The modification is accomplished on the basis of the solution known as spectral inversion (or optical phase conjugation). The device (1) comprises a medium, such as typically a dispersion shifted optical fibre (4), into which the signal to be modified is injected together with a pump signal at such a level as to induce modulation instability in the fibre (4). At the output of fibre (4) a spectrally inverted optical signal is available, which signal is translated in wavelength and is obtained with high conversion efficiency. Preferably, in order to avoid stimulated Brillouin scattering, the pump signal is subjected to spectrum broadening, for instance by means of a modulator (5).