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Check orthogonality of fiber modes
Posted Sep 18, 2019, 3:12 p.m. EDT RF & Microwave Engineering, Mesh, Results & Visualization, Studies & Solvers Version 5.4 1 Reply
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I simulated a few mode fiber in COMSOL and use maximum mesh size of 0.15 um to all domains to ensure accuracy of mode profile. I exported the electric field of the modes in x and y directions with grid step of 0.1 um and calculate the dot product between two mode pair to check the orthogonality (Ex1.conj(Ex2))+(Ey1.conj(Ey2)) but the resulted modes are not orthogonal enough as you can see in the attached fig1, there is a cross-coupling of around -40db below the maximum value. Is this normal or what I have to adjust to optimize the accuracy of the solution? and in general how to improve COMSOL accuracy when fiber has a large number of modes (e.g. 50 modes)?
I have also tried to use adaptive refinement but the dot product didn't improve as in fig2.
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Caveat: I've never tried to check orthogonality of computed modes before, and I haven't downloaded your .mph file to look at it... That said, I would be interested to know if you've tried different discretizations of the elements (e.g., comparing results using linear quartic, and cubic elements). If using higher-order elements gives noticeably better results (which it normally does, if errors are purely numerical artifacts) then that is the issue, rather than something more fundamentally-incorrect about the way you've configured your model. Have you tried doing that?
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