SignificanceStructured 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.ProgressPhase 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 ProspectsIn 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.

SignificanceLasers 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.ProgressCurrently, 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 ProspectsIn 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.

SignificanceBased 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.ProgressFirst, 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 ProspectsMethods 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.

SignificanceRecently, 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.ProgressFirstly, 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 ProspectsThis 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.

SignificanceThe 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.ProgressFirstly, 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 ProspectsThe 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.

SignificanceMulti-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.ProgressThe 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 ProspectsIn 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.

SignificanceIn 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.ProgressVisible 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 ProspectsCurrently, 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.

SignificanceThe 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.ProgressFirst, 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 ProspectsLidar 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.

ObjectiveInfrared image technology is capable of working in low-light and adverse weather conditions. Infrared vehicle detection technology is designed to use infrared sensors to monitor vehicles on roads, enabling the collection and analysis of information related to vehicle quantity and speed, which can be used to achieve traffic management and safety control. This technology can be applied not only to road vehicles, but also to rail transport, airports, and ports, providing effective technical support for the safety and convenience of the transportation industries. However, infrared vehicle detection still faces many challenges due to the low resolution, low contrast, and blurred edges of small targets in infrared images. Traditional hand-crafted image feature extraction methods are not adaptable nor robust, require substantial prior knowledge and have low efficiency. Therefore, this paper aims to explore deep learning-based vehicle detection models, which plays an important role in traffic regulation.MethodsYOLOv5 is a one-stage object detection algorithm that is characterized by its lightweight design, ease of deployment, and high accuracy, making it widely used in industrial applications. In this paper, a CFG mixed attention mechanism (Fig.2) is introduced into the model backbone to help the model better locate the vehicle area in the image and improve its feature extraction ability, due to the low resolution of infrared images. In the feature fusion part, an improved Z-BiFPN structure (Fig.5) is proposed to incorporate more information in the shallow fusion, thereby improving the utilization of shallow information. A small object detection layer is added, and the Decoupled Head (Fig.6) is used to separate classification and regression, improving the model's ability to detect small target vehicles.Results and DiscussionsIn order to improve the model's generalization ability, an infrared image dataset INFrared-417 (Fig.7) consisting of seven categories of bus, truck, car, van, person, bicycle and elecmot, was constructed by collecting data and combining existing infrared datasets. The main evaluation metrics used were AP (Average Precision) and mAP (mean Average Precision), with P (Precision) and R (Recall) as secondary metrics for the experiments. The ablation experiment results (Tab.1) confirmed the effectiveness and feasibility of the proposed improvement methods, with mAP improving by 4.0%, and AP significantly improving for the van, person, and bicycle categories, while P increased by 1.7% and R increased by 3.6%. In addition, the comparison results (Fig.10) demonstrated that the improved model reduced false alarm and missed detection rates, while improving the detection of small targets. The comparison experiment results (Tab.2) also showed that the proposed improved model had excellent performance in terms of detection accuracy and model parameter count.ConclusionsThis paper proposes an improved infrared vehicle detection algorithm. By introducing the mixed attention mechanism, the model is able to better focus on the vehicle region in the image and enhance its feature extraction ability. The improved Z-BiFPN is used in the model neck to efficiently integrate context information. At the same time, the detection head is replaced with a more advanced Decoupled Head to improve the detection ability, and a small object detection layer is added to improve the ability to capture small targets. It is hoped that this model can be applied in traffic control.

ObjectiveThe somatic cell count (SCC) in raw milk is an important basis for determining whether a cow is suffering from mastitis. Identifying cows with mastitis by testing the SCC, then isolating and treating them as early as possible, can effectively prevent the spread of bacteria in the herd to reduce consequential economic losses. However, traditional methods may lead to uneven distribution of somatic cells during milk sampling, such as cell adhesion settlement, and unrepresentative somatic cell count due to lack of matching imaging system. In this paper, a method is proposed which is based on the nine-cell grid microfluidic chip to make somatic cell evenly distributed and develop a two degree of freedom displacement platform equipped with a micro lens to improve the counting accuracy .MethodsFirstly, a simulation was performed to verify the uniformity of the somatic cell distribution within the chip observation cavities (Fig.1). And based on the simulation results, a nine palace grid microfluidic chip was prepared (Fig.2). Secondly, a two-degree-of-freedom displacement platform (Fig.6) equipped with a micro-camera lens is developed, which can automatically take images of the nine observation cavities of the chip, making image acquisition more convenient. Finally, somatic cells were counted by image processing (Fig.3), so as to verify the uniformity of somatic cell distribution, obtain the counting accuracy, and judge the health status of cow udder.Results and Discussions20 cows were randomly selected from local pastures to verify the performance of the proposed method. From the data in Tab.1, it can be seen that the standard deviation coefficient of the SCC in each group of nine images is less than or equal to 1.61%, which verifies the uniformity of the distribution of somatic cells in the nine observation cavities of the microfluidic chip. The system ensures the uniform distribution of somatic cells and renders the taken samples more representative. As can be seen from Fig.9(b), the maximum absolute value of the relative error of the system of automatic counting is 2.93%, the minimum value is 0.53%, and the average value is 1.72%. The maximum relative count error of the automatic counting is obtained as ±2.93%. The system has a very high accuracy for automatic counting.ConclusionsThe experimental results show that the somatic cell counting system developed in this paper can make the somatic cell distribution in fresh milk more uniform and count more accurately. The standard deviation coefficient of the number of somatic cells in each group of nine images was less than or equal to 1.61%, and the smaller the standard deviation coefficient is, the more uniform the distribution of somatic cells is. The accuracy of the automatic system counts ranged between 97.07% and 99.47%. This research method lays the foundation for the detection and prevention of mastitis in cows.

ObjectiveThe all-solid-state multi-wavelength red laser has significant applications in laser color large-screen displays, high-density holographic storage, measurement, and medical treatment. Its multi-wavelength characteristics also enable it to serve as a terahertz light source through difference-frequency generation. Currently, the multi-wavelength red laser can be generated by combining the emission spectrum of an inversion particle gain medium with second-order nonlinear effects. However, these methods typically have lower conversion efficiency. Stimulated Raman scattering (SRS) is a high-intensity third-order nonlinear effect that offers flexible wavelength conversion, automatic phase matching, and beam cleanup. The cascaded frequency shift property of Raman crystals is an effective method for achieving multi-wavelength output using a single pump wavelength. Diamond crystals have a high Raman gain coefficient in the visible wavelength range compared to conventional Raman crystals. Pumping diamond with a well-established 532 nm laser has great potential for obtaining efficient, high-energy, high-beam quality multi-wavelength red laser output. In this study, we investigate the generation of multi-wavelength red laser output using cascaded diamond Raman oscillators pumped by a 532 nm laser and explore their output characteristics.MethodsThe setup of the multi-wavelength red diamond Raman laser is shown (Fig.1). The pump source is a self-built 532 nm frequency doubled nanosecond laser. The pump beam is collimated by the lens group F1 and F2. A half-wave plate (HWP) is used to adjust the polarization direction of the pump to be parallel to the <111> axis of the diamond crystal for the maximum Raman gain. The diamond Raman oscillator uses a plane-concave cavity with a curvature radius of 200 mm as the output mirror. The diamond size is 2 mm× 4 mm× 7 mm. The coating parameters of the two cavity mirrors are shown (Tab.1). The cavity mirrors are high reflection coated at first-order Stokes to increase the conversion efficiency and obtain pure higher-order Stokes output. The lens F3 is used to control the pump radius in the diamond crystal to about 350 μm. The total length of the Raman cavity is 60 mm, and the distance from the output coupler to the end surface of the diamond is 7 mm. The intrinsic modes of the Raman cavity for each order of Stokes are shown (Fig.2), with a diamond between the purple dashed lines. The radius of the TEM 00 modes of the first, second, third and fourth-order Stokes are 128, 133, 139, 146 μm, respectively. Results and DiscussionsThe spectra of second-order Stokes, second- and third-order Stokes, and second- to fourth-order Stokes were collected at pump energies of 343, 437, 1165 μJ, respectively (Fig.3). The frequency shift between each Stokes order was 1 332 cm-1, consistent with the inherent Raman frequency shift of diamond. With a maximum pump energy of 1 738 μJ (Fig.4(a)), three wavelength lasing in red with energies of 143, 425, 65 μJ were obtained, with slope efficiencies of 9.7%, 31.3%, and 8.7%, respectively. The conversion efficiency increases with pump energy and levels off (Fig.4(b)). A multi-wavelength red laser output energy of 633 μJ was obtained at a maximum pump energy of 1 738 μJ, with a slope efficiency of 45.3% and an optical-to-optical conversion efficiency of 36.4%. The temporal waveform of the incident pump at 532 nm and the output Stokes of each order at maximum pump energy were measured to be 11.43, 10.41, 3.75, 2.45 ns, respectively (Fig.5). The pulse width of each Stokes order is compressed compared to the pump, with more evident compression as the Raman order increases. The near-field spot of each Stokes order has no obvious distortion. The optical-to-optical conversion efficiency can be improved by optimizing the Raman cavity mode-matching degree, and the energy ratio of each wavelength in the multi-wavelength output can be controlled by designing the mirror coating. ConclusionsIn this study, we developed a 532 nm pumped multi-wavelength diamond Raman laser and investigated its cascaded Raman laser output energy, spectrum, and pulse characteristics at different pump energies. Cascaded Raman outputs of 620, 676, and 743 nm were successfully demonstrated. With a maximum pump energy of 1 738 μJ, the output energies of 143 μJ at 620 nm, 425 μJ at 676 nm, and 65 μJ at 743 nm were achieved, with pulse widths of 10.41, 3.75, and 2.45 ns, respectively. Meanwhile, the near-field beams of all the orders exhibit good spatial distribution. The output energy of the combined multi-wavelength red laser was 633 μJ, with an optical-optical conversion efficiency of 36.4%. The results show that the visible light-pumped diamond Raman laser has tremendous potential for efficient all-solid-state miniaturized multi-wavelength lasers in red due to its extremely high Raman gain coefficient and excellent photothermal properties. This study can also provide guidance for the development of multi-wavelength Raman lasers pumped by other wavelengths.

