Potential for improving the Rubidium Frequency Standard with a novel optical pumping scheme using diode lasers

Frequency stability of a Rubidium Frequency Standard (RFS) is directly related to the signal-to-noise ratio (S/N) of the microwave-induced (6834 MHz) optical signal which is proportional to the fractional atomic population difference between the two M/sub F/=0 ground state sublevels of /sup 87/Rb. In the present RFSs this fractional population difference is small (<1%). S/N can be substantially improved by concentrating all of the atoms in one of the two M/sub F/=0 sublevels. Potentially, this could lead to a significant improvement in the short-term performance of RFSs. We have developed a novel scheme for concentrating a large fraction of the Rb atoms in one of the two M/sub F/=0 ground state sublevels. We optically pump the Rb vapor with circularly polarized light from a AlGaAs diode laser tuned to the D/sub 1/ transition (794.7 nm). Nearly all of the atoms are concentrated in one of the two high angular momentum states (M/sub F/=2 or -2 sublevels depending on the handedness of the circular polarization). The pumping laser is switched off and two radiofrequency (RF) /spl pi/-pulses are applied sequentially. The first /spl pi/-pulse transfers the atoms from the 2,2(F,M/sub F/) sublevel to the 2,1 sublevel and the second /spl pi/-pulse transfers the atoms from the 2,1 sublevel to the 2,0 sublevel. The resulting population distribution is diagnosed using a second AlGaAs diode laser (weak probe) in conjunction with a microwave field tuned to the 0-0 transition (6834 MHz). We obtain a fractional population difference of 0.7-0.9 between the two M/sub F/=0 sublevels. This should result in an improvement in the S/N by a factor of 70-90 over the lamp pumped RFSs. This could potentially be of considerable importance towards the development of future RFSs. Various relaxations and field inhomogeneities limit the transfer efficiency from being 100%. The details of the experimental technique and possible applications are discussed.<>