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RSS2015 Vol. 39, No. 3
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2015, 39(3): 289-294.
doi: 10.7510/jgjs.issn.1001-3806.2015.03.001
[Abstract]
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Abstract:
In order to overcome the backwards, such as low contrast and strong interference fringes of the images captured by terahertz(THz) continuous wave imaging system based on backward wave oscillator, to detect target objects from background and avoid the disturbance of the irregular interference fringes, the cluster method based on C-means of fuzzy local information was used to target detection and the membership function was improved so that it was suitable to terahertz images. Experiment results show that the proposed clustering algorithm can detect target objects from terahertz images corrupted with irregular interference fringes and has better accuracy than the classic image clustering algorithm.
In order to overcome the backwards, such as low contrast and strong interference fringes of the images captured by terahertz(THz) continuous wave imaging system based on backward wave oscillator, to detect target objects from background and avoid the disturbance of the irregular interference fringes, the cluster method based on C-means of fuzzy local information was used to target detection and the membership function was improved so that it was suitable to terahertz images. Experiment results show that the proposed clustering algorithm can detect target objects from terahertz images corrupted with irregular interference fringes and has better accuracy than the classic image clustering algorithm.
2015, 39(3): 295-299.
doi: 10.7510/jgjs.issn.1001-3806.2015.03.002
Abstract:
In order to reduce the impact of noise on laser optical experiments, a lock-in amplifier and an optical chopper were used. The laser was modulated at a specific frequency by the chopper and the frequency was sent to the lock-in amplifier as a reference signal. The useful signal was extracted with the lock-in amplifier based on the coherent detection method. In order to relieve the operator's labor during experiments, an automatic software system under LabVIEW software platform was developed to operate the experiments and record data automatically, which greatly simplified the operation process. The results show that measurement noise of this method reduced from 9.1% to 4.7% compared with laser intensity measurement with a power meter directly. The automatic software system saves more than 50% of time compared with manual recording of data.
In order to reduce the impact of noise on laser optical experiments, a lock-in amplifier and an optical chopper were used. The laser was modulated at a specific frequency by the chopper and the frequency was sent to the lock-in amplifier as a reference signal. The useful signal was extracted with the lock-in amplifier based on the coherent detection method. In order to relieve the operator's labor during experiments, an automatic software system under LabVIEW software platform was developed to operate the experiments and record data automatically, which greatly simplified the operation process. The results show that measurement noise of this method reduced from 9.1% to 4.7% compared with laser intensity measurement with a power meter directly. The automatic software system saves more than 50% of time compared with manual recording of data.
2015, 39(3): 300-303.
doi: 10.7510/jgjs.issn.1001-3806.2015.03.003
Abstract:
In order to achieve non-destructively measurement of optical parameters of biological tissue, optical parameters of tissue simulation phantoms were measured in experiments based on CCD technology and least square support vector machine (LS-SVM). From the experiment, the diffuse reflectance distribution of tissue simulation phantoms was measured. The LS-SVM regression model between the optical parameters and the corresponding diffuse reflectance distribution was founded. The prediction average error of the optical parameters of the tissue simulation phantoms was only 5% under small sample conditions. The results show that combination of CCD measurement technology and LS-SVM can measure the optical parameters of tissue simulation phantom accurately.
In order to achieve non-destructively measurement of optical parameters of biological tissue, optical parameters of tissue simulation phantoms were measured in experiments based on CCD technology and least square support vector machine (LS-SVM). From the experiment, the diffuse reflectance distribution of tissue simulation phantoms was measured. The LS-SVM regression model between the optical parameters and the corresponding diffuse reflectance distribution was founded. The prediction average error of the optical parameters of the tissue simulation phantoms was only 5% under small sample conditions. The results show that combination of CCD measurement technology and LS-SVM can measure the optical parameters of tissue simulation phantom accurately.
2015, 39(3): 304-307.
doi: 10.7510/jgjs.issn.1001-3806.2015.03.004
Abstract:
For the purpose that a fiber sensor system of Brillouin optical time-domain analysis (BOTDA) needs two light sources of Brillouin frequency shift, a dual-channel light source system was proposed by employing Brillouin ring cavity frequency shift and electro-optic modulation. According to several parameters, pulse width, repetition frequency rate and step of frequency sweeping, the output light spectrum of dual-channel modulation was simulated. Based on field programmable gate array (FPGA) and direct digital synthesizer (DDS) chip, experimental system was setup and both pulse detect light and sweep frequency pulse light could be output from the setup. The minimum pulse width of the detection pulse was 10ns, and the sweep frequency range reached 0MHz~90MHz with a frequency sweep step of 30Hz and a time step of 1.6s. Experimental results show that the dual channel light source can meet the demands of general BOTDA systems, without the necessity of some high speed devices, such as high frequency microwave signal source. Signal acquisition modules can adopt regular photoelectric detectors below 100MHz so as to reduce the system cost.
For the purpose that a fiber sensor system of Brillouin optical time-domain analysis (BOTDA) needs two light sources of Brillouin frequency shift, a dual-channel light source system was proposed by employing Brillouin ring cavity frequency shift and electro-optic modulation. According to several parameters, pulse width, repetition frequency rate and step of frequency sweeping, the output light spectrum of dual-channel modulation was simulated. Based on field programmable gate array (FPGA) and direct digital synthesizer (DDS) chip, experimental system was setup and both pulse detect light and sweep frequency pulse light could be output from the setup. The minimum pulse width of the detection pulse was 10ns, and the sweep frequency range reached 0MHz~90MHz with a frequency sweep step of 30Hz and a time step of 1.6s. Experimental results show that the dual channel light source can meet the demands of general BOTDA systems, without the necessity of some high speed devices, such as high frequency microwave signal source. Signal acquisition modules can adopt regular photoelectric detectors below 100MHz so as to reduce the system cost.
