噪声免疫腔增强光外差分子饱和光谱线型的理论分析

Theoretical Analysis of Saturated Spectral Line Shape of Noise Immune Cavity Enhanced Optical Heterodyne Molecular Spectroscopy

噪声免疫腔增强光外差分子光谱(NICE-OHMS)作为世界上最灵敏的光谱技术可以被应用到痕量气体检测、频率标准、原子分子光谱以及超灵敏引力波测量等领域中,高精细度谐振腔吸收池的使用在增长激光与腔内物质相互作用路径的同时,极大的提高了腔内激光功率,这就会饱和低气压下的气态样品吸收线从而获得亚多普勒光谱结构,因此NICE-OHMS技术不仅具有高灵敏、还具有超高分辨的优点。该研究基于光与二能级分子相互作用的密度矩阵理论对NICE-OHMS技术中包含亚多普勒的多普勒展宽光谱线型进行了理论推导,获得了光谱线型的表达式,同时以该表达式对光谱线型进行了数值模拟,其中调制频率、饱和参量、频率调制系数分别设置为384MHz,10和0.2。由模拟结果可见吸收光谱由两个边带的吸收信号构成,在包络上存在四个亚多普勒饱和结构;色散光谱由载频以及边带的色散三者决定,并在包络上存在五个亚多普勒饱和结构,获得了与已有实验一致的结果。最后重点分析了不同探测相位、不同饱和参量下的NICE-OHMS光谱线型尤其是亚多普勒结构的变化,由于饱和参量按照调制系数分配给载频和边带,因此虽然载频饱和参量很大,但NICE-OHMS吸收光谱幅度变化不大,主要是由于该光谱信号只与边带饱和参量有关,可以看出NICE-OHMS多普勒展宽信号具有饱和效应免疫的特性,与已有实验结果也符合较好。为更进一步的实验研究提供了必要的理论基础。

In the present paper a theoretical description of the line shapes of frequency modulated noise-immune cavity-enhanced optical heterodyne molecular spectroscopy （NICE-OHMS）under saturated conditions is presented.Expressions for both Doppler broadened and Doppler free frequency modulated NICE-OHMS signals at their absorption and dispersion phases are given.The modulation frequency,saturation parameter and frequency modulation index are set to 384 MHz,10 and 0. 2,respectively in the simulation.Based on the simulated line shape curves,in the absorption detection phase there are four sub Doppler structures ex-isting in the spectral envelope of absorption signals of two sidebands,and in the dispersion phase there are five sub-Doppler structures existing in the spectral envelope of dispersion signals of carrier and two sidebands.The sub-Doppler structure origi-nates from the fact that two counter propagating waves interact with the same group of molecules and one can act as a detecting beam and the other can act as a pumping beam.Therefore the sub-Doppler structure can appear at laser carrier and sidebands fre-quency and their middle frequencies of carrier and sidebands.Since NICE-OHMS absorption signal is only related to the absorp-tion of sidebands the small saturation parameters of two sidebands can not saturate the Doppler broadened signal too much.Al-though the NICE-OHMS dispersion signal includes the dispersion of carrier and two sidebands,the amplitude of Doppler broad-ened signal does not have so much influence by saturation except for the Doppler free signal.Therefore NICE-OHMS technique is a good candidate for the high sensitive detection of gas medium but with high selectivity under the condition of low pressure. Finally the dependences of NICE-OHMS line shape on the detection phase and saturation parameter are simulated and analyzed, the results of which nicely agree with the existing experimental results.The researches on NICE-OHMS line shapes will help ex-plain the phenomena of related experiment.