In order to study the correlation between the fucoxanthin content of Phaeodactylum tricornutum and its photosynthetic physiological indexes, different monochromatic light-emitting diodes were used as light sources to treat P. tricornutum. The fucoxanthin content, photosynthetic physiological indexes, and the expressions of photosynthesis-related genes were detected, and the correlation among the indicators was examined. The results show that purple light can significantly promote fucoxanthin and chlorophyll a production of P. tricornutum (PPPrbcL gene. Under red, blue, green, and purple light conditions, the expression level of fcpB is significantly increased compared to the control group (PfcpB gene, and negatively correlated with the expression of the rbcL gene. In conclusion, the synthesis and accumulation of fucoxanthin can be promoted by improving the photosynthetic efficiency of P. tricornutum, which can be done by regulating the parameters of rETRmax and the expression of the fcpB gene at different wavelengths.

The phase-sensitive optical time-domain reflectometer (φ-OTDR) can effectively detect the vibration information of fiber; however, the detectable vibration frequency range is generally between several hundred hertz and several kilohertz, limited by the length of the sensing fiber. To improve the vibration detection range, a φ-OTDR system that uses heterodyne coherent detection is built in this study. The phase information of the scattered optical signal of the sensing fiber is obtained using the I/Q demodulation method. Differential phase demodulation in the space and time domains is consecutively performed to achieve a distributed detection of high frequency vibration signals in the fiber. Furthermore, the feasibility of the proposed high-frequency vibration detection scheme is theoretically analyzed, and the vibration signal with a frequency of 500 kHz is effectively demodulated in the experiment, obtaining a sensing distance of 23 km.

In this study, we propose and demonstrate a distributed sensor based on weak-reflection fiber Bragg grating (WFBG) to detect the acoustic wave direction. Further, we utilize a distributed sensing fiber between two adjacent WFBGs to detect the sound vibration signals. The phase difference of the demodulation signals of two sensing fibers corresponds to the difference between the reaching times of the acoustic wave, and the sound direction can be calculated based on the time difference. A 50-m sensing fiber coil is placed in vibration liquid column, and the average sound pressure sensitivity is estimated to be -155.10 dB (re rad/μPa). Two 50-m sensing fibers are distributed on the wooden floor. Sinusoidal sound signals are received, and the root mean square error of the detection direction is 1.35°. The principle deduction and experimental results prove that the acoustic wave direction can be detected using the proposed sensor. It is expected to be carried on an unmanned underwater vehicle to detect the underwater vocal target because of the smaller size of the proposed structure when compared with that of the traditional structure of the fiber twined on substrate.

The Brillouin frequency shift extraction algorithm based on similarity matching method exhibits considerable adaptability and does not require a predefined model. In this study, the influence of the Brillouin linewidth (short for linewidt), signal-to-noise ratio, sweeping step, and sweeping range on the extraction accuracy of the frequency shift is systematically studied by varying each factor to obtain the influence of the spectral signals and sweep parameters on the performance of the algorithm. The frequency shift error is a linear function of the Brillouin linewidth. The linewidth difference between the selected detected spectrum and the selected reference spectrum does not considerably affect the accuracy of the algorithm. When the signal-to-noise ratio of the selected detected spectrum remains constant, the frequency shift error decreases exponentially with the increasing signal-to-noise ratio of the selected reference spectrum, and vice versa. When the frequency sweep step is maintained constant, the frequency shift error increases with the increasing sweep range of the selected reference spectrum; this is accompanied by a linear increase in the calculation time. The frequency shift error is the smallest when the frequency range of the selected detected spectrum is twice the linewidth. Furthermore, the obtained results can provide a reference for the accurate extraction of Brillouin frequency shift.

The phase image is inevitably affected by the halo effect because of the low spatial coherence of the extended white light source. Therefore, we propose a novel method of eliminating the halo effect based on the Hilbert transform in this study. This method does not require any prior knowledge about the measured sample and only needs Hilbert transform for obtaining the derivative of the measured phase image. Subsequently, the corrected low-frequency data and high-frequency data obtained from the acquired image after applying the Hilbert transform and from the original measured image, respectively, are carefully mixed. This method effectively eliminates the halo effects and preserves the high resolution of the white-light phase imaging system. The phase images obtained via white-light diffraction phase microscopy, including those of the standard polystyrene microspheres and the living red blood cells, are processed by applying the proposed method. The experimental results demonstrate that the proposed method can effectively and rapidly eliminate the halo effects associated with the white-light diffraction phase image.

