Interplay of laser power and pore characteristics in selective laser melting of ZK60 magnesium alloys:A study based on in-situ monitoring and image analysis

This study offers significant insights into the multi-physics phenomena of the SLM process and the subsequent porosity characteristics of ZK60 Magnesium(Mg)alloys.High-speed in-situ monitoring was employed to visualise process signals in real-time,elucidating the dynamics of melt pools and vapour plumes under varying laser power conditions specifically between 40 W and 60 W.Detailed morphological analysis was performed using Scanning-Electron Microscopy(SEM),demonstrating a critical correlation between laser power and pore formation.Lower laser power led to increased pore coverage,whereas a denser structure was observed at higher laser power.This laser power influence on porosity was further confirmed via Optical Microscopy(OM)conducted on both top and cross-sectional surfaces of the samples.An increase in laser power resulted in a decrease in pore coverage and pore size,potentially leading to a denser printed part of Mg alloy.X-ray Computed Tomography(XCT)augmented these findings by providing a 3D volumetric representation of the sample internal structure,revealing an inverse relationship between laser power and overall pore volume.Lower laser power appeared to favour the formation of interconnected pores,while a reduction in interconnected pores and an increase in isolated pores were observed at higher power.The interplay between melt pool size,vapour plume effects,and laser power was found to significantly influence the resulting porosity,indicating a need for effective management of these factors to optimise the SLM process of Mg alloys.

Broadband bidirectional Brillouin–Raman random fiber laser with ultra-narrow linewidth

We present a Brillouin–Raman random fiber laser(BRRFL)with full-open linear cavity structure to generate broadband Brillouin frequency comb(BFC)with double Brillouin-frequency-shift spacing.The incorporation of a regeneration portion consisting of an erbium-doped fiber and a single-mode fiber enables the generation of broadband BFC.The dynamics of broadband BFC generation changing with the pump power(EDF and Raman)and Brillouin pump(BP)wavelength are investigated in detail,respectively.Under suitable conditions,the bidirectional BRRFL proposed can produce a flatamplitude BFC with 40.7-nm bandwidth ranging from 1531 nm to 1571.7 nm,and built-in 242-order Brillouin Stokes lines(BSLs)with double Brillouin-frequency-shift spacing.Moreover,the linewidth of single BSL is experimentally measured to be about 2.5 kHz.The broadband bidirectional narrow-linewidth BRRFL has great potential applications in optical communication,optical sensing,spectral measurement,and so on.

Gigahertz frequency hopping in an optical phase-locked loop for Raman lasers

Raman lasers are essential in atomic physics,and the development of portable devices has posed requirements for time-division multiplexing of Raman lasers.We demonstrate an innovative gigahertz frequency hopping approach of a slave Raman laser within an optical phase-locked loop(OPLL),which finds practical application in an atomic gravimeter,where the OPLL frequently switches between near-resonance lasers and significantly detuned Raman lasers.The method merges the advantages of rapid and extensive frequency hopping with the OPLL’s inherent low phase noise,and exhibits a versatile range of applications in compact laser systems,promising advancements in portable instruments.

Effect of solution treatment on the microstructure,phase transformation behavior and functional properties of NiTiNb ternary shape memory alloys fabricated via laser powder bed fusion in-situ alloying

