Nonlinear interactions between modulated waves in a single-mode fiber

Abstract

The nonlinear interaction, induced by the Kerr effect, between a modulated wave and a carrier wave propagating in a single-mode optical fiber, has been investigated both theoretically and experimentally for three common modulation schemes: amplitude shift keying (ASK), frequency shift keying (FSK), and phase shift keying (PSK). Such interactions may lead to cross-phase modulation and to the generation of new frequencies which may have deleterious effects on coherent communication systems. Our results reveal that the Kerr effect leads to the generation of two new waves via four-wave mixing. The spectrum of one of the generated waves is a copy of the spectrum of the modulated input wave, while the spectrum of the second is an autoconvolution of the modulated input wave spectrum. The spectrum of the carrier is not changed for pure PSK or FSK. For ASK, however, the carrier becomes phase modulated. Furthermore, the depletion caused by the generation of the two new waves leads to amplitude modulation of the carrier. This depletion and thus the crosstalk are minimized for an intensity ratio of 2:1 between the modulated and the carrier waves.