coherent sources in the 3-5 μm have always been intensively demanded for a wide range of scientific and technological applications in remote sensing, spectrum analysis, materials diagnostics, aerospace fields, etc. Optical parametric oscillation is an attractive approach, especially when high energy and average power are demanded simultaneously. However, there is reverse conversion in the OPO cavity.When the pump energy is high, the signal light and idle frequency light generated are also strong. At this time, the signal light and idle frequency light will be converted to the pump light, which seriously affects the conversion efficiency of OPO. In addition, due to the high intensity of signal light in the cavity during the reverse conversion, it is easy to damage the nonlinear crystal or its coating.Therefore, how to suppress reverse conversion in the OPO cavity under high-energy pumping conditions and improve the conversion efficiency of OPO has always been the focus of research. Methods To suppress the inverse conversion in the OPO cavity, we proposed a method of inserting a frequency doubling crystal into the L-type OPO cavity to suppress the signal light intensity (Fig.1). All three mirrors of the L-shaped cavity are coated with a high-reflection coating for the signal laser, and crystals are inserted in the L-branch to achieve intracavity frequency doubling of the signal laser. When the energy density of the signal laser in the OPO cavity is high, the signal laser is converted into red light by the frequency doubling crystal and output from the L branch. At the same time, the signal laser is attenuated, reverse conversion is suppressed, and the efficiency of idle laser conversion is improved. Results and Discussions The idler laser energy was 16.18 mJ at a pump energy of 115 mJ, corresponding to an optical-to-optical conversion efficiency of 14.06% and a slope efficiency of 18.4% (Fig.2). It is the highest conversion efficiency for BaGa₄Se₇ (BGSe) OPO pumped by a 1.06 μm laser, to the best of our knowledge. The energy density of the three waves at the output of the OPO cavity is simulated. The simulation results show that the optical-to-optical conversion efficiency of the idler laser with the LiB₃O₅ (LBO) inserted in the cavity is 1.20 times higher than that without LBO in the cavity at a pump energy of 80 mJ (Fig.3). The OPO output wavelength could be tuned by adjusting the angle of the BGSe crystal (Fig.5). When the θ angle of the crystal is changed, the experimental peak wavelength agrees well with the theoretical simulation curve, and the measured $ \Delta {{\lambda }}_{2}/\Delta \mathrm{\theta } $ is −231.81 nm/(°) . When changing the φ angle of the crystal, the measured $ \Delta {{\lambda }}_{2}/\Delta {\varphi} $ of −6.25 nm/(°) deviates from the theoretical value of −1.25 nm/(°) because the incident direction of the pump laser is difficult to exactly coincide with the θ=56.3° line of BGSe. Conclusions The conversion efficiency of idler light in OPO cavity was improved by inserting a signal laser frequency doubling crystal into the L-shaped OPO cavity for the first time. When the pump energy is 115 mJ, the 16.18 mJ of the idler laser energy was obtained in BGSe OPO. The optical-to-optical conversion efficiency was 14.06%, and the slope efficiency was 18.4%, which is the highest conversion efficiency of BGSe OPO pumped by a 1.06 μm laser. The output wavelength of BGSe OPO with high conversion efficiency can also be tuned.

Research progress of laser protection technology for optoelectronic imaging system (invited)

Li Yangliang, Ye Qing, Wu Yunlong, Sun Ke, Zhang Hao, Sun Xiaoquan

2023, 52(6): 20230192. doi: 10.3788/IRLA20230192

[Abstract](357) [FullText HTML] (69) [PDF 3008KB](106)

Significance Optoelectronic imaging systems, characterized by their compact size, light weight, high reliability, resolution, and dynamic range, have been extensively employed in various fields, such as medical imaging, media production, security management, high-resolution target reconnaissance, precision guidance, fire control and targeting, and flight assistance. However, with the rapid advancements in laser technology and the widespread use of laser weapon systems, the risk of optoelectronic imaging systems being blinded or dazzled by lasers has significantly increased, resulting in a substantial decrease in information acquisition capabilities. Consequently, investigating laser protection technologies for optoelectronic imaging systems has become increasingly vital. Progress The article initially provides a brief overview of the mechanisms and limitations of laser blinding protection technologies for optoelectronic imaging systems, focusing on linear and nonlinear materials. It then delves into laser blinding protection technologies employing phase-change materials, such as vanadium dioxide, discusses their mechanisms, fabrication methods, and application progress. Subsequently, the article explores the mechanisms and preliminary application studies of laser blinding protection technologies based on computational imaging, highlights the necessity and feasibility of researching laser dazzling protection technologies for optoelectronic imaging systems in relation to laser blinding. Finally, the advantages and disadvantages of various laser protection technologies for optoelectronic imaging systems are summarized, along with potential future development directions. Conclusions and Prospects The application of computational imaging technology for laser protection offers a groundbreaking technical solution, featuring a wide protective spectrum and exceptional adaptability. This approach eliminates the need for prior knowledge of interfering laser locations, wavelengths, or polarization states, as required by linear material protection, as well as considerations of response times and protection thresholds, as demanded by nonlinear or phase-change material protection. Computational imaging technology can defend against common continuous lasers, nanosecond pulse lasers, and emerging ultra-short pulse lasers, such as picosecond or femtosecond pulses. Designing and fabricating high-precision optical field control components and ensuring high-quality image restoration are crucial future development directions for this technology. As lensless imaging technology employing mask modulation, a key research area in computational imaging progressively matures, it may fundamentally resolve the high gain caused by the optical system structure in imaging systems, thereby effectively addressing the issue of laser blinding protection in such systems. Laser dazzling protection technology exhibits broader application scenarios compared to blinding protection technology; However, current research is relatively limited, and no groundbreaking solutions have been proposed. Based on the mechanisms of laser-induced blinding and dazzling in optoelectronic imaging systems, the seperate study on blinding and dazzling technologies is incomplete and unscientific. Future research should focus on integrating laser blinding and dazzling protection for optoelectronic imaging systems, examining protection mechanisms, technical approaches, and cost-effectiveness from multiple perspectives.

Review of laser Doppler velocimeter technology for navigation and localization

Chen Lanjian, Xi Chongbin, Zhou Jian, Nie Xiaoming, Wang Qi, Huang Rong, Xiang Zhiyi, Jin Shilong

2023, 52(6): 20230143. doi: 10.3788/IRLA20230143

[Abstract](199) [FullText HTML] (81) [PDF 2433KB](41)

Significance The technology known as the laser Doppler velocimeter (LDV) has gained widespread application in both scientific research and industrial production, following years of development. This technology offers an independent means of measuring velocity, which is especially useful for the navigation and localization of vehicles. Compared to traditional velocity measuring methods such as odometers, accelerometers, and global navigation satellite systems (GNSS), LDVs have high accuracy and reliability even in all-day, all-weather conditions. These features satisfy the requirement for precise navigation and localization, thus getting the focus of attention of the researchers in this field. In order to anticipate future progress, it is essential to review the current research. Progress The frequency shift of the Doppler effect in the probe beam of LDVs depends on the velocity of the vehicle which is based on the optical Doppler effect. When LDVs are installed on vehicles, the angle between the probe beam and the road surface remains constant. By measuring the frequency shift, it is possible to determine the velocity of the vehicles with accuracy. LDVs can be classified into two types based on their optical structure of dual-beam and single-beam. For the dual-beam type, two probe beams intersect each other, and the point of intersection is referred to as the control volume. When traveling over a bumpy road, dual-beam LDVs often lose the signal due to the limited depth of the control volume. The researcher has proposed a multipoint layer-type LDV to address the limitations of dual-beam LDVs. This type of LDV consists of multiple dual-beam probes that are distributed in the vertical direction. Each probe's small depth of field is combined to form a larger depth of field. For the single-beam type, there is only one beam containing the Doppler frequency shift during the measurement, which is different from the dual-beam LDV. The reflected light of the probe beam is transmitted back to the detector after illuminating the uneven road surface and is mixed with the reference beam without any Doppler frequency shift. The single-beam LDV has a broad depth of field, ensuring accurate and stable measurements. It is particularly suitable for use in ground vehicles compared to the dual-beam LDV. Improvements have been made for single-beam LDV to enhance its performance. The reuse-type single-beam LDV utilizes part of the reference beam power to illuminate the road surface, which would otherwise be wasted by an attenuator in the traditional single-beam LDV. The Janus configuration single-beam LDV eliminates the effect of vertical velocity on the velocity parallel to the direction of the vehicle's heading when ground vehicles experience vertical jolts. The speed component in the direction of the vehicle's heading cannot accurately reflect the actual state of a moving vehicle, as there are two components in the vertical and lateral directions. To deal with this, two-dimension (2D) and three-dimension (3D) LDVs are investigated by researchers. There are two probes in 2D LDV and four probes in 3D LDV. Each probe in these two multi-dimensional LDVs can be either dual-beam or single-beam type. 　　For years, researchers have been investigating the use of LDVs in the navigation and localization of ground vehicles. By integrating LDVs with inertial measurement units (IMUs), ground vehicles can achieve significantly improved localization and navigation precision. In particular, the highly accurate velocity data provided by LDVs can effectively suppress the measurement divergence of IMUs. As a result, the accuracy of dead reckoning has already reached an impressive 0.01%. However, to fully explore the potential of LDV technology for new applications such as high-speed trains, underwater vehicles, and aerial vehicles, further research is necessary. This will require optimization of LDV technology in terms of optical design, circuit design, and system architecture. Conclusions and Prospects This paper reviews the current research on LDVs for the navigation and localization of ground vehicles. It demonstrates the process of development and concludes that LDV plays a significant role in achieving precise navigation and localization of vehicles. Additionally, the paper provides an outlook on the development trend of LDV and its potential applications for high-speed trains, underwater vehicles, and aerial vehicles. The perspectives provided here can serve as a guide for future LDV research.

