可調式鋅鎳共同擴散及鎵擴散式鈮酸鋰光波導極化分離器之研製

Abstract

本論文利用鋅鎳共同擴散及鎵擴散在Z切鈮酸鋰基板上製作可調式極化分離器。元件架構為一非對稱Y形分岔結構，輸入波導為一鋅鎳共同擴散式波導搭配指狀調變電極，直分支輸出波導為僅導通橫電模態(水平極化方向)之鋅鎳共同擴散式波導，彎曲分支輸出波導為僅導通橫磁模態(垂直極化方向)之鎵擴散式波導。 本論文所提出的製程方式，僅需要一次高溫擴散步驟，有別於先前已發表之極化分離器製程需要兩次或兩次以上高溫擴散步驟，因此能夠有效簡化製造過程。 在元件應用上，實驗結果顯示橫電模態的訊熄比為20.8dB，橫電模態的訊熄比為24.2dB，已達實際應用的標準。此外利用鈮酸鋰晶體的電光效應特性，搭配指狀電極的設計，在操作波長為1.55μm，操作電壓為 90V 下，此元件具有 37.5 %的極化轉換效率，提供了極化分離器微調整輸出光功率的機制。

In this thesis, a controllable polarization splitter with Zn and Ni co-diffusion optical waveguide and Ga diffusion optical waveguide is successfully fabricated on Z-cut LiNbO3 substrate. The basic structure of the proposed polarization splitter is an asymmetrical Y-branch with the advantage of high fabrication tolerance. Both TE and TM modes are supported in the input waveguide fabricated by Zn and Ni co-diffusion. For electrooptic modulation, a pair of interdigitated electrodes is placed on both sides of the input waveguide. In the output section, only TE modes are supported in the straight waveguide fabricated by Zn and Ni co-diffusion, whereas only TM modes are supported in the bent waveguide fabricated by Ga diffusion. As only one diffusion step is needed, instead of two or more diffusion steps used in previous works, the device fabrication process can be greatly simplified. The measured extinction ratios are 24.2dB for the TE mode and 20.8dB for the TM mode, which are good enough for practical applications. In addition, using the property of electro-optic effect in LiNbO3 crystal and design of finger-type electrodes can achieve 37.5% mode conversion efficiency in the condition of input light with 1.55 μm wavelength and 90V drive voltage. The output power ratio between two polarization components can be electrically controlled.