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Computer modelling of directional coupler based optical devices

Wongcharoen, T. (1995). Computer modelling of directional coupler based optical devices. (Unpublished Doctoral thesis, City, University of London)


This work is a study of the important parameters of synchronous and nonsynchronous, weakly and strongly coupled optical directional couplers, primarily using the finite element method. This method can be used to find accurate modal solutions of the isolated waveguides or the coupled waveguides with arbitrary shapes, index profiles, and anisotropies. Unlike the traditional coupled mode theory, the vector supermodes obtained by using the finite element method are orthogonal to each other, even when the guides are strongly coupled or when they are not identical. From an accurate knowledge of the propagation constants of the two supermodes, the coupling length of the system can also be calculated.

However, the finite element method cannot provide directly the power-transfer efficiency or the cross-talk between the coupled waveguides. Calculations of the important coupling parameters can be achieved by introducing the coupled mode approach along with the accurate modal solutions obtained by using the finite element method. In this approach, the overlap integrals of the isolated modes and the coupling factors are calculated numerically and these values are subsequently used to find the power coupling efficiency from one waveguide to another. Recently, there have been several innovative approaches to improving the traditional coupled mode theory by enforcing orthogonality of the supermodes or by maintaining the power conservation criteria. In this work, some of these new coupled mode approaches have been implemented to study the coupling parameters along with the exploitation of the accurate modal solutions obtained using the finite element method.

As an alternative, the least squares boundary residual method can be applied to find the excitation coefficients of the two supermodes by considering the butt-coupling between the input waveguide and the directional coupler section. In this approach, the continuity of the tangential electric and magnetic fields is achieved in a least squares sense at the junction discontinuity interface. Once the transmission coefficients of the two guided even- and odd- like supermodes are calculated, the power carried by the two guides along the axial direction can be easily evaluated.

In this work, the power transfer from one optical waveguides to another by the use of the finite element based propagation model has also been studied. Simulation results are presented for a wide range of directional coupler based- devices including electro-optic switches and semiconductor filters. To show the advantages of the finite element-based approaches, the power transfer efficiency between coupled waveguides with two-dimensional confinement is also presented.

Publication Type: Thesis (Doctoral)
Subjects: Q Science > QA Mathematics > QA75 Electronic computers. Computer science
T Technology > TK Electrical engineering. Electronics Nuclear engineering
Departments: School of Science & Technology > Engineering > Electrical & Electronic Engineering
School of Science & Technology > School of Science & Technology Doctoral Theses
Doctoral Theses
[thumbnail of Wongcharoen thesis 1995 PDF-A.pdf]
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