Precision Frequency Measurement of ⁸⁷Rb 5S_1/2 (F = 2)→5D_5/2 (F'' = 4) Two-Photon Transition through a Fiber-Based Optical Frequency Comb^*

The absolute frequency of ⁸⁷Rb 5S_1/2 (F = 2)→5D_5/2 (F'' = 4) two-photon transition at 778nm is measured in an accuracy of 44 kHz. A home-made erbium-doped fiber laser frequency comb with frequency stability of 5.0 × 10⁻¹³@1 s is employed for the light source. By using a periodically poled lithium niobate, the femtosecond pulse operating in 1556nm is frequency-doubled to 778nm to obtain the direct two-photon transition spectroscopy of thermal rubidium vapor. Through sweeping the carrier envelope offset frequency (f_ceo), the 5S_1/2 (F = 2)→5D_5/2 (F'' = 4) two-photon transition line is clearly resolved and its absolute frequency is determined via the peak-finding of the fitting curve. After the frequency correction, the measured result agrees well with the previous experiment on this transition. The entire system configuration is compact and robust, providing a potential candidate of optical frequency standard for telecommunication applications.