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In a linear transmission line(TEM), you can write V+ = I+ * Z, where Z is the characteristic impedance. In a non linear transmission line, there is no simple linear equation relating wave voltage to wave current. An example of a nonlinear transmission line would be V = aI + bI^2 + cI^3
You can make non linear transmission lines by loading a normal line with lumped components, like varactor diodes. Microstrip on ferrite, if the field strength is high, can be non-linear. Etc etc.
Frequency multipliers have phase curves to take a sine wave input, and form a Soliton wave by the time it reaches the output. The soliton has a rich harmonic output.