Vector modulation instability induced by vacuum fluctuations in highly birefringent fibers in the anomalous-dispersion regime

We report a detailed experimental study of vector modulation instability in highly birefringent optical fibers in the anomalous-dispersion regime. We prove that the observed instability is mainly induced by vacuum fluctuations. The detuning of the spectral peaks agrees with linear perturbation analysis. The exact shape of the spectrum is well reproduced by numerical integration of stochastic nonlinear Schr?dinger equations describing quantum propagation.