Abstract
Frequency standards in various frequency regions, from if to visible, have been built using atomic rubidium as an absorber. The optical radiation of the rubidium two-photon transition (5S-5D) at 778 nm has been recommended as the realization of meter by CIPM [1]. In the 5S-5D transition, because of the difference of the Lande g factors between S and D states, the magnetic field must be carefully shielded to avoid any possible shift of transition lines caused by Zeeman Effect [2], [3]. However, the 5S1/2−7S1/2 transition is free from such Zeeman shifts [4], since both the lower and upper levels have the same Lande g factors, and thereby does not require a magnetic shielding. The 5Sl/2−7Sl/2 transitions are capable of providing a standard with a sub MHz accuracy for frequency doubled 1520 nm light sources. For the first time, the two-photon transition of rubidium 5SI/2→7SI/2 was observed using second harmonic signal by doubling an external-cavity diode laser that is amplified by an erbium-doped fiber amplifier in a periodical poled lithium niobate waveguide. We get the efficient second harmonic signal, ~10 mW, of a 95-mW 1520-nm tunable laser system, and directly lock the laser frequency to the Rb5S1/2→7S1/2 transitions.