Herein, to achieve effective stimulated Raman conversion of the secondary Raman mode in addition to the Raman breathing mode and to obtain lasers with more coherent wavelengths, based on the rate equations of passive Q-switched internal cavity solid-state Raman lasers, the rate equations of passive Q-switched internal cavity Raman lasers with two Raman light outputs were derived and the equations were normalized. Moreover, the expressions of the normalized peak power and pulse energy of two Raman beams were derived. Numerical simulation shows the influence of each normalized parameter on output pulse the characteristics of the two Raman beams. The optimum values of these normalized parameters were numerically obtained to achieve efficient stimulated Raman scattering conversion of the secondary Raman mode. Further, the related experimental parameters were simultaneously verified and optimized the best experimental results.

Herein, the first-order vortex beam was generated from Yb∶phosphate (Yb∶QX) oscillator possessing defect-spot mirror. Moreover, the relationships among the optical-to-optical conversion efficiency of the vortex beam, defect-spot diameter, and laser mode diameter and shape inside the cavity were minutely evaluated. Results indicate that in Z-shaped solid-state lasers, when the diameter of laser mode inside the cavity is approximately 4--6 times the defect-spot diameter, the ratio of the laser spot diameter in the tangent and sagittal planes (i.e., ellipticity) is approximately 80%, and the pump power is 1.59 W, the first-order vortex beam with the highest optical-to-optical conversion efficiency of 7.7% is obtained. This study can provide a theoretical and experimental basis for generating first-order vortex beam with high conversion efficiency in Z-shaped solid-state lasers using defect-spot.

The Ti6Al4V and Inconel 718 dissimilar metals are welded using the continuous laser-induced eutectic reaction brazing technology. In this study, the influences of heat input on the microstructure and mechanical properties of the joints are systematically studied. The test results prove the formation of a large number of TixNiy intermetallic compounds in the fusion zone because of the excessive melting of the niobium interlayer at a heat input of 48 J/mm, resulting in the direct cracking of the joint. When the heat input is decreased to 40 J/mm, the unmelted Nb interlayer successfully inhabits the mixing of Ti6Al4V and Inconel 718 in the molten pool, preventing the formation of the TixNiy intermetallic compound in the joint. Further, two different metallurgical bonding interfaces, i.e., the (Ti, Nb) melting interface and the Nb/Inconel 718 eutectic reaction brazing interface, are obtained. Additionally, the (Ti, Nb) melting interface and the Nb/Inconel 718 bonding interface are separated by the unmelted Nb interlayer. The maximum strength of the joint is 205 MPa. However, an incomplete fusion defect can be observed at the Nb/Inconel 718 interface when the heat input is 34 J/mm, resulting in a tensile strength of 13 MPa.

This study design a surface response central composite design experiment to research the multiple targets technology optimization problem during the selective laser melting forming 18Ni300 die steel. Here, laser power, scanning speed, hatching distance, and powder coating thickness are considered as technology parameters and relative density, hardness, and wear resistance are considered as response targets. Combining with gray relational analysis to obtain the gray relational grade (GRG) of multiple targets and by analyzing the GRG value, we developed a regression model whose fitting value could reach 78.50%. Simultaneously, a group of optimized technology parameters, such as laser power of 250 W, scanning speed of 850 mm/s, hatching distance of 0.05 mm, and powder coating thickness of 0.02 mm, could be obtained. With the same arithmetic, optimized technology parameters group could get a better GRG value of 0.9312. Subsequent to performing confirmatory experiments, we obtained the sample with a relative density of 99.5%, hardness of 41.5 HRC, wear volume of 192000 μm 3, and GRG of 0.8895. The result is consistent with the prediction and the optimization method is reliable.

Herein, 18Ni-300 maraging steel parts were printed using selective laser melting process to examine the influence of positive defocusing distance on the printing quality and mechanical properties of 18Ni-300 forming parts. Results show that the power density is too high for a small defocusing distance (+1.5 mm), causing turbulence in the molten pool and obvious spheroidization is observed on the molten channel surface. Moreover, large pore defects appear inside the formed sample, the mechanical properties deteriorate, and the tensile fracture shows quasi-cleavage fracture characteristics. With an increase in the defocusing distance, the power density decreases and the spot and melting channel become wider, which improves the properties of the samples. Thus, the tensile fracture mechanism transforms into plastic fracture. For +2.5 mm defocusing distance, the power density is moderate, weld channels are well formed, internal structure is uniform, and interlayer bonding is enhanced. The hardness and tensile strength are 37.7 HRC and 1215 MPa, respectively. When the defocusing distance increases to more than +3 mm, the overlap ratio between the melt channels is too large and the over stacking phenomenon occurs. Furthermore, the power density is too low and the spot penetration force is weakened, thereby affecting the interlayer bonding and decreasing the sample performance.

