Select articles below Save the selection as：General references bibliographyTXTXMLEndNote
Materials and thin films
Abstract + Free to read the article if you have an account for login, or you need to pay.
Three metal (Ag, Cu, and Al) nanoparticle (NP) films are obtained by irradiating a smooth and continuous metal film with a Nd∶YAG fiber pulsed laser at room temperature. The tunability of the wavelength and intensity of localized surface plasmon resonance (LSPR) of the three metal NP films is realized by varying laser scanning speeds. The wavelength and intensity of the plasmon absorption peak of Ag NP film in visible wavelength region show a wide tuning range, while the wavelength and intensity of the plasmon absorption peak of the Cu NP film exhibit a small tuning range in the visible wavelength region. Unlike Ag and Cu NP films, the Al NP film exhibits a narrow and sharp plasmon absorption peak in the ultraviolet wavelength region, and the wavelength tuning range of LSPR is also small. Further, the stronger surface-enhanced Raman scattering signals are observed in all three kinds of laser-irradiated metal NP films than in the deposited metal films before laser irradiation. The results of the finite difference time-domain simulation with respect to the electric-field intensity distributions of all samples show good agreement with the experimental results of surface-enhanced Raman scattering.
PDF Chinese Journal of Lasers, 2020, 47(1): 0103001
Measurement and metrology
A real-time phase-sensitive optical time-domain reflectometry (Φ-OTDR) signal processing system is proposed based on the heterogeneous accelerated computing technology of field programmable gate array (FPGA) with respect to the characteristics of complexity, large-amount computation, and high real-time requirements of the Φ-OTDR signal processing system. Firstly, the heterodyne-detection-type Φ-OTDR signal processing flow is analyzed and decomposed. Subsequently, a series of accelerated computing methods based on FPGA, such as sliding window data frame segmentation, multichannel parallel fast Fourier transform calculation, frequency-domain filtering, and short-term energy summation, are proposed. Finally, the system implements long and real-time disturbance signal demodulation and display using a fiber sensing distance of 40 km, a repetition rate of 2 kHz, and a sampling interval of 1 m with a frame overlap of 80%. The FPGA system acts as a heterogeneous accelerator to mitigate the computer data processing pressure and ensure real-time operation of the sensing system at a high repetition frequency, effectively ensuring the system reliability and stability.
PDF Chinese Journal of Lasers, 2020, 47(1): 0104002
When we measure glass thickness using the traditional triangulation method, the relative position of the glass and probe should be kept fixed. When the position of the glass changes, it must be recalibrated to accurately measure thickness. To address this problem, a multi-position glass-thickness-measurement method based on laser triangulation that can automatically adapt to displacement changes is proposed in this paper. By this method, glass thickness can be measured directly when glass position changes. Firstly, the relationship between the glass thickness and spacing of imaging spots of the reflected light from the front and back surfaces of the glass is analyzed when the glass is at the standard position. Then, the relationship among the spacing of imaging spots, position of front surface of the glass, and glass thickness is analyzed when the glass displacement changes. The corresponding mathematical model is established, and a compensation algorithm is proposed herein to correct the influence of the glass displacement change on the glass-thickness measurement. Finally, a laser triangulation system based on a laser-diode-complementary metal-oxide semiconductor (LD-CMOS) is designed, and multiple glass samples with known thicknesses are used for calibration and measurement experiments. Experimental results show that when the glass position changes within the range of 1-4.5 mm, the absolute errors of glass-thickness measurement at different positions are less than 0.010 mm and the relative errors are within 0.5%. The proposed method can thus realize high-precision glass-thickness-measurement without repeated calibration when the glass is at different positions, displaying good practicability, flexibility, and versatility.
PDF Chinese Journal of Lasers, 2020, 47(1): 0104003
A segregated model for strain transfer coupling in a surface-bonded fiber Bragg grating (FBG), adhesive, and measured object was established. In addition, the correction relationship between the FBG-measured and true strains on the surface was derived. The theoretical values were verified by finite element simulation and experimental results. Its accuracy and viability under various installation conditions were also discussed. The theoretical results match the experimental results well, with errors below 1.6%. The strain sensing coefficient and its accuracy are positively correlated with the adhesive length and elastic modulus of the protective recoating, while they are negatively correlated with the mechanical strength of the measured object. In addition, the accuracy of theoretical strain sensing coefficient is negatively correlated with geometrical features of protective recoating and adhesive, with an insensitive strain sensing coefficient. This study provides reference for strain measurements using a surface-bonded FBG, error correction, and design of relevant sensors.
PDF Chinese Journal of Lasers, 2020, 47(1): 0104004
The crosstalks in detection circuits in the process of miniaturization for high-precision fiber-optic gyroscopes (FOG) become increasingly serious. In complex application conditions, it is difficult to measure crosstalks in the FOG output using traditional methods based on turntables. In this paper, we propose a method for measuring crosstalks based on simulated rotation rate phase ramp. We implement a specially designed circuit to generate a controllable phase ramp, combine it with a closed-loop phase ramp, and then connect the Y-waveguide using an adding circuit. This method can correctly estimate the crosstalk magnitude by testing gyroscope response. Related experiments are conducted, and experimental results show that the proposed method can realize the self-inspection of crosstalk in FOG detection circuits without turntables and improve the design and test efficiencies for application systems.