2015, 39(3): 308-311.
doi: 10.7510/jgjs.issn.1001-3806.2015.03.005
Abstract:
In order to improve the mass flow rate of a single discharge tube to get higher output power, a new discharge tube was designed after optimization of nozzle and loop structure of the original tube. The flow field, the output power and the mass flow rate of the discharge tube were analyzed through numerical simulation and measured in experiments. The computational results were consistent with the experimental results well. The flow field and the output power of different structure of discharge tubes were compared. The results show that the mass flow rate of the newly designed discharge tube is larger, the output power of single discharge tube is higher and the glow discharge is uniform.
In order to improve the mass flow rate of a single discharge tube to get higher output power, a new discharge tube was designed after optimization of nozzle and loop structure of the original tube. The flow field, the output power and the mass flow rate of the discharge tube were analyzed through numerical simulation and measured in experiments. The computational results were consistent with the experimental results well. The flow field and the output power of different structure of discharge tubes were compared. The results show that the mass flow rate of the newly designed discharge tube is larger, the output power of single discharge tube is higher and the glow discharge is uniform.
2015, 39(3): 312-315.
doi: 10.7510/jgjs.issn.1001-3806.2015.03.006
Abstract:
In order to fabricate colloidal crystal micro-fiber structure, monodisperse polystyrene microspheres were grown into self-assembled colloidal crystals on the surface of micro-nano fiber by Czochralski method. Microstructure of colloidal crystals and properties of transmission spectra were characterized by scanning electron microscopy and spectroscopy. The results show that polystyrene microspheres are accumulated orderly and are assembled to colloidal crystals by themselves. The colloidal crystal has [111] face-centered cubic structure and the surface of micro-fiber has facet of face-centered cubic structure. The transmission peak of micro-structured fiber is about 1400.8nm, corresponding to the photonic band gap in the [111] direction of face-centered cubic structure. This kind of micro-fiber with three-dimensional photonic crystal structure is very useful in the applications of micro-fiber sensors or filters.
In order to fabricate colloidal crystal micro-fiber structure, monodisperse polystyrene microspheres were grown into self-assembled colloidal crystals on the surface of micro-nano fiber by Czochralski method. Microstructure of colloidal crystals and properties of transmission spectra were characterized by scanning electron microscopy and spectroscopy. The results show that polystyrene microspheres are accumulated orderly and are assembled to colloidal crystals by themselves. The colloidal crystal has [111] face-centered cubic structure and the surface of micro-fiber has facet of face-centered cubic structure. The transmission peak of micro-structured fiber is about 1400.8nm, corresponding to the photonic band gap in the [111] direction of face-centered cubic structure. This kind of micro-fiber with three-dimensional photonic crystal structure is very useful in the applications of micro-fiber sensors or filters.
2015, 39(3): 316-319.
doi: 10.7510/jgjs.issn.1001-3806.2015.03.007
Abstract:
In order to meet the requirements of high stability of semiconductor laser in high precision measurement field, a semiconductor laser control system with high stability and low noise was proposed. The control system consists of a current driver and temperature controller. A negative feedback was used to maintain the current stability. A highly integrated MAX1978, as main control chip, was used to drive the thermoelectric cooler for temperature compensation. After the experiment verification, current was adjustable in the range of 200mA, current control accuracy was less than 1A. The effective value of alternating current noise voltage within 3kHz~100kHz bandwidth was less than 300nA and long-term temperature drift was less than 2m℃. The results show that the system can be used to drive distribibuted feedback external cavity LD lasers and distributed Bragg reflector LD lasers.
In order to meet the requirements of high stability of semiconductor laser in high precision measurement field, a semiconductor laser control system with high stability and low noise was proposed. The control system consists of a current driver and temperature controller. A negative feedback was used to maintain the current stability. A highly integrated MAX1978, as main control chip, was used to drive the thermoelectric cooler for temperature compensation. After the experiment verification, current was adjustable in the range of 200mA, current control accuracy was less than 1A. The effective value of alternating current noise voltage within 3kHz~100kHz bandwidth was less than 300nA and long-term temperature drift was less than 2m℃. The results show that the system can be used to drive distribibuted feedback external cavity LD lasers and distributed Bragg reflector LD lasers.
2015, 39(3): 320-324.
doi: 10.7510/jgjs.issn.1001-3806.2015.03.008
Abstract:
In order to find a method of prediction and optimization of laser dressing grinding wheel, an optimization model of process parameters for laser dressing grinding wheels was established based on the neural network and particle swarm optimization. Firstly, the neural network model mapping the relationship between the process parameters and the specimen surface roughness was constructed . Then, the process parameters were optimized by means of the particle swarm optimization algorithm based on the predication model. Finally, laser dressing experiments were carried out based on 5 groups of parameters optimized by the particle swarm algorithm. Experimental results show that the relative error between the sample value and output value from neural network is less than 3% and the relative error between the test value and the expected value is lower than 6%. In conclusion, the model has good ability of optimization.
In order to find a method of prediction and optimization of laser dressing grinding wheel, an optimization model of process parameters for laser dressing grinding wheels was established based on the neural network and particle swarm optimization. Firstly, the neural network model mapping the relationship between the process parameters and the specimen surface roughness was constructed . Then, the process parameters were optimized by means of the particle swarm optimization algorithm based on the predication model. Finally, laser dressing experiments were carried out based on 5 groups of parameters optimized by the particle swarm algorithm. Experimental results show that the relative error between the sample value and output value from neural network is less than 3% and the relative error between the test value and the expected value is lower than 6%. In conclusion, the model has good ability of optimization.
2015, 39(3): 325-328.
doi: 10.7510/jgjs.issn.1001-3806.2015.03.009
Abstract:
In order to study effect of optical route of two-photon polymerization on polymerization result, firstly, the mainly three parts i.e., beam expander, scanning galvanometer and focus lens were designed with ZEMAX software. Secondly, the whole optical route was simulated after combining the three parts together. The changes of the beam were monitored at different positions of the route with a detector. The variation of laser energy distribution could be seen in simulated diagrams and the influence of beam expander on the final focal results was analyzed. The results show that the larger the amplification multiple of the beam expander the smaller the distance between the beam expander and the light source, and the higher the energy density on the focus spot. The simulation results also provide the basis for actual experiment system construction.
