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Materials and thin films
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A novel tunable broadband metamaterial absorber/reflector is designed. Its structure consists of square single-layer graphene and a metal ground plane separated by a dielectric layer of SiO2. Due to the symmetry of the unit cell, the absorber is polarization insensitive when the light wave is incident vertically. By continuously adjusting the Fermi level of graphene, the surface conductivity of graphene can be tuned, and the working state of the proposed structure can be switched between the reflector and the absorber throughout the whole broad absorption band. The numerical simulation results show that the absorption bandwidth of the absorber reaches 4.13 THz at absorptivity larger than 90%, and the absorber maintains good broadband absorption performance within a wide incidence angle range of 65°. The proposed absorber can be widely used in high-performance terahertz devices, such as active camouflage, imaging, modulators and photoelectric switches.
PDF Chinese Journal of Lasers, 2020, 47(8): 0803001
Measurement and metrology
When the gas concentration to be measured is high, the absorption loss in the cavity is large, which limits the measurement range of cavity ring-down spectroscopy. To solve this problem, the correlation between sampling threshold and gas concentration detection is analyzed, the influence of threshold adjustment on measurement accuracy is simulated, a model describing the dynamic response ranges of threshold and gas concentration is established, and a method for detection range extension is proposed by adjusting threshold. In the experiment, the methane standard gases with different concentrations are measured by the measurement device based on cavity ring-down spectroscopy, and the measured results are consistent with the theoretical simulation results. By adjusting the measurement threshold, the range is extended to 1×10 -4 (volume fraction), the dynamic response range is 2.72×10 4, and the maximum quoted error is 0.78%. The proposed method solves the problem that low detection limit and large range cannot be met simultaneously in the technology based on cavity ring-down spectroscopy and provides an analytical method for the setting of threshold parameters.
PDF Chinese Journal of Lasers, 2020, 47(8): 0804001
The extended Cauchy model is a common model for describing the birefringence dispersion of liquid crystal mixture, however, in ultraviolet spectral ranges, the extended Cauchy model can not fit well with the experimental data. Here, we report a Sellmeier model for fitting the birefringence dispersion of liquid crystal mixture in a broad wavelength range from ultraviolet to near infrared. For this purpose, the principal birefringence values were accurately measured using spectroscopic ellipsometer and the Sellmeier coefficients were then obtained by a nonlinear fitting method. The results show that the χ2 of the Sellmeier model is 0.0019 and the χ2 of the extended Cauchy model is 0.0349. In addition, at the standard test wavelength of 589 nm and the ultraviolet wavelength of 378 nm, the fitting values show that the Sellmeier model were closer to the measured values. Both experimental result and χ2 test demonstrate that Sellmeier model has a better fit than extended Cauchy model.
PDF Chinese Journal of Lasers, 2020, 47(8): 0804002
This study builds an optical fiber surface plasmon resonance (SPR) sensor theoretical model with four-layered media based on the Kretschmann structure. Simulation results show that the phase difference between p- and s-polarized components changes almost linearly with the refractive index in the section of 1.333--1.336, deriving the sensor measurement formulas of optical fiber SPR. Using a dual-frequency He-Ne laser, an optical fiber SPR measurement system with heterodyne interferometry based on a common optical path structure is presented. A higher measurement resolution of the proposed sensor can be achieved through the signal processing method of phase demodulation. The experimental calibration data of glycerin solution show that the measurement results are consistent with the theoretical analysis. Results are in good agreement with those obtained by other methods, and the error of refractive index between the two methods is less than 8.0×10 -5. The proposed sensor can be applied in many fields such as environment detection, food safety, drug screening, and clinical medicine.
PDF Chinese Journal of Lasers, 2020, 47(8): 0804003
To improve the radiation detection performance of scintillating fiber, a silica cladding crystal scintillating fiber based on the high performance scintillation crystal LYSO∶Ce is prepared by a modified rod-in-tube method. First, the electron radiation comparison experiment is conducted by using an electron accelerator. The electron radiation responses of the silica cladding LYSO∶Ce scintillating fiber (SLCF) and existing silica cladding YAG∶Ce scintillating fiber (SYCF) are studied. Then, the influences of the fiber length and the distance from the fiber to the radiation source on the radiation sensing characteristics of SLCF are further analyzed. Finally, through the radiation experiment, the repeatability of radiation sensing characteristics of SLCF is tested. Experimental results show that SLCF has excellent electronic radiation response and sensing characteristics.
PDF Chinese Journal of Lasers, 2020, 47(8): 0804004
Considering the deviation in the time-of-flight of synthetic aperture sequential beamforming (SASB), which causes serious artifacts, this study proposed an endoscopic ultrasound-imaging algorithm (SASB-MV) combining the linear constrained minimum variance (MV) criterion with the SASB algorithm. The algorithm first focused on each virtual source point, obtained the ultrasonic echo signal vector of the imaging point, and constructed the sample covariance matrix based on the ultrasonic echo signal vector of the imaging point. The algorithm then constructed the weight vector based on the linear constraint MV criterion and finally obtained the high-resolution echo signal value of the imaging point through the weight and ultrasonic echo signal vectors. Moreover, this study conducted simulation experiments of scattering point and circular dark spot in Filed II. Experimental results show that compared with the dynamic receive focusing (DRF) and SASB algorithms, the lateral resolution of the proposed algorithm improved by 59.1% and 31.2%, and the contrast improved by 44.68% and 16.23%, respectively.
