Rayleigh length

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In optics and especially laser science, the Rayleigh length or Rayleigh range is the distance along the propagation direction of a beam from the waist to the place where the area of the cross section is doubled.[citation needed]

For a Gaussian beam, the Rayleigh length is given by

z_R = \frac{\pi w_0^2}{\lambda},

where w0 is the radius of the beam at the waist. At distance zR from the beam waist, the beam radius is increased by a factor \sqrt{2}.

A related parameter is the confocal parameter b, which is just two times the Rayleigh length:

z_R = \frac{b}{2}

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