20 Gbit/s Free-Space Optical Communications Over 1 km Link Based on Optical Mode Conversion

Free-space optical communication (FSOC) integrates the benefits of both optical fiber and radio frequency communication, offering advantages of high bandwidth, reliability and immunity to electromagnetic interference. Traditional FSOC systems that utilize multimode fibers face limitations in transmission speed, making them less compatible with high-speed optical communication modules. Hence, FSOC with single mode fiber (SMF) coupling is essential to further enhance capacity. In this work we propose and demonstrate an FSOC system utilizing mode conversion technology. Free space light is coupled into the few-mode fiber (FMF) end with high efficiency, and then converted to the fundamental mode in the SMF through a mode demultiplexer. We established a 1 km urban horizontal link in a turbulent atmosphere and analyzed the signals intensity and jitter of each mode channel. It is found that the LP01 mode accounts for 78.82% of the total received power with the lowest jitter. The demultiplexed LP01 mode was used in optical communication test to achieve 20 Gbps signal transmission with a BER lower than <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${{10}^{ - 6}}$</tex-math></inline-formula>. As the key component in this method, the mode demultiplexer has a wider bandwidth compared to the active device, which can enable a high speed and stable FSOC system in turbulent atmosphere.