PDF Chinese Journal of Lasers, 2020, 47(8): 0804005
Laser-induced breakdown spectroscopy is widely used in the material detection field because of its advantages, including online noncontact measurement and non-destructive analysis. Selecting proper analytical lines is an important prerequisite for achieving a good detection effect. This study proposed a method for adaptively selecting analytical and internal standard lines from the original spectral data of LIBS based on the global optimization ability of the genetic algorithm (GA) and the local search ability of the particle swarm optimization (PSO) algorithm. We quantitatively analyzed four major non-aluminum elements (i.e., Mg, Mn, Si, and Fe) in aluminum alloys using the analytical and internal standard lines selected using this method. The mean values of the goodness of fit, root mean square error, and relative standard deviation are 0.972, 0.35%, and 3.53%, respectively. The results obtained by traversing all other analytical lines for a quantitative analysis and comparing their calibration performances show that the analytical and internal standard lines obtained by the PSO-GA search optimization are optimal analytical spectral lines under current experimental conditions.
PDF Chinese Journal of Lasers, 2020, 47(8): 0811001
This study investigates the effect of parallel plate confinement on the spectra of the CN molecule in laser-induced PMMA plasma in the atmospheric environment. The wavelength of five measured spectral peaks are 388.29 (0-0), 387.0 (1-1), 386.14 (2-2), 385.44 (3-3), and 385.03 nm (4-4). The experimental results show that the spectral peak intensity of the CN molecule with spatial confinement is stronger than that without spatial confinement. Moreover, the vibrational temperature of the CN molecule is calculated by fitting the CN spectra. The calculated vibrational temperature of the CN molecule with spatial confinement is higher than that without spatial confinement, and the vibrational temperature at high laser energy is higher than that at low laser energy. The shock waves reflected by the parallel plate compress the plasma plume and increase its temperature and number density. Thus, the spectral intensity of the CN molecule in laser-induced PMMA plasma is improved.
PDF Chinese Journal of Lasers, 2020, 47(8): 0811002
Methane hydrate is a new clean energy source. For the cruise detection of methane hydrate, membrane separation off-axis integrated cavity output spectroscopy, M-ICOS, is applied to detect dissolved methane (CH4) and carbon dioxide (CO2). An M-ICOS system comprises a membrane separation unit, a cavity temperature and pressure controller, an optical cavity, a spectrometer unit, and an industrial personal computer. The M-ICOS system is tested for performance evaluation. The accuracies of the temperature and pressure controller are 0.0003733 ℃ and 0.6799 Pa, respectively. The limits of detection of CH4 and CO2 are 0.56×10 -9 and 0.62×10 -6, respectively. A field experiment in Shenhu area, South China Sea, found a strong fluctuation in the gas concentration at the depth of 500--700 m, which was probably because the sea flow took high concentration gas from the methane hydrate enrichment area. Experimental results reveal that the system exhibits a good sensitivity and stability, which make it usable to detect CH4 and CO2 in deep sea for cruise detection of the methane hydrate.
PDF Chinese Journal of Lasers, 2020, 47(8): 0811003
Beam transmission and control
To solve the problems of the difficulty in adjusting parameters in real time and the convergence speed being slow in the traditional stochastic parallel gradient descent (SPGD) algorithm, this study proposes an efficient SPGD algorithm based on adaptive gain and joint index optimization and establishes a numerical simulation model of this algorithm. The proposed algorithm is used for the beam cleaning of kilowatt-class slab lasers. Simulation results show that compared with the traditional SPGD algorithm, the proposed algorithm does not require a parameter adjustment, and the convergence speed and convergence effect are significantly improved. Furthermore, in the beam purification experiment of the kilowatt-class slab laser, the laser beam quality β is optimized from 7.89 to 1.95 herein.
PDF Chinese Journal of Lasers, 2020, 47(8): 0805001
Herein, a deformable lens that can directly generate tunable Airy beams was proposed. Compared with other methods, this deformable lens can be embedded in the optical path to directly generate Airy beams. A prototype deformable lens was fabricated, and an optics system based on the wavefront sensor was established to generate Airy beams. Using the fabricated deformable lens, high-quality Airy beams with an adjustable cubic phase were generated. The residual error of generated cubic phase was less than 3.4% of the corresponding target amplitude. The intensity distribution and propagation properties of the generated Airy beams were consistent with the theoretical findings, thereby demonstrating the potential application of the deformable lens in directly generating tunable Airy beams.