In order to study effect of optical route of two-photon polymerization on polymerization result, firstly, the mainly three parts i.e., beam expander, scanning galvanometer and focus lens were designed with ZEMAX software. Secondly, the whole optical route was simulated after combining the three parts together. The changes of the beam were monitored at different positions of the route with a detector. The variation of laser energy distribution could be seen in simulated diagrams and the influence of beam expander on the final focal results was analyzed. The results show that the larger the amplification multiple of the beam expander the smaller the distance between the beam expander and the light source, and the higher the energy density on the focus spot. The simulation results also provide the basis for actual experiment system construction.
2015, 39(3): 329-332.
doi: 10.7510/jgjs.issn.1001-3806.2015.03.010
Abstract:
In order to achieve a quartz filter insensitive to incident angles, based on the principle that the shifting direction of output central wavelength of a quartz birefringent optical filter is different when the filter rotate around an axis parallel and perpendicular to the quartz optical axis, a new type of two-stage modified Lyot quartz filter was presented. After theoretical analysis and experimental verification, the results show that the central wavelength of this new type filter is actually insensitive to the incident angles less than 18. The conclusion is in good agreement with the experiment results.
In order to achieve a quartz filter insensitive to incident angles, based on the principle that the shifting direction of output central wavelength of a quartz birefringent optical filter is different when the filter rotate around an axis parallel and perpendicular to the quartz optical axis, a new type of two-stage modified Lyot quartz filter was presented. After theoretical analysis and experimental verification, the results show that the central wavelength of this new type filter is actually insensitive to the incident angles less than 18. The conclusion is in good agreement with the experiment results.
2015, 39(3): 333-336.
doi: 10.7510/jgjs.issn.1001-3806.2015.03.011
Abstract:
In order to overcome defects of the traditional phase measurement hardware, such as circuit complexity, low measurement precision and narrow band width, a novel signal processing system of a laser heterodyne interferometer was designed based on AD8302 with good performance. The work theory of the system was analyzed in detail. Static and dynamic experimental data were obtained. The results show that measurement error of the system is less than 0.5 and the measurement resolution of heterodyne interference can reach 0.088nm in theory which is more conductive to measure the microvibration signal.
In order to overcome defects of the traditional phase measurement hardware, such as circuit complexity, low measurement precision and narrow band width, a novel signal processing system of a laser heterodyne interferometer was designed based on AD8302 with good performance. The work theory of the system was analyzed in detail. Static and dynamic experimental data were obtained. The results show that measurement error of the system is less than 0.5 and the measurement resolution of heterodyne interference can reach 0.088nm in theory which is more conductive to measure the microvibration signal.
2015, 39(3): 337-340.
doi: 10.7510/jgjs.issn.1001-3806.2015.03.012
Abstract:
In order to satisfy the structure stability of a large rectangle mirror(800mm400mm), a novel flexible supporting structure of space camera primary mirror with super wide coverage was designed by Bipod principle in which three supporting points were adopted in backside of the mirror. The influence of the structure parameters on the mass and stiffness of the mirror was analyzed. The parameters which had significant effect were chosen as the design variables and the optimization design of mirror lightweight structure was carried out. The finite element analysis was conducted. The results indicate that the mirror structure has better mechanical adaptability, thermal adaptability and dynamical stiffness. The results of finite element analysis are consistent with the results of vibration test.
In order to satisfy the structure stability of a large rectangle mirror(800mm400mm), a novel flexible supporting structure of space camera primary mirror with super wide coverage was designed by Bipod principle in which three supporting points were adopted in backside of the mirror. The influence of the structure parameters on the mass and stiffness of the mirror was analyzed. The parameters which had significant effect were chosen as the design variables and the optimization design of mirror lightweight structure was carried out. The finite element analysis was conducted. The results indicate that the mirror structure has better mechanical adaptability, thermal adaptability and dynamical stiffness. The results of finite element analysis are consistent with the results of vibration test.
2015, 39(3): 341-343.
doi: 10.7510/jgjs.issn.1001-3806.2015.03.013
Abstract:
Plane target positioning is one of the important parts to process the data obtained by terrestrial laser scanner measurement. In order to reduce the influence on the accuracy of multiple-scan registration, coordinate system transformation and object aided location when extracting coordinates of the target centre, a new method was proposed based on the least square method, K-means clustering method and the reflection intensity at the scanning points. After theoretical analysis and experimental verification, the positioning data with high accuracy was achieved. There are four major steps to deal with the problem: firstly, to eliminate noise points by using the total least squares algorithm; secondly, to classify the region of scanning data based on the reflection intensity; thirdly, to extract the centre coordinates of each region by means of K-means clustering method; finally, to calculate the target centre coordinates according to the centre position each region. The results show that reliable positioning accuracy can be obtained at a certain distance by means of the proposed method. The deviation of the coordinates is in the order of sub-millimeters, and the distance deviation is also in the order of sub-millimeters. The research is helpful for coordinate extraction of plane target centers.
Plane target positioning is one of the important parts to process the data obtained by terrestrial laser scanner measurement. In order to reduce the influence on the accuracy of multiple-scan registration, coordinate system transformation and object aided location when extracting coordinates of the target centre, a new method was proposed based on the least square method, K-means clustering method and the reflection intensity at the scanning points. After theoretical analysis and experimental verification, the positioning data with high accuracy was achieved. There are four major steps to deal with the problem: firstly, to eliminate noise points by using the total least squares algorithm; secondly, to classify the region of scanning data based on the reflection intensity; thirdly, to extract the centre coordinates of each region by means of K-means clustering method; finally, to calculate the target centre coordinates according to the centre position each region. The results show that reliable positioning accuracy can be obtained at a certain distance by means of the proposed method. The deviation of the coordinates is in the order of sub-millimeters, and the distance deviation is also in the order of sub-millimeters. The research is helpful for coordinate extraction of plane target centers.
