Abstract

Distributed amplification over low-doped erbium fiber^1–3 is currently of interest for a number of telecommunications applications, such as loss-less networks⁴ and soliton transmission.⁵ In the latter application it is important to keep the signal power within a range of −6 dB to + 3 dB of the fundamental soliton power in order to maintain soliton-like pulses. For low signal excursion the erbium gain must just compensate for the fiber loss. Because the erbium absorption is bleached at the pump wavelength of 1485 nm, when 65% of the ions are in the metastable level, and with the peak absorption and emission cross section being almost equal, a peak erbium absorption of about twice the background loss is required. Thus, for a signal at 1536 nm and a background loss of 0.5 dB/km, an exceptionally low erbium concentration of only 40 parts in 10⁹ is required. In addition, the dopant concentration must be uniform along the fiber length to prevent localized high optical gain.

© 1991 Optical Society of America