Quantum analysis of the nondegenerate optical parametric oscillator with   injected signal
  In this paper we study the nondegenerate optical parametric oscillator with injected signal, both analytically and numerically. We develop a perturbation approach which allows us to find approximate analytical solutions, starting from the full equations of motion in the positive P-representation. We demonstrate the regimes of validity of our approximations via comparison with the full stochastic results. We and that, with reasonably low levels of injected signal, the system allows for demonstrations of quantum entanglement and the Einstein-Podolsky-Rosen paradox. In contrast to the normal optical parametric oscillator operating below threshold, these features are demonstrated with relatively intense felds. 
B. Coutinho dos Santos, K. Dechoum, A. Z. Khoury, L. F. da Silva and   M. K. Olsen