  The spectrum of periodic differential operators typically exhibits a band-gap
structure. In this paper, we will consider perturbations to periodic
differential operators and investigate the spectrum the perturbation induces in
the gaps. More specifically, we consider the operator $$ L_0
=-\frac{1}{\eps_0(x,y,z)}\Delta $$ in $\R^3$ with $\eps_0$ periodic in all
three directions. The perturbation is introduced by replacing $\eps_0$ by
$\eps_0+\eps_1$ where we assume that $\eps_1$ is still periodic in one
direction, but compactly supported in the remaining two directions, creating a
line defect. We will show that even small perturbations $\eps_1$ lead to
additional spectrum in the spectral gaps of the unperturbed operator $L_0$ and
investigate some properties of the spectrum that is created.
