  Amplitude and phase are the basic properties of every wave phenomena; as long
as optical waves are concerned, the ability to act on these variables is at the
root of a wealth of switching devices. To quantify the performance of an
optical switching device, an essential aspect is to determine the tradeoff
between the insertion loss and the amplitude or phase modulation depth. Here it
is shown that every switching optical device is subject to such a tradeoff,
intrinsically connected to the dielectric response of the materials employed
inside the switching element itself. This limit finds its roots in fundamental
physics, as it directly derives from Maxwell's equations for linear
dielectrics, and is hence applicable to a wide class of optical components.
Furthermore it results that concepts as filtering, resonance and critical
coupling could be of advantage in approaching the limit.
