Paraxial version

The Nonlinear Envelope Equation [33] is a paraxial equation with some additional approximations related to chromatic dispersion. This equation appears to be extremely close to the paraxial version of UPPE. 

Once again we follow the general procedure and approximate the linear propagator by its paraxial version ... 

Due to space limitations, we focus in this chapter on the z-UPPE. As discussed in [1] in more detail, the time-propagated versions of UPPE are more suitable for tight-focusing scenarios when non-paraxial effects start to play a role. The -propagated equations are easier to use in situations that allow to neglect the longitudinal held components as sources of nonlinear material responses. 

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See also in sourсe #XX -- [ ]

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