Review

Review of multi-wavelength laser technology based on crystalline Raman conversion (invited)
Lv Zhiwei, Liu Zhongze, Chen Hui, Jin Duo, Hao Xin, Fan Wenqiang, Wang Yulei, Bai Zhenxu
2023, 52(8): 20230420. doi: 10.3788/IRLA20230420
[Abstract](312) [FullText HTML] (34) [PDF 2512KB](120)
  Significance   Multi-wavelength lasers that can simultaneously or alternately output different wavelengths have various applications in optoelectronic countermeasures, LiDAR, and medical treatment. However, achieving controllable and efficient multi-wavelength laser radiation is challenging due to the limitations of the emission spectrum and intensity of the laser materials. Nonlinear optical frequency conversion technology, especially stimulated Raman scattering (SRS), is an effective way to expand the laser wavelength range and enhance the laser power. SRS is a third-order nonlinear optical effect that shifts the frequency of the pump through molecular or lattice vibrations in the medium. Raman lasers can obtain high-power, high-beam-quality, and multi-wavelength laser output by utilizing the characteristics of phase conjugation, amplification, and cascade conversion of SRS. This paper introduces the basic principles of SRS and cascaded Raman conversion, summarizes the classification and structure of typical crystal Raman lasers, and reviews the current status, challenges, and opportunities of multi-wavelength laser technology based on crystal Raman conversion.   Progress  The working principle of the stimulated Raman scattering (Fig.2) and the excitation principle of cascaded Raman scattering (Fig.3) are first outlined in this article. Then the basic structure of Raman lasers was discussed (Fig.4), which can be classified into intracavity and external cavity based on the location of the Raman gain medium relative to the laser working material. A special case of intracavity Raman lasers is self-Raman lasers, where the laser working material and the Raman gain medium are the same. Next, the characteristics of different types of Raman gain media, including gas, liquid, and solid are analyzed. Among them, Raman crystals are regarded as a promising medium for multi-wavelength lasers due to their advantages such as high gain, compact structure, and good stability. Typical crystal Raman gain media were compared and their parameters are summarized (Tab.1). Finally, the current research status of multi-wavelength crystalline Raman lasers as well as their features are summarized. Based on the above research status, it is not difficult to find that linear cavities are still the most commonly used resonant cavity structure for generating multi-wavelength Raman lasers, and pulse lasers account for the highest proportion of the research. In addition, compared to intracavity Raman oscillators, external cavity Raman oscillators exhibit higher average and peak power, demonstrating stronger power scalability. Although microcavity Raman lasers currently have low output power and conversion efficiency, they have the characteristics such as high repetition rate and miniaturization.   Conclusions and Prospects   In conclusion, research on multi-wavelength lasers based on crystalline Raman conversion has made significant progress in the past decade, with the discovery of new crystals, structures, and wavelengths. The use of new crystal materials such as diamond has led to a remarkable performance in power enhancement, wavelength expansion, and miniaturization of multi-wavelength Raman lasers. Future research should focus on optimizing pump parameters and oscillator design to improve conversion efficiency, expand multi-wavelength lasers' output spectral range, and improve thermal management under high-power operation to enhance system stability and beam quality. With these advancements, we can expect that multi-wavelength solid-state lasers based on crystalline Raman conversion will play a major role in future applications.
