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Forum: Processes Involving Optical Photons
Re: Question Simulate a clear optical fiber (Simone Riggi)
Date: 21 Oct, 2009
From: Gumplinger Peter <Gumplinger Peter>

Hi Simone,

> Is this correct or is there a better way of modeling the clear fiber?

Yes, this is correct and there is at present no 'better way' of modeling clad fibers. If the fiber were wrapped with a diffuse reflector you could use the unified surface model with the 'backpainted' option. In this case, the optical boundary process model internally calculates the extra photon tracking in the thin layer; not so if the thin layer is simply open to the environment and a second internal reflection occurs. So, I cannot offer you a speed up and the thin cladding has to be defined in the geometry explicitly.

> With the scheme described above, I can get total reflections only if the

> light directly strikes the core from the fiber end. In fact when photons

> hit the fiber from the lateral side, they first encounter the clad

> layer. They can propagate through refraction into the core but cannot

> give rise to internal reflections in the core. The fiber in this case

> cannot transport photons. This is simply explained with the Snell's law.

Yes, I agree for perfectly smooth cylindrical fibers.

> It is not totally clear to me if I have to expect a light confinement
> also in the case of photons hitting the fiber from the lateral side.

Well, you just explained it; no you should not expect it for the ideal case.

> If this is the case, what is the optical mechanism responsible for that?

> Rayleigh diffusion with fiber impurities?

Yes, the only possibility for a ray to be captured is by some process to change its direction. Rayleigh is one possibility (and you can see its effect if you did not include this process in your simulation and observed a change), or a rough surface would trap some photons - but then, a rough surface also allows photons once trapped to escape again. Visualization of what the rays do is one educational exercise.

Hope this helps, Peter

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