Post-heat treatment is commonly employed to improve the microstructural homogeneity and enhance the mechanical performances of the additively manufactured metallic materials.In this work,a ternary(NiTi)91Nb9(at.%)shape memory alloy was produced by laser powder bed fusion(L-PBF)using pre-alloyed NiTi and elemental Nb powders.The effect of solution treatment on the microstructure,phase transformation behavior and mechanical/functional performances was investigated.The in-situ alloyed(NiTi)91Nb9 alloy exhibits a submicron cellular-dendritic structure surrounding the supersaturated B2-NiTi matrix.Upon high-temperature(1273 K)solution treatment,Nb-rich precipitates were precipitated from the supersaturated matrix.The fragmentation and spheroidization of the NiTi/Nb eutectics occurred during solution treatment,leading to a morphological transition from mesh-like into rod-like and sphere-like.Coarsening of theβ-Nb phases occurred with increasing holding time.The martensite transformation temperature increases after solution treatment,mainly attributed to:(i)reduced lattice distortion due to the Nb expulsion from the supersaturated B2-NiTi,and(ii)the Ti expulsion from theβ-Nb phases that lowers the ratio Ni/Ti in the B2-NiTi matrix,which resulted from the microstructure changes from non-equilibrium to equilibrium state.The thermal hysteresis of the solutionized alloys is around 145 K after 20%pre-deformation,which is comparable to the conventional NiTiNb alloys.A short-term solution treatment(i.e.at 1273 K for 30 min)enhances the ductility and strength of the as-printed specimen,with the increase of fracture stress from(613±19)MPa to(781±20)MPa and the increase of fracture strain from(7.6±0.1)%to(9.5±0.4)%.Both the as-printed and solutionized samples exhibit good tensile shape memory effects with recovery rates>90%.This work suggests that post-process heat treatment is essential to optimize the microstructure and improve the mechanical performances of the L-PBF in-situ alloyed parts.

Suppression of stimulated Brillouin and Raman scatterings using an alternating frequency laser and transverse magnetic fields

A novel scheme to suppress both stimulated Brillouin scattering(SBS) and stimulated Raman scattering(SRS) by combining an alternating frequency(AF) laser and a transverse magnetic field is proposed. The AF laser allows the laser frequency to change discretely and alternately over time. The suppression of SBS is significant as long as the AF difference is greater than the linear growth rate of SBS or the alternating time of the laser frequency is shorter than the linear growth time of SBS. However, the AF laser proves ineffective in suppressing SRS, which usually has a much higher linear growth rate than SBS. To remedy that, a transverse magnetic field is included to suppress the SRS instability. The electrons trapped in the electron plasma waves(EPWs) of SRS can be accelerated by the surfatron mechanism in a transverse magnetic field and eventually detrapped. While continuously extracting energy from EPWs, the EPWs are dissipated and the kinetic inflation of SRS is suppressed. The one-dimensional particle-in-cell simulation results show that both SBS and SRS can be effectively suppressed by combining the AF laser with a transverse magnetic field with tens of Tesla. The total reflectivity can be dramatically reduced by more than one order of magnitude. These results provide a potential reference for controlling SBS and SRS under the related parameters of inertial confinement fusion.

In-situ Raman observation on crack tips of Vickers indent in PLZT ceramics

Vickers indentation was introduced into the originally in-plane and out-of-plane poled PLZT ceramics.The Raman spectra were in-situ recorded at selected crack tips before and after the indentations,as well as after the applications of external electric field.The results show that the changes in Raman intensities of optical modes could be sensitively related to 90° domain switching around the crack tips which are strongly dependent on the directions of original polarization and geometric locations.When the direction of electric field was perpendicular to the direction of original polarization,the 90° domain switching at crack tips of the Vickers indentation on the originally in-plane poled PLZT ceramics caused most significant change in the Raman intensity,which inhibited the crack growth.However,when the direction of electric field was parallel to the direction of original polarization,the growth of crack tips became predominantly without the 90° domain switching,which led to the crack growth.

In-situ TiC Reinforced Composite Coating Produced by Powder Feeding Laser Cladding

A Ni-base alloy composite coating reinforced with TiC particles of various shapes and sizes on medium carbon steel substrate was produced by multilayer laser cladding. The chemical compositions, microstructures and surface morphology of the cladded layer were analyzed using energy dispersive X-ray spectroscopy （EDX）, scanning electron microscope （SEM）, and X-ray diffractometry （XRD）. The experimental results showed that an excellent metallurgical bonding between the coating and the substrate was obtained. The microstructure of the coating was mainly composed of γ-Ni dendrites, a small amount of CrB, Ni3B, M23C6 and dispersed TiC particles. Much more and larger TiC particles formed in the overlapping zone, which led to a slightly higher microhardness of this zone. The maximum microhardness of the coating was about HV0.21200. The effects of the laser processing parameters on the microstructures and properties of coating were also investigated.

