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Materials and thin films
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Continuous polishing is the preferred method for processing large-aperture planar optical materials, especially playing an important role in ultra-precision processing of optics. The surface accuracy of components is directly affected by the surface shape of the polishing pad. To study the removal characteristics of polishing-pad surface materials with small-tools, accurately measuring the surface shape of the polishing pad is necessary. In this paper, the height of spiral sample points on the surface of a polishing pad is collected by laser displacement sensor, and the experimental detection errors are analyzed and calibrated. Subsequently, the actual shape of the polishing pad surface is calculated. Using a computer controlled polishing method for reference, the material-removal model of the polishing pad with tool correction is established, the material removal function of the pad in the correction range of the small-tool is simulated and calculated, and the material-removal characteristics of the polishing pad are analyzed. Finally, the correctness of the simulation results is verified with a small-tool correction experiment, which provides a theoretical basis for shape modification of the polishing pad.
PDF Chinese Journal of Lasers, 2020, 47(4): 0403001
A monoatomic graphene layer exhibits considerably low absorptance, limiting its application in the optoelectronics field to some extent. In this study, we propose a method of enhancing the absorption of graphene using magneto-optical photonic crystals based on the magneto-optical effect of graphene. Subsequently, the 4×4 transfer matrix method was employed to study the influence of the relevant physical parameters on the absorption of graphene. The results show that the absorption of graphene can be effectively enhanced by adjusting the external magnetic field and that the absorption properties of graphene exhibit a certain amount of magnetic circular dichroism. However, graphene can exhibit high absorption for both left and right circularly polarized light by appropriately adjusting the magnetic induction of the external magnetic field and the Fermi energy, and near-perfect absorption can be achieved under certain conditions. The results of this study provide a theoretical basis to design and fabricate novel graphene-based optoelectronic devices such as magnetic circular dichroism sensor with high-performance, optical absorbers, and photodetectors.
PDF Chinese Journal of Lasers, 2020, 47(4): 0403003
The laser output characteristics of Ho∶YAP crystal are discrepant along different cutting directions. In this study, a-cut, b-cut, and c-cut 0.5% (atomic fraction) Ho∶YAP crystals were used to investigate laser output characteristics. Using a thulium-doped fiber laser with a central wavelength of 1915 nm and maximum power of 44.3 W to end pump the Ho∶YAP crystal along each cutting direction, continuous-wave laser power greater than 20 W was achieved. For the b-cut crystal, the central wavelength of the output laser is close to 2118 nm and the maximum continuous-wave output power and slope efficiency are 23.6 W and 61.98%, respectively. The results for the a-cut crystal are similar to those for the b-cut crystal, but the central wavelength for the c-cut crystal is 2129 nm when the maximum output laser power is achieved. When the Ho∶YAP laser operates in an in-cavity acoustic-optical Q-switched state, central wavelength shifting and multi-wavelength resonance occur in the a-cut and c-cut crystals. However, for the b-cut crystal, a stable pulsed laser output with a central wavelength close to 2118 nm can be obtained. For a pulse repetition rate of 20 kHz, the maximum average output power, pulse width, slope efficiency, and beam quality factor are 22.3 W, 20 ns, 55.22%, Mx2=1.81, and My2=1.50, respectively. From these results, it can be concluded that the b-cut Ho∶YAP crystal is more suitable for obtaining stable and efficient 2118-nm continuous-wave lasers and nanosecond-level pulsed laser output.
PDF Chinese Journal of Lasers, 2020, 47(4): 0403004
In this study, HfO2 films were deposited by the reactive electron beam evaporation technique at different oxygen partial pressures. The corresponding properties of the film such as its structure, optical property, chemical composition, absorptive property, laser damage resistance, and damage morphology are characterized and analyzed using an X-ray diffractometer, a scanning electron microscope, an ellipsometer, an X-ray photoelectron spectroscopy, a 1064-nm weak absorptivity tester, and a 1064-nm 1-on-1 damage test system. At a deposition temperature of 200 ℃, the deposited HfO2 films show a monoclinic polycrystalline structure with a grain size of approximately 10 nm. As the oxygen partial pressure increases, the oxidation degree of the films increases, thereby reducing their 1064-nm weak absorptive coefficients, which are dominated by non-stoichiometric defects. Moreover, the structure of the films becomes looser and the refractive index decreases. This study suggests that increasing the oxygen partial pressure, during the HfO2 film deposition process using reactive electron beam evaporation, can promote the suppression of nano-absorption defects in the film and subsurface cracks in the substrate, and can increase the laser damage threshold, thereby providing an important reference for preparing high performance HfO2-based optical components.
