Wavelength Division Multiplexing (WDM) is a technique to upgrade installed lightwave systems and allows for optical networking functionalities. The requirements on stability of light sources, filters, and other optical components is such that a channel spacing of 200 GHz is emerging as a viable solution. In this context, increasing the throughput leads to an increase of either the single-channel bit-rate or the total occupied bandwidth. In the first case, the achievable distance will quickly become limited by the fiber polarization mode dispersion (PMD), decreasing as the square of the bit-rate. The second case leads to an increase of the number of channels with the use of flat-gain optical amplifiers.

16 10 Gbit/s channels spanning 24 nm were transmitted over 531 km of dispersive fibre with only seven fluoride-based EDFAs, amplifier spacing varied between 60 and 93 km, and optimised dispersion management using dispersion-compensating fibre was utilised.

Noise accumulation is one of the limitations of amplified transoceanic links.1 Concerning this topic, we reported that a key amplifier feature is its spectral gain shape: if the signal is located at the gain peak, satisfactory signal-to-noise ratios are obtained for distances over 10 000 km with no need for additional in-line optical filters.2 This possibility has recently been demonstrated in a straight-line 9000-km transmission experiment.3

The authors demonstrate the distribution of 30 AM-VSB channels using three cascaded erbium-doped fiber amplifiers, with a power budget of 45 dB for a carrier-to-noise ratio of 48 dB. The power budget for a single postamplifier system is 20 dB for a 52-dB CNR. These high power budgets are obtained through an optimization of the erbium-doped fiber amplifiers' gain and noise characteristics in the saturated regime.< >