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.
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