2017 Vol. 41, No. 1
In order to obtain the residual stress field induced by laser shock processing(LSP), the numerical simulation method was used. A 2-D and axisymmetric finite element analysis(FEA)model of single laser shock processing on 35CD4 thick parts was established. History of the energies of material during dynamic analysis and surface dynamic stresses at different times were analyzed to validate the reasonability of the total time of dynamic analysis. The effect of mesh refinement and spatial distribution models of the loading on the simulation results were discussed. The results show that the element length should be around 0.03mm to get convergent results. The predicted results for single LSP with round laser spot are consistent with the available experimental data.
In order to realize the work of X-ray imaging system and testify the performance of the imaging system, the backlighting imaging system was studied by ray tracing simulation software SHADOW. By the simulation of system parameters(detection position, the size of backlighting source), the results show that the position of detection has a little influence on the relative magnification of the system. The spatial resolution of the X-ray imaging system is higher with the decrease of the size of backlighting source. The imaging system based on spherically bent crystal has high spatial resolution and better performance.
In order to achieve the laser of high power, high frequency and controllable output with the same aperture, using the method of laser amplifier, high frequency tuning Q, optical parametric oscillator(OPO), frequency doubling and scanning reflection mirror, theoretical analysis and experimental verification were carried out. Selective laser outputs 0.532μm of 40W, 1.064μm of 100W, 3.9μm of 12.6W were gotten under the condition of power supply current of 42A, Q frequency of 10kHz and the same aperture. The results show that the laser with high power, high frequency, controllable output and the same aperture can be achieved by this experimental device.
In order to achieve low-threshold optofluidic fluorescence resonant energy transfer(FRET) laser, based on Fabry-Perot (F-P) microcavity with high quality factor and stability, the generation of optofluidic FRET laser in two types of F-P resonator was studied by using indirect pumping method. Firstly, donor molecules were pumped directly, and then, the excited energies of donor molecules were transferred to acceptor molecules through FRET to realize the indirect optical pumping of the acceptor. Experimental results show that the low energy density threshold of laser pump is 0.48μJ/mm2 in F-P optofluidic cavity. The results indicate that the detection of low concentration substance can be realized by FRET laser generation in terms of laser detection.
In order to measure the position and attitude of roadheader dynamically and automatically, by analyzing and comparing the features of several existing measurement methods. a position and attitude multiple measurement method for roadheader based on vision/inertial navigation system (INS) was proposed. By getting the attitude of roadheader and the position of monocular vision measurement roadheader with strapdown inertial navigation system (SINS), the dynamic five degree-of-freedom (DOF) measurement was realized. This method was verified with the fast driving equipment in Daliuta mine. The experiment results show that the accuracy of attitude measurement is within 0.1°, the standard deviation of static drift is within 0.25° and the accuracy of position measurement is within 1cm. This method can be used for position and attitude dynamic measurement for roadheader, and possesses very broad prospect in engineering practice.
In order to study laser-induced breakdown spectroscopy(LIBS) and arc spectrum of laser-triggered vacuum switch(LTVS), and analyze the micro factors affecting delay jitter. By using single-pulse ns laser to bombard the target plasma located on switch cathode and the combined method of photoelectric multiplier tube and optical filters, the data of light intensity of discrete wavelengths and the fitting curve were gotten. After theoretical analysis, the results show that the increase of the pulsed laser energy could improve the strength of target LIBS, but almost don't affect the strength of arc spectrum. The increase of switch voltage can improve the intensity of arc spectrum. Arc plasma collides and excites in the strong electric field at the switch. The ingredient proportion of plasma changes with the change of voltage. Jitter is independent of voltage magnitude, motion status of free electrons and races of arc plasmas. The main affection takes place in the triggering period. Not laser energy but switch voltage plays a major role in the steepness of voltage drop. The results are useful for the further study about laser-target interaction, pulsed-arc and delay-jitter of LTVS.
