Aiming at a visible light communication system under the on-off keying (OOK) modulation with a rate of 100 Mbit/s and a bias current of 0.15 A, the influences of modulation degree on the performance of the communication system and the lighting effect are investigated by software simulation combined with experimental verification. The results show that the bit error rate decreases as modulation degree increases. Further, the smaller the distance is, the more sensitive the bit error rate becomes to the change in modulation degree. As for a 100 Mbit/s visible light communication system containing a 1 W phosphor-coated light-emitting diode at a set distance of 12.0 m, the modulation degree should be at least 0.1 to satisfy the threshold requirement of forward error correction. Furthermore, if the communication system requires a high reliability, the modulation degree should be between 0.1 and the modulation degree corresponding to an error rate of 0, and it should have no effect on illumination effect.

In this study, we proposed an experimental scheme for measuring the nonlinear phase shift generated by cross-phase modulation using a Fizeau interferometer. Subsequently, a formula was comprehensively obtained for evaluating the phase shift under different probe light powers, control light powers, and bias currents. The results denote that the nonlinear phase shift exhibits a monotonically increasing relation with the control light power and the bias current, and in contrast it exhibits a monotonically decreasing relation with the probe light power. Thereby, the operating points of cross-phase modulation for the semiconductor optical amplifier were optimized. Further, we proposed a method for determining the optimal phase modulation points that affect the nonlinear phase shift, and the optimal phase modulation parameters for a phase shift of π/2 were obtained. The probe light power is 0.29 mW, the control light power is 0.5 mW, and the bias current is 276 mA.

We designed and experimentally implemented a broadband light-source at 1.7 μm based on a cascaded-modulator pumping source. We generated supercontinuum by pumping a 1 km highly nonlinear dispersion-shifted fiber in the anomalous dispersion region by using a continuous light source combined with a cascaded-modulator. After filtering by using an optical fiber wavelength-division multiplexer, we obtained a broadband light source with a peak wavelength of 1748.9 nm, an output power of approximately 22 dBm, a 20 dB spectral range of 1.6-2 μm, and a corresponding spectral width of approximately 419 nm. By adding the Sagnac filter, a multi-wavelength broadband light source with a frequency period of 2.5 nm and an intensity period of 9.5 dB was obtained. In addition, we analyzed the effects of pump power, wavelength, and repetition rate on supercontinuum broadening.

The self-interference digital holographic imaging system with structured light illumination is developed to improve the axial imaging resolution and its basic principle is demonstrated. The numerical simulation and experiment are conducted to investigate the effects of the spatial frequency of the structured light and the axial distance of the sample on the tomographic imaging performances of the system. The results show that self-interference digital holographic imaging under structured light illumination can considerably improve the tomographic imaging capability.

An anti-parabolic erbium-doped fiber is proposed to effectively separate the degenerate vector modes in the second-order mode group. The proposed fiber is used as the gain medium of a laser, and the effects of erbium-dopant distribution, erbium-dopant concentration, fiber length, and pump power upon the output mode of the erbium-doped fiber laser are analyzed through numerical investigation. The TE01 or TM01 modes are obtained separately by doping erbium in different annular regions of the proposed fiber, and the slope efficiencies of the proposed fiber laser can reach 67.4% and 63.5%, with an output-mode purity reaching 99.97% and 99.99%, respectively. The proposed anti-parabolic-fiber-based erbium-doped laser has the merits of a high slope efficiency and high mode purity, and has many applications such as in high power lasers, optical-fiber communications, and optical-fiber sensing systems.

A YAG/Yb∶YAG/YAG ceramic planar waveguide is manufactured using tape casting combined with vacuum sintering and hot isostatic pressing, which is taken as the laser gain medium to investigate the characteristics of laser amplification. A 1030 nm polarization-maintained (PM) fiber laser is used as the seed laser and a 940-nm diode laser array is used as the pumping source of the amplifier. The pumping light is coupled with the planar waveguide from the end facets. Subsequently, the amplification performances under front-end-pumping and back-end-pumping are compared, and the amplification performances of dual-end pumping are experimentally tested. In case of dual-end pumping, when the seed laser power is 136 W, the laser output power is observed to be 1.41 kW and the slope efficiency is up to 41%. To the best of our knowledge, this is the superior output power worldwide for a laser with this type of ceramic planar waveguide.