PDF Chinese Journal of Lasers, 2020, 47(4): 0403005
Measurement and metrology
Bamboo resources have efficient carbon-sequestration capabilities, and their economic contributions to global carbon balance and carbon trading have attracted widespread attention. However, because of the complexity and the high canopy density of bamboo canopy structure, traditional measurement methods cannot be used to precisely measure the accumulation amount. Therefore, LiDAR point cloud data of Phyllostachys pubescens canopy are obtained using three-dimensional laser scanner to estimate the accumulation amount by the point cloud density. Experimental results show that a mathematical relation exists between the accumulation amount of the bamboo canopy and the point cloud density of bamboo poles and branches. In a sample test, the model accuracy of the accumulation amount of the bamboo pole and branches in the canopy reached 95.53% and 91.36%, respectively.
PDF Chinese Journal of Lasers, 2020, 47(4): 0404001
A star sensor is a highly accurate attitude measurement device, but it is susceptible to the thermal environment. Moreover, it is difficult to establish an optical-machine-thermo model of a high-precision star sensor by simulation. Accordingly, an experimental analysis method is proposed to simulate on-orbit thermal environment of the star sensor by heating the mounting surface and baffle in a vacuum tank and simulate the star using static star simulator. The triaxial thermostability of the star sensor can be evaluated by observing its output. A prism mounted on a mounting surface was used to remove the thermal deformation of the mounting surface by auto-collimation with an error margin of 4.5%. Results show that when the baffle is heated from 27.3 ℃ to 110.6 ℃, the drifts of the x, y, and z axis are 2.9″, 1.2″, and 2.6″, respectively. When the temperature control accuracy of the mounting bracket is (20±0.3) ℃, the optical axis drift is ±0.18″, which meets the requirements of thermostability indicators of the high-precision star sensor.
PDF Chinese Journal of Lasers, 2020, 47(4): 0404002
In this study, a high-sensitivity temperature sensor based on surface plasmon resonance (SPR) is designed by plating gold thin layer and PDMS temperature-sensitive film onto the outer layer of photonic crystal fiber (PCF). It has the advantages of a simple structure, mature process, and good reversibility. The refractive index of PDMS decreases with the increase in temperature, causing the loss peak of the core mode to move toward the short-wave direction. The full-vector finite element method is used to analyze the SPR-PCF loss spectral characteristics under the condition of a perfectly matched layer boundary, which can achieve highly sensitive and accurate temperature measurements. The temperature sensitivity of the proposed sensor has been observed to reach -8.18 nm/℃ within the temperature range of 22--47 ℃. Furthermore, the proposed measurement method is applicable in safety detection and intelligent monitoring fields.
PDF Chinese Journal of Lasers, 2020, 47(4): 0404003
The microscopic defects with respect to the cladding layer are often detected using a microscope, energy dispersive X-ray spectroscopy, and scanning electron microscope, etc. Even though cracks can be accurately observed using these methods, the obtained results are not convincing. The sample profile is used to observe the performance of the test sample; however, the local area of the section does not represent the entire cladding layer. The randomness is considerably large, and the detection area is small. Furthermore, the requirement of sample size is high. To solve these problems, we use laser-induced breakdown spectroscopy (LIBS) technology to detect the microscopic defects observed with respect to the cladding layer. Subsequently, different contents were obtained with respect to the TiC/Ni35 cladding samples, and different types of defects could be observed in the samples after cladding. The characteristic spectra of the sample surface can be observed in the 21 mm×4.2 mm region, and the region scanning results are obtained using the array matrix method. The results prove that LIBS technology can be used to rapidly characterize the microscopic defects in the cladding layer. According to the evaluation, the No. 4 sample has the most defects, the No. 1 and No. 5 sample defects are moderate, and the No. 2 and No. 3 sample defects are the lowest.
PDF Chinese Journal of Lasers, 2020, 47(4): 0411001
This paper presents the spatial distribution characteristics of a polycyclic aromatic hydrocarbon (PAH) fluorescence signal in a propane-air diffusion flame and the variation of time-resolved signal with the flame height position. Results show that the PAH for smaller rings gradually shifts toward larger rings and then toward soot particles with an increase in the flame height position, which demonstrates the formation and growth process of PAH and soot in the combustion. Based on the measurement of time-resolved fluorescence, it is found that in the same environment, the measured fluorescence decay times of larger PAH are longer than those of small PAH for which fluorescence emissions are at shorter wavelengths. As the flame height position increases, the fluorescence decay time is observed to decline owing to the increasing temperature.
PDF Chinese Journal of Lasers, 2020, 47(4): 0411002
Fiber optics and optical communication
In this study, we design a low-cost and long-perimeter security system using the weak grating array technology and phase-sensitive optical time domain reflectometer. The developed system uses the grating positions and the time-division multiplexing technique to precisely locate the external vibration points. The signal-to-noise ratio of the demodulated vibration signal is approximately 30 dB according to the principle of interference between the reflected light of the grating and the back Rayleigh scattering light of the two sides of the fiber. Herein, we use a fiber optic cable with a 5-m grating spacing to demodulate the vibration having a frequency of 10--90 Hz, and the demodulated results prove the excellent frequency response characteristics of the system. Meanwhile, the alarm algorithm of the proposed perimeter security alarm system is verified. The test results prove that the false alarm rate of the system is less than 10% and that the missing alarm rate is 0. The developed system has a maximum detection length of 50 km, which is considerably larger than those of the existing optical fiber perimeter products, and the demodulation method is simple and cheap; therefore, it has a broad application potential.
