  The purpose of this paper is to prove that the spectrum of an isotropic
Maxwell operator with electric permittivity and magnetic permeability that are
periodic along certain directions and tending to a constant super-exponentially
fast in the remaining directions is purely absolutely continuous. The basic
technical tools is a new ``operatorial'' identity relating the Maxwell operator
to a vector-valued Schrodinger operator. The analysis of the spectrum of that
operator is then handled using ideas developed by the same authors in a
previous paper.
