Discussion Forum

Caveats: I don't model a lot of fiber optics and I haven't used the beam-envelope formalism. So I could be wrong about what I say next. OK? Here goes. Anyway, my initial idea would be to include a modest volume of the surrounding medium (in your case, air). This way, the boundary on the fiber optic surface would be handled correctly. After all, in the physical world, there is an evanescent field that extends a little bit outside the cladding. And if you don't allow that to exist in your model, you probably won't have the conditions set right. Finally, to bound the outside air volume, I would use either a scattering boundary condition or PML. Alternatively, I suppose you *might* be able to get away with setting some kind impedance boundary condition on the surface of the fiber, but I wouldn't assume that you could get away with that, unless your careful review of the physics showed it was equivalent to what I described above. I encourage other readers here with more knowledge of simulating optical waveguides or fiber optics to contribute their thoughts.