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RSS2023 Vol. 47, No. 5
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2023, 47(5): 579-586.
doi: 10.7510/jgjs.issn.1001-3806.2023.05.001
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Abstract:
The extracavity pumped dual-wavelength Raman laser based on the main and the secondary Raman shifts of Raman crystal was theoretically analyzed. From the wave equation of the light field and the damped harmonic oscillator wave equation for the vibrational wave in Raman crystal, a group of coupled wave equations were deduced to describe the fundamental laser, Raman laser 1 and Raman laser 2 were then derived. The coupled wave equations were normalized by introducing several normalized parameters. The effects of the normalized Raman gain coefficient, normalized fundamental pulse width and output mirror reflectivities on the performance of extracavity dual-wavelength Raman lasers were numerically analyzed. It is found that selecting the output mirror with reflectivity less than 0.5 for the main frequency shift and larger than 0.5 for the secondary frequency shift, and the Raman crystal with small difference gain coefficients of the two Raman modes, and properly increasing the pulse width of the incident fundamental frequency light can improve the conversion efficiency of the secondary Raman frequency shift and achieve effective dual wavelength operation. The theoretical derivation and numerical calculation of this paper can be used as a theoretical tool for the design and analysis of extracavity dual-wavelength Raman lasers, and can provide a reference for the experimental research of this kind of lasers.
The extracavity pumped dual-wavelength Raman laser based on the main and the secondary Raman shifts of Raman crystal was theoretically analyzed. From the wave equation of the light field and the damped harmonic oscillator wave equation for the vibrational wave in Raman crystal, a group of coupled wave equations were deduced to describe the fundamental laser, Raman laser 1 and Raman laser 2 were then derived. The coupled wave equations were normalized by introducing several normalized parameters. The effects of the normalized Raman gain coefficient, normalized fundamental pulse width and output mirror reflectivities on the performance of extracavity dual-wavelength Raman lasers were numerically analyzed. It is found that selecting the output mirror with reflectivity less than 0.5 for the main frequency shift and larger than 0.5 for the secondary frequency shift, and the Raman crystal with small difference gain coefficients of the two Raman modes, and properly increasing the pulse width of the incident fundamental frequency light can improve the conversion efficiency of the secondary Raman frequency shift and achieve effective dual wavelength operation. The theoretical derivation and numerical calculation of this paper can be used as a theoretical tool for the design and analysis of extracavity dual-wavelength Raman lasers, and can provide a reference for the experimental research of this kind of lasers.
2023, 47(5): 587-591.
doi: 10.7510/jgjs.issn.1001-3806.2023.05.002
Abstract:
The SiGe material system demonstrates the advantages of device fabrication and performance, also the unique characteristic of Ge/SiGe multi-quantum well structure expends the application range. A design of optical integrated phase modulator based on asymmetric Ge/SiGe coupled quantum well (CQW) was proposed in this paper. According to comprehensively analysis of asymmetric Ge/SiGe CQW structure by numerical simulation, the theory was verified by implementing the fabrication and measurement. It indicates that the maximum electrorefractive index variation up to 0.01 can be achieved by using the designed asymmetric CQW, while the applied electrical field exceeds 40 kV/cm at wavelength of 1450 nm. Moreover, the fabricated device attains the electrorefractive index variation 2.4×10-3 with 1.5 V reverse bias voltage and optical wavelength 1530 nm, and the corresponding VπLπ is as low as 0.048 V·cm. The asymmetric Ge/SiGe CQW phase modulator exhibits superior performance in the same type of modulators based on SiGe, and offers new opportunities to further development of optical integrated silicon modulators.
The SiGe material system demonstrates the advantages of device fabrication and performance, also the unique characteristic of Ge/SiGe multi-quantum well structure expends the application range. A design of optical integrated phase modulator based on asymmetric Ge/SiGe coupled quantum well (CQW) was proposed in this paper. According to comprehensively analysis of asymmetric Ge/SiGe CQW structure by numerical simulation, the theory was verified by implementing the fabrication and measurement. It indicates that the maximum electrorefractive index variation up to 0.01 can be achieved by using the designed asymmetric CQW, while the applied electrical field exceeds 40 kV/cm at wavelength of 1450 nm. Moreover, the fabricated device attains the electrorefractive index variation 2.4×10-3 with 1.5 V reverse bias voltage and optical wavelength 1530 nm, and the corresponding VπLπ is as low as 0.048 V·cm. The asymmetric Ge/SiGe CQW phase modulator exhibits superior performance in the same type of modulators based on SiGe, and offers new opportunities to further development of optical integrated silicon modulators.
2023, 47(5): 592-599.
doi: 10.7510/jgjs.issn.1001-3806.2023.05.003
Abstract:
In order to tune the fluorescence performance of Yb ∶YAG crystal to be better applied in high-energy pulse lasers, based on density functional theory and crystal field theory, the electronic structure and spectroscopic properties of doped Yb ∶YAG crystal were theoretically calculated. The fluorescence properties of Yb ∶YAG crystal under the different types and occupation sites of particle doping were analyzed. According to the theoretical results, the crystal growth experiment was carried out, and the samples were prepared to verify the fluorescence performance. The results shows that, the tuning methods of Yb ∶YAG crystal fluorescence lifetime and other parameters are mastered through the above process, that is, the fluorescence lifetime of Yb ∶YAG co-doped with Cr decreases from 1.18 ms to 0.94 ms. This work lays a theoretical and experimental foundation for Yb ∶YAG crystal to realize the application of high-energy pulsed laser.
