The Microloop-Gap Resonator: A Novel Miniaturized Microwave Cavity for Double-Resonance Rubidium Atomic Clocks

Nowadays mobile and battery-powered applications push the need for radically miniaturized and low-power frequency standards that surpass the stability achievable with quartz oscillators. For the miniaturization of double-resonance rubidium (⁸⁷Rb) atomic clocks, the size reduction of the microwave cavity or resonator (MWR) to well below the wavelength of the atomic transition (6.835 GHz for ⁸⁷Rb) is of high interest. Here, we present a novel miniaturized MWR, the µ-LGR, for use in a miniature DR atomic clock and designed to apply a well-defined microwave field to a microfabricated Rb cell that provides the reference signal for the clock. This µ-LGR consists of a loop-gap resonator-based cavity with very compact dimensions (<0.9 cm³). The µ-LGR meets the requirements of the application and its fabrication and assembly can be performed using repeatable and low-cost techniques. The concept of the proposed device was proven through simulations, and prototypes were successfully tested. Experimental spectroscopic evaluation shows that the µ-LGR is well-suited for use in an atomic clock. In particular, a clock short-term stability of 7 × 10^-12τ^-1/2 was measured, which is better than for other clocks using microfabricated cells and competitive with stabilities of compact Rb clocks using conventional glass-blown cells.