Square multimode fiber with top hat

  • 25 January 2024
  • 3 replies

Does anybody know where to start modeling a square multimode fiber with top hat output?

 I have attached a paper from Optoskand where they describe a fiber with a square core. Where the beam exits the fiber the intensity profile has a square top hat distribution.

The beam has diverges out of the fiber, where the divergence angle is given by the BPP (beam parameter product) and core size.

The beam propagating out of the fiber loses its top hat distribution. Collimating this beam and focusing it back in to a spot results again in a top hat distribution in the focal plane.

What would be a good approach to model such a beam?

3 replies

Userlevel 3

What do you want to model?  Just the output from it?  Or propagation through it? 

What do you want to model?  Just the output from it?  Or propagation through it? 

I want to propagate the output from the fiber through a collimator and a focusing lens.

Userlevel 3
Badge +1

My understanding is that you’re looking at using POP. Defining a square shaped input beam then being the central question for this use case.
You actually have several options to achieve this, using different POP input beams definition options:

  • using the “file” option, either text-based or binary using for instance a Python Script. Sample codes for the latter option is available here: Python Reading Writing Binary Files (ZRD, ZBF, DAT, SDF) | Zemax Community
  • using the “file” option, with another approach. You could use the available images and convert this image to a zbf file as illustrated here: Python Converting an image into a ZBF file | Zemax Community
  • using the “DLL” option, to analytically define the desired shape. The benefit of using this approach would be flexibility as you could define your own parameters such a the square size, and why not the BPP metric from the manufacturer. I gave a quick look at the attached datasheet and it seems that the BPP combines the beam waist * divergence. For a typical Gaussian beam those parameters sound redundant but for a square beam, it might not be the case. I’d double check this with the manufacturer.

I hope this helps!