Rydberg atomic radio-optical measurement and spectrum processing techniques (invited)

Wu Jinyun, Yang Jian, Gao Weichao, Zhang Yinfa

2023, 52(6): 20230264. doi: 10.3788/IRLA20230264

[Abstract](353) [FullText HTML] (162) [PDF 12389KB](78)

Significance Rydberg atoms are highly excited atoms with large electric dipole moments. The energy difference between adjacent levels can cover an ultra-wide frequency spectrum range from DC to THz, making it possible to achieve high-sensitivity and ultra-wideband reception of electromagnetic fields. Radio-optical measurements based on Rydberg atoms are achieved by precisely controlling two laser beams, the probe laser and the control laser, to transform ground state alkali metal atoms into Rydberg atoms and induce Electromagnetic Induced Transparency (EIT) in the transmitted spectrum of the probe laser. Under the interaction of the input radio signal, Autler-Townes (AT) splitting occurs in the transparent EIT spectrum, completing the conversion of radio signals to optical signals (Fig.2-3), thereby extracting information such as frequency, amplitude, and phase of the radio signal. This technology has attracted great attention in electronic information fields such as electric field metrology, electromagnetic spectrum detection, communication, and radar in recent years. The physical implementation of this technology is simple and does not require strict physical conditions as usual quantum technologies such as single-photon sources or ultra-cold and superconducting conditions. It can be achieved at room temperature without being limited by the level of production technology. It is considered one of the fastest applicable quantum technologies with its high stability, accuracy, and repeatability that could partially replace existing radio reception technologies in the near future. Progress In the past decade, researchers have made significant progress in the study of radio-optical measurement techniques based on Rydberg atoms, from precise measurements of single-frequency static radio signals in electric field metrology applications to real-time reception of single-frequency dynamic radio signals in communication applications, and to spectrum detection and communication reception of complex multi-frequency radio signals. The key to this technology is how to quickly and accurately extract information about the radio signal from the output EIT spectrum of the atomic system. Different types of radio signals, such as static, dynamic, single-frequency, and multi-frequency radio signals, require different information extraction and spectral processing methods, as well as different experimental designs and implementations. For single-frequency static radio signals, researchers have already used Rydberg atoms in experiments to measure field strengths in the 0-320 GHz frequency range with a maximum coverage range of 780 pV·cm⁻¹ to 50 V·cm⁻¹. By using heterodyne technology (Fig.8) and critical phenomena in many-body Rydberg atomic system, the current sensitivity can reach as low as 49 nV·cm⁻¹·Hz^−1/2. Unlike measuring single-frequency static radio signals, for single-frequency dynamic radio signals, Rydberg atom systems are required to track and respond to rapidly changing radio signals in real-time and quickly read EIT spectral changes at the end point. Its primary application scenario is the communication reception. Since 2018, a large number of verification experiments on wireless communication reception principle have been carried out based on Rydberg atoms. This technology can directly convert intermediate frequency or baseband signals on the carrier into optical signals for direct demodulation. Verified communication methods include amplitude modulation (AM), frequency modulation (FM), and phase modulation (PM). When the input wireless signal becomes complex, especially for multi-frequency wireless signal input, the output EIT spectrum of the probe laser will become complicated. It will be a big challenge to quickly and effectively read and distinguish information from different frequency wireless signals. Currently, a small amount of research is focused on verifying dynamic multi-frequency wireless signals for communication reception, including using multi-harmonic Rydberg atomic level structures to finely adjust and optimize system parameters at the front end or using post-processing techniques such as deep learning at the back end to achieve recognition and reading of multiple frequency information. People have experimentally achieved simultaneous reception of five completely different frequency signals within a spectrum range of over 100 GHz (Fig.18) or 20 similar frequency signals within a range of 100 kHz (Fig.23). Conclusions and Prospects Through continuous research over the past 10 years, it has been experimentally verified that radio-optical measurements based on Rydberg atoms have unique quantum advantages in spectrum range, sensitivity, minimum field strength, signal demodulation mechanism, and other aspects. This technology has demonstrated promising prospects in applications such as electric field metrology, electromagnetic spectrum detection, communication, radar, and more. In order to further develop this technology to fully leverage the unique quantum advantages of Rydberg atoms and achieve practical applications as soon as possible, researchers need to deepen their research on the comprehensive performance improvement, anti-interference ability enhancement, miniaturization integration and simultaneously reduce the costs of radio-optical measurements based on Rydberg atoms.

Research progress of infrared stealth technology of micro-nano optical structure (invited)

Jiang Xinpeng, Du Te, Ma Hansi, Zhang Zhaojian, He Xin, Zhang Zhenfu, Chen Huan, Yu Yang, Huang Sha, Yang Junbo

2023, 52(6): 20230197. doi: 10.3788/IRLA20230197

[Abstract](635) [FullText HTML] (169) [PDF 4729KB](228)

Significance With the diversified development of infrared detection technology, the demand for infrared stealth technology is increasingly urgent. Infrared stealth technology aims to effectively control the infrared signature signals of weapons and equipment, reduce the operating range of enemy infrared detection systems, improve survival ability, penetration ability, and combat effectiveness. However, traditional infrared stealth technology is facing serious challenges of multi-band detection and multi-functional compatibility, making it of great significance to study the infrared stealth technology of micro-nano optical structure. Sub-wavelength micro-nano optical structures based on local resonance mechanism greatly enrich the modulation of light transmission behavior. They can be state-of-the-art in material and structure design for infrared radiation, so as to meet the demand of ideal emissivity spectrum for infrared stealth. It is foreseeable that infrared stealth technology based on the optical micro-nano structures will transform traditional infrared stealth technology and provide point-to-point spectral design for the multi-aspect demand, which makes the research progress of micro-nano optical structure infrared stealth technology meaningful. Progress Focusing on the progress of infrared stealth technology, this paper introduces the basic principles of thin film absorber, metal surface plasmonic, tunable absorber based on phase change materials (PCMs), and intelligent design for spectral response firstly. For example, the PCMs are widely used in tunable infrared absorbers by regulating resonance wavelength and changing infrared emissivity without the structural changes (Fig.4). And the innovatory field of the intelligent design has recently been transforming conventional micro-nano optical structure and allowing for the discovery of unorthodox optical structures via computer algorithms rather than engineered "by hand" (Fig.5-6). Secondly, the development status of optical micro-nano structure infrared stealth technology in the past decade is introduced. As an application technology driven by demand, infrared stealth technology faces many challenges such as multi-spectral compatibility, multi-functional integration, and complex changing environments. With the deepening optical micro-nano structures research, the application of infrared stealth technology has been expanding and showing the following new characteristics. (1) In order to deal with the thermal radiation detection of infrared atmospheric transparent window bands, infrared stealth technology is developing from single-band towards the multi-band infrared stealth; (2) On the basis of multi-band infrared stealth, balance and comprehensive design with infrared laser, visible light, radar and other multispectral stealth are required; (3) A new infrared stealth technology is developed that combines multiple functions such as thermal management, infrared sensor, and radiation regulation; (4) Adaptive infrared stealth technology is developed that integrates new materials such as phase change materials, two-dimensional materials of graphene, and vanadium oxides. To this end, the spectral tailoring design achieved through optical micro-nano structures endows infrared stealth technology with more new possibilities, including multispectral infrared stealth technology, multifunctional infrared stealth technology, and adaptive infrared stealth technology. According to the different requirements of multi-spectral compatibility, this paper summarizes the possible requirement of multispectral infrared stealth technology and current development status (Fig.7). Through the comprehensive survey, this paper reveals four development trends of micro-nano optical structure infrared stealth technology: multispectral compatibility, multi-function integration, large-area fabrication, and adaptive infrared stealth system. Conclusions and Prospects During the last decade, the micro-nano optical structure infrared stealth technology has made dramatic development. The multispectral infrared stealth technology has been extended from the single-band to the multi-band infrared stealth technology which is compatible with visible band, laser, and microwave. The multifunctional infrared stealth technology has been considered to integrate the thermal management, infrared encryption and infrared sensor, the adaptive infrared stealth technology has been widely studied in the dual-band infrared transparent atmosphere window by flourishing research of smart materials. The research of this paper aims to provide some reference for the infrared stealth technology of optical micro-nano structures in the future. It is expected that optical micro-nano structures will provide a promising way for the more multispectral, more versatile, and more adaptive infrared stealth technology.

