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.
The mode field diameter MFD is calculated from the effective numerical aperture NAe2 at wavelength λ as:
Diffraction limits the maximum distance of the focus, where
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