High-performance laser-pumped rubidium frequency standard for satellite navigation

Abstract

Presented is a double-resonance continuous-wave laser-pumped rubidium (Rb) atomic clock with a short-term stability of 4 × 10 213 t 21/2 for integration times 1 s ≤ t ≤ 1000 s, and a medium-to longterm stability reaching the 1 × 10 214 level at 10 4 s. The clock uses an Rb vapour cell with increased diameter of 25 mm, accommodated inside a newly developed compact magnetron-type microwave cavity. This results in a bigger signal with reduced linewidth, and thus improved short-term stability from a clock with 1 dm 3 physics package volume only. The medium-to long-term clock stability is achieved by minimising the effects of light-shift and temperature coefficient on the atoms. Potential applications of the clock are discussed.Motivation: Portable and compact atomic clocks are today indispensable for many aspects of human civilisation [1], with increasing demand for better clock precision and stability [2,3]. Application examples of such frequency standards include precise navigation, telecommunication, and space science [1,[3][4][5]. Laboratory clocks like primary caesium (Cs) fountains and optical clocks exhibit excellent stabilities of s y (t) ≤ 1 × 10 213 t 21/2 , but are bulky and expensive. Even cold atom clocks or optical clocks proposed for space applications target outlines of 1 m 3 volume, 230 kg mass, and 450 W power consumption [5]; hence a trade-off must be made between stability and portability. Recently developed portable standards, such as the passive hydrogen maser (SPHM) [4] or laser-pumped Cs beam clocks (LPCs) [6], exhibit a reasonable trade-off with volume (13 , V , 28 dm 3 ), mass (8 , m , 18 kg), power consumption (30 , P , 80 W) and stability (7 × 10 213 , s y (1 s) , 1.5 × 10 212 and 1 × 10 214 , s y (10 4 s) , 3 × 10 214 ). Here we show that our simple and compact, continuouswave (CW) laser-pumped double-resonance (DR) Rb clock stability outperforms that of LPCs standards up to 1000 s, and is comparable to coldatom portable clocks [3] or the SPHM, but from a physics package (PP) with volume of ,1 dm 3 only in our case. A previous laser-pumped Rb clock based on a magnetron-type cavity had a stability of s y ≃ 3 × 10 212 t 21/2 [7]. By increasing the cell diameter to 25 mm and redesign of the magnetron cavity, we improve on this clock stability while maintaining a very compact volume of the magnetron-type resonator. This clock can have applications in, e.g., next generation satellite navigation systems like GALILEO. In particular, a short-term stability of 6 × 10 213 t 21/2 allows reaching the 1 × 10 214 level already at timescales of 3600 s, well before the 6000 s relevant for clock error prediction and synchronisation.

A miniature frequency-stabilized VCSEL system emitting at 795nm based on LTCC modules

“…The Doppler-broadened Rb absorption signal of the microfabricated Rb cell is used to lock the laser, which is a much simpler approach than saturated-absorption spectroscopy [6] and more compact than previous studies using a thin cesium cell [13,14]. In our case, the Doppler lines are further broadened due to collisions between the Rb atoms and the buffer gas, which corresponds to the practical case encountered for miniature atomic clocks [3].…”

“…In such applications, on the one hand, there is the need of reducing the power consumption and the dimensions of the laser source, in order to facilitate the fabrication of portable devices; on the other hand, the demand for highly-stable laser sources is rapidly increasing, with the ever more widespread diffusion of applications based on, e.g. highresolution spectroscopy [1] or precise navigation systems using atomic clocks [6]. In this context, the objective of this research is to design and fabricate a compact and low-power frequency-stabilized laser, in particular for use in miniature atomic clocks [3].…”

Double-resonance in alkali vapor cells for high performance and miniature atomic clocks

“…The light power transmitted through the cell, as a function of the microwave frequency, is a measure of the atomic ground-state polarization and gives the clock signal. This signal typically has a Lorentzian shape [1,3], and stabilization of the quartz oscillato this signal realizes the clock. …”

“…In recent years, laser optical-pumping has substantially improved the achievable frequency stabilities of rubidium cell standards [1][2][3][4]. The compactness of these standards is beneficial in view of portable applications, such as GNSS, telecommunications etc.…”

“…Last developments on high-performance Rb standards have resulted in stabilities better than the lamp-pumped Rb clocks, cesium beam standards and passive hydrogen masers [3][4][5][6][7]. In this paper we will discuss the continuous-wave (CW) optical pumping approach to Rb standards.…”