3D printed microwave cavity for atomic clock applications: proof of concept

The authors present the realisation and characterisation of an additively manufactured (AM) microwave resonator cavity for double-resonance (DR) vapour-cell atomic clocks. The design of the compact microwave cavity is based on the loop-gap resonator approach, previously demonstrated for conventionally-machined aluminium components. In the present study, the resonator is fabricated by AM using a metal-coated polymer. A resonance frequency at the desired 6.835 GHz rubidium atomic frequency is obtained. When employed in an atomic clock setup, the AM cavity enables a DR signal of

Polarization response of Perovskite films for microwave tunable applications

Konstantin Astafiev

Ferroelectric thin films are widely studied nowadays as potential replacements for semiconductors in modern tunable microwave devices such as tunable filters, phase-shifters, frequency mixers, power dividers, etc. Recent progress in the deposition of complex oxide films shows promise of significant improvements in the quality of ferroelectric thin films from the point of view of dielectric loss and electrical tunability. However, there still exist certain problems which limit a wide penetration of ferroelectric thin films into the market. The most crucial issue which needs further investigation is the difference in polarization response between ferroelectric thin films and bulk materials. In comparison to bulk materials, thin films show lower dielectric constants, and consequently a smaller electrical tunability, and much higher dielectric losses at microwave frequencies. Another important question which has rarely been investigated is the problem of the reliability of tunable devices based on ferroelectric films. Therefore, a systematic, experimental investigation to understand the physics behind these problems is a task of primary importance. The present work is devoted to the detailed theoretical and experimental investigation of the polarization response in SrTiO3 and KTaO3 thin films at radio and microwave frequencies over a wide range of temperatures and applied bias fields. Specifically, the following has been performed: The polarization response of SrTiO3 and KTaO3 ferroelectric thin films has been investigated at radio frequencies over a wide temperature range using a planar capacitor structure. The sensitivity of the dielectric properties to the deposition technique, conditions of films growth and type of the substrate used, and hence to the microstructure of the films, is clearly demonstrated. While SrTiO3 is paraelectric in bulk form, the appearance of a ferroelectric phase in SrTiO3 thin films on LaAlO3 substrate has been experimentally documented, with a transition temperature lying in the range of 90-120K. It is theoretically shown in terms of Landau theory, that the behavior of the SrTiO3 thin films can not be attributed to the effect of the film/substrate mechanical coupling. The ferroelectricity of SrTiO3 thin films on LaAlO3 substrates has been investigated in detail. The hysteresis loops have been monitored in a wide temperature range using thin film planar capacitors, the driving field being predominantly in the plane of the film. The microwave dielectric loss in perovskite ferroelectric materials has been theoretically investigated. A model of the quasi-Debye loss mechanism in SrTiO3 and Ba0.6Sr0.4TiO3 materials at microwave frequencies has been developed. Experimental investigations of the dielectric loss at microwave frequencies in SrTiO3 and KTaO3 thin films have been performed using a half-wave length microstrip resonator with a resonant frequency of around 8GHz. The field and temperature dependences of the loss tangent for the SrTiO3 and KTaO3 thin-film based planar capacitors have been measured. The impact of electrical stressing on the microwave performance of SrTiO3 films deposited onto different substrates (MgO and LaAlO3) has been investigated. The most important characteristics of tunable microwave components, including tunability and loss tangent, have been monitored. The influence of the addition of a low-loss linear dielectric material to a tunable ferroelectric material has been investigated in terms of electrostatic considerations. Calculations of the dielectric loss and dielectric non-linearity of ferroelectric-dielectric composites have been performed using three different models. On the basis of the results obtained, a figure of merit for the composite material has been evaluated.

EPFL2004

Miniaturized Microwave Cavities for Industrial Applications

Maddalena Violetti

Microwave cavities, or microwave resonators, are used in a variety of applications, including filters, oscillators, frequency meters, and tuned amplifiers. The ever-increasing demand for compact and/or portable telecommunication systems drives the need for further miniaturization of such structures for improved integration. In this thesis, new miniature microwave cavity solutions are proposed, which are useful in at least two scopes of application. In the first scope, the new devices are used as microwave front-ends of a short-range radar system for on-line blade tip monitoring in gas turbines (i.e. land-based large-frame turbines, aero-derivatives and aero-engines). In general, microwave sensors are advantageous for blade tip sensing: contrary to other existing techniques, they can survive to high temperatures for extended period of operation, unaffected by contaminants, like gases and dirt. Nonetheless, the microwave front-end of such systems is of difficult implementation, due to the strict thermal-mechanical constraints of the specific field of application (i.e. a large temperature change, the presence of vibrations, dirt and combustion by-products) and the tight requirements on the size of candidate probe designs. Antenna-based solutions are currently used as tip monitoring probes, with the disadvantage of being complex structures with radiation characteristics that are not entirely well suited for short range sensing. As a valid alternative, we propose two cavity-based probe solutions, namely a miniature waveguide resonator probe and a coaxial resonator probe, which represent an elegant compromise between robustness, miniaturization and simplicity of operation. Tests in laboratory and on real turbine engines demonstrated the suitability of such devices for the aimed application and several improved characteristics with respect to other comparable techniques, like antenna-based and eddy-current sensors. In the second scope, the new proposed solutions are used as microwave resonators in miniature rubidium atomic clocks. Attempts to miniaturize such kind of systems are usually limited by the dimensional constraints of standard microwave cavity resonators (MWR) , which account for a large part of the volume of the physics package. We propose a new miniature MWR based on a loop-gap resonator structure, also referred to as the μ-LGR. This cavity design meets the desired field configuration for the atomic clock operation and achieves an interesting size-reduction. A laboratory experiment of an atomic clock integrating the μ-LGR achieved unmatched results of stability compared to other microcell clocks. Finally, we propose a miniature planar MWR for the integration of 2D microfabricated cells.

EPFL2012

New miniaturized microwave cavity for Rubidium atomic clocks

Francesco Merli, Anja Skrivervik Favre, Maddalena Violetti, Jean-François Zürcher

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 (Rb-87) 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 Rb-87) has been a long-standing issue. Here we present a newly developed miniaturized MWR, the mu-LGR, consisting of a loop-gap resonator based cavity with very compact dimensions (volume < 0.9 cm(3)). The mu-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 mu-LGR is suitable for integration in a miniaturized atomic clock.