In order to tune the fluorescence performance of Yb ∶YAG crystal to be better applied in high-energy pulse lasers, based on density functional theory and crystal field theory, the electronic structure and spectroscopic properties of doped Yb ∶YAG crystal were theoretically calculated. The fluorescence properties of Yb ∶YAG crystal under the different types and occupation sites of particle doping were analyzed. According to the theoretical results, the crystal growth experiment was carried out, and the samples were prepared to verify the fluorescence performance. The results shows that, the tuning methods of Yb ∶YAG crystal fluorescence lifetime and other parameters are mastered through the above process, that is, the fluorescence lifetime of Yb ∶YAG co-doped with Cr decreases from 1.18 ms to 0.94 ms. This work lays a theoretical and experimental foundation for Yb ∶YAG crystal to realize the application of high-energy pulsed laser.
2023, 47(5): 600-605.
doi: 10.7510/jgjs.issn.1001-3806.2023.05.004
Abstract:
In order to obtain an airborne laser radiator with compact structure, high beam quality and good optical stability, LD end-pumped configuration and orthogonal porro prism cavity technology were used to study this type of laser. Based on Jones matrix theory, the principle of realizing Q-switched with special wave-plate was analyzed, and the cavity was controlled to the large fundamental mode size through the design of Porro prism cavity mirror with suitable curvature. The results show that the laser output with single pulse energy of 72.2 mJ and beam quality of Mx2=8 and My2=6 can be obtained at 20 Hz repetition frequency under the condition of orthogonal polarization closed state. Based on the Porro prism material of K9 glass, 0.648λ wave-plate was used for Q-switched closed state. Under the working repetition frequency of 20 Hz, a high beam quality output with single pulse energy of 80 mJ, beam quality of Mx2=5.7 and My2=4.8 is obtained.The divergence angles of the laser in the x- and y-direction are 1.65 mrad and 1.35 mrad respectively. This result is helpful for the research and development of high performance miniaturized lasers.
In order to obtain an airborne laser radiator with compact structure, high beam quality and good optical stability, LD end-pumped configuration and orthogonal porro prism cavity technology were used to study this type of laser. Based on Jones matrix theory, the principle of realizing Q-switched with special wave-plate was analyzed, and the cavity was controlled to the large fundamental mode size through the design of Porro prism cavity mirror with suitable curvature. The results show that the laser output with single pulse energy of 72.2 mJ and beam quality of Mx2=8 and My2=6 can be obtained at 20 Hz repetition frequency under the condition of orthogonal polarization closed state. Based on the Porro prism material of K9 glass, 0.648λ wave-plate was used for Q-switched closed state. Under the working repetition frequency of 20 Hz, a high beam quality output with single pulse energy of 80 mJ, beam quality of Mx2=5.7 and My2=4.8 is obtained.The divergence angles of the laser in the x- and y-direction are 1.65 mrad and 1.35 mrad respectively. This result is helpful for the research and development of high performance miniaturized lasers.
2023, 47(5): 606-612.
doi: 10.7510/jgjs.issn.1001-3806.2023.05.005
Abstract:
To solve the problem that high density variation of long-range terrestrial laser scanning(TLS) point cloud, a density adaptive segmentation algorithm for extracting building plane was proposed in this paper. Firstly, the dynamic neighborhood search range was constructed based on the estimated theoretical point space, and the optimal neighborhood can be selected by internal indexes and Shannon entropy. Then, the dimensionality feature was calculated by using this neighborhood. Secondly, the region growing algorithm rules were set according to the optimal neighborhood, normal vector, dimensionality feature and point-to-plane distance to extract preliminary plane segmentation results. Finally, the segmentation result was optimized by patch merging, and then was tested on a single-site TLS data with scanning distance of 1 km. The result shows that the precision reaches 95%, the recall reaches 92%. This method can segment the building plane in the long-range TLS point cloud effectively. Compared with the traditional Shannon entropy method, the dynamic neighborhood search range used in this paper can significantly improve the efficiency of the algorithm. This method can efficiently and accurately extract the building planes from wide scene, and provide a reference for urban 3-D modeling.
To solve the problem that high density variation of long-range terrestrial laser scanning(TLS) point cloud, a density adaptive segmentation algorithm for extracting building plane was proposed in this paper. Firstly, the dynamic neighborhood search range was constructed based on the estimated theoretical point space, and the optimal neighborhood can be selected by internal indexes and Shannon entropy. Then, the dimensionality feature was calculated by using this neighborhood. Secondly, the region growing algorithm rules were set according to the optimal neighborhood, normal vector, dimensionality feature and point-to-plane distance to extract preliminary plane segmentation results. Finally, the segmentation result was optimized by patch merging, and then was tested on a single-site TLS data with scanning distance of 1 km. The result shows that the precision reaches 95%, the recall reaches 92%. This method can segment the building plane in the long-range TLS point cloud effectively. Compared with the traditional Shannon entropy method, the dynamic neighborhood search range used in this paper can significantly improve the efficiency of the algorithm. This method can efficiently and accurately extract the building planes from wide scene, and provide a reference for urban 3-D modeling.
2023, 47(5): 613-619.
doi: 10.7510/jgjs.issn.1001-3806.2023.05.006
Abstract:
In order to realize the application of laser technologies in the field of wastewater degradation, laser-induced cavitation technology was employed to degrade acid black solution in this work. The degraded solution was analyzed and characterized by an ultraviolet spectrophotometer, and the effects of laser energy and the solution initial concentration on the degradation capacity were obtained. The mechanisms of laser cavitation degradation of acid black solution were revealed based on the combindation of the theory of bubble kinetics and the chemical effect of cavitation. The results show that with the increase of laser energy, the degradation rate of acid black solution with an initial concentration of 20 mg/L gradually increases from 0.78% to 27.28%. Under the laser energy of 100 mJ, the degradation rate of the acid black solution decreases from 98.55% to 7.63% as the initial concentration increases from 5 mg/L to 20 mg/L. The higher the laser energy, the greater the degradation rate. It is difficult for laser cavitation to degrade the acidic black solution if the initial concentration is too large. This research promots the development of organic wastewater technologies such as laser cavitation to degrade dyes.
