Gaussian beam radius
= Gaussian beam radius
{c}
{title2=$w(z)=w_0\sqrt{1+(z/z_R)^2}$}
For a circular <Gaussian beam> with intensity proportional to $e^{-2r^2/w(z)^2}$, the displayed $w(z)$ is the intensity $1/e^2$ radius. Its minimum $w_0$ is the <beam waist> radius, while the larger radius at a cavity mirror is not itself a waist. The <Rayleigh range> is $z_R=\pi w_0^2/\lambda$.