Tunable diode laser absorption spectroscopy has been widely employed for gas sensing, where the gas concentration is often obtained from the absorption signal with a known or a fixed absorption path length. Nevertheless, there are also numerous applications in which the absorption path length is very challenging to retrieve, e.g., open path remote sensing and gas absorption in scattering media. In this work, a new approach, based on the wavelength modulation spectroscopy (WMS), has been developed to measure the gas absorption signal and the corresponding absorption path length simultaneously. The phase angle of the first harmonic signal (1f phase angle) in the WMS technique is utilized for retrieving the absorption path length as well as the gas absorption signal. This approach has been experimentally validated by measuring carbon dioxide (CO2) concentration in open path environment. The CO2 concentration is evaluated by measuring the reflectance signal from a distant object with hundreds of meters away from the system. The measurement accuracy of the absorption path length, evaluated from a 7-day continuous measurement, can reach up to 1%. The promising result has shown a great potential of utilizing the 1f phase angle for gas concentration measurements, e.g., open path remote sensing applications.

中文翻译：

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在波长调制光谱法中采用一次谐波相角法同时测量气体吸收和光程。

可调二极管激光吸收光谱已广泛用于气体检测，其中气体浓度通常是从具有已知或固定吸收路径长度的吸收信号中获得的。然而，在许多应用中，吸收路径的长度很难恢复，例如，开放路径遥感和散射介质中的气体吸收。在这项工作中，已经开发出一种基于波长调制光谱（WMS）的新方法来同时测量气体吸收信号和相应的吸收路径长度。WMS技术中的一次谐波信号的相位角（1f相位角）用于检索吸收路径长度以及气体吸收信号。通过在开放路径环境中测量二氧化碳（CO2）浓度，该方法已通过实验验证。通过测量来自距离系统数百米的远处物体的反射信号来评估CO2浓度。通过连续7天的测量得出的吸收路径长度的测量精度可以达到1％。令人鼓舞的结果显示出利用1f相角进行气体浓度测量（例如，开放路径遥感应用）的巨大潜力。