The Study Of Propagation And Switching Properties Of The Asymmetric Waveguide Couplers

In current opaque optical networks, signals are switched by the electronicswitching in the electronic domain. But as optical technology evolves and data ratesincrease, electronic switching becomes bottleneck. To removing completely thecurrent electronic bottleneck, the electronic switching is replaced with all-opticalcross-connects (OXCs) where signals are switched entirely in the optical domain bythe all-optical switching. So the all-optical switching, as key item of ultrahigh speedand large capacity optical networks, has attracted a lot of interests. In recent years, thenonlinear directional couplers became a hot spot, as it can act as all-optical switching.What is more, the nonlinear directional couplers have not only good switching andsolitons propagation characteristics but also fast switching speed. In previousresearches, the nonlinear directional couplers were considered to be symmetric.However, the coupler with fully identical cores is hard, and even impossible, toproduce in practice. In this paper, two types of asymmetric nonlinear directionalcouplers, which consist of two un-identical cores, are studied. The main researchworks and results are as follow:Firstly, two types of asymmetric nonlinear directional couplers are introduced.One type of coupler, which consists of two un-identical fiber cores, is called astraditional asymmetric nonlinear directional fiber coupler. The other type of coupler,which consists of one waveguide with negative-index materials (NIMs) and anotherwaveguide with conventional positive-index materials (PIMs), is call as nonlinearoppositely directed couplers. In traditional asymmetric nonlinear directional fibercouplers, the phase mismatch, group velocity mismatch, difference of dispersion anddifference effective mode area, which are the asymmetrical parameters, are introduced.In nonlinear oppositely directed couplers, the NIMs is introduce firstly, thepropagation constants mismatch is also introduced, and the relation curves betweenthe reflectivity and the propagation constant mismatch are showed.Secondly, the asymmetric nonlinear directional coupled-mode equations andnonlinear oppositely directed coupled-mode equations are introduced. And thenormalized forms of the two coupled-mode equations are deduced by usingappropriate normalized variables.Thirdly, the Lagrange variational approach and the split-step Fourier method (SSFM) are described to analyze the solitons switching and propagation performancesin the asymmetric directional fiber couplers with different asymmetrical parameters.The results show that the switching threshold power has a relationship with theasymmetrical parameters, and the pulse propagation performances change with thedifferent asymmetrical parameters. Thereby, one can choose the appropriateasymmetrical parameters to achieve good solitons switching and propagationperformances.Lastly, the finite-difference time-domain method and the finite-differencespace-domain method are introduced to analyze the reflectivity and transmissivity ofthe nonlinear oppositely directed couplers in the nonlinear case. And according to theabove methods, the solitons switching and propagation performances in the oppositelydirected coupler with different propagation constants mismatches are centered on. Theresults show that the increase of propagation constant of first waveguide can lead tothe decrease of the switching threshold power. In addition, the pulse will be ofwalk-off whether the propagation constant mismatch exists or not.