Maddalena Violetti

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.

New microwave sensing system for blade tip clearance measurement in gas turbines

Real-time clearance control in turbine engines allows improving their efficiency and safety, by enabling prognostics and optimized condition-based maintenance in the turbine hot section. This paper presents a novel 24 GHz microwave sensing system for real-time blade tip clearance monitoring suitable for small-size turbines (aero-engines and aero-derivatives). The system principle of operation is similar to the short range radar technique and employs high temperature resistant circular waveguide resonator probes mounted into the shell of the turbine. Probes concept was proven through simulation and prototypes where built and tested, showing agreement of results. The system was validated in laboratory, where the harsh turbine environment was reproduced on dedicated test rigs. Finally, the system was tested on a real engine, showing its suitability for real-time blade tip measurement.

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.

New microwave sensor for on-line Blade Tip Timing in gas and steam turbines

Real-time Blade Tip Timing (BTT) in rotating machinery enables condition-based maintenance and blade failure prevention. In this paper, we propose a new 6 GHz microwave sensing system suitable for BTT in gas and steam turbines. The system working principle is similar to the short range radar technique, using probes mounted in the turbine wall, in direct view of the passing blades. The front end of the system consists of a miniature open-ended coaxial resonator sealed by a ceramic cap. Its principle of operation is simple and robust, thus well-suited for the turbine harsh environment, while the radiated TEM mode allows for insensitivity to mounting orientation. The probe was validated through simulation and prototypes were successfully tested. Measurements on prototypes showed improved spatial resolution on the waveform compared to an existing 6 GHz patch antenna probe, and superior resolution and measurement range compared to Eddy current probes used for BTT.

Double resonance spectroscopic studies using a new generation of microfabricated microwave cavity

We evaluate the potential of a new generation microwave cavity by double resonance spectroscopic studies. Both, the cell and the microwave cavity are microfabricated. This allows a substantial size reduction for the physics package, without compromising the RF field geometry and the optical quality of the cell windows. We demonstrate short term stability of 5 × 10^-12τ^-1/2 (1-100 s), and below 1 × 10^-12 up to 10³ s. Finally, we report on the AC stark shift effect, one of the main limiting factors for long term clock stabilities.