BEAM-PROPAGATION FACTOR OF DIFFRACTED LASER-BEAMS

The recent emergence of the characterization of general optical beams by means of the variance of their transverse intensity distribution ha s given rise to the concept of the beam propagation factor (usually re ferred to as the beam quality factor), which appears as a meaningful w ay for comparing the divergences of optical beams having the same mini mum spot size. Unfortunately, a direct calculation of this factor for a beam having sharp discontinuities in its transverse intensity profil e leads to an infinite result. This difficulty is addressed by derivin g a general expression for the axial dependence of the variance of the beam's transverse intensity profile in free space. A new definition f or the beam propagation factor can be introduced, provided that the ev anescent waves of the plane-wave spectrum of the beam are ignored. Thi s modified beam propagation factor is then calculated for some specifi c diffracted intensity profiles. Finally, it is shown how the proposed definition for the variance of the plane-wave spectrum of an optical beam is connected to its far-field angular spread.