LASER DIODE OPTICALLY PUMPED CESIUM BEAM FREQUENCY STANDARD INVESTIGATIONS AT LHA

In this paper, we shall summarize our investigations concerning the laser diode optically pumped cesium beam resonator. Since 1982, theoretical and experimental work has been done on the subject. Such efforts have led to a better knowledge of the light/atom interaction processes in that kind of resonator. The results have allowed the optimization of the clock signal amplitude and consequently, the frequency performance of the clock prototype. Firstly, we briefly describe the basic atomic resonator operating with a semiconductor laser diode tuned either to the D1 or D2 cesium lines at 894 and 852 nm, respectively. The best optical transitions which give the highest clock signal amplitude are found to be sensitive to the Hanle effect. The optical pumping efficiency as well as the fluorescence yield decrease when the static magnetic field is too low in the light/atom interaction regions. We have then designed an efficient optically pumped cesium beam resonator having three magnetic field regions. When it operates with a commercially available laser diode tuned to the 3 --> 3-sigma transition at 852 nm or to the 3 --> 4-pi transtion at 894 nm, it exhibits a S/N ratio equal to 15 000 in 1-Hz bandwidth leading to a measured short term frequency stability of 2 x 10(-12) square-root-tau. The knowledge of the velocity distribution is important in order to estimate the clock accuracy. In the basic one-laser optically pumped cesium beam resonators, the velocity distribution is found to be Maxwellian. The study presented in this paper clearly points out the potentialities of the optically pumped cesium beam frequency standard.