'SPM1', 'SPM2', 'XPM', 'SS1', 'SS2', 'XS'
  'ConstantRefractiveIndex1', 'ConstantRefractiveIndex2',
  'ConstantRefractiveIndexX'
  mN2 - a matrix, or vector or value of nonlinear refractive
  index in [m2/W]
  'SelfSteepening' default 'on'
  'ConstantRefractiveIndex' the [m2/W] n2 can be transformed
  into m2/V2 with constant or wavelength and direction
  dependent refractive index (default 'off')
  this is a smaller correction than self steepening.