Collimating multimode fibers

The beam diameter Ø_beam 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}!}!}

The beam always shows divergence due to the finite core diameter d. The divergence angle θ is defined as: {!{!{\theta=\frac{d}{2f\prime}}!}!}