To study the characteristics of multi-wavelength laser-induced damages in DKDP crystals used in the inertial confinement fusion (ICF) device, a damage test facility is established, in which the DKDP crystals are under the simultaneous irradiation of 1064 nm and 355 nm lasers. The damage pinpoint morphology, density, size, and damage probability under exposure by a combination of different laser fluences are compared and analyzed. When irradiating a sample with 355-nm laser pulses in R-on-1 tests, 1064-nm laser pulses with different fluences are added. The results show that as the 1064-nm laser fluence increases, the laser damage resistance of the sample increases, the damage pinpoint morphology tends to be similar with that when the sample is irradiated by the 1064 nm laser alone, the damage pinpoint density decreases, and the damage pinpoint size increases. As a whole, the coupling conditioning effect is clearly exhibited.

The Fe901 alloy is coated on a 45 steel surface by laser cladding. The microstructure, phase compositions, and micro-hardness of the laser cladding coating are tested. The wear experiment of the laser cladding coating and the 45 steel samples are conducted using the dry sliding method. The results show that the laser cladding coating has a uniform and compact microstructure, and the phase compositions are primarily composed of martensitic phase and several inter-metallic compounds such as CrFeB and Cr7C3. The laser cladding coating shows an average micro-hardness of 718 HV, which is considerably higher than that of the substrate (269 HV). Abrasive wear, fatigue spalling, and oxidative wear are the main wear mechanisms of 45 steel. The wear mechanism of the laser cladding coating is mainly abrasive wear. Under the dry sliding condition and with the loads of 10, 20, and 30 N, the friction coefficient of the laser cladding coating is smaller than that of the 45 steel, and the relative wear resistance is 4, 18, and 20 times that of the 45 steel, respectively. Laser cladding of Fe901 alloys can considerably improve the wear resistance of 45 steel.

The finite element model for an acrylic polyurethane paint layer cleaned by a nanosecond pulsed laser on a 2024 aluminum alloy surface was established using COMSOL Multiphysics. The effects of different parameters on the laser cleaning temperature field and cleaning depth were analyzed, and the findings were verified by an experimental study. The results show that the scanning speed affects the cleaning efficiency in the form of an overlapping rate, where a low scanning speed corresponds to a reduced cleaning rate. A suitable cleaning efficiency is achieved with an overlapping rate of 50%. As the laser energy density increases, the maximum surface temperatures of the paint layer and the substrate increase linearly. When the laser energy density reaches 25 J/cm 2, the paint material in the laser irradiation region is completely removed and the ablation depth of the aluminum alloy substrate is 50 μm. For a laser energy density of 25 J/cm 2 and an overlapping ratio of 50%, the peak-to-valley height of the substrate surface groove is 50.234 μm. Thus, a suitable surface that meets the coating process requirements can be obtained with these parameters. These results provide a reference for studying nanosecond pulse laser cleaning and for selecting appropriate process parameters.

Selective laser melting (SLM) is used to rapidly form 316L stainless steels formed when the laser rotation angle is 73° and the powder layer is 30-μm thick, and the effects of bulk laser energy density and forming direction on the anisotropy of microstructure and mechanical properties of the formed parts are studied. The results show that the forming direction has a great influence on the mechanical properties, and the anisotropy of the mechanical property varies with the anisotropy of the microstructure. As the bulk laser energy density increases, the surface of the molten pool tends to be flat, the grain growth directions of formed part become singular in the x and y directions, and the grain growth direction of formed part in the z direction is obviously orientation-dependent. When the bulk laser energy density is 65-85 J·mm -3, the crystal growth direction is well aligned with the stacking direction, and the tensile strength and the percentage elongation after fracture are optimal. Therefore, the bulk laser energy density can be used for controlling the microstructure and mechanical properties of formed parts.

