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INSTRUMENTED FLEXIBLE GLASS STRUCTURE: Design of Flexure Mechanisms for Resonators and Micro-Scale Monolithic Bragg Grating Sensors

par Amez-Droz, Loïc
Président du jury Gilet, Tristan
Promoteur Collette, Christophe ;Lambert, Pierre
Publication Non publié, 2024-08-27

Thèse de doctorat

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Résumé :

The INFuSE project, funded by an FNRS PDR grant, began in 2020 to develop resonators for inertial sensors and sensor arrays incorporating photonics for force/strain and biochemical sensing. This project harnesses the unique properties of glass along with advanced 3D machining technology. Glass's low loss factor, low thermal expansion rate, optical quality surface finish, and high elastic limit-to-Young's modulus ratio make it ideal for precision mechanics and resonators. The manufacturing of fused silica structures at scales below 1 mm presents challenges. To address these challenges, the project utilizes the FEMTOprint machine, a cutting-edge device that enables 3D machining with sub-micron precision. This technology uses femtosecond laser-assisted wet etching to create monolithic structures that can integrate fluidic, optical, and mechanical functionalities at nano- and micro-scale. This work's contributions include the successful fabrication of various flexible structures using the femtosecond laser-assisted wet etching process, achieving thicknesses as low as 10 µm. The bending strength for fused silica specimens is estimated, with a maximum estimated stress of 2.6 GPa and a recommended limit of 1 GPa for micro-scale flexure specimens conception. Additionally, ring-down experiments on fused silica-based resonators showed that the damping is influenced by the air pressure at 2*10^-3 mbar, with a quality factor reaching 185,000. The project also saw the development and manufacturing of two different vertical inertial sensors, demonstrating the successful assembly of glass flexure joints with aluminium and stainless steel components. The open-loop transfer function of the first sensor showed good coherence with a reference sensor between 200 mHz and 100 Hz having its natural frequency at 2.8 Hz. Then, the main novelty of the thesis is the realisation of Bragg grating sensors inscribed in flexure specimens. Their sensitivity compares to the theoretical value of approximately 1.2 pm/με. A method for compensating for temperature gradients in Bragg grating sensors was proposed, pending further characterization. These contributions lay a strong foundation for future advancements in resonator technology and instruments with embedded photonics, enhancing applications in force/strain sensing.