TABLE OF CONTENTS A Gaussian spherical wave spreads considerably less than a plane wave diffracted by a circular aperture.
A plane wave passing through an opaque screen has an angular diameter (d) caused by diffraction of 2.44 ( ?/ d) and contains 84 percent of the beam power. A Gaussian spherical wave contains 86 percent of its power in an angular diameter of 2 ( ?/d), where d is defined as ? w o, and w o is the diameter of the waist (or smallest) size of the beam in the beam forming optics. In the examples above, the light is propagated through a circular aperture. It is worth noting that, if the aperture is equal to ? w o, 99 percent of the power in the beam is transmitted through the aperture.
The interested reader will want to read the rules in this chapter that pertain to diffraction and the "Etendue or Optical Invariant" rule (p. 286).
This result should not be overlooked in any application where laser beams are to be propagated. That is, one should not assume that the typical diffraction formula
applies for lasers.
The specific results given above derive from a definition of the "size" of the beam. Because a Gaussian beam has an extent that is not well defined, some latitude must be accepted in the power numbers that are selected. For example, Siegman [1] points out examples in which the...
