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417～445 nm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R2, 而α的选择与时延td有关, td≤4.0 μs时, α=0.3可获得最佳R2值, 之后随td的增大, α逐渐减小; 在td≥6.5 μs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2 000～3 000 K, 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 coefficient α can increase the linear regression coefficient R2 of Boltzmann plots, and the calculation accuracy of plasma temperature was improved. The LIBS spectra of low alloy steel sample region from 417 to 445 nm 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 L and α were optimized according R2, the results showed that the 8-layer db4 wavelet decomposition can gain the high R2, while the value of α associated with the delay time td, e. g., the optimum α corresponding to maximum values of R2 is 0.3 when td≤4.0 μs, and then decrease with the increasing of td, and reduced to 0 when td≥6.0 μs. The interference due to baseline on the spectral characteristic lines gradually reduced with the increasing of td, and therefore α decreased with td increase. 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 td, and the temperature fluctuation is reduced after baseline subtraction, these results are consistent with the physical process of plasma expansion.