Definition: location with minimum beam radius
Alternative term: beam focus
German: Strahltaille, Fokus
Author: Dr. Rüdiger Paschotta
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The beam waist (or beam focus) of a laser beam is the location along the propagation direction where the beam radius has a minimum. The waist radius is the beam radius at that location.
A small beam waist (more precisely, a beam waist with small waist radius), also called a tight beam focus, can be obtained by focusing a laser beam with a lens which has a short focal length and most importantly a high numerical aperture, and making sure that the lens aperture is largely filled by the input beam. A high beam quality is also an important precondition for tight beam focusing.
For non-circular beams, the longitudinal position of the beam waist can be different for different transverse directions. This phenomenon is called astigmatism, and it may be generated or removed e.g. by using cylindrical lenses.
Simple relations between the beam waist radius and the beam divergence, for example, exist for Gaussian beams.
See also: focus, laser beams, beam radius, Gaussian beams, spotlight 2007-07-11
Questions and Comments from Users
Does a beam waist occur both inside and outside laser cavity? If so, how do we calculate beam waist inside a laser cavity?
The author's answer:
One or even several beam wastes can occur inside and outside a laser resonator, depending on the used optical elements.
The beam evolution in a laser resonator (e.g. for the fundamental resonator mode) can be calculated it with software based on the ABCD matrix algorithm.
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How to calculate the spot size of a fiber laser operating in the multimode regime?
The author's answer:
That will depend on how the optical power is distributed over the fiber modes. That in turn can change e.g. you change the bending of the fiber. If you know that power distribution, you can calculate the overall intensity profile by summing up the intensities from all modes, assuming that they are not mutually coherent. (For a typical continuous-wave fiber laser, they are usually not mutually coherent.)