The terabit challenge

30 Jul 2002

The latest optical communication systems can transmit several terabits of information per second. Oliver Graydon looks into the technologies that are making it possible.

From Opto & Laser Europe June 2002

The era of terabit communication systems has truly arrived. Earlier this year Bell Labs, the research arm of Lucent Technologies, US, announced that it had broken the record for long-haul fibre-optic transmission by sending 2.56 Tbit/s a distance of 4000 km. This was achieved using 64 wavelength channels carrying 40 Gbit/s each.

A commercial version of the equipment, called the LambdaXtreme Transport, is now available on the market and is being tested by the German incumbent operator, Deutsche Telekom. The system is capable of sending either 2.56 Tbit/s (64 x 40 Gbit/s channels) a distance of up to 1000 km, or 1.28 Tbit/s (128 x 10 Gbit/s) up to 4000 km, without any electrical regeneration.

Global trialsIt is not just in Germany that such terabit systems are under test. Major telecoms operators in Japan and the US have also been running high-capacity field trials.

Japanese telecoms provider KDDI has recently performed a multi-terabit field-trial between Tokyo and Osaka. The firm's laboratories were able to successfully transmit 2.52 Tbit/s (63 x 40 Gbit/s) over a 320 km link that was made of non-zero dispersion-shifted fibre.

In March last year, Siemens and US-based telecoms service provider WorldCom announced the results of a terabit field trial that involved transmitting 3.2 Tbit/s (80 x 40 Gbit/s) of audio, video and data traffic over optical fibre installed around Dallas, US. WorldCom has also put high-capacity systems from Fujitsu (the Flashwave OADX, 1.76 Tbit/s) and Nortel (the OPTera Long Haul 1600, 1.6 Tbit/s) through their paces on a route linking New York with Washington DC.

And it looks as though even faster systems may be just around the corner. Experts at Alcatel, the French manufacturers of telecoms equipment, predict that a single optical fibre will shortly be able to transport 8 Tbit/s across a distance of 6500 km. If it happens soon, this staggering achievement will represent a 1000-fold increase in the long-haul transmission capacity of a fibre in less than a decade.

In 1995, Alcatel helped to commission the first optically-amplified submarine link (TAT-12/TAT-13), which carried just 5 Gbit/s per fibre between Europe and the US (a distance of 6300 km). Although at the time this was considered a significant achievement, far more is possible now.

Key technologiesThe emergence of terabit lightwave communication systems from companies such as Lucent, Alcatel, Nortel, Siemens, Marconi, NEC and Fujitsu is a result of the recent advances made in several key technologies. Top of the list are solitons, distributed Raman amplification, dense-wavelength division multiplexing (DWDM), 40 Gbit/s technology and forward error correction.

The latest transmission systems from Lucent, Marconi and Corvis all make use of a new transmission format known as return-to-zero (RZ). Unlike conventional communication systems, which use rectangular optical pulses to represent a digital data-bit, the RZ format exploits special bell-shaped optical pulses called solitons.

The beauty of using soliton pulses to carry data is that, as long as their power is carefully controlled, nonlinear effects in the optical fibre ensure that their pulse shape does not degrade with transmission distance. By contrast, rectangular pulses tend to spread out and smear, and must be reconstructed at regular intervals along the link.

Raman amplification is also starting to play a key role in submarine and ultra-long-haul systems. Raman scattering means that a strong optical pump signal in the transmission fibre can be used to generate gain at a slightly longer wavelength. The technique effectively turns a length of ordinary optical fibre into a cascade of micro-optical amplifiers. The result is that an optical signal travelling along the fibre receives a continual gentle amplification that helps to maintain its signal-to-noise ratio.

By combining Raman amplification with erbium-doped fibre amplifiers - devices that provide a large gain at discrete points - it is possible to greatly extend the span length of a high-capacity communications link. Researchers at US-based Tyco Telecommunications, for example, have recently used this technique to demonstrate that it is possible to send 10 Gbit/s channels across a distance of 11,000 km.

Economics of scaleDWDM technology means that by using a number of transmitters, each operating at a distinct wavelength, it is possible to squeeze more data into an optical fibre. For example, some of the latest terabit systems simultaneously transmit 160 wavelength channels separated by just 50 GHz (0.4 nm). Demultiplexers and filters separate the individual channels at the end of the link.

Last year, NEC and Alcatel independently reported that they had managed to transmit more than 10 Tbit/s over a single optical fibre link using DWDM. Although in both cases the fibre links were only about 100 km in length, these experiments proved that it is possible to transmit hundreds of wavelengths along the same fibre simultaneously. The use of 40 Gbit/s technology is also having a big impact.

Within just a few years, optoelectronic transmitters and receivers have progressed from operating at a speed of 2.5 Gbit/s to 40 Gbit/s. Each jump in channel speed instantly increases the capacity of a transmission system by a factor of four.

In fact, several telecoms operators in Europe are now trialling the 40 Gbit/s channel technology that underlies the emergence of terabit transmission. In Germany, Deutsche Telekom is currently testing Lucent's LambdaXtreme system at a line-rate of 40 Gbit/s. And KPNQwest, the pan-European operator, has used Alcatel's Optinex system to transport multiple 40 Gbit/s wavelengths over a 160 km link between the German towns of Frankfurt and Gernsheim.

Finally, forward error correction (FEC) is playing its part. By encoding data with a special algorithm prior to transmission, it is possible to dramatically reduce the number of data errors that are generated at the end of the link. Although it is not an optical technology, FEC is likely to have a crucial role to play in 40 Gbit/s transmission systems. Current systems based on the Reed-Solomon code can help to extend the distance of a link by a factor of four.