Two-photon resonant NFWM is a Doppler-free spectroscopy by using the narrow bandwidth of incident laser beams.

当用窄带宽的激光时，该技术对里德堡能级的窄光谱结构可以获得消多普勒的分辨率。

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The high resolution spectrum can be obtained through many methods, such as saturated absorption spectroscopy, Doppler free methods of two-photon spectroscopy, and etc.

提高热原子光谱的分辨率可以有许多方法，如饱和吸收光谱技术、双光子光谱的消多普勒技术等。

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