Donor-acceptor-type semiconductor polymer IPDT composed of isoindigo, propylene dioxythiophene and thiophene unit was used to constructe an organic electronic device with a configuration of Al/IPDT/ITO. It was found that the device had obvious memory resistance characteristics with ON/OFF voltages of 8 V/-7.5 V and a high-low resistance ratio over 10 2. The effect of laser irradiation under different wavelengths on the memory resistance performances of the device was studied. The results show that a laser with 632-nm wavelength and 3-mW power strongly affects the memory resistance performances. After irradiation for 60 s, the current trend of the device is reversed. The ON/OFF voltagess are reduced to -2.2 V/1.3 V and the high-low resistance ratio is increased to 10 4. The current is decreased by an order of magnitude, the fluctuation of the current-voltage curve is decreased, and the power consumption of the device is effectively reduced. The number of stability cycle tests is increased from 2000 before laser irradiation to 3500, which improves the accuracy and stability of data reading.

Motivated by the use of large-scale surface topography measurement by robots, we propose a method of point-cloud splicing based on the indoor global positioning system (iGPS). In our research, the iGPS world coordinate system is utilized as the coordinate system of point-cloud splicing to establish a mathematical model of point-cloud splicing. Furthermore, we employ the particle swarm optimization (PSO) algorithm for the iterative closest point (ICP) algorithm. The experimental results of point-cloud splicing of spherical distance measurement show that the accuracy of the measurement system is less than 0.1 mm. We also conduct a front-bumper point-cloud splicing experiment and the experimental result denote that the maximum negative deviation is -0.05189 mm and the maximum positive deviation is 0.0727 mm, which are less than 0.1 mm. It is also found that the deviation distribution is relatively uniform, which validates the proposed algorithm has a good effect on large-scale point-cloud splicing.

To meet the requirement of on-line detection of large gradient phases in machining, a single interferogram phase retrieval method based on the digital moiré phase shift algorithm and the Newton iteration algorithm is proposed herein. In the simulation, we compared the proposed method with three typical phase retrieval methods for single interferogram under different phases to be measured. The results show that the proposed method has high accuracy. We then experimentally measured the large gradient phase produced by a defocused sphere, and compared our result with the measurement result of mechanical phase shift in the interferometer produced by ZYGO corporation. The effectiveness of the proposed method is verified.

The convolution effect in phase measuring profilometry, which is caused by the point spread function in optical imaging devices, is investigated, and a method for estimating the convolution parameters of the point spread function is proposed. This method can be used to assess the point spread function parameters by utilizing the influence of convolution effect on the intensity modulation parameters in phase measuring profilometry, wherein the modulation ratio of the base frequency to the other higher spatial frequency is calculated. By adopting the resulting estimated parameter, a convolution model is established for phase correction in error areas; thus, the 3D reconstruction accuracy is improved. The simulation and experimental results both verify the efficiency of the method. Compared with that obtained before correction, the root-mean-square error of phase is decreased by 30.55%.

The double ionization of helium in the combined extreme ultraviolet (EUV) pulse and extremely short mid-infrared (MIR) laser pulse is studied by numerically simulating the time-dependent Schr dinger equation. In this process, the EUV pulse kicks off one electron, and then the produced He + is either sequentially tunneling ionized by the remaining MIR pulse, or nonsequentially impact ionized by the rescattering electron driven by the MIR laser pulse. The interference of the coexisted sequential and nonsequential double ionization events produces an unexplored electron-electron joint momentum distribution. The two electrons released via rescattering may propagate along the same direction and also propagate along the opposite directions when such an extremely short laser pulse is implemented.

