Metrological features of the rubidium two-photon standards of the BNM-LPTFand Kastler Brossel Laboratories

We have built three optical frequency standards based on the two-photon tra nsition of rubidium at 778 nm, and analysed their performance over a period of more than three years. We discuss some systematic effects that could le ad to the reproducibility Ne observe, and point out the possible improvemen ts of the devices. We also examine the short and long term stabilities of t he systems, and show that we have reached their ultimate performances.