L1-B

TRANSMISSION OF A BEAM THROUGH AN APERTURE

In some situations, it is useful to be able to calculate the ratio of the aperture radius all of those are lower case R to the beam spot size (w) from a knowledge of the beam power transmitted through a given aperture.


BEAM DIVERGENCE
The light emitted by a laser is confined to a rather narrow cone; but, as the beam propagates outward, it slowly diverges or fans out. Figure 7, greatly exaggerated, illustrates the way in which a beam of circular cross-section diverges. At the output aperture of the laser, the beam diameter is d. Its beam divergence angle is , usually expressed in milliradians. In traversing a distance . the beam diverges to a circle of diameter . Simple geometrical considerations indicate that for a small divergence angle, the diameter of the beam at a distance from the output aperture is approximated by Equation 2. The most common practice in recent years has been to specify the "full?angle beam divergence." Specifications must be checked with care in order that these different terms will not be confused.