ObjectiveThe poor thermal conductivity of Er:Yb:glass leads to poor heat dissipation in the laser system and the increased thermal load leads to severe thermal effect, which limits the laser output of high power, but there are few studies on the thermal effect of Er:Yb:glass. When the laser interacts with the gain medium, a portion of the pump light absorbed by the crystal will be converted to heat and stored in the crystal, resulting in a rise in its temperature, which becomes one of the factors limiting the output energy. The influence of thermal effect on the laser has two main aspects. On the one hand, as the laser crystal temperature increases, the fluorescence spectral line will broaden and the quantum efficiency will decrease, which will eventually lead to the decrease of conversion efficiency. On the other hand, the thermal stress and thermal lens effect generated by the temperature gradient will seriously affect the output stability and beam quality of the laser. Therefore, it is necessary to investigate the heat treatment capability of Er:Yb:glass as gain medium in order to develop a scheme for further optimization of the output performance.MethodsThe heat accumulation process inside Er:Yb:glass as the gain medium is calculated in detail by finite element analysis method. Assuming that the surface temperature of the crystal is constant with the ambient temperature, the heat generated inside the medium is mainly dissipated by heat conduction, and the outer surface is affected by natural heat convection of air. When the LD is end-pumped with Er:Yb:glass, the pump source acts as an internal heat source and operates with a temperature gradient, which triggers the heat conduction process (Fig.2). In order to compare the heat dissipation performance of the gain medium under different conditions, the simulation is carried out after reaching steady state. The crystal model was divided according to the ultra-fine grid, and the effects of non-bonded and bonded crystals as well as different pump wavelengths, powers and beam waist radius on crystal temperature distribution, thermal stress and deformation were quantitatively analyzed.Results and DiscussionsCo:MALO in the bonded crystal not only acts as a Q-switched crystal, but also as a heat sink. The bonded crystal significantly reduces the influence of thermal effects, especially the temperature of the surface after the gain medium (Fig.4). The pump wavelength of 940 nm is more penetrating to the gain medium than 976 nm, but the temperature diffusion range at 976 nm is less than 940 nm (Fig.6). The temperature of the crystal pumped at 976 nm is higher than that of 940 nm (Fig.7), but the temperature drops faster at the central wavelength of 976 nm. At this wavelength, the most dangerous effect of the temperature rise is on the surface. The increase of pump power of 100 mW corresponds to the increase of temperature of 9 K (Fig.8).The increase of the beam waist radius leads to the decrease of the optical power density and the decrease of the temperature of the bonded crystal, with only a large difference in temperature at the center of the crystal surface. The pump power is proportional to the thermal deformation. The increase of pump power of 100 mW corresponds to the increases of the thermal deformation of the crystal of 0.5 μm (Fig.17). At the same pump power, the thermal deformation will decrease with the increase of the beam waist radius, but the resulting change is not significant compared to the pump power. Co:MALO crystal bonding on the pump side can effectively reduce temperature (Fig.10), thermal stress (Fig.14) and deformation variables (Fig.18). ConclusionsThe finite element method was used to study the LD pumped Er:Yb:glass/Co:MALO based on the theory of heat conduction. Due to the heat sink effect of CO:MALO, the bonded crystal can reduce the maximum temperature, thermal stress and thermal deformation of the laser crystal. On this basis, the pump with a central wavelength of 940 nm will reduce the maximum laser temperature, while the 976 nm pump structure is a safer structure for diffusion bonding. Increasing the pump power and decreasing the beam waist radius will also lead to the thermal effect of the crystal. In the design of the laser system, the thermal effects on the output characteristics should be mitigated by preventing excessive crystal temperatures and deformations. This study provides an optimized condition for the further design of Er:Yb:glass laser with better thermal performance, and also provides a theoretical basis for the output of 1.5 μm laser with high power and beam quality.

ObjectiveExternal-cavity tunable semiconductor laser (ETSL) has been widely studied and acted as a prior selected laser source for its prestigious characteristics such as broad wavelength tuning range, single mode, narrow linewidth, and compactness. However, limited by the intrinsic operation characteristics of currently available semiconductor lasers, it is difficult to obtain a wide-range tunable laser beam output with high spectral purity directly generated by traditional monolithic semiconductor lasers. Particularly, most applications require that the output wavelength of the ETSL can be scanned continuously over time. Consequently, it is critical to build and maintain an ETSL system with a wide mode-hopping free tuning range. For this purpose, a Littman-Metcalf external-cavity oscillation structure is designed in this paper.MethodsFirst, according to the principle and characteristics of the Littman-Metcalf external-cavity oscillation structure, a 900 grooves/mm blazed grating is used as the external-cavity feedback element, single-angled facet gain chip is served as the laser gain medium (Fig.1). Then, the threshold current performance of the ETSL system is characterized by measuring the output optical power at different lasing wavelengths to determine a minimum working current (Fig.3(a)). Finally, the linewidth of the ETSL system with a wide mode-hopping free tuning range at different lasing wavelengths are compared (Fig.6).Results and DiscussionsThe designed total physical lengths of the laser cavity are changed to obtain superimposed optical spectra for different resonance wavelengths. The injection current is fixed at 410 mA and the ambient temperature is adjusted at 25 ℃, and the tuning range results are highlighted (Fig.3(c)). The single-mode operation of different lasing wavelength can be clearly identified, and the side mode suppression ratio of the system satisfies the demand of optical frequency reflectormeter. Meanwhile, the peak output power of 16.95 dBm, full range power of better than 14.96 dBm are obtained (Fig.3(d)). In the current implementation, the overall physical length of the ETSL cavity is designed to be about 50 mm, namely from the gain chip rear (left) output facet to the tuning mirror front facet, and corresponds to an axial mode spacing of 24 pm operating at 1 550 nm. The mode-hopping performance of external-cavity semiconductor laser with blazed grating is characterized by using the wavelength difference measurement method, no mode-hopping can be observed in the wavelength range of 1 480-1 580 nm (Fig.4). Stability performance of the wavelength and output power are monitored using the commercial wavelength meter (Fig.5), within a 130 mins duration, the designed ETSL has good wavelength stability (±2.5 pm) and power stability (±0.035 dB). Based on the short delay self-heterodyne interferometry, the spectral linewidth is measured to be less than 98.27 kHz within the full tuning range, the minimum spectral linewidth is 64.11 kHz around lasing wavelength of 1570 nm (Fig.6).ConclusionsA wide mode-hopping free and narrow linewidth external-cavity tunable semiconductor laser is designed, which is based on a classical Littman-Metcalf configuration. Meanwhile, the tuning characteristics and spectral linewidth of the ETSL are investigated experimentally. A wide mode-hopping free continuous wavelength tuning range of about 100 nm (namely, 1 480-1 580 nm) with a side mode suppression ratio of more than 65.54 dB and an output power of more than 14.96 dBm over the whole tuning range can be achieved in a long-term free running. The spectral linewidth performance of the designed tunable laser source measured using short delay self-heterodyne interferometry is less than 98.27 kHz. With the help of this designed tunable laser source, it is helpful to promote its application in improving the measurement accuracy of optical frequency reflectormeter. Future work shall focus on the optimization of the length of the laser cavity design to further reduce the spectral linewidth.

