2020 Vol. 44, No. 1
In order to solve the problem that modulation nonlinearity of frequency modulated continuous wave(FMCW) laser leads to the broadening of measurement signal spectrum and the reduction of laser interferometric ranging accuracy, frequency modulated interferometry ranging method based on equal optical frequency subdivision and resampling was adopted. Theoretical analysis and experimental verification were carried out. Waveform data of dual optical path ranging system after equal optical frequency subdivision and resampling of target signals at different positions were obtained. Spectrum analysis was also carried out. The results show that, by means of equal optical frequency subdivision and resampling, the subdivided clock signal points are used to resample the target measurement signal whose distance is greater than optical path difference of auxiliary interferometer. The influence of modulation nonlinearity of laser is eliminated. The problem of signal distortion caused by insufficient sampling points is avoided. Within the measurement range of 4.3m, comparing with laser interferometer, the maximum residual error of equal optical frequency subdivision resampling ranging system does not exceed ±18.46μm. The maximum standard deviation is 23.39μm. Optical path difference of auxiliary interferometer used in this method is very short. It is less affected by the environment. A stable clock signal can be obtained. It can also reduce the volume and cost of dual optical path FMCW ranging system. This study provides practical reference for long-distance and high-precision FMCW measurement.
In order to process sapphire with high efficiency and high quality by using low cost infrared fiber laser, laser-induced backside wet etching (LIBWE) was used. Theoretical analysis and experimental verification were carried out. New type of the mixed solution with high activity and stability was developed in which mass concentration of copper sulfate, sodium hypophosphite, ammonia was 28g/L, 40g/L, 45g/L respectively and whose pH value was 12. Under the same processing conditions, sapphire was cut in copper sulfate solution and the mixed solution. The results show that the cutting efficiency of sapphire in the mixed solution is about 5 times higher than that in copper sulfate solution. High-quality special-shaped sapphire parts have been processed with the mixed solution. This research has certain guiding significance for improving the processing efficiency of sapphire under the action of low power laser.
In order to study effect of wavefront aberration on focusing characteristics of ultrashort femtosecond laser pulses, based on Rayleigh-Sommerfeld scalar diffraction theory, focusing characteristics of ultrashort femtosecond laser pulses with uniform intensity distribution and Gaussian intensity distribution were compared and studied under different wavefront aberration, such as defocusing, astigmatism, coma aberration, trilobe aberration and spherical aberration. The results show that, wavefront aberration has an obvious adverse effect on intensity distribution of femtosecond pulses with uniform intensity distribution at the focal plane. The peak power of focusing femtosecond pulses is reduced. However, the influence of femtosecond pulses with Gaussian intensity distribution is relatively small. Under Gaussian intensity distribution, it is still possible to obtain better focusing spot near diffraction limit at focal plane. At the non-focal plane, even if the initial pulse has Gaussian intensity distribution, intensity distribution at the non-focal plane is also affected by various wavefront aberration. For 30fs (1/e2 half width) ultrashort pulse in this paper, the effect of wavefront aberration on pulse duration is almost negligible. The results are of practical significance for the evaluation of beam quality and the analysis of focusing characteristics of ultrashort femtosecond laser beams.
Two perpendicularly polarized transverse mode modes can be generated by a bipolarized laser resonator, namely fundamental transverse mode (TEM00) and vortex beam with orbital angular momentum (LG01). There is frequency difference in optical frequency. In order to study frequency difference tuning characteristics of two transverse modes, tuning technique combining temperature and voltage was adopted. Continuous tuning of beat signal in different frequency ranges was realized. The relationship between frequency difference of two modes vs. temperature and voltage was analyzed theoretically. Experiments showed that frequency difference can be tuned in wide range. Tuning accuracy of frequency difference was measured and analyzed. The results show that, there is good linear relationship between frequency difference vs. temperature and voltage. The tuning slopes of temperature and voltage are 3.14GHz/K and 1.76MHz/V, respectively. This study can better analyze the phenomenon of vortex beam generated directly by dual polarization resonator. It has application value in the fields of laser communication and lidar detection.
