Spectrum-induced changes in diffraction of pulsed optical beams

Propagation of ultrashort optical pulses in a linear homogeneous medium is studied using the standard diffraction theory of electromagnetic fields. Diffraction pattern of a Gaussian beam consisting of such short pulses depends on the pulse shape. In general, the pulsed beam does not remain Gaussian on propagation and its spot size is enhanced. In the visible region, changes in the diffracted intensity become noticeable only for pulse widths below 10 fs and are larger for chirped pulses and for pulses whose spectrum has long tails. We also show that the diffraction characteristics of coherent but pulsed and CW but partially coherent Gaussian beams are identical if the two have identical optical spectra. (C) 1998 Elsevier Science B.V. All rights reserved.