PDF Chinese Journal of Lasers, 2020, 47(8): 0805002
Freeform pupil illumination technology is an important photolithographic resolution enhancement technology for immersion photolithography machines locating in nodes at 28 nm and below. Arbitrary illumination mode could be realized by adjusting the angular spectrum of the beam by micro mirror array (MMA). The angular position distribution of MMA is of great significance to the application of the freeform pupil illumination technology. An angular position distribution algorithm of MMA based on genetic algorithm is proposed in this paper. Compared with the angular position distribution algorithm of MMA based on the simulated annealing algorithm, the iteration speed of the proposed algorithm is increased by more than 10 times, and the MMA angular position distribution obtained by the proposed algorithm can accurately reproduce the distribution of the target pupil intensity. Results of the photolithography performance simulation show that the root mean square (RMS) values of the asymmetry distribution of the photoresist exposure patterns of the algorithm pupil and the target pupil are basically the same, and the RMS values of critical dimension difference distribution are less than 0.5 nm.
PDF Chinese Journal of Lasers, 2020, 47(8): 0805003
Diffraction can cause beam divergence. In this study, a new type of waveguide exit port structure was constructed for a photonic crystal by adopting a two-dimensional square-lattice photonic crystal to reduce the divergence and deal with the short radiation distance and low radiation efficiency of the waveguide exit port. First, a Y-shaped channel structure was used for improving the transmittance at the output end; then, multi-branch structures was coupled for improving the convergence of the emitted light to increase the radiation distance of the light wave. Finally, the exit port period was optimized. In addition, analyses and simulations were conducted using the plane wave expansion method and the finite-difference time-domain method. The simulation result denoted that the divergence angle of the emitted light was approximately 3° and that the radiation efficiency exceeded 25% when the radiation distance of the light was 110 μm. Thus, a square-lattice photonic crystal with a Y-shaped defect, 16 exit ports, and 4 exit port periods can achieve well-directed radiation.
PDF Chinese Journal of Lasers, 2020, 47(8): 0805004
Fiber optics and optical communication
In this study, the dual-transmission properties of a two-cavity structure comprising three asymmetric fiber Bragg gratings (FBGs) are investigated. The periods or Bragg wavelengths of the three FBGs are different from each other, causing their three stopbands to partially overlap and resulting in two overlapped regions. The sizes of these overlapped regions can be controlled by tuning the FBG periods. In addition, only one wavelength can be transmitted from each of the overlapped regions. Thus, dual-transmission is achieved. However, dual transmission will exhibit an ultranarrow bandwidth if a long cavity is considered. Further, the spacing between two wavelengths can be altered by adjusting the FBG periods. The all-fiber structure proposed in this study can be extended to achieve multiple transmissions exhibiting equal or unequal spacings. The proposed structure can be potentially applied in dual- or multi-wavelength fiber lasers.
PDF Chinese Journal of Lasers, 2020, 47(8): 0806001
To achieve the flexible transmission of an ultraviolet laser, a hollow antiresonant fiber with seven capillary tubes in the cladding was fabricated. The outer diameter of the fiber is 95 μm, and the core diameter is 11 μm. The average diameter of the cladding is 5 μm, and the average wall thickness is 319 nm. Two ultraviolet laser beams with wavelengths of 266 nm and 355 nm can be simultaneously guided by the fiber with transmission losses of 0.25 dB/m and 0.8 dB/m, respectively. The bending loss in the two transmission bands at a bending radius of 5 cm is 0.05 dB/m at 266 nm and 0.023 dB/m at 355 nm. The low bending sensitivity is due to the small core diameter of the fiber. This allows further miniaturization of the device. The antiresonant fiber is expected to play an important role in laser transmission and ultraviolet laser spectroscopy.
PDF Chinese Journal of Lasers, 2020, 47(8): 0806002
Laser devices and laser physics
Sodium guide stars with high brightness are an important condition for accurate detection of atmospheric wavefront distortion. Studies have shown that the intensity of backward resonance fluorescence scattering of a sodium guide star is closely related to many factors such as atmospheric transmission efficiency, geomagnetic field, seasons, emission system parameters, receiving system parameters and pumping laser parameters. Pulsed sodium guide stars can effectively improve the signal-to-noise ratio of wavefront detection in adaptive optical systems. The return light intensity detection experiment for a pulsed sodium guide star is carried out based on pumping modes under various laser parameters. Based on the high-energy, multi-longitudinal mode, and hundred-microsecond pulsed sodium guide star laser pumped sodium layer in the atmosphere, a series of return light intensity data are obtained, which are in good agreement with the theoretical analysis results. The experimental results show that the precise control of the pumping spectral line is an important means to obtain the stable return light intensity of the sodium guide star, and the working mode of the sodium guide star based on the circularly polarized light pumping and double-line broadening pumping system is an effective way to efficiently excite the sodium layer atoms and obtain high brightness return light.
PDF Chinese Journal of Lasers, 2020, 47(8): 0801001
The Mamyshev cavity based on multi-cascade nonlinear broadening and offset filtering is used to propose a synchronous spectral overlapping multi-wavelength fiber laser. The single-cavity ring structure solves the problems of gain competition and difficulty in synchronization output of traditional multi-wavelength lasers. Through numerical simulation, the output of four-wavelength pulses with central wavelengths of 1035, 1040, 1045 and 1050 nm is realized. The spectral width of each pulse is 7 nm (greater than the interval between adjacent central wavelengths), the peak power (before external linear compression) is 0.9--1.0 kW, and the pulse width is about 0.28 ps. By optimizing the filter, the corresponding multi-wavelength output is discussed, and the influence of wavelength arrangement order on transmission function and system efficiency is analyzed.
