调谐激光吸收光谱波长偏移修正算法研究

Research on Wavelength Shift Correction Algorithm for Tunable Laser Absorption Spectrum

可调谐半导体激光器具有线宽窄、波长扫描快、室温工作等特点,基于可调谐半导体激光器构成的激光吸收光谱气体测量系统在大气环境检测、工业生产过程在线检测中得到了广泛的应用。在实际测量系统中,由于可调谐半导体激光器中心波长受温度等因素的影响发生偏移,如不进行中心波长校正,将造成序列光谱数据重叠,处理后的光谱线型发生展宽,进而影响后续的光谱线型拟合,对气体浓度的反演精度产生影响。一般采用参考光谱吸收谱线寻峰方法进行序列光谱数据偏移的对齐,但光谱数据中的随机噪声、背景噪声、漂移噪声等因素影响峰线波长的精度。为了降低上述因素的影响,提出一种改进的时域相关光谱修正算法,首先对光谱信号进行自相关,在一定程度上提高光谱信号的信噪比,然后再进行时域互相关处理,能够准确的计算出激光器波长偏移量,减少由此造成的光谱线型展宽的影响,提高了浓度反演精度和测量稳定性。在激光吸收光谱气体浓度检测实验系统中进行了实验验证,评估结果中,原始数据标准差为1.482 8,谱线寻峰方法与时域相关方法修正后数据标准差分别为0.433 9和0.293 6,改进的时域相关修正方法修正后数据标准差为0.132 5,改进的时域相关修正方法相关系数均优于0.992,欧式距离的标准差为1.726 4。系统稳定性评估中改进方法波长漂移修正后标准偏差为0.144 3。

Tunable semiconductor laser is featured by narrow bandwidth,fast wavelength scanning,and room temperature working temperature,etc.The laser absorption spectroscopy gas measurement system based on tunable diode laser is widely applied in atmospheric environmental monitoring and on-line industrial production process detection.In the actual measurement system,the center wavelength of the tunable semiconductor laser is affected by temperature and other factors.If the center wavelength is not corrected,the spectral data will be overlapped and the processed spectral line will be widened,which will affect the subsequent spectral line fitting and affect the accuracy of gas concentration inversion.Generally,the reference spectrum absorption line peak-finding method is used to align the offset of the spectral data.However,the accuracy of the peak wavelength in the spectral data is affected the random noise,the background and drift noise.In order to reduce the side effects mentioned above,an improved algorithm of the time domain correlation is proposed.Firstly,the autocorrelation of the spectral signal is carried out to improve the spectral signal-to-noise ratio to a certain extent,then the time-domain cross-correlation processing is applied,which can accurately calculate the laser wavelength offset,reduce the influence of spectral broadening,and improve the concentration inversion accuracy and measurement stability.Experiments have been carried out in the detection experimental system of gas concentration based on laser absorption.Experimental results show that the standard deviation of the original data is 1.482 8 while the standard deviation of data correction is 0.433 9 and 0.293 6,respectively.The corrected standard deviation of improved method of time domain correlation correction is 0.132 5 with the correlation coefficient higher than 0.992.The standard deviation of Euclidean distance is 1.726 4.The system stability evaluation indicates that the modified standard deviation of the wavelength drift correction is up to 0.144 3.