In order to study the types and formation mechanism of metallization products on diamond abrasive surface by laser brazing, the first-principle density functional theory was used to calculate the common carbides. Laser brazing experiments of diamond abrasives were carried out by means of Ni-Cr alloy filler metal and optical fiber laser heat source. The structure and mechanical properties of Cr3C2 and Cr7C3 carbides, as well as surface microstructures and carbide species of diamond abrasives, were obtained. The results show that both Cr3C2 and Cr7C3 are metallic, and the latter is tougher. The thickness of metallurgical reaction layer between diamond abrasives and Ni-Cr alloy brazing filler metal obtained by laser brazing is about 4μm. The surface carbides of diamond abrasives are mainly Cr3C2. The surface carbides of diamond abrasives obtained by ultrasonic-assisted laser brazing are Cr3C2 and Cr7C3. Ultrasound high frequency vibration can promote interfacial reaction, and then produce Cr7C3 with low carbon content. The research results have guiding significance for the development of laser brazing diamond technology.
In order to study application of the manipulated vector laser field in surface processing, based on Nd:YAG pulse Q-switched laser, vector control module was built. Laser ablation drilling experiments on titanium alloy and silicon was used to study effect of pulse number of cylindrical symmetric vector laser, the polarization of beam and the topological charge on laser surface treatment. The results show that the influence of laser pulse number on hole is mainly concentrated on hole depth and aperture. The polarization of beam mainly affects the roundness and smoothness of the hole. Topological loads affect the smoothness of the bottom and periphery of the hole. As a special kind of laser, the manipulated vector laser is expected to be widely used in the field of fine material processing.
In order to obtain single-order harmonics with adjustable wavelength, the method of adjusting and controlling the waveform of two-color field laser was used. Harmonic radiation was analyzed theoretically. The data of single-order harmonic enhancement ranging from the 182nd to 328th were obtained. The results show that, the enhancement of single-order harmonics comes from the folded region in short quantum path of harmonic radiation. Pulse width of the second control field has great influence on the folding region of harmonic radiation. This study provides new scheme for obtaining single-order harmonic pulses. It is helpful for the development of laser source.
In order to realize birefringent metasurface with simple structure and high transmittance, the generalized thin plate transition conditions was used to analyze the relationship between metasurface structure and incident field, reflection field and transmission field around it. Surface polarizability and magnetic susceptibility of dielectric space domain were utilized to describe the equivalent properties of the corresponding metasurfaces. A birefringent metasurface based on π-shaped metal structure unit was designed. By arranging 7 elements with gradient transmission phase in sequence, a metasurface with birefringence of x-polarized, y-polarized electromagnetic waves was formed. The results show that, in beam refraction and polarization splitting metasurfaces, the loss is less than -8dB. Full transmission is achieved in λ/4 wave-plate. The designed birefringent metasurface has high transmission property to electromagnetic wave incident vertically. Beam separation of x-polarized electromagnetic wave and y-polarized electromagnetic wave can be realized. The birefringence is realized. The research results have certain guiding significance for the design and implementation of high performance metasurfaces.
In order to realize non-contact and high-precision measurement of the radial run-out of the spindle in the machining state, the laser Doppler differential detection method based on the optical heterodyne method was used for theoretical analysis and experimental verification. The application of direct coupling and lens-coupling all-fiber optical path reduces the adjustment difficulty of the Bragg diffraction optical path, improves the measurement resolution and anti-interference ability, and reduces the noise; the optical fiber splitter realizes the differential measurement on the same axis, suppressing interference caused by the vibration of the measuring optical path itself. The radial run-out measurement result of the spindle with a relative error of 0.0838% is obtained, and the nanometer run-out error measurement is realized. The results show that the relative error and measurement uncertainty of the system are less than 0.1%. This study has a certain guiding significance for the real-time measurement of the radial run-out of the main shafts.
