FEMTOSECOND PULSES IN THE FOCAL REGION OF LENSES

A systematic theoretical and experimental study of focusing femtosecond light pulses by single lenses is presented. By evaluation of the diffraction integral the interplay of spherical and chromatic aberration is shown to determine the temporal as well as the spatial intensity distribution in the focal region of a lens. Conditions are derived under which the effect of spherical aberration dominates. Here a temporally unbroadened, in-focus pulse occurs while the spatial distribution is that expected from an annular lens aperture. If chromatic aberration is the major aberration, the in-focus pulses are considerably broadened. Both effects could clearly be measured with 100-fs pulses using a modified Michelson interferometer. In an intermediate parameter range both chromatic and spherical aberration contribute to the pulse broadening and to the spatial intensity pattern in a given plane in the focal region. From Fresnel diffraction we expect a weak-intensity distribution to precede the in-focus pulse on the axis. Another weak intensity peak is produced by the pulse traveling on axis. By measuring its separation from the main pulse in the marginal focal plane one can estimate the aberration parameters.