In this study, a sequentially coupled thermal-elastoplastic finite element model considering the contact constraints of lap interface is developed to investigate temperature field, residual stress field, and welding deformation of full and partial penetration laser welded lap joints based on ABAQUS software, and the accuracy of numerical results is verified through experimental results. Results show that the high-stress zone of the full penetration welded joint is 30% wider than that of the partial penetration welded joint because the heat input of the former is relatively higher. The transverse residual stress on lower plate lap interface of the partial penetration welded joint is 198 MPa and higher than that of full penetration welded joint because the free expansion constraint of the weld metal in partial penetration plate is stronger than that in the full penetration plate. The difference of transverse deformation between the upper plate surface and the lower plate surface of the partial penetration welded joint is significantly larger than that of the full penetration welded joint, which causes the deformation in the thickness direction of the former to be larger than that of the latter.

To study the fatigue properties of TC4 titanium alloy subjected to compound strengthening treatment by laser shock peening and shot peening, surface residual stress and cut-off value of detail fatigue rated strength (DFRcutoff) were measured for TC4 titanium alloy for various surface-strengthening cases. These cases included untreated state, shot peening, laser shock peening, and compound strengthening by shot peening and laser shock peening. Additionally, the distribution of residual stress was analyzed by ABAQUS software. Test results show that samples treated by compound strengthening exhibit the highest values of both the surface residual compressive stress and DFRcutoff, with an increment of 189.1% and 62.3% over those of untreated samples, respectively. Simulation results indicate that residual compressive stress can form on the sample surface after the strengthening process, and the surface residual compressive stress on the compound-strengthened surface is the highest, followed by the samples subjected to shot peening and laser shock peening. Moreover, the residual compressive stress layer created by the compound strengthening treatment is the deepest, followed by that for laser shock peening and finally shot peening. From the combined results of tests and finite element analysis, it can be concluded that the DFRcutoff of TC4 titanium alloy is positively correlated with the surface residual compressive stress and depth of the residual compressive stress layer.

In this study, we investigate the effects of longitudinal magnetic field on the welded joint formation characteristics, ferrite/austenite microstructures, and fatigue crack propagation in the laser-MIG welding of 316L austenitic stainless steels. The experimental results indicate that the upper hybrid layer exhibits shallower penetration and larger width, whereas the lower hybrid layer exhibits a better symmetry with the addition of the magnetic field. Further, the melting area of the two hybrid layers and the remelted area between the two hybrid layers remain almost constant. The agitation of the solid-liquid front of the molten pool is promoted by the addition of the magnetic field, resulting in different fusion line shape characteristics. Furthermore, the addition of the magnetic field changes the growth direction of the microstrucuture near the fusion line, improves the thermal cycle of the base metal near the fusion line, reduces the width of the heat-affected zone, and restrains the coarsening of the grains. The magnetic field can promote the transition of cellular grains to the dendrites in the interlayer, change the ferrite dendritic morphology, and refine the interlayer and intralayer austenite structures. The fatigue crack growth resistance increases owing to the refinement of the microstructure, reducing the crack sensitivity of the joint.

The selective laser melting (SLM) technology is used to obtain the 24CrNiMo alloy steel for studying the influence of the laser scanning process parameters on its microstructure, relative density, hardness, and tensile properties. The results demonstrate that the microstructure of the SLM-formed alloy steel comprises tempered martensite and a small amount of retained austenite. The molten pool volume increases, whereas the cooling rate decreases with increasing laser power and decreasing laser scanning speed, resulting in the coarsening of the tempered martensite lath and the widening of the heat affected zone; further, the microhardness of the steel is observed to decrease. Meanwhile, the pores induced by the non-melted particle decrease, improving the relative density. The highest relative density of 99.93% can be obtained when the laser power and scanning speed are 320 W and 750 mm/s, respectively. When the laser power is 320 W and scanning speed is 950 mm/s, the SLM-formed alloy steel exhibits optimal tensile properties, its tensile strength and yield strength are 1362 MPa and 1252 MPa, respectively, and its elongation is 16.2%. Under reasonable SLM parameters, the comprehensive mechanical properties of the 24CrNiMo alloy steel formed via SLM are significantly better than those of the as-cast steel.

