Self-amplitude-modulation of optical pulses in nonlinear dispersive fibers

We consider propagation of optical pulses in the anomalous dispersive region of single-mode fibers after including the effects of Kerr-type nonlinearity. It is found that the pulse develops an internal structure with deep amplitude modulation. We show that this self-amplitude-modulation of pulses is related to the modulation instability of continuous-wave optical beams in nonlinear dispersive fibers. The initiation of this phenomenon does not require an external probe or spontaneous emission. Numerical results show that self-phase-modulation broadens the power spectrum enough to encompass the frequency at which self-amplitude-modulation provides the maximum gain. The amplification of these sidebands manifests as a breaking of the optical pulse into several subpulses. For negligible loss, the pulse breakup may also be viewed as the internal structure of a high-order soliton. However, modulation instability appears to provide a simpler interpretation and explains the features of numerical simulation and experiments.