2015, 39(3): 344-348.
doi: 10.7510/jgjs.issn.1001-3806.2015.03.014
Abstract:
Sensors are widely used in railway safety maintenance. In order to achieve real-time online gauge measurement, an on-board machine vision system of gauge detection was established with the combination of laser technique and CCD image sensors. Firstly, the working principle and structure of the system were introduced. A fast extraction method of rail contour centerline was proposed based on pre-noising and distance transform (PNDT) algorithm. Then, the image was enhanced with the strong contrast stretching method and exponential transform method. Fast pre-noising processing was conducted with the combination of Gaussian smoothing and dynamic region of interest (ROI). Finally, precise threshold segmentation processing was carried out for the images. The rail contour centerline was gotten and the gauge measurement point was located with the distance transform method. Experimental results show that the detection accuracy of the system is -1mm~+1mm and image frame processing speed is 14.35ms. The system can meet the requirement of the robustness, detection speed and accuracy in real-time online gauge detection.
Sensors are widely used in railway safety maintenance. In order to achieve real-time online gauge measurement, an on-board machine vision system of gauge detection was established with the combination of laser technique and CCD image sensors. Firstly, the working principle and structure of the system were introduced. A fast extraction method of rail contour centerline was proposed based on pre-noising and distance transform (PNDT) algorithm. Then, the image was enhanced with the strong contrast stretching method and exponential transform method. Fast pre-noising processing was conducted with the combination of Gaussian smoothing and dynamic region of interest (ROI). Finally, precise threshold segmentation processing was carried out for the images. The rail contour centerline was gotten and the gauge measurement point was located with the distance transform method. Experimental results show that the detection accuracy of the system is -1mm~+1mm and image frame processing speed is 14.35ms. The system can meet the requirement of the robustness, detection speed and accuracy in real-time online gauge detection.
2015, 39(3): 349-352.
doi: 10.7510/jgjs.issn.1001-3806.2015.03.015
Abstract:
In order to study the influence of defocusing amount on laser ultrasonic signal amplitude, thickness measurement experiment was carried out. Aluminum test blocks in different thickness were irradiated by the same pulse laser while using plano-convex spherical lens whose focal length was 100mm. The lens was fixed at a 5-D optical stage and the horizontal axis knob was adjusted to change the defocusing amount. The ultrasound signals were received by piezoelectric probe with 5MHz center frequency and the ultrasonic signal data of all test blocks under each defocusing amount were recorded. The thickness of each aluminum test block was obtained according to the time interval between the signals. The results indicate that relative errors between the measurement value and the actual value are in the range of 3% and the maximum signal amplitude can be obtained when the defocusing amount is -10mm under the same laser irradiation.
In order to study the influence of defocusing amount on laser ultrasonic signal amplitude, thickness measurement experiment was carried out. Aluminum test blocks in different thickness were irradiated by the same pulse laser while using plano-convex spherical lens whose focal length was 100mm. The lens was fixed at a 5-D optical stage and the horizontal axis knob was adjusted to change the defocusing amount. The ultrasound signals were received by piezoelectric probe with 5MHz center frequency and the ultrasonic signal data of all test blocks under each defocusing amount were recorded. The thickness of each aluminum test block was obtained according to the time interval between the signals. The results indicate that relative errors between the measurement value and the actual value are in the range of 3% and the maximum signal amplitude can be obtained when the defocusing amount is -10mm under the same laser irradiation.
2015, 39(3): 353-356.
doi: 10.7510/jgjs.issn.1001-3806.2015.03.016
Abstract:
In order to reduce the influence of temperature on output wavelength and power stability of semiconductor lasers, a constant current module was designed to drive thermoelectric cooler. The cooling capacity of the thermoelectric cooler was controlled by changing the current of the constant current module. The optimal control parameters of proportion-integration-differentiation algorithm were set to realize high precision temperature control. The system consists of high precision temperature measurement circuit, control core of DSP F28335, thermoelectric cooler control circuit, human-computer interaction and communication module. Constant temperature control was realized for a 50W high power laser diode at 5℃~26℃ ambient temperature, the temperature control accuracy reached 0.02℃ at 15℃~45℃. The results show that this system has a wide temperature control range and high control precision, which satisfies the requirement of temperature control of high power semiconductor lasers.
In order to reduce the influence of temperature on output wavelength and power stability of semiconductor lasers, a constant current module was designed to drive thermoelectric cooler. The cooling capacity of the thermoelectric cooler was controlled by changing the current of the constant current module. The optimal control parameters of proportion-integration-differentiation algorithm were set to realize high precision temperature control. The system consists of high precision temperature measurement circuit, control core of DSP F28335, thermoelectric cooler control circuit, human-computer interaction and communication module. Constant temperature control was realized for a 50W high power laser diode at 5℃~26℃ ambient temperature, the temperature control accuracy reached 0.02℃ at 15℃~45℃. The results show that this system has a wide temperature control range and high control precision, which satisfies the requirement of temperature control of high power semiconductor lasers.
2015, 39(3): 357-360.
doi: 10.7510/jgjs.issn.1001-3806.2015.03.017
Abstract:
In order to realize the real-time and accuracy of an aircraft parking system and ensure the reliability of laser scanning data, a laser scanning system based on dual galvanometer was established. Error of the laser scanning system and noise of the laser measuring data were analyzed by using the data processing technology such as data reduction, de-noising, coordinate transformation, distance compensation. The laser scanning data of airplane parking was obtained after field test of an airport. The results show that airplanes can be parked accurately through processing of laser scanning data, and he distance between stop line and plane can be assured to less than 100mm.