Research progress of aerosol particle aggregation model (invited)

Gu Youlin, Zhang Xi, Hu Yihua, Meng Fanhao, Chen Guolong, Ding Wanying, He Haihao

2023, 52(6): 20230243. doi: 10.3788/IRLA20230243

[Abstract](176) [FullText HTML] (67) [PDF 1753KB](31)

Significance Aerosol particles refer to the solid, liquid or solid-liquid mixture particles suspended in the atmosphere caused by naturally formed or artificial factors, which are composed of metal powder, bioparticle, dust particle and so on. The analysis of the spatial structure and motion status of aerosol particles is of great significance for ecological environmental protection, climate change control and functional materials development. As an important means to study the spatial structure and motion status of aerosol particles, the aggregation model of aerosol particles is used to simulate the aggregated procedure of aerosol particles under different conditions, which is employed to explain its aggregation and physical mechanism. Aggregation models are widely used in optics, astronomical physics, dynamics and so on. The investigation on the mechanism, influential factors and application of aerosol particles aggregation model is beneficial to the optimization of aerosol particles aggregation model and the preparation of novel extinction materials. Progress Firstly, the mechanism and main influential factors of aerosol particle aggregations are analyzed, such as particle spatial structure characteristics, the interaction of particles, and atmospheric environment. The theory of fractal, mathematical statistics and molecular simulation used in the study of aerosol particle aggregation are summarized. Secondly, based on the implementation methods of aerosol particle aggregation simulation, the classifications and characteristics of aggregation models are described. In addition, the application of the aerosol particle aggregation model in optics, astrophysics, and dynamics is mentioned. The model optimization such as algorithm efficiency, simulation modes, and application errors reduction is analyzed. Finally, in view of the current application status and challenges faced by aerosol particle aggregation models, the trend of aggregation models is proposed, such as the construction of a non-spherical particle aggregation model, application of multi-factor coupled aggregation model and simulation of real-time spatial distribution of aggregation particles, etc. Conclusions and Prospects In recent years, the aerosol particle aggregation models have been used in a variety of areas. The aerosol particle aggregation model can be used to simulate the visual procedure of particle aggregation, study the formation mechanism and aggregation dynamics of particles, and carry out an in-depth analysis of aggregation characteristics. The aerosol particle aggregation model is important to analyze dynamics, morphology and other properties of aggregates. These models can be used to explain the phenomena such as gas mixture explosions and comet polarization. It can also provide a means for the screening and controllable preparation of extinction materials. However, there are still some shortcomings. Firstly, the complex morphology and structures of non-spherical particles are an important part of simulating and analyzing more realistic aerosol particle applications. The aggregation model of randomly oriented non-spherical aerosol particles with controllable particle shape and size has not yet been established. Secondly, the analysis of the aggregation mechanism of the model is relatively simple. To improve the accuracy of the simulation, the influence of multiple factors on the aggregation procedure needs to be considered. In addition, the simulation of real-time spatial distribution of the particles is acquired to further investigate in future. Therefore, the model can be optimized as followings. On the one hand, the aggregation models for of randomly oriented non-spherical aerosol particles can be analytically established. On the other hand, it can be revised in terms of multi-factor coupling and real-time spatial distribution of particles.

Research progress of low-quantum-defect fiber laser at 1 μm band (invited)

Xu Jiangming, Zhang Yang, Ma Xiaoya, Ye Jun, Ke Yanzhao, Li Sicheng, Liang Junrui, He Junhong, Huang Liangjin, Pan Zhiyong, Yao Tianfu, Leng Jinyong, Zhou Pu

2023, 52(6): 20230267. doi: 10.3788/IRLA20230267

[Abstract](152) [FullText HTML] (46) [PDF 3434KB](69)

Significance Power scalability of fiber lasers have attracted a great deal of attention for its remarkable features, such as excellent beam quality, high conversion efficiency, flexible operation, and wide applications in biomedicine, intelligent manufacturing, energy exploration, defense and security. However, there have been no reports of significant operation power breakthroughs of near-single-mode fiber laser since the first demonstration of 10 kW-level system in 2009. The wasted heat accumulation, which can induce thermal lens and transverse mode instability effects, is one of the most important limitation factors. Quantum defect, defined as (1-λp/λs), where λp is the pump wavelength and λs is the lasing wavelength, has always been a key parameter in high-power fiber lasers. High quantum defect not only limits the conversion efficiency but also increases the thermal load in fiber lasers. In hence, much research on low quantum defect fiber laser has been reported in the past decades. Progress This paper first introduces the performance exploration of high-power fiber laser at 1 μm band, including the power scaling and corresponding quantum defect decrease. It can be said that the power scaling progress of fiber lasers is also a continuous struggle against waste heat and other factors. As to low quantum defect fiber laser, the reported works mainly focus on two different technical schemes based on rare earth doped fiber and passive fiber. For the convenience of description, this article stipulates that the quantum defect of low quantum defect fiber lasers is ≤ 4.50%, and the quantum defect of ultra-low quantum defect fiber lasers is ≤ 1%. 　　Then, ytterbium-doped fiber lasers with low quantum defect are summarized. In 2011, Wirth et al. demonstrated a 2.9 kW fiber laser operating at 1071 nm that is tandem-pumped by a 1030 nm thin-disk laser, and the corresponding quantum defect is about 3.83% (Fig.1). In 2014, Chang et al. presented a fiber laser with a maximal output power of 5.7 W and a quantum defect of 1.9% (Fig.2). To further reduce the quantum defect of fiber lasers, some specially designed active fibers and high pumping intensity methods are adopted. For example, in 2018, Yu et al. demonstrated a 400 mW-level fiber lasers with less than 1% quantum defect via ytterbium-doped multicomponent fluorosilicate fibers (Fig.5). 　　Additionally, Raman fiber lasers with low quantum defect are reviewed. Based on common silicon fiber, a maximal power of 3 kW-level with a quantum defect of 4.42% (Fig.7) and a maximal power of 6.2 W with a quantum defect of 0.56% were achieved. To further improve the operation power of ultra-low quantum defect fiber laser, the scheme enabled by boson peak in phosphosilicate fiber was presented and realized by Zhang et al. in 2020 (Fig.14). In 2021, Ma et al. demonstrated a 100 W-level ultra-low quantum defect fiber laser with a quantum defect of 0.97%. What's more, cladding pump scheme was also been validated (Fig.15). Conclusions and Prospects The important progress of low quantum defect fiber laser operating at 1 μm band is reviewed. And the manuscript mainly focuses on two different technical schemes based on rare earth doped fiber and passive fiber. In rare earth doped fiber based lasers, the utilization of tandem-pumping, multi-component doping and strong pumping schemes can reduce the quantum defect, and the related ytterbium-doped fiber lasers with quantum defect ≤1% have achieved 400 mW-level output power. In Raman fiber lasers, the maximal output power of 100 W-level with a quantum defect of ≤1% has been demonstrated with the aid of techniques such as special doping, pump spectrum regulation, and gain competition suppression. The feasibility of cladding pumping scheme has also been verified successfully, indicating its significant potential in achieving high-power and low quantum loss output.

Lidar point cloud expansion and identification method for masking targets based on time-spectra information

Xu Shilong, Xia Yuhao, Dong Jiajie, Qian Qishu

2023, 52(6): 20230213. doi: 10.3788/IRLA20230213

[Abstract](97) [FullText HTML] (15) [PDF 1721KB](25)

Objective With rich data and wide scanning range, three-dimensional lidar is widely used in obstacle detection, terrain reconstruction, target detection, classification and tracking, as well as forestry and agricultural remote sensing. When using point cloud data for object description, people usually describe the details of objects through point cloud interpolation, and distinguish the types of objects through point cloud classification identifiers. The above functions can be achieved with high spectral full waveform lidar through waveform decomposition and spectral reconstruction. However, when there are multiple targets in the laser beam forming a shielding relationship, due to the close spacing and light spot splitting, it is difficult to accurately obtain target time-spectral information to reverse the target position and reflectivity distribution. For this purpose, we design a hyperspectral waveform decomposition method and corresponding point cloud expansion and identification method. Methods Considering the spatial correlation and shape characteristics of the echo waveforms of various spectral channels, a new hyperspectral full waveform lidar waveform data processing method is proposed (Fig.1). Based on the prior knowledge obtained from multi-channel echo waveform comparison, inter channel correction is used to further improve the extraction accuracy of multi-target waveform components. Based on this new waveform decomposition method, the time-spectral information of the target can be accurately obtained. Accordingly, a point cloud extension and point cloud identification methods based on principal component analysis and random forest algorithm are proposed (Fig.2). Results and Discussions With a full-waveform hyperspectral lidar, camouflage nets, and two diffuse reflective plates with known reflectivity, point cloud expansion and identification verification experiments were conducted. The experiment broke through the range resolution limited by pulse width under dense shielding conditions, resulting in a triple expansion of point cloud data on the target board. The target reflectance spectrum recovered by the proposed algorithm has a high similarity to the actual reflectance spectrum of the target, and the point cloud identification result perfectly distinguishes two target plates at the same distance. Conclusions A new hyperspectral waveform decomposition scheme is proposed to solve the problems encountered in the generation of three-dimensional point clouds for hyperspectral full waveform lidar during the detection of obscured targets, such as the difficulty in resolving close range targets, and the inaccurate acquisition of spectral information caused by light spot splitting. Based on this, a method for expanding and identifying point cloud data is proposed. Experimental results show that even under dense camouflage nets, accurate waveform decomposition and spectral reconstruction can be achieved with the proposed waveform processing method, thereby achieving point cloud expansion and target type identification. When the distance between the shield and the target is smaller than the resolution distance determined by the laser pulse width, the method still has good point cloud expansion ability.