The cladding layer of K418 alloy blade after remanufacturation was easy to crack and the mechanical properties of bonding interface were poor. In order to solve these problems, the pulsed laser with the advantages of adjustable input, controllable heat input and lower temperature of molten pool and heat affected zone was used. Good metallurgical bonding was formed between K418 matrix and Inconel718 cladding layer under the conditions of laser power 2.5kW, powder feeding rate 37.5g/min, scanning rate 8mm/s and carrier gas flow 3L/min. The results show that the microstructure of the cladding layer is composed of plane crystal at the interface, cell crystal at the bottom, dendrite in the middle and equiaxed crystal at the top. Under the optimized process parameters, Inconel718 cladding layer with good forming quality and without obvious cracks and pore defects is obtained. After the test of the hardness of the matrix and the coating, overall hardness of the coating is about 300HV and the distribution is uniform. Average hardness of the matrix is above 400HV, and the hardness of the bonding interface is 460.46HV, which is 12% higher than that of the matrix. Phase analysis shows that, the properties of Inconel718 cladding layer and matrix K418 match well. The solidification process of laser remanufacturing is L→γ→(γ+MC)→(γ+laves). Solid solution and precipitation of γ′ phase in heat affected zone of matrix K418 alloy are completed by heat input of the pulsed laser. A small amount of secondary precipitates is precipitated along the grain boundary at the interface. Laves phase and MC phase can pin the grain boundary and hinder the slip of the cladding layer and the grain boundary at the interface. The related process and parameters can be used for reference and analysis of K418 blade laser remanufacturing.
Laser measurement was the main method for velocity measurement of the moving objects such as conveyor belt and hot rolled steel plate. In order to improve measurement accuracy and response speed, system transfer correlation analysis velocity measurement technology was proposed. System transfer function of two signals between transmitter and receiver was calculated. Theoretical verification and experimental analysis were carried out by correlation analysis. The results show that, relative error is less than 0.1%. Laser velocity measurement system is constructed by correlation analysis of system transfer function instead of demodulation circuit to simplify signal conditioning circuit and reduce the uncertainty caused by circuit part. Fast correlation analysis is realized by cross-power spectral function method and transfer function of the system is obtained in frequency domain to improve the response speed of the system. By reasonably setting the distance between transmitters, sampling integration improves the measurement signal-to-noise ratio and reduces environmental uncertainty. The uncertainty of measurement is less than 0.2%. This research has certain guiding significance for the real-time measurement of conveyor belt speed.
Elemental imaging technology is an element spectral intensity imaging technology based on laser-induced breakdown spectroscopy (LIBS). The technology has the ability of multielement analysis, in situ imaging and large area sample scanning. The sample preparation is simple and the imaging speed is fast. Firstly, the principle of LIBS element distribution imaging technology is introduced. Secondly, the commonly-used LIBS imaging equipment is introduced. Finally, the applications of LIBS imaging technology in paleoclimatology, pathological analysis, drug metabolism and botanical analysis are reviewed.
In an optical fiber perimeter alarm system, when analyzing and identifying the fiber vibration signal, there are a series of limitations such as network broadband, storage capacity and computing speed in the process of sampling, storage, transmission and signal processing of high-frequency large-scale signal. In order to solve this problem, an adaptive compression sensing method of optical fiber perimeter alarm signal based on wavelet packet was proposed. Firstly, multi-scale wavelet packet decomposition was used to decompose the optical fiber vibration signal. By calculating the mathematical expectation of the high frequency part of the wavelet packet coefficients at different scales as the threshold value, wavelet packet coefficients were set to zero. The wavelet packet decomposition scale was adaptively selected so that the signal sparse in frequency domain. Then, wavelet packet coefficients were classified according to the mathematical expectation and information entropy of the wavelet packet coefficients. According to different types of coefficient blocks, the corresponding processing methods were designed to improve the speed of signal transmission and processing. The results show that this method can effectively reduce the observation data of optical fiber vibration signal. At the same sampling rate, it can improve the accuracy and speed of signal reconstruction.
