Frequency stability of an optical frequency standard at 192.6 THz based on a two-photon transition of rubidium atoms

Abstract

We have developed two frequency standards at 192.6 THz (1556.2 nm) based on a two-photon transition in rubidium at 385.2 THz (778.1 nm). These standards use a high power DFB laser at 1556.2 nm and second harmonic generation (SHG) in a periodically poled lithium niobate (PPLN) crystal. The linewidth of the DFB is reduced to the kHz level using optical feedback from a confocal cavity. The SH light is used to injection-lock a 778.1 nm laser diode which allows to observe the (5S 1/2, F g=2−5D 5/2, F e=4) two-photon transition in 87 Rb and lock the 1556.2 nm laser. Allan variance measurements between two identical standards show a beat stability of 2.5×10 −13/ τ 1/2 for observation times between 100 ms and 10 s and a level of 5.8×10 −14 for 100 s. Systematic effects shifting the locked frequency of the standards from that of the Rb transition are detailed and some experimental measurements are presented. Finally, absolute frequency measurements were performed at the INMS/CNRC in Ottawa allowing the determination of the absolute frequency of the standard to be 192 642 283 183.7±0.5 kHz.