A brief overview of the basic principles and main applications of ultrafast photoemission electron microscopy is presented in this paper. Then, the progress of our research on ultrafast photoemission electron microscopy, including near-field imaging, near-field spectroscopy, and time-resolved ultrafast dynamics, in the field of nanophotonics, particularly in the field of surface plasmonics, is highlighted. These studies not only help to deepen understanding of the fundamental properties of surface plasmons and the interaction among different modes but also benefit the design and development of surface plasmonic applications. Finally, further potential applications of this technique are prospected.

The time-stretch dispersion Fourier transform (TS-DFT) technology promotes the study of transient phenomena in mode-locked lasers and is of great significance for revealing the dissipative dynamics in complex systems. In this paper, we first introduce the fundamental principles of TS-DFT technology and its key issues in data acquisition and data processing. Then we review the applications of TS-DFT technology in the study of various ultrafast phenomena in passively mode-locked fiber lasers, such as noise-like pulse and rogue wave, soliton explosion, soliton bound state, mode-locked self-starting, and vector solitons.

Terahertz phonon polaritons can be generated in ferroelectric crystal LiNbO3 using femtosecond laser pulses. When the crystal thickness becomes comparable with or less than the terahertz wavelength, LiNbO3 functions as an integrated chip that integrates the processes of generation, propagation, control, and detection of terahertz wave and its interaction with materials or microstructures. This provides a platform for the research and applications of terahertz waves. Moreover, by using the spatio-temporal super-resolution imaging technology, the transmission in the chip and interaction of terahertz waves with microstructures can be visualized and quantitatively analyzed. This paper reviews some works on the LiNbO3 chip, such as the research on terahertz propagation characteristics, generation of frequency-tunable terahertz sources, and terahertz modulation using microstructures. These results demonstrate that the subwavelength LiNbO3 chip is a promising terahertz integrated device.

In this paper, the research works related to femtosecond laser based artificial atmospheric modulation are reviewed. The contents start by describing the research progress on photo-oxidation byproducts, energy deposition, aerosol formation, water condensation, and precipitation that are all induced by femtosecond filamentation. Subsequently, the research progresses on laser induced water condensation and precipitation as well as manual lightning control are reviewed. Finally, a preliminary physical picture of femtosecond laser based atmospheric modulation is proposed, and the technical challenges and their possible solutions towards the practical applications of artificial rainmaking in the future are discussed.

Recent research progress of terahertz (THz) radiation generation based on the ultrafast spin dynamics is reviewed. The transient spin-charge conversion based on the inverse spin Hall effect and the Rashba-Edelstein effect is introduced, and it is pointed out that the ferromagnetic/non-magnetic heterostructure has been used to design a low-cost and high-efficiency THz radiation source. The efficiency and bandwidth of a spintronics-based THz emitter can be improved by optimizing layer thickness, growth conditions, substrates, and construction. The applications of the THz emission spectrum are outlined in the study of the ultrafast formation dynamics of the spin Seebeck effect.

In this study, a fluorotellurite glass fiber with relatively good thermal and chemical stability was developed. Using this glass fiber as a nonlinear medium, the broadband supercontinuum (SC) generation of 0.6-5.4 μm was experimentally obtained. An SC light source with an average power of about 20 W and a spectral range of 1-4 μm was also obtained. This study mainly focuses on the recent progress on the high-power mid-infrared SC light sources, including the material characteristics of fluoride glass fibers and fluorotellurite glass fibers and the SC laser sources based on the former. The future development of such SC laser sources is prospected.

High-power ultrafast lasers have a wide range of applications, such as precise industrial processing, ultrafast spectroscopy, high-field physics, and military applications. Fiber lasers have the advantages of convenient operation, thermal load insensitivity, and good beam quality. The recent research progress of high-power ultrafast fiber lasers is reviewed, including the emerging passive mode locking and chirped pulse amplification technologies. The advantages of high-power fiber lasers on nonlinear optics are discussed via an example application, namely using high-power ultrafast fiber lasers to generate high-order harmonics. Further, the potential future research directions have also been prospected.

