Extra phase noise from thermal fluctuations in non-linear optical crystals

Recent experiments with optical parametric oscillators have shown that an unpredicted extra noise on the phase of the fields reduces the level of squeezing or quantum correlations. In some cases, it can hinder the attempts to observe entanglement in those systems. It is probably the main reason why signal and idler entanglement above threshold remained unobserved by almost 20 years after its prediction. This paper investigates that the extra noise can be explained by phonon scattering of the central carrier of the intense field. In this Brillouin process, the scattered light acts as an extra noise source in the sideband where squeezing is generally observed. With this model, the linear dependence of the excess noise with the intracavity power and its behavior with temperature can be explained. Adding this noise source to the analysis of the quantum features in the OPO above threshold, recent results in the quest for tripartite entanglement in the OPO above threshold can be explained.