Feasibility investigation on deep ocean compact autonomous Raman spectrometer developed for in-situ detection of acid radical ions

A newly developed Deep Ocean Compact Autonomous Raman Spectrometer （DOCARS） system is introduced and used for in-situ detection of acid radical ions in this paper. To evaluate the feasibility and capability of DOCARS for quantitative analysis of the acid radical ions in the deep ocean, extensive investigations have been carried out both in laboratory and sea trials during the development phase. In the laboratory investigations, Raman spectra of the prepared samples （acid radical ions solutions） were obtained, and analyzed using the method of internal standard normalization in data processing. The Raman signal of acid radical ions was normalized by that of water molecules. The calibration curve showed that the normalized Raman signal intensity of SO4^2-, NO3^-, and HCO^-3 increases linearly as the concentration rises with correlation coefficient R^2 of 0.99, 0.99, and 0.98 respectively. The linear function obtained from the calibration curve was then used for the analysis of the spectra ,data acquired in the sea trial under a simulating chemical field in the deep-sea environment. It was found that the detected concentration of NO3 according to the linear function can reflect the concentration changes of NO~ after the sample was released, and the detection accuracy of the DOCARS system for SO^2-_4 is 8%. All the results showed that the DOCARS system has great potential in quantitative detection of acid radical ions under the deep-sea environment, while the sensitivity of the DOCARS system is expected to be improved.

Microstructure and properties of in-situ chromium carbide composite coating by laser cladding

Using Ni/Cr/graphite powder blends as raw powders,a Ni matrix composite coating reinforced by in-situ carbide,was fabricated on the surface of Q235 by means of laser cladding. These microstructure and properties were discussed. The result of phase analysis( XRD) and microstructure investigation( SEM) showed that the coatings consist mainly of Cr_3 C_2,Cr_7 C_3 and γ-( Ni,Cr),which are consistent with the thermodynamic calculations. The wear morphology of the coatings was also examined. The results of dry sliding wear tests of different Cr/C ratio show that the wear resistances of the Cr_3 C_2-reinforced coating,respectively,are 13. 4,9. 5,9. 1 and 6. 5 times higher than that of the substrate and the main wear mechanisms of the coatings are adhesion and abrasive wear with slight oxidation.

Development of an In-Situ Laser Machining System Using a Three-Dimensional Galvanometer Scanner

In this study, a three-dimensional (3D) in-situ laser machining system integrating laser measurement and machining was built using a 3D galvanometer scanner equipped with a side-axis industrial camera. A line structured light measurement model based on a galvanometer scanner was proposed to obtain the 3D information of the workpiece. A height calibration method was proposed to further ensure measurement accuracy, so as to achieve accurate laser focusing. In-situ machining software was developed to realize time-saving and labor-saving 3D laser processing. The feasibility and practicability of this in-situ laser machining system were verified using specific cases. In comparison with the conventional line structured light measurement method, the proposed methods do not require light plane calibration, and do not need additional motion axes for 3D reconstruction;thus they provide technical and cost advantages. The insitu laser machining system realizes a simple operation process by integrating measurement and machining,which greatly reduces labor and time costs.

In-situ deposition of apatite layer to protect Mg-based composite fabricated via laser additive manufacturing

Biodegradable magnesium(Mg) and its alloy show huge potential as temporary bone substitute due to the favorable biocompatibility and mechanical compatibility. However, one issue deserves attention is the too fast degradation. In this work, mesoporous bioglass(MBG)with high pore volume(0.59 cc/g) and huge specific surface area(110.78 m^(2)/g) was synthesized using improved sol-gel method, and introduced into Mg-based composite via laser additive manufacturing. Immersion tests showed that the incorporated MBG served as powerful adsorption sites, which promoted the in-situ deposition of apatite by successively adsorbing Ca2+and HPO42-. Such dense apatite film acted as an efficient protection layer and enhanced the corrosion resistance of Mg matrix, which was proved by the electrochemical impedance spectroscopy measurements. Thereby, Mg based composite showed a significantly decreased degradation rate of 0.31 mm/year. Furthermore,MBG also improved the mechanical properties as well as cell behavior. This work highlighted the advantages of MBG in the fabrication of Mg-based implant with enhanced overall performance for orthopedic application.