In order to provide measurement accuracy evaluation for 3-D laser scanning devices, a laser radar measurement system (LRMS) was choosed as the research object and uncertainty model of point cloud fitting plane was proposed. After experimental verification, a single-point error ellipsoid model for the LRMS based on error ellipsoid theory was established. According to the distribution characteristics of plane error ellipsoid of point cloud, uncertainty model of point cloud fitting plane was proposed to evaluate and fit the dimension measurement accuracy associated with plane. Verification experiments were performed by using the LRMS to measure the height of a cubic object, and the measurement accuracy results were evaluated and compared with the simulated results using the proposed uncertainty model. The results show that the model can accurately estimate the uncertainty of height measurement. The research has the validity and practical significance.
In order to optimize the design of wide field-of-view high-gain optical antenna in free space, the finite element method was utilized to analyze the effect of different structure parameters on transmission enhancement characterization of bull's eye structure, horn-shaped structure and bowl-shaped structure respectively. The relationship between the numbers of slit and the coefficient of surface-plasmon-enhanced extraordinary transmission was obtained, and the optimal structure parameters of bowl-shaped optical antenna with subwavelength structure was gotten. The results show that for horn-shaped optical antenna, when the single silt can excite the strongest surface plasmon polaritons(SPP), the coefficient of transmission enhancement has lager value. By optimizing the structural parameters, the value of horn-shaped structure enhancement coefficient varies from 20 times to 140 times when the incident angle θ is within ±5°. The smooth enhanced gain is gotten at the incident angle in the range from ±5°to ±26°. The average value of the enhancement coefficient is 10. The performance is doubled compared with the performance of present structural parameters. Bowl-shaped structure has a good enhanced transmission character at the incident angle in the range of ±60° and the average value of the enhancement coefficient is 10 in the smooth enhanced transmission area. The wide field-of-view transmission character of the bowl-shaped is better than of the horn-shaped.
In laser cladding, it was difficult to accumulate heat on the pure copper surface for forming effective metallurgical bonding between copper and Ni60 coatings. In order to overcome the difficulties, a method of pulsed laser cladding assisted with preheating was adopted. Laser cladding experiments on copper substrate were conducted, and a 3-D transient thermo-elastic-plastic model was developed to simulate the thermal process and residual stress. When the preheating temperature reached 573K, the cracks in Ni60 cladding layer were completely eliminated. The processing efficiency of laser cladding was increased 2.2 times when preheating temperature was 673K. The average hardness of the Ni60 coating obtained by laser cladding assisted with preheating was 800HV0.2. Under normal temperature, the relative wear resistance of Ni60 coating and ASTM52100 steel was 4.45. Friction coefficient of Ni60 coating was 57% of ASTM52100 steel or copper. The results show that with the increase of the preheated temperature, the number of cracks in Ni60 coating decreases and processing efficiency of laser cladding increases. Also, the surface hardness of Ni60 coatings improves and friction coefficient decreases effectively. By pulse laser cladding assisted with preheating, Ni60 cladding layer on pure copper without cracks and air holes is obtained.
In order to compare and select the technology of locking frequency of pump source, by using intra-modulation technology and Pound-Drever-Hall(PDH) technology separately, and choosing periodically poled KTP(PPKTP) crystal as nonlinear crystal, stable frequency operation of 894.6nm continuous light two-mirror standing wave frequency doubling cavity was realized. The results show that frequency doubling cavity has the same blue light power output using two locking technologies and under different input light powers. 447.3nm blue light of 178mW is obtained when the maximum fundamental light power of 350mW and the corresponding conversion efficiency is 50.8%. Under the condition of the maximum fundamental power, the power fluctuation of blue light obtained by intra-modulation technology and PDH technology are 3.4% and 2.3% in 2h respectively. The research is helpful for preparing stable output of 447.3nm blue light to obtain nonclassical light source at D1 line of cesium atom.
