  By using an analogy with axionic like systems, we study light propagation in
periodic photonic topological insulator (PTI). The main result of this paper is
an explicit expression for the PTI band structure. More specifically, it was
found that for nonzero values of the topological phase difference
$\gamma=\theta_2-\theta_1$ a finite gap $\delta \propto\gamma^2$ opens in the
spectrum which is equivalent to appearance of nonzero effective photon mass
$m^{*}(\delta)\propto \frac{\sqrt{\delta}}{\delta +2}$.