PDF Chinese Journal of Lasers, 2020, 47(1): 0104005
In this paper, the effect of temperature on the near-infrared spectroscopy model of cement raw meal was investigated by developing two models (model I and model II) with different calibration sets. Model I and model II were developed by applying the partial least squares method using same-temperature and different-temperature samples, respectively. In comparison with model I, the root mean square error of prediction (RMSEP) of SiO2, AlO3, Fe2O3, and CaCO3 in model II was reduced by 78.3%, 26.4%, 42.9%, and 60.4%, respectively. Experimental results show that the temperature of the cement raw meal sample has a certain influence on the prediction results of the near-infrared spectroscopy model. The influence of temperature on the prediction results can be effectively reduced by modeling with temperature gradient samples in the calibration set, and thus the infrared spectroscopy model can better be applied to production site.
PDF Chinese Journal of Lasers, 2020, 47(1): 0111001
A carbon monoxide (CO) measurement system based on photoacoustic spectroscopy with a 2.3 μm mid-infrared tunable diode laser is built. The CO absorption line at 4300.699 cm -1 is selected as the sensing object. In order to eliminate the influence of long relaxation time of CO molecule on measurement, water vapor is added into experimental gas to enhance CO photoacoustic signal. By optimizing the modulation parameters, the optimal modulation amplitude and modulation frequency of the system are determined to be 4.29 cm -1 and 785 Hz,respectively. Under the optimal experimental conditions, there is a good linear relationship between the second harmonic signal of the selected spectral line and the CO concentration, and the linearity is 0.994. The 2.13×10 -6 volume fraction of CO in the air can be retrieved from the relationship. Finally, by using the Allan variance to analyze the long-term stability of the system under dry and wet conditions, the detection limits under dry and wet conditions are 1.18×10 -7 and 0.58×10 -7, respectively. It is proved that the addition of water vapor can effectively improve the CO detection sensitivity of the system.
PDF Chinese Journal of Lasers, 2020, 47(1): 0111002
To improve the applicability of methane gas detection in complex environments, a hollow-core band gap photonic crystal fiber is used as the optical chamber,realizing the inserted detection of homologous methane concentration. Tunable diode laser absorption spectroscopy with a 0.5-m hollow-core band gap photonic crystal fiber is used for the online detection of methane gas. The detection limit of the designed detection system is approximately 1.92×10 -5 and the long-term stability of detection fluctuation is less than ±2.18%. The single-ended total reflection design and homologous detection method make inserted detection possible in complex environments. This work provides the research foundation for distributed detection using a single light source.
PDF Chinese Journal of Lasers, 2020, 47(1): 0111003
Beam transmission and control
In adaptive optics systems, the traditional proportional-integral control model relies on the response matrix of the deformable mirror, which is sensitive to changes in the system state. When the response matrix is altered, the wavefront correction performance is degraded. In this paper, the output of control signal from Hartman slope data is realized by redefining the back-propagation neural network structure, and a control model is established. Experimental results show that the proposed model eliminates the limitation of the traditional fixed model and acquires the characteristics of an online real-time update response model. The control model delivers high convergence performance, can adapt to environmental changes, and is robust. It also improves the control precision and the control performance to a certain extent.
PDF Chinese Journal of Lasers, 2020, 47(1): 0105001
Nonuniform lateral temperature distribution causes the thermal lens effect on the far-field slow-axis divergence angle. To alleviate this effect and improve the slow-axis beam quality, we incorporated a method of adiabatic package by implanting an air gap between the chip and the transition heat sink to reduce the conduction heat dissipation on both sides. Herein, the finite element analysis software ANSYS 18.0 was used to analyze the temperature distribution of chips with the edge adiabatic package. The results show that a chip with contact width 200 μm can reduce the slow-axis divergence angle by approximately 28%, from 11.5° to 8.2°, when the working current is 1.6 A. Likewise, it can reduce the beam parameter product and the beam quality factor by 28% and 24%, respectively. The increase in thermal resistance is 6%. Finally, the implementation of edge adiabatic package has little effect on the lasing wavelength, threshold current, and electro-optical conversion efficiency of the device.
PDF Chinese Journal of Lasers, 2020, 47(1): 0105002
This study presents the reflection and transmission characteristics of Laguerre Gaussian beam (LGB) from the periodic layered film with topological insulator (TI) based on angular spectrum expansion and 4×4 matrix transfer theory. For the incidence of linearly polarized LGB on the periodic layered film, the intensity distributions of the reflected and transmitted fields are evaluated and discussed. It is shown that the intensity distributions are greatly influenced by the topological magneto-electric polarizability (TMEP) of TI and the periods of the TI periodic layered film. This investigation provides a new method to manipulate the light field of vortex beams by changing the TMEP value or the periods of TI films. The methods presented herein can be extended to different TI-layers and provide reference for the research on photonic band structures and band gaps in TI photonic crystals in further research.