In this study, a method for fabricating embedded submicron metal wires on glass surfaces is demonstrated. First, grooves with submicron linewidths were ablated on glass surfaces using femtosecond laser direct writing. Then, metal films were deposited on the laser-treated glass surfaces using continuous-flow electroless plating. Subsequently, the plated samples were annealed by a thermal treatment process. Finally, an additional mechanical polishing process enabled the controllable fabrication of the embedded submicron metal wires on the glass surfaces. By combining the threshold effects of femtosecond laser ablation and continuous-flow electroless plating, metallic silver lines with a minimum linewidth of approximately 0.66 μm can be prepared. Moreover, four-probe measurement results indicate that the resistivity of the fabricated submicron metal wires is only approximately 1.2 higher than that of bulk silver, thus indicating their good electrical conductivity.

Carbon fibers (CFs) reinforced 316L stainless steels are prepared using laser cladding (LC) and the effects of scanning speed on the microstructure, microhardness, and wear resistance of carbon fibers reinforced 316L stainless steel are investigated in this study. Results show that the laser cladded 316L stainless steel without carbon fibers is composed of γ-Fe phase, the phases of the laser cladding carbon fibers reinforced 316L stainless are mainly composed of M23C6, γ-Fe and α-Fe, and M23C6 homogeneously distributes between γ-Fe and α-Fe dendrites. As the scanning speed increases, the distance between the dendrite arms decreases, while the microhardness first increases and then decreases. As a result, the wear resistance first enhances and then decreases. When the scanning speed is 12 mm/s, the laser cladding carbon fibers reinforced 316L stainless steel exhibits the highest wear resistance, and its wear resistance increases by approximately 25.3% compared with that of the laser cladded 316L stainless steel without carbon fibers.

This paper proposes a cavity adjustment method that corrects the relative misalignment of cavity parameters caused by the conversion of the initial cavity and test cavity structure during cavity ring-down high reflectance measurements. The proposed method monitors the shape of the ring-down cavity-transmission spot and also improves the efficiency of cavity adjustment. The cavity adjustment is based on the aspect ratio of the circumscribed rectangle and the area ratio of the spot to the circumscribed rectangle. The method suppresses the excitation of the high-order transverse mode caused by the misalignment of cavity parameters and maintains the transmission spot modes of the initial and test cavities in the TEM00 mode. In experimental tests, the adjustment based on the aspect ratio and area ratio not only distinguished different spot modes but also effectively dealt with special spot patterns between modes (such as elliptical spot and multimode jump). When the initial cavity and the test cavity spots were adjusted to the TEM00 mode, the measured mean reflectance of the sample was maximized and root mean square was minimized. Results show that this method can help to improve the cavity adjustment efficiency in cavity ring-down high-reflectance measurement technology.

In this study, we compare the accuracy and applicability of laser interferometer and electromagnetic acoustic transducer (EMAT) with respect to the detection of the metal laser ultrasonic signals to detect the thickness loss and surface-breaking cracks with respect to the metals during the production and usage processes. The surface and longitudinal wave signals were detected using the laser interferometer for predicting the thickness of the aluminum plate, and the longitudinal wave signal on the back of the aluminum plate was detected using the laser interferometer to detect the depth of the defect. Surface wave signal of EMAT is used to determine the position of crack defect of aluminum plate, and longitudinal wave signal of EMAT is used to determine the thickness of aluminum plate. The results show that, compared with the laser interferometer's complex laser ultrasonic signal receiving optical path, the errors of EMAT aluminum plate thickness measurement and crack defect location detection are less than 4%. And the prediction of the crack location will not increase with the decrease of the surface crack depth. Furthermore, the obtained results prove that the combination of the laser ultrasonic technology and the EMAT method can effectively reduce the complexity associated with the detection conditions and improve the applicability of laser ultrasonic technology.

To explore the applicability of the fiber Bragg grating (FBG) sensing technology in the penetration characteristics of jacked piles with different pile diameters, FBG strain and pressure sensors were calibrated using a tensile testing machine and sand calibration method, respectively. The linearity and testing accuracy of sensors are found to be good. Further, the jacked pile test of the test model with sensors was conducted in a large model box. Experiment results indicate that the requirement for FBG sensing technology to test the penetration characteristics of jacked piles with different pile diameters can be satisfied. FBG sensors demonstrate high linearity and sensitivity; moreover, the test data are reliable, and the installation method of the sensor is feasible. During the penetration process, FBG sensors can dynamically and accurately monitor the pile driving force, pile end resistance, side friction resistance, pile axial force, and unit side friction. Moreover, they can reflect the differences and change rules of penetration characteristics of the model piles with different pile diameters in the process of isostatic pressing.