Coherent Raman scattering is extensively used in various fields, such as biomedical tissue imaging and pharmacokinetics, because of its significant advantages, including non-invasive detection, label-free operation, and chemical specificity. Further, we introduce the implementation and characteristics of the fiber laser sources for coherent Raman scattering (CRS) microscopy, and review the most recent advances in improving the output power, tuning range, and spectral resolution based on the dual-color synchronized ultrashort pulses via supercontinuum generation, soliton self-frequency shift, and four-wave mixing. Additionally, the latest advances in four-wave-mixing-based fiber optical parametric oscillators are introduced. Subsequently, the temporally synchronized, spatially overlapped, and wavelength-tunable dual-color ultrashort pulses are obtained based on dispersion filtering and polarization manipulation using the polarization-maintaining fiber and the photonic crystal fiber. Furthermore, the generated laser pulses can be used to achieve non-invasive as well as label-free spectroscopic detection and microscopic imaging for lipids, proteins, and nucleic acid, which could provide an effective methodology to realize compact, user-friendly, and environmentally stable coherent Raman scattering imaging.

Owing to the developments in laser processing technology and liquid crystal alignment technology, a novel compartmentalized out-of-plane alignment technology of liquid crystals based on femtosecond laser direct writing is introduced. Femtosecond multiphoton polymerization based laser direct writing is used to prepare microstructures comprising polymeric ribbons. The liquid crystal cells are fabricated by filling different kinds of liquid crystals that can then function as an electro-optical switch, a magneto-optical switch, and light field control. The surface relief grating structures are distributed on the sidewalls of each polymer ribbon, which make it possible for liquid crystal orientation within the polymeric ribbon channel. The technology can realize the physical isolation of liquid crystals in different regions and complete phase separation of liquid crystals and polymers. The proposed technology is fast and simple, which has great application value in tunable diffraction gratings, special light field generation, and light field control in photonic crystals.

This paper summarizes the working principles of all-fiber mode converters such as mode selective couplers and acoustically-induced fiber gratings. Combining the advantages of a mode-locked fiber laser and mode converters is a simple and effective method to generate ultrafast vector beams and optical vortex beams. The generated ultrafast laser with high-order modes (HOMs) has high peak powers and high mode purity. The experimental results demonstrate that the fast response and broadband mode conversion characteristics of these mode converters. In addition, the future development directions and application prospects of ultrafast HOMs are discussed.

An ultrafast pulsed laser has important applications in some fields, such as laser processing and strong field physics, because of its ultrashort response time and ultrahigh peak power. Accordingly, the ultrafast pulsed laser has been rapidly developed in the visible range with the development of the blue laser diodes and the praseodymium-ion-doped laser gain media. In this study, we mainly review the research progress and status of ultrafast pulsed lasers in the visible range, and describe the applications of the mode-locking technology in the visible range, including Kerr-lens mode-locking, high repetition rate GHz-frequency self-mode-locking, and mode-locking based on saturable absorbers. Further, the futher development direction and perspectives of ultrafast visible lasers are prospected.

This article introduces the basic principle of chirped pulse amplification (CPA) for high-power femtosecond fiber laser amplifiers, along with a discussion of the development status and bottlenecks of the key stretchers and compressors in the CPA structure. It provides a description of the structure and working principle of a typical large-mode-area fiber, and briefly discusses the development status of the corresponding CPA system based on this kind of fiber. Further, it introduces the nonlinear amplification technologies, and discusses an amplification scheme for narrower pulse duration and higher pulse quality. Finally, it presents an analysis of advanced technologies such as an all-fiber structure, coherent beam combination, single-crystal fibers as active elements, and picosecond seed sources, and provides a summary of the development trend of high-power femtosecond fiber laser amplifiers.

