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 byz-axis. In the far-field regime (), it can be approximated by a linear equation, when
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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