A passively-mode-locked laser generator having an active optical fibre and fibre pumping means adapted to cause the generation of an emission signal in the fibre, a polarization control means, signal extraction means and at least one optical component with transmissiveness depending on polarization is disclosed. The components are inserted in an optical fibre path that, for a pumping energy lower than a predetermined value corresponding to a linear propagation condition of the signal, gives rise to a polarization rotation of the emission signal through an angle causing the extinction of the signal itself within the optical component with transmissiveness depending on polarization. For a pumping energy of a higher power than the predetermined value, the generator causes a polarization rotation of the emission signal through an angle corresponding to the transmission with low losses of only pulses of high peak power of the emission signal.