Buried Metal Grating for Vertical Fiber-Waveguide Coupling with High Directionality

• Main Authors: Wu, Che-Yao, 吳哲嶢
• Other Authors: Lee, Po-Tsung
• Format: Others
• Language: en_US
• Published: 2012
• Online Access: http://ndltd.ncl.edu.tw/handle/29221035236697212228

碩士 === 國立交通大學 === 光電工程學系 === 101 === Coupling efficiency of the grating coupler which vertically couples fiber-waveguide is affected by directionality and fiber mode overlap. Although the directionality can be improved by poly-silicon overlay grating structure or additional bottom mirror, these structures need additional fabrication procedures like deposition and lithography. But metal grating structure can be fabricated by easier lift-off process. In this thesis, we design a metal grating which is buried under silicon waveguide, and used the photonic band gap effect of photonic crystal with the grating structure to reduce the substrate loss, and improve the directionality of grating structure. First, we investigate the dependence of directionality on grating height, filling factor and silicon oxide thickness by simulation. The results show that the grating structure can reach 90% coupling directionality when grating is 1000 nm in height. A metal grating of 200 nm in height, buried in oxide layer of 1450 nm in thickness, can also reach 80% coupling directionality and 65% coupling efficiency. And this high directionality grating structure is adapted to lift-off technique. We also investigate the alignment tolerance of our grating structure. A 2° alignment error in angle results in 1.19dB additional coupling loss. And 1 μm alignment error in lateral only results in 0.32dB additional coupling loss.