Distortion of femtosecond laser pulses in lenses and lens systems.

Owing to the difference between the phase and group velocity, the pulse front may be delayed with respect to the phase front by several picoseconds when traversing a lens or a lens system. The delay is a parabolic function of the input radius. The delay is calculated for singlet lenses, achromats, compound lenses, telescopes. The effect may be two to three orders of magnitude larger than the broadening due to group velocity dispersion in the materials of the lenses