In order to improve the doubling frequency efficiency, the influence of beam waist position in frequency doubling crystal on the frequency doubling efficiency was analyzed. According to the propagation property of fundamental mode Gaussian beam, combining with the characteristic that equal phase plane of end mirror and cavity mirror having the same radius of curvature in the stable three-mirror-folded cavity, cavity mirror with the same radius of curvature was placed on the proper position and three-mirror-folded cavity with two Rayleigh length was realized. The efficiency of frequency doubling was improved. The stability regions of the cavity with one and two Rayleigh length were compared and the same stability regions were obtained. The experimental results show that when 5W fiber coupling 880nm laser diode is used to pump 3mm×3mm×5mm Nd:YVO4 and quasi-phase matched PPMgOLN with dimension of 10mm×2.1mm×0.5mm is used as frequency doubling crystal, the overall conversion efficiency is improved about 18%. For the two types of cavity, beam quality is the same. The polarization direction of doubling frequency light is consistent with that of fundamental frequency light. Stable low noise green laser is obtained. The effectiveness of resonant cavity design is verified. The study is helpful to improve the efficiency of intracavity frequency doubling.
To design optical filter and optical switch with high quality and performance, by using the transfer matrix method theory and computational simulation method to draw the transmission spectrum band of 1-D photonic crystal, the degeneracy effect of the distance between two defects and the thickness of the defect on the transmission band spectrum of photonic crystal were studied and analyzed. The results show that the transmission band degeneracy of the photonic crystal is sensitive to the distance between two defects, the transmission band degeneracy is more obvious with the greater distance between two defects. When the distance between two defects is larger, the degenerate effect of transmission band spectrum is more obvious. When the distance increases to a certain value, complete degenerate of discrete transmission peak appears. When the defect thickness increases with integer times, the tendency of degenerate of transmission band spectrum of photonic crystal appears. When defect thickness increases with odd and even number of times, the degenerate speed of transmission spectrum is not the same. The former is shorter than the latter. No matter defect thickness increases with odd or even number times, the degeneracy of transmission band spectrum of photonic crystal reduces gradually. Degeneracy effect can provide the guidance for photonic crystal design of the new optical filter, optical switch and modulation mechanism.
In order to investigate the characteristics of pulsed CO2 laser triggering discharge, a high voltage capacitance discharge experimental platform was set up by a pair of hemisphere graphite electrode with 10mm radius and 8mm gap. The real-time data of discharge voltage and current were obtained. Discharge voltage and discharge current were fitted by two-order oscillation circuit model. The impedance of laser induced discharge plasma was obtained. And the discharge time, discharge delay and jitter data were collected. The results show that the impedance of laser trigger discharge plasma is very little, and of 1Ω~2Ω. The fitted plasma impedance slightly decreases with the increase of laser energy, discharge voltage and capacitance. The delay varies from 2μs to 10μs with the change of experimental condition. Delay and delay jitter decrease with the increase of laser energy and discharge voltage. Discharge capacitance has no obvious effects on delay and delay jitter. Laser pulse and high voltage with high stability are helpful for the stability of laser triggered discharges.
In order to detect and analyze the mixed gas, according to specific absorb spectrum peak of different kinds of gas in the mixed gas, based on the theory of laser resonant cavity enhanced spectroscopy and the technology of Raman scattering spectrum, and using active laser resonant cavity and avalanche photodiode, an online analysis system of the composition of the mixed gas was designed. Experimental results show that real time on-line detection of mixed gas can be realized and the mixed gas containing volume fraction of N2 and O2, respectively 0.502 and 0.498, can be detected effectively. The system is convenient, easy, safe, and reliable for operation. And the system can be used to detect the composition of the mixed gas.
To study the influence of laser shock peening number and pulse energy on microstructure and surface hardness of TC17 titanium alloy, TC17 titanium alloy samples were laser shock peened with different process parameters. The results show that severe plastic deformation and a great deal of high-density dislocations, such as proliferate, pile up and tangle, are generated in the material surface layer in the process. The maximum depth of micro-pits created by single pulse can be 21.4μm. When pulse energy is 5J, and overlap impacts increase from 1 time to 4 times, the surface hardness of materials increases by 8.3%, 17.2%, 24.3% and 24.5% respectively, compared with parent metal. In the meantime, when overlap impact is 1 time, and pulse energy is 5J and 7J, the surface hardness increases by 8.3% and 14.2% respectively. The conclusion is that the surface hardness is enhanced with the increase of impacts and pulse energy. High density dislocation on material surface by laser shock processing is the key reason for the increase of surface hardness.
