Semiconductor distributed feedback (DFB) laser has become a key component in optical fiber communication and free-space optical communication due to its excellent spectral characteristics, modulation characteristics, as well as the low cost and volume-production characteristics, which will play an irreplaceable role in a variety of applications such as 5G, data center and microwave photonics. This paper reviews the DFB lasers in the optical communication band based on their applications and characteristics. The principle and recent progress of diverse DFB lasers are introduced and reviewed respectively, including high speed directly modulated DFB laser, high power DFB laser, and low noise (narrow linewidth and low relative intensity noise) DFB laser.

The continuous growth of data centers and 5G wideband wireless communication is driving toward an ever increasing demand for short-distance broadband fiber transmission, which has greatly promoted the development of high-speed optoelectronic devices. In short-distance applications, although directly modulated lasers have the advantages of low cost and low power consumption, the modulation bandwidth and transmission distance are limited by the relaxation oscillation frequency and frequency chirp. On the other hand, an electroabsorption modulated laser (EML) exhibits wide modulation bandwidth and low frequency chirp, thus allowing transmission with higher bit-rate and over longer distance. In this paper, the integration schemes and device structure of EML integrated light source are discussed. Recent progress in high-speed, high power, and low cost EML devices is summarized. Finally, a prospect for future development of EML is presented.

With the rapid expansion of the communication industry and the development of technologies, such as optical interconnection and on-chip experiments, there is a growing demand for integration and miniaturization of devices such as lasers. Semiconductor nanowire lasers have been widely studied in the field of micro/nano-lasers due to their unique one-dimensional structure and flexible bandgap control performance. Semiconductor nanowire lasers with single-mode output are of great significance in the fields of optical interconnection, sensing, spectroscopy, and interferometry. This article reviews the basic technology and research progress of single-mode semiconductor nanowire lasers. First, the common materials of semiconductor nanowire lasers are introduced, and the basic mode characteristics are analyzed using the circular dielectric waveguide model. Subsequently, the main methods and development status of semiconductor nanowires achieving single-mode laser output are elaborated. Finally, the challenges faced by each scheme are summarized.

As semiconductor nanocrystals, colloidal quantum dots (CQDs) are widely used as gain materials for micro/nanoscale lasers owing to their advantages of high quantum yields (~100%), tunable emission wavelengths, excellent stability, high refractive index, solution processability, and low-cost synthesis. Based on nanoscale sizes (2--20 nm) and solution processability, CQDs can construct high refractive index micro/nanoscale structures through self-assembly methods. Here, three kinds of CQD micro/nanoscale lasers are summarized based on manufacturing methods of CQD resonators, and their characteristics are discussed in details. In addition, the on-chip integrations of CQD lasers and waveguides are also introduced. Finally, the outlook for the development of CQD lasers and its on-chip integration in photonic integrated circuits (PICs) is provided.

Vertical-cavity surface-emitting lasers (VCSELs) provide several advantages, such as small footprint, low power consumption, high efficiency, broad modulation bandwidth, long lifetime, circular beam, wafer-level test, and two-dimensional array arrangement. VCSELs have been widely deployed in fields such as data communication, sensing, light detection and ranging (LiDAR), and material processing. These applications require VCSELs that have superior modal properties, high data rate, high energy efficiency, and high temperature stability. Many novel oxide-confined VCSELs based on separated confinement for single-mode operation have been reported. Progress in data rate, power consumption, and temperature stability of VCSELs has been achieved using novel VCSEL structures, improving the active region, reducing the parasitic effect, and reducing the thermal effect. Photonic metastructures with unique optical properties can be used to control the modes and modal fields of VCSELs and integrate VCSELs with in-plane photonic circuits.

Quantum cascade lasers (QCL) emission wavelength spans from infrared to terahertz (THz), lead to many applications such as pollution monitoring, industrial process control, medical diagnostics, narcotic drugs and biochemical dangerous goods sensitive detection, and free space optical communications. Invented in 1994 following many years of research on band-structure engineered semiconductors and devices grown by molecular beam epitaxy, this fundamentally new laser has rapidly advanced to a leading position among infrared and terahertz semiconductor lasers in terms of practical wavelength agility as well as power and temperature performance. In this paper, we review the the progresses in designing concept of QCL, expanding wavelength range, increasing output power, realizing broadly tunable single-mode operation, and enhancing the beam quality in sequence. Finally, we give the brief conclusion and possible outlook.