PDF Chinese Journal of Lasers, 2020, 47(4): 0406001
Traditional iterative signal clipping (ISC) techniques can reduce the effect of nonlinear clipping noise in nonlinear LED optical-orthogonal frequency division multiplexing (O-OFDM) systems. However, the ISC system requires multiple LEDs to emit light at the same time, which is complicated in synchronization and hardware applications, as the bit error rate (BER) increases with the channel gain difference. An O-OFDM symbol decomposing serial transmission system based on single LED is proposed in this study. The O-OFDM symbol is decomposed by amplitude clipping, followed by serial framing, and it is finally input to single LED. The expression of the system theoretical signal-to-noise ratio is derived and the simulation models using the Monte Carlo bit error rate and the error vector magnitude (EVM) are built. Results demonstrate that the performance of EVM and BER enhance significantly as the number of symbols decomposing increases; however, the communication rate slightly decreases, and the direct current biases will affect the performance of EVM and BER. Furthermore, the proposed system is simple to implement and avoids the problem of BER degradation caused by the channel gain difference.
PDF Chinese Journal of Lasers, 2020, 47(4): 0406002
Laser devices and laser physics
In the preparation of oxide confinement vertical-cavity surface-emitting lasers, selective internal corrosion often occurs when GaAs/AlGaAs is etched, which directly affects the subsequent oxidation process and electrode passivation. To address this problem, the wet and the dry etching processes are thoroughly investigated in this study. Results show that the hollows caused by selective internal corrosion can be effectively eliminated by adjusting the volume ratio of the etching solution and bottom electrode radio frequency (RF) power of the inductively coupled plasma (ICP) etching. We obtain good steepness and smoothness in the wet etching process when the volume ratio of the H3PO4-H2O2-H2O solution is 1:1∶10. In the ICP dry etching process, the dynamic balance of the chemical and physical etching in the chamber is adjusted by changing the RF power of the bottom electrode; the hollow phenomenon is hardly observed, and the sidewall steepness is greater than 80° at a power of 100 W.
PDF Chinese Journal of Lasers, 2020, 47(4): 0401001
In this study, a Kirkpatrick-Baez (KBA) X-ray microscope is developed to diagnose laser-produced plasmas and research the inertial confinement fusion. The microscope comprises two sets of bimirrors. Herein, the ray-tracing program for grazing incident reflectors is studied. The KBA reflector is designed by using this program. It is found that the bimirror would be compact if the aperture stop is placed on the second mirror. Furthermore, the influences of field curvature and pitch angle of KBA microscope on the image quality are studied, and the optimal defocusing distance and the pitch angle error of the system are determined.
PDF Chinese Journal of Lasers, 2020, 47(4): 0401002
In this study, a step-detection method for the time synchronization of short pulses, based on spatial-spectral interference and far field, is proposed. A large range of time synchronization is first captured using the spatial-spectral interference, and the high-precision detection of which is then realized using far field. The proposed method is then simulated and analyzed. Results show the feasibility of this proposed method, suggesting that it exhibits advantages such as a large range and high precision. Moreover, the low requirement of energy makes it suitable for a short-pulse laser system as most of the energy can be applied to physical experiments. Furthermore, it can be used for the measurement and control of the time synchronization in plasma parameter diagnosis, coherent beam combination, and other fields.
PDF Chinese Journal of Lasers, 2020, 47(4): 0401003
The fiber lasers operating in a radiation environment will suffer from radiation damage, which will reduce the output power and generate radiation-induced heat. This will affect the normal application of the lasers and the safety of the system. In this study, we intend to establish the relation between the radiation environment characterization parameters and the power, thermal effect, and temperature distribution with respect to the gain fiber of the fiber lasers. Subsequently, we conduct a numerical simulation with respect to the power characteristics and thermal effects of the γ-ray radiated double-clad fiber lasers. Additionally, we study the effects of the radiation-induced loss, radiation dose, fiber length, and pumping style on the output power of the lasers. Furthermore, we analyze the thermal effects of the fiber lasers under different pumping styles. The results of this study could provide a reference to study the radiation effects of the fiber lasers.
PDF Chinese Journal of Lasers, 2020, 47(4): 0401004
We use inductively coupled plasma (ICP) etching equipment to study the etching process for GaAs/AlGaAs materials used in vertical-cavity surface-emitting lasers. During the ICP etching process, photoresist is used as the etching mask, whereas Cl2/BCl3 is used as the etching gas. The ICP antenna power, radio frequency bias power, and cavity pressure in case of the GaAs/AlGaAs materials and masks are analyzed and summarized through experiments. Further, scanning electron microscopy is used to investigate the effects of different parameter conditions on the verticality and bottom flatness of the pattern sidewalls. Finally, a round-table structure with smooth sidewalls and flat bottom is obtained by adjusting and optimizing the parameters during each process to ensure a high etching rate.
