Collimating multimode fibers
Collimated beam diameter of a multimode fiber
The beam diameter Øbeam is given by the focal length of the collimating lens f‘ and by the numerical aperture NA of the multi-mode fiber.
\emptyset_{beam}=2\cdot f\prime\cdot NA
Beam divergence multi-mode
Beam divergence of a collimated multimode fiber
The beam always shows divergence due to the finite core diameter d. The divergence angle θ is defined as:
\theta=\frac{d}{2f\prime}