Gaussian beam radius ($w(z)=w_0\sqrt{1+(z/z_R{)}^2}$)

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 $z_R=\pi w_0^2/\lambda$.