PDF Chinese Journal of Lasers, 2020, 47(4): 0401005
In this work, we introduce a high-band-gap GaAsP between the barrier layer and the waveguide layer on both sides of the active region to improve the output power and reliability of a semiconductor laser diode. The leakage of carriers in the active region is suppressed, and the device''s performance is greatly improved. Results show that the characteristic temperature of the device increases from 150 to 197.37 K (-75.76 ℃) in the temperature range of 10--40 ℃, and the temperature-dependent drift coefficient of peak wavelength is 0.207 nm/℃. The maximum output power of a 9XX-nm laser diode with a strip width of 200 μm and cavity length of 2000 μm is as high as 14.4 W. The device achieves a maximum electro-optical conversion efficiency of 71.8% for an injection current of 7 A; the slope efficiency is 1.21 W/A. An accelerated aging test of the device at constant current shows that the laser diode has a reliable operating life of over 20000 h.
PDF Chinese Journal of Lasers, 2020, 47(4): 0401006
In full-aperture annular polishing, material removal rate increases from the center toward the edge of square optical, which leads to a collapse angle surface profile and affects the precision of the final surface. Several types of element motion trajectories were studied herein, and the material removal rate distribution of a full-aperture surface was calculated based on the Preston equation. Results show that the square optical element moves along the radial direction or in direction of a certain angle with respect to the radial direction while maintaining its original rotation, and the additional speed of individual points on the entire polishing surface remains consistent, making the material removal rate distribution of square optical element more uniform. Therefore, the collapse angle surface profile of the square optical element can be effectively controlled by the additional motion trajectories of elements.
PDF Chinese Journal of Lasers, 2020, 47(4): 0402001
The technique of laser surface remelting was carried out at different power parameters on an A356 aluminum alloy surface to analyze the microstructure and microhardness of the remelting layer. A three-point bending test was then used to study the mechanical properties of the remelting layer as well as the bonding properties between the layer and the substrate. Compared with that of the substrate, we found the microhardness of the remelting layer greatly increased. Regarding other properties, both the fine grain strengthening and second phase dispersion strengthening are found to increase the strength of the remelting layer, which delays the appearance of the initial cracks in remelting layer. The remelting layer and substrate had good metallurgical bonding ability and no cracks appear at the interface. The remelting samples also show favorable surface mechanical properties with no delamination appearing between the remelting layer obtained at laser power of 1500 W or 2000 W and the substrate.
PDF Chinese Journal of Lasers, 2020, 47(4): 0402002
In laser cutting, the auxiliary gas and laser beam axis are typically coaxially arranged to ensure the same cutting quality in each cutting direction. It is found that when the axes of the auxiliary gas and laser beam are not coaxial (i.e., in an off-axis state), the cutting efficiency can be effectively improved. However, the influence mechanism of off-axis state on the cutting process remains unclear. This study established a three-dimensional symmetrical model of off-axis laser cutting with cut slits and simulated the laser cutting process with nitrogen as the auxiliary gas using the finite element method. By changing the off-axis amount, the structure of the airflow field structure of the auxiliary gas is analyzed, meanwhile, the influence of the off-axis amount on the dynamic performance of the auxiliary gas was investigated during the laser cutting process. In addition, the influence mechanism of the off-axis amount on the laser cutting was clarified by analyzing the simulation results, and it was verified in a cutting experiment. Results show that the off-axis amount affects the dynamic performance of the auxiliary gas, and the appropriate off-axis amount can effectively improve the quality of laser cutting.
PDF Chinese Journal of Lasers, 2020, 47(4): 0402003
In this study, AISI304 powder processed with solution-nitriding in super pure nitrogen flow as raw material was used to prepare austenite stainless steels cladding layer with different nitrogen contents to obtain the laser cladding layer of nitrogenous austenitic stainless steel with excellent comprehensive performance. Results show that nitrogen precipitates during laser cladding process and pores appear in the cladding layer. As the nitrogen content in powder increases, the nitrogen content in the cladding layer gradually increases,the porosity first increases and then decreases and the strengths significantly enhances, while the elongation gradually decreases. For the cladding layer prepared with power with nitrogen mass fraction of 0.41%, its nitrogen mass fraction is 0.25%, tensile strength is (856±20) MPa, yield strength is (747±15) MPa, elongation is (22±3)% and microhardness is 280.68 HV0.2. For the cladding layer prepared with power with nitrogen mass fraction of 0.16%, its corrosion potential and corrosion rate are 0.0777 V and 1.6198×10 -5 g/(m 2·h) respectively, which are higher than that of the AISI304 cladding layer.