Phase unwrapping technology for structured light three-dimensional measurement: A review (invited)
Zhang Zonghua, Li Yanling, Gao Feng, Gao Nan, Meng Zhaozong, Jiang Xiangqian
2023, 52(8): 20230126. doi: 10.3788/IRLA20230126
[Abstract](520) [FullText HTML] (98) [PDF 1983KB](227)
  Significance   Structured light three-dimensional (3D) shape measurement methods are increasingly used in reverse engineering, aerospace, biomedicine, cultural relics protection and other fields. As a key link in structured light 3D measurement, phase unwrapping plays an important role in accuracy, speed and reliability. This paper reviews the basic principles of phase unwrapping technology, the research status at home and abroad, the advantages and disadvantages of various methods and the future development direction. Firstly, according to the different calculation methods, the existing phase unwrapping methods of structured light 3D shape measurement technology are divided into the following four categories for detailed introduction: temporal phase unwrapping, spatial phase unwrapping, deep learning-based phase unwrapping and other phase unwrapping. Then, the advantages and disadvantages of various technologies are compared in detail. Finally, the characteristics of phase unwrapping technology are summarized and the future research direction of this technology is prospected. Based on the review in this paper, the principles and progress of various phase unwrapping methods can be understood. Moreover, according to the characteristics of different technologies, combined with application requirements and actual measurement conditions, the most effective phase unwrapping method can be selected to achieve accurate 3D shape measurement.  Progress   Phase unwrapping is a key technique involving multiple application fields, and the phase unwrapping method applied in structured light three-dimensional shape measurement technology is mainly reviewed. According to different calculation methods of phase unwrapping, it can be divided into the following four types: temporal phase unwrapping, spatial phase unwrapping, phase unwrapping based on deep learning and other phase unwrapping.　　The methods proposed during the development of temporal phase unwrapping can be summarized into four categories: phase unwrapping methods based on gray codes, multi-frequency methods, multi-wavelength methods and phase unwrapping methods based on number theory. Spatial phase unwrapping method mainly introduces the principle and development of the quality guided phase unwrapping and the branch-cut phase unwrapping. The phase unwrapping method based on deep learning mainly analyzes the advantages and disadvantages of various methods in terms of improving measurement efficiency and reducing the number of projection patterns. Other phase unwrapping methods mainly include space-time phase unwrapping, geometric constraints and photometric constraints, etc. The application scenarios and main advantages and disadvantages of the methods are introduced.　　In order to describe several phase unwrapping methods in more detail, this paper compares and summarizes the performance of several typical phase unwrapping methods. The comparison is made in terms of the number of projected fringe patterns required, measurement speed, noise immunity performance and calculation accuracy. In addition, some conclusions are made to the comparison results.　　Finally, the future development direction of the phase unwrapping method is summarized, aiming to provide reference for the development and research of fringe projection technology.  Conclusions and Prospects  In this paper, the methods of phase unwrapping are classified and summarized. The temporal phase unwrapping is more suitable for high measurement accuracy and no restrictions on measurement time. Due to the advantage of fast measurement speed, the spatial phase unwrapping method is more suitable for high-speed applications. The phase unwrapping method based on deep learning can solve the shortcomings of the temporal phase unwrapping and spatial phase unwrapping methods to a certain extent. In addition, other phase unwrapping methods are proposed for specific measurement scenarios and measurement requirements. In view of the importance of the phase unwrapping method in the fringe projection 3D shape measurement process and the factors affecting the accuracy, the future development directions include the following five points: reduce the number of projection patterns, enhance noise resistance and robustness, reduce the complexity of calculation, improve the accuracy of phase unwrapping, and realize high-speed real-time measurement.
Research progress of high-power free-space Raman amplification technology (invited)
Bai Zhenxu, Hao Xin, Zheng Hao, Chen Hui, Qi Yaoyao, Ding Jie, Yan Bingzheng, Cui Can, Wang Yulei, Lv Zhiwei
2023, 52(8): 20230337. doi: 10.3788/IRLA20230337
[Abstract](271) [FullText HTML] (52) [PDF 2089KB](76)
  Significance   Lasers with special wavelengths, high power, and high beam quality have significant applications in the fields such as sodium guide star, laser ranging, and free-space communication. One of the effective approaches to extend the spectral range of lasers is based on stimulated Raman scattering (SRS), which can amplify Stokes beam with a desired wavelength using conventional pump sources. This method can produce high-power and high-quality lasers with special wavelengths, and has advantages such as flexible wavelength selection, simple structure, and strong power scalability. In recent years, SRS-based amplifiers have been applied to generate sodium guide star laser sources, and have potential for further development in other areas. This article reviews the main principles, characteristics, and research progress of high-power free-space Raman amplification technology, and discusses its future trends and application prospects.   Progress  Currently, the commonly used gain media for Raman amplifiers include gases and crystals. Gas Raman media have advantages such as a large Raman frequency shift, low self-focusing threshold, low optical coupling wave loss, and almost unlimited size. However, they also have disadvantages such as low gain, large volume, and susceptibility to optical breakdown. Compared to gas Raman media, crystal Raman media have advantages such as high Raman gain coefficient, good thermal conductivity, stable performance, and easy miniaturization. However, there are still bottlenecks in the output power and energy of crystalline Raman amplifiers due to factors such as crystal size and damage threshold. Beam combination based on Raman amplification is also an important way to break through the power bottleneck of a single beam and achieve power scaling. This method has advantages such as simple structure, flexible design, and high expandability, and is expected to be further developed and applied in the field of high-power special wavelength lasers. The parameters of gas Raman amplifiers with free-space structures are summarized (Tab.1). At present, the peak laser power output has reached the megawatt level, and the single pulse energy has reached the joule level. The experimental parameters of some crystal Raman amplifiers are summarized (Tab.2). The pulse width of crystal Raman amplifiers is mainly in the nanosecond, picosecond, and femtosecond levels, with peak power reaching the gigawatt level and single pulse energy reaching the millijoule level.   Conclusions and Prospects  In recent decades, Raman amplifiers in free space have made many outstanding achievements in the field of high-power special wavelength lasers. However, the output power of Raman amplifiers is still limited by factors such as the Raman medium and amplifier structure. To overcome these limitations, future developments in Raman amplification technology will focus on developing new Raman media, optimizing the preparation technology of large-size Raman crystals, improving the conversion efficiency of Raman amplifiers, and expanding the beam combination structure of high-power Raman lasers. In the future, Raman amplification technology is expected to achieve even greater results in the field of high-power special wavelength lasers.
