具有鋅鎳鎵共同擴散波導及側壁延伸電極之鈮酸鋰極化分離器

Abstract

本論文利用鋅鎳鎵共同擴散法在鈮酸鋰基板上製作可調式極化分離器。元件的基本架構為一非對稱Y形分岔結構，輸入波導為一鋅鎳共同擴散式波導，其特點是製作一脊形結構上，並搭配指狀電極。在輸出部分，直分支波導亦為鋅鎳共同擴散式波導，但利用不同厚度使其僅導通橫電模態(水平極化方向)；彎曲分支輸出波導則為僅導通橫磁模態(垂直極化方向)之鎵擴散式波導。 先前已發表極化分離器之製程均需要兩次以上的高溫製程，本論文所提出之製程僅需一次高溫擴散，可有效節省製程時間，簡化製程步驟。 在元件應用上，實驗結果顯示在操作波長為1.55μm之環境下，橫電模態的訊熄比為27.8dB，橫磁模態訊熄比為24.4dB，遠高於實際應用之需求(13dB)。 在調變效果上，本論文利用指狀電極之設計，比較在相同脊形深度以及電極寬度下，有無側壁延伸電極之調變電壓差異，證實在相同調變率下側壁延伸式電極成功降低了41%的電壓。 最後本論文進一步應用側壁延伸電極於6μm高之脊形元件上，在外加電壓47V時可達成兩極化模態的完全轉換，與之前已發表結果比較降低12.9%之調變電壓。

Controllable polarization splitters with Zn-Ni diffusion and Ga diffusion optical waveguides are successfully fabricated on Z-cut LiNbO3 substrates. The basic structure of the proposed polarization splitter is an asymmetrical Y-branch. Both TE and TM modes are supported in the input waveguide branch fabricated by Zn-Ni diffusion. In particular, the input waveguide branch is fabricated on a ridge structure with finger-type electrodes to modulate the polarizations. In the output section, only TE modes are supported in the straight waveguide fabricated by another Zn-Ni diffusion, whereas only TM modes are supported in the bent waveguide fabricated by Ga diffusion. Though three kinds of metal with various thicknesses are deposited, only one diffusion step is needed, instead of two or more diffusion steps as used in previous works. Thus, the fabrication process can be greatly simplified. Experimental results show, the extinction ratios are 27.8dB and 24.4dB for the TE and TM modes at 1.55μm wavelengh, which are good enough for practical applications (13dB). To study the of tunabilities of splitters, finger type electrodes are designed and fabricated in this work, sidewall-extended and conventional electrodes with the same ridge depth and electrodes gap are compared. Experimental results show the driving voltage of splitter with sidewall-extended finger type electrodes is 41% reduced, less than those without. Finally, polarization splitters on ridges of the depth 6μm with sidewall-extended electrodes are fabricated for demonstration. Measured results show polarization states can be completely converted by an applied voltage of 47V, which is 12.9% less than those reported.