Laser spectral gas sensing technologyIt plays an important role in many fields such as climate warming, Mars exploration, marine exploration and biomedicine. The continuous deterioration of global environment, ecology and energy problems puts forward higher requirements for the analysis ability of multi-material and even unknown components of optical gas sensing. However, due to the lack of ideal coherent light source, it is difficult to obtain fine spectral information quickly and accurately in a wide spectral band.
optical frequency comb (optical frequency comb, OFC) provides a ruler for measuring frequency and time, which fundamentally solves the problem of optical frequency measurement and greatly promotes the development of cutting-edge basic physics research. OFC is represented as a series of comb spectral lines with equal frequency interval in the frequency domain. After interacting with gas molecules, it can be analyzed in the frequency domain. It can not only obtain wide spectral coverage, but also obtain high spectral resolution, which provides a new technical means for high-precision spectral measurement.However, this technology often relies on high bandwidth photodetectors and complex spectral analysis technology, and requires a long interaction path between laser and gas to improve the detection sensitivity, which seriously limits the wide application of optical frequency comb spectroscopy in the field of gas sensing.
Double comb photothermal spectroscopy
In order to break through the bottleneck of this technology, fromWang Qiang’s research team from Changchun Institute of optics, precision machinery and physics, Chinese Academy of Sciences and Ren Wei’s team from the Chinese University of Hong KongCreatively put forwardDouble comb photothermal spectroscopy方法（DC-PTS），首次实现了基于光频梳的气体分子光热光谱测量。
Research resultsDual-comb Photothermal Spectroscopy Published in international authoritative journalsNature Communications。 Among them, researcher Wang Qiang of the Chang Guang Institute of the Chinese Academy of Sciences and Dr. Wang Zhen of the Chinese University of Hong Kong are the co first authors of the paper. The team of Professor Jin Wei of the Hong Kong Polytechnic University and the team of Professor Wang Yingying of Jinan University provided key anti resonant hollow fiber devices.
图1：Double comb photothermal spectroscopy方法概念图
The principle of dc-pts is shown in Figure 1. The double optical comb light source is used as the pump light source, and one of the optical pulses moves for the same time within the duration of the other optical pulse to periodically modulate the optical pulse. In the frequency domain, the heterodyne beat frequency of each pair of comb teeth of the double light comb light source can absorb the gas molecules and realize the intensity modulation of a specific frequency. Because the photothermal effect caused by intensity modulation will periodically modulate the refractive index of the medium, when the double optical comb passes through the gas medium and is absorbed, the refractive index of the medium carries a series of modulation frequencies. Using optical interferometry, refractive index modulation and Fourier transform, the corresponding spectral information in a wide band can be obtained.
图2：乙炔气体宽波段Double comb photothermal spectroscopy
In the principle verification experiment, the researchers used an electro-optic modulator to generate a double comb pump laser with natural intrinsic mutual coherence. An all fiber Fabry – P é rot interferometer was constructed with a 7 cm anti resonant hollow fiber. KW · cm ⁻ can be realized with only mW laser ² Magnitude of pump light intensity. In hollow fiber 28 μ In the spatial scale of M, the optical comb can modulate the gas refractive index at more than 100 different frequencies at the same time μ L sampling volume of gas realizes the detection sensitivity of ppm level and photothermal spectrum measurement over 1 THz spectral width (as shown in Figure 2).
研究人员所提出的Double comb photothermal spectroscopy方法不仅具备单波长激光光谱测量的高选择性和快速响应特点，同时光频梳和光热光谱技术的融合使得同时具备宽光谱、高分辨率、极低耗气量和高灵敏度成为可能，为分子探测提供丰富的光谱信息，针对大气监测、深空探测、海洋科学、呼气诊断等不同领域对精密气体探测的需求提供多功能的光谱气体传感技术。
随着光学微腔、量子级联激光器等先进光梳光源和中红外空芯光纤技术的迅速发展，Double comb photothermal spectroscopy方法有望进一步拓展到气体分子的中红外指纹光谱带，同时结合光学腔增强、高性能相位解析技术，可以实现更强的气体分子探测能力和更小的集成尺寸，为基于激光光谱的前沿科学探索和工程应用研究提供前所未有的可能性。
Article information Wang, Q., Wang, Z., Zhang, H. et al. Dual comb photothermal spectroscopy Nat Commun 13, 2181 (2022).
https://doi.org/10.1038/s41467-022-29865-6 The research was supported by the projects of the National Natural Science Foundation of China (62005267, 51776179), etc.