Buildup of modulational instability from wave breaking in birefringent fiber propagation

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Optical modulational instability (MI), the spontaneous generation of intensity modulation on a continuous wave, has received much recent attention as a novel nonlinear process and as a source of terahertz repetition-rate subpicosecond pulses. The MI phenomenon was first observed for anomalous group-velocity dispersion,1 and it has recently been observed with coupled nonlinearly visible waves in a birefringent fiber.2 For use of generated terahertz pulse trains it is desirable to have oscillations with stable relative delay. However, MI, being a parametric process, typically builds up from noise. As a result, the oscillations are not coherently generated, and their timing relative to the input is not controlled. In this paper it is observed that when coupled, visible picosecond pulses propagate in a birefringent fiber, the pulses first form "wave-breaking" oscillations, which then, owing to MI gain, grow exponentially. When cross-correlation with a 100 fs probe pulse was employed, terahertz oscillations were observed to form on orthogonally polarized pulses copropagating in the fiber. The correlations were measured by averaging over many pulses, and they demonstrate the stability and relative coherence of the oscillations. Numerical simulations confirm this description, showing the formation and amplification of wave-breaking oscillations, and they also indicate the formation of dark soliton trains.