Two-dimensional transition-metal dichalcogenides (TMDC) have unique advantages and can be used as gain materials for laser emission. The strong Coulomb interaction and weak dielectric screening effect of TMDC materials induce a large exciton binding energy to achieve stable exciton emission at room temperature. Moreover, TMDC can extensively increase the capability of light confinement owing to its high refractive index of up to 6--7. The atomic layer surface of TMDC materials has no dangling bonds, so they can avoid lattice mismatch when connected with silicon-based semiconductor devices. These unique properties render TMDC as potential gain materials that can be coupled with silicon-based microcavities to form laser devices. Furthermore, their atomic thickness and near-infrared spectral radiation ensure promising interconnection with integrated devices. This study summarizes the research progress of lasers based on TMDCs in recent years with the classification of optical microcavities and laser principles. Moreover, current challenges and their future application prospects are also discussed.

Based on the intersubband transitions of multiple quantum wells, the quantum cascade laser (QCL) can be considered to be an unipolar semiconductor laser. The lasing frequency can be observed in the mid-infrared, far-infrared, and terahertz regions. Terahertz QCL is the most effective electrically pumped terahertz radiation source in the frequency range of 1--5 THz and exhibits the advantages of compactness, easy integration, high output power, and high conversion efficiency. In this study, the active regions, waveguide structures, and materials associated with terahertz QCLs are initially described. Next, the high-performance terahertz QCLs are presented under the conditions of extreme lasing frequency, high operating temperature, and high output power. Subsequently, the recent advances in terahertz QCLs are reviewed from the aspects of photonic engineering, including one-dimensional gratings, two-dimensional photonic structures, and metasurfaces. Additionally, the progress in case of frequency combs based on terahertz QCLs in our laboratory, such as active-frequency combs, passive-frequency combs, and terahertz dual-combs, is reviewed.

The development of laser technique has promoted the progress of modern science and technology, and changed the life of human beings. Among them, the miniaturized laser light source has become one of the research hotspots. Due to the effect of the intensity confinement of metal plasmon, plasmonic nano lasers are able to achieve ultra-small physical size which can break through the optical diffraction limit, and achieve the high modulation speed and minimum lasing threshold, so it is widely concerned. In this review, the recent progress of plasmon nanolasers at home and abroad is discussed. The gain media, metal species, and device structures are compared and summarized. Finally, the future development potential of the plasmon nanolasers is discussed and prospected.

Semiconductor micro/nanowire lasers can be used as integrated coherent light sources. Further, they can be extensively applied in various fields, including optical communication, optical computing, sensors, and biological studies. In this study, we review the domestic and international research progress on wavelength tuning and single-mode lasing of the semiconductor micro/nanowire lasers. Moreover, the Bose-Einstein condensation phenomenon of exciton-polariton based on the strong photon-exciton coupling effect is introduced as a new method to develop micro/nanowire lasers with low lasing thresholds. In addition, the operating mechanism of new lasers based on exciton-polariton and new achievements are presented.

GaN-based vertical-cavity surface emitting lasers (VCSELs) have undergone rapid advancement in the past 20 years and are considered a research hotspot for next generation semiconductor lasers. GaN is a versatile material for fabricating optoelectronic devices in ultraviolet, blue-violet, and green bands. The characteristics of VCSELs include output beam with circular symmetry, low thresholds, low divergence angles, and high-modulation speeds. In this paper, the development history of GaN-based VCSEL was first reviewed and some of their applications were discussed. Moreover, key challenges in the design and manufacturing of mirrors and cavities were discussed. Further, the heat dissipation mechanism of GaN-based VCSELs with three different structures and their optimization strategies were analyzed and discussed. Finally, research progress and latest advancements in GaN-based blue, green, and ultraviolet VCSELs were reviewed.

