Rayleigh length

In optics and especially laser science, the Rayleigh length or Rayleigh range, ${\displaystyle z_{\mathrm {R} }}$, 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.¹ A related parameter is the confocal parameter, b, which is twice the Rayleigh length.² The Rayleigh length is particularly important when beams are modeled as Gaussian beams.

For a Gaussian beam propagating in free space along the ${\displaystyle {\hat {z}}}$ axis with wave number ${\displaystyle k=2\pi /\lambda }$, the Rayleigh length is given by²

${\displaystyle z_{\mathrm {R} }={\frac {\pi w_{0}^{2}}{\lambda }}={\frac {1}{2}}kw_{0}^{2}}$

where ${\displaystyle \lambda }$ is the wavelength (the vacuum wavelength divided by ${\displaystyle n}$, the index of refraction) and ${\displaystyle w_{0}}$ is the beam waist, the radial size of the beam at its narrowest point. This equation and those that follow assume that the waist is not extraordinarily small; ${\displaystyle w_{0}\geq 2\lambda /\pi }$.³

The radius of the beam at a distance ${\displaystyle z}$ from the waist is⁴

${\displaystyle w(z)=w_{0}\,{\sqrt {1+{\left({\frac {z}{z_{\mathrm {R} }}}\right)}^{2}}}.}$

The minimum value of ${\displaystyle w(z)}$ occurs at ${\displaystyle w(0)=w_{0}}$, by definition. At distance ${\displaystyle z_{\mathrm {R} }}$ from the beam waist, the beam radius is increased by a factor ${\displaystyle {\sqrt {2}}}$ and the cross sectional area by 2.

The total angular spread of a Gaussian beam in radians is related to the Rayleigh length by¹

${\displaystyle \Theta _{\mathrm {div} }\simeq 2{\frac {w_{0}}{z_{R}}}.}$

The diameter of the beam at its waist (focus spot size) is given by

${\displaystyle D=2\,w_{0}\simeq {\frac {4\lambda }{\pi \,\Theta _{\mathrm {div} }}}}$.

These equations are valid within the limits of the paraxial approximation. For beams with much larger divergence the Gaussian beam model is no longer accurate and a physical optics analysis is required.