About this calculator

This calculator computes the $1/e^2$ spot diameter of a collimated Gaussian beam whose wavelength is $\lambda$ and diameter at lens$D$ when it is focused with a lens having a focal length $f$. Additionally, the beam quality parameter $M^2$ can be specified if the beam deviates from an ideal Gaussian. With these inputs, the diameter of the focal spot is

To get this diffraction-limited beam diameter, the lens should not have strong aberrations. Additionally, the lens diameter should be at least twice the beam diameter input value not to clip the "wings" of the Gaussian profile.