  The Su-Schrieffer-Heeger (SSH) model is the simplest example of a lattice
with non-trivial topology. It supports mid-gap topologically protected states,
whose energies are unaffected by disorder. We show that photonic multilayer
structures provide an exact implementation of an SSH lattice, provided each
layer has the same propagation thickness. From this, it follows that the cavity
mode in a conventional semiconductor microcavity is a protected SSH mid-gap
state. We demonstrate this experimentally using controlled disorder in a
mathematically equivalent system, a radio frequency transmission line made from
sections of coaxial cable with high and low impedances. We also show
theoretically that transmission lines connected to form networks map onto
topologically interesting lattices in in higher dimensions.