In order to realize the real-time and accuracy of an aircraft parking system and ensure the reliability of laser scanning data, a laser scanning system based on dual galvanometer was established. Error of the laser scanning system and noise of the laser measuring data were analyzed by using the data processing technology such as data reduction, de-noising, coordinate transformation, distance compensation. The laser scanning data of airplane parking was obtained after field test of an airport. The results show that airplanes can be parked accurately through processing of laser scanning data, and he distance between stop line and plane can be assured to less than 100mm.
2015, 39(3): 361-365.
doi: 10.7510/jgjs.issn.1001-3806.2015.03.018
Abstract:
In order to study the propagation characteristics of short laser pulses in partially stripped plasma, parameters evolution equation of short laser pulses in partially ionized plasma was derived using variational method. The effect parameters of wakefield, relativistic self-focusing and intensity of partially stripped plasma were analyzed. The coupled evolution equations of the laser spot size and the pulse length were derived and the propagation of a short laser pulse in partially stripped plasma was analyzed under the effects of transverse wakefield(TWF) and longitudinal wakefield(LWF). The results show that laser pulse can be allowed to propagate in patitally stripped plasma only when a certain condition is satisfied. The evolution of the pulse length should be considered in partially stripped plasma. When laser pulse and plasma density is constant, with the increasing of ionization degree, wakefield will enhance the self-focusing of the laser pulse further. Longitudinal wakefield has more obvious effect on self-focusing of laser pulse than transverse wakefield. The results may be significant theoretically to the mechanism of ionization-induced injection and acceleration by an intense laser pulse.
In order to study the propagation characteristics of short laser pulses in partially stripped plasma, parameters evolution equation of short laser pulses in partially ionized plasma was derived using variational method. The effect parameters of wakefield, relativistic self-focusing and intensity of partially stripped plasma were analyzed. The coupled evolution equations of the laser spot size and the pulse length were derived and the propagation of a short laser pulse in partially stripped plasma was analyzed under the effects of transverse wakefield(TWF) and longitudinal wakefield(LWF). The results show that laser pulse can be allowed to propagate in patitally stripped plasma only when a certain condition is satisfied. The evolution of the pulse length should be considered in partially stripped plasma. When laser pulse and plasma density is constant, with the increasing of ionization degree, wakefield will enhance the self-focusing of the laser pulse further. Longitudinal wakefield has more obvious effect on self-focusing of laser pulse than transverse wakefield. The results may be significant theoretically to the mechanism of ionization-induced injection and acceleration by an intense laser pulse.
2015, 39(3): 366-371.
doi: 10.7510/jgjs.issn.1001-3806.2015.03.019
Abstract:
In order to meet the demand that different users sent at different rates in an optical code division multiple access (OCDMA) system,the interval-set with no repetitive data was designed based on MATLAB and the multi-length address codes with ideal correlation was obtained. The code family will be obtained corresponding to the given code weight and corresponding capacity. The formula of bit error rate (BER) was derived based on the characteristics of the codes and BER diagram with the change of user numbers was given. The multi-rate OCDMA system was designed and simulated. The bit error rate and system performance were analyzed. The results show that the codes have the good correlation and good performance of BER. The system can meet the needs of users at different rates. The original sign can be restored after decoding and the excellent eye diagrams can be obtained. The results are helpful for the development of multi-rate OCDMA systems.
In order to meet the demand that different users sent at different rates in an optical code division multiple access (OCDMA) system,the interval-set with no repetitive data was designed based on MATLAB and the multi-length address codes with ideal correlation was obtained. The code family will be obtained corresponding to the given code weight and corresponding capacity. The formula of bit error rate (BER) was derived based on the characteristics of the codes and BER diagram with the change of user numbers was given. The multi-rate OCDMA system was designed and simulated. The bit error rate and system performance were analyzed. The results show that the codes have the good correlation and good performance of BER. The system can meet the needs of users at different rates. The original sign can be restored after decoding and the excellent eye diagrams can be obtained. The results are helpful for the development of multi-rate OCDMA systems.
2015, 39(3): 372-376.
doi: 10.7510/jgjs.issn.1001-3806.2015.03.020
Abstract:
Micropulse lidar (MPL) is an effective tool for aerosol detection. To verify the accuracy of detection, MPL was used to calculate the aerosol optical depth (AOD) in the northern suburb of Nanjing with the Fernald inversion method. The inversion results were compared with those obtained with a sun-photometer,a Raman-Rayleigh-Mie lidar and a standard aerosol detection instrument (a moderate-resolution imaging spectroradiometer). The results show good correlation among them. The MPL is useful for AOD inversion and can be used to verify other measurement data effectively.
Micropulse lidar (MPL) is an effective tool for aerosol detection. To verify the accuracy of detection, MPL was used to calculate the aerosol optical depth (AOD) in the northern suburb of Nanjing with the Fernald inversion method. The inversion results were compared with those obtained with a sun-photometer,a Raman-Rayleigh-Mie lidar and a standard aerosol detection instrument (a moderate-resolution imaging spectroradiometer). The results show good correlation among them. The MPL is useful for AOD inversion and can be used to verify other measurement data effectively.
2015, 39(3): 377-380.
doi: 10.7510/jgjs.issn.1001-3806.2015.03.021
Abstract:
In order to obtain 1064nm laser with high power and high beam quality, a laser resonator was designed to solve thermal lens effect with a concave lens as compensation lens. The selection of the compensation lens was analyzed. A Nd:YAG laser with a compensation lens and a plano-plano cavity was verified in the experiment. High beam quality 55W 1064nm laser was obtained when the focal length of the concave lens was 250mm and the transmittance of the output mirror was 30%. This research is helpful to the design of laser resonators with high power and beam quality.
In order to obtain 1064nm laser with high power and high beam quality, a laser resonator was designed to solve thermal lens effect with a concave lens as compensation lens. The selection of the compensation lens was analyzed. A Nd:YAG laser with a compensation lens and a plano-plano cavity was verified in the experiment. High beam quality 55W 1064nm laser was obtained when the focal length of the concave lens was 250mm and the transmittance of the output mirror was 30%. This research is helpful to the design of laser resonators with high power and beam quality.