PDF Chinese Journal of Lasers, 2020, 47(1): 0105003
The polarization characteristics of the polarized light propagating in a turbid medium are of considerable importance in various polarization-related technologies. Here, a light propagation model is established for a turbid medium comprising randomly oriented non-spherical particles. Further, the polarization characteristics of the backscattering light after multiple scattering are numerically evaluated for a turbid medium comprising different non-spherical particles based on the T-matrix method and the vector Monte Carlo simulations. During this process, the medium is categorized as a Rayleigh-scattering-particle medium or a Mie-scattering-particle medium based on the non-spherical particle size in the medium. The results show that in the Rayleigh-scattering-particle medium, the influence of particle shape on the polarization characteristics of the backscattering light of the linearly-polarized light and circularly-polarized light is considerably small, the polarization preservation ability of the linearly-polarized light is better than that of the circularly-polarized light, and the linearly-polarized direction is maintained, whereas the circularly-polarized rotation is reversed. Conversely, in the Mie-scattering-particle medium, the different particle shapes considerably affect the polarization characteristics of the backscattering light; the polarization preservation ability of the circularly-polarized light is better than that of the linearly-polarized light at large optical thicknesses, and the opposite is true at small optical thicknesses. Furthermore, the particle shapes significantly influence the spatial distribution of the polarization degree and backscattering intensity of the polarized light.
PDF Chinese Journal of Lasers, 2020, 47(1): 0105004
Fiber optics and optical communication
In this study, we design an angle diversity optical receiver using photodiodes with two different fields of view (FOV) for indoor multiple-input and multiple-output visible light communication systems. The system combines the advantages of the receiver with two different FOV (2-FOV) and traditional angle diversity receiver (ADR) to achieve better reception performance. Furthermore, simulation of a typical indoor visible light communication scenario using light-emitting diode lamps as the data transmitters is performed. In our simulation, the minimum signal-to-noise ratio (minSNR) at the output end of an equalizer in the proposed system is higher than those of the 2-FOV receiver and the conventional ADR, which achieves the minSNR of over 45 dB in 97% of indoor locations. The ratio is increased by 96% and 32% compared to those of the 2-FOV receiver and conventional ADR, respectively. Finally, the total bit error rate is calculated for the system using asymmetrically clipped optical orthogonal frequency division multiplexing as the modulation scheme, and the results of the zero-forcing equalizer and minimum mean square error equalizer are given. The results demonstrate that the proposed receiver has the lowest bit error rate for the indoor locations under consideration.
PDF Chinese Journal of Lasers, 2020, 47(1): 0106001
Herein, we study the out-of-lock frequency-tracking problem in a resonant fiber-optic gyro (RFOG). First, we analyze the reason and mechanism of out-of-lock frequency tracking, and find that the change of the current in frequency-tracking synchronization and the symmetry change caused by non-reciprocal noises, such as backscatter and polarization coupling, are the main reasons for the peak pulse and zero-bias change, respectively. Second, we propose a scheme for out-of-lock frequency-tracking control of the RFOG based on temperature closed-loop feedback of the semiconductor laser. The long-term tracking synchronization of the laser central frequency with the fiber resonator''s single-resonant frequency can be realized by temperature closed-loop control; thus, the gyro output error caused by out-of-lock frequency tracking is eliminated. The overall technical scheme, signal processing flow, and implementation method of out-of-lock control are described in detail. Finally, we construct a successful RFOG prototype and test the static performance of the RFOG both before and after implementing the out-of-lock control. The test results show that this out-of-lock frequency-tracking control scheme can reduce the output pulse amplitude mutation of the RFOG from 3000 (°)/h to 200 (°)/h and the output zero-bias change of the RFOG from 600 (°)/h to 0 (°)/h, which completely eliminates the zero-position change in the frequency secondary locking process; consequently, the gyro precision is significantly reduced to 4.9 (°)/h (for 100-s smooth integration time).
PDF Chinese Journal of Lasers, 2020, 47(1): 0106002
In this work, the polarization-dependent loss (PDL) of a SiO2/Si arrayed waveguide demultiplexer (AWG DEMUX) is optimized. The physical factors causing the polarization dependence of the AWG and the process methods and conditions required to eliminate this dependence are analyzed theoretically. AWG DEMUX chips are fabricated by semiconductor processes, such as chemical-vapor deposition, photolithography, and etching. The boron and phosphorus contents in the cladding material are optimized and adjusted according to theoretical analysis. The PDLs of the chips are successfully reduced to 0.12 dB so that the PDL parameters meet the chip''s commercialization requirements.
