  Autocorrelation measurement based on second-harmonic generation (SHG), the
best-known technique for measuring the temporal duration of ultrashort pulses,
could date back to the birth of ultrafast lasers. Here, we propose and
experimentally demonstrate that such well-established technique can also be
used to measure the orbital angular momentum of ultrashort Laguerre-Gauss (LG)
pulses. By analysing the far-field pattern of the SHG signal, the full spatial
structure of ultrashort LG pulses, including both azimuthal and radial indices,
are unambiguously determined. Our results provide an important advancement for
the well-established autocorrelation technique by extending it to reach its
full potential in laser characterization, especially for structured ultrashort
pulses.