PDF Chinese Journal of Lasers, 2020, 47(8): 0801002
4.3 μm mid-infrared fiber lasers are produced by pumping Dy-doped InF3 with dual wavelengths of 1.7 μm and 2.3 μm. The spatial distributions of pump light power, signal light power and population density in the cavity are calculated, and the influences of pump light power, reflectivity of the output mirror, fiber length, and fiber loss on output laser power are numerically analyzed. The calculation results show that the self-termination effect can be effectively eliminated by dual-wavelength pumping, and the 4.3 μm laser output can be realized. The laser output power mainly depends on the power of 2.3 μm pump laser. When the 1.7 μm pump laser power is fixed, the laser output power increases linearly with the increase of the 2.3 μm pump laser power. When the 2.3 μm pump laser power is fixed, the optimal 1.7 μm pump laser power is obtained, which makes the laser output power maximum. The study results provide a feasible scheme for the operation of a 4 μm high-power continuous-wave fiber laser, which is instructive for obtaining 4 μm lasers via dual-wavelength pumped Dy∶InF3 fiber laser.
PDF Chinese Journal of Lasers, 2020, 47(8): 0801003
Attosecond transient absorption (ATA) spectroscopy is a very useful technique for studying the ultrafast dynamics of electrons in atoms or molecules on sub-femtosecond timescale. The time-dependent Schr?dinger equation is numerically solved to simulate the evolution of hydrogen molecular ion ( H2+) in the intense near infrared (NIR) and extreme ultraviolet (XUV) composite laser field and to examine the ATA spectroscopy under the nuclei-fixed or nuclei-movable condition. The research shows that when the nuclei is fixed, the resulting ATA spectroscopy is similar to those of atoms. In contrast, when the nuclei is not fixed, the ATA spectroscopy exhibits richer periodically-modulated absorption line structures, whose modulation period is just equal to half of the NIR laser period. By analyzing the ionization-dissociation characteristics of H2+, we clarify the origin of such half-periodic modulation as quantum interference among different quantum transition paths from ground to excited states. Comparing the ATA spectroscopy under different situations, one can easily distinguish the obvious influence of nuclear motion on molecular ATA spectroscopy.
PDF Chinese Journal of Lasers, 2020, 47(8): 0801004
Currently, spot welding technology is widely used in the overlap welding of galvanized steel plates for automobile applications. However, the novel laser screw welding technique is expected to gradually replace resistance spot welding because of its high efficiency and flexibility. In this study, the effects of inter-sheet gaps on the formation and mechanical properties of laser screw welding joints are investigated. The escape process of high-pressure zinc vapor and the causes of defects are analyzed at different gaps. For ST12 galvanized steel welded joint with 1.8 mm thick single-sheet and 6.7 mm spot weld diameter, this new technique offers an overlap gap window of 0.2--0.4 mm, suggesting wide adaptability for inter-sheet gaps. Using a gap greater than 0.1 mm, welding defects caused by the escape of zinc vapor can be effectively avoided. The best weld appearance and mechanical properties can be obtained at a gap of 0.2 mm because of the fluent escape of zinc vapor.
PDF Chinese Journal of Lasers, 2020, 47(8): 0802001
In this paper, a KrF excimer laser is used to decompose 4H-SiC substrates to prepare graphene layers. The research is focused on the influence of the crystal orientation of 4H-SiC on the quality of the graphene produced. The effects of laser energy density, pulse number, and crystal orientation on graphene quality are analyzed. With a laser energy density of 1.06 J/cm 2 and a pulse number of 8000 shots, the graphene obtained on the polar Si-plane (0001) and on the non-polar a-plane (11?20) of the 4H-SiC sample are both of the best quality. We find that a buffer layer that provides a template for the growth of graphene is formed between Si-plane (0001) and 4H-SiC substrate. The graphene obtained from the buffer layer is consequently more ordered and has fewer defects. In contrast, there is no buffer layer between the photo-generated graphene on a-plane (11?20) and 4H-SiC substrate, which results in the generated graphene being disordered and more sensitive to the laser parameters.
PDF Chinese Journal of Lasers, 2020, 47(8): 0802002
In this paper, the 6A01-T5 aluminum alloy hollow extrusion profiles with lock bottom for high-speed trains was welded by oscillating fiber laser-CMT arc hybrid welding. With optimized welding parameters, joints with uniform surface formation and without defects were obtained. Furthermore, the porosity distribution characteristics, microstructure and mechanical properties of the joint were investigated.The experimental results show that the joint formed by the oscillating laser hybrid welding has excellent forming quality. The average porosity rate of cross-section and longitudinal section is 0.63% and 0.06%, respectively. The round porosities with regular shape and diameter less than 50.0 μm are dominant, and the technological porosities in the weld bead are obviously restrained, compared to the non-oscillating laser hybrid welding. The central microstructure of the weld bead is dendrites, and the secondary dendrite is obviously weakened along the weld centerline. The microhardness of the fusion zone and heat affected zone is lower than that of base metal, which means the two zones are the softening zone.The fatigue limit of the hybrid joints is 105.0 MPa and the average tensile strength of joints is 223.19 MPa, while the tensile strength is up to 84.22% of the base metal. The tensile fracture shows an obvious dimple shape, which is the typical characteristic of ductile fracture.
