A method comprising using a pulse shaper in the spectral domain to generate multiple-color pulses directly at the output of the laser amplifier. The delay can thus be controlled directly in the spectral domain and there is no need for an optical delay line. The method allows reducing the number of optical components and insures insensitivity to alignment, vibrations and turbulence on long distance propagation and filamentation, particularly in air. The method allows programmable and tunable interaction, since the pulse shaper is able to control the laser spectral amplitude and phase.