GEO radius and enclosed energy for fiber size coupling

Question

Hi,
I have a question on the relation between GEO radius in spot diagram and radius for enclosed energy. I would like to couple a fiber to my beam and would like to understand the size of the fiber to choose and its acceptance angle.
If I look at the 86% enclosed energy, using Huygens PSF, it is about 17um radius. Therefore I thought the MFD is 2x17um=34um which sets the limit of roughly 30um on the fiber core size.
However, if I look at the spot diagram the GEO radius is about 1mm. Meaning, I need 2mm fiber size to cover all the beam to my understanding. This is far above 30um size which made me very confuse.
I checked geometrical image analysis. For 1.5mm image size, 97% of the total power included. For 34um image size, the efficiency is only 2%. Also when I look at the Fiber coupling option of MMF, the coupling efficiency is max 1.6%. I am not sure what fiber size it assumes to calculate the fiber coupling efficiency as I only selected 0.5 NA as the input. Also there is no difference for the coupling efficiency output for SMF or MMF selection in the tool, which I donot understand.
I am coming to the conclusion of using 1.5mm core size, but I donot understand the 34um diameter for 86% enclosed energy.
In regards to 
When I am looking at the afocal image on the spot diagram, it is about 7degrees. This is the divergence of the beam, but does that mean the fiber input acceptance angle is 7degrees? I am not sure I understand it and how to limit it to only accept 0.01degrees.
Lastly, to incorporate coupled fiber do I have to switch to non-sequential mode? 
I would highly appreciate your help.
Best regards,
Fatemeh

Sandrine Auriol · Accepted Answer

HiFatemeh,After writing my answer (apologies for the delay), I realized that you had also open a case in the support. So I’ll work on that now, but here is my answer in case it may be useful for other users.The spot diagram is only based on ray-tracing. No diffraction is taken into account. In the spot diagram, there is an option to display the Airy disk, which is useful to check that your system is not close to being diffraction limited.The Huygens PSF takes into account the diffraction from the exit pupil to the image plane. Rays are converted into spherical or plane wavelets and interfere at the image plane.Your results are very different, so I would think that your system is not diffraction limited. Check the OPD for example. You can also open the Huygens PSF plot. If the system is far from being diffraction limited, there are chances that the sampling is too low, and the result you get using that tool is meaningless.Then to calculate the fiber coupling, there are mainly two tools:Multi-Mode Fiber coupling / Geometric Image Analysis: it works for MMF and is only based on the aperture at the image plane and the NA. If rays are above a certain angle, we don’t consider them in the efficiency. This article may help:https://support.zemax.com/hc/en-us/articles/1500005486601-How-to-model-multi-mode-fiber-couplingSingle-Mode Fiber coupling : it is based on a diffraction PSF (FFT or Huygens). The software checks the overlap between the wavefront from the system and the receiving fiber.https://support.zemax.com/hc/en-us/articles/1500005576542-Single-mode-fiber-coupling-in-OpticStudioSandrine

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