Femtosecond pulse imaging: ultrafast optical oscilloscope

A nonlinear optical processor that is capable of real-time conversion of a femtosecond pulse sequence into its spatial image is introduced, analyzed, and experimentally characterized. The method employs nonlinear spectral domain three-wave mixing in a crystal of LiB3O5, where spectral decomposition waves of a shaped femtosecond pulse are mixed with those of a transform-limited pulse to generate a quasi-monochromatic second-harmonic field. By means of this nonlinear process, the temporal-frequency content of the shaped pulse is directly encoded onto the spatial-frequency content of the second-harmonic field, producing a spatial image of the temporal shaped pulse. We show that, unlike the commonly used autocorrelator, such time-to-space conversion carries both amplitude and phase information on the shape of the femtosecond pulses.