To improve the environmental stability and self-starting ability, researchers have developed the nonlinear optical-loop mirror and nonlinear amplifying-loop mirror (NALM) mode-locking technologies. The developing process of these technologies can be summarized as the following three phases: non-polarization-maintaining (PM) structures with polarization controllers, PM figure-of-8 cavity structures with dual-gain, and PM figure-of-9 cavity structures with a phase shifter. Among these, the newly invented figure-of-9 NALM mode-locked fiber laser is particularly advantageous owing to its easy self-starting ability, long-term stability, concise structure, and cost-effectiveness. Such a mode-locked fiber laser is increasingly employed in applications such as optical frequency combs, generation of THz radiation, and advanced material micromachining.

A hollow-core anti-resonant fiber (HC-ARF) has advantages such as wide transmission bandwidth, low transmission loss, high damage threshold, and high mode purity. The HC-ARF is widely applied in high-power pulse transmission and compression, ultrafast nonlinear frequency conversion, short-haul high-speed and high-capacity optical communications, bio-chemical analysis, and quantum storage. Herein, the development history of a hollow-core photonic crystal fiber(HC-PCF)was briefly reviewed, focusing on several new HC-ARFs that have emerged in recent years. In addition, the key technologies and the recent advances for the applications gas-filled HC-ARFs in the field of novel Raman laser frequency conversion were discussed.

Femtosecond optical parametric oscillators (OPOs) are capable of generating tunable femtosecond laser pulses in the ultraviolet, mid- and far-infrared spectral range. These oscillators are widely used in the fields of frequency metrology, biomedicine, gas detection, broadband communications, and defense. This study conducts the research on the OPO synchronously pumped by a femtosecond Ti:sapphire laser, and realizes the operation of high-power femtosecond OPOs with a stabilized wavelength of 1053 nm. Based on these, the OPOs synchronously pumped with high-power femtosecond Ytterbium-doped all-solid-state lasers are further investigated. With the relevant research progress on femtosecond OPOs worldwide, the future development of OPOs synchronously pumped by all-solid-state femtosecond lasers is prospected.

Mode-locking and ultrashort pulse synchronous pumping are the main ways to obtain ultrashort pulses from Raman fiber lasers. In the mode-locked Raman fiber lasers, high performance output can be achieved in the structure with equivalent saturable absorbers, but the limitation caused by the long gain fiber remains. The method of synchronous pumping can overcome this problem effectively and achieve excellent ultrafast Raman laser output. There is a great potential for the ultrafast Raman fiber laser technology, and future research will focus on further improving the overall performance of various ultrafast Raman fiber lasers and investigating new phenomena observed in these laser systems.

Herein, we review the recent progress in the electron dynamics of optical ionization using ultraintense laser fields. Further, the photoelectron momentum distributions at the tunnel exit during nonadiabatic tunneling ionization are analyzed. The electron angular distributions are obtained in terms of molecular coordinates, allowing the imaging of the molecular inner orbits. The two-color (400 nm+800 nm) circularly polarized laser fields are used to achieve a double-pointer attoclock in which the phase and amplitude of the electronic wavepackets can be characterized. Furthermore, the photoelectrons with a high degree of spin polarization can be produced using circular laser fields based on the ionization of atoms with large spin-orbit coupling effects. Finally, the applications of ultrafast strong field physics and its developmental trend are illustrated.

Herein, an all-solid-state widely tunable Ti:sapphire laser pumped by a home-made 532 nm laser with high power and high conversion efficiency is demonstrated. First, a maximum output power of 37.8 W at 532 nm is obtained by Q-switching and intracavity frequency doubling via a Nd∶YAG crystal that is symmetrically pumped by three 1 kHz laser diode (LD) arrays. The envelope of each LD pulse contains five Q-switched pulses with a duration of 90 ns and repetition rate of 5 kHz. Using the green light to pump the Ti∶sapphire crystal, a 733.5-871.1 nm widely tunable laser is then obtained. The maximum output power at 771 nm is 8.26 W, the optical-optical conversion efficiency is 42%, and the pulse duration is 14 ns. Moreover, the output power stability is superior to ±4.4% within 30 min.