Research progress of all-solid-state intra-cavity vortex beam generation in visible laser (invited)
Qi Yaoyao, Li Junchen, Zhang Yu, Yan Dapeng, Chang Feng, Yu Xiufen, Bai Zhenxu, Ding Jie, Yan Bingzheng, Wang Yulei, Lv Zhiwei
2023, 52(8): 20230424. doi: 10.3788/IRLA20230424
[Abstract](287) [FullText HTML] (40) [PDF 7496KB](99)
  Significance   In recent years, visible vortex laser carrying orbital angular momentum (OAM) has been widely used in the fields of astronomy, optical manipulation, microscopic imaging, sensing, quantum science and optical communication. Especially for the underwater communication or the super-resolution imaging, further optimizing the output of vortex beams in the visible range is of great significance in enhancing imaging resolution and communication capacity. This not only holds importance in scientific research but also holds vast potential for wide-ranging applications in real-life scenarios, paving the way for advancements in high-resolution imaging, high-speed communication, and other fields.   Progress  Visible vortex beams can be generated through both extra-cavity and intracavity conversion methods. This study focuses on the intracavity conversion approach to obtain visible vortex beams. With the development of the vortex lasers operating at 1 μm, nonlinear frequency doubling has become a common technique for generating visible vortex beams. By utilizing techniques such as intracavity thermal lens effect, etched point defects, and design of a hemispherical resonator cavity, combined with frequency doubling method, visible vortex beams can be generated without the need for additional components. Alternatively, an extra-cavity mode converter can be used to generate vortex beams, which is then combined with frequency doubling method to produce visible vortex beams. Compared to nonlinear frequency conversion techniques, direct pumping the visible laser crystals to obtain visible vortex beams in the visible range can improve conversion efficiency. For the LD pumped Pr3+ doped all-solid-state laser, visible vortex beams can be generated through intracavity mode conversion techniques such as off-axis pumping, annular light pumping, spherical aberration mode selection. Visible vortex fiber lasers offer advantages of compact structure and high conversion efficiency. They mainly utilize techniques such as fiber core misalignment fusion splicing or specially designed mode selectors to generate visible vortex lasers.   Conclusions and Prospects   Currently, visible vortex solid-state lasers are mainly achieved by combining near-infrared vortex beams with frequency doubling or by utilizing LD direct pumped Pr3+-doped crystals combined with intracavity vortex beam conversion technology. The former approach typically requires the insertion of laser crystals and frequency doubling crystals inside the cavity, leading to a complex system structure and lower optical-to-optical conversion efficiency. In the future, visible vortex solid-state lasers have great potential for development in terms of tunability, multi-wavelength operation, high power, and single longitudinal mode characteristics. Achieving multi-wavelength visible vortex beam output and generating ultra-short pulse vortex beams (such as picosecond and femtosecond vortex beams) are among the directions for further advancement. Furthermore, if visible vortex solid-state lasers can be extended to the realm of spatiotemporal mode locking, it will inject new vitality into the development of vortex beams.
