Abstract: In order to reduce amplified spontaneous emission noise in the output signal of an amplifier of the kind in which amplification introduces amplified spontaneous emission, for example an erbium-doped fiber amplifier, the amplified signal is filtered using a loop mirror filter. The loop mirror filter may comprise a 3-dB fiber coupler with a loop of linear active fiber connected between two of its ports. The amplified signal and the filtered signal may be applied to, and extracted from, one of its other ports, conveniently by way of a circulator.