Strapdown vehicle autonomous gravimetry method based on two-dimensional laser Doppler velocimeter

Wei Guo, Yang Zekun, Gao Chunfeng, Zhou Jian, Yu Xudong, Luo Hui, Deng Bin, Zhou Wenjian, Cheng Jiayi

2023, 52(6): 20230174. doi: 10.3788/IRLA20230174

[Abstract](213) [FullText HTML] (74) [PDF 2487KB](26)

Objective As one of the basic physical fields of the earth, gravity field reflects the distribution of underground materials and the changes of space and time. It has important value in resource exploration, military application and space science research. At present, the measurement methods of earth gravity field include aerial gravimetry, marine gravimetry, satellite gravimetry and ground gravimetry, et al. As an important method of gravity field measurement, ground gravimetry is mainly used for local fine construction of earth gravity field, which can be divided into ground static gravimetry and ground dynamic gravimetry. Due to the high cost and low efficiency of ground static gravimetry, ground dynamic gravimetry is usually adopted, namely ground vehicle gravimetry. At present, strapdown inertial navigation system (SINS)/global navigation satellite system (GNSS) integrated system is usually used in vehicle gravimetry, which lacks autonomy and has limited accuracy in the special environment where GNSS signal is blocked. To solve this problem, this paper proposes a strapdown vehicle autonomous gravimetry method based on two-dimensional laser Doppler velocimeter (LDV). Methods In this paper, a high-precision autonomous vehicle gravimetry method is designed. In order to improve the autonomy of the system, SINS/LDV integrated system is adopted in this paper, which does not need to rely on external signal sources. In order to ensure the measurement accuracy of the system, the LDV adopted by the system is two-dimensional, which is sensitive to the velocity of the vertical direction, so as to reduce the measurement error. The systematic errors are analyzed, and the constraints on the device accuracy and measurement scheme of LDV are proposed (Tab.1). The data processing flow of SINS/LDV integrated gravimetry system is also proposed. The system can finally realize the high-precision gravimetry in special environment. Result and Discussions The experiment was conducted in a special environment. During the experiment, the altitude changed greatly (Fig.4) and the GPS was seriously blocked (Fig.5). There are altogether 6 repeated lines in the experiment, each of which is about 11 km. During the experiment, the maximum horizontal error of SINS/LDV integrated navigation is about 17 m (Fig.8), and the maximum height error is about 2 m (Fig.9), which meets the requirements of system standard (Tab.1). Gravity anomaly were calculated according to the results of integrated navigation. The six lines based on SINS/LDV integrated system had a good consistency, and the maximum and minimum internal coincidence accuracy of a single survey line are 1.05 mGal and 0.47 mGal, and the total internal coincidence accuracy is 0.70 mGal (Tab.2). However, the consistency of the six lines based on SINS/GNSS integrated system are relatively poor. The maximum and minimum accuracy of internal coincidence of a single line are 2.46 mGal and 1.02 mGal, and the total internal coincidence accuracy is 1.53 mGal (Tab.3). The accuracy of SINS/LDV integrated system is generally better than that of SINS/LDV integrated system, and the total accuracy of SINS/LDV integrated system is about 54% higher than that of SINS/GNSS integrated system. Conclusions In this paper, a strapdown vehicle autonomous gravimetry method based on two-dimensional laser Doppler velocimeter is studied. The measurement principle and error model of the system are analyzed, and the corresponding index and data processing flow of the system are given. The vehicle gravimetry experiment shows that the consistency of the six lines in SINS/LDV integrated gravimetry system is high, while that of the six lines in SINS/GNSS integrated gravimetry system is relatively poor when the satellite signal is seriously blocked. Accordingly, the single internal coincidence accuracy of SINS/LDV integrated system is generally better than that of SINS/GNSS integrated system, and the total internal coincidence accuracy of SINS/LDV integrated system is nearly half higher than that of SINS/GNSS integrated system. The experimental results show that SINS/LDV integrated gravimetry system can realize gravimetry, and the gravimetry accuracy is higher than SINS/GNSS integrated gravimetry system in special measuring environment. The research of this paper provides technical support for the vehicle gravimetry in the environment when the GNSS signal is blocked, and the relevant results can be applied in geological exploration, gravity matching and the refinement of the earth's local gravity field.

Calibration of RLG-INS/EML integrated navigation system considering current velocity

Zhao Yingwei, Li Xiangyuan, Zheng Jiaxing, Tan Wenfeng

2023, 52(6): 20230142. doi: 10.3788/IRLA20230142

[Abstract](144) [FullText HTML] (31) [PDF 1756KB](19)

Objective The velocity output of an electromagnetic log (EML) is actually a velocity relative to the water. The current velocity will affect the velocity output of an EML in marine applications. If a ring laser gyroscope inertial navigation system (RLG-INS) is integrated with an EML to construct an integrated navigation system, the current velocity will produce negative effects on the calculation of calibration parameters, which thus leads to a degraded navigation performance in the following navigation process. In order to reduce the current velocity's effect on the calibration of a RLG-INS/EML integrated navigation system and improve the calibration accuracy of the relevant parameters, a calibration method of considering the current velocity effect is introduced. Methods Based on the integrated navigation principle, the current velocity can be augmented as an extra state in the calibration Kalman filter, while the GNSS position and velocity outputs are implemented as the Kalman filter's measurements to estimate these calibration parameters. The proposed method is beneficial in improving the calibration accuracy while estimating the current velocity concurrently. In such a case, the states estimated in the Kalman filter include attitude error, velocity error in eastern, northern and up directions, position error (longitude, latitude and height), gyroscope biases (three directions), accelerometer biases (three directions), installation error, scale factor error and current velocity in eastern and northern directions. Based on the calibration function, the effects of the current velocity on the EML scale factor and installation error are analyzed, which also reveals the coupling relationship among different calibration parameters. It is indicated that the attitude error related terms can be eliminated from the calibration equation especially when the attitude error is very large at the beginning of the navigation Kalman filter, which may cause a negative effect on the parameter estimation. An analysis of the current velocity's observability is also conducted, which is helpful in determining a proper calibration path to estimate the relevant states. Based on this observability analysis, a turn can help speed up the current velocity convergence. Therefore, it is suggested that some turns should be conducted during the calibration process. Results and Discussions The proposed method is further examined through the simulation and experiment. In the simulation, due to the absence of the EML, the EML velocity output is generated by converting the GNSS velocity in the navigation frame to the EML frame based on a real ship trajectory (Fig.2), while some errors such as scale factor error and installation error are also added to the generated simulation data. The simulation result indicates that the calibration parameters' residues can be reduced to a very small quantity after three iterations (Fig.3-8), if there are several turns in the trajectory. In the experiment, an EML and an INS are mounted together in a ship. The total experiment period is about 9.4 h, while the first 2.4 h are utilized to calibrate these parameters (Fig.9). All the parameters can be well estimated after 3 iterations and several turns, which verifies the effectiveness of the proposed method (Fig.10-12). The whole experiment period is used to examine the correctness of the calibration parameters. Compared with the result without calibration, the navigation performance can be greatly improved with the proposed calibration method (Fig.13). The maximum position drift is much smaller than the case without calibration. Conclusion The calibration of RLG-INS/EML integrated navigation system is very important in maintaining the navigation performance. Due to the effect of the current velocity, the calibration parameters will be polluted by this extra velocity. In this manuscript, a calibration method of considering the current velocity effect in proposed. The current velocity is augmented as an extra state in the calibration Kalman filter. The coupling relationship among different parameters is analyzed in theory, while a calibration strategy of improving the observability is also introduced. The simulation and experiment results verify the effectiveness of the proposed method. The proposed method can simplify the calibration process, which has potential in improving the calibration efficiency while maintaining the calibration accuracy.

Optimal calibration method for nonorthogonal errors in dual-axis rotational RLG INS

Li Ding, Yu Xudong, Wei Guo, Luo Hui

2023, 52(6): 20230148. doi: 10.3788/IRLA20230148

[Abstract](100) [FullText HTML] (42) [PDF 2277KB](22)

Objective In the rotational ring laser gyro inertial navigation system, the rotary mechanism propels the inertial measurement unit (IMU) to rotate periodically concerning the vehicle. It takes work to get the attitude information directly. High-accuracy attitude information is essential for long-endurance high-accuracy autonomous navigation. High-accuracy vehicle attitude decoupling techniques for dual-axis rotational inertial navigation systems are of great practical importance. To extract the vehicle attitude information in the rotational laser inertial navigation system, decoupling the IMU attitude from the dual-axis attitude is necessary. However, there are inevitable nonorthogonal angles in the dual-axis rotational inertial navigation system. Installing the inner and outer axis frames cannot guarantee the inner axis zero frames, and the outer axis frame cannot be completely orthogonal. Similarly, when mounting the IMU on an inner axis frame, there is no guarantee that the IMU frame is entirely orthogonal to the inner axis frame. By calibrating the nonorthogonal errors, an accurate attitude transfer model can be established, and the accurate attitude information of the vehicle can be obtained. Methods An optimal calibration method for nonorthogonal errors in dual-axis rotational RLG INS is proposed. The proposed calibration method allows accurate calibration of nonorthogonal angles between the rotational axes of the dual-axis rotational inertial navigation system. Based on the mechanical structure of the dual-axis rotational inertial navigation system and the definition of the coordinate frame, the spatial relative position model between the coordinate frame was established, including the spatial position relationship between the outer and inner axis frame and the spatial position relationship between the IMU and the inner axis frame. The IMU's attitude to the vehicle's attitude transfer model is constructed, which includes the nonorthogonal angles. Furthermore, the calibration problem is converted into an optimization problem based on the attitude transfer model. A fitness function is constructed using the vehicle attitude error as the fitness. The particle swarm optimization algorithm finds the optimal global solution to the created fitness function. The nonorthogonal angles calibration of the dual-axis rotary mechanism is achieved. Results and Discussions Calibration tests and navigation tests were conducted to verify the effectiveness of the proposed nonorthogonal angles calibration method. The test equipment consisted of a dual-axis rotational modulated laser inertial navigation system self-developed by the National Defense University of Science and Technology (Fig.4) and a set of GNSS equipment to obtain the reference position of the test site. The specifications of the inertial measurement unit are shown (Tab.1). The parameters of the particle swarm optimization algorithm are set (Tab.2). The 30 Monte Carlo tests were conducted to verify that the PSO algorithm did not fall into a local optimum or fail to converge completely. The result shows the convergence of the fitness values for the 30 Monte Carlo tests (Fig.5). As the number of iterations increases, the fitness values converge well and do not fall into a local optimum. The results from a randomly selected set of 30 Monte Carlo tests are shown (Tab.3). The dual-axis rotational modulated laser inertial navigation system is placed on a static base platform. An initial alignment is performed for 600 s to obtain the initial attitude, followed by a 16-order navigation rotational modulated phase. The calibration results from Tab.3 are then substituted into the attitude solution process to get the vehicle's attitude information. The vehicle's attitude error is shown (Fig.6-8), where the red curve is the attitude without nonorthogonal angles calibration, and the blue curve is the vehicle attitude after calibration compensation using the conventional method. The green curve is the vehicle attitude obtained after compensating for the proposed calibration method. The solved vehicles attitude is shown (Fig.9). In the case of a static base, the actual vehicle attitude remains unchanged. If the nonorthogonal angles are not compensated, the vehicle's attitude calculated by IMU has a significant fluctuation due to the oscillating attitude error generated by the nonorthogonal angles. The statistical values of the vehicle attitude error are show (Tab.4). Compared with the conventional method, the pitch error is reduced by 89.29% with the proposed method, the roll error is reduced by 73%, and the yaw error is reduced by 81.39%. The repeatability of the proposed method was verified with the Monte Carlo tests (Fig.10). Conclusions An optimal calibration method for nonorthogonal errors in dual-axis rotational RLG INS is proposed. The experimental results show that the nonorthogonal angles can be effectively achieved with the proposed nonorthogonal angles calibration method, which can significantly reduce the vehicle attitude error and decouple the attitude information of the IMU from the rotating mechanism to obtain the vehicle attitude information with high accuracy. The proposed calibration method is more accurate than conventional methods and does not require a specific rotation path. The technique is simple to operate. The inertial navigation system can calibrate the nonorthogonal angles in situ by executing 16-position navigation paths during the standby phase. With the proposed calibration method, the dual-axis rotational inertial navigation system can output high-accuracy attitudes, guaranteeing high-accuracy autonomous navigation.

