High-resolution spectroscopic studies in glass-blown and micro-fabricated cells for miniature rubidium atomic clocks

Abstract

Buffer-gas alkali vapour cells form the heart of essentially all types of commercial compact or miniaturized atomic clocks. They reliably hold and confine a vapour of alkali atoms, which provides the atomic transition frequency serving as stable reference for the clock oscillator. The desire to bring atomic clock stability to portable applications such as telecommunication and navigation increased the need for more compactness and lower power consumption. This motivated the present thesis work on the cells miniaturization and the novel clocks that could be realized with such cells. More than 150 glass blown cells were produced and tested and more than 30 microfabricated cells evaluated. We present the fabrication process for each type of them. We restrict ourselves to the spectroscopic analysis of certain cell types only, which are more oriented towards the miniaturization of an atomic clock. These evaluation techniques, developed in the frame of this thesis, allowed to tests the innovative (micro-) cells fabrication processes elaborated at the LTF and SAMLAB in Neuchâtel. In particular, the buffer gas mixture characterization with a resolution of ±1%, and the leak rate detection with a limit of 1.5 x 10-13 mbar l/s were achieved. This allowed the validation of two distinct sealing processes: the “classic” anodic bonding, and an innovative low temperature sealing technique using thermocompression of indium. As an alternative to the buffer gas, the use of certain types of wall coating also allows the atomic polarization preservation. Four different types have been used: Parylen C and N, Tetracontane and OTS. While the Parylene appears to be inadequate for use with rubidium atoms, excellent antirelaxing properties are obtained with tetracontane and OTS. The successful in-house fabrication of wall coated cells allowed the observation of the ripening process by double resonance spectroscopy. The results are presented and an interpretation is given. A microfabricated OTS wall coated cell was produced at Neuchâtel too, by R. Straessle at SAMLAB. We present its spectroscopic analysis and demonstrate truly antirelaxing properties of the coating in a 4.2 mm3 vapour volume. Finally, a spectroscopic and metrologic study of an innovative "cell-microwave resonator" assembly is presented. Both the cell and the resonator are microfabricated. The cell vapour volume is of the order of 50 mm3 only. Systematic shifts limiting the metrologic performances are characterized, with a focus on the light shift. A detailed, theoretical and analytical, analysis is presented in both, the D1 and D2, lines and for various optical pumping schemes. Frequency light shift is found to be one of the main stability limiting factor in the medium to long term regime. The limit is at a level of few 10-12. We demonstrate fractional frequency stability < 10-11 from 1 s up to one day of integration time with a resonator physics package volume of less than 0.9 cm3. An alternative interrogation scheme, suppressing completely the light shift, is discussed to improve the medium to long term performances. This scheme would allow the fabrication of an atomic clock extremely compact with highly competitive stability performances.