One of the most irritating expressions people can use, “broadband at the speed of light”, is a little closer to coming true thanks to researchers from the University of Southampton, who have demonstrated air-filled fibres with propagation happening at 99.7 percent of c.
In a conventional fibre, the glass acts as a waveguide: the core and its cladding have different refractive indices, which means the optical signal follows the path you want. In the solid core, light can only propagate at roughly 70 percent of the speed of light in a vacuum.
Through air, light moves more quickly, so there's been a long-running thread in research to do just that: propagate the light through air, and use the fibre to confine it – rather like metal waveguides are used to guide microwave-frequency signals.
This has been known for ages, and a search turns up years of research into air waveguides for optical signals. What the University of Southampton researchers are claiming (abstract) in Nature Photonics is to have solved the problems of loss and coupling that the air solution encounters.
By getting propagation speed up to 99.7 percent from 70 percent of light-speed, the best-case trip from Australia to the US would be cut from about 43 milliseconds to about 30 milliseconds (ignoring router hops and regeneration). In the world of long-distance communications, the lower latency would be beloved of gamers, and also in the world of high-speed financial trading.
The researchers also note that success with air-filled fibre would be of benefit “inside the box” – for example, to help ship data between elements in supercomputers (since electricity can only limp through copper at about two-thirds light-speed).
They claim to have achieved 3.5 dB per kilometre loss, and the 160nm wide channel was enough to carry were able to send 37 WDM channels at 40 Gbps each. ®