波长调制光谱检测下限计算方法的研究

在基于波长调制光谱技术的大气痕量气体检测下限计算方法的研究中,全面准确地描述吸收信号及噪声的大小极为重要。为了探讨半导体二极管激光器在进行波长直接调制时伴随产生的残余振幅调制噪声对检测下限的影响,在理论推导中采用了一种同时考虑波长和振幅调制的二次谐波信号分析及提取方法,以二次谐波信号峰谷差值作为系统的检测信号,对可调谐二极管激光器吸收光谱检测系统的信噪比和检测下限进行了理论分析,取得了洛伦兹吸收线型条件下残余振幅调制噪声对系统信噪比和检测下限的影响的精确计算数据。结果表明,在吸收线的线宽较大的检测条件下,残余振幅调制噪声是影响检测下限的重要因素。

Study on calculation method of detection limit based on wavelength modulation spectroscopy

In the study of calculation method of atmospheric trace gas detection limit based on wavelength modulation spectroscopy, it is extremely important to describe the magnitude of the absorption signal and noise objectively and accurately. In order to study the effect of residual amplitude modulation noise on the detection limit when the wavelength of a semiconductor diode laser is modulated directly, the second harmonic signal analysis and extraction method were used in theoretical derivation taking into account the frequency modulation and amplitude modulation simultaneously. The difference value between the peak and valley of the second harmonic signal was utilized as detection signal. The signal-to-noise ratio (SNR) and detection limit of the tunable diode laser absorption spectroscopy detection system were theoretically analyzed. Under the condition of Lorentzian absorption lineshape, accurate calculation data of the impact of the residual amplitude modulation noise on the SNR and the detection limit were obtained. The results show that the residual amplitude modulation noise is an important factor affecting the detection limit under the condition of large absorption linewidth.