Figure 41 shows a different cross section through a Gaussian beam: it plots the beam size as a function
of the position on the optical axis.
Such a beam profile (for a beam with a given wavelength ) can be completely determined by two
parameters: the size of the minimum spot size
(called beam waist) and the position
of the beam
waist along the z-axis.
To characterise a Gaussian beam, some useful parameters can be derived from and
. A
Gaussian beam can be divided into two different sections along the z-axis: a near field – a region around the
beam waist, and a far field – far away from the waist. The length of the near-field region is
approximately given by the Rayleigh range
. The Rayleigh range and the spot size are related by
The angle between the z-axis and
in the far field is called the diffraction
angle6
and is defined by
Another useful parameter is the radius of curvature of the wavefront at a given point z. The radius of curvature describes the curvature of the ‘phase front’ of the electromagnetic wave – a surface across the beam with equal phase – intersecting the optical axis at the position z. We obtain the radius of curvature as a function of z:
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