ObjectiveThe stimulated Brillouin scattering phase conjugated mirror (SBS-PCM) has garnered significant attention in the laser field due to its ability to compensate for both static and dynamic wavefront distortion in real time and enhance beam quality. However, there remain concerns regarding optical breakdown and degradation of output beam quality under high power pumping. Liquid gain medium is currently the most widely used SBS medium due to its characteristics of high gain, high damage threshold resistance and strong size expansion. However, with the increase of injection power, thermal convection caused by absorption of liquid medium will cause wavefront distortion in reflected Stokes light, resulting in reductions of beam quality.MethodsThe finite element method was involved, and the 2-dimentional thermal convection at the focus section was solved by coupling the continuity equation, momentum equation, energy equation and the internal heat source equation. The boundary condition was adiabatic, and the numerical model of thermal convection in the medium cell under high power pump was developed. The dimensionless Se number is introduced to calculate the eddy flux to quantify the thermal convection intensity in the medium cell. Results and DiscussionsThe variation of the Se number with the interaction time is quantitatively analyzed, and the influence of the pump light repetition rate on the thermal convection intensity distribution is emphatically discussed. The results show that, starting from the pump light injection medium, the Se number firstly increases and then decreases, and finally tends to be stable. In addition, when the repetition rate increases from 10 Hz to 250 Hz,the maximum Se number increases from 10 to 49, and the stable Se number increases from 6 to 31, but the time taken for the Se number to reach the maximum value decreases from 9 s to 3 s. The time taken to reach the stable value is reduced from 37 s to 19 s (Fig.4). The contour of thermal convection velocity and density distribution and corresponding experimental observed spatial profiles at different repetition rates were shown (Fig.5-8). With the increase of repetition rate, the intensity of thermal convection increases, and the distribution of low-density areas in the medium cell expands, leading to the increase of the horizontal and vertical deformation of light spots. ConclusionsThe relationship between pump light repetition rate and thermal convection in liquid medium is analyzed from liquid medium flow. The pump light repetition rate is an important factor affecting the thermal convection intensity, the thermal convection intensity is positively correlated to the repetition rate, and the time for the thermal convection intensity to reach the extreme value and the stable value is negatively correlated with the repetition frequency. With the increase of thermal convection intensity, the degree of spot migration increases gradually. This study provides a new perspective for perfecting the model of photothermal effect.

ObjectiveBrillouin laser is an important technological approach for achieving high coherence and low noise lasing, among which Brillouin lasers in free space have been proven to generate high-power single-frequency laser radiation. However, unlike the widely studied guided-wave-based Brillouin lasers, no studies on the linewidth properties have been reported for Brillouin lasers in free space. In this paper, a series of research works have been conducted on the generation, parameter regulation, and performance optimization of the Brillouin lasers in free space using diamond as gain media. We experimentally studied the feasibility of realizing linewidth narrowing of the Brillouin laser in free space.MethodsThe structure of the spatial Brillouin laser and the corresponding linewidth measurement device is shown respectively (Fig.1(a), (b)). The Brillouin laser uses a diamond crystal as the Brillouin gain medium, which has the highest known thermal conductivity and transmission range. A directly pumped ring cavity structure is used for the experiments, where the linewidth of the pumped light is 7.36 kHz. The linewidth behavior of the Stokes light is comparatively investigated by choosing three different sets of coupled mirror reflectivity: R1 = 96%, R1 = 97% and R1 = 98.5% for the experiments. Results and DiscussionsThe measurement results (Fig.2) show that the Stokes linewidth becomes narrower as the coupler reflectivity increases. The Stokes linewidths corresponding to three sets of coupler reflectivity are 3.2 kHz, 2.43 kHz and 1.77 kHz, respectively, and all of them realize linewidth compression compared with the pump, with the highest compression ratio of 4.1. Theoretically, the output efficiency and linewidth compression can be improved at the same time by decreasing the insertion loss of the intracavity element, and the analysis shows that, at the pump power of 60 W and coupled-mirror reflectivity of 96%, the linewidth of 1.6 kHz and up to 80% can be achieved by decreasing the insertion loss of the intracavity element. The analysis shows that at a pump power of 60 W and a coupling mirror reflectivity of 96%, a linewidth of 1.6 kHz and a Stokes output with an optical conversion efficiency of up to 80% can be realized by reducing the insertion loss of the intracavity components. In the future, when realizing ultra-narrow linewidth laser radiation, the technical noise introduced in the system will be the main obstacle limiting the further reduction of the fundamental linewidth.ConclusionsFor the first time, we have verified the feasibility of realizing linewidth-narrowed Brillouin laser output in a free-space optical transport structure. The study provides a feasible technical solution for obtaining high-power, narrow-linewidth lasing with a wide wavelength range. The result is of great significance for promoting the development of diamond laser technology and advancing the application of highly coherent light sources.

SignificanceFlat optics elements based on geometric phase, owing to their low cost, integrability, and versatility, have been widely used in shaping of light's spatial structure. Notably, current SOC (spin-orbit coupling) devices, such as the best-known q-plates, provide only spatial phase modulation with SoP (state of polarization)-switchable behavior. The absence of amplitude control prevents research scholars from accessing light's full spatial degrees of freedom, thus limiting their application in corresponding studies. This team demonstrates a series of novel flat optics elements with liquid-crystal geometric phase, which unlocks the full-field control of paraxial structured light, providing a powerful toolbox for relevant experimental studies and especially for high-dimensional classical/quantum information.ProgressTo control a paraxial SOC state in all its spatial degrees of freedom, spin-dependent complex amplitude modulation provides an essential alternative. But up to now, it has remained elusive with flat optics. This paper fills this gap by putting forward a new type of geometric phase element termed structured geometric-phase grating (SGPG), featuring a spatially-varying grating cycle, depth and orientation (Fig.1). In addition, the joint team also demonstrated the vector wavefront control technology based on the geometric phase of liquid crystals, and developed a series of liquid crystal geometric phase elements (e.g., mode convertor (Fig.2(a)) and high-order spatial mode generator (Fig.2(b)) with the full dimensional control ability.Conclusions and ProspectsSuch a crucial advance, compared with the present geometric phase elements, unlocks the control of paraxial structured light in all spatial dimensions, and paves the way for arbitrary SOC conversion via flat optics. This capability makes it a key extra-/intracavity component to build a structured laser that has greater tunability in beam structure, compared with reported systems based on q-plate and metasurface. For quantum optics, the proposed reciprocal SOC interface allows to implement a Bell measurement for arbitrary SOC states, which is the basis for the teleportation scheme for SOC photon pairs. Moreover, owing to the capability of full-field spatial mode control, the device also paves the way for quantum control of high-dimension photonic skyrmions. Beyond single-beam vector mode control, this principle can further realize multiple vector mode control through the addition of a Dammann grating structure. This represents a promising way to develop information exchange and processing units working for photonic SOC states, that is, vector-mode multiplexers and demultiplexers.

SignificanceDriven by the recent gradual maturity of digital holography and flat optics with geometric phase, great advances have been made in shaping and application of structured light in the linear optics. In comparison, relevant study based on nonlinear optics, although enabling many crucial functions, such as information exchange between light fields or photons, is still in its infancy. Focusing on this frontier topic of structured nonlinear optics-i.e., nonlinear generation, transformation and interface of classical/quantum states encoded by complex spatial modes-some theoretical and technical bottlenecks to parametrically control all the spatial dimensions of light have been broken. These results lay a solid foundation for future relevant studies on high-dimensional quantum optics experiments.ProgressAdvances in parametrically controlling all the spatial dimensions of light can be divided into theoretical and applied aspects. On the theoretical side, a parameter transformation theory for the full-field selection rule of spatial modes in cylindrical coordinates during small signal three-wave mixing (Fig.1(a)) and a theoretical model of spin-orbit-coupling-mediated nonlinear polarizations (Fig.1(b)) were first proposed. These two theoretical tools can be used together to describe and predict the nonlinear propagation of the vector spatial structure of light fields (amplitude, phase, and polarization) in any paraxial second-order parameter process. Guided by theoretical tools, a nonlinear astigmatism frequency interface has been proposed (Fig.1(c)). In this parametric astigmatism system, an unexpected new physical effect called anomalous orbital angular momentum conservation has been uncovered. This discovery renewed the perception of the nonlinear orbital angular momentum conservation. On the applied side, a frequency conversion technique based on a Sagnac nonlinear interferometer pumped by a super-Gauss mode was first proposed to achieve a spatial polarization independent conformal frequency interface (Fig.2(a)). On this basis, the apparatus can also act as a spatial-amplitude independent frequency interface for orbital angular momentum, which enables a simultaneous conversion of frequency and orbital angular momentum without impacting on the radial mode of signals, with the introduction of vortex super-Gaussian modes (Fig.2(b)). What's more, the new theory has also inspired a new application called spatially-resolved autocorrelation technique, which is based on spatially multimodal nonlinear optical effects. This technique allows for the characterization of the temporal envelope and spatial modes of ultrafast light simultaneously (Fig.2(c)).Conclusions and ProspectsThese systematic research results fill the key theoretical gaps in the field of nonlinear control of structured light in all spatial degrees of freedom. They also provide inspiration for new ideas in the field of light field shaping and have significant implications for related studies, such as structured laser technology, modulation of high-dimensional quantum states and polarization-resolved up-conversion imaging.

