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

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