Controllable fabrication and characterization of suspended graphene/hexagonal boron nitride heterostrcuture Joule heating infrared radiation devices (invited)

Liu Qiang, Luo Fang, Deng Xiaojiang, Zhu Mengjian, Zhu Zhihong, Qin Shiqiao

2023, 52(6): 20230218. doi: 10.3788/IRLA20230218

[Abstract](205) [FullText HTML] (45) [PDF 2032KB](31)

Objective Graphene exhibits superior optical, electrical, thermal, and mechanical properties, while the suspended structure avoids external factors such as wrinkles, carrier scattering and doping caused by rough substrates, and can maximize the intrinsic physical properties of graphene, which is of great significance in the research of high-performance graphene microelectronics and optoelectronic devices. However, the current research on suspended graphene devices is yet limited by the complicated fabrication methods, low yield, and unstable electrical and thermal properties of devices. Methods In order to improve the yield rate of suspended graphene nano devices and the comprehensive performance of the device, this paper develops a method by using two-dimensional material hexagonal boron nitride (h-BN) to pick up graphene, then transfers graphene directly to the surface of pre-fabricated metal electrodes, and finally prepares suspended graphene Joule heating infrared radiation devices (Fig.1). In order to further reduce the defects and improve the device quality, a high-vacuum thermal annealing treatment was performed on the suspended graphene device. Based on the high-quality suspended graphene device after annealing, we used Raman spectroscopy and luminescence spectroscopy to study the temperature characteristics and thermal radiation spectral characteristics of the device under the Joule heating effect caused by bias voltage. Result and discussion The experimental results show that the h-BN covers the upper surface of the graphene and plays a critical role in supporting and suspending the graphene, which effectively improves the stability of the suspended graphene and avoids device failures such as collapse and fracture. After the thermal annealing at 400 ℃/3 h in high vacuum of 4.5×10⁻⁴ hPa, the resistance of suspended graphene decreased to one-sixth of that before annealing, and the carrier mobility increased eighteen times compared with that before annealing (Fig.4). When the bias voltage is 8 V, the temperature of suspended graphene measured by Raman spectroscopy is 836 K, and it shows a strong infrared radiation signal at 955 nm wavelength (Fig.5). Conclusions This paper presents a controllable fabrication method of high-quality suspended graphene Joule heating radiation devices, and investigates the electrical, temperature, and thermal radiation characteristics of suspended graphene devices. The h-BN in the device structure demonstrates a good support and adhesion effect for suspended graphene, which greatly improves the device performance. The impurities attached to the surface of graphene can be effectively removed through high vacuum thermal annealing, which greatly improves the electrical performance of suspended graphene devices. It was observed that the temperature of graphene increased with the increase of bias voltage, showing a blue shift in the Raman spectrum and strong thermal radiation emission. The research results of this paper provide an important reference for deepening the understanding of the intrinsic physical properties of suspended graphene and developing optoelectronic applications based on suspended graphene devices.

Control of photoelectric properties in all-inorganic CsPbBr₃ thin films with two-dimensional interface modification

Zhou Qingwei, Wu Fan, Luo Fang, Huang Xianyan, Guo Chucai, Zhu Zhihong

2023, 52(6): 20230219. doi: 10.3788/IRLA20230219

[Abstract](130) [FullText HTML] (31) [PDF 3067KB](26)

Objective As a new generation photovoltaic technology, perovskite solar cells (PSCs) have achieved a comparable efficiency to commercial silicon-based solar cells, demonstrating great application potential. However, these photoactive layers based on organic-inorganic hybrid perovskites are very unstable, which seriously hinders their commercial application. Therefore, all-inorganic CsPbBr₃ perovskite has attracted enormous attention due to its outstanding environmental tolerance to heat, moisture, oxygen and UV light. Unfortunately, the device efficiency based on CsPbBr₃ perovskite is relatively lower compared to that of the PSCs device with organic-inorganic hybrid perovskites. The recombination of charge carriers at the interface between the charge transport layers and the perovskite layer in all-inorganic CsPbBr₃ PSCs is the key factor that restricts the further improvement of its photoelectric conversion efficiency (PCE). In recent years, two-dimensional transition metal dichalcogenides (TMDCs) materials represented by MoS₂, MoSe₂, WS₂ and WSe₂ have attracted more and more attention due to their unique physical and chemical properties. With the advantages such as adjustable band gap and band edge, high carrier mobility, stable chemical properties and matching energy level with perovskite materials, two-dimensional TMDCs are regarded as effective interface modification materials to promote interface charge extraction in all inorganic CsPbBr₃ PSCs. However, the current research on using two-dimensional materials as interface modification layers and charge carrier transport layers in all-inorganic PSC is still in its infancy. Methods Through interface engineering, various two-dimensional TMDCs (MoS₂, MoSe₂, WS₂, and WSe₂) materials are introduced at the interface between the perovskite layer and the electron transport layer of the all-inorganic CsPbBr₃ PSCs with an FTO/SnO₂/TMDCs/CsPbBr₃/C structure. The two-dimensional TMDCs here act as both interface modification materials and carrier transport layers. Through interface level compensation and barrier reduction with TMDCs interlayers, the carrier extraction and transport in all-inorganic CsPbBr₃ PSC devices are promoted (Fig.1). Moreover, by constructing TMDCs/CsPbBr₃ van der Waals heterostructure, high-quality CsPbBr₃ perovskite thin films with large and compact grains are grown via lattice matched van der Waals epitaxy (Fig.5). Results and Discussions The interface charge loss between the perovskite layer and the electron transport layer is reduced, and the carrier extraction in the all-inorganic CsPbBr₃ PSC devices are enhanced, so that the PCE of the devices is increased from the initial 7.94% to 10.02%, and the open-circuit voltage is increased from 1.474 V to 1.567 V (Fig.8). In addition, Mott–Schottky curves are measured in the dark by performing capacitance-voltage characterization, which demonstrates an enhanced built-in potential, indicating the enlarged driving force for charge transportation with TMDCs/CsPbBr₃ heterostructure (Fig.9). Finally, the time-resolved photoluminescence decay measurements are performed to investigate the photoluminescence decay lifetimes, which are closely related to the electron extraction ability. Due to better energy level matching of TMDCs with perovskite layers, CsPbBr₃ perovskite layers show shorter carrier lifetimes on TMDCs interlayer (Fig.10), which further confirms the enhanced electron extraction ability from perovskite to the electron transport layer. Conclusions A novel strategy is developed to prepare high-quality perovskite films by constructing TMDCs/CsPbBr₃ van der Waals heterostructure and achieve high-performance photoelectric devices through interfacial energy level matching, which provides a new way for the development of all-inorganic perovskite optoelectronic devices based on two-dimensional TMDCs materials.

Polarization characteristics of Weyl metamaterial based on interfacial reflection eigenmodes

Wang Hanyu, Xu Wei, Zhu Zhihong, Yang Biao

2023, 52(6): 20230233. doi: 10.3788/IRLA20230233

[Abstract](209) [FullText HTML] (44) [PDF 8621KB](29)