2015, 39(3): 381-385.
doi: 10.7510/jgjs.issn.1001-3806.2015.03.022
Abstract:
In order to reduce the influence of noise on the detection results of hyperspectral anomaly detection and improve the rate of anomaly detection,a new anomaly detection process based on improved minimum noise fraction (MNF) transformation was proposed. Firstly, to improve the traditional MNF transform, the weighted neighborhood averaging method was used to estimate the noise matrix,a specific weight was given to each pixel of the neighbor matrix for increasing the portion of background pixels and suppressing the noise pixels in the sample matrix. It was an effective way to extract noise information by calculating the difference. Secondly, improved MNF transform was used to reduce the dimension of hyperspectral image data and to separate the noise from signals effectively.Finally, anomaly detection algorithm was implemented on low-dimensional denoised data. After actual test of AVIRIS data, the results show that the improved algorithm has better effect of reducing the dimension and separating the noise, and the rate of anomaly detection is improved significantly.
In order to reduce the influence of noise on the detection results of hyperspectral anomaly detection and improve the rate of anomaly detection,a new anomaly detection process based on improved minimum noise fraction (MNF) transformation was proposed. Firstly, to improve the traditional MNF transform, the weighted neighborhood averaging method was used to estimate the noise matrix,a specific weight was given to each pixel of the neighbor matrix for increasing the portion of background pixels and suppressing the noise pixels in the sample matrix. It was an effective way to extract noise information by calculating the difference. Secondly, improved MNF transform was used to reduce the dimension of hyperspectral image data and to separate the noise from signals effectively.Finally, anomaly detection algorithm was implemented on low-dimensional denoised data. After actual test of AVIRIS data, the results show that the improved algorithm has better effect of reducing the dimension and separating the noise, and the rate of anomaly detection is improved significantly.
2015, 39(3): 386-390.
doi: 10.7510/jgjs.issn.1001-3806.2015.03.023
Abstract:
In order to improve the power conversion efficiency, dynamic response of the output current and the load voltage adaptability of a traditional linear laser diode driver, a 2kW laser diode driver was designed based on double-phase dual chopping conversion. The static and dynamic characteristics of laser diode driver were tested and analyzed. The power efficiency of the double-phase dual chopping conversion current source was measured. The results show that the power system has good adaptability to the load and outputs rectangular waveforms. The power conversion efficiency of the current source is up to 93% when driving laser diode modules directly. Typical rise and fall time of output current in the range of 0A~100A is approximately 0.5ms. Current ripple factor is less than 0.03%.
In order to improve the power conversion efficiency, dynamic response of the output current and the load voltage adaptability of a traditional linear laser diode driver, a 2kW laser diode driver was designed based on double-phase dual chopping conversion. The static and dynamic characteristics of laser diode driver were tested and analyzed. The power efficiency of the double-phase dual chopping conversion current source was measured. The results show that the power system has good adaptability to the load and outputs rectangular waveforms. The power conversion efficiency of the current source is up to 93% when driving laser diode modules directly. Typical rise and fall time of output current in the range of 0A~100A is approximately 0.5ms. Current ripple factor is less than 0.03%.
2015, 39(3): 391-394.
doi: 10.7510/jgjs.issn.1001-3806.2015.03.024
Abstract:
In order to measure glass bubble size in real time, a dynamic system for measuring glass bubble was built in laboratory with a linear laser diode, a linear CCD and a movement control system based on the laser vision principle. The laser spot projected onto the glass surface was imaged on the photosensitive array of the camera through its zoom lens. Gray images of glass bubbles were grabbed by Sapera CamExpert software with the combination of the transverse scanning of the camera and the longitudinal scanning of the movement platform. The transverse and longitudinal precision of gray images was discussed in theory. The criterion for distinguishing the bubble images from distortion was confirmed and the matching relationship between the clock frequency of work pixels and the longitudinal velocity of the float glass was deduced. Bubble gray images were grabbed at the different longitudinal velocities. The transverse and longitudinal pixel number of glass bubbles was measured by the Sapera Architect. The transverse and longitudinal size of bubbles was acquired by theoretical calculation. Comparing with bubble size measured with a ruler, the relative error of the longitudinal size is 0.18 and of the transverse size is 0.05. The results show that in the case that camera work distance, focal length and pixel cell size are fixed, the transverse size of glass bubbles can be measured by this system in the range of permitted errors.
In order to measure glass bubble size in real time, a dynamic system for measuring glass bubble was built in laboratory with a linear laser diode, a linear CCD and a movement control system based on the laser vision principle. The laser spot projected onto the glass surface was imaged on the photosensitive array of the camera through its zoom lens. Gray images of glass bubbles were grabbed by Sapera CamExpert software with the combination of the transverse scanning of the camera and the longitudinal scanning of the movement platform. The transverse and longitudinal precision of gray images was discussed in theory. The criterion for distinguishing the bubble images from distortion was confirmed and the matching relationship between the clock frequency of work pixels and the longitudinal velocity of the float glass was deduced. Bubble gray images were grabbed at the different longitudinal velocities. The transverse and longitudinal pixel number of glass bubbles was measured by the Sapera Architect. The transverse and longitudinal size of bubbles was acquired by theoretical calculation. Comparing with bubble size measured with a ruler, the relative error of the longitudinal size is 0.18 and of the transverse size is 0.05. The results show that in the case that camera work distance, focal length and pixel cell size are fixed, the transverse size of glass bubbles can be measured by this system in the range of permitted errors.
