Cross Beam Saturated Absorption Spectroscopy: A novel technique for optimization of power broadening

Saturated Absorption Spectroscopy (SAS) is a technique commonly used for recording Doppler broadening free absorption spectra of atomic vapours and also to estimate the natural linewidth of the atomic transitions. However, in conventional SAS two counter-propagating laser beams are used as pump and probe and the linewidths of the absorption spectra show strong dependences with the power of the laser beams following the saturation theory. The present work demonstrates a unique technique of Cross-Beam spectroscopy for recording saturated absorption spectra near their natural linewidth by deploying pump and probe beams in orthogonal orientation instead of their collinear position. In the case of orthogonal orientation, the recorded absorption linewidth hardly varies with the power variation of the laser beam and does not follow the Saturation theory. Following the above technique, transition frequencies for different isotopes of ytterbium (Yb) have been measured, and the transition frequency for the most abundant isotopes, i.e., Yb¹⁷⁴, estimated to be 751526556.04(60) MHz with natural linewidth of 32.0(8) MHz.