In order to realize the application of laser technologies in the field of wastewater degradation, laser-induced cavitation technology was employed to degrade acid black solution in this work. The degraded solution was analyzed and characterized by an ultraviolet spectrophotometer, and the effects of laser energy and the solution initial concentration on the degradation capacity were obtained. The mechanisms of laser cavitation degradation of acid black solution were revealed based on the combindation of the theory of bubble kinetics and the chemical effect of cavitation. The results show that with the increase of laser energy, the degradation rate of acid black solution with an initial concentration of 20 mg/L gradually increases from 0.78% to 27.28%. Under the laser energy of 100 mJ, the degradation rate of the acid black solution decreases from 98.55% to 7.63% as the initial concentration increases from 5 mg/L to 20 mg/L. The higher the laser energy, the greater the degradation rate. It is difficult for laser cavitation to degrade the acidic black solution if the initial concentration is too large. This research promots the development of organic wastewater technologies such as laser cavitation to degrade dyes.
2023, 47(5): 620-626.
doi: 10.7510/jgjs.issn.1001-3806.2023.05.007
Abstract:
In order to quickly and non-destructively detect and assess mildew and moisture content of timber, the hyperspectral data of timber was actively acquired by hyperspectral light detection and ranging (LiDAR), and a method was designed to analyze mildew characteristics and establish timber moisture content prediction model. Firstly, timber sample(white pine) hyperspectral data was measured at monthly intervals for four months, and the spectral characteristics of mildew occurrence and development(normal, wet and mildew state) were analyzed. Then based on analyzing the spectral characteristics of sample moisture content, competitive adaptive reweighted sampling, successive projections algorithm and competitive adaptive reweighted sampling-successive projections algorithm were employed to extract feature wavelength. Finally, prediction models were established with partial least squares regression respectively. The results show that, the spectral reflectance of normal state is highest and the mildew's is lowest; when the mildew state is stable, the spectral reflectance changes slowly with time and tends to stabilize; and the model based on combined algorithm achieved the best predictive performance, the correlation coefficient and root mean square error of the prediction set are 0.9073 and 0.7564, respectively. The active acquisition of hyperspectral information by hyerspectral LiDAR can be used to assess mildew and predict timber moisture content, providing new ideas for rapid non-destructive detecting of wood buildings.
In order to quickly and non-destructively detect and assess mildew and moisture content of timber, the hyperspectral data of timber was actively acquired by hyperspectral light detection and ranging (LiDAR), and a method was designed to analyze mildew characteristics and establish timber moisture content prediction model. Firstly, timber sample(white pine) hyperspectral data was measured at monthly intervals for four months, and the spectral characteristics of mildew occurrence and development(normal, wet and mildew state) were analyzed. Then based on analyzing the spectral characteristics of sample moisture content, competitive adaptive reweighted sampling, successive projections algorithm and competitive adaptive reweighted sampling-successive projections algorithm were employed to extract feature wavelength. Finally, prediction models were established with partial least squares regression respectively. The results show that, the spectral reflectance of normal state is highest and the mildew's is lowest; when the mildew state is stable, the spectral reflectance changes slowly with time and tends to stabilize; and the model based on combined algorithm achieved the best predictive performance, the correlation coefficient and root mean square error of the prediction set are 0.9073 and 0.7564, respectively. The active acquisition of hyperspectral information by hyerspectral LiDAR can be used to assess mildew and predict timber moisture content, providing new ideas for rapid non-destructive detecting of wood buildings.
2023, 47(5): 627-631.
doi: 10.7510/jgjs.issn.1001-3806.2023.05.008
Abstract:
In order to study the intrinsic relationship between the polarization flipping effect and the phase retardation based on laser feedback, the output intensity and polarization state of the laser were observed during the tuning of the feedback cavity length by using a microchip Nd∶YAG laser (central wavelength of 1064 nm) as the light source and two relative rotating λ/4 wave-plates in the feedback cavity to change the phase retardation. The results indicate that, with the increase of phase retardation in the feedback cavity, the indentation depth of the polarization flipping position gradually deepens, and the width of the light intensity modulation waveform of the two polarization states tends to be the same. When the phase retardation in the feedback cavity is 90°, the duration of two orthogonal polarization states to be the same in an intensity modulation period, and each polarization flipping corresponds to a change of λ/4 feedback cavity length. The variation of the characteristic point of polarization flipping caused by different phase retardation provides a potential measuring method for phase retardation of waveplate at 1064 nm wavelength. Meanwhile, the change of phase retardation shapes the output light intensity waveform of laser, which provides a basis for optical subdivision of laser feedback fringe (self-mixing interference fringe).
In order to study the intrinsic relationship between the polarization flipping effect and the phase retardation based on laser feedback, the output intensity and polarization state of the laser were observed during the tuning of the feedback cavity length by using a microchip Nd∶YAG laser (central wavelength of 1064 nm) as the light source and two relative rotating λ/4 wave-plates in the feedback cavity to change the phase retardation. The results indicate that, with the increase of phase retardation in the feedback cavity, the indentation depth of the polarization flipping position gradually deepens, and the width of the light intensity modulation waveform of the two polarization states tends to be the same. When the phase retardation in the feedback cavity is 90°, the duration of two orthogonal polarization states to be the same in an intensity modulation period, and each polarization flipping corresponds to a change of λ/4 feedback cavity length. The variation of the characteristic point of polarization flipping caused by different phase retardation provides a potential measuring method for phase retardation of waveplate at 1064 nm wavelength. Meanwhile, the change of phase retardation shapes the output light intensity waveform of laser, which provides a basis for optical subdivision of laser feedback fringe (self-mixing interference fringe).
