An optical fiber transmission system for soliton signals with wavelength multiplexing in which the bit times of the various channels &lgr;1 to &lgr;n of the multiplex are substantially synchronous, at least at one point. At least one saturable absorber is located at this point. The channels can be made synchronous by an appropriate selection of wavelengths, or by using optical delay lines, or the like. The saturable absorber then modulates the intensity of the soliton signals in the various channels of the multiplex. It is possible to use a clock for remotely driving the saturable absorber. The invention also relates to a corresponding transmission method. The invention makes it possible to transmit over long distances without using active components.