The Second Cesium Fountain Clock NIM5: Construction and the First Evaluation in 2007

We have locked the 9.19 GHz reference frequency to the MOT and OM clock transition respectively on our second cesium fountain clock NIM5. A master laser is stabilized through AOM frequency shift/modulation to produce modulation-free output for injection locking and for detections. A slave laser, a single-pass AOM and a custom SM PM 1times3 fiber-optics integrated coupler compose each of the identical two MOT/OM optical units (upward and downward). Each fiber outlet from the coupler provides more than 11 mW optical power for one of the three MOT/OM laser beams. The NIM5 adopts the (1,1,1) MOT/OM configuration with less than 2 arc minutes angle accuracy to load atoms by MOT or by direct OM. An alternative sampling-comparison is utilized to improve the frequency locking stability by reducing the locking period to one fountain circle for NIM5. The first evaluation of NIM5 MOT operation in April to May of 2007 showed a combined uncertainty of 3E-15 and an evaluation of NIM5 OM is on its way.