M. Pellaton

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 approximate to 3 x 10(-11) at 1 s of integration time, which compares well to the performance of compact commercial Rb atomic clocks.

Compact microwave cavity for high performance rubidium frequency standards

The design, realization, and characterization of a compact magnetron-type microwave cavity operating with a TE(011)-like mode are presented. The resonator works at the rubidium hyperfine ground-state frequency (i.e., 6.835 GHz) by accommodating a glass cell of 25 mm diameter containing rubidium vapor. Its design analysis demonstrates the limitation of the loop-gap resonator lumped model when targeting such a large cell, thus numerical optimization was done to obtain the required performances. Microwave characterization of the realized prototype confirmed the expected working behavior. Double-resonance and Zeeman spectroscopy performed with this cavity indicated an excellent microwave magnetic field homogeneity: the performance validation of the cavity was done by achieving an excellent short-term clock stability as low as 2.4 × 10(-13) τ(-1/2). The achieved experimental results and the compact design make this resonator suitable for applications in portable atomic high-performance frequency standards for both terrestrial and space applications.

Microfabricated alkali vapor cell with anti-relaxation wall coating

We present a microfabricated alkali vapor cell equipped with an anti-relaxation wall coating. The anti-relaxation coating used is octadecyltrichlorosilane and the cell was sealed by thin-film indium-bonding at a low temperature of 140 °C. The cell body is made of silicon and Pyrex and features a double-chamber design. Depolarizing properties due to liquid Rb droplets are avoided by confining the Rb droplets to one chamber only. Optical and microwave spectroscopy performed on this wall-coated cell are used to evaluate the cells relaxation properties and a potential gas contamination. Double-resonance signals obtained from the cell show an intrinsic linewidth that is significantly lower than the linewidth that would be expected in case the cell had no wall coating but only contained a buffer-gas contamination on the level measured by optical spectroscopy. Combined with further experimental evidence this proves the presence of a working anti-relaxation wall coating in the cell. Such cells are of interest for applications in miniature atomic clocks, magnetometers, and other quantum sensors.

Low-temperature indium-bonded alkali vapor cell for chip-scale atomic clocks

A low-temperature sealing technique for micro-fabricated alkali vapor cells for chip-scale atomic clock applications is developed and evaluated. A thin-film indium bonding technique was used for sealing the cells at temperatures of ≤140 °C. These sealing temperatures are much lower than those reported for other approaches, and make the technique highly interesting for future micro-fabricated cells, using anti-relaxation wall coatings. Optical and microwave spectroscopy performed on first indium-bonded cells without wall coatings are used to evaluate the cleanliness of the process as well as a potential leak rate of the cells. Both measurements confirm a stable pressure inside the cell and therefore an excellent hermeticity of the indium bonding. The double-resonance measurements performed over several months show an upper limit for the leak rate of 1.5 × 10−13 mbar·l/s. This is in agreement with additional leak-rate measurements using a membrane deflection method on indium-bonded test structures.

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

We present two lines of investigations on vapor cell based laser-microwave double-resonance (DR) rubidium atomic frequency standards: a compact high-performance clock exhibiting σ_y(τ) <; 1.4×10^-13 τ^-1/2 and a miniaturized clock with σ_y(τ) <; 1×10^-11 τ^-1/2. The applications of these standards are emphasized towards portable applications such as next generation GNSS, deep space missions and telecommunications. Other techniques for DR clocks are discussed in brief.

Pulsed optically pumped Rb clock with optical detection: First results

In this work we present the first results related to a prototype of Rb clock working in pulsed regime, developed in the frame of an ESA contract (“Next generation compact atomic clocks”, 21504/08/NL/GLC). The contract is a joint participation between Istituto Nazionale di Ricerca Metrologica, Optics Division (INRIM, Italy) and Université de Neuchatel, Laboratoire Temps - Frequence (LTF, Switzerland).

Compact and frequency stabilized laser heads for Rubidium atomic clocks

We present the development and complete spectral characterization of our compact and frequency-stabilized laser heads, to be used for rubidium atomic clocks and basic spectroscopy. The light source is a Distributed Feed-Back (DFB) laser diode emitting at 780 nm or 795 nm. The laser frequency is stabilized on a sub-Doppler absorption peak of the 87Rb atom, obtained from an evacuated rubidium cell. These laser heads, including the electronics for the light signals detection, have an overall volume of 0.63 liters. We also present a variant of the laser head into which is integrated an Acousto-Optical Modulator (AOM) that precisely detunes the laser frequency in order to minimize the AC Stark shift in Rb atomic clocks.

High performance vapour-cell frequency standards

We report our investigations on a compact high-performance rubidium (Rb) vapour-cell clock based on microwave-optical double-resonance (DR). These studies are done in both DR continuous-wave (CW) and Ramsey schemes using the same Physics Package (PP), with the same Rb vapour cell and a magnetron-type cavity with only 45 cm3 external volume. In the CW-DR scheme, we demonstrate a DR signal with a contrast of 26% and a linewidth of 334 Hz; in Ramsey-DR mode Ramsey signals with higher contrast up to 35% and a linewidth of 160 Hz have been demonstrated. Short-term stabilities of 1.4×10-13 τ-1/2 and 2.4×10-13 τ-1/2 are measured for CW-DR and Ramsey-DR schemes, respectively. In the Ramsey-DR operation, thanks to the separation of light and microwave interactions in time, the light-shift effect has been suppressed which allows improving the long-term clock stability as compared to CW-DR operation. Implementations in miniature atomic clocks are considered.

New miniaturized microwave cavity for Rubidium atomic clocks

Nowadays there is an increasing need for radically miniaturized and low-power atomic frequency standards, for use in mobile and battery-powered applications. For the miniaturization of double-resonance (DR) Rubidium (87Rb) atomic clocks, the size reduction of the microwave cavity or resonator (MWR) to well below the wavelength of the atomic transition (6.835 GHz for 87Rb) has been a long-standing issue. Here we present a newly developed miniaturized MWR, the μ-LGR, consisting of a loop-gap resonator based cavity with very compact dimensions (volume <; 0.9 cm3). The μ-LGR meets the requirements of the atomic clock application and its assembly can be performed using repeatable and low-cost techniques. The concept of the proposed device was validated through simulations and prototypes were successfully manufactured and tested. High-quality DR spectra and first clock stabilities were demonstrated experimentally, proving that the μ-LGR is suitable for integration in a miniaturized atomic clock.

Low-temperature indium hermetic sealing of alkali vapor-cells for chip-scale atomic clocks

We present a low-temperature indium hermetic bonding technique on wafer level without using flux, active atmosphere or other pretreatment of the indium. Its simplicity and low temperatures allow encapsulation of sensitive MEMS devices. Bonding stronger than 18 MPa was accomplished with temperatures never exceeding 140°C. Leak rate measurements revealed leak rate below 2.5 × 10-12 atm cc/s. This bonding technique is then applied to fabricate rubidium vapor-cells for chip-scale atomic clocks (CSAC). Saturated absorption spectroscopy performed two and five months after fabrication confirms less than 1 mbar of gas contamination, and the retrieved clock signal demonstrates the suitability of the cell for clock applications.