Division-of-amplitude photopolarimeter(DOAP) was the key part of Stokes ellipsometer. To reach the complex and strict processing demands of beam splitter in DOAP, two waveplates are inserted respectively in the reflection light path and transmittion light path of classic DOAP to form an improved DOAP. The instrument matrix of new DOAP was derived and influence of waveplate parameters on the condition number of instrument matrix was also analyzed. From theoretical analysis and experimental verification, the optimal parameters and their relationships of waveplates were gotten. The results show that, measurement standard errors of film thickness and refractive index of the optimized Stokes ellipsometer are 0.1nm and 0.001 respectively. By choosing the optimal azimuthal angles or phase retardations of waveplates, the instrument matrix can be optimized to improve the accuracy and stability of measurement.
In order to improve the plane positioning accuracy of selective compliance assembly robot arm (SCARA) robot, the method combined compensation strategy based on grid model and the minimum error approximation principle was proposed. Firstly, a simplified model for SCARA robot was established and grid model principle was summarized. Secondly, an initial grid model was constructed by analyzing the location relationship between the first-time reached actual point and the desired point. The starting point of the variable parameter grid model in the next step was constructed by using the minimum distance error approximation method. Finally, the convergence and update direction were determined by the location of the desired point in grid model. The results show that the positioning compensation strategy of visual guidance makes up the phenomenon that the plane positioning accuracy is not high because of the inaccuracy of the model. The accuracy of the positioning compensation strategy is much better than 1mm~3mm of the spatial interpolation compensation method. The proposed method has brief parameter regulation, clearly mobile times and strong industrial application.
In order to research multiangular reflectance polarization characteristics of minerals, the reflectance polarization spectrum of minerals in visible light waveband was measured by the method of changing the incident angle and detection angle. After theory analysis and experimental verification, the results show that the degree of polarization (DOP) of minerals is less affected by wavelength in visible light region, and DOP is stable. Incident angle and detection angle have significant influence on polarization spectrum. With the increase of incident angle and detection angle, DOP firstly increases and then decreases. The maximum angle is nearing Brewster angle. DOP of minerals have great differences when incident angle and detection angle vary between 55° and 65°. DOP of chalcedony is significantly higher than the other three minerals. DOP of chalcedony with smaller particles and higher crystallinity is the highest and DOP of opal with amorphous structure is the smallest. The study is feasible and reliable to identify and classify minerals by the degree of polarization and provides new way for minerals detection.
In order to solve the power fluctuation problem of 3kW RF slab CO2 laser at high power operating conditions, a turning mirror adjusting method of out light path was adopt to compensate the light beam deviation caused by thermal deformation of cavity mirrors. The key technology of laser input power control by pulse width modulator based on TMS320LF2407 chip was studied. End mirror sampling power detection system of shaping light path was designed. And the compensation theory of out light adaptive regulation system was analyzed. The result shows that, the end mirror sampling laser power detection system of shaping light path has high linearity and the maximum error compared with reference power meter is ±2.59%. when using a piezoelectric ceramics microdisplacement driver of PA 100/T 14 to adjust and compensate the adaptive turning mirror with micro-motion distance less than 54μm, the light beam deviation will be less than 0.8mm of slit width of spatial filter. After adjustment and compensation, the output power fluctuation declines from 18% to 2%, and reaches the compensation demand of beam deviation caused by thermal deformation of cavity mirror. The sidelobe of shaping light disappears and the best output mode is gotten. The research has the practical value for improvement of laser stead output power.
In order to solve the problem of dual wavelength laser stability and achieve the purpose of narrowing the line width, the unwanted oscillation modes was limited by adding ring filter into the laser structure. After simulation and calculation of filter, the dressing spectrum was obtained. Single frequency dual-wavelength laser with linewidth of 5.7kHz was obtained by experiments. After output power stability test, the power fluctuation within 1h was 0.6dB. The result shows that the effect of ring filter is very obvious.
