可调谐二极管激光吸收光谱(Tunable Diode Laser Absorption Spectroscopy, TDLAS)技术是一种具有高灵敏度、高分辨率的气体吸收光谱检测技术, 具有响应快、精度高、单模特性优秀、通用性强等优势。TDLAS直接吸收法通过测量绝对吸收强度来计算待测气体温度和浓度, 但容易受到颗粒物浓度、激光强度波动等影响。TDLAS波长调制法采用高频正弦信号对激光器进行调制, 使得激光输出频率和强度同时受到调制, 具有高信噪比和灵敏度的特点, 但是需要通过标定实验或复杂的算法来确定气体参数。因此, 通过吸收光谱理论和波长调制理论, 推导出蕴含分子吸收信息的谐波通项表达式, 并在此基础上分析谐波信号与待测气体绝对吸收强度之间的关系, 建立了一种基于谐波信号的绝对吸收强度测量算法。以NH3分子在1 531 nm附近的谱线为例进行数值分析, 发现调制幅度达到a=0.032 cm-1(调制系数m=2)时, 仿真结果与理论计算结果(a=0)相对误差不超过2%, 进一步验证了算法的可靠性与准确性。

Tunable diode laser absorption spectroscopy(TDLAS) is a high sensitive and high resolution gas absorption spectrum detection technology, with fast response, high precision, excellent single mode characteristics and strong versatility. The temperature and concentration of the detected gas were calculated by measuring the absolute absorption intensity in the direct absorption method of TDLAS, but was easily affected by particle concentration and fluctuation of laser intensity. The laser was modulated through high frequency sinusoidal signal in the wavelength modulation method of TDLAS, making the laser output frequency and intensity modulated simultaneously, with the characteristics of high signal-to-noise ratio and sensitivity, but the gas parameters were determined by calibration experiments or complex algorithms. Therefore, the expression of harmonic terms containing molecular absorption information was deduced through the theory of absorption spectrum and wavelength modulation, and the relationship between harmonic signal and absolute absorption intensity of detected gas was analysed, establishing the measurement algorithm of absolute absorption intensity based on harmonic signal. The absorption spectral line of NH3 molecule near 1 531 nm was taken as an example for numerical analysis, and the relative errors between theoretical calculation (a=0) and simulation values were no more than 2% when the modulation amplitude reached up to a=0.032 cm-1(m=2), which further proved the reliability of on-line measurement algorithm.