Download Beam Divergence-Advanced Physics-Lab Report and more Exercises Advanced Physics in PDF only on Docsity! 2 Introduction 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. 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 we know the beam diameter at two separate points (D1, D2), and the distance (L) between these points. The beam divergence is given by θ~tan-1(r2-r1/L) If the beam has been collimated using a lens or other focusing element, the divergence expected can be calculated from two parameters: the diameter, Dm, of the narrowest point on the beam before the lens, and the focal length of the lens, f. The divergence is then given by Divegence= rm/f 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. docsity.com 3 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 Note that the beam will have an initial beam diameter and that diameter will increase over distance. This increase is known as beam divergence. The following Figure shows the half-angle divergence of a Gaussian laser beam is defined via the asymptotic variation of the beam radius (blue) along the beam direction. Note, however, that the divergence angle in the figure appears much larger than it actually is, since the scaling of the x and y axes is different . docsity.com
