Bidirectional wavelength-division-multiplexing fibre-free-space optical communications using polarisation...

To address the growing demand from emerging applications, high transmission capacity is essential for both fibre backbones and last-mile communications. This can be achieved by integrating optical fibre with optical wireless technologies, facilitating the development of fibre-free-space optical communications. Here we report a bidirectional wavelength-division-multiplexing fibre-free-space optical communication employing polarisation multiplexing technique and tunable optical vestigial sideband filter. The transmission capacity is considerably increased by integrating the polarisation multiplexing technique with the wavelength-division-multiplexing scheme. The transmission performance is extensively enhanced by using a tunable optical vestigial sideband filter and vestigial sideband-four-level pulse amplitude modulation. Moreover, the optical wireless link is substantially extended through the operation of triplet lenses. Low bit error rates and clear vestigial sideband-four-level pulse amplitude modulation eye diagrams are attained with a high aggregate transmission capacity of 480 Gb/s for downstream/upstream transmission. This capability of bidirectional fibre-free-space optical communications holds substantial potential for enhancing advanced wired-wireless communications.