PDF Chinese Journal of Lasers, 2020, 47(8): 0802003
The laser metal deposition process is accompanied by the interaction of the forming environment and molten pool, which influences the solidification conditions and formed microstructure to a certain extent. To achieve the rapid production of large, complex, and high-performance metal structures, high-deposition-rate laser metal deposition (HDR-LMD, deposition rate ≥1 kg/h) technology was developed by simultaneously improving the energy and mass inputs based on the conventional laser metal deposition (C-LMD, deposition rate ≤0.3 kg/h) technology. Compared with the C-LMD process, HDR-LMD can change the shape and temperature of the molten pool, further influencing the forming environmental effect on the molten pool. Moreover, as the dimension of the forming part increases, the cost of establishing an environment chamber also increases. Based on the aforementioned background, the forming environmental effect (air and argon environments) on the solidification condition, single-layer geometry, and microstructure of HDR-LMD GH4169 superalloy was studied. Results show that although the heat dissipation of different environments influence solidification condition during the HDR-LMD process, their influences on the oxidation are very small. Moreover, compared with argon environment, air environment exhibits a greater cooling rate, finer primary dendrite arm spacing, less Laves phase content, and lower degree of microsegregation. This study indicates that the solidification condition and microstructure of HDR-LMD GH4169 alloy does not significantly improve upon being shielded with argon, which can provide a reference for industrial production.
PDF Chinese Journal of Lasers, 2020, 47(8): 0802004
Drag reduction is of great importance for ship transportation. Various micro-nano groove structures with different section shapes were fabricated on the surfaces of 6061 aluminum alloy using an ultrafast laser. Further, the femtosecond laser fabrication of the drag reduction micro-nano structure, the surface drag reduction effect, and the surface corrosion resistance of the micro-U-shaped groove structures were investigated. Various microgroove structures were quickly fabricated by modulating the processing parameters and the scanning route. Microgroove structures covered with nanoparticles exhibit superhydrophilicity. Compared with the pristine surface, two types of micro-U-shaped groove-structured surfaces with structural period widths of 28 μm and 50 μm and depths of 20 μm and 25 μm shortened their motion time in tank moving experiments by 4.3% and 11.6%, respectively, indicating their effective surface drag reduction. After forming a superhydrophobic surface, the surface corrosion rates of these two micro-U-shaped groove structures tested in 3.5% NaCl solution were decreased by 87% and 73%, respectively. The micro-nano structures fabricated by an ultrafast laser exhibit good drag reduction effect and corrosion resistance, which indicates their valuable application potential.
PDF Chinese Journal of Lasers, 2020, 47(8): 0802005
Herein, AlSi10Mg alloy with different Sc contents were prepared by laser melting using a spot size of 2 mm. Further, the influences of Sc content on the microstructure and mechanical properties of the AlSi10Mg alloy were studied using metallographic microscope, scanning electron microscope, X-ray diffractometer, micro-hardness tester, and electronic universal material testing machine. The results showed that α-Al phase and Si phase were the main phases in the alloy. The α-Al dendrites in the alloy were refined and gradually transformed to equiaxed grain when the Sc mass fraction was increased from 0 to 0.2%. Meanwhile, the morphology of the Si phase changed from fibrous to particulate. In addition, the densification, mechanical properties, and thermal stability of the alloy were improved significantly. When the mass fraction of Sc was 0.2%, the densification of the alloy reached about 98.39%, the main rare earth phase in the alloy was Al3Sc, and the microhardness, plastic compression Rpc0.2 and compressive strength corresponding to 25% of the deformation were improved by 19.4%, 23.1%, and 17.5% compared with the alloy without Sc, respectively. When the Sc mass fraction exceeded 0.2%, the grain of the alloy coarsened and the optimization effect of Sc with respect to the alloy decreased. The alloy exhibited a finer microstructure and better mechanical properties when compared with those alloy prepared using the traditional process because of the high energy and fast cooling associated with the laser melting process. With the addition of rare earth Sc, the mechanical properties of the alloy were further improved with the help of solid solution strengthening. The mechanical properties of the alloy were worse than those of the alloy prepared using a laser with a micrometer spot size because of the large spot size considered in the experiment.
PDF Chinese Journal of Lasers, 2020, 47(8): 0802006
Intelligent modeling of directed energy deposition contributes in improving the manufacturing accuracy of deposition. We designed an experiment with the deposition process parameters (laser power, powder feeding rate, scanning speed, and distance of the nozzle) as inputs, and the width and height of the cladding tracks as ouputs. A support vector regression (SVR) model based on radial basis function (RBF) was established to predict the size of the cladding tracks. Then, an improved particle swarm optimization algorithm was used to determine suitable values for the hyperparameters of SVR. Results indicated that the average relative errors of RBF-SVR for predicting the width and height of the cladding tracks were 4.58% and 5.33%, respectively, which were better than the results obtained using the back propagation (BP) neural network where the average relative errors were 6.72% and 7.96%, respectively. The RBF-SVR is suitable for predicting the size of cladding tracks and provides a reference for selecting process parameters in a directed energy deposition.
