Observation of modulation instability in the normal dispersion region over nonpolarization preserving fibers

Modulation instability (MI) can be interpreted as four-wave mixing between an intense CW pump and noise Fourier components within a certain spectral region where phase matching is satisfied. The dispersion contribution to the phase of the waves can be compensated by the nonlinear phase contribution (self- and cross-phase modulation), only in the anomalous dispersion region (i.e. group velocity dispersion (GVD) parameter D>0). In the normal GVD region, MI has been observed in birefringent or bimodal fibers. In these fibers phase matching can be assisted by wave-vector differences from either different polarization states or fiber modes. For nonpolarization preserving single mode fibers it has been theoretically predicted that MI can occur in the normal GVD region if a second pump wave is simultaneously propagating in the anomalous GVD region. The MI gain spectrum in the normal GVD was predicted to be a replica of the MI spectrum in the anomalous GVD region, i.e., with two typical MI sidelobes on each side of the laser line. In this paper we present experimental verification of these predictions.