2023, 47(5): 632-638.
doi: 10.7510/jgjs.issn.1001-3806.2023.05.009
Abstract:
In order to simulate the laser threat in complicated condition, a new type of laser source with combined multi-wavelength and multi-coding beam out was presented. Three wavelengths of 650 nm, 808 nm and 905 nm were adopted here. The microcontroller STC8H was selected as core of the coding module to modulate laser beams. By dichroic mirrors, three channel lasers were combined to one channel laser output. And relevant theoretical analysis and simulation verification were carried out. The results show that the energy of pulse laser is up to 2.45 mJ, the diameter of beam is 6 mm. It can modulate laser driving signal in the precise frequency coding, the variational duty cycle coding and the variational space coding. The precision of frequency and duty cycle are 0.5 Hz and 0.1%. The device can provide laser beam with different coding messages in different wavelengths. This research presents a new detection device of laser warning devices, which has reference for the improvement of detection.
In order to simulate the laser threat in complicated condition, a new type of laser source with combined multi-wavelength and multi-coding beam out was presented. Three wavelengths of 650 nm, 808 nm and 905 nm were adopted here. The microcontroller STC8H was selected as core of the coding module to modulate laser beams. By dichroic mirrors, three channel lasers were combined to one channel laser output. And relevant theoretical analysis and simulation verification were carried out. The results show that the energy of pulse laser is up to 2.45 mJ, the diameter of beam is 6 mm. It can modulate laser driving signal in the precise frequency coding, the variational duty cycle coding and the variational space coding. The precision of frequency and duty cycle are 0.5 Hz and 0.1%. The device can provide laser beam with different coding messages in different wavelengths. This research presents a new detection device of laser warning devices, which has reference for the improvement of detection.
2023, 47(5): 639-645.
doi: 10.7510/jgjs.issn.1001-3806.2023.05.010
Abstract:
In order to study the effect of process parameters on the surface roughness of CuCrZr alloy micro-channel formed by selective laser melting (SLM) technology, CuCrZr alloy sample with micro channel was prepared by orthogonal experiment. The roughness and micro morphology of the inner surface of the sample were measured by 3-D topography instrument and scanning electron microscope. The effects of laser power, scan speed and hatch distance on the surface roughness of the overhang surface and side surface of the microchannel were analyzed. The results show that the scan speed has the greatest influence on the roughness of the overhang and side surfaces, and the overhang surface is affected by the quality of the molten pool itself, resulting in the phenomenon of "slag hanging"; while the side surface roughness is greatly affected by the Marangoni effect. After optimization, under the process parameters of laser power 380 W, scan speed 520 mm/s, and hatch distance 0.12 mm, micro-channel samples with minimum side surface roughness (16.91 μm) can be processed; laser power is 320 W, scan speed is 560 mm/s, under the process parameters of 0.14 mm hatch distance, the micro-channel sample with the smallest overhang surface roughness (24.86 μm) was obtained. This research provides the basis for SLM forming surface precision from the perspective of laser processing window, and gives the preliminary process parameter scheme.
In order to study the effect of process parameters on the surface roughness of CuCrZr alloy micro-channel formed by selective laser melting (SLM) technology, CuCrZr alloy sample with micro channel was prepared by orthogonal experiment. The roughness and micro morphology of the inner surface of the sample were measured by 3-D topography instrument and scanning electron microscope. The effects of laser power, scan speed and hatch distance on the surface roughness of the overhang surface and side surface of the microchannel were analyzed. The results show that the scan speed has the greatest influence on the roughness of the overhang and side surfaces, and the overhang surface is affected by the quality of the molten pool itself, resulting in the phenomenon of "slag hanging"; while the side surface roughness is greatly affected by the Marangoni effect. After optimization, under the process parameters of laser power 380 W, scan speed 520 mm/s, and hatch distance 0.12 mm, micro-channel samples with minimum side surface roughness (16.91 μm) can be processed; laser power is 320 W, scan speed is 560 mm/s, under the process parameters of 0.14 mm hatch distance, the micro-channel sample with the smallest overhang surface roughness (24.86 μm) was obtained. This research provides the basis for SLM forming surface precision from the perspective of laser processing window, and gives the preliminary process parameter scheme.
Study on noise feature analysis and processing algorithm of magneto-optical image of welding defects
2023, 47(5): 646-652.
doi: 10.7510/jgjs.issn.1001-3806.2023.05.011
Abstract:
In order to solve the problem that it was hard to extract the outline information from the magneto-optical image of welding defects, an image denoising and contour detection method was proposed. The crack defect samples were obtained by laser spot welding, and the magnetic flux leakage field of defects was simulated by finite element method. The gray continuity, concentration, and noise characteristic of magneto-optical images were compared and analyzed. The fast non-local mean filtering algorithm was used to remove noise, and compared with the traditional filters. According to the distinction in gradient value between flaw information and background information in magneto-optical images, an operator was used to carry out Otsu threshold segmentation and the edge detection based on the first order gradient graph of the image. The results show that the standard deviation and image entropy of the magneto-optical image processed by this method reaches 30.0465 and 6.0395 respectively, and the image aggregation degree is better, which is closer to the simulation magnetic field curve, and the defect contour information could be better extracted. This result is helpful for defect recognition and target detection.
In order to solve the problem that it was hard to extract the outline information from the magneto-optical image of welding defects, an image denoising and contour detection method was proposed. The crack defect samples were obtained by laser spot welding, and the magnetic flux leakage field of defects was simulated by finite element method. The gray continuity, concentration, and noise characteristic of magneto-optical images were compared and analyzed. The fast non-local mean filtering algorithm was used to remove noise, and compared with the traditional filters. According to the distinction in gradient value between flaw information and background information in magneto-optical images, an operator was used to carry out Otsu threshold segmentation and the edge detection based on the first order gradient graph of the image. The results show that the standard deviation and image entropy of the magneto-optical image processed by this method reaches 30.0465 and 6.0395 respectively, and the image aggregation degree is better, which is closer to the simulation magnetic field curve, and the defect contour information could be better extracted. This result is helpful for defect recognition and target detection.
