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 Ø_beam 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 Ø_beam = 1.92 mm

Beam divergence θ = 0.22 mrad