Improved Short-Term Frequency Stability of a Rubidium Atomic Fountain Clock Using a Cryogenic Sapphire Oscillator

This work demonstrates the integration of a cryogenic sapphire oscillator (CSO) into a rubidium atomic fountain clock. After stabilization of its frequency and output power, the free-running CSO output is compared with that of a reference hydrogen maser and phase-locked to it with a 400 s servo time constant. The 100 MHz CSO signal locked to the hydrogen maser is transmitted to the laboratory housing the rubidium fountain clock via fiber-optic microwave frequency transfer. A frequency synthesizer uses this reference signal to generate the 6.834 GHz Ramsey interrogation signal, replacing the original local oscillator in the fountain clock. As a result, the frequency stability is improved from 2.7× 10^-13τ^-1/2 to 9.0× 10^-14τ^-1/2. The dominant noise sources currently limiting further improvement are also identified and analyzed. Background atoms are found to be the primary limitation under the present experimental conditions.