Two-photon resonant NFWM is a Doppler-freespectroscopy 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。
2
The high resolution spectrum can be obtained through many methods, such as saturated absorption spectroscopy, Dopplerfree methods of two-photonspectroscopy, and etc.