[en] A powerful experimental method, based on holographic microscopy, is used to retrieve the spatiotemporal distribution of small (<10^-3) refractive index perturbations induced by ultrafast laser pulses in transparent media. A numerical iterative wavefront propagation approach is used to accurately reconstruct both the phase and the amplitude of the perturbed probe beam wavefront, while Abel inversion recovers the exact three dimensional distribution of the perturbation. For demonstration, a laser filament induced plasma string in air is fully characterized