In the conventional laser marking systems, the marking area is usually limited due to the short focal length. Herein, an alternative method is proposed to pursue large-area marking based on femtosecond laser filamentation, and the marking range and resolution are analyzed in detail. The experimental results show that the femtosecond laser filamentation can effectively increase the focal depth and complete marking on large-area flat or curved surface samples. Furthermore, the resolution uniformity is improved within the effective working range of the marking system.

A time-resolved shadowgraphic imaging system based on femtosecond laser pump and probe is established for directly imaging the ultrafast process of femtosecond laser ablating of micro-holes in silica glass. The time-delay dependent phenomena of plasma channel decay, shock wave expansion, and micro-hole stretch are observed when the energy density, time-delay, and pulse number change. The experimental results show that the proposed system is very efficient for the in situ observation of the micro-nanostructures in transparent media induced by femtosecond laser ablation.

In this study, the supercontinuum generation in calcium fluoride (CaF2) crystals are measured during high-intensity femtosecond laser filamentation under different incident laser energies and various crystal orientations. The experimental results show that the maximum blue-shifted cut-off wavelength of the supercontinuum spectrum remains constant at 300 nm for a broad range of incident laser energies because of the intensity clamping effect. Moreover, the maximum blue-shifted cut-off wavelength is also independent on the crystal orientation, but the intensity of supercontinuum significantly varies with the crystal orientation.

Two phase gratings are used as beam splitter and combiner to generate a femtosecond non-cylindrical vector beam exhibiting continuously varying polarizations. Further, by adjusting the orientation of the grating vector of the beam combiner, a scalar beam can be transitioned to a vector beam, and the polarization distribution of the femtosecond laser beam can be modulated. Two-dimensional periodic surface structures comprising curved ripples are fabricated on a tungsten surface using a femtosecond non-cylindrical vector beam. By adjusting the beam polarization distribution, the bottom length of the curved ripples, i.e., the period along the vertical direction, can be decreased to 4 μm while maintaining the horizontal period at 560 nm. The microreflectance spectral measurements denote that the periodic ripples significantly reduce the reflectivity in the visible and near-infrared ranges and that the reflectivity increases as the vertical period of the curved ripples decreases. Furthermore, femtosecond laser ablation does not change the chemical components of the tungsten surface, therefore, the variation in reflectance can be solely attributed to the change in surface morphology.

The effects of laser pulse duration and polarization on the femtosecond filament-induced fluorescence of combustion intermediates were studied herein. The variations in the fluorescence spectra of combustion intermediates, such as OH, CH, CN, C2, and C, were observed by varying laser pulse duration and polarization. The results show that the fluorescence intensities of all the observed combustion intermediates decrease with the increase of laser pulse duration and polarization ellipticity. This behavior is explained by the fact the clamped intensities and plasma densities inside the filament core are strongly dependent on laser pulse duration and polarization, leading to the changes in the multiphoton-excited signals of combustion intermediates.

This study presents the design of a normal-dispersion polarization-maintaining Yb-doped fiber laser based on nonlinear amplification loop mirror. Stable output of a picosecond pulse train with an average power of 7.8 mW is realized from the Yb-doped mode-locked laser at a pump power of 80 mW. The output pulse train has a repetition rate of 9.9 MHz, a central wavelength of 1064 nm, a pulse duration of ~18 ps, and the corresponding spectral width of 0.18 nm. The designed laser has the advantages of a simple structure, self-starting operation, and high stability.

In this study, we demonstrate a wavelength-tunable passively mode-locked fiber laser based on a 45° tilted fiber grating and a tapered fiber using the nonlinear polarization rotation technique. Further, stable mode-locked pulses with a central wavelength of 1568.8 nm and output power of 2.31 mW can be obtained when the input pump power is increased to 454 mW. Accordingly, the 3-dB spectral bandwidth and pulse width are 4.5 nm and 1.3 ps, respectively. Subsequently, the central wavelength of the pulse can be continuously tuned from 1568.8 nm to 1560.24 nm using the tapered fiber as an adjustable attenuator to modulate the cavity loss. The demonstrated fiber laser can be used in several fields, including sensing, spectroscopy, and telecommunications.

