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Laser communications are expected to enable wide deployment of high capacity telecommunication networks. To ensure that components used in such architectures are competitive both in terms of costs and performances, coupling into a single-mode fiber at the reception side is mandatory as most off the shelf components (COTS) have already been developed for optical fiber networks.
However, in free-space optical communication through the atmosphere, turbulences modify the wavefront profile which degrades coupling towards a single-mode fiber. Multi-Plane Light Conversion (MPLC) is proving to be a new effective technique to mitigate turbulence effect. The degraded beam is decomposed on a mode basis, typically Hermite-Gaussian modes, each mode being passively demultiplexed towards a single-mode fiber. The incoming turbulent spatial mode undergoes phase and intensity fluctuations, but, as the MPLC is a passive component, this only leads to phase and intensity fluctuations of the signal inside the corresponding single-mode fiber. The complexity of the architecture is transferred from correcting actively the wavefront to signal processing inside single-mode fibers.
Here, we investigate the performance improvement of the MPLC technique and mode collection compared to direct single-mode fiber coupling. We evaluate theoretical and experimental collection efficiency for SMF only and the summation of the 15 first Hermite-Gaussian modes for D/r0 from 1 to 14. Results show that 15 modes MPLC appear to be a good compromise between the number of modes and the complexity of the device. This configuration typically improves the collection efficiency by >7 dB in the case of strong turbulence when D/r0 >4. Moreover, the minimum collection efficiency that would correspond to a link failure is dramatically improved compared to SMF fiber alone. Finally, power distribution over the modes seems to be similar which will facilitate the implementation of this technique.
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