A high-speed optical communication link between a nano-satellite in low orbit and a commercial optical ground station has been demonstrated for the first time.
The milestone was achieved through the ‘Keraunos’ project, a collaboration between the French Defence Innovation Agency, beam shaping firm Cailabs, and Unseenlabs, a developer of nano-satellites.
One of the key aims of the project was to tackle atmospheric turbulence, which hinders the adoption of space-to-ground optical communications systems.
Cailabs developed the ground receiver that overcomes this challenge while being sufficiently reliable and robust to be offered commercially.
Laser communication and air turbulence
Laser communication promises the next generation of secure, high-throughput free-space communications. However, its commercial deployment is still hindered by air turbulence, which impacts the phase and intensity of the light beam and degrades the quality of the link.
This becomes a particular issue at data rates exceeding 10 Gbps, where optical communication competes with radio frequency systems. To establish a high-throughput optical link, it is essential to use standard telecom equipment, which primarily consists of single-mode optical fibre.
However, coupling light into a single-mode fibre requires focusing the signal on a high signal-to-noise ratio (SNR) detector – and the size of the detector decreases with increasing data throughput. This turn, results in increased sensitivity to turbulence.
Cailabs’ beam shaping technology, which it calls multiplane light conversion (MPLC), enables complex beam shaping through a succession of spatial phase profiles and free space propagation.
“You can modify several beams at the same time, simultaneously, to a range of forms,” said David Allioux, Project & Product Manager at Cailabs, during a webinar explaining the technology.
Adaptive optics without the active mechanical components
Within ground stations, the technology couples the multimode signal into a single-mode fibre. It first decomposes the collected beam into a limited number of spatial modes, and then coherently recombines them into a single-mode fibre on an integrated photonic chip.
MPLC functions in a similar way to adaptive optics without the active mechanical components, making turbulence mitigation more affordable and robust.
"Adaptive optics are used to monitor the wavefront profile with a wavefront sensor, and this wavefront profile is then modified with deformable mirrors,” Allioux explained. “What we do is, in a sense, similar, but we modify the whole field – amplitude and phase. With adaptive optics, you only play with the phase.”
Combining multiple signals into a single mode fibre is done two-by-two using a cascade Mach-Zehnder architecture, explained Allioux, using two phase shifters and two 50/50 beamsplitters.
The successful demonstration opens the door for the use of space-based laser communications on mobile, land-based, naval and airborne platforms, Cailabs envisions.
