Farjady, Farshid; (1997) General strong coupled mode theory for multiwaveguide nonlinear directional couplers. Doctoral thesis (Ph.D.), University College London (United Kingdom).

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Abstract

Three main topics are covered in this thesis. The first part of this thesis concerns new theoretical work on strong coupled mode theory for the multiwaveguide nonlinear directional coupler (NLDC). The coupled mode equations for the general case including all the guides interacting are derived for the first time and expressed in the form of a new single-line matrix equation. The matrix form allows the physics of the NLDC to be immediately apparent. We discuss an error in the literature regarding the strongly coupled two guide case. Two new identities for the NLDC are derived. We propose an improvement on the coupled mode theory model by taking into account the power held in the overlap. We show for the first time analytical solutions for the strong coupling case, and derive a new accurate analytical formula for the switching power. We numerically check our equations against other methods and against beam propagation method simulations for both weak and strong coupling regimes. We also study in detail the variation of the coefficients appearing in the equations against different parameters. The second part concerns a new soliton generator based on graded nonlinearities. It is shown that this device leads to cleaner and more efficient single-soliton generation, and can be used to improve the efficiency of soliton couplers. Our device has the advantage that it prevents multisoliton emission effects normally associated with uniform nonlinear devices. It is also a linear scanner in contrast with previous uniform nonlinear devices which were angular scanners. We next propose a novel three guide graded-nonlinear soliton coupler. It is simpler to operate and more efficient than a previous published device which used uniform nonlinearity. It operates solely by means of the power of the propagating optical beam, whereas the previous published device required an additional control beam.

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