  Type II optical parametric oscillators are amongst the highest-quality
sources of quantum-correlated light. In particular, when pumped above
threshold, such devices generate a pair of bright orthogonally-polarized beams
with strong continuous-variable entanglement. However, these sources are of
limited practical use, because the entangled beams emerge with different
frequencies and a diffusing phase-difference. It has been proven that the use
of an internal wave-plate coupling the modes with orthogonal polarization is
capable of locking the frequencies of the emerging beams to half the pump
frequency, as well as reducing the phase-difference diffusion, at the expense
of reducing the entanglement levels. In this work we characterize theoretically
an alternative locking mechanism: the injection of a laser at half the pump
frequency. Apart from being less invasive, this method should allow for an
easier real-time experimental control. We show that such an injection is
capable of generating the desired phase locking between the emerging beams,
while still allowing for large levels of entanglement. Moreover, we find an
additional region of the parameter space (at relatively large injections) where
a mode with well defined polarization is in a highly squeezed vacuum state.