Research progress of high-frequency and high-energy solid state lasers at 1.6 µm (invited)
Li Pengfei, Zhang Fei, Li Kai, Cao Chen, Li Yan, Zhang Jiachao, Yan Bingzheng, Bai Zhenxu, Yu Yu, Lv Zhiwei, Wang Yulei
2023, 52(8): 20230403. doi: 10.3788/IRLA20230403
[Abstract](184) [FullText HTML] (44) [PDF 3640KB](77)
  Significance   The laser near 1.6 µm is not only the safe band of human eyes, but also the transmission window in the atmosphere. The high-frequency and high-energy laser close to 1.6 µm can also carry information with high resolution and large amount of data at a longer distances. In recent years, with the improvement of crystal preparation and lens coating technology, the 1.6 µm band laser obtained by directly pumping gain media and frequency conversion technology has greatly improved the parameters such as repetition frequency, energy and beam quality. In this paper, the principles and research progress of 1.6 μm laser generated by erbium-doped crystal direct pumping, optical parametric oscillation and stimulated Raman frequency shift are introduced, the advantages and disadvantages of the above three schemes in 1.6 μm laser are analyzed, and their application prospect in 1.6 μm high-repetition rate and high-energy laser is prospected. The problem of poor output beam quality when high-frequency and high-energy lasers is obtained near 1.6 µm is also analyzed, and several enhancement examples are given. The development prospect of obtaining better beam quality and high-frequency and high-energy lasers by optical parametric oscillation near 1.6 µm is discussed.  Progress  Firstly, the energy level conversion process of the laser near 1.6 µm directly generated by pumping Er3+ doped crystals is given. However, the low absorption efficiency of pump light, the small photon transition cross section, the high number of parasitic lasers in the crystal and the low thermal conductivity of the crystal make the thermal load on the crystal very high. All these reasons limit its application in obtaining high-repetition rate and high-energy lasers at about 1.6 µm band. Then the process of obtaining stokes light by stimulated Raman frequency shift is described. Raman lasers based on conventional Raman gain materials such as BaWO4, SrWO4, Ba(NO3)2, BaTeMo2O9, GdVO4, YVO4 and KGd(WO4)2 are analysed, as their low Raman gain coefficients and the low thermal conductivity and thermal expansion coefficients of the crystals lead to the inability of these non-linear crystals to obtain high re-frequency, large-energy wavelength band lasers near 1.6 µm. In contrast, the high and low thermal expansion coefficients of diamond and its transparency over a wide wavelength range make up for some shortcomings of traditional Raman crystals, but the Raman frequency shift is only 1 332.3 cm−1, so it is still impossible to convert the existing and technically mature high-power 1 µm band lasers to the 1.6 µm band with second-order Stokes frequency shift. These reasons limit the application of stimulated Raman shifts to obtain high-frequency and high-energy lasers near 1.6 µm. Finally, the OPO technique based on KTA and KTP crystals is presented for application in obtaining a human-safe laser output in the wavelength band near 1.6 µm with wide wavelength tuning, higher beam quality, high heavy frequencies and large energy. Although the spot quality of laser output of OPO technology is poor in the wavelength band near 1.6 µm, it is possible to obtain laser output with high repetition rate, high energy and good beam quality in the wavelength band near 1.6 µm with reasonable resonator design, phase matching method of nonlinear crystal, selection of pump wave shape and pulse width, and use of a Gaussian mirror and a quasi-monolithic 90° image rotation, which is certainly what researchers in OPO technology are working hard to achieve.   Conclusions and Prospects  The high-frequency, high-energy laser near 1.6 µm is of great significance because it meets the needs of long-distance and high-data transmission without causing unintentional harm to people nearby. The main methods for obtaining lasers in the 1.6 µm band are pump light direct pumping of Er3+ doped crystals, SRS and OPO techniques. However, the low absorption efficiency of Er3+ crystals, the low thermal conductivity of the gain medium and the short lifetime of the energy level of the crystals make them unable to meet the requirements of high-repetition rate and high energies. The SRS technique is only capable of shifting the 1 µm band to near 1.49 µm due to the low thermal conductivity of the existing Raman medium and the limited Raman frequency shift, while the OPO technique is capable of achieving high-frequency and high-energy output near 1.6 µm by adjusting the parameters of the pump light and resonant cavity with a good nonlinear crystal. Although the beam quality of the output light is not good, laser pulses with good beam quality can be obtained through proper optimization, and there is much room for improvement in the current methods to solve this problem.
