Comments on the picosecond pulse width measurement by the single-shot second harmonic beam technique

We show that the measurement of the ultrashort light pulse duration by the single-shot second harmonic beam technique requires caution. It is found that the far-field distribution of the second harmonic radiation from non bandwidth-limited pulses shows a pronounced structure even if the near-field pattern is without structure. This interference effect is interpreted as being due to the special symmetry properties of the spatial coherence of the second harmonic beam. A simple method for a quick determination of the existence of the bandwidth-limited relation in an individual picosecond light pulse is proposed.