2015, 39(3): 395-398.
doi: 10.7510/jgjs.issn.1001-3806.2015.03.025
Abstract:
In order to study periodic structures on Si surface induced by femtosecond laser, the relationship between the ripple period and the incident laser wavelength was analyzed. After theoretical analysis and experimental verification, the ripple period of monocrystalline silicon was calculated to be about 800nm. The ripple structures irradiated by 1kHz femtosecond pulses (center wavelength of 800nm, pulse width of 35fs) were studied. The results show that a set of liner ripple structure with parallel and equal spaces is found on the silicon surface irradiated by pulse laser under the condition that power density is slightly higher than damage threshold of the workpiece. The period of ripples is about 750nm by measurement, close to the center wavelength of laser. When the polarization direction of incident laser is changed by a rotatable Glan-Taylor prism, the direction of periodic ripples is changed accordingly. The direction of ripple is perpendicular to the electric vector direction of the femtosecond laser. The processing images show that the irradiated area is much cleaner than the non-irradiated area. The investigation is significant for further study of laser-induced periodic surface structures.
In order to study periodic structures on Si surface induced by femtosecond laser, the relationship between the ripple period and the incident laser wavelength was analyzed. After theoretical analysis and experimental verification, the ripple period of monocrystalline silicon was calculated to be about 800nm. The ripple structures irradiated by 1kHz femtosecond pulses (center wavelength of 800nm, pulse width of 35fs) were studied. The results show that a set of liner ripple structure with parallel and equal spaces is found on the silicon surface irradiated by pulse laser under the condition that power density is slightly higher than damage threshold of the workpiece. The period of ripples is about 750nm by measurement, close to the center wavelength of laser. When the polarization direction of incident laser is changed by a rotatable Glan-Taylor prism, the direction of periodic ripples is changed accordingly. The direction of ripple is perpendicular to the electric vector direction of the femtosecond laser. The processing images show that the irradiated area is much cleaner than the non-irradiated area. The investigation is significant for further study of laser-induced periodic surface structures.
2015, 39(3): 399-404.
doi: 10.7510/jgjs.issn.1001-3806.2015.03.026
Abstract:
In order to improve the surface property of H13 steel, Co42 anti-wear resistance coating was performed by laser cladding method on the surface of H13 steel. The dilution ratio, surface roughness, and the surface morphology of the samples obtained under different parameters were investigated. The proper parameters for laser cladding were as following: defocus value of 20mm, laser scan speed of 1.7mm/s, pulse width of 2.0ms, and pulse frequency of 30Hz. The microstructure of the coating was characterized as fine dendrites with a thickness of 230m. The microhardness presented a gradient decreased from the coating to the substrate. The microhardness in the coating was 650HV0.1, which was about 2.7 times of that of the substrate. The results show that the parameters including defocus value, laser scan speed, pulse frequency, and pulse width influence the above three properties of the coating. This result is helpful for improving friction stir welding technology.
In order to improve the surface property of H13 steel, Co42 anti-wear resistance coating was performed by laser cladding method on the surface of H13 steel. The dilution ratio, surface roughness, and the surface morphology of the samples obtained under different parameters were investigated. The proper parameters for laser cladding were as following: defocus value of 20mm, laser scan speed of 1.7mm/s, pulse width of 2.0ms, and pulse frequency of 30Hz. The microstructure of the coating was characterized as fine dendrites with a thickness of 230m. The microhardness presented a gradient decreased from the coating to the substrate. The microhardness in the coating was 650HV0.1, which was about 2.7 times of that of the substrate. The results show that the parameters including defocus value, laser scan speed, pulse frequency, and pulse width influence the above three properties of the coating. This result is helpful for improving friction stir welding technology.
2015, 39(3): 405-409.
doi: 10.7510/jgjs.issn.1001-3806.2015.03.027
Abstract:
With continuous change of the incidence angle, the transmittance of a Glan-Foucault prism fluctuates a lot. In order to suppress such fluctuation, experiments were carried out for measurement of the transmittance of the Glan-Foucault prism based on a polarized light experimental system. The transmittance was simulated under the assumption that there was multiple beam interference between the prism air gaps. Simulation results and experimental results matched very well. The relative quantitative reason of the transmittance fluctuating with the incidence angle was put forward for the first time. Two methods were proposed to inhibit the fluctuation. The results showed that the reason of fluctuation is the strengthened or weakened multiple beam interference induced by the change of path difference of interference beam between the air gap with the rotation of the stepping motor with the prism as depolarizer. The fluctuations are stronger just because the transmitted beam for the Glan-Foucault prisms is s component on the incident plane which has greater reflectance. The study is helpful for suppressing the transmittance fluctuation of a Glan-Foucault prism.
With continuous change of the incidence angle, the transmittance of a Glan-Foucault prism fluctuates a lot. In order to suppress such fluctuation, experiments were carried out for measurement of the transmittance of the Glan-Foucault prism based on a polarized light experimental system. The transmittance was simulated under the assumption that there was multiple beam interference between the prism air gaps. Simulation results and experimental results matched very well. The relative quantitative reason of the transmittance fluctuating with the incidence angle was put forward for the first time. Two methods were proposed to inhibit the fluctuation. The results showed that the reason of fluctuation is the strengthened or weakened multiple beam interference induced by the change of path difference of interference beam between the air gap with the rotation of the stepping motor with the prism as depolarizer. The fluctuations are stronger just because the transmitted beam for the Glan-Foucault prisms is s component on the incident plane which has greater reflectance. The study is helpful for suppressing the transmittance fluctuation of a Glan-Foucault prism.
2015, 39(3): 416-418.
doi: 10.7510/jgjs.issn.1001-3806.2015.03.029
Abstract:
In order to confront time delay integration charge coupled device (TDI-CCD) imaging equipment efficiently, theoretical analysis and experiments about pulse laser interference on TDI-CCD with pulse width of 10ns and wavelength of 1.06m were conducted. Interference phenomena and mechanisms were analyzed. Saturation threshold was measured by digital image processing method. The results show that saturation threshold is 1.3mJ/cm2. Interference will be more effective if pulse repetition frequency is higher.
