Abstract
Abstract Because of its generality and usefulness, the coupled-mode theory (CMT) plays an important role in analyzing many guiding wave optical devices. The CMT is a kind of variational principles. There are various formulations to take different factors into account, particularly in the case of strong coupling, and thereby the CMT and the related manipulations sometimes become sophisticated or even tedious. Among optical waveguides, their properties are similar, but the step-index slabs are the most basic waveguides where the ray-optic analysis is also interesting. The purpose of the thesis is to propose an alternative way of thinking the light coupling in step-index slab couplers. In conjunction with the ray theory and the light tunneling in slabs, we formulate a theory in computing the coupling coefficient in slab couplers. The coupling coefficient derived by this method is compared with those of the conventional CMT and the improved CMT. For a large slab separation, they are proven to be the same. For a small slab separation and strong coupling, the coupling strengths based on the present method and the improved CMT show good agreement. In this thesis, we also numerically analyze a complex coupler with an absorbing medium. It turns out to be very problematic for the CMT, but it is easy using the present method. The examples suffice to show that our formulation is superior to the CMT in some respects.