Fiber coupling and wavelength

  • 18 September 2023
  • 3 replies


I’m trying to investigate the effect of different parameters on fiber coupling (receiving fiber only), such as focal length of the injection lens, wavefront aberrations, fiber NA, etc…

The system is  a 19mm parallel beam, a  paraxial lens, and a single mode fiber at the focal plane.

It seems that the wavelength value has no effect on the fiber coupling efficiency (FICL operand). This seems not physical to me…

Any idea?



Best answer by Jeff.Wilde 19 September 2023, 18:48

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Userlevel 3

Hi Julien,

As paraxial lens will not give any optical aberration, all  the wavelength will focus at same location with same converging angle, this will leads to no change in coupling efficiency for different wavelength. However, if you will use a real lens with slight chromatic aberration you can analyse the effect of different wavelength on coupling efficiency as different wavelength will focus at different location with slightly different cone angles, this will leads to defocus for fixed position of fibre tip and change in coupling efficiency.

Hope this helps!


Hi, yes of course, but the diffraction will depend on the wavelength, so the injection should vary. I think it is the fiber NA that in reality depends on the wavelength, so I mat use different fiber NA for each wavelength.


Userlevel 7
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@Julien Dejonghe 

Yes, in your case when using FICL, the focused beam NA does not vary with wavelength, and if the fiber NA is assumed to be constant (independent of wavelength, which is not realistic) then the coupling will not change either.

You may want to consider using POP.  In this case you can define the input beam based on its spatial profile and transverse size, and specify the fiber based on its MFD.  The coupling vs wavelength is then easy to generate.

Here’s an example of the POP settings when using a paraxial lens (diameter of 20 mm and a focal length of 100 mm) with an input Gaussian beam having a 5 mm waist and a fiber with a 4 um waist: