Field demonstrations of coherent WDM via a diode-pumped erbium-doped fibre amplifier repeater

The authors report the results of field demonstrations of DPSK and FSK coherent transmission of 2 WDM channels via a single optical repeater. They indicate the scope for expansion to longer cascades spanning hundreds of kilometres between digital regenerators, or to many more wavelength channels. The optical repeater employed comprised two erbium-doped fibre amplifiers in cascade. Both were optically-pumped by diode lasers emitting at 1.48 mu m wavelength: the first was co-pumped (producing low noise) whilst the second was counter-pumped (giving a high saturated output power). An optical isolator separated the two amplifiers, to suppress laser oscillations induced by reflections from the two pump sources. The demonstration route comprised two links between nodes in the BT network located at Edinburgh, Galashiels, and Newcastle. The link lengths were 70 and 130 km, with losses of 21.5 and 43.5 dB, giving a total system length of 200 km and a loss of 65 dB.