Highly nonlinear photonic crystal fibers have the characteristics of small core and large refractive index contrast. Due to its periodic air hole structure, the phonons generated by the guided acoustic-wave Brillouin scattering (GAWBS) are tightly trapped in the core area and interact significantly with photons. The refractive index of the fiber will be modulated by phonons, resulting in a phase modulation on optical waves. Using the Sagnac interferometry to transform phase modulation to intensity modulation, we demonstrate the generation and detection of phonons by GAWBS in the photonic crystal fiber in the 1550 nm and 1060 nm bands, respectively. The experimental results show that the fundamental mode frequency of acoustic phonons is 1.24 GHz for both cases with the pump wavelengths of 1550 nm and 1060 nm, respectively, which verifies the theory that the phonon frequency in forward Brillouin scattering is independent of the pump wavelength.

The effects of the parent pulse parameters on the width and quality of the synthesized pulse are investigated using an analytical method. The research results show that the time-domain quality of the synthesized pulse can be improved when the two parent pulses exhibit similar widths and energies, and the substrate of the synthesized pulse can be reduced by decreasing the time-domain widths of parent pulses. In order to verify the optimizing method for pulse coherent synthesis, the pulse output obtained from the Yb 3+-doped fiber femtosecond laser system is first divided into two branches, then they are treated by nonlinear frequency conversion in different nonlinear fibers, respectively, and finally two pulses with pulse durations of less than 30 fs are obtained. These two pulses are further coherently synthesized. Consequently, after coherent synthesis, a few-cycle laser pulse with a pulse duration of 8 fs is realized.

This study proposes a fast and high-precision distance measurement method suitable for moving targets. By combining the sinusoidal reference time interval measurement method with the vernier clock controlled pulse emission technology, this method could achieve high-precision ranging for moving targets. The time-of-flight of a laser pulse from a range finder to a target is used as the initial value of the estimated distance; this value is measured using a sinusoidal signal as a reference. Using the vernier clock controlled pulse emission technology, the high-resolution measurement can be obtained by selecting a linear segment at zero point of the sinusoidal signal as the timing feature point. The high-accuracy ranging can be realized by averaging multi-pulse measurements based on the vernier moment corresponding to the timing feature point as the time of the fixed-point pulse emission. The experimental results show that for an average laser power of 1 mW, the range accuracy is ±(3 mm+2×10 -6×D), where D is the measured distance. Moreover, the measurement time is shortened to 5 ms within 300-m ranging without a cooperation target. The proposed system has a simple structure, is inexpensive, and can be easily implemented.

Herein, light detection and ranging data were collected as remoting data sources by terrestrial laser scanning (TLS). Metasequoia, palm, sapindus, bamboo, and rubber trees were selected as research objects. Three effective features are proposed, which are relative clustering features of trees, features of point cloud distribution of trees, and apparent features of trees. 68 feature parameters are listed. A support vector machine (SVM) classifier was then used to verify and calculate the training dataset and to determine the optimal feature parameters in cross-validation. Finally, the tree species is classified in the test dataset. The research results show that the average classification accuracy of tree classification based on the optimal parameters of relative clustering features of trees is low (45%), that based on the optimal feature parameters of point cloud distribution slightly increases (58.8%), that based on the optimal parameters of tree appearance features is relatively high (63.8%), and that based on the 13 optimal parameters of three types of features is the highest (87.5%). In addition, due to the difference between metasequoia and other tree species is obvious, the metasequoia is outstanding in classification and its misjudgement rate is the lowest (6.5%). The proposed method has high feasibility and provides a powerful tool for obtaining a more accurate distribution of forest species.