Objective As a massless relativistic fermion, Weyl fermions play a crucial role in quantum theory and the standard model. To mimic the physical properties of Weyl fermions, constructing Weyl points in momentum space needs breaking the inversion or time-reversal symmetry, and those Weyl points are topologically protected. Weyl points possess unique characteristics, including positive and negative chiralities corresponding to the sources and sinks of the Berry curvature, respectively. Consequently, Weyl points are regarded as magnetic monopoles in momentum space. Weyl points have also attracted significant attention due to their scattering and transport properties. For example, Weyl semimetal exhibits chiral zero modes and corresponding chiral magnetic effects in condensed matter physics. It is worth mentioning that Weyl points are widely acknowledged as singularities in the reflection phase, but there has been relatively little study on the reflection eigenmodes of the interface between a Weyl metamaterial and air. Methods This article utilizes the ideal Weyl metamaterial as a research platform, which offers a relatively large frequency range for exploring the fundamental properties of Weyl points. By applying the effective media theory, the constitutive relation of saddle-shaped metallic structures (Fig.1) can be described concisely. Additionally, the band structure of Weyl metamaterial can be calculated using simulation software. Results and Discussions For a totally reflected interface, the wave vectors of the incident and reflected state space are different. Before solving for the reflection eigenmodes, it is necessary to define the basis separately for the two different state spaces of the incident and reflected electromagnetic fields. Since the electric field is a polar vector and the magnetic field is an axial vector, the mirror operation introduces different responses in the directions perpendicular and parallel to the interface. By applying the mirror operation, we can connect the incident and reflected state spaces, allowing us to solve for the eigenmodes using conventional methods and determine their matrix representation. Given this definition, energy conservation ensures that the reflection coefficient matrix must be unitary, while Lorentz reciprocity guarantees that the reflection coefficient matrix must be symmetric. Such a unique reflection coefficient matrix must have real eigenstates, resulting in both reflection eigenmodes being linearly polarized. Using this method to analyze the reflection eigenmodes of Weyl metamaterials, it is found that all reflection eigenstates are linearly polarized. The two eigen electromagnetic fields are perpendicular to each other, forming a cross shape. As the scanning path changes continuously in the Brillouin zone, the orientation of the cross shape also varies. When the scanning path surrounds the Weyl points in momentum space, the eigen field (cross shape) undergoes an additional phase shift of $ \mathrm{\pi }/2 $. A quadratic Möbius strip can describe this feature. Conclusions In the case of total reflection at an interface, the incident state space and the reflected state space are bridged via a mirror operator. Combining the interface reflection operator with the mirror operator allow us to define the eigenstates of the total reflection interface. When the incident basis is chosen as linear polarized, this unique definition results in the reflection eigenstates of the total reflection interface being linear polarization modes protected by energy conservation and Lorentz reciprocity. Taking the Weyl metamaterial as an example, even if the interface reflection between the Weyl metamaterial and air exhibits polarization conversion characteristics, given this unique definition, the eigenmodes of the interface reflection can be obtained as linear polarized. Since the two linear polarization eigenstates are perpendicular to each other, a rotation angle could be defined to characterize the change in the rotation angle of the eigenmode. In addition, when scanning a loop path around the Weyl points in momentum space, the eigen field acquires an additional phase shift of $ \mathrm{\pi }/2 $, which can be described using a quadratic Möbius strip.

Photodetection properties of van der Waals vertical heterostructures based on photogenerated carrier-dominated FN tunneling

Liu Ping, Xu Wei, Xiong Feng, Jiang Jinbao, Huang Xianyan, Zhu Zhihong

2023, 52(6): 20230217. doi: 10.3788/IRLA20230217

[Abstract](108) [FullText HTML] (37) [PDF 5935KB](36)

Objective Compared with traditional 3D bulk semiconductors, 2D layered semiconductors (e.g. transition metal dichalcogenides) have the features of large exciton binding energy, strong light-matter interaction and layer-dependent band structure, due to the intrinsic quantum confinement effect in the out-of-plane direction. Owing to such special photonic and photo-electronic properties, transition metal dichalcogenides and their van der Waals heterostructures have great potential for high-performance photodetector applications. In recent years, photodetector devices based on mechanisms such as photogating effect, photoconductive effect, photovoltaic effect, and photothermoelectric effect have been proposed and widely studied. Transition metal dichalcogenides planar optoelectronic devices based on photogating effect have similar device structures with transistors and compatible fabrication, together with high responsivity, but suffer from slow response speed and large dark current without applying gate bias, which limits the improvement of the device performance. Therefore, improving the response speed and reducing the dark current of transition metal dichalcogenides optoelectronic devices becomes an urgent issue. Methods With mechanical exfoliation and dry transfer methods, van der Waals photodetectors with a graphene/MoS₂/h-BN/graphene vertical heterostructure are constructed (Fig.1). In the devices, MoS₂ performs as the photoabsorber with graphene as both top and bottom electrodes. The h-BN insulating layer is inserted between MoS₂ photoabsorber and the bottom graphene electrode as an effective and tunable barrier. Both AFM and Raman characterizations are taken to confirm the thickness of the materials and the device structures. The tunneling current from the top graphene electrode to the bottom graphene electrode through MoS₂ and h-BN under dark and laser illumination is measured with the home-built photocurrent measurement system. Results and Discussions Based on the I-V characteristics of the vertical heterostructure device under both dark and laser illumination, together with the Fowler-Nordheim (FN) tunneling fitting of the I-V curves, the transport mechanism of FN tunneling is verified in the graphene/MoS₂/h-BN/graphene vertical heterostructure device (Fig.2). With the inserted wide bandgap h-BN insulating layer between the graphene electrode and MoS₂ photoabsorber, dark current was highly suppressed, while photogenerated carriers (holes in MoS₂) contributed effectively to the photocurrent through FN tunneling (Fig.3), which matches well with the observation of clear photocurrent when applying positive bias (Fig.2(b)). Detailed measurement of the photocurrent under laser illumination with various powers reveals the responsivity of the device of ~140 mA/W at laser power of 4.5 mW/cm² and I_light/I_dark ratio of ~2.3 at laser power of 41.4 mW/cm² (Fig.4) are achieved. A low dark current in the order of picoamperes and relatively high photodetection response speed with the response time of ~0.3 s are achieved, which is nearly two orders of magnitude higher than that of traditional graphene/MoS₂ heterostructure with the response time of ~20 s (Fig.5). The achieved low dark current and high response speed confirm the principle design of van der Waals vertical heterostructures based on FN tunneling effect in promoting the photodetection performance of the devices. Conclusions A novel van der Waals vertical heterostructure with graphene/MoS₂/h-BN/graphene is developed to achieve high-performance photodetector properties with a low dark current and relatively high photodetection response speed, which verifies the significance of FN tunneling of photogenerated carriers for the development of van der Waals heterostructure photodetectors based on 2D materials.

Research progress of laser dazzle and damage CMOS image sensor (invited)

Wen Jiaqi, Bian Jintian, Li Xin, Kong Hui, Guo Lei, Lv Guorui

2023, 52(6): 20230269. doi: 10.3788/IRLA20230269

[Abstract](334) [FullText HTML] (99) [PDF 2874KB](112)

Significance Complementary Metal Oxide Semiconductor (CMOS) image sensors are currently the most mainstream solid-state image sensors. They have the characteristics of low power consumption, high integration, and fast imaging. In the past decade, breakthroughs have been continuously made in their performance, surpassing Charge Coupled Device (CCD) image sensors in market share and product iteration speed. It is widely used in the fields such as digital cameras, security monitoring equipment, mobile phones, drones, medical detection, and autonomous driving. As the core component of an optoelectronic imaging system, image sensors strongly absorb laser energy within their working wavelength, making them more susceptible to laser damage compared to other components of the optoelectronic system. However, the new back side illumination CMOS and stacked CMOS have significant structural differences from traditional front side illumination CMOS image sensors, and their ability to resist laser interference and damage has been greatly improved. Therefore, the laser interference effect and damage mechanism of CMOS image detectors have received widespread attention from scholars at home and abroad. Progress Firstly, the structure and working principle of CMOS image sensor according to its development history are introduced. The pixel structure of CMOS has evolved from passive pixels to active pixels, where each pixel can independently collect, amplify, and output signals. SiO₂ deep trench isolation (DTI) structure (Fig.3(d)) is used between pixels for crosstalk suppression. The chip structure of CMOS has evolved from front illuminated to back illuminated and stacked, and the position of the metal wiring layer has buried deeper, making it more difficult to cause destructive damage. On this basis, the weaknesses of CMOS image sensor in the process of laser irradiation are briefly analyzed. CMOS uses a correlated double sampling (CDS) circuit to output signals, which uses the difference between two signals to output, and interfering with both signals causes pixel oversaturation; The use of the same column line to transmit the reference signal of a column of pixels provides the possibility of large-scale crosstalk. The damage at different stages is related to the depth of laser action. It can be concluded that the key to causing large-scale damage to CMOS image sensors is the severe damage to the internal circuit layer.　　CMOS image sensor is used more and more widely. More attention has been paid to the experimental study of laser-induced dazzle and damage of CMOS. The evaluation methods of interference and the main measurement methods of damage threshold are summarized. The representative measurement results of interference and damage threshold are summarized (Tab.1-2). By comparing the results of interference, the conditions of oversaturation and crosstalk are summarized, and the conclusion is verified that the above-mentioned CDS circuit is susceptible to interference. Compared with CCD, CMOS has better anti-damage ability, especially the back-illuminated CMOS, which is difficult to cause large area damage. This is because the back-lit CMOS circuit layer is deeper, above a thicker layer of silicon-based material, forming a certain inherent protective layer. With the wide application of backlit and stack CMOS chips, how to improve the damage efficiency of laser-illuminated CMOS chips is an urgent problem to be solved in the next research.　　Finally, the development status and prospects of using new laser systems to improve the damage ability of CMOS image sensors are discussed. The composite laser can be made up of two pulses with different parameters. The ablation and damage of the composite laser on the single material target has been well studied. If the laser parameters are matched properly, the absorption rate of laser energy can be improved effectively. It has been proved that the composite laser can improve the efficiency of damaged CMOS to some extent, but the effect is limited. To further improve the laser damage efficiency, we can consider to further increase the adjustable parameters of the laser, the combination of three or more pulses into the pulse string form. Conclusions and Prospects CMOS image sensors are booming, which have become the most mainstream image sensors. As an important countermeasure, the research of laser jamming and damage CMOS image sensor needs to be further explored. The purpose of this paper is to provide some references for the future research of laser jamming and damage CMOS, and the idea of using the new laser system to improve the damage efficiency is proposed.