PDF Chinese Journal of Lasers, 2020, 47(1): 0106003
Laser devices and laser physics
This paper reports the experimental study of the single-pass and cascaded stimulated Raman scattering (SRS) of deuterium gas in anti-resonance hollow-core fibers (AR-HCFs). The process of SRS of deuterium gas in AR-HCF is analyzed in detail and the variations in the output spectrum and Raman power with the pressure of deuterium gas and the pump laser power are studied as well. It is found that the rotational SRS can be effectively suppressed and the efficiency of vibrational SRS is improved by reducing the pressure and using pump laser pulses with a relatively low peak power. In addition, by further designing and fabricating an AR-HCF with transmission bands in the right position and narrow bandwidths, efficient first-order vibrational Stokes light (1561 nm) and second-order cascaded vibrational Stokes light (2925 nm) can be output via pumping with a 1064 nm pulsed laser.
PDF Chinese Journal of Lasers, 2020, 47(1): 0101001
This paper proposes and demonstrates a method to produce narrow-linewidth, narrow-pulse-width, and high-repetition-rate pulses with actively Q-switched ring-cavity all-fiber lasers based on acoustic-optic modulators. We use a double-clad gain fiber as the cladding power stripper''s input fiber in the ring cavity to shorten the cavity length and reduce reflections from the forward amplified spontaneous emission (ASE) at the output end of the gain fiber, which can efficiently suppress the ASE gain self-saturation so that the effective cavity gain can be enhanced. The narrow-linewidth Q-switched laser pulse can be established quickly, providing a narrow pulse width and high repetition rate for the pulses. Our experimental results demonstrate that the laser can produce 150-kHz Q-switched laser pulses with a narrow linewidth and pulse width of 0.16 nm and 10.4 ns, respectively, at a pump power of 7 W.
PDF Chinese Journal of Lasers, 2020, 47(1): 0101002
This paper reports on a passively mode-locked ytterbium-doped fiber laser with a 1502-m total cavity length operating in the dissipative soliton resonance (DSR) region using the nonlinear optical loop mirror (NOLM) technique. High-energy square pulses with an ultra-low repetition rate of 133.18 kHz are observed. Both the duration and single pulse energy of the output DSR pulse increase linearly with increasing pump power. The maximum output pulse duration reaches 761.6 ns at a pump power of 414.47 mW, while the single pulse energy at the same power level reaches 60.2 nJ. In addition, the influence of cavity length on the pulse duration and single pulse energy is investigated by changing the length of the single mode fiber in the NOLM. The results indicate that the longer the cavity length is, the larger the pulse duration is, and the lower the peak power is.
PDF Chinese Journal of Lasers, 2020, 47(1): 0101003
To avoid deterioration in the ranging accuracy caused by pulse-timing errors, a pulsed laser ranging method using cyclostationary random sequences is proposed. In this method, echo pulse-timing signals are mapped onto periodic reference signals to construct cyclostationary random sequences, which converts measurements of pulse-timing moments into parameter estimation of these sequences. The use of periodic stationary changes in time for cyclostationary stochastic processes enables the accurate estimation of parameters from the measurement data with timing jitter errors. High-precision target distances can then be obtained. An ergodic undersampling method is also proposed to overcome the difficulty of using equivalent and equally-spaced sampling for obtaining the measurement data of cyclostationary random sequences when the sampling and signal frequencies are very different. A pulsed laser rangefinder based on these principles is developed with the advantages of high accuracy and simple structure. When the laser exit pupil average power is 1 mW and the signal-to-noise ratio is 10, tests show that the non-cooperative target range is 300 m and the range accuracy is ±(2 mm+2×10 -6D ).
PDF Chinese Journal of Lasers, 2020, 47(1): 0101004
High mode discrimination and large mode area in fibers are key technical challenges for the generation and transmission of high-power radially polarized field (RPF) laser. Based on this, a novel method for the design of a radially polarized field fiber is proposed. By introducing a circularly symmetrical radially distributed thermal stress field into the core of the fiber, a radial birefringence effect can be realized in the core, which effectively breaks the degeneracy between polarization modes in conventional optical fibers. The effective index difference among TM01, TE01, and HE21 modes is of the order of 10 -4. This allows the TM01 radial polarization mode to be separated. In addition, a large mode area design for the TM01 mode field can be achieved by using the proposed RPF fiber.
PDF Chinese Journal of Lasers, 2020, 47(1): 0101005
Femtosecond laser has many potential applications in various fields, such as industrial processing, precision measurement, defense, and scientific research. A high-power and high-pulse-quality femtosecond chirped-pulse amplifier system based on spectrum control and dispersion optimization is reported. In the proposed system, the chirped fiber Bragg grating (CFBG) with tunable dispersion is designed to match the compressor and can be fine-tuned. Thus, the net dispersion of the system approaches zero by controlling the residual dispersion of CFBG dispersion compensation system. Furthermore, the degradation of the pulse quality in the amplification processing is avoided by controlling the spectral shape via spectral filtering before the main amplifier to prevent distortion. Finally, high-quality femtosecond laser pulses as short as 198 fs with an average power of 24 W at a repetition rate of 50 MHz are generated.
