Draka, Bell Labs go ultra-long on 40G

This article originally appeared on fibresystems.org.

Although network operators are keen to deploy 40 Gbit/s wavelengths to upgrade their capacity on terrestrial routes, submarine cable systems have stayed at 10 Gbit/s because of the distances involved. But tomorrow in a post-deadline paper to be presented at the European Conference on Optical Communications (ECOC), researchers will describe an experiment that demonstrates the feasibility of transmitting 40 Gbit/s traffic over transoceanic distances.

The experiment involved sending 81 channels at 40 Gbit/s over a distance of 11,520 km — setting a new distance-capacity record.

Key to the breakthrough was a new design of optical fibre, from Amsterdam-based fibre and cable maker Draka. Called LongLine, the fibre has a large effective area, which helps to reduce the effect of non-linearities. In turn, this enables higher launch powers to be used, so the signal can go further before it disappears in the noise.

Large effective area fibre is not a new concept. In fact, Corning unveiled the first large effective area fibre, trademarked LEAF, almost exactly 10 years ago. But to put things in perspective, LEAF has an effective area of about 72 ?m2, while LongLine has an effective area of 120 ?m2 — almost twice the effective area of standard non-zero dispersion shifted fibre (NZ-DSF).

Typically as the mode size is increased in a fibre, the optical field spreads further into the cladding, which leads to higher attenuation and bending loss. To overcome this, Draka applied the technology from its bend-insensitive fibres, using a “trench” in the index profile of the fibre to keep the light contained.

“You can control the bend performance, or you can control the size of the effective area without losing light,” explains Alain Bertaina, marketing director, optical fibre, for Draka.

The other challenge in designing the fibre was keeping the fibre characteristics compatible with negative dispersion fibre. Submarine systems typically use alternating lengths of positive and negative dispersion fibre throughout their length to compensate for chormatic dispersion.

The manufacturing process also has a important role to play, says Bertaina. Draka uses chemical vapour deposition, which allows flexible and very precise definition of the refractive index profile of the fibre. “When we target an [index] profile, even a very complex one, we are able to precisely match the profile we target,” he claims.

• UPDATE 25/09/2008 The transmission experiment was carried out by researchers at Alcatel-Lucent Bell Labs, France, and IRISA/INRIA at the Campus Universitaire de Beaulieu, France.

The channels were spaced on a 50 GHz grid, and used polarisation division multiplexed (PDM) binary phase shift keying (BPSK) modulation scheme with coherent detection. More details are provided in the ECOC post-deadline paper Th 3.E.2, which was presented on Thursday 25 September.

Reproduced with permission. © Institute of Physics and IOP Publishing Ltd.