Recent progress in laser-pumped rubidium gas-cell frequency standards

This paper presents the current results of our development of a laser-pumped passive rubidium frequency standard. With a vapor cell containing isotopic Rb⁸⁷ and a mixture of buffer gas we obtained a double resonance signal compatible with a short-term stability of 2.10^-13 τ^-1/2 (shot noise limit). Measurements of the effect of the interrogating phase noise demonstrated that our microwave synthesizer did not limit this potential short-term stability. Two types of monochromatic light source lasers have been used: broad-band solitary lasers and extended cavity lasers. We found that their main limitation on the frequency stability was due to the AM noise detected by the photocell. In order to improve the S/N of the clock, an all-electronic AM noise cancellation technique has been successfully employed. Light-shift measurements allowed tuning of the laser frequency to the zero light-shift point. Presently, our clock has a short-term stability of 7.10^-13 τ^-1/2 (2<τ<40 s) with the solitary laser and 5.10^-13 τ^-1/2 (4<τ<40 s) with the extended cavity laser. These are the best reported performances for passive rubidium clocks