ED Sciences Physiques et de l'Ingénieur
Hybrid quantum accelerometer for onboard applications
by Cyrille DES COGNETS (Laboratoire Photonique, Numérique & Nanosciences)
The defense will take place at 14h00 - Amphitheatre A.Ducasse Institut d'Optique d'aquitaine, 1 rue françois mitterrand, 33400 Talence
in front of the jury composed of
- Baptiste BATTELIER - Ingénieur de recherche - Université de Bordeaux - Directeur de these
- Tim FREEGARDE - Professor - university of Southampton - Rapporteur
- Yannick BIDEL - Directeur de recherche - ONERA - Rapporteur
- Christophe COLLETTE - Professeur - Université de liège - Examinateur
- Baptiste ALLARD - Maître de conférences - LCAR - Examinateur
- Olivier GAUTHIER LAFAYE - Directeur de recherche - LAAS - Examinateur
This thesis focuses on the correction of rotational and vibrational effects that affect acceleration measurements in cold-atom interferometers. Such instruments can notably be employed to improve the long-term stability of inertial navigation systems used to estimate a vehicle's position, but they remain bulky and sensitive to various perturbations, which limits their use in embedded environments. The manuscript first presents the principles of our tri-axis hybrid quantum accelerometer: Raman transitions, atom-based Mach–Zehnder interferometry, and hybridization with classical accelerometers. The hybridization scheme merges the measurements of both technologies and includes real-time correction of the laser phase and frequency to compensate Doppler shifts and to reconstruct fringes blurred by vibrations. Two operation modes are described: the gravimeter mode, providing discrete gravity measurements, and the strapdown accelerometer mode, offering continuous measurements with a wide dynamic range and long-term stability. In this latter mode, measurements can be performed regardless of the instrument's orientation. The experimental aspects are then detailed. To make the system compatible with onboard strapdown operation, several technological choices have been implemented: a compact fiber-based laser system relying on advanced modulation techniques to generate the required frequencies, and hybridization with gyroscopes to compensate for rotations and changes in orientation. The strapdown mode also introduces constraints related to sensor tilt, particularly regarding detection and laser-beam geometry. After validating the sensor under static conditions, the work of this thesis focuses on its operation in dynamic environments. In particular, rotations degrade the measurement of the atomic accelerometer by reducing the interferometer contrast and introducing significant phase shifts. To correct these effects, two approaches were investigated: a rigid mode, limited to low rotation rates (<4.6°/s) due to contrast loss, and an inertial-pointing mode in which the reference mirror is stabilized via a tip–tilt platform driven by a real-time system using fiber-gyroscope measurements. This compensation suppresses contrast loss and enables operation under high rotation rates. A theoretical model has been developed to describe contrast loss (rigid mode) and rotation-induced phase shifts. These solutions have been validated experimentally using a manual rotation platform, leading to the first demonstration of an atomic accelerometer operating up to 14°/s with a 10-ms interrogation time and a sensitivity of 35 µg/shot. We have then used a hexapod capable of generating controlled motion to test the sensor under realistic dynamic conditions. During motion, the atomic fringes become blurred due to strong vibrations transmitted to the reference mirror, and the correction fails because of insufficient correlation with the classical accelerometer. To mitigate this issue, mechanical dampers have been added, and several solutions for improving real-time correction have been explored through the assessment of classical accelerometer performance. Despite progress, further improvements are still required to achieve good dynamic performance. Finally, I present gravity measurements obtained with the accelerometer on the hexapod. With an interrogation time of 29 ms, the sensor reaches a static sensitivity of 1.33 µg/√τ and a stability of 31 ng at 3000 s, enabling the detection of Earth tides. Dynamic measurements, however, show significant degradation and reveal the limits of our gravity-tracking method, as well as the technical challenges that remain for mobile gravimetry.