2023, 47(5): 653-658.
doi: 10.7510/jgjs.issn.1001-3806.2023.05.012
Abstract:
To improve the corrosion resistance of 316L substrate and extend its service life in marine environment, Ni-Cu-WC cladding layers with different copper content were prepared on 316L stainless steel by laser cladding technology. The microstructure, hardness, corrosion resistance, and corrosion behavior of the cladding layer were analyzed by scanning electron microscope, energy dispersive spectroscopy, microhardness tester, and electrochemical workstation. The results show that the coating has good forming quality, and the dendrites of the cladding layer is induced by excessive Cu addition. The addition of Cu partially inhibits the formation of carbide reinforcing phase, resulting in a decrease in the hardness of the cladding layer. At the same time, the Cu element in the cladding layer can effectively improve its corrosion resistance in seawater. The experimental results show that the corrosion resistance is the best when the mass fraction of Cu is 0.1, the corrosion potential increases to -0.864 V in the NaCl solution(mass fraction of 0.035), and the corrosion current density decreases to 0.1049 μA/cm2, respectively. This study provides a theoretical reference for the preparation of seawater corrosion resistant coatings by laser cladding.
To improve the corrosion resistance of 316L substrate and extend its service life in marine environment, Ni-Cu-WC cladding layers with different copper content were prepared on 316L stainless steel by laser cladding technology. The microstructure, hardness, corrosion resistance, and corrosion behavior of the cladding layer were analyzed by scanning electron microscope, energy dispersive spectroscopy, microhardness tester, and electrochemical workstation. The results show that the coating has good forming quality, and the dendrites of the cladding layer is induced by excessive Cu addition. The addition of Cu partially inhibits the formation of carbide reinforcing phase, resulting in a decrease in the hardness of the cladding layer. At the same time, the Cu element in the cladding layer can effectively improve its corrosion resistance in seawater. The experimental results show that the corrosion resistance is the best when the mass fraction of Cu is 0.1, the corrosion potential increases to -0.864 V in the NaCl solution(mass fraction of 0.035), and the corrosion current density decreases to 0.1049 μA/cm2, respectively. This study provides a theoretical reference for the preparation of seawater corrosion resistant coatings by laser cladding.
2023, 47(5): 659-665.
doi: 10.7510/jgjs.issn.1001-3806.2023.05.013
Abstract:
In order to improve the receiving sensitivity of the wireless optical communication system, an AdaBoost weak-light signal detection algorithm based on the improved base classifier coefficient was adopted to solve the signal detection problem of multi-pixel photon counter (MPPC) under weak-light conditions. In this algorithm, k-nearest neighbor (KNN) was used as the base classifier to build a strong classifier. A W-AdaBoost algorithm based on the weights of incorrect and correct classification samples was proposed to solve the problem of that the traditional AdaBoost algorithm's base classifier coefficients are only related to the error rate, which causes redundant base classifiers to consume system resources. The W-AdaBoost algorithm transforms the issue of signal demodulation into classification, a 450 nm semiconductor laser and MPPC photoelectric conversion device are used to build a wireless optical communication system. The experimental results show that the sensitivity of the improved W-AdaBoost-KNN algorithm is about 1.6 dB and 4.8 dB higher than that of the traditional AdaBoost-KNN algorithm and the single KNN algorithm respectively, when the communication rate of the system is 2 Mbit/s and the bit error rate is 3.8×10-3. The research results show that W-AdaBoost-KNN algorithm can improve the signal detection efficiency under weak-light conditions and improve the receiving sensitivity of the wireless optical communication systems.
In order to improve the receiving sensitivity of the wireless optical communication system, an AdaBoost weak-light signal detection algorithm based on the improved base classifier coefficient was adopted to solve the signal detection problem of multi-pixel photon counter (MPPC) under weak-light conditions. In this algorithm, k-nearest neighbor (KNN) was used as the base classifier to build a strong classifier. A W-AdaBoost algorithm based on the weights of incorrect and correct classification samples was proposed to solve the problem of that the traditional AdaBoost algorithm's base classifier coefficients are only related to the error rate, which causes redundant base classifiers to consume system resources. The W-AdaBoost algorithm transforms the issue of signal demodulation into classification, a 450 nm semiconductor laser and MPPC photoelectric conversion device are used to build a wireless optical communication system. The experimental results show that the sensitivity of the improved W-AdaBoost-KNN algorithm is about 1.6 dB and 4.8 dB higher than that of the traditional AdaBoost-KNN algorithm and the single KNN algorithm respectively, when the communication rate of the system is 2 Mbit/s and the bit error rate is 3.8×10-3. The research results show that W-AdaBoost-KNN algorithm can improve the signal detection efficiency under weak-light conditions and improve the receiving sensitivity of the wireless optical communication systems.
2023, 47(5): 666-671.
doi: 10.7510/jgjs.issn.1001-3806.2023.05.014
Abstract:
In order to obtain a robust near infrared spectral model, a method based on variate selection and model updating was adopted. 240 Red Fuji apples were used to obtain near infrared diffuse transmission spectra and soluble solids content data, and a partial least squares regression model was developed to predict apple soluble solids content. The modelling variates were selected by using backward interval partial least squares and competitive adaptive reweighting algorithms. The model was updated by adding some samples from the new batch to the old batch and recalibrating. The results indicate that the model performance can be improved by variable selection, with the prediction coefficient of determination increasing to 0.7915, the root mean square error of prediction decreasing to 0.5810 and the prediction bias decreasing to 0.2627. Combining the model update strategy, the root mean square error of prediction and the prediction bias were further reduced. Model updating using only 20 samples has already led to a significant improvement in model performance, with the prediction coefficient of determination improving to 0.8506, the root mean square error of prediction decreasing to 0.4358 and the prediction bias decreasing to 0.1045, the result that is useful for robust near infrared spectroscopy modelling of a wide range of fruits.