It is difficult to extract feature points in the underwater environment owing to light refraction and water turbidity. To solve this problem, an underwater binocular measurement system that detects visual features using a line-structured light array is designed in this study. A large number of feature points are obtained by extracting the center of light strip. The refraction model of the underwater binocular's optical path is established, and an improved epipolar matching method is proposed to obtain the three-dimensional coordinates of feature points to realize the measurement and the restoration of three-dimensional morphology of underwater objects. The experimental results show that the measurement accuracy of the proposed method for underwater objects is comparable to that for land, and the measurement error is within 0.3 mm, which meets the measurement accuracy requirements. The measurement process of the proposed system is fast and accurate, and it is suitable for underwater real-time positioning measurement.

In this study, we theoretically investigate the high-order harmonic and isolated attosecond pulse generation with respect to the helium ion in the spatially inhomogeneous chirped field. The results demonstrate that the harmonic spectral cutoff of the spatially inhomogeneous chirped field is significantly extended when compared with the harmonic spectra generated by the single inhomogeneous field and the chirped field. Subsequently, an ultrabroad supercontinuum with a bandwidth of 1073 eV is successfully obtained. Furthermore, the spatiotemporally synthesized field can suppress a long path and select a short path to efficiently contribute to supercontinuum generation. Therefore, an isolated 13 as pulse is directly generated by superposing a wide range of continuous harmonics. Finally, a single attosecond pulse with pulse width of 7.4 as can be easily obtained by adjusting the chirp parameter.

A basic content of the morphological analysis of space objects is obtaining the volume parameter. LiDAR can be effectively used to scan space objects to obtain the laser point cloud and calculate its volume. First, a three-dimensional (3-D) LiDAR was used to scan the object to obtain the original point cloud; then, after 3-D space transformation, the point cloud is repaired with missing data, and the filtering and down-sampling processes were conducted to denoise and reduce the point cloud data. Finally, an implicit surface reconstruction algorithm was used to build the mesh model of the 3-D point cloud, and the volume was obtained from the mesh model. Two experimental objects were scanned using the LiDAR for experimental verifications. Compared with the actual volumes, the errors of the two experimental volumes are only 0.456% and 0.394%, indicating that the proposed volume calculation method demonstrates good surface reconstruction effect and volume calculation accuracy.

In this work, a model for the visualization of active laser imaging of underwater targets is established. The model can be used to analyze the effect of equipment parameters, water optical transmission properties, and target reflection characteristics on the active underwater laser imaging system in a direct, comprehensive, and systematic manner and provide a visual reference for further optimization of underwater optical imaging systems. The model employs the Monte Carlo method to track the state changes of photons during the entire underwater imaging process. The scattering of light by random rough surface was simulated using geometrical optics approximation. Furthermore, the random collision principle was used to simulate the process of scattering of light by particles suspended in water. Finally, a two-dimensional image was obtained using the Gauss formula, which enables the visualization of underwater target laser imaging. To verify the simulation results, the actual images obtained in the laboratory's controlled water tank were compared with the simulated images under the same parameters. The results show that the simulation images have the same characteristics and change rules as the actual images. The results show that the model can simulate the process of underwater laser active imaging well, with high accuracy and availability.

Laser-additive manufacturing technology is a rapidly developing key technology for the today's developed countries and pave a new technological way for the design and manufacture of high-performance metallic aerospace components. Metallic aerospace components have the characteristics of lightweight, difficult-to-process and high-performance, which poses significant challenges to the material design, structural optimization, process control, performance, and application evaluation of laser additive manufacturing. In this study, three categories of metallic materials typically applied in aerospace fields (i.e., Al-, Ti-, and Ni-based alloys and their metal matrix composites) and four kinds of typical structures (i.e., large-scale metal structure, complex integrated structure, lightweight lattice structure, and multi-functional bionic structure) are introduced. The recent research progress of laser additive manufacturing, both at home and abroad, in terms of new material preparation, new structure design, structure and performance control of laser additive manufacturing, high-performance/multi-functional components manufacturing, and aerospace applications, is presented. The coordination mechanisms of macro/micro cross-scale structure and performance control in laser additive manufacturing of high-performance metallic components are proposed. Furthermore, future research and development strategies of laser additive manufacturing technology in the direction of material-structure-process-performance integration are suggested.