Nanooptics is a new frontier and fundamental direction generated by the intersection of photonics and nanotechnology, which enables people to manipulate the interaction between light and matter at the nanoscale and explore new physical phenomenons. Nanolaser is a new kind of light source and its research is an important branch in the field of nanooptics. Since the feature of small size and strong confinement, nanolasers have attracted more and more attentions from researchers in recent years. In this paper, some exciting progresses in laser miniaturization are reviewed. First, we briefly describe various new types of lasers realized in recent years and their characteristics. Second, the new physical problems of micro- /nano-lasers are analyzed and the latest progresses are presented. Finally, some technical challenges in the application of nanolasers are introduced and considered.

Photonic crystal surface-emitting laser (PCSEL) is a new type of semiconductor laser, which can produce high-power laser emission with ultralow divergence (less than 1°). It has an important application prospect in light detection and ranging equipment, space communication, sensing and material processing. It can be divided into two types: defect mode and band-edge mode. The band-edge mode PCSEL has the following advantages: it has single-mode and easy two-dimensional integration. Therefore, the basic principle and research progress of band-edge mode of a PCSEL are briefly introduced in this study. Based on the theoretical derivation and several reported examples, how to improve the output power of a band-edge mode PCSEL is discussed and a new method of power enhancement is proposed. Finally, the development trend of a band-edge mode PCSEL is prospected.

Compared with noble metal nanoparticles, all-dielectric nanoparticles have attracted much attention because of their support to Mie resonance with low loss in optical frequencies, which have been the important material in nanophotonics to enhance the interaction between light and matter. Semiconductor nanoparticles with high refractive indexes are important all-dielectric nanoparticles, which support both electrical and magnetic resonances and can be used to construct low-loss metamaterials and metasurfaces in optical frequencies. Based on the recent research on semiconductor sub-wavelength particles with high refractive indexes, the related works and applications of the fluorescence manipulation of semiconductor micro-nano particles based on Mie resonance are mainly introduced, and the development of this field is prospected.

Owing to the strong three-dimensional quantum-confinement effect of semiconductor quantum dots (QDs), QD lasers exhibit superior performances with low threshold current, high modulation rate, high temperature stability, low linewidth enhancement factor, and high antireflection. They are expected to have important applications in high-speed optical communication, high-speed optical interconnection, and other fields. At the same time, a QD structure is insensitive to dislocations, making QD lasers powerful candidates for the efficient light sources that are urgently needed for silicon optical integration. First, we briefly review the research progress on 1.3-μm semiconductor QD lasers, and then focus on the excellent characteristics exhibited by GaAs-based QD lasers, including their threshold current density, temperature stability, modulation rate, and antireflection characteristics. We also introduce QD lasers grown directly on GaAs and Si (001) substrates.

A 1.31 μm square-Fabry-Perot (FP) coupled cavity semiconductor laser is theoretically analyzed and experimentally prepared. The square cavity acts as a reflection end for the FP cavity, and its reflectivity is adjustable by changing the current injected into the square cavity. The mode coupling between the square mode and the FP mode can suppress other side modes, and it is easy to realize single-mode lasing. The maximum single-mode lasing side mode suppression ratio obtained in the experiment is 38 dB, the wavelength tuning range is 6 nm, and the estimated characteristic temperature T0 is 46 K.

This study thoroughly investigates the influences of n-type AlGaN cladding layers and upper waveguide layers (UWG) on the optical field distribution and electrical characteristics of GaN-based green laser diodes (LDs). It is found that the optical field leakage can be evidently suppressed by increasing the thickness of the AlGaN cladding layer or the indium content of the InGaN upper waveguide layer. Moreover, compared with LDs with In0.02Ga0.98N UWG, the LDs with a higher indium content of In0.05Ga0.95N UWG perform better because of the larger optical confinement factor. Therefore, coregulation of the n-type cladding layer and the UWG can improve the optical field distribution, enhancing the performance of GaN-based green LDs.

