# Beam divergence

## Beam divergence

### Beam divergence of a single-mode fiber

Beam path of a collimated beam exiting a single-mode fiber with angled connector FC APC.

From principle, a collimated beam has a divergence greater than zero, i.e. the beam diameter ­varies with distance A from the ­fiber collimator. The beam ­divergence θ depends (for large distances of A) on the beam ­diameter Øbeam at the position of the fiber collimator and on the wavelength λ.

{!{!{\theta=\frac{2\cdot\lambda}{\pi\cdot\emptyset_{beam}}}!}!}
{!{!{\theta=\frac{\lambda}{\pi\cdot f'\cdot{\rm NA}_{e^2}}}!}!}

### Example:

Wavelength λ = 660 nm

Focal length f’ = 12 mm

Numerical aperture NAe2 = 0.08

Beam diameter Øbeam = 1.92 mm

Beam divergence θ = 0.22 mrad