Quadratic dependence on temperature of Cs 0–0 hyperfine resonance frequency in single Ne buffer gas microfabricated vapour cell

Abstract

Presented is the observation of a quadratic temperature dependence of the Cs 0 -0 ground state hyperfine resonance frequency in a single Neon (Ne) buffer gas vapour microcell. The inversion temperature, expected to be theoretically independent of the buffer gas pressure, is measured to be about 808C for two different samples. A proposal to develop chip scale atomic clocks with improved long-term frequency stability, simpler configuration (a single buffer gas instead of a buffer gas mixture) and then relaxed constraints on pressure accuracy during the cell filling procedure is presented.Introduction: In the last decade, the significant advances in micromachining technologies and semiconductor lasers combined with the coherent population trapping phenomenon [1] has allowed the development of chip scale atomic clocks (CSAC) [2,3]. These miniature time references, exhibiting typical frequency stability of the order of 10 210 at 1 s and 10 211 at 1000 s to 1 day for a volume of a few tens of cm 3 and a power consumption of 150 mW, are of great interest in various portable battery-operated applications, such as navigation receivers and telecommunication systems.The heart of these miniature frequency standards usually consists of a microfabricated alkali vapour cell, formed in a wafer of silicon with glasses bonded to both sides. The cell is filled with buffer gases in order to prevent wall relaxation, increase the atom -light interaction time, and reduce the Doppler broadening [4]. Nevertheless, slight interactions taking place between alkali atoms and buffer gases cause a frequency shift of the hyperfine transition of the atom. This shift Dn, of the order of several hundreds of Hz per Torr, is expected to be dependent on the nature of the gas, pressure and operating temperature T, and can be expressed as in [5]:

Temperature Dependence Cancellation of the Cs Clock Frequency in the Presence of Ne Buffer Gas

“…This yields to T i = (80 ± 4)°C. [15] Recently [14], the inversion temperature was observed in millimeter size Cs cells with a 75 Torr pressure of Ne buffer gas. The results are in agreement with ours.…”

First-order cancellation of the Cs clock frequency temperature-dependence in Ne-Ar buffer gas mixture

“…It is clearly shown that there is no way to obtain any inversion temperature above about 80 • C for Cs cells filled with a Ne-Ar mixture. For a = 0 (pure Ne cell), the temperature-dependence of the Cs frequency is expected to be cancelled at about 80 • C as recently observed experimentally in chip-scale cells [13] and cm-scale glass blown cells [9]. Compared to pure Ne cells, the addition of Ar always shifts the inversion temperature to lower temperatures.…”

“…The buffer gas pressure shift measurement procedure is similar to the one described in [13]. The microcell temperature is changed.…”

“…Recently, O. Kozlova et al established first measurements of the quadratic coefficient for various buffer gases (Ne and N 2 ) [9] thanks to a high-resolution CPT Cs clock combining an original excitation scheme, a temporal Ramsey interrogation [10,11] and allowing a great reduction of the light shift sensitivity [12]. At the same time and particularly, it has been measured in microcells [13] and cm-scale cells [9] that the Cs clock transition frequency shows a quadratic dependence on temperature around an inversion temperature of about 80 • C using only Ne as a unique buffer gas. In this case, it has been shown experimentally that the inversion temperature does not depend on the buffer gas pressure [14] as expected theoretically.…”

Metrological characterization of custom-designed 8946 nm VCSELs for miniature atomic clocks

“…The Cs microfabricated cell, realized according to the process described in [20], is 2 mm in diameter, 1.4-mm long, and is filled with Ne buffer gas. The cell is temperature-stabilized to ≈ 80 • C where the temperature-dependence of the Cs clock frequency is cancelled [21][22][23]. A static magnetic field of several μT is applied to the cell, placed in a single-layer mu-metal magnetic shield.…”