ObjectiveOptical interferometry metrology techniques and devices are pillars of modern precision metrology. With the development of laser and light field shaping technologies, their performance has achieved significant improvements across multiple orders of magnitude. However, they are still limited by the wavelength of the light source. Due to the easy absorption and difficult manipulation of extremely short-wavelength optical fields, the resolution of interferometers cannot be infinitely improved by simply reducing the wavelength. "Phase superresolution" refers to the technological means to overcome the limitation imposed by the light source wavelength. Currently, research on phase superresolution mainly focuses on manipulating N-photon entangled states to achieve this goal. However, the extremely high difficulty in preparing and controlling N-photon entangled states, as well as the low efficiency of coincidence counting, renders this approach impractical for actual measurements. Therefore, it is necessary to realize real-time phase superresolution measurements to meet practical application requirements. MethodsTo overcome these aforementioned cutting-edge challenges, the collaborative team has taken a novel approach of utilizing the modal structure evolution of orbital angular momentum (OAM) coherent states during parametric conversion processes to simulate the behavior of N00N states in SU(2). Consequently, they have achieved a more efficient means of actively preparing multi-photon amplitude signals carrying interferometer arm phase information.Results and DiscussionsPhase superresolution signal carried by coherent states with an N-fold enhancement (N=4) has been achieved in a single artificial metamaterial crystal using multiple quasi-phase matching in quasi-periodic optical superlattice (Fig.1(a)). By cascading parametric conversions of the superresolution signal, a phase superresolution interference signal with enhanced resolution of up to N=12 has been realized. Remarkably, the signal intensity remains visible to the naked eye, and real-time recording can be achieved with low-cost photodetectors. In the near future, using this scheme with appropriate technical improvement (Geometric phase elements) with N>100, corresponding to an extreme-ultrviolet de Broglie wavelength, is expected to be an attainable goal. ConclusionsThe preparation of N-photon entangled states typically involves the use of spontaneous parametric down-conversion (SPDC) process to convert the short-wavelength pump light into a photon stream with extremely low efficiency. The target signal is then selected using inefficient photon coincidence counting systems. As a result, the performance of such systems is significantly lower compared to interferometers that directly utilize the pump light source for sensing. In the approach proposed by the collaborative team, spatial modes are employed to encode phase information into the pump light field. By actively constructing multiphoton amplitudes through a strong stimulated parametric process, the signal power loss incurred in cascaded nonlinearities can be regained through phase-sensitive amplification, leading to a significant improvement in system performance. Therefore, their result paves a promising way for the development of practical phase superresolution interferometry techniques and instruments for metrology.

ObjectiveStimulated Brillouin scattering (SBS) is a powerful tool for serving as a phase conjugation mirror (PCM) due to its inherent properties of high gain, small frequency shift, and phase conjugation. Solid-state gain media offer the advantages of high stability and high repetition rate SBS, compared to liquid and gas gain media. However, solid gain media face the challenge of recovery once breakdown occurs. Currently, there is limited research on achieving high-efficiency and high-energy SBS generation in solid media, which restricts the application of solid gain media in high-energy SBS. In this study, we experimentally investigate an SBS generator based on bulk fused silica to provide guidance for the development and application of all solid-state SBS systems with high efficiency.MethodsThe experimental setup is illustrated (Fig.1). A passively Q-switched nanosecond laser, based on a ring cavity, is used as the pump source, delivering a pulse width of 10 ns. A Fabry-Perot etalon is inserted into the cavity to control the number of longitudinal modes. Lenses L1 and L2 are utilized to adjust the beam diameter from 3.2 mm to 5.6 mm, while the focal length of L3 is 250 mm. Fused silica, with a length of 200 mm, serves as the Brillouin gain medium. The output characteristics of SBS generation, including threshold, slope efficiency, damage threshold, and beam profile, are studied by varying the pump mode and pump intensity. Results and DiscussionsCompared to a single longitudinal-mode (SLM) pump, the SBS threshold for a multi-longitudinal-mode (MLM) pump is 14% higher, and the damage threshold is only 34 mJ (Fig.2(a)). A phase-conjugate reflectivity of up to 81.0%, with a slope efficiency of 85.8%, is achieved when the pump single pulse energy is 183.1 mJ. The results indicate that MLM pulse spikes are the key factor causing optical breakdown in the SBS process, while SLM pumping can effectively prevent the optical breakdown in solid media. The narrowest Stokes pulse width of 5.5 ns is obtained at an energy reflectivity of 15%; While the waveform maintains good fidelity at the highest input pump energy (Fig.2(b)). The Stokes beam profile exhibits a good cleanup effect under low-energy pumping conditions (Fig.2(c)). However, it gradually evolves towards the pumping profile as the energy increases. This suggests that high reflectivity also leads to high beam quality fidelity of Stokes.ConclusionsIn this study, we have demonstrated the feasibility of achieving high-efficiency and high-energy SBS output in fused silica. A Stokes energy of 183.1 mJ with a slope efficiency of 85.8% was obtained when the pump energy was 226 mJ. This research lays the foundation for optimizing the characteristics of SBS-PCM based on solid Brillouin gain media, as well as its expansion in pulse compression, Brillouin amplification, and beam combination. It has important implications for achieving high-power all-solid-state SBS lasers.

ObjectiveThe high-energy, high-repetition-rate sub-nanosecond lasers have been widely applied in various fields such as industry, military, and scientific research due to their superior peak power compared to nanosecond lasers and enhanced stability compared to femtosecond lasers. Researchers have discovered that sub-nanosecond lasers have a lower threshold for causing complete damage to optoelectronic devices compared to nanosecond and femtosecond lasers. Therefore, high-repetition-rate, high-energy sub-nanosecond solid-state lasers offer significant advantages in the field of optoelectronic countermeasures. Currently, under conventional water cooling conditions, the single-pulse energy of high-repetition-rate lasers has surpassed the hundred-millijoule level. However, for optoelectronic countermeasure applications, higher repetition rates and higher single-pulse energies are required to improve the hit rate on rapidly moving targets. Thus, the breakthrough of higher repetition rates and high-energy sub-nanosecond lasers is urgently needed.MethodsThis paper presents the realization of Joule-level sub-nanosecond laser output by combining end-pumped microchip crystal picosecond laser generation technology and multi-pass multi-stage slab laser amplification techniques. Initially, the microchip laser is pre-amplified through a three-stage end-pumping process, scaling the microjoule-level energy to millijoule-level. Subsequently, the shaped laser beam with a size of 2×18 mm2 is injected into a first-stage single-end-pumped slab amplifier system. The amplified laser is then transmitted through a first-stage imaging and beam expanding system before being injected into a second-stage dual-end-pumped double-pass amplifier system. Finally, the laser is further amplified through a third-stage single-pass booster amplifier in the slab configuration. This study presents the design of a dual-end pumping structure (Fig.1), with the omission of the isolation system. Results and DiscussionsIn the dual-end-pumping structure, the energy of the leaked pump light can directly cause damage to the pumping module. In this study, experimental results revealed significant fluctuations in the energy of the leaked pump light with variations in the pump current and pump module cooling temperature (Fig.2(a)). Therefore, by controlling the cooling temperature of the pumping module, it is possible to regulate the energy of the leaked pump light at different pump currents, thereby avoiding damage to the pumping module and eliminating the need for complex isolation devices. Using the temperature-controlled dual-end pumping technique, the research team amplified the seed light from 3.12 mJ at 500 Hz to 952 mJ (Fig.2(b)) with pulse width of 680 ps, under the conditions of a first-stage and second-stage pump cooling temperature of 25 ℃, and a second-stage pump cooling temperature of 23 ℃. The amplification energy levels in Fig.2(b) were measured after the output of the third-stage slab amplifier. The sub-nanosecond laser output with Joule-level energy and a repetition rate in the hundreds of hertz achieved by this system represents the highest parameters currently achieved in the field of slab lasers.ConclusionsThis paper reports a high-repetition-rate, high-energy Nd:YAG low-doped slab laser. The laser utilizes a high-power master oscillator power amplifier (MOPA) structure, with a single longitudinal mode microchip laser as the seed source, and achieves amplification through a three-stage slab system with beam shaping. The study demonstrates that the leaked pump light in the dual-end-pumped slab amplifier can damage the pumping module. However, precise control over the energy of the leaked pump light can be achieved by controlling the cooling temperature of the pumping module, effectively avoiding damage to the pumping module. In this work, using temperature-controlled dual-end pumping technique, we amplify the seed light from 3.12 mJ at 500 Hz to 952 mJ with a pulse width of 680 ps. This work provides an effective pump cooling solution for high-energy, short-pulse lasers, ensuring their stable operation, thereby paving the way for the application of high-repetition-rate, high-energy sub-nanosecond slab lasers in the field of optoelectronic countermeasures.