Ultrafast laser filamentation is a unique nonlinear optical phenomenon that occurs during high-power ultrafast laser propagation in transparent optical media. In filamentation, a plasma channel, whose length can exceed the confocal length limit, is generated and referred to as filament. Following filamentation, very rich optical effects, such as supercontinuum generation, super clean fluorescence emission and amplification, and self-pulse compression, occur. The applications of this filamentation in various domains, ranging from remote sensing to ultrafast laser technology, weather control, laser fabrication, etc., have also been highly anticipated. This review gives a brief introduction of the history of the study of filamentation, including the research progress in experimental techniques, fundamental physics, and control methods. Furthermore, future challenges for the application of filamentation in several representative directions, for example, atmospheric long-distance propagation dynamics, physical chemistry inside filaments, and optimization of THz generation, are highlighted.

Vortex beam is a new type of structured light field carrying orbital angular momentum, with a spirally distributed phase surface. It has great application potential in fields such as quantum entanglement, quantum telecommunication, and microparticle control. With further research, structured light carrying more complex topological structures, phase singularities, orbital angular momentums, and polarization singularities than traditional vortex beams has been produced and has drawn the attention of many researchers. Focusing on two distinct mechanisms, i.e., generating spatially structured light straight from a solid-laser cavity and controlling the light field out of solid-laser cavity, we introduce methods on off-axis pumping and astigmatism conversion from the cavity, modulating elements from the cavity, pump shaping from the cavity, customizing the optical field out of cavity by a spatial light modulator, mode superposition out of cavity, and controlling the metasurface microstructures out of cavity. Moreover, the strengths and weaknesses of these methods and the trend in the development of spatial structured light have been described in this article.

Precise timing has become a necessary technology for many large-scale advanced scientific facilities, and timing precision is a crucial for these instruments to achieve their ultimate goals. This study reviews recent advances in timing technologies, including timing characterization for different kinds of sources (lasers, microwaves, and X-ray pulses) and large-scale timing synchronization systems based on free-space links and optical fibers. Furthermore, the primary limitations of timing systems and the future research prospects are discussed herein.

Millisecond and nanosecond lasers have become the preferred processing tools for alumina and aluminum nitride ceramics owing to the lasers' unique advantages of high reliability, high efficiency, low cost, and ability to process hard, brittle, and difficult-to-machine materials. These lasers play an irreplaceable role in the industrialized processing of group holes on ceramic substrate surfaces. This paper introduces the removal mechanisms with respect to laser processing ceramic materials of electronic substrates, including material ablation threshold, photothermal effect, and photochemical effect. The effects of the lasers' processing parameters and ambient environments on the hole size of ceramic materials, such as diameter, depth, and taper, are discussed. Current problems in the industrial application of laser hole drilling to ceramic substrates are summarized, and future development trends are presented.

Owing to their reduced weight, size, and high electronic-optic conversion efficiency, rare earth (RE) doped active fiber lasers or amplifiers are crucial in space-based applications, such as space laser communication, space laser radar, and space waste disposal and military. However, the radiation-induced attenuation of the active (RE-doped) fibers is approximately 1000 times larger than that of passive (RE-free) fibers under the same radiation condition, which poses a severe challenge to the long-term stability of active fiber lasers or amplifiers in space. First, this study briefly introduces the space radiation environment, the application requirements and challenges of silica-based optical fibers in space. Second, the latest research progress in the field of radiation-resistant active fibers, both in China and elsewhere, are systematically introduced from three aspects: 1) the mechanism of radiation-induced darkening of active fibers; 2) the primary factors influencing the radiation resistance of active fibers; 3) the methods to improve the radiation resistance of active fibers. Finally, the potential issues that require further investigation are suggested.

Eye-safe 1550-nm laser is located in an excellent atmospheric transmission window and detection-sensitive region of Ge and InGaAs photodiodes operating at room temperature. Therefore, such laser can be widely used in many fields, including lidar, laser ranging, and remote sensing. Diode-pumped Er 3+-doped crystals are considered as an effective method for directly obtaining the compact 1550-nm all-solid-state laser. In this paper, recent research progress in 1550-nm all-solid-state lasers based on Er 3+-doped crystals is reviewed. Moreover, further development of 1550-nm all-solid-state lasers is discussed.

