COMPARISON OF PUMPING A CESIUM BEAM TUBE WITH D1 AND D2 LINES

The operation of a cesium beam tube optically pumped with cesium D1 and D2 lines is theoretically and experimentally investigated. In a single laser diode configuration using the D2 line, the most efficient excitation scheme is found to be the 3 --> 3-sigma-(D2) scheme leading to a clock signal-to-noise ratio equal to 15 000 in a one-hertz bandwidth. In the case of the D1 line, the predicted best excitation schemes are the 4 --> 3-pi-(D1) and the 4 --> 3-sigma-(D1) schemes. However, these two configurations lead to an extra fluorescence noise presumably due to unpumped atoms. The 3 --> 4-pi-(D1) scheme is experimentally found to be the best with S/N performances comparable to the 3 --> 3-sigma-(D2) configuration. Nevertheless, in the case of the 3 --> 3-sigma-(D2) scheme, optical pumping efficiency decreases whe the magnetic field is too low whereas 3 --> 4-pi-(D1) features are independent of the magnetic-field amplitude. Moreover, the well-separated fluorescence lines in the case of the D1 line allow a more accurate locking of the laser frequency on the top of the fluorescence line than in the case of the D2 line. A perfectly symmetrical microwave spectrum results when the 3 --> 4-pi-(D1) scheme is used.