In order to obtain a robust near infrared spectral model, a method based on variate selection and model updating was adopted. 240 Red Fuji apples were used to obtain near infrared diffuse transmission spectra and soluble solids content data, and a partial least squares regression model was developed to predict apple soluble solids content. The modelling variates were selected by using backward interval partial least squares and competitive adaptive reweighting algorithms. The model was updated by adding some samples from the new batch to the old batch and recalibrating. The results indicate that the model performance can be improved by variable selection, with the prediction coefficient of determination increasing to 0.7915, the root mean square error of prediction decreasing to 0.5810 and the prediction bias decreasing to 0.2627. Combining the model update strategy, the root mean square error of prediction and the prediction bias were further reduced. Model updating using only 20 samples has already led to a significant improvement in model performance, with the prediction coefficient of determination improving to 0.8506, the root mean square error of prediction decreasing to 0.4358 and the prediction bias decreasing to 0.1045, the result that is useful for robust near infrared spectroscopy modelling of a wide range of fruits.
2023, 47(5): 672-677.
doi: 10.7510/jgjs.issn.1001-3806.2023.05.015
Abstract:
In order to control the thermal stability of water-laser coupling in water-jet guided high-power laser and to ensure the effective transmission of laser energy to the workpiece surface, the thermal loss problem of water-jet guided high-power laser was investigated. The finite element method was used to establish a numerical model of water-laser coupling, and ray tracing was used to simulate the temperature distribution of water jets with different incident laser power, jet velocity, and diameter. The model's effectiveness was verified by experimental data, and continuous and pulsed laser temperature distribution was compared under the same power. The results show that the water jet' s temperature increases with the water-laser coupling length. When the incident laser power increases, the pressure of the coupling cavity decreases, the jet diameter decreases, and the temperature of the jet increases, which seriously affects the stability of the jet. Due to the water jet' s cooling effect, the pulse laser' s temperature difference with an average power of 300 W is about 5 ℃ compared with that of the continuous laser. The research results provide certain reference significance for controlling the thermal effect of water-laser coupling in water-jet guided high-power laser and improving the transmission efficiency of laser energy.
In order to control the thermal stability of water-laser coupling in water-jet guided high-power laser and to ensure the effective transmission of laser energy to the workpiece surface, the thermal loss problem of water-jet guided high-power laser was investigated. The finite element method was used to establish a numerical model of water-laser coupling, and ray tracing was used to simulate the temperature distribution of water jets with different incident laser power, jet velocity, and diameter. The model's effectiveness was verified by experimental data, and continuous and pulsed laser temperature distribution was compared under the same power. The results show that the water jet' s temperature increases with the water-laser coupling length. When the incident laser power increases, the pressure of the coupling cavity decreases, the jet diameter decreases, and the temperature of the jet increases, which seriously affects the stability of the jet. Due to the water jet' s cooling effect, the pulse laser' s temperature difference with an average power of 300 W is about 5 ℃ compared with that of the continuous laser. The research results provide certain reference significance for controlling the thermal effect of water-laser coupling in water-jet guided high-power laser and improving the transmission efficiency of laser energy.
2023, 47(5): 678-685.
doi: 10.7510/jgjs.issn.1001-3806.2023.05.016
Abstract:
In order to solve the problems of low optimization accuracy and weak obstacle avoidance ability of agricultural unmanned aerial vehicle (UAV) when carrying out path planning tasks in the environment of obstacle interference, a wireless ultraviolet(UV) light flight path guidance system was established by using the characteristics of all-weather non-direct communication, covert communication and various special occasions of wireless ultraviolet light communication guidance equipment. An improved bat algorithm based on exponential decline strategy (EDS-IBA) was proposed. The dynamic inertia weight based on exponential decline strategy and the artificial potential field of adjacent path points was introduced to ensure the smooth path of the UAV while avoiding falling into the local optimum. The diversity and authenticity of the flight environment were guaranteed through the cost of obstacle interference and the cost of ultraviolet communication. The results show that, compared with the improved bat algorithm based on differential evolution algorithm and the traditional bat algorithm, in 2-D environment, the average path length of EDS-IBA is shortened by 10.7% and 16.3% respectively; in 3-D environment, the planned average path is shortened by 13.7% and 36.2% respectively. When the algorithm reaches the convergence state, the fitness value of EDS-IBA is also small, which has good feasibility and effectiveness in path planning ability and algorithm performance. This research has certain practical significance for the practical application and development of UAV path planning.
In order to solve the problems of low optimization accuracy and weak obstacle avoidance ability of agricultural unmanned aerial vehicle (UAV) when carrying out path planning tasks in the environment of obstacle interference, a wireless ultraviolet(UV) light flight path guidance system was established by using the characteristics of all-weather non-direct communication, covert communication and various special occasions of wireless ultraviolet light communication guidance equipment. An improved bat algorithm based on exponential decline strategy (EDS-IBA) was proposed. The dynamic inertia weight based on exponential decline strategy and the artificial potential field of adjacent path points was introduced to ensure the smooth path of the UAV while avoiding falling into the local optimum. The diversity and authenticity of the flight environment were guaranteed through the cost of obstacle interference and the cost of ultraviolet communication. The results show that, compared with the improved bat algorithm based on differential evolution algorithm and the traditional bat algorithm, in 2-D environment, the average path length of EDS-IBA is shortened by 10.7% and 16.3% respectively; in 3-D environment, the planned average path is shortened by 13.7% and 36.2% respectively. When the algorithm reaches the convergence state, the fitness value of EDS-IBA is also small, which has good feasibility and effectiveness in path planning ability and algorithm performance. This research has certain practical significance for the practical application and development of UAV path planning.
