Abstract

One of the major problems related to wavelength division multiplexing (WDM) systems with in-line erbium-doped fibre amplifier (EDFA) repeaters is the non-uniform gain spectrum of EDFAs. A conventional EDFA typically has a sharp peak around the wavelength of 1.532 nm with a reduced gain at longer wavelengths. This can cause significant optical power difference between channels of different wavelengths. In long distance optical fibre systems where optical signals see a cascade of EDFAs, this power difference can be accumulated exponentially as the number of cascaded EDFAs increases, resulting in significant optical signal-to-noise ratio (SNR) difference between different wavelength channels. Recently there is growing attention paid to seek means of equalisation to overcome this problem [1]. These include end-to-end servo-loop-controlled equalisation by adjusting input power level of each channel at the transmitter [2], and improvement of optical gain flatness of individual EDFA by equalising filters [3-5].

© 1994 Optical Society of America