We propose a new monolithic widely tunable semiconductor laser?multi-channel interference (MCI) laser. Multi-arm interference is used for mode selection, which is simple to manufacture and has large tolerance. The wavelength tuning range of the MCI laser based on the electro-optic effect is more than 40 nm, the side mode suppression ratio (SMSR) is more than 40 dB in the whole tuning range, and the fiber output power of the device integrated with the laser and the semiconductor optical amplifier is more than 13 dBm. In the meanwhile, the wavelength tuning range of the MCI laser based on thermo-optical effect is greater than 45 nm, the SMSR is larger than 48 dB, the line width is less than 250 kHz, and the total thermal tuning power consumption is less than 50 mW. In this paper, the theoretical design, fabrication, and test of the device are described in detail.

In the medical field, the 1160-nm wavelength vertical-external-cavity surface-emitting semiconductor laser (VECSEL) is the fundamental frequency lasing source of orange laser. However, the strain accumulation effect induced by the high strain InGaAs quantum well in the luminous zone limits high output power. In this study, the secondary compensation method is proposed for a high strain InGaAs quantum well using the GaAsP material in a single luminescent zone to achieve high material growth quality of optical absorption layers. The structure of the absorption layer containing Al is designed to reduce the photogenic carrier-blocking effect caused by GaAsP barrier and improve the injection efficiency of photogenic carriers. The lasing wavelength and output power of the prepared VECSEL devices are 1160 nm and 1.02 W, respectively. The lasing spot shows symmetrical morphology, and the divergence angles of the spot at orthogonal directions are 10.5° and 11.9°.

High-power semiconductor lasers operating at a wavelength of 800 nm are one of the preferred laser sources for long-distance illumination. However, owing to poor beam quality and brightness, it is difficult for 800-nm semiconductor lasers to transmit over long distances. Therefore, it is essential to improve the beam quality and brightness. For spectral beam combining method, the output power and brightness are improved while preserving the beam quality of a laser unit. Based on this method, a high-power laser source comprising 10 800-nm laser arrays is developed by optimizing the spectral width and the combining structure according to the gain spectra of laser bars, with a continuous power of 363.5 W, beam quality of 4.17 mm·mrad, brightness of 212 MW/(cm 2·sr), and electro-optic conversion efficiency of 40%. Through further structural optimization and polarization coupling, a kW-class high-power 800-nm semiconductor laser can be obtained, providing a high-performance laser source for long-distance laser illumination.

In this study, we propose a scheme to generate tunable and broadband two-channel optical frequency comb (OFC) based on a gain-switched 850 nm vertical-cavity surface-emitting laser (850 nm-VCSEL) under orthogonal optical injection. In addition, the influences of some operation parameters on the performances of the generated OFC are numerically investigated. For developing this scheme, an 850 nm-VCSEL is modulated using a large signal current and is subsequently driven into a gain-switching state. In this case, the Y polarization component of the laser oscillates and behaves in a periodic pulse state, whereas the X polarization component is suppressed. Therefore, a one-channel OFC is generated in the Y polarization direction (Y-OFC). The X polarization component of the gain-switched 850 nm-VCSEL can be oscillated to achieve a periodic pulse state with a spectral intensity equivalent to that of the Y polarization component by introducing optical injection polarizing along the X direction (orthogonal optical injection). Thus, a two-channel OFC can be obtained. The simulated results are obtained based on an 850 nm-VCSEL under current modulation with a modulation depth of m=0.75 and a modulation frequency of fm=4.2 GHz as well as orthogonal optical injection with frequency detuning of Δv=10.0 GHz; these results indicate that a two-channel OFC having bandwidths of greater than 105.0 GHz with an amplitude variation of less than 10 dB can be obtained under the optimized injected field amplitude Einj. Subsequently, the ranges of Einj and Δv are determined by examining the optical spectra of the X and Y polarization component outputs obtained from the gain-switched 850 nm-VCSEL subjected to orthogonal optical injection for obtaining a two-channel OFC. Finally, the tunability of comb spacing is demonstrated by analyzing the OFC performance under different values of fm.

The wet oxidation process is a key technology for fabricating a vertical cavity surface emitting laser (VCSEL), but the stability and controllability of the oxidation process require further improvement. Herein, the oxidation temperature, which is the core factor in the oxidation process, was studied in depth. By establishing a control experiment, the effect of the oxidation temperature on oxidation rate and shape of the oxidation pore was explored. Understanding this effect is vital to accurately control the size and shape of the oxidation pore and improve the electro-optical characteristics of the device. Moreover, the oxidation temperature control curve was simultaneously optimized based on the oxidation reaction mechanism of AlGaAs. Experimental results indicate that samples oxidized based on the oxidation temperature control curve exhibit excellent thermal stability and high overall structural reliability.

