A ray of light [free space optical transmission]

The author looks at the role of free-space optical transmission as a solution to the problem of broadband access. The last few years have seen a dramatic resurgence of interest in nonmilitary uses of free-space optical transmission, driven by the pressing demand for high-bandwidth connections to business and commercial premises. To date, development activity has been concentrated in the US, with San Diego-based AirFibre and Seattle-based Terabeam the two principal players. The AirFibre system employs a point-to-point mesh architecture, with a receiver/transmitter at both ends of each link. The transmission wavelength is 780 nm, the transmission range is around 200 to 500 m, depending on atmospheric conditions, and the access bit rate is 622 Mbit/s. AirFibre has conducted city-wide product trials, and has a world-wide distribution agreement with Nortel. The Terabeam approach is more of a point-to-multipoint system, with the single laser beam emanating from the hub shared over multiple customers. UK interest in free-space optical communications is represented by quantumBEAM. The quantumBEAM system architecture is a radical departure from the approaches adopted by its US rivals. It's basically a point-to-multipoint system, but each link has only a single laser source, located at the customer premises end of the connection. The novelty of the system is concentrated in the quantumBEAM hub, where a pixellated electro-optical array detects signal transmissions from customers, and reflects the laser beam back along the same path so that it can be used for hub-to-customer transmissions.