Seems L is loss, what is U?
what software you have? If you have FDTD, you can simulate the whole device; If you have FDE, you may integrate the loss with different radius of curvature to get one value (may need to normalize the result).
You know FDE can give loss at different bending radius. But depending on the bending radius, you may need to well design the simulation region if you want to sweep. If the bending radius is too small, special care should be taken.
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