ObjectiveVisible and ultraviolet laser output can be realized by solid-state lasers using second-harmonic generation (SHG) technology, which have a wide range of applications in many fields, including atmospheric exploration, biomedicine and industrial processing. The spatial walk-off effect that occurs inside the crystal in the SHG process can cause the spatial phase mismatch between the fundamental laser and the frequency-doubling laser, which lead to obvious loss of SHG efficiency restricting the SHG conversion efficiency. In order to further optimize the frequency doubling efficiency, the SHG theoretical model combined with influencing factors including the spatial walk-off effect needs to be improved. Therefore, combined with the spatial walk-off effect and SHG laser analysis, a hybrid model is proposed to study the parameters in the conversion process of the internal walk-off effect of nonlinear crystals. In this model, the influence of various parameters such as SHG crystal length, laser power density, SHG crystal type, laser wavelength on the SHG efficiency are carefully analyzed, which is of great significance for the optimization of SHG structure and the improvement of conversion efficiency.MethodsBased on the ideal SHG conversion efficiency model, this study discussed the energy conversion and beam separation in the process of space walk-off effect, and establishes the SHG conversion efficiency model of two different pump structures, including focused beam and parallel transmission beam (Fig.1-2). The specific effects of various parameter changes of SHG crystals and fundamental laser have been discussed.Results and DiscussionsAccording to the established frequency-doubling efficiency model, the influence of the departure effect on the frequency-doubling efficiency has been discussed (Fig.3). The frequency-doubling efficiency was analyzed by using the fundamental frequency optical parameters of fundamental frequency light, including beam quality, beam radius, focal length of focusing lens, beam waist size and divergence angle (Fig.4-5). The frequency-doubling efficiency was analyzed by taking the crystal parameters of frequency-doubling crystal type and crystal cutting angle as variables, and it was found that the theoretical optimal frequency-doubling efficiency of KDP, LBO and KTP crystals was 58%, 80% and 97% (Fig.6). In addition, the influence of frequency-doubling crystal length on frequency-doubling efficiency is studied, and the optimal length selection model of frequency-doubling crystal is obtained (Fig.7).ConclusionsA SHG efficiency model of nonlinear crystals with the walk-off effect is established. Aiming at the two different situations of parallel transmission pump beam and focused pump beam, the nonlinear transformation efficiency model of the SHG process is improved through the simulation analysis of the spatial walk-off effect, and the influence of various pump parameters and crystal parameters changes on the SHG efficiency has been discussed. The theoretical optimal SHG efficiency of KDP, LBO and KTP crystals is 58%, 80% and 97% due to the influence of different spatial walk-off distances. Compared with the traditional SHG efficiency model, this model has a more complete study of the spatial walk-off process, and comprehensively refines the analysis of various factors in the SHG process, which can more accurately predict the change of SHG efficiency. It would be helpful to achieve SHG efficiency optimization by adjusting pump parameters and crystal parameters in the future. The model can also be used to realize the selection of the optimal length of SHG crystal and the best SHG crystal type in different conditions, which can be applied to the nonlinear crystal selection process of scientific research and commercial short-wave solid-state lasers to achieve conversion efficiency improvement and cost reduction.

ObjectiveBrillouin scattering is a third-order nonlinear light scattering phenomenon resulting from the interaction of a pump beam incident on a medium with elastic acoustic phonons within the medium. The stimulated Brillouin scattering (SBS) technique is widely used for its high-energy conversion efficiency, small Brillouin frequency shift, and phase conjugation. Currently, the SBS technique has been widely used in Brillouin spectroscopy, pulse width compression, and beam combination, in which the frequency shift and linewidth are two essential parameters of Brillouin scattering. The frequency shift and linewidth have been successfully applied in Brillouin spectroscopy to differentiate the information about medium type, concentration, and temperature characteristics. Previous studies mainly focus on measuring time-domain pulse width in SBS pulse-width compression. The study of frequency-domain linewidth variation is less involved. In contrast, the linewidth variation characteristics are closely related to the information about medium viscosity, temperature, and refractive index, so it is of great significance to investigate the linewidth variation characteristics during SBS compression.MethodsThe experiment probes the linewidth change of the medium FC-770 during SBS compression by focusing a single-cell setup, which is mainly composed of a generator and a long focusing lens and is characterized by controllable incident energy and a simple structure. The linewidth change incident to the second-stage generator is controlled by the compact two-cell setup, which has high-energy conversion efficiency, and the time-domain waveform of the incident pulse generated by the compact two-cell setup can maintain a better shape to avoid the effect of the time-domain waveform on the linewidth change. Due to the low laser repetition frequency during SBS compression, the Fabry-Pérot (F-P) combined with the COMS beam profilers (CBP) is selected to measure Stokes linewidth at low repetition frequency. The interference scattering plot is obtained by processing the interference circle acquired by CBP, and the Stokes intrinsic linewidth value is obtained after non-linear fitting.Results and DiscussionDuring the SBS compression, pump energy, lens focusing parameters, and incident laser linewidth influence the Stokes linewidth output. With the increase of the pump energy, the energy reflectivity increases rapidly, and the Stokes linewidth first increases rapidly and then gradually narrows. When the pump energy is 35 mJ, the Stokes linewidth can be narrowed to about 400 MHz (Fig.5). As the focal length of the lens increases, the energy density at the focal point decreases, but the increase in focal length of the lens increases the effective interaction length of the pump light with the back-transported Stokes light, and the Stokes linewidth increases rapidly (Fig.6). By taking a compact two-cell setup to control the linewidth incident to the generator, it can be seen that as the value of the linewidth incident to the second-stage generator gradually increases, the value of the output Stokes linewidth gradually increases, and when the value of the linewidth incident to the second-stage generator varies from 280 MHz to about 450 MHz, the value of the output Stokes linewidth varies from 500 MHz to about 680 MHz (Fig.8).ConclusionsDuring the SBS compression process, the variation of the Stokes linewidth of the medium FC-770 output shows a tendency of first increasing and then rapidly narrowing with the increase of the pump power density. As the lens's focal length in front of the generator decreases, the Stokes linewidth becomes narrower. When the focal length of the lens is small, the Stokes linewidth variation is less effective by the pump energy. The input laser linewidth is controlled by secondary compression, and the Stokes output linewidth gradually broadens as the incident linewidth value increases. From the experiments, it can be seen that the linewidth variation in the SBS compression process has a specific law, and the frequency domain linewidth variation contains rich information about the medium properties, so the study of Stokes linewidth variation in the SBS compression process is of great significance for the study of medium properties.

ObjectiveThe cavity length adjustment configuration has important applications in optical resonators. A cavity length adjustment configuration of double optical wedge (DOW) is proposed, which can adjust the cavity length independently of the cavity mirror. DOW configuration is composed of two right-angle wedges with beveled planes placed in parallel opposites. The optical path inside DOW is changed by driving the wedges to move in the vertical direction, and then the optical path in the resonator is changed. The theoretical formula for calculating the change of optical path of DOW configuration is established. According to the formula, the change of optical path is positively correlated with the wedge angle, the refractive index of wedge and the wedge displacement in the vertical direction. The wedge angle and refractive index determine the optical path adjustment efficiency of DOW configuration. According to the theoretical design, the YAG DOW configuration with wedge angle of 29° and refractive index of 1.81 has higher adjustment efficiency and less optical loss, and the adjustment coefficient is 0.53. In the experiment, the double corner cube retroreflector (DCCR) ring cavity is used to verify the cavity length adjustment, and the feasibility and effectiveness of DOW configuration to adjust the cavity length are verified. The deformable structure of DOW configuration is discussed and analyzed. The optical path adjustment properties of DOW configuration with beveled planes placed in parallel opposites, regular optical wedges and cascaded DOW configuration are discussed. The performances of DOW configuration and its deformed configuration in optical path adjustment efficiency, optical loss and the complexity of the optical path construction are compared, and the advantages of these DOW configurations in practical application are determined, which provides a reference for the design and selection of DOW configuration.MethodsIn theory, by geometric calculation, the calculation formula of the optical path adjustment of the DOW configuration is derived, and the results are shown in Eq.(3). According to Eq.(3), there are three factors affecting the adjustment ΔL, namely wedge angle α, wedge refractive index n, and wedge displacement Δh1+Δh2 in the vertical direction. The larger the wedge angle value of αis, the higher the adjustment efficiency is, and the data results are shown (Fig.4); The greater the refractive index nis, the higher the adjustment efficiency is, and the data results are shown (Fig.5). The larger the displacement Δh1+Δh2is, the higher the adjustment efficiency is. According to these factors, a DOW configuration with wedge angle α = 29° and material YAG is designed, and its adjustment coefficient is 0.53. Results and DiscussionsThe adjustment effect of DOW configuration to the cavity length is verified experimentally by using the DCCR ring cavity. The reflector of DCCR ring cavity is corner cube retroreflector, which can not be drived by PZT directly, thus the cavity length adjustment of DCCR ring cavity can be realized with DOW configuration. The experimental setup is shown (Fig.10). DOW configuration is inserted into the DCCR ring cavity, and 1.6 μm laser is injected into the cavity. When DOW configuration is operating, 1.6 μm laser will form a resonance signal and output from M3, through which the cavity length adjustment value ΔL caused by DOW configuration can be determined. The experimental results are shown (Fig.11). The appearance of resonance signal proves that the cavity length changes, and the change value ΔL is consistent with the theoretical expectation. The above experimental results prove that DOW configuration is effective in adjusting the cavity length. ConclusionsIn this paper, a DOW configuration is proposed, which can be used in the special scenario where the cavity length cannot be adjusted by driving the cavity mirrors. The formula for calculating the adjustment value and adjustment coefficient of DOW configuration is given theoretically. The influence of wedge angle and refractive index on the adjustment efficiency is analyzed. DOW configuration with wedge angle of 29° and matrix of YAG is designed. DOW configuration has a large adjustment efficiency (adjustment coefficient is 0.53) and a small light loss, and is the better choice in various DOW configurations. The cavity length adjustment of the length of DOW configuration is realized experimentally in a double-corner cone ring cavity, which verifies the feasibility and effectiveness of the DOW configuration. Finally, different deformation structures of DOW configurations are given, and the property parameters of each deformation structure are compared. Compared with the traditional cavity length adjustment configuration, cavity length adjustment configurations of DOW has low adjustment efficiency and certain insertion loss, but it provides an adjustment mode independent of the cavity mirror, and provides a new choice for the cavity length adjustment in special application scenarios.