PDF Chinese Journal of Lasers, 2020, 47(4): 0402004
To enable efficient and uniform ablation of stains and cleaning of targets using laser in the two-dimensional plane, a laser spiral filling path is designed by changing the laser progressive scanning mode in this study. By controlling the galvanometer motor, the laser is deflected to distribute the ablation in a network shape over the plane. At any ablation point in the laser filling field, compared with the line-by-line ablation, the results confirmed a longer cooling time and a weaker laser thermal accumulation during spiral ablation. The line-by-line scanning speed is the motor speed of a single galvanometer, whereas the spiral scanning speed is the vector speed of two motors. Therefore, the effect of laser spiral cleaning is more uniform and effective as compared with the cleaning in the line-by-line scanning mode.
PDF Chinese Journal of Lasers, 2020, 47(4): 0402005
In this study, Ni60A-based alloy coating was prepared on a 42CrMo alloy structured steel substrate surface using laser cladding strengthened by follow-up feed pulse current. Moreover, the effect of pulse current on laser cladding coating is investigated. Results demonstrate that the follow-up feed electrode can increase the utilization efficiency of the pulse current by exploiting its skin effect, directing most of the current into a melting pool. However, the relatively close electrode distance can significantly increase current density and strengthen the improvement effect, which is beneficial for preparing high-quality cladding coatings with few defects. Besides demonstrating higher efficiency than conventional laser cladding, the pulse current-strengthened laser cladding also refines the crystal grain size of the cladding layer, reduces porosity, and increases the uniformity of microstructures in the cladding layer. Hence, dendritic crystals in conventional laser cladding are transformed into isometric crystals. The porosity of the pulse current-strengthened laser cladding is 0.39% and the average grain size is approximately 4 μm, which are better than the corresponding values of 0.62% and 6 μm, respectively, for a conventional laser cladding. Moreover, the residual thermal stress and cracking sensitivity on the cladding layer are lowered by using the follow-up feed electrode. In particular, the streaming effect and Joule thermal effect produced by the unique high-energy density pulse current across a small electrode distance at the crack tip of the cladding layer can facilitate self-healing of cracks in the cladding layer. Therefore, a crack-free cladding layer can be obtained. Thus, the proposed method solves the easy-cracking problem of the laser cladding layer.
PDF Chinese Journal of Lasers, 2020, 47(4): 0402006
The weld morphologies, droplet transfer, and spatters of laser pulsed-arc hybrid welding were studied using high-speed photography, macro-metallograph, and welding electrical signal acquisition. Results show that laser pulsed-arc hybrid welding can improve the welding morphologies. The weld morphologies were improved. The welding current, the arc voltage, and the droplet transfer mode were changed significantly during the laser pulsed-arc hybrid welding. The liquid bridge formed between the droplet and the molten pool as well as the wire and the droplet explored under the action of excessive electromagnetic contraction force, and the droplets on the surface of molten pool exploded under the action of the arc force and expansion of gases, so the welding spatters were produced. Spatters are significantly reduced during laser pulsed-arc hybrid welding, mainly because the pulse arc reduces the short circuit transfer current and pool oscillations.
PDF Chinese Journal of Lasers, 2020, 47(4): 0402007
The thickness of cladding layers on free-form surface tends to be nonuniform owing to an instable overlapping ratio in the laser cladding process. In this paper, a trajectory generation algorithm for a laser cladding with an equivalent overlapping ratio on a free-form surface is proposed to solve this problem. A depth camera was used to collect the point cloud data on the whole surface. A point cloud beam was fitted as initial cladding layer, and a processing point set was obtained using the equidistant step method. The contour along the vertical direction of the cladding layer was identified by cross-section slicing, and equidistant points of processing points were fitted for ensuring the equality of the overlapping ratio. The next processing trajectory was fitted from the obtained equidistant point set until it traversed the entire surface. The posture of a cladding nozzle was determined along the normal vector direction of the processing points to keep the axis of the nozzle perpendicular to the machined surface. The results of the surface cladding experiment conducted on a hemispherical end socket reveal that cladding layer with equal overlapping ratios can be obtained by combining the equidistant trajectory algorithm and posture adaptation of the nozzle. In addition, the cladding layer on the free-form surface exhibit uniform thickness and high microhardness and have no obvious defects such as inter-channel bulging or depression.
