  The article considers a parametric effect which takes place when the velocity
of signal propagation in a long line changes. We found the analytical solution
describing the form of the transformed signal for a line with losses, when line
parameters change symmetrically. We also considered lines without losses, with
asymmetrical change of parameters. Our theoretical results comply with
experimental data. In certain conditions, such a line can be used as an
amplifier. The parametric effect in optics is described by Maxwell's equations,
while in case of a long line, the analysis is based on telegrapher's equations.
However, it turns out that in the end, both in optics and electronics, the
parametric effect is described by wave equations that are mathematically
similar. This is because fundamentally, the parametric effect is physically
based on energy interchange between the controlling (pump) signal and the
transformed one, when the parameters of the propagating medium change. So, the
obtained results can be used for analysis of parametric effects in optics and
electronics.
