Laser technology and application
2019, 48(S1): 1-5.
doi: 10.3788/IRLA201948.S106001
Laser systems often cope with complex ambient air flow. As the size of the laser beam expanding system increases, its crystal window is difficult to achieve. Then the ambient air flow can easily enter the system, thus it affects the beam quality. CFD was used to solve the problem of aero-optical effects caused by ambient air flow. With the aid of fluid mechanics software FLUENT CFD model, the internal flow field of the laser beam expander was simulated. Various parameters of the internal flow field of the system were obtained under different inlet angles. Through the Gladstone-Dale relationship, the refractive index could be calculated from the flow density field. Using the ray tracing method, the beam transmission path could be got in the non-uniform refractive index flow field. Finally, the optical aberration caused by turbulence was calculated by numerical analysis. The results show that the ambient airflow will introduce more vortices to the flow field of the primary mirror, the secondary mirror and the deflecting mirror. The influence of the optical effect cannot be ignored, which must be considered in the overall design. Therefore, a method to increase the length of the mirror tube of the beam expanding system was proposed to reduce this effect. After the length of the mirror tube was lengthened by 0.5 m, the eddy current in the beam expanding system can avoid the main path of beam propagation. Meanwhile, the RMS can be reduced from the 0.317m to about 0.078m. The study proves the necessity of avoiding the aero-optical effect caused by ambient airflow. And it provides reference data and ideas for the optical and structural design of the beam expanding system.
2019, 48(S1): 6-14.
doi: 10.3788/IRLA201948.S106002
Laser ablation driven is one of the most promising active debris removal technologies. However, the rotation state of the target has a very complicated impact on the effect of laser ablation, which is one of the problems to be solved. In this paper, the laser driving impulse under the continuous irradiation for rotating plate target was analyzed by theoretical calculation, and the effect of pulse frequency and target rotation speed on it was also discussed. And with the surface triangulation and three-dimensional reconstruction calculation method, the magnitude and direction of laser driving impulse under the continuous irradiation for rotating typical shape target, such as cylinder, cuboid and cone, and irregular shape target were analyzed under different pulse frequency and different rotational angular velocity. The results show that the rotation of the target will make the driving effect of continuous pulse laser change periodically with the increase of pulse number; Its magnitude and periodic variation are affected by the rotation speed, pulse frequency and the shape of itself; By the rational selection of laser irradiation strategy, the influence of rotation on the impulse direction of laser pulse continuous irradiation can be minimized or eliminated.
2019, 48(S1): 15-20.
doi: 10.3788/IRLA201948.S106003
The temperature stability of the vapor cell plays an important role in improving the performance of the nuclear magnetic resonance gyroscope. It's meaningful to explore an approach for temperature stabilization of the vapor cell based on the light intensity of nuclear magnetic resonance gyroscope's probe laser. The negative correlation between the light intensity of the probe laser and the temperature of the vapor cell was analyzed theoretically. Then the concrete corresponding fitting curve about temperature and the light intensity was obtained experimentally. Furthermore, a LabView program for the feedback control of the temperature was designed, which achieved the temperature stabilization of the vapor cell. Finally, the experimental results validated the proposed approach in stabilizing the temperature of the vapor cell, which had advantages of high sensitivity and accuracy. The achieved long-term temperature drift is within 0.02℃/h.
2019, 48(S1): 21-26.
doi: 10.3788/IRLA201948.S106004
A high integration density front-end readout circuit(ROIC) was presented and designed in allusion to the kind of light laser detection and ranging(LiDAR) using avalanche photodiode(APD) detector. The front-end readout circuit consisted of high-gain broadband transimpedance amplifier (TIA), fixed gain voltage amplifier, high-speed real-time comparator, low voltage differential signaling (LVDS)output interface et al. TIA adopted resistance feedback structure. The output voltage signal of TIA connected to next stage through the way of pseudo-difference signaling, therefore obtaining better anti-interference ability. Fixed gain voltage amplifier was a non-inverting amplifier with resistance feedback network. Then a decompensated open-loop amplifier was used for the comparator in the chip. The ROIC can connect with FPGA directly profiting from the LVDS, again improving the integration density of the chip. The circuit was designed with chrt 0.18 m process. The area of chip was about 0.9 mm1.9 mm. The voltage of the power supply to the chip was 3.3 V. Through the test, the gain of TIA can achieve 83.6 dB, the bandwidth can reach 120 MHz. At the same time, the whole readout circuit can make an immediate response for input pulse signal with 5 ns pulse width. The LVDS which was designed as the output of whole readout circuit can meet the request of FPGA's interface.
