High-efficiency free-space optical communication link with refractive adaptive optics

Free-space optical communication links are vulnerable to beam wandering and turbulence-induced aberrations, which degrade optical wavefront quality and fiber coupling efficiency. These systems are usually designed to completely illuminate the receiver aperture to reduce the scintillation generated by the wandering of the laser beam, generating antenna losses. This work introduces an innovative receiver employing refractive adaptive optics elements to dynamically correct beam wandering and wavefront aberrations. We present the results obtained from a laboratory test replicating the conditions present in a 270-meter ground-to-ground link used to perform quantum key distribution in the center of Rome. The beam wandering is artificially generated using a Fast-Steering Mirror, while a Deformable Mirror introduces high-order aberrations. Corrections for beam wandering and residual tip-tilt errors are achieved with Fast Steering Prisms, while high-order aberrations are corrected using a Multi-Actuator Lens. This design makes the Adaptive Optics system fully refractive, which has the potential to enhance compactness compared to conventional systems based on Deformable Mirrors. Experimental results demonstrate an average improvement in the Strehl Ratio of up to 87% and an average single-mode fiber coupling efficiency of up to 37%, preserving all the light sent from the transmitter.