Hi Abdallah,
Thanks for the project file. Just from looking at the output of the Parabolic fiber though I can see that you are indeed generating many modes, however I do not see a clear LP21 mode at the output of the parabolic fiber.
Here’s what I suggest:
1) In the LED change the mode type to Laguerre-Gaussian.
2) Change the Mode X (m,n) index array to include 2,1, so for example write “0 1, 0 2, 1 1, 2 1”. This should generate LP01, LP02, LP11 and LP21.
Hope this helps, if it doesn’t please also include your fiber data file, so I can look at it!