Alloptical signal processing in quadratic nonlinear materials  Steffen Kjęr Johansen
 Abstract  The focal point of the research presented here is alloptical signal processing via nonlinearities. The objective has been to investigate the interaction between optical signals via nonlinearities and how these nonlinearities can be engineered to serve specific purposes.
The nonlinear response of materials with a second order nonlinearity, the socalled X(2) materials, is faster and stronger than that of more conventional materials with a cubic nonlinearity. The X(2) materials support spatial solitons consisting of two coupled components, the fundamental wave (FW) and its second harmonic (SH).
During this project the interaction between such spatial solitons has been investigated theoretically through perturbation theory and experimentally via numerical simulations. The outcome of this research isnew theoretical tools for quantitatively predicting the escape angle, i.e. the angle of incidence below which the solitons will fuse and above which they will move apart. Headon collision experiments are not comprised within the model assumptions, but even so expressions predicting the socalled inwards escape angle are proposed and numerically veriŻed for certain cases. Chapter 2 and paper 1 are dedicated to this part of the research. In chapter 4 the generality of the theoretical approach is emphasised with the derivation and verification of equivalent tools for media with a saturable nonlinearity.
The strength of the X(2) nonlinearity strongly depends on the phase mismatch between the FW and the SH. Via quasiphasematching (QPM) the phase mismatch and hence the nonlinearity is eŁectively brought under control through periodic sign reversal of the nonlinearity. On theaverage QPM changes the quadratic nonlinearity and induces new cubic nonlinearities in the system. The engineering and exploitation of these cubic nonlinearities in twoperiod QPM waveguides has been another area of investigation. Introducing the second period might make practical engineering of the nonlinearities possible. A major result is the discovery that cubic nonlinearities leads to an enhancement of the bandwidth for soliton generation. This part of the research is presented in Chapter 3 and paper 2.  Keywords  Nonlinear Optics, Physical Optics, Quadratic Nonlinearity, Saturable
Nonlinearity, Solitons, Soliton generation, Soliton Interaction,
QuasiPhaseMatching, Engineering of Optical Nonlinearities, AllOptical Signal Processing.  Type  Ph.D. thesis [Academic thesis]  Year  2002  Publisher  Informatics and Mathematical Modelling, Technical University of Denmark, DTU  Address  Richard Petersens Plads, Building 321, DK2800 Kgs. Lyngby  Series  IMMPHD2002105  Electronic version(s)  [pdf] [ps]  BibTeX data  [bibtex]  IMM Group(s)  Mathematical Physics 
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