PDF Chinese Journal of Lasers, 2020, 47(8): 0802007
To evaluate the elevated-temperature service performance of aero-engine components of GH3536 alloy formed via selective laser melting (SLM), tensile specimens were manufactured via SLM with optimized parameters and were treated using stress relieving and hot isostatic pressuring (HT+HIP). The tensile properties of the specimens were tested at 20--815 ℃ and the microstructures, fracture morphology, and fracture mechanism were analyzed. The results show that after HT+HIP treatment, (Cr,Mo)23C6 precipitates at the grain boundaries, a columnar-to-equiaxed transition partially occurs, however, the directional solidification characteristic is still present. There are finer grains and more grain boundaries on the transverse section. Fracture analysis shows chains of precipitated carbides along the grain boundaries weaken the grain boundaries; in addition, intergranular fracture was observed. Owing to the anisotropy of the grain size, the mechanical properties and fracture morphologies vary with the specimen building direction. As the temperature increases, the tensile strength decreases and the elongation first increases and then decreases. The tensile failure mechanism changes as the temperature increases; this is characterized by stronger trend of intergranular fracture, smaller plastic deformation, and longer cracks at elevated temperature.
PDF Chinese Journal of Lasers, 2020, 47(8): 0802008
Tin-based Babbitt metals are widely used as wear-resistant materials on the surface of high-performance bearing parts because of its low friction coefficient, good wear resistance and excellent anti-adhesion. To improve the defects in tin-based Babbitt alloy prepared by traditional casting and reduce material waste, laser cladding deposition (LCD) was used to fabricate tin-based Babbitt metal on the surface of 20 steel substrate, and the laser cladded Babbitt metal was compared with the alloys prepared by other two preparation processes, namely, the static casting and the centrifugal casting. The microstructure, element distribution, microhardness, friction and wear performance, and internal quality of the alloy layers were tested and analyzed. Results show that the microstructure size of tin-based Babbitt metal prepared by LCD is smaller than those of cast alloys, and the microstructure along the deposition direction is characterized by banded distribution of light alternated with darkness. Besides, the microhardness of the alloy layer prepared by LCD is significantly higher than that of the alloy prepared by static casting process and similar to that of the alloy prepared by centrifugal casting process. Nevertheless, in the case of dry friction, the friction coefficient of tin-based Babbitt metal prepared by LCD is greater than that of the alloys prepared by traditional casting process, and its wear mechanism is mainly abrasive wear. Additionally, ultrasonic and dye-penetration nondestructive tests indicate that there are no obvious defects in the tin-based Babbitt metal layer prepared by LCD, and it is well bonded to the steel back.
PDF Chinese Journal of Lasers, 2020, 47(8): 0802009
The cause and mechanism of tin spattering in the free flux laser jet solder ball bonding (LJSBB) process were analyzed using a right-angle Au/Sn-3.0Ag-0.5Cu/Au micro solder joint by removing the surface contamination and the volatiles on the Au bonding pads and observing their inner microdefects of Au layer. Results reveal that the prompt expansion of the gas in the microvoids distributed randomly in the Au layer is the key factor leading to spattering. Moreover, if the positions of the microvoids are different, the shape of defects are different. When the microvoids are near the edge of the Au bonding pad, tin spattering occurs easily, whereas the microvoids far away from the edge lead to a gas hole, evenly bring to the tin spattering and the gas hole simultaneously. Because of the solidification time and the limited floating speed of the gas bubble, the larger initial gas bubble easily results in the gas hole or tin spattering, and the smaller one leads to the inner gas hole.
PDF Chinese Journal of Lasers, 2020, 47(8): 0802010
In order to study the influence of matrix temperature state on the surface quality of workpieces during the interaction process between additive and subtractive manufacturing, the additive workpieces with different cooling time are milled, and the surface roughness, surface morphologies, surface residual stress, surface microstructures and surface hardness of workpieces are tested. When the workpiece is milled at a matrix temperature of 482 ℃, its surface roughness is about 2.226 μm. The surface has the defects of pits and protrusions, the residual stress is the residual tensile one, and the hardness of the top deposited layer is about 200 HV. When the workpiece is milled at matrix temperatures of 205 ℃ and 164 ℃, its roughness is reduced to 1.192 μm and 0.844 μm, respectively. The surface morphology is composed of uniform milling textures. The surface residual stress is transformed from the residual tensile one to the residual compressive one, and the hardness of the top deposited layer is 309 HV and 286 HV, respectively. The results show that, with the decrease of matrix temperature, the surface roughness of the workpiece decreases, the surface defects are reduced, the surface residual stress is converted from the residual tensile one to the residual compressive one, the hardness increases, and the surface quality improves.
