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