Everything about The Beam Divergence totally explained
The
beam divergence of an electromagnetic beam is an angular measure of the increase in
beam diameter with distance from the optical
aperture or antenna
aperture from which the electromagnetic beam emerges. The term is relevant only in the "far field", away from any
focus of the beam. Practically speaking, however, the far field can commence physically close to the radiating aperture, depending on aperture diameter and the operating wavelength.
Beam divergence is often used to characterize electromagnetic beams in the optical regime, for cases in which the aperture from which the beam emerges is very large with respect to the
wavelength. That said, it's also used in the Radio Frequency (RF) regime for cases in which the antenna is operating in the so-called optical region and is likewise very large relative to a wavelength.
Beam divergence usually refers to a beam of circular cross section, but not necessarily so. A beam may, for example, have an elliptical cross section, in which case the orientation of the beam divergence must be specified, for example with respect to the major or minor axis of the elliptical cross section.
The divergence of a beam can be calculated if one knows the beam diameter at two separate points (
Di,
Df), and the distance (
l) between these points. The beam divergence is given by
» .
Like all electromagnetic beams,
lasers are subject to divergence, which is measured in
milliradians (mrad) or
degrees. For many applications, a lower-divergence beam is preferable. Neglecting divergence due to poor beam quality, the divergence of a laser beam is proportional to its wavelength and inversely proportional to the diameter of the beam at its narrowest point. For example, an ultraviolet laser that emits at a wavelength of 308 nm will have a lower divergence than an infrared laser at 808 nm, if both have the same minimum beam diameter. The divergence of good-quality laser beams is modeled using the mathematics of
Gaussian beams.
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