iXAtom - Inertial Guidance and Navigation using Cold Atoms
The joint laboratory iXAtom brings together the knowledge of the French company iXBlue—experts in optical gyroscopes, photonics and inertial navigation—and personnel from LP2N specialized in atom interferometry. The aim of this collaboration is to make technological advances using cold atoms to develop the next generation of inertial sensors for industrial, military and Space applications, with anticipated improvements in their performance. In the near future, we plan to develop a compact gyroscope and a three-axis accelerometer based on new techniques in atom interferometry. The ultimate goal of iXAtom is to build a new autonomous device which can compete with technologies based on global positioning systems without the drawback of external communication for recalibration.
Our quantum accelerometer is based on cold 87-rubidium atom interferometer and velocity-sensitive Raman transitions.
A 3D magneto-optical trap is loaded from background vapor and the cloud is then cooled to a few micro-Kelvin using standard optical molasses techniques. Atoms are prepared in a single internal state and are interrogated by Raman beams in a Mach-Zehnder interferometer during free fall. The population of the internal states is probed by resonant fluorescence. The resulting interference fringes exhibit a phase shift that depends on the acceleration a of the atoms relative to the mirror, and scales as the square of the interrogation time T.
Compact and robust systems are crucial for onboard applications such as inertial navigation. We have constructed a field-deployable sensor head with a compact form factor designed for multi-axis accelerometry.
A retro-reflected beam geometry allows us to form phase gratings for counter-propagating Raman transitions along each axis. These beams are controlled in polarization with liquid crystal waveplates. Classical accelerometers, which are fixed to the rear of the mirrors, are used to correct for vibrations and to form a hybrid sensor.
Our laser architecture, which utilizes telecom components for their robustness and reliability, combines an all-fibered IQ modulator operating at 1560 nm and a wavelength conversion module to 780 nm.
Using carrier-suppressed dual single-sideband (CS-DSSB) modulation, the IQ modulator generates two optical sidebands that can be independently controlled in frequency, phase and power. The full performance and utility of modern RF sources can then be transferred to the optical signal using electro-optic modulation. Compared to standard phase modulators, this architecture presents strong attenuation of lines that generates parasitic Raman transitions and avoids additional acceleration bias.
Three-Axis Rotation Platform
We recently acquired a three-axis rotation table, which allows us to tilt the experiment. This table will enable us to perform sequential multi-axis measurements in different orientations, test various systematic effects, and take steps toward trully mobile implementations of the prototype.
The Kalman filter is a robust predictor-corrector algorithm commonly used in data fusion applications, such as inertial navigation. We have developed a Kalman filter for atom interferometry which is capable of tracking the complete interference fringe (phase, offset, contrast) as they vary in time. Additionally, we implemented a hybridization scheme between classical and quantum sensors that is capable of operating even while in motion.
Here, the correlations between classical and quantum accelerometers is used to determine and subsequently reject the bias of a mechanical accelerometer.
A noisy environment reminiscent of active navigation has been simulated by cycling the temperature of the mechanical accelerometer over 5 °C (equivalent to 1 mg of bias drift), modulating the laser beam intensity by 10% and, with a loudspeaker, generating 5-mg-amplitude vibration noise on the reference mirror. Under these conditions, the hybrid accelerometer reaches sub-micro-g levels after only 10 s of integration. The Kalman filter performs better than sine-fitting the interferences fringes over the entire spectral band.
In a quiet environment, the hybrid sensor realizes a sensitivity of 3.2 micro-g per shot and a bias stability of 10 ng after 11 h of integration.