| Summary: | 博士 === 國立臺灣大學 === 光電工程學研究所 === 91 === In this thesis, improved branching structures for reducing the propagation loss of multibranch waveguides are proposed. For practical application, only two-branch, three-branch, and four-branch optical waveguides, are stressed. Five structures for two-branch waveguides are proposed. First, the transmitted power of symmetric Y-branch waveguide is increased by adding a chamfered and a bulged triangular region for phase compensation. The adequate sizes of the triangular regions are then found. Secondly, we use the coherent coupling effect between guided and radiation modes to design a novel wide-angle low-loss symmetric Y-branch with multisectional bending waveguide segments. Thirdly, when an asymmetric two-branch waveguide is considered. The proposed asymmetric Y-branch waveguide with a coupling gap between straight and bend waveguide is chamfered and bulged at the branching point to increase the transmitted power. Fourthly, another asymmetric Y-branch using low-index microprisms is proposed. A microprism for bending the phase front of incident wave and a microprism for dividing the optical power into two branching arms are designed by the bend and the Y-branch phase compensation rule, respectively. The phase mismatch at the branching point is then reduced and the output power ratio can be adjusted by changing the length of the microprism. Finally, a couple-type power divider with a low-index diamond-like microprism between output waveguides is proposed. The output power ratio is designed by changing the coupling length between two parallel straight waveguides.
For three-branch waveguide, a fork-type three-branch waveguide with a low-index trapezoidal microprism located at the branching point is proposed. The central part of phase front of incident optical field is accelerated more than the outer one by the microprism and therefore more power is directed into the side waveguides. Besides, for the sake of wide angle, a coupler-type power divider consists of a straight and two bending waveguides with a high-index microprism situated at bend corners of the output waveguides is proposed. The designed output power ratio can be obtained by adjusting the coupling length between waveguides.
For four-branch waveguide, proposed is a single-mode wide-angle power divider consisting of a symmetric Y-branch waveguide with a phase front retarder around the junction and a central V-shaped two-branch waveguide located a coupling gap apart. Low loss, wide angle, and uniform output power distribution can be achieved by only adjusting the gap between Y-branch and V-shaped two-branch waveguides. And the effect on power dividing ratio is insensitive to fabrication tolerance.
|
|---|
