The ALASCA (Advanced Laser Communication System Analysis) project, coordinated by the European Space Agency (ESA) with the support of ESO, the European Southern Observatory, aims to develop a robust, high-throughput optical feeder link between ground stations and satellites, enabling continuous and resilient communication beyond the limits of radio-frequency systems.
A central challenge in uplink optical communication is the distortion introduced by atmospheric turbulence, which ALASCA addresses through the use of a Laser Guide Star Adaptive Optics (LGS-AO) system. Developed in the frame of the ESA/ESO CaNaPy project, backbone of ALASCA, this system projects a sodium laser at 589nm into the upper atmosphere to create an artificial reference star. Wavefront distortions caused by the atmosphere are then measured and corrected in real-time, significantly improving the optical signal quality and addressing the Point Ahead Angle (PAA) issue.
This groundbreaking project explores the use of a monostatic launch scheme for the first time on a large 1-m aperture telescope for satellite communications. In a monostatic scheme, laser launch (guide star laser + communications laser) and wavefront sensing of the sodium LGS happen through the same aperture. Due to the high sensitivity of the wavefront sensor to stray light in the setup, this requires a pulsed sodium guide star laser to toggle between laser launch and wavefront detection. A major benefit is the AO correction of the uplink laser beam resulting in a diffraction limited laser guide star, boosting the laser intensity in the mesosphere. In this high-intensity regime, spectral hole burning is expected to become noticeable, which is the depletion of the resonant velocity class of the Doppler-broadened velocity distribution of the mesospheric sodium atoms due to the light pressure of the laser. One of our contributions to ALASCA is a laser frequency chirping system, which shifts the laser frequency in tune with the Doppler shift of the excited sodium atoms, thus, counteracting spectral hole burning.
This week we are at ESA’s Optical Ground Station (OGS) on Tenerife for the commissioning of our frequency chirping setup and the first pre-operational tests of some of the sub-systems of this complex setup. We are looking forward to the exciting results coming out of this project in the next years.
