Loss of temporal overlap of Pulses caused by Group Velocity mismatch.

Nonlinear Frequency conversion works well with ultrashort pulses, because these can have high peak intensities even when the average optical powers are small. However, the useful interaction length in a nonlinear crystal can be limited by the so-called temporal walk-off: as the pulses of the involved frequency components have different group velocities, they lose their temporal overlap after some propagation distance. This effect limits the conversion efficiency, and IF the temporal walk-off within the crystal length is significant, it can also lead to modified pulse shapes.