Research progress of vector optical beam with longitudinally varying polarization (invited)
Wang Wenyue, Li Jinsong, Guo Jixiang, Lv Jiaqi
2023, 52(8): 20230362. doi: 10.3788/IRLA20230362
[Abstract](162) [FullText HTML] (57) [PDF 2017KB](67)
  Significance   Based on the spatial manipulation technique of polarization, the unique spatially structured properties of polarization make vector optical beam show great research values and application potentials in optics and its interdisciplinary fields. Previous studies mainly focused on the polarization manipulation in the transverse plane, but the longitudinal (propagation) direction is also an important dimension for manipulating optical field. The unique beam, with customized polarization distribution in the longitudinal direction, has attracted increasing attentions in recent years. Beyond enriching the diversity of vector beam, the variation of polarization along the direction of propagation provides increased scope for the light-matter interaction, especially in the optical nonlinear effect and spin-orbit coupling. Moreover, it also offers applied advantages in remote polarimetry, material deep processing and three-dimensional micromanipulation.  Progress   First, the generation principles of vector beam with longitudinally varying polarization are introduced. In order to realize the variation of polarization in the longitudinal direction in free space, the direct method to modulate the propagation environment of the polarized beam are unsuitable and the possibility of modulating the beam at the initial plane to indirectly control the longitudinal distribution of polarization should be taken into consideration. Two main methods reported to achieve longitudinally varying polarization are the construction of varying phase difference and amplitude difference in the propagation direction. The longitudinally varying phase difference is achieved by discrepant initial radial phase modulations on the orthogonally polarized components, while the construction of varying amplitude difference in the propagation direction is achieved by different spatial spectrum filters for the customized amplitude relations between orthogonally polarized components. Relevant experimental methods are summarized which can be divided into the modulation of phase and the filtering of the spatial spectrum. The phase modulation method includes the single-path generation method based on phase mask (Fig.3-6) and double-path generation method based on holographic gratings (Fig.7-9). As the most common method to generate vector beam with longitudinally varying polarization, the phase modulation method has the problem of controllability on the polarization variation. On the one hand, this controllability is reflected in the accuracy of the longitudinal manipulation of polarization. Due to the different initial phase modulation, corresponding variation occurs in the amplitude of orthogonally polarized components during propagation, which will inevitably affect accuracy of polarization manipulation especially for the high-frequency longitudinal variation of polarization. On the other hand, the controllability is reflected in the flexibility of the longitudinal spatial modulation of polarization. Except for the variation of polarization along the equator and meridian of the Poincaré sphere, the continuous longitudinal variation of polarization can also track other trajectories on the Poincaré sphere. The recent works to improve the controllability of longitudinal manipulation of polarization are discussed in detail.   Conclusions and Prospects  Methods to generate vector beam with longitudinally varying polarization along propagation direction have been rapidly developed in recent years. Although many approaches to achieve the longitudinal manipulation of polarization have been demonstrated, there are still some problems to be solved. On the one hand, the generation method which balances efficiency and flexibility will contribute to research and practical applications of vector beams with longitudinally varying polarization. On the other hand, based on the novel spatial manipulation dimension of polarization, the interaction between the longitudinally varying polarization and matter still need to be further studied to give full play to its longitudinal polarization "ruler" role, and the application of unique vector beam in laser depth machining, laser measurement and optical micromanipulation needs to be expanded. The research of this paper aims to provide some reference for the design and generation of vector beam with longitudinally varying polarization. The generation theories and experimental methods of vector beam with longitudinally varying polarization are summarized, and the development prospects are also forecasted, which may be helpful for the manipulation techniques of optical field and its applications in laser fabrication, laser measurement and optical micromanipulation.
Research progress of AlGaN-based DUV μLED (invited)
Liu Zhaoqiang, Jia Tong, Xu Xiangyu, Chu Chunshuang, Zhang Yonghui, Zhang Zihui
2023, 52(8): 20230390. doi: 10.3788/IRLA20230390
[Abstract](290) [FullText HTML] (69) [PDF 2703KB](55)
  Significance   Recently, deep-ultraviolet (DUV) communications based on DUV micro-LED technology have drawn a significant interest. This is because deep-ultraviolet (DUV) communications possess a number of advantages such as the low back-ground noise, a non-line-of-sight (NLOS) link and high security. However, its development is constrained by the lack of light sources with high power and high modulation bandwidth. In recent years, the rapid advancement of low-cost, high-output power AlGaN-based DUV LEDs has greatly accelerated the development of UVC communication and its application in various fields. Moreover, the DUV μLEDs with small chip size have the advantages of high modulation speed and low power consumption, making them attractive for implementing high-speed UVC systems. However, the low luminous efficiency of AlGaN-based μLED seriously affects the data transmission rate in deep ultraviolet communication. Therefore, we provide a review and comprehensive analysis of the size effect on the optical, electrical, thermal and modulation properties for AlGaN-based μLED, including its underlying physics mechanism. In addition, we also review various approaches to improve the light extraction efficiency and thermal characteristics of DUV μLED, which is of great significance for the study of DUV μLED.   