小波变换基线扣除对激光诱导等离子体温度计算的改善

Accuracy Improvement of Temperature Calculation of the Laser-Induced Plasma Using Wavelet Transform Baseline Subtraction

温度是激光诱导等离子体特性研究最重要的参数之一,为降低光谱连续背景对Boltzmann平面法计算等离子体温度的精密度的影响,利用小波变换对等离子体光谱进行分解,并采用软阈值法对代表光谱基线的低频信号进行扣除。选择合适的小波分解层数L及阈值系数α能有效提高Boltzmann图的线性相关度,即有更高的拟合系数R2,从而提高等离子体温度的计算精密度。对低合金钢样品417~445nm波段LIBS光谱采用db4小波函数进行分解、基线扣除和信号重构,选用12条Fe原子谱线建立Boltzmann图,由Boltzmann图拟合直线的斜率计算得到等离子体温度。对L和α系数进行了优化选择,研究发现,采用8层小波分解时Boltzmann图具有较高的R^2,而α的选择与时延t_d有关,t_d≤4.0μs时,α=0.3可获得最佳R^2值,之后随t_d的增大,α逐渐减小;在t_d≥6.5μs后,α=0,即光谱低频信号被完全扣除,说明基线对光谱特征谱线的干扰随时延的增加逐渐减弱。基线扣除后各时延的等离子体温度降低约2 000~3 000K,温度随时延的增加逐渐降低,与等离子体膨胀过程中温度逐渐下降的物理过程相吻合,且变化过程中的波动变小。

Temperature is one of the most important parameters in studying of laser-induced plasma characteristics.To reduce the impact of continuous background on the calculation of temperatures using Boltzmann plots,the wavelet transform was used to decompose the spectrums,and the low-frequency signals represented the spectral baseline were deducted by using soft-threshold method.Selecting the appropriate wavelet decomposition level L and threshold coefficientαcan increase the linear regression coefficient R^2 of Boltzmann plots,and the calculation accuracy of plasma temperature was improved.The LIBS spectra of low alloy steel sample region from 417 to 445nm were decomposed by using db4 wavelet,and then baseline subtraction and signal reconstruction were carried out,respectively.Twelve Fe atomic lines were chosen to establish Boltzmann plots,and the temperatures were calculated from the slope of the fitted lines in the plots.The value Landαwere optimized according R^2,the results showed that the 8-layer db4 wavelet decomposition can gain the high R^2,while the value ofαassociated with the delay time t_d,e.g.,the optimumαcorresponding to maximum values of R^2 is 0.3when t_d≤4.0μs,and then decrease with the increasing of t_d,and reduced to 0when t_d≥6.0μs.The interference due to baseline on the spectral characteristic lines gradually reduced with the increasing of t_d,and thereforeαdecreased with t_dincrease.After the baseline was deducted,the temperature calculated by Boltzmann plot decrease of about 2 000 to 3 000 K.The temperature gradually decreased with the increasing of the t_d,and the temperature fluctuation is reduced after baseline subtraction,these results are consistent with the physical process of plasma expansion.