  We study theoretically and experimentally the quantum properties of a type II
frequency degenerate optical parametric oscillator below threshold with a
quarter-wave plate inserted inside the cavity which induces a linear coupling
between the orthogonally polarized signal and idler fields. This original
device provides a good insight into general properties of two-mode gaussian
states, illustrated in terms of covariance matrix. We report on the
experimental generation of two-mode squeezed vacuum on non-orthogonal
quadratures depending on the plate angle. After a simple operation, the
entanglement is maximized and put into standard form, \textit{i.e.} quantum
correlations and anti-correlations on orthogonal quadratures. A half-sum of
squeezed variances as low as $0.33 \pm 0.02$, well below the unit limit for
inseparability, is obtained and the entanglement measured by the entropy of
formation.