2023, 47(5): 686-690.
doi: 10.7510/jgjs.issn.1001-3806.2023.05.017
Abstract:
In order to design and manufacture a new type of phase retarder, a multi-period binary 1-D photonic crystal composed of zinc sulfide (ZnS) and cryolite (Na3AlF6) was prepared on the glass with a refractive index of 1.52 by using the characteristic 1-D photonic crystal. Numerical simulation calculation and theoretical analysis were carried out. The analysis results show that, the effective refractive index of one-dimensional photonic crystal is the virtual equivalent refractive index in the band gap range. At oblique incidence, the phase of the reflected light of the p light and s light in the band gap increases, the phase delay occurs, and the polarization state changes from linear polarized light to elliptical (circular) polarized light. When total reflection occurs, the effective refractive index of the optical sparse medium is the virtual equivalent refractive index. The phase of reflected light increases and phase delay is observed at the same time. Its polarization state changes from linear polarized light to elliptical (circular) polarized light. The propagation direction of light and the phase of polarization state can be changed by using the retarder, and the defect of thin film λ/4 wave-plate is solved.
In order to design and manufacture a new type of phase retarder, a multi-period binary 1-D photonic crystal composed of zinc sulfide (ZnS) and cryolite (Na3AlF6) was prepared on the glass with a refractive index of 1.52 by using the characteristic 1-D photonic crystal. Numerical simulation calculation and theoretical analysis were carried out. The analysis results show that, the effective refractive index of one-dimensional photonic crystal is the virtual equivalent refractive index in the band gap range. At oblique incidence, the phase of the reflected light of the p light and s light in the band gap increases, the phase delay occurs, and the polarization state changes from linear polarized light to elliptical (circular) polarized light. When total reflection occurs, the effective refractive index of the optical sparse medium is the virtual equivalent refractive index. The phase of reflected light increases and phase delay is observed at the same time. Its polarization state changes from linear polarized light to elliptical (circular) polarized light. The propagation direction of light and the phase of polarization state can be changed by using the retarder, and the defect of thin film λ/4 wave-plate is solved.
2023, 47(5): 691-699.
doi: 10.7510/jgjs.issn.1001-3806.2023.05.018
Abstract:
In order to solve the phase demodulation problem caused by cyclic jump, an automatic phase unwrapping algorithm based on field-programmable gate array (FPGA) was proposed. Using the data pipeline structure, the number of jump cycles was automatically found by the state machine, and the automatic phase compensation was realized by the number of jump cycles. The proposed algorithm was theoretically analyzed and verified by experiments. The results show that when the width of the counting bit of the jump cycle number is 8 bits, the FPGA can be applicable to the situation of less than 256 jumps. When the digital width of the jump cycle number increases, more jumps are applicable. The automatic phase unwrapping algorithm almost does not occupy storage resources, and can solve the arctangent phase jump problem caused by the increase of the vibration modulation range. The phase demodulation error is within 1‰, which meets the real-time requirements of high-precision vibration detection. The automatic phase unwrapping algorithm provides a more concise solution to the problem of cyclic jump in the calculation results of arctangent phase in laser Doppler vibration measurement.
In order to solve the phase demodulation problem caused by cyclic jump, an automatic phase unwrapping algorithm based on field-programmable gate array (FPGA) was proposed. Using the data pipeline structure, the number of jump cycles was automatically found by the state machine, and the automatic phase compensation was realized by the number of jump cycles. The proposed algorithm was theoretically analyzed and verified by experiments. The results show that when the width of the counting bit of the jump cycle number is 8 bits, the FPGA can be applicable to the situation of less than 256 jumps. When the digital width of the jump cycle number increases, more jumps are applicable. The automatic phase unwrapping algorithm almost does not occupy storage resources, and can solve the arctangent phase jump problem caused by the increase of the vibration modulation range. The phase demodulation error is within 1‰, which meets the real-time requirements of high-precision vibration detection. The automatic phase unwrapping algorithm provides a more concise solution to the problem of cyclic jump in the calculation results of arctangent phase in laser Doppler vibration measurement.
2023, 47(5): 700-707.
doi: 10.7510/jgjs.issn.1001-3806.2023.05.019
Abstract:
In order to solve the problem that it is difficult to extract the local feature information of point cloud in 3-D point cloud completion, a multi-resolution point cloud completion network structure based on fusion graph attention was proposed. The method of data processing with generative adversarial network framework was adopted. The structure of the point cloud image was constructed by the generator through the graph attention layer, the feature information of different resolutions with grid data was fused, and the folding operation was combined to reconstruct the missing structure and output the stepwise completed point cloud data. The truth and falsity of the point cloud was discriminated by the discriminator. The accuracy was improved through feedback, and the generator was optimized, so that the generated data has a fine geometric structure, which is similar to the real point cloud. The proposed method was verified experimentally and analyzed theoretically with four related methods on the shape dataset, and the optimal results were obtained. The results show that the proposed method can effectively complete the missing part of point cloud shape and obtain a complete and uniform point cloud shape, the network performance is improved by about 1.79% compared with point fractal network, the proposed method also achieves the expected effect on the completion of the measured data. The proposed point cloud completion network structure not only extracts the global shape features of point cloud, but also better extracts the local geometric feature information of point cloud, making the completed point cloud shape more refined. This study provides a reference for 3-D modeling of smart cities.