2019, 48(S1): 27-34.
doi: 10.3788/IRLA201948.S106005
For stainless steel-carbon steel sector laminated plates, the laser concentric circular scanning method was used to bend the plates. The geometrical characteristics measurement of deformed sample were measured by laser scanner. The bending angle of each position of the laser scanning lines and the deformation characteristics of the curved surface on both sides of the scanning line were analyzed. The effects of laser processing parameters and sheet geometry parameters on laser arc scanning bending of sector laminates were studied. The results show that deformation zone of the scanning line after laser circular scanning is three-dimensional curve in space and the bending angles at the scanning line are different. The angle in central line is the smallest and increases gradually to the left and right sides of the plate, reaching the maximum at the edge of the plate. The angle is basically symmetrical along the center line of the plate; The surface is obviously deformed by a curved surface and the area from the scanning line to the outer circular line of the sector plate is inwardly recessed, and the area from the scanning line to the inner circular line of the sector plate is convex outward. The research results provide a basis for in-depth understanding of the three-dimensional deformation mechanism of laser circular scanning.
A comprehensive study of coherent coupling of low-amplitude spatial solitons that co-propagate in biased photorefractive crystals with both the linear and quadratic electro-optic effects was presented. Our results were shown that coherently coupled bright-bright and dark-dark spatial soliton pairs in the low-amplitude regime could be formed under appropriate conditions. The evolution equations, analytic solutions and the expressions of soliton pair widths of these low-amplitude coherently coupled spatial soliton pairs were obtained. It was proven that the existence, property and propagation of these soliton pairs due to co-effects of both the linear and quadratic electro-optic effects was greatly influenced by the photorefractive effects which could be enhanced, weakened or even counteracted because of the interaction of these two electro-optic effects. Moreover, the effects of three physical factors, i.e., the initial phase difference between two incident coherent beams, the intensity ratio and various external bias field on the existence conditions, properties of these low-amplitude coherently coupled soliton pairs have been discussed in detail. Finally, the self-deflection of low-amplitude bright-bright soliton pairs had also been investigated by means of perturbation procedure.
2019, 48(S1): 43-49.
doi: 10.3788/IRLA201948.S106007
Atmospheric aerosol, a main pollutant source, mainly exists in planetary boundary layer, and the precise detection of its vertical distribution, especially in the near ground, has an important application value. Backscattering lidar is a powerful tool for aerosol extinction coefficient profile detection, but no signals or only weak signals could be received in near distance because of the blind zones and transition regions. The above problems were resolved by combining CCD detection with backscattering lidar, and the aerosol extinction to backscattering coefficient (lidar ratio) was retrieved accurately with the combined measurement of Raman scattering channel. In order to validate CCD detection, two different CCD lidars with different CCD position distance and focal length were designed respectively for comparison. The relative difference of aerosol extinction coefficients from two CCD lidars was less than 3% under about 1.2 km height, which indicates that the detection results are in good agreement. Case study shows that aerosol extinction coefficient near the ground is inhomogeneous with height, and sometimes increases with time and sometimes decreases.
2019, 48(7): 706001.
doi: 10.3788/IRLA201948.0706001
A new all-weather outdoor Raman-Mie scattering lidar system was developed and used in the atmospheric aerosol pollution monitoring for the continuous observation of atmospheric boundary layer, tropospheric aerosol and cloud optical characteristics. In lidar system design, this lidar system adopted the existing mature technology of Mie scattering, polarization and Raman lidar remote sensing, and its structure was compact and convenient for transportation. In lidar control design, it was easy to operate with one-touch button and the system was available with manual and automatic working modes as well as network control and data transmission functions. The automatic and manual retrieval software were applied in the lidar data processing. The former can automatically process and display the observed results in real time according to the system settings, while the latter can retrieve and display the accurate observation data according to the parameters set by the user. This lidar system has achieved the requirements of production application and it can be widely used in the fields of atmospheric environment monitoring and atmospheric science.