In order to study effect of pressure and buffer gas on photoacoustic signal and resonance frequency, a trace gas detection system was designed based on photoacoustic spectroscopy. Taking NH3 standard gas as an example, filling buffer gas into the photoacoustic cell to change the pressure in the photoacoustic cell, with the pressure as a single variable, the change of photoacoustic signal and resonance frequency was obtained in pressure range from 0.03MPa ~0.1MPa. And then, different kinds of buffer gases were filled into the photoacoustic cell respectively. The change of photoacoustic signal and resonance frequency in pressure range from 0.03MPa ~ 0.1MPa were obtained under different buffer gas conditions. The results show that, with the increase of pressure, the amplitude of photoacoustic signal increases. The heavier the buffer gas, the greater the increase of photoacoustic signal. The increase of pressure makes the resonance frequency shift. The shift of resonance frequency is inversely proportional to molar mass of the mixed gas molecules in the photoacoustic cell. The change of pressure and background gas makes the environment more complex and affects the detection results. This study provides a reference for solving this problem.
In order to measure the surface topography of micro transparent objects quickly and effectively, a two-step phase-shifted digital holography system based on Mach-Zehnder interferometer was designed. The system used two identical CCDs to collect interferometric images at different distances at the same time. An optical phase shifting unit was used to form a π phase shift between the interferograms recorded by each CCD. Then the phase reconstruction algorithm was given by the space transfer function and Fourier transform of light. A two-step phase-shifted interferometric optical path was constructed. The microlens array was used as the object to be measured. The feasibility of the system was verified. The results show that, the system saves more than half of the time compared with the four-step phase-shifting method. It can achieve the phase reconstruction results consistent with the four-step phase-shifting method. This method is helpful to improve the efficiency of phase reconstruction.
Oxygen parameters of subcutaneous tissue are the important basis to reflect the survival of skin flaps after skin flap transplantation. In order to monitor the oxygen saturation of skin tissue noninvasively, continuously and in real time, theoretical analysis and experimental verification were carried out by near infrared spectroscopy. A current-voltage (I-V) conversion method was proposed and a measurement system was built. The system was used to validate the stability and simulate the vascular embolization of forearm vein occlusion flaps.The results show that, there is a slight difference in tissue oxygen saturation in different parts of different human bodies, it is about 0.05. Under venous occlusion, oxygen parameters of the system tissue change significantly. The maximum decrease is 0.25. The system has high sensitivity. It can continuously measure tissue oxygen saturation parameters and reflect the trend of tissue oxygen change. It can provide important reference for clinical detection of oxygen transport status of skin flaps after operation.
In order to improve conversion efficiency and output energy of a picosecond optical parametric generator/amplifier (OPG/OPA), walk-off compensation structure and special design of lens film system were used to verify the experiment. The evolution of beam quality M2 of OPA signal light at different pumping energies and the tuning output performance of OPA signal light ranging from 430nm to 680nm were studied. The results show that, when pumped by 6.9mJ at 355nm, maximum output energy of 2.7mJ at 510nm signal is obtained. The corresponding optical-optical conversion efficiency is 39.1%. Photon conversion efficiency is 56.2%. This method can effectively improve output energy and conversion efficiency of OPG/OPA. This study is helpful for the characterization of ultraviolet and deep ultraviolet crystals.
In order to study lift-off effect of polyimide films under 308nm excimer laser irradiation, by means of experimental research, the effects of laser energy density, spot overlap rate, pulse frequency and substrate temperature on laser lift-off were investigated. The morphologies of the stripped substrates and films were observed by microscopy. The results show that, the threshold of laser lift-off energy is about 160mJ/cm2. When laser energy density is about 180mJ/cm2~190mJ/cm2 and spot overlap rate is 68.33%, lift-off effect is better. Increasing the substrate temperature is beneficial for laser lift-off process. This study has certain significance for the industrial application of polyimide films in the field of flexible electronics.
In order to get night vision images with natural color, color night vision technology has become a hotspot in the field of night vision technology. The development of night vision technology and the technical means of obtaining natural color night vision images are introduced. It can be divided into hardware mode and software synthesis mode.The software methods are mainly based on multi-sensor image fusion and color transfer.The technical schemes of both the modes are introduced in detail.The technical prospect of color night vision technology is also given.
