Nonlinear optics in birefringent fibers: Modulational instability for normal dispersion

Abstract

Optical modulational instability (MI), the spontaneous generation of intensity modulation on a continuous wave, has been investigated as a novel nonlinear optical process and a source of high-repetition-rate subpicosecond pulses. Straightforward extension of a previous analysis shows that MI occurs even for normal dispersion. At low power, gain exists in a band of frequencies near Ω ≅ δ/β, where δ is the group-velocity mismatch of the two polarization modes and β is the dispersion, and at higher power the gain band broadens and shifts to a lower frequency. Each wave develops intensity modulation, and, in the spectral domain, the slow wave develops a single sideband frequency downshifted by Ω (whereas for the fast wave it is upshifted by Ω). Physically, this means that each sideband occurs at a frequency such that its group velocity is approximately synchronized with the pump of the opposing polarization wave. This behavior is in contrast to previously observed optical MI, where the pump develops dual sidebands, and thus the present system represents a new class of MI. The experimental observation of this behavior is described.