The End of World Wide Waiting

Until now, optical fibre transmission systems have used light of a specific wavelenght only, mostly in ranges of 1.3 or 1.55 micrometers. These are single-channel systems with transmission rates up to 2.5 G.bit/s over ranges of 40 to 80 km between two repeaters that `freshen up' the signals. At higher transmission rates and longer distances, a phenomenon referred to as `dispersion' starts disrupting transmission by deforming data-bearing light pulses.

The scientists have shown that a technique called `dispersion compensation' pushes the point at which these distortions become disruptive back much further than had been imagined possible till now. The flashes of light can become shorter and they remain independent of each other, running through the glass fibres in various colours, increasing capacity several times.

The data througput of 160 Gbit/s was possible with four wavelenghts of 1.55 um and transmission rates of 40 Gbit/s each, achieving ranges of 100 km without repeaters on standard fibres. Optical multiplexing of the four 40 Gbit/s every 2nm was achieved by means of an arrayed waveguide grating (AWG) developed at HHI, demultiplexing of the wavelength at the end of the transmission system with a tunable optical filter.

Demultiplexing the signal into 4 bit streams of 10 Gbit/s each is achieved with a high-speed optical switch. This switch consists of an interferometer circuit with semiconductor amplifiers, wich are best integrated onto an InP chip.

According to theoretical analyses and numerical calculations this process can be expanded beyond the four channels used in the trials, to an Nx40-Gbit/s WDM transmission system. Capacity increases with a combination of WDM and TDM are likely to be more cost-effective than bit-rate increases with exclusively TDM systems.