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Microclocks at NIST - Long term frequency stability
Atomic Vapor Cells with Improved Long-term Frequency Stability

A novel technique for microfabricating alkali atom vapor cells is described, in which alkali atoms are evaporated into a micromachined cell cavity through a glass nozzle. A cell of interior volume 1 mm3, containing 87Rb and a buffer gas, was made in this way and was integrated into an atomic clock based on coherent population trapping. A fractional frequency instability of 6 × 10-12 at 1000 s of integration time was measured. The long-term drift of the F=1, mF = 0 → F=2, mF = 0 hyperfine frequency of atoms in these cells is below 5 × 10-11 / day. This high stability results from an improvement of both the linear frequency drift and temperature-related effects. The measurements presented here show that it is possible to reach long-term stabilities below 10-11 at 1 hour of integration in microfabricated cells. This makes chip-scale atomic clocks a viable technology for applications that require small size and low power consumption in combination with long-term frequency stability.

Beam filling technique

 

Novel filling method for chip-scale alkali vapor cells. (a) The silicon and glass preform and the cover glass are baked at 300 °C inside the vacuum chamber. (b) The mixture of barium azide and alkali chloride is reacted inside a glass ampoule to create barium, chlorine, alkali atoms, and nitrogen; the nitrogen is pumped away. (c) The alkali atoms are diffused through the glass nozzle into the cell preform. (d) The chamber is filled with a combination of buffer gases. The cover glass is anodically bonded to the silicon preform at a temperature of 300 °C and with 200 V applied across the sample.

 
Allan deviation of beam-filled cell
 

Fractional frequency instability measured in the Cs D2 CSAC (squares) and the cell made with the and the clock using the cell made with the new evaporation method, based on the D1 line of 87Rb (triangles).

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References:

Requires Adobe Acrobrat Reader S. Knappe, V. Gerginov, P. Schwindt, V. Shah, L. Hollberg and J. Kitching, "Atomic vapor cells for chip-scale atomic clocks with improved long-term frequency stability," Opt. Lett. 30, 2351 (2005).