3.4: Universality of the NSE
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Above, we derived the NSE in detail for the case of dispersion and self-phase modulation. The input for the NSE is surprisingly low, we only have to admit the first nontrivial dispersive effect and the lowest order nonlinear effect that is possible in an isotropic and homogeneous medium like glass, gas or plasmas. Therefore, the NSE and its properties are important for many other effects like self-focusing [19], Langmuir waves in plasma physics, and waves in protein molecules [20]. Self-focusing will be treated in more detail later, because it is the basis for Kerr-Lens Mode Locking.