Coherent population oscillations spectroscopy, which is based on the interaction between atoms and the phase locked laser, is a kind of atomic population modulation spectroscopy. When the laser frequency difference is less than natural width of energy level, the coherent oscillation of atomic population will be induced by laser intensity modulation so that the probe laser transmission with narrow bandwidth can be realized. For a closed two-level system (TLS), the spectral line-width is limited mainly by the spontaneous emission lifetime of the upper atomic energy level. As for a three-level atomic system of Λ configuration, the two linearly polarized beams with both σ+ and σ- polarization component, the laser-atom interaction satisfies the selection rule. The spectral line-width mainly depends on the ground-state relaxation time, and the dependence on the line-width of spontaneous radiation is eliminated. In this paper, the laser from a external-cavity diode laser has its frequency locked to Cesium <inline-formula><tex-math id="M1">\begin{document}$6{{\rm{S}}_{1/2}}\left( {F = 3} \right) \to 6{{\rm{P}}_{3/2}}\left( {F' = 3} \right)$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20210405_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20210405_M1.png"/></alternatives></inline-formula> transition. The frequencies of the two beams are shifted down by two independent double-passed acousto-optic modulators (AOM) to nearly resonate to Cesium <inline-formula><tex-math id="M2">\begin{document}$6{{\rm{S}}_{1/2}}\left( {F = 3} \right) \to 6{{\rm{P}}_{3/2}}\left( {F' = 2} \right)$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20210405_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20210405_M2.png"/></alternatives></inline-formula> transition. The probe beam and the coupling beam are superposed at polarization beam splitter (PBS) cube and transmitted through the magnetically shielded cesium vapor cell in the same direction. The two beams have approximately the same Gaussian diameter of 6.6 mm. The beams are separated by another PBS behind the vapor cell, and the probe beam is detected by a photodiode. We realize the coherent population oscillation spectroscopy through the Cesium vapor cell at room temperature without buffer gas. The spectral linewidth is typically less than 50 kHz which is far below the spontaneous radiation linewidth(~5.2 MHz). The linewidth of coherent population oscillation spectroscopy of the Λ-type atomic energy level structure depends only on the population associated with the oscillation of multiple degenerate level systems except phase correlations of atomic states. Coherent population oscillation is beneficial to the obtaining of the narrow linewidth spectroscopy through the Rydberg atomic system with long excited state lifetime. Considering the importance of electric field measurement using Rydberg atoms, the method of coherent population oscillation can be used to improve the sensitivity of precise measurements based on Rydberg atoms.