ObjectiveOptical microscopy technology is to observe and record images of the microstructure of objects at a scale indistinguishable from the human eye, and has become an important tool for human observation of the microscopic world. Among them, electron microscopy has a resolution that breaks the limit of optical diffraction and can reach the nanometer level. However, it needs to provide a vacuum environment for electron acceleration, so it is not conducive to the observation of living samples. Optical microscopes are easy to operate and inexpensive, and are widely used in scientific research, industry, medicine and other fields. Conventional light microscopes rely on the contrast produced by differences in the optical properties of the sample for imaging, and do not require labeling or staining, but do not have sufficient specificity. In contrast, nonlinear optical microscopy not only realizes specificity imaging, but also has higher imaging depth and resolution. Among them, fluorescence microscopy technology with the help of fluorescent probes to label different components within the biological sample, through the detection of fluorescence signal to achieve imaging of the labeled components in the sample, to obtain its distribution within the sample. TPF (two-photon fluorescence) microscopic imaging technology based on the nonlinear effect of two-photon absorption, the fluorescence signal will not be excited outside the focal plane, and therefore has a high spatial resolution. TPF microscopy mostly uses infrared wavelength light source, which has lower phototoxicity and photobleaching to biological samples and higher imaging depth. In summary, this paper builds a TPF microscopy system based on femtosecond pulsed light source to study the imaging performance of the system on biological samples.MethodsWe build a TPF microscopic imaging system (Fig.1), using a Ti: sapphire femtosecond laser as the excitation laser source, with a central wavelength of 800 nm, a repetition frequency of 80 MHz, and a pulse width of 100 fs. The scanning optical path of the system was formed by lens and a scanning oscillator to complete the collimation, beam reduction, and two-dimensional deflection of the excitation light. The fluorescence signal is converted into an electrical signal and processed by a computer to obtain the imaging results.Results and DiscussionsThe output spectrum of the femtosecond laser, and the fluorescence spectrum of rhodamine B were obtained using a TPF spectroscopic measurement system (Fig.2). The central wavelength of the femtosecond laser was 1 030 nm, and the half-height width of the spectrum was 14.47 nm. While the spectral range of the fluorescence covered from 620 nm to 710 nm, the intensity increased steeply from 620 nm, with a peak at 630 nm, and then the intensity decreased slowly with increasing wavelength, so the laser could effectively excite the TPF signal of the sample. The relationship between the two-photon fluorescence intensity and the excitation pulse power was analyzed by adjusting the power of the excitation pulse (Fig.4). The fluorescence intensity was linearly related to the square of the excitation power in the region of different concentrations of the samples. The ratio coefficient of fluorescence intensity to the square of the excitation power was larger in the region with higher concentration for the same excitation power. The fluorescence intensity distribution of the samples within 0-14 μm depth was obtained by 3D TPF microscopic imaging experiments of mouse brain sections (Fig.5). It was obtained that the gray matter portion within the mouse brain sample was located in the superficial layer within 6 μm of the sample, while the white matter portion was more widely distributed longitudinally. The depth distribution curves of the fluorescence intensity of different tissues were obtained by curve fitting, which led to an imaging depth of 12.9 μm for the system. The intensity distribution curves of the narrow slits of multiple samples were plotted, and by analyzing the minimum distance that the imaging system could resolve, a lateral resolution of at least 2.25 μm was derived.ConclusionsA femtosecond laser was used as the excitation laser source. The fluorescence spectra of the rhodamine B solution samples were measured under excitation at 800 nm. Thus a detection window of 636-703 nm was selected for subsequent microscopic imaging experiments. TPF microscopic imaging experiments of mouse brain sections stained by rhodamine B were carried out to obtain the fluorescence intensity distribution of biological samples in the depth of 0-14 μm by tomography imaging. After three-dimensional reconstruction of the images, it was concluded that the gray matter portion within the mouse brain sample was located in a shallow layer within 6 μm of the sample, while the white matter portion was more widely distributed longitudinally, while the gray matter of the mouse brain had higher fluorescence intensity and had a higher density than the white matter portion. The experimental results demonstrate that the constructed microscopic imaging system has excellent spatial resolution and imaging depth, with an imaging depth of 12.9 μm and a lateral resolution of at least 2.25 μm.

ObjectiveWith the rapid development of modern information technology, optical three-dimensional (3D) profiling measurement technology has gradually matured. Among numerous optical 3D profiling measurement technologies, due to its non-contact and high-accuracy measurement, digital fringe projection（DFP） technology is increasingly applied in the fields such as biomedical monitoring, virtual reality, and computer vision, and has broad prospects for development due to its non-contact and high measurement accuracy. However, this technology still faces some technical challenges: 1) Due to the limited depth of field of the system equipment (such as cameras and projectors), only the 3D shape of objects within a limited depth of field can be reconstructed; 2) Nonlinearity problems caused by the γ-effect of commercial projectors may affect measurement accuracy. To overcome these problems, this paper proposes a method to extend the measurement depth range, which can achieve high-accuracy measurement of multiple objects at different depths or objects with a large depth range.MethodsThe paper proposes a novel method for measuring the 3D shape of objects with a large depth range. Firstly, defocus technique is used to measure the dithering pattern in a simulated sinusoidal mode, avoiding the influence of projector non-linear errors on 3D measurement of fringe projection and increasing the measurement speed. Then, by analyzing the relationship between the degree of defocusing of the fringe and the depth (Fig.1-2), this paper analyses the relationship between fringe defocus and depth and finds that the defocus degree of fringes at different frequencies is inconsistent with the depth variation nodes. Based on this, a multi-frequency phase selection method is proposed in this paper. The optimal frequency mode determination algorithm (Fig.4) is used to select the bayer dithering algorithm and the floyd-steinberg dithering algorithm to generate dithering patterns. After comparing the phase error distribution of the fringe images within the range of 12-60 pixel in period at 25 defocus levels, the defocusing selection range of the corresponding fringe frequency is screened to determine the optimal selection of fringe frequency at different defocusing degrees. Then, in order to obtain a binary pattern with the highest quality sinusoidal structure, 8 different scanning orders are used based on the selection results of the optimal frequency mode which is to select the optimal dithering mode for the current frequency (Tab.1). Finally, the method uses the selected dithering fringe pattern within the optimal frequency range to obtain the 3D shape of the object. The proposed method can extend the measurement range of object depth by selecting multi-frequency dithering fringe and determining the optimal frequency at different defocus degrees.Results and DiscussionsThis paper presents qualitative and quantitative comparison experiments between the standard sinusoidal fringe and the proposed method. In the qualitative experiment, both methods are used to reconstruct the 3D shape of an object with a depth of 22.5 cm (Fig. 9). The measurement results of the proposed method are better than those of the standard sinusoidal fringe method with complete shape, clear details and without ripple phenomenon. Moreover, in the quantitative experiment, the maximum absolute error of the proposed method is 0.033 mm (Tab.2), which is comparable to the measurement accuracy of traditional DFP technology. Therefore, the proposed method not only ensures measurement accuracy but also extends the measurement depth range, and effectively solves the problem of measuring the 3D shape of objects with a large depth in the DPF field.ConclusionsThis paper proposes a MFPS method based on dithering algorithms to solve the limited measurement depth range and nonlinearity problem of the existing DFP technology. By using defocusing dithering techniques, the impact of projector nonlinearity error is overcome. Moreover, the MFPS method is used to generate dithering fringe patterns for measurement, which extends the measurement depth range. Experimental results demonstrate that the proposed method effectively extends the measurement depth range and achieve 3D shape measurements of objects in a large depth range.

