The Passive Optically Pumped Rb Frequency Standard: the Laser Approach

This paper provides an overview of the efforts accomplished during the last few decades in the attempt of implementing a Rb-cell frequency standard using optical-microwave double resonance and laser optical pumping. The results of a new analysis based on a three-level model are summarized. Pumping rates, line widths, light shifts, cell radiation transmission and resonance signal size relative to background are calculated making use of the coherence property of the pumping laser. The expected frequency stability is calculated. The results obtained by several research groups are reviewed, compared to the analysis, and evaluated in the context of the implementation of a practical frequency standard. It is concluded that compromises must be made between the goal of obtaining a frequency stability close to the shot noise limit and that of minimizing complexity. Two directions that address such compromises are outlined.