PDF Chinese Journal of Lasers, 2020, 47(1): 0101006
Inconel 718 alloys with premade grooves are repaired layer by layer with Inconel 718 spherical powder via the laser additive manufacturing process. The repaired Inconel 718 alloys are then subjected to δ aging treatment at 800 ℃ for different time (4, 8, 16, and 32 h) to study the effect of the aging time on the microstructure and tensile properties of the repaired layer. The results show that the Laves and strengthened γ″ phases in the repaired layer gradually disappear with the increase of the aging time, whereas the δ phase nucleates and grows on the basis of the stacking faults of the γ″ phase on the close-packed plane through the shearing mechanism. In addition, needle-like precipitates appear in the δ phase of the repaired zone around the residual Laves phase and become larger with aging time. However, precipitates in the substrate metal preferentially nucleate and grow at the grain boundaries and eventually grow in parallel within the grains. Although the aging treatment can effectively improve the microhardness and tensile strength of the repaired zone and the substrate metal of Inconel 718 alloys, the hardness and mechanical properties decrease as the aging time continues to increase. After aging treatment for 4 h, the microhardnesses of the repaired zone and the substrate metal reach the highest values of 361 HV and 465 HV, respectively, and then gradually decrease with the aging time. Furthermore, with different aging treatments, all the tensile fractures of the repaired parts are located in the repaired zone. The fracture surfaces are flat, showing typical brittle fracture characteristics.
PDF Chinese Journal of Lasers, 2020, 47(1): 0102001
In this study, a direct transition TA15/GH4169 dual alloy composite structure is prepared using the laser deposition manufacturing technique at preheating substrate temperatures of 100 ℃ and 400 ℃. Further, the effect of the preheating temperature on the microstructure, residual stress, and microhardness of the TA15/GH4169 interface is investigated. Based on the results, substrate preheating can be concluded to considerably influence the forming quality of the composite structure, whereas the non-preheated substrates produce defects at the composite structure interface. The forming quality improves when the preheating temperature is 400 ℃, and a gradient transition zone of approximate 0.33 mm is formed at the composite structure interface. The residual stress on the GH4169 side significantly decreases when the preheating temperature is increased from 100 ℃ to 400 ℃. The residual stress value reduces by 21.59% when the substrate is preheated to 100 ℃, whereas it reduces by 33.19% when the substrate is preheated to 400 ℃. The microhardness of the composite structure slightly decreases with an increase in the preheating temperature. At the same preheating temperature, the microhardness of the transition zone is the highest. At 400 ℃, the microhardness of the transition zone increases by 91.2% compared with that of the TA15 side.
PDF Chinese Journal of Lasers, 2020, 47(1): 0102002
In this study, laser polishing experiment of the additive manufactured TC4 titanium alloy with abrasive blasting is conducted in argon gas. Further, a polarization curve is plotted to verify the corrosion resistances of the surface before and after laser polishing and to analyze the effect of laser polishing on the corrosion resistance of the TC4 titanium alloy based on the surface roughness, grain size, surface residual stress, and microstructures. The results denote that the self-corrosion potential and self-corrosion current density of the polished surface are greater than those of the initial surface, indicating that the polished surface exhibits a lesser corroded tendency when compared with that exhibited by the initial surface. However, if the polished surface corrodes, its corrosion rate is observed to be slightly greater than that of the initial surface. The increase in self-corrosion potential can be attributed to the decrease in the surface roughness of the material. Furthermore, the increase in self-corrosion current density can be attributed to grain refinement and residual tensile stress on the surface.
PDF Chinese Journal of Lasers, 2020, 47(1): 0102003
The stability of coaxial powder feeding laser cladding process is affected by many factors, which makes it difficult to estimate the optimal process parameters. This study designs a central composite experiment, which considers the process parameters (laser power, powder feeding speed, and scanning speed) as input and outputs the characteristic parameters that reflect the cladding morphology and quality. The regression model and neural network in the response surface method are applied to the prediction of the single-pass cladding results, and their effects are compared. Based on this, a multi-objective optimization algorithm, i.e., the non-dominated sorting genetic algorithm II (NSGA-II), is used to optimize the three aforementioned process parameters. The results denote that the optimized process parameters can improve the surface hardness of the repaired parts by 17.11%, reduce the depth of the base heat-affected zone by 13.90%, and improve the cladding efficiency by 6.10%.
PDF Chinese Journal of Lasers, 2020, 47(1): 0102004
The butt-joint laser welding of magnesium and titanium alloys is achieved by shifting laser pulses on the magnesium side and adding copper interlayers. The diffusion of elements and reaction characteristics at the interface are investigated. Furthermore, the effect of the thickness of the copper interlayer on the microstructure and mechanical properties of the joint is investigated. In addition, the primary causes of joint fractures are analyzed. The results show that the copper interlayer improves the microstructure at the weld/titanium interface and increases the content of Ti-Cu compounds near the interface. As the thickness of the copper interlayer increases, the thickness of the Ti2Cu reaction layer at the interface also gradually increases and becomes continuous. Moreover, the fracture position of the joint moves from the interface reaction layer to the weld zone. When the thickness of the copper interlayer reaches 30 μm, the tensile strength of the joint reaches 121 MPa.
