Researchers at Nortel Networks in England have demonstrated a way of sending a dense electrical time-division-multiplexing (ETDM) signal using an existing fiber type and commercial erbium-doped fiber amplifiers (EDFAs) spaced 160 km or more apart. This spacing, which the team claims is a record, does not require optical regeneration. Instead, Raman-pumping provides both additional amplification and compensation for the EDFA gain tilt. The transmission, with Q-factors varying from 10.1 to 13.1 dB, is good enough to give a 10-12 bit-error rate (BER) with just moderate forward-error correction.
In the Nortel demonstrator, amplification stages consist of two commercial EDFAs, a Raman pump module, and a dispersion-compensation module (DCM). These stages can be widely separated because they provide two different kinds of amplification with gain tilts that compensate for each other.
The experimental system, developed at Nortel's Harlow Laboratories (Essex, England), used 32 x 40-Gbit/s channels at wavelengths from 1535.04 to 1559.79 nm with 100-GHz spacing (see figure). As they travel through the 1000 km of ordinary non-dispersion-shifted fiber (NDSF), the optical carrier-suppressed return-to-zero (RZ) pulses are amplified by pairs of EDFAs. Unfortunately, the EDFAs amplify the longer wavelengths more than the shorter ones—a phenomenon known as gain tilt.
At the amplification stages, Raman pump beams are injected into the NDSF to counter-propagate with the signal wavelengths. These beams nonlinearly interact through stimulated Raman scattering in the fiber to produce signal amplification. When the lower-wavelength, higher-energy pump photons are absorbed at the same time as the higher data-carrying wavelengths, two of the latter photons are emitted (along with some vibrational energy phonons). The extent of this amplification is determined by the relationship between the wavelengths of the pump and signal beams. Thus, Raman pumping can also exhibit gain tilt; the Nortel team was able to engineer that tilt to compensate for that of the EDFAs.
The use of these two complementary amplification techniques means that the amplification stages can be widely separated. In the current system, which has a 40-dB span loss, just six were used: with the last two spaced 200 km apart. This gave a raw BER of 5 x 10-6 over 1000 km. With high-performance error correction, the addition of further pumping wavelengths, and the optimization of dispersion compensation, researchers say this should allow for optical links that are even longer. For more information, contact Yanjun Zu at firstname.lastname@example.org.
Sunny Bains is a scientist and journalist based in London, England.