2019, 48(7): 706002.
doi: 10.3788/IRLA201948.0706002
According to the requirements of surface microstructural manufacture, the ablation effect of ultrashort pulses on metal materials was studied. By using the double temperature equation, a multi-pulse irradiation analysis model was established. And then the temperature variations rule of single-pulse and multi-pulse ablation metal iron were quantitatively calculated. The results show that some key parameters, such as laser fluence, pulse width, and pulse interval, have noticeably influence on the electron/lattice temperature variation. Finally the number of pulses that needed for the material to reach the ablation threshold under different laser fluence is plotted, which provides a theoretical basis for the laser control in the process.
2019, 48(7): 706003.
doi: 10.3788/IRLA201948.0706003
Based on high intensity laser systems theory and self-focusing research, the nonlinear medium under the beam transmission process was described using the beam propagation method and ray tracing method. Besides, it was found that some ways were suitable for nonlinear medium, such as Adams method under the ray tracing method and recovery algorithm based on gradient information of intensity distribution. Finally, the simulation results revealed the relationship between the light intensity and these factors, including lens thickness, the refractive index of medium, the lens curvature radius and the radius of the incident beam. And combined with optical design software, it not only reflected the relationship between the product of the refractive index and the light intensity and the focus position changes, but also the results of image quality intuitively through the data of spot diagram. And two kinds of main optical methods, the ray tracing method and the beam propagation method, were made for the comparative analysis. According the simulation results, it could be determined that bits of the system was easily affected by self-focusing, so the size of certain parameters could be improved, the adverse effect could be reduced and eliminated, and an optimal method which was suitable for strong laser system could be found.
2019, 48(7): 706004.
doi: 10.3788/IRLA201948.0706004
The finite element method was employed to simulate the dynamic stress waves and residual stresses induced by laser impacting the cylindrical surface of round rod made of 2024 aluminum alloy. During simulation, the code ABAQUS/Explicit was firstly utilized to simulate the process of the stress wave propagation in the rod with the diameter 16 mm, which was induced by the laser shock wave with peak value 2 GPa. Subsequently, the other code ABAQUS/Standard was used to calculate further the residual stresses distributed in the cylindrical surface of rod. Based on the simulation results, the effects of the rod diameter on the attenuation of the peak pressure of the stress wave and the residual stress distribution were investigated. Corresponding experiments were carried out to validate the calculated results as well. The results indicate that the peak pressure of the stress wave induced by the 2 GPa shock wave decreases rapidly to 250 MPa within 400 ns in the round rod with diameter 16 mm. After laser shocking, an uneven residual stresses are distributed in central region of the impacted surface, and tensile residual stresses are formed at the center, where the value of residual stresses S11 reaches 42 MPa. While compressive residual stresses are formed in other impacted areas(radius from 0.5 mm to 1.5 mm), where the compressive residual stresses S11 are roughly 250 MPa. During the propagation of the stress wave, the decay rate of its peak pressure decreases with the enlarging rod diameter, and the compressive residual stresses distributed on the cylindrical surface increase with the increasing rod diameter.
2019, 48(7): 706005.
doi: 10.3788/IRLA201948.0706005
In order to improve calibration accuracy of the liquid crystal spatial light modulator (LC-SLM), strip grating phase image were proposed to calibrate the LC-SLM. Firstly, the relation between the phase contrast of the strip grating and the intensity of the zero-order diffraction spots was simulated based on Fourier optics. Then the experimental optical path was set up and the strip grating phase image was loaded on the LC-SLM to measure the relation between the gray level and the intensity of the zero-order diffraction spots. Through calculation and analysis, the relationship between the gray level and the amount of phase delay was obtained. And finally, calibration LUT file of LC-SLM for 488 nm laser was also obtained. After calibration, a higher linearity of 0.996 4, and a lower calibration error of 0.224 0 rad were got. The experimental results show that, the double-helical spot generated by the complex high-order phase was basically consistent with the simulation results when the LUT was presented on LC-SLM. The results show that for a specific wavelength, LUT file can be obtained by calibration of LC-SLM by using the strip grating phase image and the beam can be modulated effectively according to the loading phase image, and the method is simpler.
