The gain spectra for a bidirectionally pumped 30 km length of lightly doped dispersion shifted erbium fibre and the amplification performance in saturation of a 54 km length of similar fibre are presented. The longer length comprised six spliced fibres with an average erbium concentration in the core of ∼60ppb and could be rendered lossless with 2 × 30 mW of pump power for low power signals. With 2 × 55 mW of pump power a peak fibre gain of 17dB at 1536nm was realised; as the input signal power was increased the gain was suppressed but the fibre remained transparent for signal powers up to –2 dBm.
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Williams et al. (1991) studied this question.