Absolute frequency measurement of the hyperfine structure of the 5S1/2 – 5D3/2 two-photon transition in rubidium using femtosecond frequency comb

•Optical frequency comb is used to measure the hyperfine components of a transition.•This transition is for the two-photon absorption 5S1/2 – 5D3/2 in rubidium.•Their absolute frequencies are reported with uncertainties as low as 3.4 kHz.•A laser at 1556 nm is locked to the hyperfine components after frequency doubling.•A stabilized frequency reference for telecommunication application is introduced. In this paper, the absolute optical frequencies of the hyperfine components of the 5S1/2 – 5D3/2 two-photon transition in rubidium is measured using a femtosecond frequency comb (OFC). In order to Interrogate this transition, a laser light at a wavelength of 778.2 nm is used. This wavelength is obtained from the frequency doubling of a telecom wavelength at 1556.4 nm emitted from a narrow linewidth fiber laser. The absolute frequency measurement with OFC is performed while the laser frequency is locked to each hyperfine component of the transition. The measurement uncertainty is estimated to be as low as ±3.4 kHz at 2 σ after correcting the main systematic shifts. The stability of the 85Rb 5S1/2(Fg = 3) − 5D3/2 (Fe = 4) hyperfine component shows a Standard Allan Deviation of 3 × 10−12 at 1 s, which reaches 7 × 10−13 at 1000 s.