The optical system comprises: means for introducing a controlled negative or positive chirp to an incoming ultrashort laser pulse to be characterized; a nonlinear optical medium through which said chirped ultrashort laser pulse is propagated, wherein as a result of said propagation: different chirp values are introduced in the ultrashort laser pulse at different propagation distances along the nonlinear optical medium, and a transverse nonlinear signal is generated in a direction perpendicular to the propagation axis; analyzing means configured for recording a single-shot spectral image of said generated transverse nonlinear signal; and a processing module comprising one or more processors configured to execute a numerical iterative algorithm to said single-shot spectral image to retrieve the electric field, amplitude and phase, of the ultrashort laser pulse.