In order to evaluate the ability of chemical fractionation treatments, including water (H2O) washing, ammonium acetate washing (NH4Ac) and hydrochloric acid (HCl) washing, for the removal of alkali metal, and analyze the feasibility and accuracy of laser induced breakdown spectroscopy (LIBS) measuring the alkali content in coal, a sequence of solutions including deionized water, ammonium acetate and hydrochloric acid were employed to treat Zhundong coal. The contents of sodium and potassium in treated coal were measured by LIBS and compared with the measurements of inductively coupled plasma (ICP) spectroscopy. The experimental results of confirming the ability of LIBS in measuring the alkali content in coal were obtained.The results show chemical treatment such as washing can effectively remove the alkali metal from Zhundong coal, and LIBS technique has good repeatability of Na, K measurement for different samples. The measurements of Na and K by LIBS had high sensitivity and low detection limit. The relative error between LIBS and ICP was less than 7%. LIBS could be a valid method to achieve the online measurement of alkali metal in coal.
Morphological operators used in the automated morphological endmember extraction (AMEE) algorithm didn't acquire correct result in the area of pure pixel concentration distribution. The dilation operation only chose one candidate endmember from each structure element and would lose some important pixels. In order to solve the problem, the AMEE algorithm was modified by an improved morphological operator and new structural element. The improved morphological operator was proposed after introducing the concept of reference spectral vector, and a new calculation method of morphological eccentricity index was given. To avoid information loss, four candidate endmembers were chosen from each improved even-numbered structure element. The modified automated morphological endmember extraction algorithm was tested based on a hyperspectral data set. The experimental results show that the improved method can obtain correct candidate endmembers from the area of pure pixel concentration distribution, and information loss in the procedure of dilation is also avoided. The proposed method produces more accurate results of endmember extraction and the spectral unmixing.
In order to make the quality of fringe spacing and fringe visibility of interference pattern to meet the strict demands of active coherent laser detecting system of far distance, the distribution characteristics of coherent Gaussian beams based on Mach-Zehnder interferometer were analyzed. Optical intensity distribution of coherent Gaussian beams was deduced. Then interference pattern of scanning coherent Gaussian beams was gotten. The expressions of optical path difference, fringe spacing and fringe visibility based on Mach-Zehnder interferometer were derived by physical optics theory. The influences of rotation angle of beam splitter, detection distance and scanning speed on distribution characteristics of coherent light of far distance were analyzed and simulated numerically. As a result, the quantitative relation of rotation angle of beam splitter, detection distance and scanning speed was found. The results show that the distribution of coherent light of far distance is mainly controlled by rotation angle of beam splitter, detecting distance and scanning speed etc. Rotation angle of beam splitter in the interferometer has the influence on both fringe spacing and fringe visibility. The conclusion is helpful to choose the rotation angle in the experiment. Fringe spacing and fringe visibility should be taken into consideration simultaneously and the rotation angle of beam splitter is adjusted. It's useful to make the fringe visibility as large as possible.
In order to eliminate Glan-Taylor prism transmission spectra curve fluctuation and improve the measurement accuracy of extinction ratio, the quadratic curve fitting method was adopted to confirm the extreme point position of transmission curve by using polarizing prism as analyzer. The secondary light intensity was measured. And the incident end and transmission end of prism, reflection end and exit end of the cementing layer were analyzed theoretically. Then, variation law of the transmission spectrum of prism at different angles was used to explain the degree of interference. The measurement results show that this method eliminates the wave interference effect and improves the accuracy of the extinction ratio. The results solve the measurement accuracy problem of extinction ratio of air-spaced polarization prisms, and it provides the reference to use polarizing prism correctly.