Progress  Firstly, the current research status of DUV μLED as a solar blind UV communication source is introduced. The performance for UV communication system utilizing LED as a light source is summarized (Tab.1). It can be seen that under the same modulation mode, larger bandwidth and higher data transmission rate can be achieved with DUV μLED. In addition, due to the rapid attenuation of ultraviolet light power in the atmosphere, the transmission rate decreases for long distance communication. Therefore, ensuring both a large modulation bandwidth and a high optical output power for DUV μLED are very crucial for the high-speed propagation of DUV μLED optical communication system. The optical and electrical properties of DUV μLED are significantly affected by its size. The smaller size of the μLED enable them to withstand a higher current density, while the capacitance decreases as the size decreases. Consequently, the μLED with smaller size exhibits a higher modulation bandwidth. However, the reduction of the active area results in a decrease in output power as the size decreases. Additionally, the severe self-heating effect induces a thermal droop in EQE, making it challenging to achieve high power with high work currents. The low light extraction efficiency (LEE) and increased series resistor further deteriorate the self-heating effect. Therefore, to break the bottle of the light output power of μLED, it is necessary to improve the LEE, the series resistor and heat dissipation. Various micro-nano structures for nAlGaN, pAlGaN and sapphire can be used as scatter centers to improve the LEE. The patterned pAlGaN exhibits the most significant effect in improving LEE due to its proximity to the active region. However, it generally brings in a higher work voltage. Increasing the ohmic contact area and only patterning the area around the p-electrode can avoid the disadvantage. In addition, the inclined sidewall technology shows a significant potential for enhancing the LEE of DUV LED. And the shape and the sidewall reflector for the inclined sidewall have a substantial influence on the LEE of DUV μLED. Furthermore, to mitigate the self-heating effects of the device, the ohmic contact resistivity of DUV μLED device should be decreased, and the reflectivity of electrode should be increased. Therefore, the designed electrode needs to possess excellent ohmic contact and high reflectivity. Meanwhile, the device heat dissipation can be improved by increasing the electrode contact area and the device side wall area. Various technologies, such as a rectangle chip shape and a metal radiator can be utilized to enhance the device heat dissipation of DUV μLED.   Conclusions and Prospects  This paper presents a systematically review of the research status of DUV μLED in the field of wireless optical communication. And the size effect on the modulation characteristics, light extraction efficiency, current and voltage characteristics, optical power characteristics and side wall defect ratio are comprehensive analyzed and its underlying physical mechanism is also shown. Various technologies for improving the efficiency of light extraction and heat dissipation are summarized and discussed in detail. Although a great progress have been made in the development of DUV μLED, further research should be dedicated to enhancing the LEE and heating dissipation of DUV μLED. Especially, the electrode and the chip shape need to be designed to ensure high reflectivity and excellent ohmic contact, high efficiency scatter and good heating dissipation.
Advancements in fusion calibration technology of lidar and camera
Wang Shiqiang, Meng Zhaozong, Gao Nan, Zhang Zonghua
2023, 52(8): 20230427. doi: 10.3788/IRLA20230427
[Abstract](321) [FullText HTML] (100) [PDF 1657KB](140)
  Significance   The data gathered by a singular sensor is inherently incomplete. For instance, the point clouds obtained by lidar lacks texture and color information, and the picture captured by camera lacks depth information. The data fusion of lidar and camera enable the harnessing of complementary information between the sensors, resulting in the acquisition of precise three dimensional (3D) spatial perception, which is widely applied in various fields, including autonomous driving and mobile robotics. In recent years, a lot of scholars at home and abroad have made significant research advancements in the field of sensor fusion, especially in the fusion of lidar and camera. However, there is a lack of a comprehensive paper summarizing the research achievement in the field of sensor fusion by scholars from various backgrounds. This paper provides a comprehensive summary of the research outcomes pertaining to the calibration method for lidar and camera fusion, which serves as a valuable reference for future researchers working in this field. Additionally, this paper serves as a helpful resource for beginners seeking a concise introduction to the subject, allowing them to quickly familiarize themselves with the calibration method for lidar and camera fusion.   Progress  First, the fundamental principles and techniques involved in the calibration of lidar and camera systems are presented. The fundamental principles of camera calibration is introduced. Moreover, a succinct overview of the existing camera calibration methods is provided, accompanied by a delineation of their individual characteristics. Simultaneously, the principle and classification of lidar are introduced, and the characteristics of different types of lidar are analyzed. A mathematical model for mechanical lidar is established and the calibration methods for internal parameters of mechanical lidar are summarized. Furthermore, the principle of joint calibration for lidar and camera is introduced. 　　Secondly, the calibration process of lidar and camera systems involves two main stages of feature extraction and feature matching. The processing methods of point cloud and image are briefly introduced, then extrinsic calibration methods of lidar and camera are emphatically introduced. The extrinsic calibration methods of lidar and camera systems can be categorized into target-based calibration, targetless-based calibration, motion-based calibration and deep learning-based calibration. The existing research results of each calibration method are summarized. The target-based calibration approach achieves high precision. However, it entails a complex calibration process. The targetless-based calibration method is simple and convenient, allowing for online calibration, but it exhibits lower calibration accuracy compared to the target-based calibration. The motion-based and deep learning-based calibration methods are considered as pivotal research directions for future advancements. 　　Finally, We conclude the paper and highlight the future development trends. Feature extraction and matching are the key progress in the calibration of lidar and camera. Although there have been many kinds of calibration methods for lidar and camera, it still needs a better way to improve the accuracy and robustness of the calibration results. In recent years, the development of deep learning technology has provided new opportunities for the fusion of lidar and camera data, and proposed new directions for online calibration in natural scene.   Conclusions and Prospects  Lidar and camera calibration has emerged as a significant research area, aiming to compensate for the limitations of individual sensor information and enable accurate perception of 3D information. The calibration technology primarily encompasses point cloud processing, image processing, and calibration methods. The crux of the calibration process lies in identifying corresponding features and subsequently matching them. In this paper, the characteristics of four distinct methods of targeted-based calibration, targetless-based calibration, motion-based calibration, and deep learning-based calibration are summarized. The accurate online calibration in diverse scenarios emerges as a prominent research focus in the future. In conclusion, the future research direction of calibration focuses on enhancing accuracy, improving robustness, online calibration, automating calibration, and establishing a unified verification standard. These advancements aim to further enhance the calibration process and its applicability in various domains.