Semiconductor laser has been half a century since its birth, tremendous progress has been made in theory, practice, and applications, and the market occupies more than half of the entire laser field. It is widely used in communication networks, industrial processing, medical and beauty, laser sensing, aviation and defense, security protection, and even consumer electronics. On the basis of reviewing the development history of early domestic and international semiconductor lasers, this article mainly focuses on GaAs-based 8xx nm and 9xx nm semiconductor lasers in the field of high-power pump sources, 905 nm tunnel junction lasers and 940 nm vertical cavity surface emitting lasers in the field of three-dimensional sensing, and GaSb-based infrared lasers and InP-based quantum cascade lasers in the field of spectral analysis and infrared sensing, for a brief review. The content includes the main application scenarios, the main goals pursued, the latest developments in the past 10 years at home and abroad, and the possible development trends and directions in the future.

Neural networks, as one of the most representative techniques in artificial intelligence, have been in rapid development towards high computational speed and low power cost. Due to intrinsic limitations brought by electronic devices, it can be hard for electronic implemented neural networks to further improve these two performances. Optical neural networks can combine both optoelectronic technique and neural network model to provide ways to break the bottleneck. In order to have a brighter view on the history, frontiers and future of optical neural networks, optical neural networks of feed-forward, recurrent and spiking models are illustrated in this paper. Challenges and future trends of optical neural networks on in situ training, nonlinear computing, expanding scale and applications will thus be revealed.

Air lasing refers to the coherent emission produced with air as the gain medium. Air lasing has numerous advantages such as high directionality, high coherence, high intensity, and free-space propagation. Therefore, air lasing provides a novel pathway for remote sensing. Air lasing, which is generated by the interaction between a strong ultrafast laser and atoms or molecules in air, includes many new strong-field effects. In this paper, we reviewed the major advances in air lasing in recent years. First, generation methods and basic characteristics of three types of air lasing were introduced. Next, new physical effects involved in air lasing were revealed based on two aspects, gain mechanism of molecular nitrogen ion lasing and quantum coherence. Moreover, applications of air lasing in remote sensing were discussed. Conclusively, the research significance of air lasing was summarized, and the opportunities and challenges in this topic were highlighted.

Optical waveguides are fundamental elements in integrated optics. Compact lasers based on the waveguide platforms are miniature light sources that have gained increasing research attention. Waveguide lasers are expected to play important roles in photonics. Laser crystals are major gain media for solid-state lasers. In this study, we review the state-of-the-art advances in solid-state waveguide lasers, including operation of continuous wave and pulse (Q-switched or mode-locked). The lasing wavelength covers a wide spectral range from visible light to the mid-infrared. Furthermore, a brief perspective on future research directions is provided.

In the past decade, mid-infrared ultrafast mode-locked lasers have made significant progress, effectively promoting their applications in many fields, including mid-infrared frequency combs, molecular spectroscopy, material processing, laser surgery, molecular organism, and chemistry. Herein, starting with the progress of mid-infrared ultrafast fiber lasers in recent years, we introduced the development of various optical fiber lasers in this band and discussed femtosecond fiber laser solutions achieving shorter pulse widths and farther wavelengths. Further, we introduced chirped pulse amplification and nonlinear amplification. Thus, mid-infrared ultrafast fiber lasers are in the high-speed development stage and will have abundant applications in the future.

X-rays produced through picosecond petawatt lasers have high flux, short pulse duration, and tiny focal spot. High spatial and temporal resolution X-ray point-projection backlight radiography developed using such high flux X-rays is an important diagnostic technique for measuring the dynamic response of materials under intense laser load, the inertial confinement fusion, and other high energy density physics. Short pulse X-rays are generated through the picosecond petawatt laser beams on TITAN and OMEGA-EP systems, as well as the upgraded SHENGUANG-II facility and other large picosecond watt laser facilities. The spectrum, conversion efficiency, and resolution of the X-ray backlights produced using the interactions between the picosecond petawatt lasers and solid targets are characterized in the experiments conducted herein. Furthermore, the technology of point projection backlighting is developed, and the dynamic demonstration experiment is performed. Images of the compressed fuel of inertial confinement fusion targets and the ejecta of shock-loaded materials are successfully obtained using backlight radiography.