In this study, an intermediate monomer and a branched tris (diethylamino) phosphine (TDP) were employed in a CsPbBr3 perovskite nanocube solution to synthesize CsPbBr3 perovskite nanorods, ensuring both the slow growth rate and the orientation growth. Transmission electron microscopy, X-ray diffraction, ultraviolet-visible absorption spectroscopy, and photoluminescence (PL) spectroscopy were used to characterize the obtained material. It is found that the CsPbBr3 perovskite nanorods exhibit good crystal quality, low defect density, and favorable optical property. The gain coefficient (860 cm -1) and threshold (17.5 μJ/cm 2) were measured through the variable length stripe method and PL spectra under different pumping densities. Furthermore, the photostability of the sample at a high temperature and high relative humidity (85 ℃, 85%RH) was tested. It is found that the stability improved. Results provide experimental basis for the practical application of advanced perovskite-based lasers.

High-power blue lasers based on gallium nitride (GaN) have significant prospects in numerous applications, such as laser display, laser lighting, and metal processing. In this study, the epitaxial growth temperature of the p-AlGaN cladding layer of blue lasers is optimized to suppress the thermal degradation of the quantum wells (QWs) and QW structures are optimized to improve the carrier distribution to achieve high-power blue lasers. Upon varying the front facet-coating reflectivity, the internal optical loss and carrier injection efficiency are deduced to be 6.8 cm -1 and 90%, respectively. Under pulsed operations, the threshold current density of the blue lasers is 1 kA/cm 2 and the efficiency gradient is 1.65 W/A. Thus, the expected output power is 4 W at a current density of 6 kA/cm 2. Under continuous-wave operations, the threshold current density of the blue lasers is 1 kA/cm 2 and the efficiency gradient drops to 1 W/A for poor packaging heat dissipation perform. Thus, the output power reaches 2.2 W at a current density of 6 kA/cm 2.

Based on the I-type quantum well of the GaSb system, lasers with a lasing wavelength of 2。75 μm was fabricated. The valence band level of the barrier was effectively reduced, and the valence band order was increased using the quinary barrier material AlGaInAsSb. Additionally, the luminescence wavelength of the quantum well red shifted to 2。75-μm band. The optimal epitaxial parameters of the quantum well were obtained by optimizing the growth parameters of molecular beam epitaxy, and a Fabry-Perot laser device with a cavity length of 1。5 mm, ridge width of 50 μm, and central wavelength of 2。75 μm was fabricated. The laser realizes continuous lasing at room temperature, and its maximum output power and threshold current are 60 mW and 533 A·cm -2, respectively.

Presently, the manufacturing technology of silicon-based light sources that are compatible with complementary metal oxide semiconductor processes and have high luminous efficiency is immature. To address this problem, a new structure for a poly-silicon light-emitting device is proposed in this paper. The possible avalanche modes (interband transition, bremsstrahlung, intraband transition of holes between light and heavy mass bands, ionization under high-field conditions, and indirect interband reorganization) of the device under reverse bias voltage are studied. The mechanism is analyzed theoretically; moreover, the drift and diffusion of holes and electrons inside the device under reverse bias voltage are studied, which reveals that carrier injection increases the number of carriers involved in the avalanche multiplication process and can lead to an increase in the impact ionization rate. Furthermore, the electric field, spectrum, current, and light intensity of the device are analyzed. The quantum efficiency and photoelectric conversion efficiency are calculated; it is concluded that the impact ionization rate is improved by the carrier injection. The improvement in the device''s luminous efficiency is achieved by the improvement in the impact ionization rate, for which the quantum efficiency is 5.9×10 -5 and photoelectric conversion efficiency is 4.3×10 -6.

