  We analyze the polarization of a quantum radiation field under a
de-Gaussification process. Specifically, we consider the addition of photons to
a two-mode thermal state to get mixed non-Gaussian and nonclassical states
which are still diagonal in the Fock basis. Stokes-operator-based degrees of
polarization and two distance-type measures defined with Hilbert-Schmidt and
Bures metrics are investigated. For a better insight we here introduce a
polarization degree based on the relative entropy. Polarization of the thermal
states is fully investigated and simple closed expressions are found for all
the defined degrees. The evaluated degrees for photon-added states are then
compared to the corresponding ones for the two-mode thermal states they
originate from. We present interesting findings which tell us that some popular
degrees of polarization are not fully consistent. However, the most solidly
defined degrees, which are based on the Bures metric and relative entropy,
clearly indicate an enhancement of polarization through de-Gaussification. This
conclusion is supported by the behavior of the degrees of polarization of the
Fock states, which are finally discussed as a limit case.