ObjectiveDue to its high speed and high precision, fringe projection profilometry has been widely used in many fields, such as automatic online inspection of mechanical parts, automobile manufacturing, cultural heritage protection. However, traditional fringe projection uses a single exposure time or a single projection intensity to measure objects with high dynamic range (HDR). Overexposure will occur in areas with large reflectance, which exceeds the maximum brightness range of the camera sensor, resulting in the failure to obtain true intensity and accurate three-dimensional (3D) data. To solve this issue, this paper proposes a HDR object surface 3D measurement method utilizing the different color channel responses of a color camera based on monochrome fringe projection.MethodsIn this paper, a 3D measurement method of HDR object surface based on monochrome fringe projection is proposed. In this method, the blue fringe patterns are projected onto the surface of the measured object, and the color camera captures the color image from another perspective. The two fringe patterns corresponding to the blue and green channels from the captured fringe images are separated. Mask image of blue and green channels are generated by selecting a group of pixels with unsaturated and maximum modulation from the blue-green channel fringe patterns. The HDR image is synthesized by the mask images of blue and green channels and fringe patterns of blue and green channels (Fig.3). Then, phase calculation methods and system calibration are applied to achieve 3D measurement of objects with high dynamic range.Results and DiscussionsTo demonstrate the effectiveness of the proposed method, a metal flat plates and a metal spherical part with HDR surface were tested. Comparative experiments were conducted between the separated blue channel fringe patterns and the synthesized HDR images to verify the effectiveness of the proposed HDR method (Fig.8, Fig.10). The proposed method can provide accurate 3D measurement results without measurement errors caused by pixel saturation. To quantitatively evaluate the accuracy of the method proposed, an artificial standard step surface were measured by Zhang's method and the proposed HDR images (Fig.12). 3D data of the step surface measured by the coordinate measuring machine (CMM) can be used as the ground truth. The difference between the measured data and the ground truth are shown (Tab.1). It can be seen that the accuracy of the proposed method is slightly higher than Zhang's method. Compared with the existing HDR methods, the proposed method has the advantage of fewer images and without additional hardware facilities.ConclusionsThe 3D measurement technique for HDR object surface based on monochrome fringe projection is proposed by utilizing the different color channel responses of a color camera. The blue fringe patterns are projected onto the surface of the tested object, and captured by the color CCD camera. Fringe patterns corresponding to the blue and green channels are separated from the captured color fringe patterns. A group of pixels with unsaturated and maximum modulation from the blue and green channel fringe patterns are selected to generate mask images of the blue and green channels. The HDR image is synthesize by the mask images of the blue and green channels and fringe patterns of blue and green channels. Then the absolute phase is obtained by the phase calculation method, and the HDR object is measured by the calibrated system. The three-step phase-shifting method and an optimal three-fringe selection method are applied to obtain the wrapped phase and unwrapped phase, respectively. A total of 9 color images are required to reconstruct the 3D shape of HDR objects. Compared to traditional methods, the proposed method has the advantages of reducing the number of projected images and improving measurement efficiency.

ObjectiveDue to the advantages of high precision, easy recognition and high degree of automation, the three-channel phase measurement profilometry in optical three-dimensional measurement has gained increasing attention in both scientific research and engineering applications. For three-channel phase measurement profilometry, the chromatic aberration between projector channels is the key factor affecting the measurement accuracy. Most of the existing chromatic aberration correction methods of projectors regard projectors as "reverse cameras". Therefore, the accuracy of correction results will be dependent on the imaging quality of the camera. Moreover, the existing chromatic aberration measurement and correction methods still have shortcomings, so it is significant to improve the measurement accuracy of the system. Therefore, this study carries out the research on the projection chromatic aberration modeling and correction of phase target-based phase measurement profilometry.MethodsIn this paper, the projection chromatic aberration modeling and correction method using the LCD screen with holographic projection film as the phase target is proposed (Fig.3). Firstly, the unfolded phase of LCD display fringes and projector projection fringes are calculated respectively. Next, binary fitting on display phase and projection phase are carried out. The green channel is regarded as an ideal channel, and the ideal pixel values of red and blue channels is calculated. Then the ideal pixel is substituted into the projection equation, and the ideal phases of the red and blue channels are obtained. Thus, the mathematical model of the chromatic aberration of the projector is established. Finally, the pre-compensation of projection fringes is implemented with the established chromatic aberration model(Fig.5). Then, the pre-compensated fringes are projected in three channels, so that the chromatic aberration of the projector is corrected.Results and DiscussionsThe experimental results demonstrated the performance of the proposed method. The average chromatic aberration of the blue and green channels is corrected from 0.325 5 pixel to 0.106 3 pixel. The average chromatic aberration of the red and green channels is corrected from 0.365 1 pixel to 0.111 4 pixel (Fig.10). This method can effectively improve the projection quality for three-channel phase measurement profilometry. The average error of the measured step is reduced from 0.489 mm to 0.038 mm (Tab.2). The experimental results verified the effectiveness of the chromatic aberration modeling and correction method of projector. This method can improve the overall measurement accuracy of three-channel phase measurement profilometry. Compared with the existing methods, the proposed method can be calibrated to avoid the impact of camera errors and effectively shorten the calculation time. Moreover, this method can be applied to the measurement and correction of different projector chromatic aberration.ConclusionsA phase-measurement contouring chromatic aberration modeling method using an LCD display as a phase target is designed and calibrated for study. This method eliminated the coupling error of the camera while measuring and calibrating the projector chromatic aberration, and enabled measurement of the projector chromatic aberration at global pixel points, while using mathematical modeling to model the projector chromatic aberration in a chromatic way to shorten the calculation time. By measuring the 3D shape of the actual object for accuracy comparison experiments and comparing the accuracy error before and after correcting the chromatic aberration of CP270 projector and PRO4500 projector, it can be concluded that the projection chromatic aberration modeling and correction study based on phase target proposed in this paper can better improve the projection quality in phase contour measurement and enhance the measurement accuracy of commercial projectors with poor accuracy. For the projectors with low accuracy, the method of correcting chromatic aberration in this paper can greatly improve the measurement accuracy of projectors. For the projectors with high accuracy, the proposed projector chromatic aberration modeling and correction method can further improve the measurement accuracy.

ObjectiveWater impurity in organic solvent strongly affects the process of chemical reaction. However, current methods for detecting trace water still suffer from disadvantages such as complex operation, high toxicity of experimental reagents, low detection sensitivity, and the inability to monitor in real-time. In order to overcome these challenges, a D-type fiber sensor which combines silver and silk protein in the visible wavelength range is designed. It is designed for high-sensitivity detection of trace water in organic compounds, and the performance of the sensor is optimized and analyzed.MethodsFirst of all, the circular air hole in the middle of the fiber is replaced by the grapefruit type, which increases its size by about ten times (Fig.1), which can reduce the difficulty of the sensor in the actual production process with high efficiency. In addition, the lower metal silver film on the side profile of D-type fiber can form an energy channel with the nearby large air hole of grapefruit type, which promotes the energy leakage of the fiber core into the plasma and enhance the SPR effect. Secondly, a certain number of silver grating structures are added to the lower part of the top silver film, which can make the incident bright part confined to the slit cavity. Through reasonable structural design and size optimization, the local surface plasmon resonance (SPR) wave and surface plasmon wave can further resonate, thus realizing the enhancement of SPR phenomenon.Results and DiscussionsTheoretical analysis shows that compared to a single-layer silver structure, the double-layer silver structure on the surface of the D-type fiber sensor can induce strong localized light. In the subsequent size selection process, the initial thickness of the silk protein and the height of the air hole (Tab.1), the height of the silver gratings (Tab.2), the amount of the silver gratings (Tab.3), the spacing of the silver gratings (Tab.4) and the thicknesses of the underlayer silver film (Tab.5) are optimized to obtain the optimal sensing structure. Through the above optimization, the optimal structural parameters of the sensor are as follows: the height of the fiber air hole is 4.5 μm, the height of the silver grating is 10 nm, the number of the silver grating is 58, the spacing of the silver grating is 50 nm, and the thickness of the underlying silver film is 31 nm. Compared with other structures or methods of trace water detection equipment in sensitivity or detection limit performance, the results show that the detection performance of the grapefruit-type three-core fiber sensor for trace water based on silver-silk protein-silver structure is much better than the previous design.ConclusionsBased on metal insulator metal structure and SPR principle, D-type fiber sensor is designed to detect trace water in organic matter. The sensing materials mainly use silver metal and silk protein, and the sandwich structure composed of silver metal can significantly enhance the local electric field, thus improving the sensitivity of the sensor. Silk protein has good optical properties and can quickly and accurately make reversible volume changes in response to external stimuli. Therefore, water absorption of silk protein can be calculated by its expanded volume combined with Darcy's law, and the water content in organic matter can be further obtained. The high sensitivity measurement of trace water in organic matter is realized, and the sensitivity can reach 1.39 nm/ppt (1 ppt=10-12). Moreover, the fitted line has an R2 value greater than 0.999, achieving the expected performance. And the sensor is designed to have a long service life, and is not affected by temperature.