In this study, theoretical analysis and experimental investigation of the temperature characteristics of a 974 nm dual-fiber Bragg grating laser are conducted. The effect of grating pitch on reflectivity is theoretically simulated. First, the spectrum of the device is measured at room temperature (25 ℃). Compared with the spectrum of a device without a dual-fiber grating, the secondary peak in the spectrum of the dual-fiber grating laser is significantly suppressed, and the peak wavelength (974.07 nm) of the test laser is locked near the center wavelength of the grating at 974 nm. The power current voltage characteristics characteristics of the device are also measured. When the operating current reaches 400 mA, the output power of the pigtail is greater than 253 mW. Then the wavelength change rate and power change rate of the device at full temperature are analyzed, and it is found that the wavelength change rate is less than 8.2×10 -3 nm/℃. Finally, the differential structure function curve of the device is obtained, and the thermal resistance distribution is analyzed. By optimizing the heat sink sintering process, a device power change rate of less than 1.06% is obtained.

Concerning the problem of current low indoor positioning accuracy and high deployment cost, this paper proposes a visible light communication/improved pedestrian dead reckoning (VLC/IPDR) smartphone-based particle filtering fusion indoor positioning technology. The method fist decodes the image information of the light emitting diode (LED) light source captured by the smartphone CMOS camera to determine the LED area of the point to be located. Second, the specific position information is determined according to the illuminance model and the direction angle obtained by the mobile phone gyroscope. Finally, the position coordinates obtained by VLC are used as the observation values, and IPDR is used as the state transition equation of particle filtering, the particle filter is used to fuse the two for joint positioning. Experimental results show that the average positioning error of this method is less than 6 cm under a small space of 3 m×3 m×3 m and a single light source, the average positioning error of this method is less than 0.2 m in the multi motion mode positioning test on the vertical intersection path of 120 m and 45 m.

A scheme to generate a tunable optical frequency comb (OFC) based on coupled radio frequency (RF) signals and Mach-Zehnder modulator (MZM) is proposed. The coupled RF signal generated using a cheap signal source can be used to replace the expensive and high-frequency signal sources, thereby reducing the cost of OFC generators. The theoretical model of OFC generated using various coupled RF signals is established, and the scheme is verified using OptiSystem 13.0. The effects of the modulating voltages of upper and lower arms of MZM, output frequency of the RF signal source, and coupling mode on the comb number, power flatness, and frequency interval are analyzed. Simulation results indicate that for 5-GHz output RF signal and 4 times multiplicative-couple, OFC with frequency interval of 10 GHz, comb line number of 7, and power flatness of 1.88 dB can be obtained. Moreover, the proposed scheme can generate OFC with additional comb-lines, better flatness, and lower cost compared with the same frequency interval OFC generation scheme based on a single MZM. The proposed scheme provides guiding significance for the design of low-cost and high-performance OFC generators.

By using the generalized nonlinear Schr?dinger equation, the supercontinuum characteristics of fs level double Gaussian pulses bound-state and double Airy pulses bound-state in highly nonlinear single-mode optical fiber are simulated. Simulation results demonstrate that, because of the self-healing properties of Airy pulse, the output supercontinuum generated by the double Airy pulses bound-state is wider and flatter than that generated by the bound state of double Gaussian pulses bound state. The influence of sub-pulse interval of the double pulses bound-state on supercontinuum generation is studied. It is found that when the sub-pulse interval increases, the spectrum widens first, then narrower, and finally remains unchanged. It is shown that there is an optimal sub-pulse interval to produce the widest supercontinuum, which is of great significance to select the appropriate sub-pulse interval of the double pulses bound-state and optimize the supercontinuum

To address the deterioration of the joint performance owing to the formation of brittle Mg-Al compounds, this paper proposes a laser fusion welding technology by adding Ni interlayer to magnesium and aluminum alloys'' interface layer. A lap joint is applied with a magnesium alloy sheet on the top and an aluminum alloy sheet at the bottom. The interlayer element is selected based on the calculations of thermodynamic properties and mechanical properties calculations. Finally, the effects of Ni interlayer on the microstructure and properties of fusion welded joints are studied. Results show that the surface of the joint with an Ni interlayer exhibits a regular fish scale, the weld has good formability, and the maximum shear strength (linear load) of the joint is 78.92 N/mm under laser power of 1450 W, a welding speed of 1200 mm/min, a laser head deflection angle of 20°, a defocus amount of 0 mm, and side blowing and back protection using the Ar protective gas. The Ni interlayer foil acts as a barrier to limit the upward diffusion of the Al element in the lower aluminum alloy liquid into the upper magnesium alloy molten pool; therefore, the formation of brittle Mg-Al compounds is reduced. In addition, the reaction of Ni and Al in the molten pool region of alloys joint to form ductile AlNi and Al3Ni compounds, AlNi and Al3Ni compounds have better ductility, and their structures are more stable than those of Mg-Al compounds, implying that the joint brittleness is reduced. The above synergistic effect is an important reason why adding Ni interlayer improves the shear performance of welded joints of magnesium/aluminum alloys.

