A Low Temperature-Sensitive Ramsey Cavity for Rb Fountain Clocks

A low temperature-sensitive microwave-vacuum integrated Ramsey cavity (MVIC) used for Rb fountain clocks is proposed to solve the problems that the clocks have a small operating-temperature range and are sensitive to the ambient temperature changes. This cavity mainly consists of a tube made of titanium (Ti) and two end caps made of oxygen-free copper (OFC), forming a cylindrical microwave resonator (CMwR) working at TE011 mode. Utilizing the different thermal expansion of the tube and the end caps, the change of the cavity resonance frequency (ReF) to temperature can be self-compensated. To improve this cavity quality factor Q, the tube inner surface is coated by a thin copper film fist and then by a thin gold film on the top. There are two coupling holes (C-holes) face to face on the tube wall for microwave coupling. Each C-hole is vacuum-sealed with a ceramic window (CW) first. And then it is covered by a rectangular waveguide. Two microwaves are fed into the waveguides then coupled into the CMwR through the CWs, respectively. For the prepared MVIC for Rb fountain clocks, its Q-factor is measured to be 11500 and its ReF matches the Rb clock frequency within -49 kHz. And its ReF thermal-coefficient is measured to be -16.3 kHz/degrees C, which is 7.1 times lower than that of a commonly used OFC Ramsey cavity with the same sizes in the temperature range of 22.48 degrees C-34.48 degrees C. Additionally, this cavity ReF can be tuned about 130 kHz after performing a further bake-out for the fully assembling clock physical package.