Quantum entanglement in strongly non-degenerate OPOs: The doubly and singly resonant cases

Many theoretical and experimental approaches to quantum entanglement in Optical Parametric Oscillators (OPO) concern the degenerate or the slightly non-degenerate cases where a single local oscillator can be used to measure quadrature correlations for the characterization of fluctuations below the Standard Quantum Limit (SQL) in the frequency domain. In the strongly non-degenerate case, alternative characterizations of quantum entanglement are needed. We provide evidence that correlation functions corresponding to coincidence measurements of the intensity difference between signal and idler fields should be able to assess quantum entanglement and at the same time be experimentally viable in the case of strongly non-degenerate OPO.