This study proposes a V-shaped seam tracking algorithm based on particle filter with a histogram of oriented gradient (Hog) to address the problem of accuracy loss due arc light, spatter, and other noises in the process of weld tracking. The Hog has good invariance in geometric and optical deformation, and it is used to describe the directionality of the laser stripe at the flex point. The Hog of the target region of the weld seam is extracted to establish the target template. The candidate region state of the weld seam is predicted by the continuity of the position information of the weld seam image sequence. The Hog feature of the candidate region state is calculated through particle sampling. The similarity in the features of the candidate state and the target template is calculated, the optimal weld position is then obtained, and the weld seam tracking is realized. The tracking experiment of typical V-shaped weld image is performed. Experimental results show that the proposed algorithm can accurately locate the target position of weld under the environment of strong arc light, splash and other noise interferences, and the tracking error is less than 0.24 mm.

Vortex radii of perfect vortex beams are independent of the topological charge and carry orbital angular momentum, and these beams have wide applications in optical communications, quantum optics, and laser manufacturing. In this study, radial phase-shifted spiral zone plates that can generate perfect vortex beams have been fabricated using two-photon photopolymerization direct laser writing. The vortex radii of perfect vortex beams are adjusted by changing the control parameter of the radial phase-shift. Moreover, interference patterns verify that the orbital angular momentum carried by vortex beams is consistent with the designed topological charge. Thus, this study can provide a reference for the rapid design and manufacture of photonic chip integration.

In this study, the 3D morphology of a keyhole after droplets fill the molten pool and the metal flow behavior of the molten pool during the process of droplets of different size filling the molten pool by corresponding numerical simulation are studied. Numerical simulation results show that the droplets above and below the keyhole that fall into the molten pool have a greater influence on the morphology fluctuation of the keyhole. Larger droplets increase the keyhole depth fluctuation. After a droplet with a radius of 0.9 mm enters the molten pool, the tendency of the liquid metal on the front wall of the keyhole to flow from the top to the bottom of the molten pool is increased, and a downward flow trend is observed near the bottom of the keyhole. The flow trend before a droplet enters the molten pool is diametrically opposite. After a droplet with a radius of 0.6 mm enters the molten pool, the liquid metal flow trend in the front wall of the keyhole is more complicated, the upper part of the front wall of the keyhole has a tendency to flow from the surface to the bottom of the molten pool. The flow trend from left to right in the middle of the front wall of the keyhole and a trend from the bottom of the molten pool to the top of the bottom of the keyhole occurs。

This study aims to realize the real-time control of the white light color temperature in a laser display system. For this purpose, first, we calculated the optical power ratio of an RGB laser (R: 638 nm; G: 520/532 nm; and B: 450 nm) when mixing white light of color temperature 5500--7500 K according to CIE1931 and CIE1964 standard colorimetric observers. Subsequently, we analyzed the effects of the light power proportion, center wavelength, and spectrum width on the color temperature, chromatic coordinates, and luminous flux of the mixed white light. Finally, we realized a real-time optical feedback and control system based on the AS73211 color sensor and achieved a color temperature control error of less than ±50 K in the 5500--7500 K range.

