Absolute frequency measurement of the hyperfine structure of the 5S_1/2 – 5D_3/2 two-photon transition in rubidium using femtosecond frequency comb

In this paper, the absolute optical frequencies of the hyperfine components of the 5S_1/2 – 5D_3/2 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 5S_1/2(F_g = 3)− 5D_3/2 (F_e = 4)hyperfine component shows a Standard Allan Deviation of 3 × 10⁻¹² at 1 s, which reaches 7 × 10⁻¹³ at 1000 s.