PDF Chinese Journal of Lasers, 2020, 47(4): 0402008
The railroad switch is an important part of turnouts. The surface of the switch rail could be easily worn and peeled because of its poor working condition. Laser cladding technology can significantly improve the surface hardness of the railroad switch, and the wear and surface damage resistance as well as service life are accordingly improved. A Fe-W-Cr composite coating was successfully prepared on the surface of U71Mn railroad switch using high-power semiconductor laser cladding technology. The microstructures, phase formation and elemental distribution of the coating were tested, and the hardness, impact toughness, and friction and wear properties of the coating were analyzed. Results show that there is no pore, crack or other defect in the coatings, and the metallurgical bonding between the coatings and the substrates are effectively established. The dendrite structures exist in most of the region on the coating, and networks of carbides are distributed around the grain boundaries. The average hardness of the coatings is 876.8 HV (the hardness of railroad switch rail is 252.3 HV), the impact toughness is 2.30 J/cm 2, and the friction coefficient is 0.31(the friction coefficient of railroad switch rail is 0.63). Under the same friction and wear condition, the wear loss of the coating is 0.0043 g, which is only 10.54% of the substrate (the wear loss of substrate is 0.0408 g). The hardness and wear resistance of the railroad switch are significantly enhanced using laser cladding, effectively improving its service life.
PDF Chinese Journal of Lasers, 2020, 47(4): 0402009
The high-quality welding of aluminum alloy/steel dissimilar alloys is difficult owing to the great variation in their physical and chemical properties. In this work, the laser beam wobble welding process is adopted to achieve good connection between 6061 aluminum alloy and 316L stainless steel. Moreover, the effects of laser wobble mode and wobble frequency on weld formation characteristics, microstructure, and tensile-shear strength have been investigated. The results indicate that laser wobble welding can increase interfacial bonding area, decrease weld penetration, and effectively suppress weld pores and cracks. Laser wobble can make the interfacial temperature uniform and enhance the stirring effect on the weld pool; it also alleviates metallurgical reactions and suppresses the formation of brittle intermetallic compounds. The interfacial microstructures in a welded joint obtained with wobble laser mainly comprise Fe(Al) solid solution and a small amount of FeAl3 phase; the maximum tensile-shear strength of the joint obtained with the wobble laser can reach 117.5 N/mm, which is 45% more than that of conventional laser welding. Welded joint fractures are also observed at the interface between stainless steel and the weld metal.
PDF Chinese Journal of Lasers, 2020, 47(4): 0402010
To study the effects of cryogenic laser peening (CLP) on the damping characteristics and vibration fatigue life of TC6 titanium alloy, laser peening experiments at room temperature (25 ℃) and cryogenic temperature (-130 ℃) are performed in this study. Then, the microstructures of the samples prior and subsequent to laser peening are observed using transmission electron microscopy (TEM). The damping ratios of the samples prior and subsequent to the experiment are tested using the frequency response method. The vibration fatigue test of laser-peened samples is performed using the hydraulic vibration fatigue test system. The influence mechanism of CLP treatment on damping characteristics of the samples and its relationship with fatigue life are analyzed. Finally, the vibration fatigue fractures of the samples are observed using scanning electron microscopy (SEM). Results show that the CLP treatment can produce a large number of dislocations and deformation twins on the surface of the samples, effectively improving the damping ratio and vibration fatigue life of TC6 titanium alloy.
PDF Chinese Journal of Lasers, 2020, 47(4): 0402011
Low cost nanosecond lasers can be used to fabricate various micro and nanostructures on metal surfaces. After chemical modification, superhydrophobic surfaces with different water adhesion are obtained. However, only few studies are reported on the droplet impact performance of these superhydrophobic surfaces. Therefore, we investigate the droplet impact performance of the typical superhydrophobic surfaces fabricated by nanosecond laser with different pulse duration. Results indicate that the superhydrophobic surfaces fabricated by different pulse duration nanosecond lasers exhibit extremely short solid-liquid contact time when droplets are impacted. Under the same parameters, superhydrophobic surface fabricated by shorter pulse duration laser has higher microstructures, which results in an increase in the solid-liquid adhesion force during droplet impact and a relatively long solid-liquid contact time.
PDF Chinese Journal of Lasers, 2020, 47(4): 0402012
In this study, a 550-W single-frequency all-fiber laser with near diffraction-limited beam quality is achieved by simultaneously suppressing the stimulated Brillouin scattering and mode instability effects with a long-tapered gain fiber and employing the ability of high mode instability threshold of 1030-nm signal laser. The slope efficiency of the whole amplifier is observed to be as high as 80% and the measured beam quality M2 is approximately 1.47 at the maximum output power. Further brightness scaling is limited owing to the mode instability effect.
PDF Chinese Journal of Lasers, 2020, 47(4): 0415001
In order to accurately measure the characteristics of attosecond pulses, we have independently developed a set of attosecond streaking camera with high energy resolution. The device adopts an electronic time-of-flight spectrometer with a magnetic bottle structure and an electronic flight distance of up to 2 m. Based on the above design, the spectrometer has high energy resolution and collection efficiency. The stability accuracy of delayed scanning of NIR femtosecond pulses and XUV attosecond pulses is smaller than 20 as (root-mean-square) in optical system of the device. A double optical gating technology was used to shape the optical electric field of the femtosecond pulses, and an isolated attosecond pulse was generated in the Ne gas cell. The attosecond streaking spectrogram was obtained by the attosecond streaking camera. An isolated 159-as attosecond pulse was achieved through phase retrieval by omega oscillation filtering (PROOF).
