Wavelength-diverse transmission for turbulence-resistant free-space optical communication

Free-space optical communication (FSOC) links are susceptible to outages caused by atmospheric turbulence-induced fading. Wavelength diversity can mitigate fading by transmitting correlated information across substantially uncorrelated channels. In this study, we demonstrate a wavelength diversity system to mitigate the effects of turbulence on a signal propagated in free space. A modified polarization-multiplexed coherent optical transceiver transmits a 56.8 GBaud signal at a client rate of 200 Gbps across two carrier frequencies within the C-band over a bench-top free-space link with a turbulence emulator. Two synchronized single-wavelength coherent receivers capture the received waveforms at each carrier frequency, which are subsequently digitally combined using maximal ratio combining for offline processing. Our results demonstrate a reduction in outage probability by nearly a factor of 20 compared to the single carrier transmission scenario. Finally, we observe fading-dependent correlation between wavelengths in the C-band, which is exploited to reduce the outage probability by 86%.