Review on the discipline of high power fiber laser in China
Zhou Pu
2023, 52(7): 20230071. doi: 10.3788/IRLA20230071
[Abstract](305) [FullText HTML] (94) [PDF 969KB](76)
  Significance   High power fiber laser is one of the hot topics in the laser field, and fast development and significant milestones have been achieved in China recently. Although there have been plenty of review papers of high power fiber laser published, most of them focus on the scientific achievements. It is to be noted that, the discipline construction, which includes (but not limits to) education and training, basic and applied research, academic communication, practical application and so on, lays the foundation for the field of high power fiber laser.  Progress  The development of discipline of high power fiber laser in China could be summarized into four stages. In the 1990s, the basic investigation on fiber laser was initiated. In the 2000s, high power fiber lasers based on double clad fiber dominates the field of high power fiber laser, and more than 1 kW output power was achieved by several independent groups. From 2010 to 2017, fast progress was made in the discipline of high power fiber laser, the layout of scientific research was optimized, and different research groups began to focus on key components, high performance fiber laser, fiber laser technology and system integration, respectively, and leading results in single frequency fiber laser, Raman fiber laser, fiber supercontinumm and coherent beam combining, were achieved. Since 2018, the discipline of high power fiber laser develops steadily, high power fiber laser with more than 20 kW output power and multimode fiber laser system with more than 100 kW were developed, which promotes the fast development of advanced manufacturing. However, there are still several challenges for the discipline of high power fiber laser in China. For example, as the most representative result of high power fiber laser, 10 kW single mode fiber laser was still under development, also, the performance lacks in the field of mid-infrared fiber laser and ultrafast fiber laser. In addition, it also lacks in original theoretical results such as transverse mode instability and multimode nonlinear fiber optics, the communication between the academic community and application community could be further enhanced.  Conclusions and Prospects  Discipline of high power fiber laser in China has been developed in a fast and stable way in the past few decades, from the current state that includes scientific research, education, academic communication and application. It is suggested that, cooperation in multidiscipline, education, training and course material, and high-quality communication could be enhanced to ensure further development.
Quality characteristics and nondestructive test and evaluation technology for laser additive manufacturing alloy steel components (invited)
Xu Binshi, Dong Shiyun, Men Ping, Yan Shixing
2018, 47(4): 401001. doi: 10.3788/IRLA201847.0401001
[Abstract](580) [PDF 2656KB](210)
Nondestructive test(NDT) technology is the important technical support for laser additive manufacturing alloy steel components, the key technology to ensure laser additive manufacturing production quality and in-service safety and the important technical composition to the production safety guarantee through life cycle. The formation, texture and mechanics properties of alloy steel components made by laser additive manufacturing are different from those made by traditional technologies, so NDT technology faces many challenges. The forming quality characteristics of laser additive manufacturing alloy steel were summarized, including forming flaws and mechanics properties; Based on the development of NDT technologies, the applications of NDT technologies in laser additive manufacturing were reviewed, especially in applications of material mechanics properties and flaws; Based on ultrasonic and micro-magnetic techniques, micro-magnetic sensor design scheme, calibration method and principles of evaluating the material mechanics properties were outlined; Finally, the challenges and prospects of NDT in laser additive manufacturing alloy steel components were discussed.