In order to confront time delay integration charge coupled device (TDI-CCD) imaging equipment efficiently, theoretical analysis and experiments about pulse laser interference on TDI-CCD with pulse width of 10ns and wavelength of 1.06m were conducted. Interference phenomena and mechanisms were analyzed. Saturation threshold was measured by digital image processing method. The results show that saturation threshold is 1.3mJ/cm2. Interference will be more effective if pulse repetition frequency is higher.
2015, 39(3): 419-422.
doi: 10.7510/jgjs.issn.1001-3806.2015.03.030
Abstract:
Laser speckle has important applications in many areas, such as optical coherence tomography, holography, multimode optical fiber communication and metrology. In order to evaluate the laser speckle, the power spectrum width of speckle image in frequency domain was proposed as the evaluation method of laser speckle. The experiment device of power spectrum speckle evaluation was designed based on optical fiber vibration. A 0.5W, 532nm laser was used as light source and a voltage-driven voice coil motor was used to vibrate the fiber and control laser speckle. A CCD image acquisition card was used to collect images. Speckle contrast method and power spectrum method were respectively used to evaluate the speckle images under different driving voltages. The results show that the method of power spectrum has higher sensitivity and wider range of evaluation.
Laser speckle has important applications in many areas, such as optical coherence tomography, holography, multimode optical fiber communication and metrology. In order to evaluate the laser speckle, the power spectrum width of speckle image in frequency domain was proposed as the evaluation method of laser speckle. The experiment device of power spectrum speckle evaluation was designed based on optical fiber vibration. A 0.5W, 532nm laser was used as light source and a voltage-driven voice coil motor was used to vibrate the fiber and control laser speckle. A CCD image acquisition card was used to collect images. Speckle contrast method and power spectrum method were respectively used to evaluate the speckle images under different driving voltages. The results show that the method of power spectrum has higher sensitivity and wider range of evaluation.
2015, 39(3): 423-427.
doi: 10.7510/jgjs.issn.1001-3806.2015.03.031
Abstract:
In order to solve the undersampling problem of phase unwrapping caused by fast changing of phase, the theory of lateral shearing interferometry was introduced to build optical field and weighted discrete cosine transform was used to solve Poisson equation. On this basis, a new improved least square algorithm based on lateral shearing interferometry(LSBLS) algorithm was put forward. The specific phase unwrapping method was given based on theoretical analysis and the feasibility of the improved algorithm was proved after experimental verification. The results show that the improved algorithm can effectively avoid the undersampling problem caused by the fast changing of phase.
In order to solve the undersampling problem of phase unwrapping caused by fast changing of phase, the theory of lateral shearing interferometry was introduced to build optical field and weighted discrete cosine transform was used to solve Poisson equation. On this basis, a new improved least square algorithm based on lateral shearing interferometry(LSBLS) algorithm was put forward. The specific phase unwrapping method was given based on theoretical analysis and the feasibility of the improved algorithm was proved after experimental verification. The results show that the improved algorithm can effectively avoid the undersampling problem caused by the fast changing of phase.
2015, 39(3): 428-431.
doi: 10.7510/jgjs.issn.1001-3806.2015.03.032
Abstract:
In order to improve the fusion quality of visible and infrared images, a fusion algorithm was put forward based on support value transform. At first, the visible and infrared images were transformed by support value. And then the low frequency coefficient was fused by using local energy ratio modulation weighted rule while the high frequency coefficient was fused by using local variance ratio weighted rule. Finally the fused image was obtained through the reconstruction algorithm and the fusion performance was gotten after theoretical analysis. The results show that, compared with wavelet transform, contourlet transform and the algorithm provided in the literature, the method improves the fine details, contained information and visual effect of the image significantly.
In order to improve the fusion quality of visible and infrared images, a fusion algorithm was put forward based on support value transform. At first, the visible and infrared images were transformed by support value. And then the low frequency coefficient was fused by using local energy ratio modulation weighted rule while the high frequency coefficient was fused by using local variance ratio weighted rule. Finally the fused image was obtained through the reconstruction algorithm and the fusion performance was gotten after theoretical analysis. The results show that, compared with wavelet transform, contourlet transform and the algorithm provided in the literature, the method improves the fine details, contained information and visual effect of the image significantly.
2015, 39(3): 432-436.
doi: 10.7510/jgjs.issn.1001-3806.2015.03.033
Abstract:
In order to solve the problem of signal-to-noise ratio decreasing with the decreasing of cavity length during the miniaturization of ring laser, narrow band filter was designed for suppressing the discharge glow of He-Ne gas without affecting on laser output. Based the unique step automatic coating design method of OpenFilters software and using the initial film thickness as the clue, a number of non-structured membrane structures were designed for the requirements of practical application of spectral characteristics. An optimized narrow band filter for crystal monitoring was selected under the consideration of layer number, thickness and stress. When s polarization light was incident from the air at angle of 45, the filter had peak transmittance of 99.9% at central wavelength of 632.8nm, full width at half maximum of 5.2nm and average transmittance at cutoff band of 1.35%. The results show that the filter can suppress the discharge glow of He-Ne gas effectively and improve the signal-to-noise ratio of miniaturized ring laser.
In order to solve the problem of signal-to-noise ratio decreasing with the decreasing of cavity length during the miniaturization of ring laser, narrow band filter was designed for suppressing the discharge glow of He-Ne gas without affecting on laser output. Based the unique step automatic coating design method of OpenFilters software and using the initial film thickness as the clue, a number of non-structured membrane structures were designed for the requirements of practical application of spectral characteristics. An optimized narrow band filter for crystal monitoring was selected under the consideration of layer number, thickness and stress. When s polarization light was incident from the air at angle of 45, the filter had peak transmittance of 99.9% at central wavelength of 632.8nm, full width at half maximum of 5.2nm and average transmittance at cutoff band of 1.35%. The results show that the filter can suppress the discharge glow of He-Ne gas effectively and improve the signal-to-noise ratio of miniaturized ring laser.
