A transportable fountain clock with high reliability is important for high-precision time-frequency measurements. Because of its relatively small cold atoms’ collision frequency shift and ease of attaining high quantum state preparation efficiency, the rubidium atomic fountain clock has an indicated higher stability and reliability. This paper reports the design and operation of a transportable rubidium atomic fountain clock developed by the Shanghai Institute of Optical and Fine Mechanics, Chinese Academy of Science. After being transported more than 1000 km from Shanghai to the Changping Campus of the National Institute of Metrology, China, the optical platform and other hardware of the fountain clock did not need to be adjusted. The rubidium fountain clock maintained a stability of 4.0 × 10−13τ1/2, reaching 5.0 × 10−16 at 300 000 s. After transportation, the rubidium fountain clock and a cesium fountain clock (NIM5) were operated together against the reference frequency of a hydrogen maser. In three separate operating periods, over a total of nearly three months, the average frequency repeatability of the rubidium fountain was less than 3.8 × 10−15.
In this paper, we designed, assembled, and tested a reliable laser system for 87Rb cold atom fountain clocks. The laser system is divided into four modules according to function, which are convenient for the installing, adjusting, maintaining, and replacing of the modules. In each functional module, all optical components are fixed on a baseplate with glue and screws, ensuring the system’s structural stability. Mechanical stability was verified in a 6.11g RMS random-vibration test, where the change in output power before and after vibration was less than 5%. Thermal stability was realized by optimizing of the structure and appropriate selection of component materials of the modules through thermal simulation. In the laser splitting and output module, the change in laser power was less than 20% for each fiber in thermal cycles from 5 to 43℃. Finally, the functionality of the laser system was verified for a rubidium fountain clock.
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