Researchers from Aston University and Glasgow University have created signal-processing algorithms to help mitigate the impact of turbulence in free-space optical experiments. They used commercially available photonic lanterns — optical fiber devices that enable a single-mode photonic function to take place within a multimode fiber — and a commercial transponder and a spatial light modulator to emulate turbulence to perform experiments that could bring free-space internet a step closer to reality. Free-space optical technology wirelessly transmits data as light through the air for use in telecoms or computer networking. However, because the data is sent as pulses of light, ambient weather conditions can cause problems. A bright sunny day or thick fog can diffract or scintillate the beam of light, creating turbulence that causes data to be lost. By applying a successive interference cancellation digital processing algorithm, the researchers reported that they achieved record results. The researchers adapted a method used in radio communications, in which redundant receive channels are used to improve bit error rate performance. The team simultaneously transmitted multiple data signals using different spatially shaped beams of light using a photonic lantern. Turbulence changes the shape of the beams, often losing the signal if only a single simple shape is transmitted detected. “Using a single beam, when a single beam was transmitted, turbulence similar to a hot sunny day destroyed the signal 50% of the time,” said Andrew Ellis of Aston University. By detecting light with multiple shapes using a second lantern, more of the light is collected at the receiver, and the original data can be unscrambled. This greatly reduced the impact of the atmosphere on the quality of the data received. The method, called multiple input multiple output (MIMO) digital signal processing, holds advantages over previously tried adaptive optics methods by shifting the complexity from the optical domain into the digital. Further, the results indicate that performance can be further improved by increasing the number of redundant receive channels. In South Africa, researchers from Aston University and Glasgow University are working with the University of the Witwatersrand on a project investigating the real-world applications of free-space optics technology. The project aims to provide the internet performance of a pure fiber connection without the need to install cables. It uses a free-space optical communication system that can link to remote sites using a wireless optical line-of-site signal to link to nearby fiber sources. The research was published in IEEE Journal of Lightwave Technology (www.doi.org/10.1109/JLT.2022.3209092).