PDF Chinese Journal of Lasers, 2020, 47(4): 0415002
Basis dependence and statistical fluctuation of light sources are problems for the measurement-device-independent quantum key distribution protocol based on heralded single-photon source (HSPS). To solve these problems, in this work, the quantum key distribution protocol based on HSPS in Poisson distribution and orbital angular momentum (OAM) was studied. Moreover, its statistical fluctuation was analyzed. The relationship among the transmission efficiency, key generation rate, and safe transmission distance of the protocol under symmetric and asymmetric channels was examined. Furthermore, the effect of statistical fluctuation on the key generation rate and transmission distance was simulated. Simulation results show that the problem of basis dependence is solved by OAM coding, and the key generation rate and transmission distance of the protocol are improved. The effect of statistical fluctuation on the key generation rate of the protocol increases with the transmission distance. For the same number of pulses, the key generation rate and safe transmission distance under the asymmetric channel are greater than those under the symmetric channel.
PDF Chinese Journal of Lasers, 2020, 47(4): 0412001
Holography and information processing
In this study, we investigate the axicon-focusing characteristics of a cylindrical vector beam based on the Rayleigh-Sommerfeld vector diffraction theory and the amplitude transmittance function. The results prove that the vector focus field of a radially polarized beam comprises radial and longitudinal components, whereas that of an azimuthally polarized beam only comprises an azimuthal component. Further, the focus depth of the focus field increases with the increasing waist radius of the beam. When compared with the ideal case, the focal depth of the focus field remains almost unchanged when the axicon apex is hyperbola; however, its intensity oscillates because of the interference effect, which is dependent on the waist radius of the incident beam. This oscillation phenomenon disappears when the waist radius of the radially and azimuthally polarized beams are 4 mm and 3.5 mm, respectively. The obtained results provide a reference for the practical application of the axicon systems and the improvement of the theory of cylindrical vector beam focusing.
PDF Chinese Journal of Lasers, 2020, 47(4): 0409001
In this study, we compare the wavefront reconstruction error (WRE) of the slope zeroing and subaperture removal recovery methods for the lack of light in subapertures. For lack of light in a single subaperture, results show that the WRE of the slope zeroing method increases with the increase in the radius of the subaperture center relative to the center of the full aperture. The WRE owing to the lack of light in the subaperture in the central part of the aperture is small, whereas this error along the edge of the aperture is large. In contrast, the WRE of the subaperture removal method owing to the lack of light in the subaperture is observed to be uniform and small in the aperture. The WRE increases with the increase in the number of subapertures lacking light. In addition, we investigate and compare such cases of multiple subapertures that lack light. Results show that although the calculation of the subaperture removal method for lack of light in subapertures is complex, the WRE of the subaperture removal method is much smaller than that of the slope zeroing method.
PDF Chinese Journal of Lasers, 2020, 47(4): 0409002
To obtain three-dimensional full-field measurements of the surface defects of optical elements, a method of digital holographic microscopic scanning imaging is proposed in this study. Based on the numerical reconstruction algorithm of digital holography angle spectrum, the phase distribution of scratches on the surface of optical elements is obtained, and the whole-field measurement of scratches is obtained using the principle of scanning and splicing. Then, a two-dimensional precision scanning component is developed based on the digital holographic micro-experimental device. For the standard scratch having a width of 50 μm and a depth of 50 nm, we measure a width of 49.2 μm and a depth of 48.9 nm. Next, the full-field three-dimensional morphology of the scratch is obtained by splicing. The experimental results show that the defect panorama with larger scale can be obtained by phase stitching. The relative errors of width and depth are 1.6% and 2.2%, respectively.
PDF Chinese Journal of Lasers, 2020, 47(4): 0409003
Biomedical photonics and laser medicine
In this work, by combining a polarization beam-splitter prism and a wave-plate, four sinusoidal signals with a phase difference of 90° were generated. The wave-plate was used for synchronous phase delay. The operation range of the diffraction grating displacement sensor was increased to one-half of the coherence length of the laser. By differentially processing two signals with a phase difference of 180°, it is possible to avoid the effects of the laser power stability, background light, and the temperature drift of the operational amplifier. Finally, an interpolation circuit was used to construct the arctangent function of the output signal to realize the nonlinear A/D conversion of the sinusoidal function and a displacement resolution of 2.54 nm. In the future, this type of high-resolution and large-range displacement sensor will promote the development of automation equipment, electronic manufacturing, and the industrial intelligence field.