PDF Chinese Journal of Lasers, 2020, 47(8): 0802011
PDF Chinese Journal of Lasers, 2020, 47(8): 0816001
Herein, the pulse synchronization between Nd∶YVO4 picosecond and Er-doped fiber mode-locked oscillators was examined, which is the basis for both optical parametric amplification and generation of mid-infrared. In the experiment, the repetition rates of 1555-nm Er-doped fiber mode-locked oscillator and 1064-nm Nd∶YVO4 picosecond oscillator were detected by a photodiode. The timing delay between the two lasers was detected using a frequency mixing and low-pass filter circuit. The repetition rates of two lasers were synchronized by controlling the piezoelectric transducer and stepper motor in the Er-doped fiber mode-locked oscillator. The proposed configuration exhibits advantages of simple structure, high integration and reliability.
PDF Chinese Journal of Lasers, 2020, 47(8): 0815001
The high-finesse optical microcavity is the core of a strongly coupled cavity quantum electrodynamics (QED) experimental system. However, due to the limited intervention space of an optical microcavity, it is difficult to obtain an effective initialization treatment to the atomic internal states trapped by the optical cavity. By selecting the light field that interacts with the ground state and a higher-order excited state of the atom, the limitation of the microcavity mirror on the intervention space is effectively avoided, and the optical pumping of the atomic internal states and the preparation of the atomic state (spin polarization) in the optical microcavity are realized. At the same time, based on the difference in coupling strength between optical microcavity and different internal states of atoms, a model for describing and optimizing the atomic polarization rate in the cavity is established and the state preparation efficiency of 85% of cesium atoms in the cavity is finally obtained.
PDF Chinese Journal of Lasers, 2020, 47(8): 0812001
This paper presents an adaptive iterative denoising algorithm based on noise estimation for terahertz images. First, the noise standard deviation of the real experimental image is obtained by using the noise level estimation method, and then the noise image is denoised by using quadtree based weighted double square non-local mean denoising method to improve the quality of the reconstructed image. Finally, the noise estimation and denoising of the denoised image are carried out again, and the best denoising effect is obtained through repeated iterations. Experimental results show that the method can retain the details of the image, effectively removes the background noise caused by the imaging system, and has a good denoising effect on terahertz image.
PDF Chinese Journal of Lasers, 2020, 47(8): 0814001
Based on terahertz time-domain spectroscopy, we analyze the curing properties of the adhesive layer and study the variation trend of the extinction coefficient and refractive index of the adhesive layer during the curing period. To characterize the solidification properties of multi-layer bonded structure, we employ the mean value as well as half peak half width of time-of-flight (TOF) and energy integral value of multi-layer adhesive structure after the Gaussian curve fitting. After curing for 96 h, the statistical results of the two eigenvalues gradually are stabilized. The extinction coefficient and refractive index of the adhesive layer exhibit the same trend, which proves that the curing time of the adhesive layer is approximately 96 h. We can use the two characteristic values of TOF and energy integral value adopted in this experiment as the evaluation index of the endpoint of solidification in the bonding process indirectly so as to improve the testing level of materials.
PDF Chinese Journal of Lasers, 2020, 47(8): 0814002
Terahertz holographic reconstructed images are prone to boundary blur. Therefore, this study proposes a segmentation method based on optimized region growth by evolutionary algorithms. First, the proposed method is used to perform bilateral filtering and morphological erosion on the original images to obtain the seeds of the region growth. Second, genetic algorithm and differential evolution algorithm are used to perform threshold optimization to limit the region growth. Subsequently, the segmentation results of the terahertz holographic images are obtained. Average structure similarity (MSSIM) is used as an objective evaluation for assessing the algorithm''s effectiveness. Segmentation results show that the region-growing algorithm optimized by the evolutionary algorithm has a good segmentation effect. Moreover, the MSSIM can reach 0.8 or higher. Finally, to compare the optimization performance of two evolutionary algorithms, the algorithms are applied to visible light images. According to the segmentation results of the images, it is concluded that the differential evolution algorithm is superior to the genetic algorithm in terms of speed and searchability.
PDF Chinese Journal of Lasers, 2020, 47(8): 0814003
This study proposes a terahertz high-sensitivity sensor with a combined structure of double split-ring resonators and a metal strip as a basic unit. The destructive interference of the electromagnetic waves of the resonance rings and the metal strip with different radiation losses in far field makes the line width of the reflected resonance narrow, showing a typical asymmetric Fano-type reflection spectrum. Moreover, we verify that the quality factor of the tunable sensor can be tuned by changing the coupling distance between the double split-ring resonators and the metal strip through numerical simulations. When the coupling distance is 22 μm, the quality factor reaches 83. In addition, the sensitivity of the proposed metamaterial sensor to the analyte thickness is studied. The frequency shift exponentially increases as the thickness of the analyte increases and gradually saturates when the thickness exceeds 10 μm. Finally, we also investigate the impact of the analyte with different refractive indices on the proposed sensor. The proposed sensor exhibits extremely high refractive index sensitivity and the figure of merit, i.e., 171 GHz/RIU and 18.3, respectively, when the analyte thickness is 22 μm. This high-sensitivity sensor has extensive application prospects in biochemical detection.
