Modulation instability leading to self-amplitude modulation of optical pulses in nonlinear dispersive fibers

Abstract

Propagation of a cw optical beam in fibers1 is inherently unstable in the anomalous dispersion regime, a phenomenon referred to as modulation instability (MI). Its observation2 has required a noise input or an independent beam at the maximum-gain frequency. In this paper we show that self-phase modulation of optical pulses can broaden the spectrum enough to encompass the maximum gain frequency and to induce self-amplitude modulation through MI. We solve the nonlinear Schrodinger equation numerically and find that in appropriate conditions a 100-ps pulse breaks into a sequence of pulses whose period is determined by the peak power and the fiber dispersion. This occurs for moderate peak powers (~100 mW) for fiber lengths ~5-10 km. We present the results showing the evolution of the pulse shapes and spectra along the fiber length for Gaussian and secant hyperbolic pulses.