ObjectiveAccurate determination of camera-to-reference frame parameters is crucial. Traditional systems are limited by camera field of view, constraining object size measurability. Omnidirectional cameras offer wide view and high imaging quality by using rotation systems or combining with LiDAR for scene modeling. This paper proposes a calibration method for omnidirectional camera and rotation axis. It uses omnidirectional cameras to capture rotation of QR code chessboards. A reliable mathematical model and nonlinear fitting optimize initial results for accurate parameter estimation. This method has low equipment requirements, considering board placement within the camera's view. The experimental results indicate that the average optimized reprojection error of this method can be controlled below 0.15 pixel, satisfying the requirements of experimental measurements and demonstrating promising application performance in various scenarios.MethodsA reliable system is proposed to calibrate the extrinsic parameters between the camera and the rotation axis. A omnidirectional camera with a resolution of 4 000 pixel×3 000 pixel is utilized to capture the dual ChArUco calibration boards (Fig.2). For the extrinsic calibration, an algorithm is designed to fit the rotation plane and different methods for establishing the axis coordinate system are introduced (Fig.5). The accuracy of the system is evaluated using the distance from the optical center to the origin of the axis coordinate system (Fig.9) and the reprojection errors under different conditions (Fig.11).Results and DiscussionsIn this method, the Perspective-n-Point algorithm is employed to determine the camera's optical center coordinates. Subsequently, a nonlinear least squares fitting technique is applied to fit the rotation plane and sphere of the optical center (Fig.8). The circularity fitting standard deviation for the intersection between the plane and the sphere is measured to be 0.021 8 mm, while the flatness fitting standard deviation is 0.030 1 mm. The range of distances from the camera's optical center to the axis is found to be 0.085 mm, with a standard deviation of 0.021 mm (Fig.9). Additionally, the maximum reprojection error between the experimental reference group and the other two control groups is 0.141 6 pixel (Fig.12), thereby validating the accuracy of the proposed method.ConclusionsTo address the issue of pose uncertainty between the camera and the rotation axis, this paper proposes a calibration method based on a omnidirectional camera and dual ChArUco calibration boards. The method captures multiple sets of images containing the dual targets to obtain the position information of the camera's optical center at each shooting position. By establishing a mathematical model for coordinate system transformation, the pose relationship between the camera and the rotation axis is computed and optimized, effectively suppressing the influence of random errors in the experiments. Experimental results demonstrate that the proposed method achieves sub-millimeter-level accuracy in the distance between the camera and the rotation axis, with an average optimized reprojection error controlled below 0.15 pixel. Compared to other methods, the method presented in this paper has lower system complexity, improved accuracy by use of two calibration boards, and effectively mitigates random errors caused by placement variations. The results indicate that this method exhibits good robustness and convenience, making it reliably applicable to shooting tasks in diverse scenarios.

ObjectiveTuned laser absorption spectroscopy (TLAS) technology has advantages such as non-contact, anti-interference, and high sensitivity, which can be used for gas concentration, temperature, and pressure measurement. In the existing pressure detection models, limited feature points of spectral lines are mostly extracted and calculated, which can lead to problems such as susceptibility to interference in measurement results and significant measurement errors. Therefore, it is necessary to establish a new anti-interference and stable pressure detection model. To solve this problem, a mathematical model was proposed for fitting the pressure and spectral line shape function within the low and high pressure ranges based on the gas pressure measurement method of absorption line width.MethodsSimulation research on the second harmonic absorption lines under different pressures was conducted based on the principle of spectral line broadening. In order to simulate the pressure changes by adjusting the Gauss/Lorentz halfwidth ratio, the second-order derivative signal was obtained by convolution of Gauss and Lorentz functions to simulate the second harmonic of the absorption spectral line. By establishing a mathematical model of the Gauss/Lorentz line fitting ratio and pressure, the fitting relationship between the two was obtained under ideal conditions and the influence of laser line width, white noise, and background interference. The comparative analysis on the stability of the fitting ratio with eigenvalues used to calculate pressure in existing models such as the peak width and 2f/4f amplitude under dynamic noise and background interference were conducted. Finally, the measured signal at 1 580 nm of CO2 gas was processed to verify the simulation results. Results and DiscussionsThe simulation results show that under ideal conditions and the influence of laser linewidth, white noise, and background interference, there is a third-order fitting relationship between the Gauss/Lorentz line fitting ratio and pressure, and the fitting degree remains above 0.998 0 (Fig.3-6). Compared with traditional models, it has better stability under dynamic noise and background interference (Tab.1). The experimental results show that the third-order fit between the Gauss/Lorentz line fitting ratio of the actual detection spectral line and the pressure is 0.986 3 (Fig.9), slightly lower than the simulated fit of 0.998 7 (Fig.2), which is consistent with the simulation analysis results.ConclusionsIn order to establish a more effective pressure detection method, based on the principle of spectral line broadening, the pressure change is simulated using the ratio of Gauss function half width to Lorentz function half width, and the Voigt function is used to describe the absorption spectral line shape. A mathematical model was established for the pressure to fitting ratio under ideal conditions, laser linewidth, white noise, and background interference. Through simulation analysis, the relationship between pressure and fitting ratio satisfies a third-order fitting relationship, which is not only affected by laser linewidth, white noise, and background interference, but also maintain stability under dynamic noise and background interference, which exhibits advantages in pressure detection compared to traditional models. The experimental validation was carried out using CO2 absorption spectra. The curve fitting obtained from analyzing the experimental data was slightly lower, but its trend was consistent, which indicated the effectiveness of the established mathematical model. The proposed method has certain theoretical significance and practical value in pressure measurement, providing new ideas for pressure detection.

ObjectiveIn recent years, terahertz radar has been widely used in the fields such as human security and non-destructive testing due to its advantages of high resolution, high penetration, and high safety. Researchers have proposed terahertz MIMO array radar from the perspective of improving imaging speed. This radar combines the spatial division multiplexing technology of MIMO arrays to achieve fast and real-time imaging. However, due to the short wavelength of terahertz waves and the sparse design of MIMO arrays, the array element spacing is too large, resulting in high gate sidelobe levels in the radar beam pattern, which affects imaging quality. Optimizing the array position through optimization algorithms can effectively solve this problem, but previous research is mainly focused on optimizing low-frequency MIMO array radars, while high-frequency terahertz MIMO array radars may encounter more severe high gate sidelobe level problems. Therefore, it is necessary to design optimization algorithms with higher optimization accuracy for this band. Therefore, from the perspective of solving this problem, this paper first abstracts the optimization model of terahertz MIMO linear array, and then proposes a dual strategy adaptive differential evolution for array optimization based on the optimization characteristics of the model.MethodsA multi-constraint optimization model is established with the goal of reducing the peak sidelobe level ratio based on the optimization characteristics of terahertz MIMO arrays. Using Kent chaotic sequences to generate an initial population, this method can make the distribution of initial individual genes more uniform in the solution space. A dual mutation strategy was proposed to enable the algorithm to select appropriate mutation strategies based on the number of iterations and individual fitness values. Adaptive improvements have been made to the parameters, allowing them to be autonomously adjusted based on individual evolution. The convergence performance of the algorithm was tested through standard functions, and the effectiveness of the algorithm for terahertz MIMO array optimization was tested through simulation experiments.Results and DiscussionsThe DSADE algorithm proposed in this paper has the best optimization effect on the 8 transmitting and 8 receiving terahertz MIMO array antenna, and the optimized minimum peak sidelobe level ratio is 1.32 dB lower than the ISMADE algorithm (Tab.5). It can be clearly seen from Fig.4 that the DSADE algorithm effectively suppresses the gate sidelobe level in the directional synthesis map of the MIMO array. The comparison of the 50 optimization results (Fig.6) also shows that the optimization performance of the DSADE algorithm is significantly better than other algorithms. It has been proven that this algorithm can effectively optimize terahertz MIMO arrays, suppress gate sidelobe levels, and improve imaging quality.ConclusionsA portable infrared target simulation system is designed with working wavelengths of 3-5 μm and 8-14 μm. This system has the characteristics of simple structure, adjustable wavelength, rich targets, and clear and stable imaging. The wavefront quality of the system was analyzed using Zemax software, and at 4 μ the PV value of the center field of view in the m-band is 0.0132λ. The root mean square value is 0.0038λ, at 12 μ the photovoltaic value of the center field of view in the m-band is 0.0044λ.The mean square difference is 0.0013λ. An optical mechanical thermal analysis was conducted on the collimation system, and at a temperature difference of 30 ℃, the deformation caused by the mechanical structure was much greater than that of the primary and secondary mirrors themselves, reaching 10% μ. In the order of m, the imaging results have significant defocusing errors, which can be compensated for by temperature changes through refocusing the target disk in an adjustable three-dimensional position. The imaging function of the system was tested. For targets of different shapes, the system can generate clear and recognizable images, providing stable simulated targets for infrared detection equipment.