Irradiation effect of 2.79 μm mid-infrared laser on CMOS image sensor

Wang Xi, Zhao Nanxiang, Zhang Yongning, Wang Biyi, Dong Xiao, Zou Yan, Lei Wuhu, Hu Yihua

2023, 52(6): 20230168. doi: 10.3788/IRLA20230168

[Abstract](198) [FullText HTML] (63) [PDF 1394KB](60)

Objective The CMOS image sensors are widely used in aerospace, security monitoring, industrial control, navigation and guidance, image recognition systems and other fields. Most of researches on laser irradiation effect of CMOS image sensor mainly focus on visible and near infrared bands. With the application of more and more lasers with different wavelengths, there is a great risk of damage to optical systems irradiated by out-of-band lasers, and it is necessary to conduct systematic experimental studies on the interaction between out-of-band lasers and photo detectors. In the photoelectric countermeasure, it is very important to study whether the interference and damage can be effectively caused to the detector when the interference and damage are irradiated by the out-of-band laser, and what its mechanism is. The wavelength of 2.79 μm mid-infrared laser is in the atmospheric window, which has the characteristics of small air scattering and long propagation distance. This band is also the working band of most reconnaissance satellites, surveillance satellites, early warning satellites and other space-based systems. In the future space applications, the high-power 2.79 μm mid-infrared laser has a broad application prospect. Therefore, it is of great reference value in the laser attack and defense field to study the irradiation effect of mid-infrared laser on CMOS image sensor. Methods In the experiment, the CMOS image sensor irradiated by 2.79 μm mid-infrared laser is carried out (Fig.1). The computer is connected to the output signal of CMOS image sensor to observe and record the effect of laser irradiation. In order to study the damage effect of laser irradiation on CMOS image sensor, the experiment is divided into two stages. In the first stage, the laser energy is directly irradiated on the sensor without the sapphire focus lens, and the interference effect of 2.79 μm mid-infrared laser on CMOS image sensor is studied. In the second stage, the sapphire focus lens is placed in the optical path to study the damage effect of 2.79 μm mid-infrared laser on CMOS image sensor. The differential interference contrast (DIC) microscope is used to observe the damage morphology of CMOS sensor samples. Results and Discussions The experimental results of laser interference show that saturation and oversaturation appears on the CMOS image sensor with the increase of laser energy (Fig.3). After stopping laser irradiation for a period of time, CMOS can automatically return to the normal working state. The experimental results show that with the increase of the repetition frequency, CMOS image sensor needs less laser energy and less time to achieve saturation or oversaturation of full screen (Fig.5). The experimental results of laser damage show that the phenomenon of saturation, oversaturation, black screen, green screen and bright line are observed with different laser repetition frequency (Fig.8-9). The damage morphology shows that obvious melting damage occurs in the irradiation area of laser spot, and the high laser energy in the center of beam leads to the ablation and evaporation of a large area of pixel material, and the periphery of the spot area is obviously heated, but no cracks appear (Fig.10). It shows that the damage of 2.79 μm mid-infrared laser on CMOS sensor is mainly due to the thermal melting of materials, and the thermal effect is obvious. Conclusions The experimental results indicate that the CMOS image sensor has good anti-interference and anti-damage ability. The damage thresholds of CMOS image sensor irradiated by 2.79 μm mid-infrared laser at a 10 Hz pulse repetition frequency are 0.44 J/cm² for saturation, 0.97 J/cm² for oversaturation, and 203.71 J/cm² for damage, respectively. It can be seen that the damage threshold of the CMOS image sensor is much higher than its interference threshold. The experimental results show that the damage mechanism of CMOS image sensor is mainly melting damage, and the thermal effect is obvious.

Optimization of structural parameters of Si₃N₄/WS₂/Al₂O₃ sandwich nanolaser

Liu Ning, Zhou Guyu, Yang Xi, Xu Jipeng, Hong Qilin, Huang Xianyan, Zhang Jianfa, Liu Ken, Zhu Zhihong

2023, 52(6): 20230196. doi: 10.3788/IRLA20230196

[Abstract](196) [FullText HTML] (57) [PDF 1949KB](31)

Objective High-performance on-chip nanolasers are very important for the development of communication, sensing, quantum and so on. On-chip nanolasers can be realized by integrating layered two-dimensional (2D) transition metal chalcogenides (TMDs) with optical microcavities. However, the integration of traditional 2D materials and microcavities is achieved by transfer methods, which limits the scale fabrication of on-chip nanolasers. Based on the above background, we propose a prototype of a TMDs-based microcavity nanolaser array prepared by direct growth method. High optical confinement factor in nanolasers can ensure a larger mode gain and a lower laser threshold. It is necessary to analyze the influence of various geometric parameters on the optical confinement factor of nanolaser by simulation and to optimize the structure, so as to lay a certain theoretical foundation for high-performance nanolasers that can be prepared on a large scale in integrated optical chips. Methods Suspended silicon nitride (Si₃N₄) microdisk resonators with high quality factor were prepared using complementary metal oxide semiconductor (CMOS)-compatible fabrication process; Different from traditional transfer methods to realize the integration of 2D material and microcavity, we propose to use physical vapor deposition (PVD) method to directly grow monolayer tungsten sulfide (WS₂) on the surface of Si₃N₄ microdisk as gain material, realizing the conformal covering of the microdisk; In order to ensure that monolayer WS₂ can work stably under the pump of a laser, and to ensure a larger confinement factor in the monolayer gain material, the method of atomic layer deposition (ALD) was used to deposit alumina (Al₂O₃) with a certain thickness, and a nanolaser with sandwich structure Si₃N₄/WS₂/Al₂O₃ was formed; A simplified 3D simulation model of the nanolaser was constructed in Comsol software, and the effects of Al₂O₃ coating thickness T, Si₃N₄ microdisk diameter D and thickness H on the optical confinement factor were analyzed; The devices were characterized by fluorescence and scanning electron microscopy. Results and Discussions When constructing the simulation model, the silicon oxide (SiO₂) pillar structure and the circular notch of monolayer WS₂ caused by the SiO₂ pillar are omitted (Fig.2); The effects of Al₂O₃ coating thickness T (Fig.4), Si₃N₄ microdisk diameter D (Fig.6) and thickness H (Fig.7) on the optical confinement factor were analyzed. Within the range of selected parameters, the optical confinement factor first increases and then decreases with the increase of Al₂O₃ coating thickness T and Si₃N₄ microdisk diameter D, the decrease of the thickness H of the Si₃N₄ microdisk can also significantly increase the optical confinement factor of the nanolaser; The feasibility of this direct growth method was demonstrated by fluorescence and scanning electron microscopy after monolayer WS₂ was grown onto the Si₃N₄ microdisk (Fig.8); After the deposition of completion of Al₂O₃, time-space images of the nanolaser above and below the threshold were shown (Fig.9). Conclusions Nanolaser with a sandwich structure Si₃N4/WS₂/Al₂O₃ was proposed. The preparation process of the sandwich nanolaser was expounded. Suspended Si₃N₄ microdisk resonators with high quality factor were prepared using CMOS-compatible fabrication process, PVD method was used to directly grow monolayer WS₂ on the surface of Si₃N₄ microdisk as gain material, and ALD method was adopted to deposit Al₂O₃ with a certain thickness after monolayer WS₂ was grown. Thus, the nanolaser with a sandwich structure Si₃N₄/WS₂/Al₂O₃ was formed; In the simulation software, the geometry of the nanolaser was simplified and the parameters were simulated and optimized, the effects of Al₂O₃ coating thickness T, Si₃N₄ microdisk diameter D and thickness H on the optical confinement factor were analyzed. Within the range of selected parameters, the optical confinement factor first increases and then decreases with the increase of Al₂O₃ coating thickness T and Si₃N₄ microdisk diameter D, the decrease of the thickness H of the Si₃N₄ microdisk can also significantly increase the optical confinement factor of the nanolaser; The characterization results of some devices were displayed, which lays a good simulation foundation for the further optimization of device parameters in the later period, and has certain guiding significance for the large-scale preparation of high-performance nanolasers in the field of optical communication and so on.

High-power vortex Raman fiber laser

Li Yang, Fan Chenchen, Hao Xiulu, Ma Xiaoya, Yao Tianfu, Xu Jiangming, Zeng Xianglong, Zhou Pu

2023, 52(6): 20230292. doi: 10.3788/IRLA20230292

[Abstract](262) [FullText HTML] (79) [PDF 1608KB](54)

