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Definition: light beams propagating dominantly in one direction
In most cases, a laser emits light in the form of a laser beam. This means that the light dominantly propagates in a certain direction, typically with most of the light power concentrated to a small area of the order of a square millimeter. Laser beams are often close to Gaussian beams, where the transverse profile of the optical intensity of the beam can be described with a Gaussian function, the width of which varies along the propagation direction. This variation of beam size can be very small for beams with large width or very fast for tightly focused beams.
Figure 1: Snapshot of the electric field distribution around the focus of a Gaussian beam. In this example, the beam radius is only slightly larger than the wavelength, and the beam divergence is strong.
Generally, laser beams exhibit a high degree of spatial coherence, which is related to a high beam quality. As a result, such beams exhibit a good focusability and the potential to form collimated beams with very low beam divergence.
Laser beams often have a small optical bandwidth, so that the temporal coherence is also high. An often unwanted consequence of the high level of coherence is the tendency to form speckle patterns.
The optical power of a laser beam may hardly change during propagation in a transparent medium, or quickly decay in an absorbing or scattering medium. Inhomogeneous media (i.e., media with a locally varying refractive index) can also distort the shapes of laser beams. This can happen e.g. due to thermal effects in a gain medium. Some lasers emit continuously, but a laser beam can also consist of a fast sequence of pulses.
Most laser beams are linearly polarized, i.e., the electric field oscillates in a certain direction perpendicular to the propagation direction.
A laser beam of visible light with sufficiently high power, propagating in air, is sometimes visible as an illuminated line. This is because a tiny portion of the optical power is scattered in air and can thus get to the observing eye. When the laser beam hits some diffusely scattering object, such as a white screen, a much brighter spot is seen on that screen, since most of the optical power is scattered at this point.
Bibliography
| [1] | H. Kogelnik and T. Li, "Laser beams and resonators", Appl. Opt. 5 (10), 1550 (1966) |
| [2] | A. E. Siegman, "Defining, measuring, and optimizing laser beam quality", Proc. SPIE 1868, 2 (1993) |
| [3] | A. E. Siegman, "Lasers", University Science Books, Mill Valley, CA (1986), ISBN 0-935702-11-3 |
See also: laser light, collimated beams, coherence, beam quality, Gaussian beams, beam divergence, speckle, polarization of laser emission
Categories: general optics, lasers
