PROPAGATION LAW AND QUASI-INVARIANCE PROPERTIES OF THE TRUNCATED 2ND-ORDER MOMENT OF A DIFFRACTED LASER-BEAM

The propagation of the truncated second-order moment (i.e. integrated over a finite interval enclosing a constant fraction of the total powe r) of a diffracted beam is analyzed. An asymptotic analysis, supported by numerical simulations, shows that the propagation law becomes near ly perfectly parabolic as the power fraction increases. It is also dem onstrated, on a general basis, that a parabolic propagation law implie s the invariance of the infered beam-propagation factor.