Main features of space rubidium atomic frequency standard for BeiDou satellites
Wuhan Institute of Physics and Mathematics (WIPM) has been developing the space rubidium atomic frequency standard (RAFS) for BeiDou navigation satellite system since the late of 1990's. Design of the RAFS aims to realize high stability, small size, long lifetime, high reliability and space environmental adaptability. The physics package was designed in SFT scheme. A slotted tube cavity with small size and TE011 resonant mode was used to construct the cavity-cell assembly, and an argon lamp with long lifetime was used as the pumping source. The microwave chain includes a 10MHz VCXO, a 5.3125MHz synthesizer, a ×9 RF multiplier and a ×76 SRD multiplier. Electronic circuits were designed by using analog circuitry to minimize the risk of element failure in space. The day stability of the space RAFS's developed for BeiDou regional system distributes within 2~5×10-14. To meet the needs of BeiDou global system, design of the RAFS has been improved further. A slotted tube cavity with larger size and better resonant mode was used, the argon lamp was substituted with a filtered xenon lamp, and the phase noise of microwave chain was depressed further. A prototype of new generation space RAFS was built recently. Preliminary test showed that the day stability of the prototype has reached 3×10-15.
Mesoscopic physics in vapor-cell atomic clocks
In the early 1980s, it was common to hear that Rb atomic clocks were limited to Sy(τ) = 10<sup>−11</sup>/τ<sup>1/2</sup> at best, and that their ultimate frequency stability could never be better than low-parts of 10<sup>−13</sup>. Twenty-first century Rb clocks put the lie to those pronouncements. Today, it is not uncommon for vapor-cell clocks to exhibit short-term stabilities at low-parts of 10<sup>−13</sup>/τ<sup>1/2</sup>, and ultimate stability levels in parts of 10<sup>−15</sup>. Here, we discuss what it will take to improve those clocks further (e.g., getting the long-term stability to break into the 10<sup>−16</sup> range). Specifically, it is our contention that next steps will require a better understanding of mesoscopic physics in vapor-phase systems: atomic phenomena occurring on a scale between the microscopic level of quantum dynamics and the macroscopic level of vapor temperature and pressure. Though at present we have some understanding of mesoscopic physics and its effects, much more needs to be understood. Further, while past and present advances have been driven by “one-off” phenomenological discoveries, it is our opinion that rapid advancement will be best facilitated by a more overarching vision of mesoscopic vapor-phase physics, which at present is lacking.
Microfabricated rubidium vapour cell with a thick glass core for small-scale atomic clock applications
This paper presents a new fabrication method to manufacture alkali reference cells having dimensions larger than standard micromachined cells and smaller than glass-blown ones, for use in compact atomic devices such as vapour-cell atomic clocks or magnetometers. The technology is based on anodic bonding of silicon and relatively thick glass wafers and fills a gap in cell sizes and technologies available up to now: on one side, microfabrication technologies with typical dimensions ≤ 2 mm and on the other side, classical glass-blowing technologies for typical dimensions of about 6–10 mm or larger. The fabrication process is described for cells containing atomic Rb and spectroscopic measurements (optical absorption spectrum and double resonance) are reported. The analysis of the bonding strength of our cells was performed and shows that the first anodic bonding steps exhibit higher bonding strengths than the later ones. The spectroscopic results show a good quality of the cells. From the double-resonance signals, we predict a clock stability of ≈3 × 10−11 at 1 s of integration time, which compares well to the performance of compact commercial Rb atomic clocks.