High-power linearly-polarized tunable Raman fiber laser

In this study, we demonstrate an all-fiber high-power linearly-polarized tunable Raman fiber laser system. An in- house high-power tunable fiber laser was employed as the pump source. A fiber loop mirror （FLM） serving as a high reflectivity mirror and a flat-cut endface serving as an output coupler were adopted to provide broadband feedback. A piece of 59-m commercial passive fiber was used as the Raman gain medium. The Raman laser had a 27.6 nm tuning range from 1112 nm to 1139.6 nm and a maximum output power of 125.3 W, which corresponds to a conversion efficiency of 79.4%. The polarization extinction ratio （PER） at all operational wavelengths was measured to be over 21 dB. To the best of our knowledge, this is the first report on a hundred-watt level linearly-polarized tunable Raman fiber laser.

Raman crystal lasers in the visible and near-infrared

Raman lasers based on potassium gadolinium tungstate and lead tungstate crystals pumped by a≈120 ps Nd: YAG laser at 1.064/μm were developed. High reflection mirrors for the Stokes wavelength have been used to generate near-infrared and eye safe spectral region of 1.15 - 1.32/μm. Second harmonic generation of the generated Raman lasers was observed. Eifficient multiple Stokes and anti-Stokes picosecond generation in 64 crystals have been shown to exhibit stimulated Raman scattering on about 700 lines covering the whole visible and near-infrared spectrum. All stimulated Raman scattering (SRS) wavelengths in the visible and near-infrared spectrum are identified and attributed to the SRS-active vibration modes of these crystals.

Q-Switched Raman Fiber Laser with Molybdenum Disulfide-Based Passive Saturable Absorber

We demonstrate a Q-switched Raman fiber laser using molybdenum disulfide （MoS2） as a saturable absorber （SA）. The SA is assembled by depositing a mechanically exfoliated MoS2 onto a fiber ferrule facet before it is matched with another clean ferrule via a connector. It is inserted in a Raman fiber laser cavity with a total cavity length of about 8kin to generate a Q-switching pulse train operating at 1560.2nm. A 7.7-kin-long dispersion compensating fiber with 584 ps.nm-i km-1 of dispersion is used as a nonlinear gain medium. As the pump power is increased from 395 m W to 422 m W, the repetition rate of the Q-switching pulses can be increased from 132.7 to 137.4 kHz while the pulse width is concurrently decreased from 3.35μs to 3.03μs. The maximum pulse energy of 54.3 nJ is obtained at the maximum pump power of 422 roW. These results show that the mechanically exfoliated MoS2 SA has a great potential to be used for pulse generation in Raman fiber laser systems.

In-situ electrochemical surface-enhanced Raman spectroscopy in metal/polyelectrolyte interfaces

Polyelectrolyte becomes more and more popular in electrocatalysis.The understanding of electrode/polyelectrolyte interfaces at the molecular level is important for guiding further the polyelectrolyte-based electrocatalysis.Herein,we demonstrate an in-situ surface-enhanced Raman spectroscopic method by using a three-electrode spectroelectrochemical cell towards characterizing the electrode/polyelectrolyte interfaces.The Ag/AgCl and Ag/Ag_(2)O electrodes are used as the reference electrode in the acidic and the alkaline systems,respectively.The working electrode is made of a transparent carbon thin film which loads the electrocatalysts.The applications of this method are demonstrated through the in-situ characterizations of the p-methylthiophenol adsorbed on the Au and Pt and the electrochemical oxidation of Au on polyelectrolyte membranes.The potential-dependent spectral features of these two systems show that this method is a powerful tool for investigating the electrode/polyelectrolyte interfaces in electrocatalysis.

