An ultra‑stable optical frequency standard for telecommunication purposes based upon the 5S_1/2 → 5D_5/2 two‑photon transition in rubidium

In this study, we report the development of a frequency standard for optical fiber communication applications based on a two-photon transition in rubidium at 385.2 THz. This standard kills two birds with one stone in the sense it is capable of providing us with two highly stable serviceable wavelengths at 778.1 and 1556.2 nm. In this system, we exploit the narrow line-width of a fiber laser emitting at 1556.2 nm in conjunction with an erbium-doped fiber amplifier to generate a sufficient second harmonic laser beam at 778.1 nm in a periodically polled lithium niobate waveguide mixer in order to probe and frequency-lock the laser to the 5S_1/2 (F g = 3)–5D_5/2 (F e = 5) hyperfine two-photon transition component in ⁸⁵Rb. The metrological performance of the standard is evaluated with the aid of an optical frequency comb synthesizer. Allan variance measurement shows a stability of 4 × 10⁻¹² at 1 s (limited by the comb stability), reaching a floor of 6.8 × 10⁻¹³ at 1000 s. After correction of all the major systematic frequency shifts including the light shift, the absolute frequency is found to be 385 285 142 374.0 (5.0) kHz. Moreover, the absolute frequencies of most of the hyperfine components of the 5S_1/2–5D_5/2 transition of the two naturally existing rubidium isotopes are measured using a femtosecond frequency comb synthesizer after stabilizing a laser on each component.