Herein, a polymeric thermo-optic (TO) mode switch based on an asymmetric directional coupler was fabricated. The device comprised a single-mode waveguide (SMW) and two-mode waveguide (TMW). The metal heater was deposited on TMW to achieve switching from the LP01 mode in SMW to LP11a mode in TMW by driving the voltage supplied to the heater. Using the traditional semiconductor microfabrication technology, the TO switch was fabricated on a Si substrate using EpoClad and SU-8 polymers as the cladding and core layers, respectively. When the operating wavelength was 1550 nm, the TO switch extinction ratio was 14.6 dB, driving power was 29.6 mW, and rise and fall times were 660.0 and 480.0 μs, respectively. The device has numerous potentials in reconfigurable mode-division-multiplexing systems.

Local geometric structure Fisher analysis (LGSFA) utilizes neighbor points and corresponding reconstructions to determine the intrinsic manifold structure of hyperspectral data, and it can improve the classification accuracy of hyperspectral image (HSI). However, LGSFA uses original sample and reconstruction points to construct graphs together, which cannot effectively preserve the global structure of nonlinear manifolds in low-dimensional spaces. To address this issue, this paper proposes a local reconstruction Fisher analysis (LRFA) method for HSI classification. The proposed method first reconstructs each data point from its intraclass neighbors to learn the global structure of manifolds. Then, intrinsic graph and penalty graph are constructed based on these reconstructions. In the low-dimensional space, the intraclass compactness and the interclass separability are improved by minimizing the intraclass distance and maximizing the interclass distance, respectively. Thus, the distinction in features is enhanced for HSI classification. Experimental results on the Pavia University and Urban datasets prove the effectiveness of the proposed method. Compared with other state-of-art methods, the proposed method achieves higher classification accuracy. When 1% of samples are randomly selected for training, the overall accuracy of 86.07% and 83.77% are obtained and increased by 7.84 percentage points and 1.27 percentage points, respectively, in comparison with the results of LGSFA.

An empirical method for synthetic aperture radar (SAR) sea significant wave height (SWH) inversion based on the extreme learning machine (ELM) model is proposed in this study. Spatial-temporal-matched ENVISAT ASAR image and ECMWF reanalysis sea SWH dataset are collected and analyzed to establish the empirical method. Two cases of mass- and less-matched datasets are used to investigate the capability of the ELM model to establish the empirical relationship from the SAR image parameters to wave SWH parameters. In addition, the CWAVE method is compared with established empirical method as a reference. Results show that the training precision of empirical method is 0.87 in the case of mass-matched dataset, which is slightly worse than that of the CWAVE algorithm (0.91). However, in terms of the method training efficiency, the empirical method (0.022 s) behaves better than the CWAVE algorithm (0.514 s). Moreover, in the case of less-matched dataset, the training accuracy of empirical method is 0.59 and its training efficiency is 0.008 s, which is much better than the CWAVE algorithm (-0.38 and 0.318 s), revealing that the ELM-based empirical method can achieve high retrieval precision of SAR wave SWH results under less-matched dataset.

The aerosol optical parameters can be quantitatively observed with high precision using a high-spectral-resolution lidar (HSRL) without considering the lidar ratio. In this study, an airborne HSRL was developed to observe the optical characteristics of aerosols and lidar was used for performing airborne observation experiments. Herein, the aerosol optical parameter inversion algorithm was improved accordingly. The experimental data of different flight areas and different flights were selected for inversion. Subsequently, the aerosol characteristics were analyzed by combining the solar photometer, hybrid single-particle Lagrangian integrated trajectory model, and satellite data. The analysis results denote obvious differences among the values of the aerosol optical parameters in different regions. The aerosol extinction coefficient exceeds 1.2 km -1 in areas with frequent human activities such as towns and factories.The lidar ratio varies significantly with the altitude, and its maximum value is 60 sr. The aerosol optical depth is concentrated between 0.7 and 1 in the altitude from 0 to 3 km. In mountainous and marine areas, the aerosol extinction coefficient is concentrated in the range of 0.2 to 0.8 km -1; further, the lidar ratio varies only slightly with the altitude and is concentrated between 5 and 30 sr. The aerosol optical thickness is concentrated between 0.3 and 0.7 in the interval from 0 to 3 km. Furthermore, the aerosol distribution is affected by the meteorological conditions, such as the wind and weather conditions, as well as the pollution processes; the optical characteristics of the aerosols in the same area on different dates are affected by various factors, including the movement and diffusion of atmospheric pollutants.