Fiber lasers have the advantages of high photoelectric conversion efficiency, transmitting laser with flexible medium, high power output, high beam quality, high compactedness and reliability. They are especially suitable as heat sources for two types of metal additive manufacturing technologies: powder bed fusion (PBF) and directed energy deposition (DED). This paper reviews the types, output power, and operating modes of fiber laser heat sources in those two additive manufacturing technologies, and the PBF and DED research status of fabricating typical metal materials. The relative density and mechanical properties of specimen manufactured by PBF and DED are higher than that of traditional casting and close to the level of forging. Finally, the research direction of fiber laser additive manufacturing technology is prospected, and the development trend of fiber laser is prospected according to the demand from those two kinds of metal additive manufacturing technology.

Light can carry spin angular momentum and orbital angular momentum. The spin angular momentum is associated with the circular polarization state of the beam, while the orbital angular momentum is related to the spiral phase of the beam. Since Allen et al. theoretically confirmed the physical concept of the orbital angular momentum of photons in 1992, this novel type of light field with a special spiral phase wavefront has attracted many research interests and found many important applications in both classical and quantum optical realms. This study, from both the fundamental and applied physics, reviews the preparation and detection methods of the orbital angular momentum beam, especially the recent progress in a variety of fields with the orbital angular momentum ranging from spiral-phase contrast imaging and remote sensing of the rotational Doppler effect to optical micromanipulation.

The Shanghai Soft X-ray Free-Electron Laser (SXFEL) Facility is the first coherent X-ray light source in China with a wavelength covering the water window. An elliptically polarized undulator (EPU) will be installed to meet the needs of different users, so that the output laser can be switched between linear polarization and circular polarization. This paper presents the polarization control design for the user facility, the evaluation of bunch energy jitter, and analysis of radiation power stability. Moreover, a switching structure of the circular polarization direction is also designed, so the polarization of the output laser can be switched between opposite directions on the base of permanent magnet oscillator at the rate of tens of Hz.

The phase reconstruction technique plays an extremely important role in various fields, such as materials science, biomedicine, and astronomy. In different phase retrieval technologies and applications, different light sources are needed. The wavelength, coherence, and energy of a light source can all affect the final phase reconstruction. In previous studies, in terms of spatial coherence, a light source has often been considered completely coherent light. However, in the actual experiments, X-ray and electron beams both correspond to partially coherent light. Spatial coherence of fully coherent light propagated through a medium will reduce accordingly. Therefore, it is especially important to identify appropriate ways to realize correct reconstruction under partially coherent illumination. In this paper, we provide a review of the research background and progress in developing phase reconstruction methods under partially coherent illumination. Some common methods, such as mode decomposition, transport of intensity equation, self-reference holography, and focus variation, are introduced and their advantages and disadvantages are compared. Moreover, the application of the self-reference holography method to the measurement of the correlation function for a specially correlated partially coherent beam and the determination of the corresponding topological charge of a partially coherent vortex beam is also summarized.

Recently, air lasing has attracted significant attention owing to its promising applications in atmospheric sensing and environmental monitoring. Air lasing usually refers to strong-laser-induced population inversion of atmospheric constituents, resulting in no-cavity light amplification over a remote distance. It has been revealed that with the excitation of intense laser pulses, molecules of the two primary atmospheric constituents, i.e., nitrogen and oxygen, can exhibit lasing behaviors. The gain media can be atomic oxygen and nitrogen, as well as neutral nitrogen molecules and nitrogen molecular ions. Herein, we present the phenomena, underlying mechanisms, and potential applications of air lasing with a focus on the gain media of neutral and single-ionized nitrogen molecules. Moreover, the influence of the laser polarization state on the N2+ lasing is discussed in more detail.

This paper uses laser-induced breakdown spectroscopy and support vector machine to analyze the content of Mn in soil. Forty-four soil samples were collected in Huaibei, Anhui. The samples were divided into training set (34 samples) and test set (10 samples) using Kennard-Stone (K-S) method. Multiple linear regression (MIR), grid search method (GSM), genetic algorithm (GA), particle swarm optimization (PSO), and least squares method (LS) were used to establish quantitative analysis models. The results show that the correlation coefficients Rtra2 of the training set of the MIR, GSM, and PSO models are only 0.861, 0.866, and 0.862, respectively. The correlation coefficients Rt2 of the test set of corresponding models are lower than 0.9, the relative error is greater than 8.6%, and the error is larger. The Rtra2 of the GA model is greater than 0.93, and Rt2 is less than 0.9. The training time of the GA model is long, so the training time must be reduced, and the correlation of the test set must be improved. The LS model works well with Rtra2 0.998 and Rt2 0.967, and the relative error is small. The training time is greatly shortened year-on-year, correlation is good, and generalization ability is strong. The LS model is more suitable for the rapid detection of the Mn element in soil.