# Laser beam spot size calculator

## About this calculator

This calculator computes the spot diameter of a collimated Gaussian beam whose wavelength is and diameter at lens when it is focused with a lens having a focal length . Additionally, the beam quality parameter 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.

This calculator also computes the depth of field () of the focused beam defined as two times the Rayleigh range :

where the parameter has been added to the usual definition of Rayleigh range to account for the increased divergence. The depth of field is relevant for example when focusing the beam into a nonlinear crystal for second harmonic generation.