Modal crosstalk and differential mode delay in mode-division multiplexing in few-mode fiber must be compensated for with adaptive equalization algorithm. For the purpose of decreasing complexity of adaptive equalization algorithm in long-haul few-mode fiber communication system, a multiple-input multiple-output equalizer with variable step-size frequency-domain-block least mean square(VS-FBLMS) algorithm was proposed to uncouple a 2×2 mode-division multiplexing system. Compromising the conflicting requirements of convergence speed and convergence performance, FBLMS adaptive algorithm was employed to update equalizer taps and modify step-size factor with variable step-size function. Furthermore, the algorithm decreased the computational complexity by using fast Fourier transform. In a 112Gbit/s high-speed communication simulation system over 1000km in few-mode fiber, signal Q2 factor increased 3.7dB with same convergence speed. The performance of the algorithm was tested on field programmable gate array with a 100km few mode fiber communication system. The study demonstrates that the algorithm successfully uncouples the signals of mode-division multiplexing system and realizes fast convergence and low disorder.
In order to meet the needs of structure health monitoring, the application of quasi-distributed fiber Bragg grating(FBG) sensor in crack monitoring and damage localization was studied in experimental method. By choosing the simply supported reinforced concrete beam and an H-shaped steel beam as the test object, the key location of the beam was divided into several units of multi-point monitoring to obtain the data of the strain. The data was analyzed to achieve the crack monitoring and damage location. The results show that quasi-distributed FBG sensor can accurately locate the occurrence of cracks and damage of the measured area.
In order to solve the insufficient of signal process algorithms during the detection of atmospheric trace gas concentration by wavelength modulation laser spectroscopy technique, a new method of gas concentration inversion based on the random sample consistency (RANSAC) algorithm was proposed. By choosing the simulation signal and the actual measurement signal of formaldehyde in the atmosphere as examples, theoretical analysis and experimental study were carried out and compared with the traditional least square method. The results show that the proposed algorithm has better immunity to noises and outliers. Especially under the conditions of low signal-to-noise ratio (SNR), the measurement accuracy can be improved by one order of magnitude. The algorithm shows better reliability and superiority.
In order to research the effects of frequency bandwidth on the propagation and applications of broadband laser, the propagation formula of broadband laser with rectangular spectrum passing through a bifocal dispersion lens system was derived by using diffraction integral. Bandwidth-induced focal switch in broadband laser with rectangular spectrum is studied by numerical calculation. The effects of bandwidth on the intensity distribution and focal switch were analyzed. The results show that bandwidth is an important factor affecting intensity distribution and focal switch. Primary maximum intensity of the focused broadband laser can rapidly bounce from one place to another with the variety of bandwidth and then the focal switch is generated. The results are helpful for further applications of broadband laser.
To satisfy the requirement of thermal adaptability and limited space, a kind of bipod flexure mounts for space reflector (Ø355mm) made of super low expansion coefficient glass was designed by finite element analysis (FEA) method. Firstly, the basic design principles of bipod flexure were studied. The advantage of bipod structure, compared with three points backside structure, was discussed from the view of freedom. Secondly, simulation analysis and optimization design were carried out for the influence of dimension parameters of support structure and flexure hinge on the surface figure accuracy. The intersection position of supporting foot extension line should be the key design parameters and the bonding position should be designed respectively. The results indicate that backside bipod flexure mounts after improved design has fine thermal adaptability and can effectively discharge the load caused by thermal variation, and has fine supporting ability and dynamic stiffness at the same time. root mean square of surface figure accuracy reaches 3.68nm, after reflection mirror mounting, and the first order frequency of assembly is 123.41Hz. The data can meet the design requirements. This study can supply the meaningful reference for future structure design of backside bipod mounts.
In order to investigate laser disturbing effects under different incident angles, experimental study and theoretical analysis were carried out by gradually adjusting the optical axis pointing. Spot positions, receiving energies and spot shapes of laser on the detector were achieved under different incident angles. The results show that spot positions shift linearly with the increase of incident angle. The spot would deviate 16pixel from the center of detector with the increase of 0.1° of incident angle. Receiving energy decreases with the increase of incident angle and the reduction is less than 1% for the optical system with viewing angle of 8°. Spot shape meets space shift invariance under different incident angles. The study is helpful for the experiment of laser jamming electro-optical imaging system.