具高只向性之下埋式金屬光柵光纖波導耦合器

Abstract

在垂直式的光柵耦合器中，耦合效率受到指向性以及光纖模態與繞射光模態的重疊程度所影響。雖然說我們可以利用局部區域的多晶矽的沉積或是在基板與光波導之間加上反射鏡來提升耦合器的指向性，然而這些結構還需要而外的沉積與黃光製程。如果利用金屬來製作我們的光柵結構，我們就可以用較簡易的剝離(lift-off)製程來製作我們的金屬光柵結構。因此在本論文，我們利用金屬光柵並引入了光子晶體之光能隙的概念，將光柵埋至光波導下方，用以降低結構繞射至基板的損耗，因而提升耦合器的指向性。 在模擬當中首先我們探討了，光柵高度、金屬占據比例與氧化層厚度對指向性的影響。從模擬結果中我們可以得知，當光柵的高度為1000奈米時，光柵耦合器的指向性能高達90%。對於一個200奈米高的光柵，埋在1450奈米厚的二氧化矽裡指向性能達到80%，此時的耦合效率能達到65%。而如此高指向性的光柵結構是可以利用較簡易的剝離製程來完成的。緊接著我們又探討了光纖與耦合器的對準對耦合效率的影響，當光纖擺放的角度差兩度時，只會增加1.19dB的耦合損耗。而在水平面上的對準上，1微米的誤差也只會增加0.32dB的耦合損耗。

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.