An optical repeater is comprised by an amplifying optical fibre (5) and directional coupler means (3). An input pulse (1) is reflected, by a respective grating (7) written in the optical fibre, in dependence on the spectral content of the pulse (1). The output pulse (8) is thus reshaped (amplified) and narrowed spectrally. The gratings have different spacings, at least in use of the repeater, so that output pulses with predetermined characteristics can be achieved.