Observation of modulational instability for normal dispersion

Abstract

Optical modulational instability (MI), the spontaneous generation of intensity modulation, has received much attention recently as a novel nonlinear-optical process and as a source of high-repetition-rate sub picosecond pulses. In a detailed analysis1 it was pointed out that MI would only be seen for anomalous group-velocity dispersion(GVD,β = 0). This effect was first observed in the nonlinear propagation of infrared light in optical fibers.2 The situation of waves nonlinearly coupled through cross phase modulation in a biréfringent fiber was treated3 for anomalous GVD. A straightforward extension of this analysis shows that MI occurs even for normal dispersion.4 We find that gain exists in a band of frequencies centered about Ω ≅ δ / β , where δ = v g ( s l o w ) − 1 − v g ( f a s t ) − 1 is the group-velocity mismatch of the two polarization modes. Each wave develops intensity modulation, and the slow wave develops a single sideband frequency downshifted by Ω, and the fast wave is upshifted by Ω. This behavior is in contrast to previously observed optical MI,2 in which the pump develops dual sidebands and thus represents a new class of MI.