Development of beam combining of high power high brightness diode lasers
Wang Lijun, Peng Hangyu, Zhang Jun, Qin Li, Tong Cunzhu
2017, 46(4): 401001. doi: 10.3788/IRLA201746.0401001
[Abstract](679) [PDF 2795KB](780)
Diode lasers have advantages of high efficiency and compact structure. The low power and poor beam quality of diode lasers limits its application fields. The new developments of diode lasers in power and beam quality were introduced. The new combining techniques and the experimental results of diode lasers were summarized. The high brightness diode laser achievements acquired by CIOMP were also presented.
Discussion of the laser ranging with polarization spectral imaging observations and communication technology for space debris
Jiang Huilin, Fu Qiang, Zhang Yalin, Jiang Lun
2016, 45(4): 401001. doi: 10.3788/IRLA201645.0401001
[Abstract](482) [PDF 1862KB](354)
With the increase of human exploration of space activities, space debris detection is particularly important. In this paper, the space debris hazard and detection significance were firstly introduced. Then, the main difficulties and the development trend of space debris detection were analyzed. On this basis, combined with space debris detection difficulties, a new scheme of detection for space debris and information transmission was proposed, which integrated laser ranging, spectral polarization imaging, laser communication features. And the key technology to decompose and feasibility analysis were made, which may provide a new way of thinking for space debris detection.
Development of new inertial technology and its application in aerospace field
Wang Wei
2016, 45(3): 301001. doi: 10.3788/IRLA201645.0301001
[Abstract](639) [PDF 999KB](438)
Dynamic precise measurement of movement information constructs the foundation of guidance,navigation and control of various vehicles. The inertial technology is the only independent means to establish the position and attitude reference of a vehicle in all kinds of environments, so it is the basis of dynamic precise measurement of movement information. The development of inertial technology, including optical gyroscope and its inertial navigation system, MEMS gyroscope and accelerometer, atom gyroscope and accelerometer, other kinds of gyroscope and accelerometer, micro-technology for positioning, navigation and timing, and inertial executer, was reviewed in this paper, and the main technology problems of these gyroscopes, accelerometers and inertial navigation systems which are needed to be resolved in aerospace field were proposed. The main application of these inertial technologies in domain aerospace field was described, including application in satellites, launch vehicles, manned aerospace and moon explore. Finally, three development trends of the inertial technology in aerospace field were given.
Progress on sodium laser guide star
Xu Zuyan, Bo Yong, Peng Qinjun, Zhang Yudong, Wei Kai, Xue Suijian, Feng Lu
2016, 45(1): 101001. doi: 10.3788/IRLA201645.0101001
[Abstract](738) [PDF 2925KB](579)
Astronomical telescope is always one of the vital tools that help human kind to unveil hidden natural laws in the universe. However, spatial resolution of the large ground-based telescope was severely limited because the turbulence of atmosphere degenerates the perfect wavefront from stars into an aberrated one, which was the key science and technology difficult problem to achieve high resolution astronomical observation. So adaptive optics(AO) was being pursuit by many teams internationally to correct the wavefront aberration and make large ground-based telescope resolving power to near diffraction limitation. It indicates that the ground-based optical telescope was currently turning into the AO telescope age. Sodium laser guide star(LGS) generated by laser exciting sodium atoms in the mesospheric layer at an altitude of about 90 km, as a beacon of AO correction, was the cutting-edge technology for the AO telescope. The theory, methods and development status of the sodium LGS were described in this paper. Especially in our lab, micro-second sodium LGS laser system suitable for efficient excitation of the sodium layer was developed with spectral format matched to the mesospheric D2 line. The sodium LGS laser system was successful to apply in some domestic and overseas large telescopes.
Progress of low level light video technology
Jin Weiqi, Tao Yu, Shi Feng, Li Benqiang
2015, 44(11): 3167-3176.
[Abstract](1049) [PDF 2816KB](2072)
As one of the key technologies to expand human eye's night visual perception, low level light (LLL) night vision technology has wide application in military and civilian fields. With the development of digital image processing technology, LLL video devices not only improve the night vision image quality through the image processing, but also provide a broad space for image information fusion with the infrared thermal imaging and the improvement of nighttime target detection/recognition, scene understanding ability and so on, become one of the important directions of current night vision technology both at home and abroad. This paper reviews the development of LLL video devices, analyzes the characteristics and development trends of electronic vacuum + solid LLL video imaging devices(such as ICCD/ICMOS, Electron Bombardment EBCCD/EBCMOS, etc.), all solid-state LLL video imaging devices (such as electron multiplying EMCCD, Extreme Low-Light CMOS, etc.), analyzes and discusses the development of LLL night vision technology combined with PHOTONIS LYNX program.

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