Operation of a compact cesium beam tube: how to improve the stability towards the 10/sup -12/ /spl tau//sup -1/2/ level

We report the improvement of the frequency stability of a small sized optically pumped Cs beam clock Cs4. This is essentially due to the increase of the signal to noise ratio through detection with a cycling transition. Pumping and detection are respectively performed by means of the (F=4 /spl rarr/ F'=4) and (F=4 /spl rarr/ F'=5) transitions of the Cs D2 line. An excellent frequency stability is obtained when an extended cavity laser with line-width 400 kHz is used: for /spl tau/ in the range 1 to 5800s, /spl sigma//sub y/(/spl tau/) /spl ap/ 1.5 10/sup -12/ /spl tau//sup -1/2/ instead of 4.10/sup -12/ /spl tau/ /sup -1/2/ in the basic configuration. Preliminary results indicate that /spl sigma//sub y/(/spl tau/) is already improved by a factor 1.4 when a commercial DBR diode laser (line-width less than 1 MHz) is used.