# Collimated beam diameter of a singlemode fiber

## Collimated beam diameter of a singlemode fiber

### Beam diameter

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

The collimated beam diameter Øbeam is a function of the ­collimating focal length f’ and the ­numerical aperture NA of the ­single-mode fiber.

For a specified fiber NAe2, the beam diameter Øbeam (defined at its 1/e2-level) for a focal length f’ is given by:

{!{!{\emptyset_{beam}=2\cdot f'\cdot{\rm NA}_{e^2}}!}!}
If the effective numerical aperture NAe2 of the fiber is not known, then the beam diameter (defined at its 1/e2-level) can be calculated from the nominal numerical aperture NA  and the focal length f’using:
{!{!{\emptyset_{beam}=\frac{1}{F_{NA}}\cdot f'\cdot NA}!}!}

The nominal fiber NA corresponds to the Gaussian angle distribution at a level 1% - 5 %, requiring the factor FNA to correct for the different definitions of the NA.

### Factor FNA

Factor FNA depending on the nominal NA %-Level.

LevelFNA
1 %0.76
3 %0.66
5 %0.61

### Example:

Focal length: f'   = 12 mm
Effective numerical aperture of fiber: NAe2 = 0.08
Beam diameter: Øbeam= 2 ⋅12 mm ⋅ 0.08 = 1.92 mm