The purpose of the study on experiment technology of laser induced fluorescence in water tunnel is to provide a practical, intuitive, and effective method for flow visualization on separation and vortex structure. Rhodamine B was selected as the stain and fluorescence agent which matched the optical parameter and the characteristics of green laser sheet with 532 nm wavelength in 1 m1 m water tunnel. The study focused on the key technical problems such as the constitution and preparation method of stain and fluorescent agent, the selection of the optimum intensity of continuous laser, the matching relationship between flow velocity and the pore pressure of dyeing line and the matching relationship between camera field of view and the flow structure in the contrast of fine adjustment. The experiments were carried out to investigate the flow field around a cylinder, the wing of a transport plane model and the cargo door of a transport plane model together with the wake by means of the dedicated laser induced fluorescence experiment technology. The typical experiment results verify that the dedicated laser induced fluorescence technique is suitable for flow visualization on flow separation and vortex structure including boundary layer, spatial separation and wake field which is a promising method worthy of popularization and application.
2019, 48(7): 706007.
doi: 10.3788/IRLA201948.0706007
Improving the energy concentration of the main lobe and suppressing the grating lobe are the key to the application of optical fiber laser phased array technology. In this paper, the cutting point was the reason why laser phased array was difficult to satisfy the element spacing of /2 and then many grating lobes were produced. Considering the problem that the aperture of the phased array was enlarged by the method of suppressing the unequal-spacing grating lobe, a new idea of introducing genetic algorithm into the optimization of element distribution of optical fiber laser phased array was proposed. The main method was to propose a fitness function related to the main lobe energy concentration and the main lobe/side lobe contrast from the perspective of the maximum suppression of the lobe. The characteristic parameters of genetic algorithm were corresponded to the main parameters of fiber laser phased array. Then, taking two kinds of linear arrays as an example 20 line array element/array element interval as 3 times wavelength and 50 line array element/array element interval as 20 times wavelength, the traditional equally spaced array elements and unequal spacing, the array element distribution and the genetic algorithm optimized the array element distribution were simulated respectively. The distribution of the far-field energy, the main lobe energy concentration and the contrast between the main lobe and the maximum lobe of the fiber laser phased array were calculated and compared. The results show that the energy concentration of the main lobe is increased by 9.69% and 3.33% respectively, and the energy contrast between the main lobe and the first grating lobe is increased by 13.12% and 9% respectively. It can be seen that the phased array optimized based on genetic algorithm is expected to obtain a longer working distance under the same total power of laser emission.
2019, 48(7): 706008.
doi: 10.3788/IRLA201948.0706008
Lidars to observe ozone and aerosol vertical profile are widely used in environment monitor area recently. A new lidar system with both functions was introduced. The 532 nm and 266 nm light beam were generated separately with second and fourth harmonic generators using Nd:YAG laser oscillator. The 289 nm and 299 nm Raman spectrums were obtained using two Raman cells based on the theory of stimulated Raman scattering, in which one was pumped with deuterium and another was pumped with hydrogen. The ozone concentration profile was retrieved by DIAL algorithm while the aerosol concentration profile was retrieved by Mie scattering algorithm. The lidar detection results agree well with that of ozone analyzer according to the horizontal scanning experiment where the maximum relative error is less than 10%. Results observed on the science island in Hefei City, Anhui province show that the detection height is 3 km in daytime and 5 km in night time for ozone while the height is 10 km in daytime and 15 km in night time for aerosol.
2019, 48(7): 706009.
doi: 10.3788/IRLA201948.0706009
A high power 25/400 m ytterbium doped double-cladding fiber(YDF) with excellent beam quality was successfully fabricated by MCVD in conjunction with solution doping technology. The dopants of the silica fiber core include Yb2O3, Al2O3 and P2O5. Al2O3 could reduce the clustering of Yb3+ and increase the doping concentration. P2O5 was helpful to restrain photodarkening phenomenon. The core-cladding refractive index difference is 0.001 2, corresponding to the core numerical aperture of 0.06. The value of the cladding absorption is 2.1 dB/m at 976 nm. Master oscillator power amplifier configuration based on two end pump was constructed to test the Yb-doped fiber performance. In the experiment, the power of the 1 080 nm seed is 235 W, the total power of the pump power is 3 706 W. The maximum output power is 3 243 W with a slope efficiency of 81.1%. The beam quality factor was measured to be 1.7. Stimulated Raman scattering(SRS) has not been found in the output laser spectrum. The fiber laser continues to work for one hour, the output power is stable at 3 240 W. In addition, a kind of 25/400 m imported fiber was tested in the same experiment schematic. Comparative experimental results show that the performance of the domestic fiber is close to the imported fiber.
