Abstract

Repeaterless transoceanic optical communication links are the subject of active research and development. Recently several groups^1,2 have reported error free soliton transmission over more than the trans-Pacific distance (9,000 km) at GHz bit rates. Different techniques (passive filters^2,3 and pulse retiming¹) have been used to overcome the Gordon-Haus limit⁴ (accumulation of amplified spontaneous emission in the erbium-doped fibre amplifiers regularly spaced along the optical link). Soliton pulses have been generated from mode-locked diode lasers² and from gain-switched diode lasers¹. Mechanical instability and fixed repetition rates of mode-locked external cavity lasers seem to be a problem for practical applications. Unlike the mode-locking techniques, gain-switching does not require an extension of the laser cavity. However, its output is severly chirped and a narrow-band optical filter must be used to limit the bandwidth of the signal. Despite the fact that the τ Δ f product obtained with such a source is apparently satisfactory (0.4), gross excess pulse broadening is observed for distances greater than a few thousand kilometers⁵.

© 1993 Optical Society of America