Mitsubishi Electric pushes fibre transmission higher

In a breakthrough that could lead to cheaper international telephone calls and Internet leased lines, engineers at Mitsubishi Electric Corp. have succeeded in sending 1.3Tbps of data down a fibre-optic cable 8,400 kilometres in length.

While the speed does not represent an overall speed record, it is a record for the distance, which is equivalent to that from Tokyo to Southern California – the path that fibre-optic cables between Japan and the U.S. often run.

Getting more data to travel along a fibre-optic cable is important for the cable operators. The high cost of laying underseas cables and keeping them in operating order all adds to the price telecommunication carriers pay to use the cables. If more data can be sent along a single fibre, the construction and running costs can be shared between more customers and the price to each customer can be reduced.

One of the standard technologies employed commercially on such cables is dense wavelength division multiplexing (DWDM), a system that allows multiple beams of light to travel along the fibre at the same time without interfering with each other. Researchers at Mitsubishi Electric used DWDM to group 65 signals, each of 20Gbps, together to get their total speed up to 1.3Tbps.

To be able to send the data over greater distances, the engineers worked to refine the amplifiers used in the system, said Takashi Mizuochi, manager of the lightwave transmission team at Mitsubishi Electric’s research centre in Kanagawa outside of Tokyo. At intervals along the route of the fibre, amplifiers must be placed to boost the light signal and clean up any interference with the signal.

First, the team improved the amplifiers to produce a stronger light beam that can travel up to 75 kilometres without the need for amplification. Current systems can only manage gaps of around 45 kilometres before an amplifier station is needed, said Mizuochi.

Secondly, the team expanded the bandwidth of the amplifiers so that they would be able to handle more channels. The new amplifiers have a 36-nanometer bandwidth compared to a 30-nanometer bandwidth on normal amplifiers, thus allowing around 10 more channels to be carried down the fibre.

With the 8,400-kilometre barrier broken, Mizuochi is turning his attention to longer distances.

“Now we are trying to expand the distance to 9,000 kilometres,” he said. Why that distance? A transpacific fibre cable often runs in a ring with different paths being taken by the northern and southern halves of the ring. The northern half covers a distance of 9,000 kilometres, he said.

Mitsubishi Electric plans to disclose more details about the transmission system during a presentation at the Optical Fiber Communication Conference scheduled to be held in Anaheim, Calif., on March 20.

Mitsubishi Electric Corp., in Tokyo, can be contacted at