PDF Chinese Journal of Lasers, 2020, 47(8): 0814004
Based on the transfer matrix method, we calculate the impedance of a one-dimensional finite photonic structure to study the existing condition of interface states of layered composite structures with inversion symmetry. For two photonic structures with different inversely symmetric centers, the imaginary parts of their impedance have the opposite sign in all the odd bandgaps and the same sign in the even bandgaps. Therefore, The composite structure composed of the two individual photonic structures exhibits one interface state at the characteristic frequency where the sum of the impedance imaginary parts in the odd bandgaps of the two individual structures is equal to zero, and meanwhile the imaginary part of impedance of the composite structure is also zero at the characteristic frequency. If one composite structure is composed of multiple finite layered structures with different inversely symmetric centers in the way of alternating arrangement, and then in the same odd bandgaps, multiple interface states will appear. The imaginary part of impedance of the composite structure at the characteristic frequency is zero, and the electric field distribution of the interface state optical wave shows significant localization at the composite interface. In addition, the positions of multiple interface states can be adjusted to meet different application requirements by changing the unit cell number of one part of the composite structure.
PDF Chinese Journal of Lasers, 2020, 47(8): 0813001
Remote sensing and sensors
An adaptive, dynamic method based on the navigation and location applications of 2D lidar was proposed for extracting the center of cylindrical artificial landmarks (reflector marks) using laser echo intensity. The influence of different application scenes on the accuracy of extraction of the landmark center was analyzed. The proposed method was tested using different diffuse reflectivity background plates, incidence angles between the lidar and background plates, and specular reflection background plates. The recognition results for three algorithms were acquired using an experimental platform for different application scenarios. We then compared and analyzed the experimental results, which reveal that the modified algorithm can reliably and accurately extract the centers of the artificial landmarks in various application scenarios and that it is highly robust and practical.
PDF Chinese Journal of Lasers, 2020, 47(8): 0810001
Point cloud classification is an important stage in the application of airborne LiDAR point cloud in urban modeling and road extraction. Although there are many methods for point cloud classification, there are still some problems such as multi-dimensional feature vector information redundancy and low accuracy of point cloud classification in complex scenes. To solve these problems, a point cloud classification method is proposed based on multi- entity eigenvector fusion. The method extracts the feature vectors based on point entity and object entity and classifies the point cloud data by using random forest combined with color information. The experimental results show that the proposed multi-entity classification method is more accurate than the single-entity classification method. In order to further analyze the validity of random forest for point cloud classification, the support vector machine (SVM) and the back propagation (BP) neural network are used for a comparative analysis. The experimental results show that the three groups of point cloud classification results obtained by the random forest method are higher than those by the other two methods in the recall rate and F1 score.
PDF Chinese Journal of Lasers, 2020, 47(8): 0810002
The wind field data in near space is of great significance to the attitude control and flight safety for aircrafts. As for the principle prototype of a direct detection Doppler wind lidar (DWL) developed by Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, which can be mounted on aircrafts in near space, its working principle and system structure are introduced, and its relevant technical parameters are given. The DWL principle prototype is self-tested on the ground and the inversion results are compared with those from the coherent Doppler lidar (CDL) and the sounding balloons. The results show that the wind speed and wind direction measurement results retrieved by the DWL principle prototype agree well with those measured by the CDL and the sounding balloons near the ground. Within the height range of 0.3--1.1 km, the root mean square error (RMSE) of wind speed retrieved by the DWL principle prototype is 1.38 m·s -1, and the RMSE of wind direction is 9° based on the CDL measurement results. Within the height range of 0--3 km, the RMSE of wind speed retrieved by the DWL principle prototype is not more than 2.1 m·s -1, and the RMSE of wind direction is not more than 10° based on the sounding balloon measurement results.
PDF Chinese Journal of Lasers, 2020, 47(8): 0810003
A three-dimensional (3D) wind field inversion method of a coherent Doppler wind lidar based on genetic algorithm for spectrum estimation is proposed. The method can obtain 3D wind vectors directly from the multi-directional power spectral density without estimating the radial wind velocities, which can improve the data inversion accuracy in the case of low signal-to-noise ratio (SNR). The genetic algorithm adopted is an improved genetic algorithm for the coherent lidar, which can accurately, quickly, and parallelly retrieve the wind vector. Simulation is performed by the proposed genetic algorithm, and results show that the improved genetic algorithm has a significant improvement in convergence speed and global optimization ability compared to the traditional genetic algorithm. In the low SNR signal simulation comparison, the wind field inversion result of this method is better than that of the traditional nonlinear least squares method. The method has been applied to actual lidar systems. In the comparison of the measured data of the lidar and sounding balloons, the root mean square error of the horizonal wind speed is less than 0.7 m/s, and the standard deviation of the horizontal wind direction is less than 6°. The accuracy of the wind field inversion results is verified. By comparing the results of the proposed method with the results of the least squares wind field inversion of the measured data, we find that under the atmosphere conditions at that time, the proposed method increases the detection range by about 12.3%. The comparison between simulation and measured data fully proves the ability and effectiveness of the proposed method to retrieve the 3D wind field.
PDF Chinese Journal of Lasers, 2020, 47(8): 0810004