Two-colour quantum entanglement in a singly resonant optical parametric oscillator approaching threshold

Following the analytical work of Ref. [1], a numerical analysis of squeezing and quantum entanglement in a continuous wave singly-resonant optical parametric oscillator approaching threshold is provided. The singly resonant case is mainly relevant to largely non-degenerate signal and idler modes (two-colour output). As the threshold of oscillation is approached the numerical spectra of the intensity difference confirm squeezing of quantum fluctuations and a progressive line-narrowing in the linear case. In the nonlinear case entanglement is confirmed although progressively reduced when approaching threshold with the squeezing spectra still displaying a narrowing of the spectral line. Modification of quantum entanglement approaching threshold is also evaluated via the condition of state inseparability.