基于谱线频率特征的调谐二极管激光吸收光谱参数分析

调谐激光二极管吸收光谱(TDLAS)技术因其高分辨率、高灵敏度和快速测量等优点在工业生产、环境污染监测等方面受到广泛应用。波长调制光谱（wavelength modulation spectrum, WMS）的二次谐波信号经常用作气体浓度反演的检测信号。TDLAS检测性能与系统参数，如锁相放大器的时间常数、扫描幅度、扫描频率、调制幅度、调制频率等的选取紧密相关，实际测量中各参数的选择多以谱线形态特征为依据，参数之间的关联性未能得到较好体现。由于信号的采样与处理均在频域对谱线产生作用，各参数之间的作用相互关联。然而很少有研究参数对谱线频域的影响，针对此问题，在一定的理论基础上通过实验分别观察各调制参数对二次谐波信号的影响。通过保持其他参数不变，只改变一个参数的方法，得出各个参数对信号线型、频率特征及噪声引入的影响规律，继而分析并验证了多参数联合变化对谱线频带的决定作用。与基于时域特征的传统方法相比，基于谱线频率特征分析一方面具有与谱线信号采集检测处理机理相近的优点，另一方面可以直观得到各参数对主频带的影响和不同频率信号的衰减趋势。总结出基于频率特征的各参数的基本选取方法，以谱线频带和截止频率相互关系为判定标准，截止频率的大小由锁相放大器时间常数决定。通过设置合适的时间常数和扫描参数使信号频带与截止频率相近但不相交，使谱线频带内频率分量不产生衰减，频带外噪声得到最大抑制；再根据锁相放大器的性能和信号信噪比来确定调制参数，使谱线主频幅度最大；最后根据系统需求确定采样率。单周期采样点不变时，低扫描频率时检测精度相对提高但耗时较长；反之，扫描频率提高，速度变快但检测精度下降。通过联合影响规律调整关联参数，减小硬件限制对参数最优值选取造成的影响。可在考虑系统检测需求与硬件条件限制的前提下，通过参数选择得到最优二次谐波信号，为此技术的实际应用提供了参数优化的实验依据与参考方法。

Parameter Analysis of Tunable Diode Laser Absorption Spectroscopy Based on Spectral Frequency Characteristics

Tunable diode laser absorption spectroscopy (TDLAS) technology is widely used in the industrial production and environmental pollution monitor due to its high resolution, high sensitivity and fast measuring speed. The second-harmonic signal of wavelength modulation spectroscopy (WMS) is often used as the detection signal for gas concentration inversion. The detection performance of TDLAS is closely related to the system parameters, such as the time constant of lock-in amplifier, scanning amplitude, scanning frequency, modulation amplitude and modulation frequency. The selection of each parameter in practical measurement is mostly according to the spectral line shape characteristics, in which method the relevance between each parameter is not considered completely. Since the signal sampling and processing affect the spectral line in the frequency domain, the mechanism of influence between parameters is interrelated. However, the influence of each parameter on the frequency domain of the signal was rarely researched. Aiming at this problem, the influence of modulation parameters on second harmonic signals was observed by experiment based on a certain theory in the present paper. The influence of each parameter on signal line shape, frequency characteristics and introduced noise was obtained when only one parameter was changed with all other parameters unchanged. Then the determinant of multi-parameter joint changes on spectral frequency band was analyzed and verified. Compared with traditional methods based on time-domain features, spectrum analysis from frequency characteristics has the advantages not only similar to the principle of spectrum signal acquisition, detection and processing, but also intuitive to get the influence of various parameters on the main frequency band and the attenuation trend of different frequencies signals. The basic selection method of each parameter was summarized based on spectral frequency characteristics. Firstly, the time constant and the scanning parameters should be reasonably set with the judgment criteria of the relationship between the spectral frequency band and the cutoff frequency, which is determined by the time constant of the lock-in amplifier. The appropriate time constants and scanning parameters should be set so that the signal frequency band is close to but not intersected with the cut-off frequency, the in-band components are not attenuated, and the out-band noise is maximally suppressed. Then, the modulation parameter should be determined to maximize the main frequency component amplitude of the spectrum on the basis of the function of lock-in amplifier and the spectrum signal noise ratio. Finally, the sampling rate should be determined according to the system requirements. When the sampling points of one period remain unchanged, the detection accuracy is relatively improved at low scanning frequency but the sampling time increases, on the contrary, the sampling speed increases and the detection accuracy decreases at a higher scanning frequency. The interrelated parameters should be adjusted simultaneously to reduce the influence of the hardware limit on the parameter optimal selection. In this paper, the optimal second harmonic signal can be obtained by parameter selection in the premise of sufficiently considering the system detection requirements and hardware condition limitations, which provided experimental basis and reference method of parameter optimization for the practical application of such technology.