2019, 48(6): 606001.
doi: 10.3788/IRLA201948.0606001
Laser effective utilization rate is low because the gap between adjacent pixels in APD detectors used in laser 3D imaging is too large. Aiming to the question, the array beam laser 3D imaging technology was proposed. A laser beam of the laser radiation source was diffracted into sub-beams in array with a liquid crystal spatial light modulator, so that the laser beam can be divided into sub-beams corresponding to the array APD detector. The positions of the laser beam and array APD detectors were adjusted, so that the laser beam can irradiate on an object and focus on the effective array pixel of APD detector, the utilization efficiency of laser beam was improved. The composition and working principle of array beam laser 3D imaging system were introduced. The scheme to realize the array beam by using liquid crystal spatial light modulator was proposed The prototype of the array beam laser 3D imaging system was studied, and the result of laser array beam using the prototype was verified. The experimental result shows that 3D imaging range reaches 510 meters with peak power 10 kW, pulse width 8 ns, and 88 APD detector fill factor 2/3, the effect distance increases by 39.1%, compared with the laser 3D imaging system without array beam.
2019, 48(6): 606002.
doi: 10.3788/IRLA201948.0606002
Pulsed laser ranging system is widely used in laser radar, laser guidance and other fields due to its advantages of high precision, strong anti-interference ability, etc. However, the commonly used timing discrimination method has errors and restricts the improvement of dynamic ranging accuracy, which was mainly caused by the attenuation and widening of the echo pulse. A new method of timing discrimination was presented in view of this problem, which was based on the dual-channel timing discrimination method. The dual-channel timing discrimination method was composed of constant threshold timing discrimination method and peak timing discrimination method. Through the dual-channel timing discrimination method, accurate echo timing discrimination can be achieved without being affected by attenuation and broadening. By introducing the theoretical equations of the laser emission pulse, the time domain distribution model of the echo waveform was established. The experimental results show that the short ranging error can be controlled within 3 cm by adopting the dual-channel timing discrimination method, and the accuracy can be further improved by multiple measurements, which solves the bottleneck problem that the timing discrimination error restricts the dynamic ranging accuracy.
Initiative control technology based on periodic temperature excitation was used for mixing gasdynamic CO2 laser(MGDL). Through numerical simulation, influences of periodic temperature excitation on mixing characteristic of donor and assistant flows and small signal gain coefficient of mixing gasdynamic CO2 laser were studied. Investigation results indicate that compared with the case that has no periodic temperature excitation, mixing efficiency of donor and assistant flows can be enhanced and small signal gain coefficient in mixing nozzle can be improved when periodic temperature excitation with certain excitation amplitude and excitation frequency is imposed at the outlet of assistant nozzle. Excitation amplitude and excitation frequency have important effects on mixing efficiency of donor and assistant flows and small signal gain coefficient. As the excitation amplitude increases, the small signal gain coefficient first increases and then decreases. In downstream area of mixing nozzle, small signal gain coefficients under the condition of the six excitation frequencies selected by this paper are all higher than the state without periodic temperature excitation. With the increase of excitation frequency, mixing efficiency of donor and assistant flows becomes better and better; but when the excitation frequency increases up to a certain value, mixing efficiency of donor and assistant flows will not change any more.
A high stability and compact structure 488 nm light generated by intra-cavity frequency doubling in an optically pumped semiconductor disc laser was designed. In order to obtain a 488 nm laser with good beam quality and stable performance output, a semiconductor gain medium chip with 13 QWs and 808 nm/976 nm Double Band Mirror was pumped vertically by 808 nm LD on the top surface of the chip, and the chip with double diamond heat spreaders bonded on the both sides was introduced. 488 nm laser was generated by doubling frequency with I phase matched LBO crystals inserted in the cavity. 111 mW 488 nm laser with 1.3 nm spectral line width was obtained, the optical to optical efficiency was 1.2%, the beam quality of Mx2、My2 were 1.03 and 1.02 respectively, and the instability is less than 0.6% with continuously work for more than 3 h.
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