# Refocusing the collimator to produce a focus spot

## Producing a focus spot

### When do you need a separate micro focus optics?

For spots < 10 times the mode field MFD of the fiber, a good quality spot can no longer be achieved by simply refocusing the collimation optics. Instead, a combination of collimation and micro focus optics is needed. For single-mode fibers the Gaussian intensity distribution and beam shape are maintained in any case.

### Producing spots by adjusting the fiber collimator

Beam path of a refocused collimated beam exiting a single-mode fiber with angled connector FC APC.

The adjustment of the colli­mating lens generates a focused beam. At distance A, relative to the fiber collimator, a beam waist with diameter Øspot is formed.

{!{!{\emptyset_{spot}=MFD\times\left(\frac{A}{f'}-1\right)}!}!}

The mode field diameter MFD is calculated from the ­effec­tive numerical aperture NAe2 at wavelength λ as:

{!{!{MFD=\frac{2\cdot\lambda}{\pi\cdot{\rm NA}_{e^2}}}!}!}

Diffraction limits the maximum distance of the focus, where

{!{!{A\le A_{max}=f'+\frac{\emptyset_{beam}^2\cdot\pi}{8\lambda}}!}!}

and Øbeam is the collimated beam diameter.

Øspot:	Beam diameter in focus
A:		Working distance
f’:		Focal length of collimating lens
MFD:	Mode field diameter of single-mode fiber