Frequency stability of an optically pumped caesium-beam frequency standard at the KRISS

An optically pumped caesium-beam frequency standard (known as the KRISS-1) has been constructed at the Korea Research Institute of Standards and Science (KRISS) and the first measurements of long-term frequency stability have been made. Two methods were used to make the frequency standard immune to variations in the environmental temperature: an F = 4 → F' = 3 (σ + π)-polarized laser was used as a pumping source, and the difference between two sequential error signals was used as a feedback signal in the digital servo loop. A frequency stability of σy(τ) = 2.5 × 10-14 at τ = 105 s for a temperature variation of 4 °C was achieved with the KRISS-1.