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The Photonics Spotlight

Nonlinearities in Fiber Amplifier Modeling

Ref.: spotlight 2010-03-03; encyclopedia articles on nonlinearities, Brillouin scattering, Raman scattering, fiber amplifiers, laser modeling

Dr. Rüdiger Paschotta

I am often asked whether my fiber amplifier and laser modeling software RP Fiber Power can be used for modeling nonlinear effects. Strictly speaking, it can't, as it is not able to calculate the generation of power at new wavelengths through nonlinear effects, including the back-action (depletion) on the original wavelength components. What it can do, however, is to calculate the expected Raman and Brillouin gain. If the nonlinear gain is below the corresponding threshold value, you know that power extraction by these effects is negligible. If not, you know that the calculated powers are actually not realistic, because in reality the nonlinear effects would change the results.

In practice, it is very often fully sufficient to find out whether or not nonlinear effects become important. You do not need to know exactly what would happen in the strongly nonlinear regime, simply because you want to avoid operation in this regime anyway!

One might think that it would be not that difficult to fully take into account nonlinear effects in such a numerical model. After all, the corresponding equations are not necessarily very complicated. This reasoning is not valid, however – for several reasons:

Therefore, in many cases of interest such nonlinear modeling would be extremely difficult, and often at the same time not particularly useful. If anyone claims that his fiber amplifier software can do such things, you should be quite cautions.

Of course, there are cases where the modeling of strong nonlinear conversion processes makes sense. For example, fiber Raman lasers can be modeled with reasonable accuracy, even though it may be difficult to reliable predict the resulting optical spectrum. In the area of fiber amplifiers and lasers, I think it would not make much sense to include nonlinear conversion, and therefore such extensions are not planned for the RP Fiber Power software.

This article is a posting of the Photonics Spotlight, authored by Dr. Rüdiger Paschotta. You may link to this page, because its location is permanent. See also the Encyclopedia of Laser Physics and Technology.

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