According to the invention, the optical signal (SE) is spatially divided into N elementary optical signals (SE.1, SE.2, . . . , SE.N), the spectral ranges thereof being adjacent in pairs and forming, substantially by juxtaposition, the spectral range of the optical signal; these N elementary signals are amplified respectively by means of N elementary amplifiers (4.1, 4.2, . . . , 4.N), the spectral ranges thereof respectively comprising the spectral ranges of said N elementary signals; the N amplified elementary signals (Ss.1, Ss.2, . . . , Ss.N) are assembled to form an amplified optical signal (Ss), the spectral range thereof substantially coinciding with a predetermined spectral range, and finally the spectral phases of the N initial elementary signals (Ss.1, Ss.2, . . . , Ss.N) are adjusted before amplification on the basis of the spectral phase of said amplified signal (Ss).