TABLE OF CONTENTS A laser beam's full divergence angle is approximately the wavelength divided by the diameter of the transmitter aperture, or
| where | ? | = transmitter beam width (full angle) in radians |
| ? | = wavelength | |
| d | = aperture diameter in the same units as the wavelength |
This is a basic result drawn from diffraction theory and does not include any of the additional aberrations that occur in real systems. However, this rule is widely used to estimate the size of a laser beam that has propagated through a vacuum and is also frequently used as a first estimate even in atmospheric applications. The rule works fine in environments in which scattering is small as compared with absorption, because, in those cases, the beam shape is not affected.
This rule provides quick estimations of minimum beam divergence. A more accurate presentation is provided below.
In the far field, the full beam width of a Gaussian beam can be approximated by
where ? o = Gaussian beam waist radius in meters
This definition of beam width is based on the location of the 1/ e points in the beam. This is one of several common ways that beam spread is defined for Gaussian laser beams. Siegman describes several, including the more conservative 99 percent criterion in which the size of the beam is defined as the area that includes 99 percent of the energy in the beam. The reader will find that this definition of beam size...
