NEW DESIGN FOR A HIGH-PERFORMANCE OPTICALLY PUMPED CESIUM BEAM TUBE

The replacement of state selecting magnets by optical interaction regions in a cesium beam resonator provides means to improve the clock frequency stability and accuracy. Among the various annoying frequency shifts, that related to the Rabi pulling effect disappears as a consequence of the microwave spectrum symmetry. This gives the opportunity for decreasing the amplitude of the static magnetic field B0 in the microwave interrogation region to values less than the conventional ones used in cesium beam frequency standards with magnetic state selection. Nevertheless, it was pointed out recently that, for the transitions of interest, coherent population trapping may occur in the two optical interaction regions of the resonator at a low static magnetic field. In order to avoid the resulting decrease of the clock signal amplitude, a magnetic field of the order of 3 . 10(-5) Tesla must be created in the light-atoms interaction regions. An optically pumped cesium beam resonator has been designed including three successive magnetic field regions. The optical interactions take place in the first and third regions, where the magnetic field has the required value of 3 x 10(-5) T. The microwave interaction occurs in the intermediate region, where the value of the C-field is typically set to 4 x 10(-6) T. It has been verified that the magnetic field profile along the cesium beam does not induce Majorana transitions. Using a single laser diode emitting at 852 nm with a linewidth of about 30 MHz, the resonator gives an excellent amplitude signal to noise ratio equal to 20 000 in a 1-Hz bandwidth.