Bimetal Temperature-Compensated Ramsey Cavity for Atomic Fountain Frequency Standards

A typical Ramsey cavity, commonly used in atomic fountain frequency standards (AFFSs) for microwave interrogation, is sensitive to ambient temperature variation. Then, it becomes one of the main limiting factors for the AFFSs operating in a narrow operating-temperature range and a temperature-well-controlled environment up to now. In this article, a new design of bimetal temperature-compensated Ramsey cavity (BTCRC) based on the thermal expansion self-compensating mechanism is presented and experimentally demonstrated. This cavity is a typical TE011-mode cylindrical microwave resonator with a titanium tube and two oxygen-free copper (OFC) caps. The resonance frequency (RF) of the cavity can be immune to temperature variations if the dimensions of the tube and caps are properly designed. To validate this self-compensating mechanism, a BTCRC resonating at the Rb clock frequency with a <inline-formula> <tex-math notation="LaTeX">$Q$ </tex-math></inline-formula>-factor of about 17000 is constructed. And its RF thermal coefficient is measured to be 0.73 kHz/°C, which is about 150 times superior to −115.9 kHz/°C of a typical OFC Rb cavity in the temperature range of <inline-formula> <tex-math notation="LaTeX">$22.48~^{\circ }\text{C}$ </tex-math></inline-formula>–<inline-formula> <tex-math notation="LaTeX">$37.48~^{\circ }\text{C}$ </tex-math></inline-formula>. During the operation of AFFSs, this BTCRC ensures that the microwave field amplitude inside the cavity, which dictates the transition probability and thus the contrast of the Ramsey fringes, will be stable in time. In addition, the potential temperature variation-induced frequency shifts due to cavity pulling are significantly minimized.