  In this paper we study the uniqueness and the increasing stability in the
inverse source problem for electromagnetic waves in homogeneous and
inhomogeneous media from boundary data at multiple wave numbers. For the unique
determination of sources, we consider inhomogeneous media and use tangential
components of the electric field and magnetic field at the boundary of the
reference domain. The proof relies on the Fourier transform with respect to the
wave numbers and the unique continuation theorems. To study the increasing
stability in the source identification, we consider homogeneous media and
measure the absorbing data or the tangential component of the electric field at
the boundary of the reference domain as additional data. By using the Fourier
transform with respect to the wave numbers, explicit bounds for analytic
continuation, Huygens' principle and bounds for initial boundary value
problems, increasing (with larger wave numbers intervals) stability estimate is
obtained.
