= Beam waist
{title2=$w_0$}
The <beam waist> is the narrowest cross-section of a freely propagating <Gaussian beam>. Its <radius> $w_0$ uses the <wave amplitude> $1/e$, or <light intensity> $1/e^2$, convention; its <diameter> is $2w_0$. Specifying a “spot size” without saying <radius> or <diameter> can introduce a factor-of-two error. Within the scalar <paraxial approximation>, the <wavefront> at the <beam waist> is planar. The minimum <radius> sets both the <Rayleigh range> and the far-field <beam divergence>.
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