## 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 Ø_{beam} is a function of the collimating focal length f’ and the numerical aperture NA of the single-mode fiber.

For a specified fiber NAe^{2}, the beam diameter Ø_{beam} (defined at its 1/e^{2}-level) for a focal length f’ is given by:

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

If the effective numerical aperture NAe^{2}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 F

_{NA}to correct for the different definitions of the NA.### Factor FNA

Factor F_{NA} depending on the nominal NA %-Level.

Level | FNA | ||
---|---|---|---|

1 % | 0.76 | ||

3 % | 0.66 | ||

5 % | 0.61 | ||

### Example:

Focal length: f' = 12 mm

Effective numerical aperture of fiber: NAe^{2} = 0.08

Beam diameter: Ø_{beam}= 2 ⋅12 mm ⋅ 0.08 = 1.92 mm