Integration of a fused silica capillary and in-situ Raman spectroscopy for investigating CO_(2) solubility in n-dodecane at near-critical and supercritical conditions of CO_(2)

To determine the solubility of CO_(2)in n-dodecane at T=303.15-353.15 K,P≤11.00 MPa,an integrated fused silica capillary and in-situ Raman spectroscopy system was built.The Raman peak intensity ratio(I_(CO_(2))/IC-H)between the upper band of CO_(2)Fermi diad(I_(CO_(2)))and the C-H stretching band of n-dodecane(IC-H)was employed to determine the solubility of CO_(2)in n-dodecane based on the calibrated correlation equation between the known CO_(2)molality in n-dodecane and the I_(CO_(2))/IC-Hratio with R^(2)=0.9998.The results indicated that the solubility of CO_(2)decreased with increasing temperature and increased with increasing pressure.The maximum CO_(2)molality(30.7314 mol/kg)was obtained at 303.15 K and7.00 MPa.Finally,a solubility prediction model(lnS=(P-A)/B)based on the relationship with temperature(T in K)and pressure(P in MPa)was developed,where S is CO_(2)molality,A=-8×10^(-6)T^(2)+0.0354T-8.1605,and B=0.0405T-10.756.The results indicated that the solubilities of CO_(2)derived from this model were in good agreement with the experimental data.

Supercontinuum fiber laser-based coherent anti-Stokes Raman scattering microscopy for label-free chemical imaging

Coherent anti-Stokes Raman scattering(CA RS)microscopy can resolve the chemical compo-nents and distribution of living biological systems in a label-firee manner and is favored in several disciplines.Current CA RS microscopes typically use bulky,high-performance solid-state lasers,which are expensive and sensitive to environmental changes.With their relatively low cost and environmental sensitivity,supercontinum fiber(SF)lasers with a small footprint have found increasing use in biomedical applications.Upon these features,in this paper,we homebuilt a low-cost CARS microscope based on a SF laser module(scCA RS microscope).This SF laser module is specially customized by adding a time synchronized seed source channel to the SF laser to form a dual-channel output laser.The performance of the scCARS microscope is evaluated with dimethyl sulfoxide,whose results confirm a spatial resolution of better than 500nm and a detection sensitivity of millimolar concentrations.The dual-color imaging capability is further demonstrated by imaging different species of mixed microspheres.We finally explore the potential of our scCARS microscope by mapping lipid droplets in different cancer cells and corneal stromal lenses.

Mode decoupling in solid state ring laser based on stimulated Raman effect in polar crystals

In this paper we study the gain saturation induced mode-coupling control in solid state ring laser devices based on the stimulated Raman effect of the polar crystals in.order to realize solid state ring laser gyroscopes. We theoretically investigate the mode coupling induced by gain saturation between clockwise （CW） and counterclockwise （CCW） propa- gating laser modes. Because the CW and CCW running waves are pumped with counter-propagating lasers respectively, the independent coexistence can be ensured.

Modelling of passively Q-switched lasers with intracavity Raman conversion

We present a model of passively Q-switched Raman lasers by utilizing the rate equations. The intracavity fun-damental photon density, Raman photon density and the initial population-inversion density of the gain medium are assumed to be of Gaussian spatial distributions. These rate equations are normalized by introducing some synthetic parameters and solved numerically, and a group of general curves are generated. Prom these curves we can understand the dependence of the Raman laser pulse characteristics on the parameters about the pumping, the gain medium, the Raman medium and the resonator. An illustrative calculation for a passively Q-switched Nd^3＋：GdVO4 self-Raman laser is presented to demonstrate the usage of the curves and related formulas.

STUDY ON THE MICROSTRUCTURE AND PROPERTY OF PET FIBER BY LASER RAMAN MICROSCOPE

In this paper, the microstructure change of one step-draw PET fiber has been studied byvarious methods, such as, Laser Raman Microscope, Wide-angle X-ray, Density-gradient andPolarizing Microscope. The computer has been used to resolve overlapped bands in the Ramanspectra. Then the band changes have been correlated with trans, gauche and stressed trans-conformations indicated by a conformational index. Based on these indices, the relationshipbetween the conformation change of glycol units in the fiber structure and the macromechanicalproperties of fiber is expounded.