PDF Chinese Journal of Lasers, 2020, 47(1): 0102005
In this study, the relation between keyhole oscillation and depth is analyzed based on the internal pressure balance conditions of the keyhole to allow real-time monitoring of the laser penetration welding process. Then, based on the coupling of keyhole behavior with plasma behavior and consistency of plasma oscillation characteristics with plasma electrical signal fluctuation characteristics, we use a short-time autocorrelation analysis method to analyze the relation between the oscillation period of a plasma electrical signal and weld depth during laser penetration welding of A304 stainless steel and Q235 carbon steel. Results show that the plasma electrical signal''s oscillation period increases with an increase in the weld depth, and the relations between the plasma electrical signal''s oscillation period and weld depth differ when the welding materials are different. Finally, in a variable heat input continuous welding verification test, we obtain a good correspondence between the short-time autocorrelation analysis results of plasma electrical signals and weld penetration when the welding process is stable, which is consistent with the keyhole oscillation characteristic equation we analyzed.
PDF Chinese Journal of Lasers, 2020, 47(1): 0102006
To investigate the generation mechanism of anisotropy of tensile properties, we tested the tensile properties of samples from different sampling angles and analyzed the crystal orientation of formed parts using electronic back-scattering diffraction. The results demonstrate that the elongation of the 90° sample (angle between the long axis direction of the tensile sample and the horizontal direction is 90°) is 32% higher than that of the 45° sample, and the tensile strength and yield strength are 9% and 8% lower, respectively. The Schmid factor corresponding to the 90° sample''s main texture has the highest value of 0.485, and the angle θ between external tensile stress σ and shear stress along the sliding direction τ is 38°. The first moving slip system is a prismatic slip system. The Schmid factor of the 45° sample is the lowest (0.415), and θ is 28°. The first slip system of the sample is a pyramidal slip system. The 90° sample has the smallest proportion of small angular grain boundaries (1%), whereas the 45° sample has the highest proportion (29.2%). Additionally, the 90° sample has optimal plasticity, whereas the 45° sample has the highest strength.
PDF Chinese Journal of Lasers, 2020, 47(1): 0102007
A high-temperature wear-resistant self-lubricating composite based on TiC/CaF2/Inconel 718 superalloy is fabricated using the laser melting deposition technique. The composite''s microstructure, microhardness, and high-temperature dry-sliding friction and wear properties are investigated. Furthermore, its high-temperature wear mechanism is studied. The results demonstrate that the composite''s microstructure comprises TiC, CaF2, Cr7C3, γ″-Ni3Nb, and γ-(Ni, Fe). In-situ synthesized TiC primary phases and fine CaF2/TiC eutectics are uniformly scattered on a matrix of γ-(Ni, Fe) solid solution, which is strengthened by super-fine Cr7C3 and γ″-Ni3Nb high-temperature strengthening phases. The composite''s average microhardness is approximate 820 HV. Compared with the Inconel 718 reference specimen fabricated by laser melting deposition technique, the composite has good high-temperature wear-resistance and a low and stable friction coefficient. The composite''s excellent high-temperature friction and wear properties are derived from its reasonable microstructure.
PDF Chinese Journal of Lasers, 2020, 47(1): 0102008
PDF Chinese Journal of Lasers, 2020, 47(1): 0116001
PDF Chinese Journal of Lasers, 2020, 47(1): 0116002
An all polarization-maintaining fiber laser is established based on a nonlinear amplifying loop mirror, and thus a stable optical oscillator source is applied to an optical comb system which works outdoors. In this experiment, supercontinuum covering from 1000 nm to 2300 nm is produced by using two-level optical preamplification and high nonlinear fibers. The optimal signal-to-noise ratio of carrier-envelope offset frequency f0 detected by the collinear f-2f interferometer is 35 dB. Furthermore, the relationship between the net cavity dispersion and the linewidth of f0 is investigated. The narrowest linewidth of f0 in the free-running mode is narrowed to 5 kHz. Finally, the repetition rate fr and f0 are both phase-locked to a rubidium atomic clock. The standard deviations for fr and f0 are 780 μHz and 308 mHz, respectively. The fiber optic comb system guarantees compact size and excellent integration level.