Objective In recent years, vortex beams carrying orbital angular momentum (OAM) have attracted much attention due to their important research value and application prospects in optical communication, particle acceleration, particle manipulation, super-resolution imaging, and other fields. At present, vortex optical field can be mainly divided into two types, one is generated by spatial devices, such as the spatial light modulator, and the other is generated in fiber. Fiber lasers possess the advantages of compact structure, convenient thermal management, and high efficiency, which is conducive to realize the output of high-power and high-stability vortex beam. With the rapid development of big data, cloud computing, Internet of Things, and other technologies, it is urgent to expand the information capacity of communication systems. Vortex beams can greatly improve the capacity of communication systems because of their infinite orthogonality. At present, the output vortex beams of fiber lasers are mainly concentrated in the emission bands of rare earth ions such as ytterbium-doped, erbium-doped and thulium-doped, and the combination of space division, mode division and wavelength division multiplexing technology urgently needs to expand the wavelength range of the vortex beam. For this purpose, an all-fiberized vortex Raman fiber laser (RFL) is designed. Methods At present, many devices can realize vortex beam output in a fiber laser. Among them, the acoustically-induced fiber grating (AIFG) has the advantages of simple structure, wide wavelength tuning range, fast response speed and low insertion loss. Combined with the AIFG and RFL, when the output wavelength is converted by Raman frequency shift, there is no need to redesign and replace the mode conversion device. The RFL is built (Fig.1). The laser resonator is composed of a pair of fiber Bragg gratings and gain fiber. The AIFG is fused after the cavity. To control the polarization state of the output mode, a three-loop polarization controller (PC) is connected after AIFG. And the transmission spectrum of the LP₀₁ mode is tested (Fig.2(a)), which indicates that there is a linear relationship between the frequency and the wavelength. Once the suitable electrical signal is loaded on the AIFG, the output mode is converted to LP₁₁ mode, and the ring-shaped radially polarized light and vortex beam with topological charge l=±1 output can be realized by precise polarization control. Results and Discussions The variation curve of the output signal optical power with the pump optical power is shown (Fig.3(a)), where the black and red curves correspond to the output mode of LP₀₁ mode and LP₁₁ mode, respectively. The maximum output power of LP₁₁ mode is 69.6 W, with a slope efficiency of 90.6% and total optical efficiency of 83%. The output spectrum at the highest power is shown (Fig.3(b)). The central wavelength of the output laser is 1 134.72 nm, and the second-order Raman light with a central wavelength of 1 194.16 nm restricts further power improvement. At the highest power, the spectral purity of signal light reaches 99.72%. By adjusting the PC, the radially polarized light output can be obtained. The mode field distribution of the output beam is detected by rotating the linear polarizer (Fig.4(b1)-(b4)), which verifies the radially polarized TM₀₁ mode. Once there is a π/2 phase difference between the $ HE_{21}^{even} $ and $ HE_{{\text{2}}1}^{odd} $mode, the vortex beam can be realized through the superposition of the two modes, and the "Y-shaped" interference fringe can be detected through the self-interference (Fig.4(c)-(d)), which proves that the vortex beam with topological charge l=±1 is generated. Conclusions An all-fiberized Raman fiber laser with radially polarized light and vortex beam with topological charge l=±1 output is realized based on AIFG. It has the advantages of compact structure, wide wavelength tuning range, fast response speed and low insertion loss. The output central wavelength is 1 134.72 nm, with the spectral purity of 99.72%. The maximum output power is ~70 W, and the efficiency is 83%. As an all-fiberized mode conversion device, the AIFG is expected to be the key device to fill in the gaps in the spectrum of vortex beams due to its ultra-wide wavelength tuning ability and high power tolerable capacity, which could provide a reliable light source for the application and exploration of vortex beams in special waveband. By replacing the pump source and the gain fiber, the wavelength coverage can be extended further through cascaded Raman shift.

Recent progress in beam steering by fiber laser phased arrays

Shu Bowang, Zhang Yuqiu, Chang Hongxiang, Chang Qi, Leng Jinyong, Ma Pengfei, Zhou Pu

2023, 52(6): 20230250. doi: 10.3788/IRLA20230250

[Abstract](276) [FullText HTML] (80) [PDF 3165KB](100)

Significance Fiber laser phased arrays have been applied in multiple regions, such as adaptive correction of wavefront phase distortion, dynamic beam steering technology, laser coherent beam combination and remote detection imaging, etc.. Among them, great progress has been achieved in dynamic beam steering owing to its tiling structures, organizable sub-apertures and available high power laser of laser coherent beam combination, which means it could be beneficial for flexible deflection, fast implementation of access addressing long-distance transmission. To further improve the performances of the whole scanning system and spatial scanning characteristics, this review discussed effective strategies provided by laser optical phased arrays, such as accurate regulation of array units, diverse array designs, flexible and free phase controlling methods, constant update of phase controlling algorithm and broad coverage of spectrum. These related advantages promote the development towards the direction of non-mechanical, large-angle, high precision and strong anti-interference. Progress With the development of coherent beam combination, high power and more channels of system could be available in practice. Owing to these outstanding results, more possibility could be realized in dynamic beam steering, which was recognized to be an important application based on coherent beam combination. Since 2005, DARPA have tried to break through limitation of scanning speed with purely electronic-controlled laser phased arrays, which is caused by disadvantages of mechanical scanning device. In the same year, they put forward 'APPLE' system configured by distributed sub-apertures, which could be used for adaptive wavefront compensation and adaptive phase-locking controlled by stochastic parallel gradient descent algorithm (Fig.1). This novel structure provides strategies for flexible, non-mechanical and fast addressing beam steering. In 2013, DARPA kept on carrying out the project of SWEEPER, which aimed to realize 10 W laser scanning within a wide field of view up to 45 degrees. Recently, Civan Lasers develop multiple dynamic beam laser solutions to output power controlling, beam shaping and light field pattern customization (Fig.2). Moreover, their products could be applied in cutting, welding, metal additive manufacturing and drilling, such as OPA 6 laser and OPA 12 laser. It is worth noting that these device could work at high power. 　　Interiorly, Xidian university have developed multiple beam steering system for realizing fast beam deflection with LiNbO₃ phase shifters in 2021. Institute of Optoelectronic Technology, Chinese Academy of Sciences have achieved great progress in continuous beam steering with the combination of piezoceramics phase-modulators and adaptive fiber optical collimators in the same year (Fig.9). To further judge the effects of beam steering, some indexes have been given including scanning precision, range, speed, energy concentration and effect of sidelobes. Although the works were still limited in dynamic beam steering based on coherent beam combination, it showed potential in large scientific devices with high power, expending functions and large array integration. Conclusions and Prospects Coherent beam combination possess great potential in information transmission, defense security, laser processing and material technology, etc.. There is still wide space in developing dynamic beam steering based on fiber laser phased arrays. Firstly, the dynamic instantaneous tracking scanning is hopefully realized by such platforms with large array areas and relatively high power. Secondly, scanning ability should be focused on wide field of view, high precision, large active range to achieve the sufficient details in the full region. Lastly, more light-weight device is required to be adaptive in all kinds of environment. Although great progress has been made in multiple indexes of coherent beam combination, large-scale application is still a challenge in dynamic beam steering.

Coherent polarization beam combination of two ultrafast laser channels based on fiber stretcher phase locking

Wang Tao, Li Can, Liu Yang, Ren Bo, Tang Zhenqiang, Chang Hongxiang, Xie Gehui, Guo Kun, Wu Jian, Xu Jiangming, Leng Jinyong, Ma Pengfei, Su Rongtao, Li Wenxue, Zhou Pu

2023, 52(6): 20220869. doi: 10.3788/IRLA20220869

[Abstract](234) [FullText HTML] (52) [PDF 1978KB](48)

Objective High-power ultrafast fiber lasers have broad applications in the frontier science and industry fields such as high-energy physics, high-order harmonic generation, advanced manufacturing and so on. Currently, the well-known fiber chirped pulse amplification (CPA) scheme has realized kW-level average power and multi-mJ single pulse energy of ultrafast laser, whilst further development is hindered by the influence of nonlinear effects and mode instability. At present, coherent beam combination (CBC) of ultrafast fiber laser is an effective way to break the power limitation of single-channel fiber, and has attracted much research interest. Essentially, the CBC system requires that each channel of amplifiers is phase locked, which is conventionally realized based on the electro-optical effect of lithium niobate, whereas with the compromise of large insertion loss and low damage threshold. In this study, we propose to utilize the fiber stretcher to control the laser phase by stretching the fiber based on piezoelectric ceramics. Compared with the lithium niobate modulator, the fiber stretcher has a larger dynamic adjustment range, lower insertion loss, higher damage threshold, as well as the additional merits of compactness and robustness. Methods The ultrafast fiber laser with a repetition rate of 50 MHz is firstly broadened by a chirped fiber Bragg grating, and then reduced to a repetition rate of 2 MHz by a pulse picker. After a single-mode amplifier, the pulsed laser signal is divided into two channels. Then, the average power is scaled to 6.1 W through two parallel-configured polarization-maintaining fiber amplifiers. For one of the channels, a spatial delay line consisting of a polarizing beam splitter prism, a quarter wave plate and a mirror placed on a high precision displacement platform is inserted in front of the main amplifier to effectively compensate the optical path difference between the two channels. The amplified lasers are collimated and combined through the polarization beam combining mirror. The combined laser is sampled by a photodetector, processed by the phase-locked control system, and converted into a voltage signal, which is fed back to the fiber stretcher to realize effective phase locking (Fig.1). Results and Discussions The effective coherent polarization beam combination of two ultrafast fiber lasers is realized based on the fiber stretcher and the stochastic parallel gradient descent (SPGD) algorithm. The highest combined power is 10.9 W with a combining efficiency of 90.1 % (Fig.3). According to the normalized temporal intensity fluctuation before and after phase locking at the highest power, the phase noise of the system is effectively suppressed in the closed-loop state with a phase residue error of λ/31, and the output power shows good long-term stability. When the system is in the open-loop state, the output beam profile is unstable and changes randomly. However, after the phase control system is turned on, the beam profile tends to be stable (Fig.2). The central wavelength and 3 dB bandwidth of the combined beam at the highest power are 1 036.1 nm and 6.5 nm, respectively. The combined beam can be compressed to 494 fs (assuming the pulse is Gaussian profile) with a compression efficiency of 73.3% (Fig.4). Conclusions In this study, the coherent polarization beam combination of two ultrafast laser channels is successfully realized based on fiber stretcher and SPGD algorithm. Compared with the conventional electro-optical phase modulator, the fiber stretcher not only avoids the spectral modulation of the pulse signal, but also has smaller insertion loss, larger phase adjustment range and higher damage threshold. In the experiment, the highest combined power is 10.9 W with a combining efficiency of 90.1%, and the phase residue error is about λ/31 in the closed-loop state. The combined beam can be compressed to 494 fs with a compression efficiency of 73.3%, and the corresponding single pulse energy is 3.99 μJ. The above experimental results verify the feasibility of the fiber stretcher to control the phase in fiber CBC system. The next step involves expanding the system to more channels and higher combined power.

2023 Vol. 52, No. 6

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