In order to synthesize X-ray image information of bronze mirrors into the same image, source image was decomposed by lifting wavelet. Different fusion rules were used for image fusion at low and high frequencies respectively. The method of combining regional energy with regional variance was used in low frequency. Spatial frequency combined with the method of neighborhood pixels to standardize intermediate pixels was used in high frequency. Finally, the target image was obtained by lifting inverse wavelet transform. Theoretical analysis and experimental verification were carried out. Information entropy, average gradient and standard deviation of the fused image were obtained. The results show that, in three groups of experiments, compared with the other three algorithms, information entropy of the proposed algorithm in this paper is increased by 5.76% on average. Average gradient is increased by 28.70%. Standard deviation is increased by 7.70% on average. The algorithm effectively preserves the information of the source image. Edge transmission effect is better. This result is helpful for the fusion of X-ray images of bronze mirrors.
In order to achieve rapid and simultaneous quantitative detection of C, H, S in coal, laser-induced breakdown spectroscopy(LIBS) was used. A Nd:YAG solid-state laser with wavelength of 1064nm was used as excitation source. Nine kinds of national standard coal samples were ablated in air environment. Combined with partial least squares regression, the spectrum with wavelength ranges of 188.885nm~308.008nm and 655nm~660nm were selected to detect C, H and S non-metallic elements in coal simultaneously. The correction model and prediction model data of partial least squares regression were obtained. The theoretical analysis and experimental verification were carried out. The results show that, determination coefficients R2 of the predicted and real concentrations of C, H and S are 0.9421, 0.9894 and 0.9840. Root mean square error of calibration is 2.2772, 0.2356 and 0.1678. Average relative error is 2.6348%, 7.1185% and 8.8600%. LIBS combined with partial least squares regression can be used to quantitatively detect non-metallic elements in coal.
To solve the problem of low efficiency of leukocyte recognition in artificial microscopy, automatic recognition of white blood cells was studied on computer micro vision platform. After filtering the image color model, precise stripping of white blood cells and image background was realized by region growing algorithm. The extraction of nucleus and cytoplasm of leucocytes was realized by Otsu method, which is valley threshold segmentation method of gray histogram. According to the morphological, color and texture characteristics of cells, a large number of white blood cells were identified and classified by artificial neural network classifier. The results show that, white blood cell image segmentation and intelligent identification algorithm have high accuracy and efficiency. The final accuracy can reach 95.6%. It meets the need of automatic detection of leukocytes in clinical microscopic vision.
Traditional digital image correlation method based on Newton-Raphson iteration is greatly influenced by the initial value of iteration. In order to overcome the problem, a digital image correlation method based on genetic algorithm was proposed. The data point to be measured was chosen as the center and several valuation points in the neighborhood were selected. The coordinates of estimated points before and after deformation were matched by digital image correlation method based on genetic algorithm. Three or more non-collinear pairs of valuation points were randomly selected to be substituted for the affine transformation model. The initial deformation value was estimated based on affine transformation results and it was used as the initial value of Newton-Raphson iteration. Finally, the sub-pixel displacement was calculated by Newton-Raphson iteration method. The results show that the matching time of this method is 37.54% less than that of the traditional method. Compared with the traditional digital image correlation method, it is more reliable in search performance and matching accuracy. This study provides a reference for the effect of iterative initial value optimization on matching speed and accuracy in digital image correlation method.
In order to improve the uniformity of etching, continuous fluid model and heat transfer model of gas flow were established for 400mm reactive ion etching(RIE) chamber. Pressure, velocity and temperature distribution in the reaction chamber were studied. When cooling plate was kept at 285K, the distributions of velocity, pressure and temperature near the wafer in the cavity were analyzed by changing the inlet flow rate and outlet pressure in turn. Then the distance between the plates (30mm~60mm), the diameter of the intake port (300mm~620mm) and the diameter of the exhaust port (50mm~250mm) were changed in turn. Air flow and temperature distribution in the reaction chamber were analyzed. The results show that, the distribution of air pressure is characterized by low edge and high center. The flow velocity is characterized by high edge and low center. The uniformity of air pressure is better at low flow rate. The uniformity of pressure distribution increases with the increase of the distance between the chamber plates. It also increases with the decrease of the outlet area and the increase of the inlet area of the chamber gas. The temperature field near the top of the wafer is uniform and stable. It is almost unaffected by the fluctuation of inlet flow. Thermal stability is good. The research results are of great significance to the structural design improvement of large-aperture RIE chamber and the control of large-aperture reactive ion etching process.