In order to solve the problem that it is difficult to extract the local feature information of point cloud in 3-D point cloud completion, a multi-resolution point cloud completion network structure based on fusion graph attention was proposed. The method of data processing with generative adversarial network framework was adopted. The structure of the point cloud image was constructed by the generator through the graph attention layer, the feature information of different resolutions with grid data was fused, and the folding operation was combined to reconstruct the missing structure and output the stepwise completed point cloud data. The truth and falsity of the point cloud was discriminated by the discriminator. The accuracy was improved through feedback, and the generator was optimized, so that the generated data has a fine geometric structure, which is similar to the real point cloud. The proposed method was verified experimentally and analyzed theoretically with four related methods on the shape dataset, and the optimal results were obtained. The results show that the proposed method can effectively complete the missing part of point cloud shape and obtain a complete and uniform point cloud shape, the network performance is improved by about 1.79% compared with point fractal network, the proposed method also achieves the expected effect on the completion of the measured data. The proposed point cloud completion network structure not only extracts the global shape features of point cloud, but also better extracts the local geometric feature information of point cloud, making the completed point cloud shape more refined. This study provides a reference for 3-D modeling of smart cities.
2023, 47(5): 708-715.
doi: 10.7510/jgjs.issn.1001-3806.2023.05.020
Abstract:
In order to accurately reconstruct the 3-D chlorophyll distribution of adaxial and abaxial leaves, the leaves and plants of epipremnum aureum in different growth states were selected as the experimental samples, whose spatial-spectral point cloud data were acquired by hyperspectral light detection and ranging (LiDAR), and a reconstruction method based on classification and prediction was designed. Chlorophyll content prediction models of adaxial and abaxial leaves were constructed by the partial least squares regression modeling method, to calculate the chlorophyll of different leaves, adaxial and abaxial leaves were classified with the spectral adaptive threshold selection method, and then the plant 3-D distribution of chlorophyll was reconstructed. The results show that this reconstruction method can obtain the predicted value with a coefficient of determination 0.69 and a root mean square error 4.97, which is closer to the true value. This result will provide a basis and theoretical approach for plant phenotypes research.
In order to accurately reconstruct the 3-D chlorophyll distribution of adaxial and abaxial leaves, the leaves and plants of epipremnum aureum in different growth states were selected as the experimental samples, whose spatial-spectral point cloud data were acquired by hyperspectral light detection and ranging (LiDAR), and a reconstruction method based on classification and prediction was designed. Chlorophyll content prediction models of adaxial and abaxial leaves were constructed by the partial least squares regression modeling method, to calculate the chlorophyll of different leaves, adaxial and abaxial leaves were classified with the spectral adaptive threshold selection method, and then the plant 3-D distribution of chlorophyll was reconstructed. The results show that this reconstruction method can obtain the predicted value with a coefficient of determination 0.69 and a root mean square error 4.97, which is closer to the true value. This result will provide a basis and theoretical approach for plant phenotypes research.
2023, 47(5): 716-722.
doi: 10.7510/jgjs.issn.1001-3806.2023.05.021
Abstract:
In order to evaluate the relationship between detection efficiency and meteorological elements in different scanning modes of coherent wind light detection and ranging(LiDAR), the detection data of coherent wind LiDAR at Guanghan Airport from August 2020 to July 2021 were used for analysis and verification. The results show that the detection efficiency decreases linearly after the detection range is larger than 3 km under plane position indicator scanning mode; When scanning at 90°, the detection efficiency decreases linearly after 500 m; The overall detection efficiency was highest in November, lowest in July, and decreased from November to following year in July, and increased from July to November; The detection efficiency is low from sunset to sunrise, and the detection efficiency is highest at noon; In summer and autumn, LiDAR detection efficiency is positively correlated with PM2.5 concentration, and negatively correlated with logarithm of precipitation, it provides an important basic guarantee for the accuracy of airport LiDAR identification of low-level wind shear.
In order to evaluate the relationship between detection efficiency and meteorological elements in different scanning modes of coherent wind light detection and ranging(LiDAR), the detection data of coherent wind LiDAR at Guanghan Airport from August 2020 to July 2021 were used for analysis and verification. The results show that the detection efficiency decreases linearly after the detection range is larger than 3 km under plane position indicator scanning mode; When scanning at 90°, the detection efficiency decreases linearly after 500 m; The overall detection efficiency was highest in November, lowest in July, and decreased from November to following year in July, and increased from July to November; The detection efficiency is low from sunset to sunrise, and the detection efficiency is highest at noon; In summer and autumn, LiDAR detection efficiency is positively correlated with PM2.5 concentration, and negatively correlated with logarithm of precipitation, it provides an important basic guarantee for the accuracy of airport LiDAR identification of low-level wind shear.
2023, 47(5): 723-728.
doi: 10.7510/jgjs.issn.1001-3806.2023.05.022
Abstract:
In order to reduce the influence of weld shape and color diversity on segmentation accuracy in laser weld semantic segmentation, an image semantic segmentation method based on attention mechanism was used to extract weld. The image in the weld was converted from RGB(red, green, blue) to HSV(hue, saturation, value) color space, and the weld surface color was recognized in HSV. The effects of three kinds of welds on region segmentation and color recognition were analyzed. The results show that the average pixel accuracy of the weld segmentation region is about 91.2%, and the segmentation effect of the attention U-Net model with attention mechanism is better. The results of automatic identification of weld surface color meet production requirements, and have broad application prospects in industrial production.
In order to reduce the influence of weld shape and color diversity on segmentation accuracy in laser weld semantic segmentation, an image semantic segmentation method based on attention mechanism was used to extract weld. The image in the weld was converted from RGB(red, green, blue) to HSV(hue, saturation, value) color space, and the weld surface color was recognized in HSV. The effects of three kinds of welds on region segmentation and color recognition were analyzed. The results show that the average pixel accuracy of the weld segmentation region is about 91.2%, and the segmentation effect of the attention U-Net model with attention mechanism is better. The results of automatic identification of weld surface color meet production requirements, and have broad application prospects in industrial production.