PDF Chinese Journal of Lasers, 2020, 47(1): 0115001
Holography and information processing
The classical dark channel prior defogging algorithm easily loses image details. Alternatively, the defogging algorithm based on edge-preserving filtering can effectively protect image details; however, it is time-consuming. Aiming at the aforementioned problems, this paper proposed an adaptive exponentially weighted moving average filtering algorithm that protected image edge details while taking less time. Combined with an improved dark channel, this method achieved fast and precise defogging. First, the improved dark-channel algorithm was applied to obtaining a rough distribution of atmospheric transmittance. Second, the transmittance was optimized by employing the adaptive exponentially weighted moving average filtering algorithm. Subsequently, the transmittance of the bright region was repaired to avoid color distortion. Finally, the defogged image was processed using the transformation of the atmospheric scattering model. The experimental results show that the proposed algorithm has a high execution speed; moreover, the defogged image processed using the proposed algorithm has good performance under the following three nonreference objective evaluation indexes: effective edge intensity, color reproduction ability, and structural information.
PDF Chinese Journal of Lasers, 2020, 47(1): 0109001
A pseudoscopic problem often exists in traditional integral imaging display, and is typically solved using two-step shooting process. In this paper, a one-step shooting method for integral imaging without depth inversion is proposed. In this method, the off-axis paralleled shooting scheme is adopted to shoot a three-dimensional (3D) scene and generate an orthoscopic elemental image array (EIA) by reasonably designing the shooting parameters and rearranging the set of elemental images. The generated EIA can be directly used for integral imaging display without depth inversion. The proposed method avoids complex image correction and the tedious two-step shooting process and can quickly generate the EIA with correct depth information. In 3D integral image display experiment, the 3D images reconstructed based on EIA are vivid and clear. The experimental results validate the efficacy of the proposed method.
PDF Chinese Journal of Lasers, 2020, 47(1): 0109002
Biomedical photonics and laser medicine
Herein, we propose an imaging method based on frequency modulation and spatial coding to overcome the problem associated with fluorescence molecular tomography data acquisition methods, improve the data acquisition scheme, and reduce data acquisition time. In this method, the excitation beam is split into several sub-beams and used as multipoint excitation source. These sub-beams are modulated to different frequencies and then simultaneously incident at different points on the target surface. At the detection side, the emergent light of the target is first directed to a single photomultiplier through a spatially coded mask. According to the compressed sensing theory, the distribution of fluorescence signals on the target surface can be obtained by changing the mask mode and conducting sparse reconstruction and recovery. We design the corresponding simulation experiment to evidence the proposed methodology. Result shows that the method can better restore the original image, which proves the feasibility of the proposed method.
PDF Chinese Journal of Lasers, 2020, 47(1): 0107001
This paper proposes a tomographic short-time integral imaging method. In terms of terahertz tomography, the experimental results show that the data quality and imaging effect of the proposed short-time integral imaging method are better than those of the traditional method. In the wavelet denoising theory, a δ-σ evaluation rule is proposed based on the characteristics of the terahertz signal, and the optimal wavelet denoising combination (e.g., the sym7 wavelet with a decomposition scale of 5) is selected using the evaluation rule. Based on this, the short-time integral imaging experiment of nondestructive detection tomography of phenolic plastic samples is set up, and different wavelet denoising combinations are compared. The effect of wavelet denoising is compared from two subjective evaluation indexes of defect number and defect recognition rate and the objective evaluation index of Weber contrast. Results prove that the sym7 wavelet (with the decomposition scale of 5, soft-threshold processing) is effective in wavelet denoising of nondestructive detection signals of phenolic wedge defects. The background noise in the nondestructive detection image of the sample after signal preprocessing is effectively suppressed. The contrast effect between prefabricated defects and the background area is more obvious, and the internal structural changes in the sample can be detected easily and accurately.
PDF Chinese Journal of Lasers, 2020, 47(1): 0114001
Herein, a double-periodic photonic crystal structure array is designed by double-cone interferometry using 3+3 beams, and nano-focusing effect is studied as well. The small period of photonic crystal lattice and large period of photonic array can be changed by adjusting the incremental angle between inner and outer cones. The structure and shape of gradient photonic crystal lattice can be changed by adjusting the intensity and polarization direction of the beam. Fourier transform method is used to study the nano-focusing effect of the designed photonic crystal structure. The structure is very important for improving the extraction efficiency, optical display, optical coupling, and optical integration of light emitting diodes.
PDF Chinese Journal of Lasers, 2020, 47(1): 0113001
Remote sensing and sensors
We proposed and demonstrated an optical fiber temperature sensor based on rare-earth-doped double-fiber peanut (RDDFP). We used fibers doped with rare-earth elements to fabricate a peanut-shaped structure that can realize high temperature sensitivity. The sensitivity of temperature to the cladding mode and core mode interference are used and the strong optothermal effect of rare-earth element ions is combined in the proposed structure. We have theoretically and experimentally investigated and compared the mode interference and thermosensitive effect of the erbium-doped double-fiber peanut (EDDFP) and ytterbium-doped double-fiber peanut (YDDFP). Experimental results show that comparing with a single-mode fiber peanut, RDDFP exhibits higher temperature sensitivity because of its stronger optothermal effect. The temperature sensitivity of EDDFP and YDDFP are 1286 pm/℃ and -2343 pm/℃, respectively. The optical fiber temperature sensor based on RDDFP has the advantages of high sensitivity, high repeatability, all fiber, simple fabrication, compact structure, and so on. It has good application prospects in the fields of power systems, architecture, aerospace, and ocean development.
