Polarization dependence of the direct two photon transitions of 87Rb atoms by erbium: Fiber laser frequency comb

• The two-photon transitions in thermal gas of 87Rb are obtained using an Er: fiber laser frequency comb.
• The signal-to-noise of the transition spectrum is improved by equally dividing the laser powers.
• The polarization dependence of two photon transition based on optical frequency comb has been studied in theory and experiments.
• The two photon transition can be enhanced or degrade by an appropriate choice of polarization.
• The study offers a useful technique in the fields of precision spectroscopy.

The femtosecond fiber-based optical frequency combs have been proved to be powerful tools for investigating the energy levels of atoms and molecules. In this paper, an Er-doped fiber femtosecond optical frequency comb has been implemented for studying the polarization dependence of 5S–5D two-photon transitions in thermal gas of atomic rubidium 87 using an entirely symmetrical optical configuration. By changing the polarization states of the counter-propagating light beams, the polarization dependence of direct two photon transition spectrum is demonstrated, and a dramatic variation (up to 5.5 times) of the two-photon transitions strength has been observed. The theory for the polarization dependence of two photon transition based on the second-order perturbation was established, which is in good agreement with the experimental results. The measurement results indicate that the polarization state manipulation with the existing frequency comb is used for femtosecond optical frequency comb based two photon transition spectroscopic purposes, which will improve the precision measurement of the absolute transition frequency and related applications.