Soliton or soliton-like optical pulses with characteristics adapted to propagation in an optical line for RZ type transmission are generated by modulating a continuous optical signal (1) by means of a modulator (2) which is substantially devoid of chirping, in which the modulator drive signal comprises at least one frequency (4) and one harmonic (5) thereof superimposed on each other (7).A high-speed optical transmission system is rendered independent of the type of transmitter or of signals sent to it if it comprises an adaptation unit (32) receiving the original optical pulses (30) and capable of generating corresponding pulsed signals (63) of RZ type.