PDF Chinese Journal of Lasers, 2020, 47(4): 0403002
In this study, we propose an adaptive optics closed-loop control model based on the far-field index gradient, which can be used to stabilize the response matrix based on the recursive least square values. Further, the current system state can be rapidly self-learned using the far-field index gradient. The experimental results denote that the proposed model exhibits real-time online update characteristics; furthermore, the proposed model can adapt to the state of H-S subaperture lack of light or non-ideal centroid detection, which improves the control performance to some extent.
PDF Chinese Journal of Lasers, 2020, 47(4): 0405001
Based on the transmission characteristics of surface plasmon polaritons and the optical properties of periodic photonic crystals, a hybrid structure of sub-wavelength dielectric grating-metal Ag thin film-periodic photonic crystal is proposed, which produces discrete state in sub-wavelength dielectric grating-metal Ag thin film structure and continuous state in periodic photonic crystal structure. Through the theoretical analysis and transmission characteristics of the structure, the generation mechanism of Fano resonance in the structure is expounded, the Fano-resonance sensing structure model based on angle modulation is established, and the influence of the structural parameters on the reflection spectrum curve is quantitatively analyzed. Results show that when the periodic photonic crystal periodic layer number is N=4, the grating period is Λ=258 nm, and the Ag-film thickness is d0=27 nm, the quality factor FOM value of the structure can be as high as 2.11×10 4, and the angular sensitivity is S=40 (°)/RIU. The proposed structure provides an effective theoretical reference for achieving Fano resonance in a sub-wavelength dielectric grating structure, and it has certain guiding significance to the design of the optical refractive index sensing structure.
PDF Chinese Journal of Lasers, 2020, 47(4): 0413001
Remote sensing and sensors
Aiming at the problem of low accuracy in semantic segmentation of three-dimensional laser point clouds in road scene, an end-to-end multi-feature point clouds semantic segmentation method based on convolutional neural network is proposed. Firstly, the feature images such as point cloud distance, adjacent angle and surface curvature are calculated based on spherical projection to apply to convolutional neural network; then, a convolutional neural network is adopted to process multi-band depth images to obtain pixel-level instance segmentation results. The proposed method combines traditional point cloud features with the deep learning method to improve the result of point cloud semantic segmentation. Using KITTI point cloud data set test, simulation results show that the multi-feature convolutional neural network semantic segmentation method has better performance than other semantic segmentation methods without combining with point cloud features such as SqueezeSeg V2. The precision obtained with proposed method for car, bicycle and pedestrian segmentation is 0.3, 21.4, 14.5 percentage points higher in comparison with the SqueezeSeg V2 network.
PDF Chinese Journal of Lasers, 2020, 47(4): 0410001
In this study, non-hydrogenated oxygen-doped diamond-like carbon (DLC) films were grown by femtosecond laser, and the influence of oxygen atmosphere on the infrared (IR) properties of the films was investigated. The evolution of the properties of oxygen-doped DLC films at the microscale was determined by investigating their doping content, atomic bonding, and crystal structure. The results show that the oxygen atmosphere increases the diamond-phase content and reduces the absorption of the DLC films, thus increasing their IR transmittance. The refractive index of the films can be freely controlled by the oxygen atmosphere, which provide a suitable method for the design of multilayer optical films. The oxygen in the environment does not change the amorphous structure of the DLC films and does not hinder their IR transmittance. A model is proposed for the rearrangement of the atomic bonds in the carbon films in the oxygen atmosphere, which is of benefit to the research on oxygen-doped DLC films and provides theoretical analysis and a practical basis for improving the anti-reflective protection of IR windows by DLC films.
PDF Chinese Journal of Lasers, 2020, 47(4): 0410002
The existing laser-correlated imaging echo based on full waveform sampling experiences the problem of requiring a large amount of collected data for echo, and the limitation of the sampling rate to ranging accuracy and resolution. We propose a new ghost imaging technology based on time-over-threshold (TOT) technology to address this problem. A time width or peak inverse method based on the TOT technology is proposed to obtain the intensity information of the echo signal. The influences of the threshold choice of TOT, laser pulse width, and the measurement error of the ghost imaging reconstruction performance are also studied. Numerical simulation and experimental results demonstrate that the proposed technology can achieve laser correlation imaging. When the threshold is set to 35% of the maximum echo signal, the laser width is wider than 30 sampling intervals, and the root-mean-square error of the measurement error is not greater than 1 sampling interval, the reconstruction quality can be guaranteed. In addition, the method of obtaining the intensity information of echo signal based on peak inverse is more accurate than the time width method based on TOT.
PDF Chinese Journal of Lasers, 2020, 47(4): 0410003
Target alignment and beam-target coupling technologies are the most critical technologies in laser fusion experiments, and they are the important with respect to the success of the laser fusion experiment. This study reviews the technical solutions pertaining to the target alignment and beam-target coupling of several representative high-power laser facilities from the 1970s to the present, discusses the basic principles of the technical solution design, summarizes the advantages and defects of different facilities'' solutions, and suggests to some new methods.
PDF Chinese Journal of Lasers, 2020, 47(4): 0400001