PDF Chinese Journal of Lasers, 2020, 47(1): 0110001
To meet the demand for the real-time collision-avoidance detection of close obstacles by an unmanned surface vehicle (USV) on the sea surface, this study establishes an obstacle adaptive grid representation method for the USV based on three-dimensional (3D) lidar. According to the distribution of the lidar point cloud of environmental obstacles around the USV, a functional relationship among obstacle density, obstacle expression time, and grid map resolution is established for adaptively determining the moderate map resolution and constructing a grid map. The 3D lidar point cloud data are subjected to the process of dimensionality reduction and projected onto the grid map to reduce the amount of data and improve obstacle detection efficiency. Furthermore, a method validation experiment is conducted using 3D lidar; consequently, the lidar point cloud data of three different obstacle scenarios are obtained. The results show that the desired resolution of the obtained grid map and number of details regarding the obstacle increase with an increasing number of obstacles. Conversely, the desired resolution of the obtained grid map is lower and obstacle representation is faster with fewer obstacles in the environment, and the obstacle adaptive grid representation can be realized. The follow-up research of USV local collision avoidance path-planning can be supported by the established obstacle adaptive grid representation method.
PDF Chinese Journal of Lasers, 2020, 47(1): 0110002
Self-encoding and common markers are designed to improve the capacity and efficiency of automatic encoding of the markers in a large-sized target calibration field. An indoor target calibration field is established, where the self-encoding markers are adopted as the encoding reference and the common markers are adopted as the control-point targets. Based on the principle of “homologous points with same number,” an automatic encoding method is proposed to deal with the self-encoding and common markers. This method can automatically realize the automatic coding mapping between the image markers and the object control-point markers with the same name, avoid setting the number of image markers manually, and improve the calibration efficiency. Subsequently, the general form of indirect adjustment model is derived for the calibration of a multi-view combined camera with implicit constraints. This model is then applied to calibrating a developed multi-view camera comprising webcams. The experimental results denote that the proposed method exhibits the advantages of large encoding capacity (up to 65535), high decoding efficiency, and automatic decoding. Because the relative orientation parameters exhibit less fluctuation and the back-projection root-mean-square (RMS) error in the image point plane is less than 0.20 pixel, the robustness and accuracy of the calibration can be improved using the proposed calibration method. Furthermore, the usage of the proposed method can be easily extended to other similar multi-view or panoramic cameras to obtain appropriate calibration.
PDF Chinese Journal of Lasers, 2020, 47(1): 0110003
Real-time dynamic target tracking and recognition has become an urgent issue that should be resolved with respect to streak tube laser imaging radar. In this study, we propose a sampling interpolation algorithm for target three-dimensional (3D) reconstruction to solve the problem of long operation time associated with the peak detection 3D reconstruction algorithm. First, the streak image is sampled with a certain sampling rate at equal intervals and is interpolated; subsequently, the peak feature is extracted. When the sampling rate is 4%, the sampling bilinear interpolation reconstruction method is approximately 2.3 times faster than the original reconstruction method, and the correlation coefficient of the 3D reconstruction matrix is about 0.99 for targets with a detection distance of 706 m; further, in case of the nearest neighbor sampling interpolation method, the reconstruction speed is 3.5 times faster than that of the original reconstruction method, and the correlation coefficient of the 3D reconstruction matrix is about 0.98. The experimental results denote that the proposed algorithm can improve the 3D object reconstruction speed; the larger the number of frames of the target streak image is, the better the effect of improving the 3D target reconstruction speed will be.
PDF Chinese Journal of Lasers, 2020, 47(1): 0110004
Laser lift-off (LLO) is a technique used to transfer devices to terminal substrates through the ablation of materials by pulsed laser irradiation. Recently, LLO has become the major technique for the fabrication of flexible electronic devices because of its wide material applicability and process compatibility. Further, the representative research achievements of LLO in case of the fabrication of flexible electronics are investigated and presented in this study with respect to the basic mechanisms and technological features, and novel theories and application techniques are given particular emphasis. Accordingly, the application prospects of the LLO technique, especially the possibility of ultrafast laser applications, are summarized and forecasted.
PDF Chinese Journal of Lasers, 2020, 47(1): 0100001
The liquid crystal optical devices that are used in fusion ignition, optoelectronic countermeasure, laser radar, and laser communication encounter a performance failure that can be attributed to the near-infrared high-power laser irradiation. In this study, the basic constituent components and working principle of the liquid crystal optical devices are initially introduced. Subsequently, the laser damage characteristics and mechanisms of the conductive film, alignment film, and liquid crystal material, which are the main constituent components of the liquid crystal optical devices, are reviewed. Finally, the laser damage characteristics and mechanisms of the whole liquid crystal optical under the radiation of near-infrared laser are summarized.
PDF Chinese Journal of Lasers, 2020, 47(1): 0100002