# Collimating multimode fibers

## 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}}!}!}