Graduate School in Nonlinear Science

Sponsored by the Danish Research Academy

MIDIT                               OFD                           CATS
Modelling, Nonlinear Dynamics       Optics and Fluid Dynamics     Chaos and Turbulence Studies
and Irreversible Thermodynamics     Risø National Laboratory      Niels Bohr Institute and 
Technical University of Denmark     Building 128                  Department of Chemistry
Building 321                        P.O. Box 49                   University of Copenhagen 
DK-2800 Lyngby                      DK-4000 Roskilde              DK-2100 Copenhagen Ø
Denmark                             Denmark                       Denmark


by Stefano Trillo
Fondazione Ugo Bordino
Rome, Italy

MIDIT-seminar 455

Tuesday June 29, 1999, 11.00 h
at MIDIT, IMM, Bldg. 305, Room 027, DTU

Abstract: The theory of modulational instability (MI) in optical fibers is briefly reviewed, with specific focus on the polarization arrangements which allow for MI to take place in the normal dispersion regime in bimodal (birefringent) fibers. Specifically, we wish to address the question concerning the most favourable conditions for frequency conversion from a strong pump to the modulational sidebands. According with the standard linearized MI analysis, this condition is obtained at the MI peak gain or phase-matching of the underlying four-photon process. We show, however, that the MI analysis can be prolonged into the nonlinear (strongly depleted) regime revealing a complex homoclinic structure of the system of coupled nonlinear Schroedinger equations which govern the nonlinear mixing process. We show that, as a consequence, the optimum conversion strongly deviates from the phase-matching condition, being obtained even outside the MI gain or parametric bandwidth. A clear experimental evidence for this phenomenon has been recently provided.