Fig. 1. Physical system of ⁸⁵Rb fountain clock. (a) Structural diagram; (b) photo

Fig. 2. Working levels and timing sequence of ⁸⁵Rb fountain clock. (a) Ultra-fine level structure and related transition of ⁸⁵Rb D2 line; (b) operation timing sequence of fountain clock

Fig. 3. Scheme for large-range frequency shift

Fig. 4. Optical system of ⁸⁵Rb fountain clock

Fig. 5. Aluminum profile gridded platform. (a) Building of platform; (b) fixing of optical devices; (c) 50 cm×50 cm single layer optical panel; (d) mechanical simulation of non-gridded platform (top: side view; bottom: bottom view); (e) mechanical simulation of gridded platform (top: side view; bottom: bottom view)

Fig. 6. Optical path in four pass module built on aluminum profile. (a) Simulation; (b) photo

Fig. 7. Spectrum of four pass module in optical system of ⁸⁵Rb fountain clock ^[12]

Fig. 8. Photo of ⁸⁵Rb optical system

Fig. 9. Power fluctuation of one laser in ⁸⁵Rb optical system

Fig. 10. Physical experimental results of ⁸⁵Rb fountain clock. (a) Falling TOF signal; (b) Ramsey transition line