鋅鎳共同擴散式鈮酸鋰光波導元件之特性與應用

Abstract

本論文探討在Z切鈮酸鋰基板上以鋅鎳共同擴散的方式設計製作光波導及元件，並首次針對其製程特性、定量擴散行為、折射率模型、以及相關的應用與優勢，作深入的系統性研究與分析。 在製程特性方面，有鎳金屬作為附著層，解決了原本鋅金屬對於鈮酸鋰基板附著力不佳的問題，使得製程成功率大為提升。在擴散過程中，高溫不超過攝氏950度，因此沒有鋰離子外擴散的問題，並使用兩段式升溫，增加鋅金屬氧化的程度，以提升製程穩定性。在擴散行為分析方面，採用二次離子質譜儀縱深分析，配合電子微探儀表面測定，考慮升降溫的效應，分別建立鎳、鋅單獨擴散，以及鋅鎳共同擴散的定量擴散模型，並討論共同擴散與單獨擴散之間的差異。 根據所得之定量擴散模型，結合稜鏡耦合系統對於光波導等效折射率的量測，並進一步利用逆WKB法重建折射率分佈，可以計算出各個擴散源的折射率模型。實驗結果顯示單位濃度鎳所造成的折射率變化量高於鋅，而由於不同極化方向上的折射率變化趨勢不同，因此造成了製程相依極化特性。在鋅鎳共同擴散的情況下，其折射率分佈可成功地由相關的擴散與折射率模型計算出，並且由於鋅的擴散速度較慢，可以改善原本鎳擴散式光波導光場侷限性不佳的問題。 在元件的應用上，本研究首先將鋅鎳共同擴散式光波導應用於波導極化器，實驗結果顯示非普極化方向的極化訊熄比為24dB，普極化方向可達29dB，且不需要任何額外的結構設計。其次，本研究也結合製程相依極化特性以及鈮酸鋰晶體的電光效應，設計及實作出一可調式極化分離器，極化分離訊熄比在非普極化與普極化方向分別為18dB與23dB，極化轉換率可達60%以上。這樣的元件提供了極化分離器一個調整輸出光功率比例的機制，在光通訊元件或光感測器上，均有一定的實用性。

In this dissertation, optical waveguides and devices made by Zn and Ni co-diffusion on Z-cut lithium niobate (ZNI:LiNbO3) are systematically investigated from their fabrication characteristics, (co-)diffusion behaviors, refractive index models, to unique applications for the first time. In the waveguide fabrication process, the required diffusion temperature is no higher than 950 Celsius degree, and thus free of lithium out-diffusion problem. The adhesion of deposited Zn layer to substrate is significantly improved by the presence of Ni. A two-step thermal cycle is programmed in the diffusion process to enhance oxidation of Zn and yield stable waveguide conditions. To study the diffusion behaviors, secondary ion mass spectrometer (SIMS) and electron probe x-ray microanalyzer (EPMA), along with a FDTD-based diffusion simulator, are used to build and verify quantitative diffusion models for Ni, Zn, and ZNI. The diffusion parameters are successfully retrieved, and major effects of co-diffusion are also discussed. With the quantitative diffusion models, prism-coupler measurement and IWKB methods are combined to construct refractive index models for related diffusants. Experimental results show that index change induced by unit concentration of Ni is higher than that of Zn. Different polarizations exhibit different trends in refractive index models so that the fabrication-dependent polarizations can be achieved. The index profiles of ZNI:LiNbO3 waveguides can be derived by co-diffusion model and refractive index models of individual diffusants. It is also found that the optical confinement of ZNI waveguides is better than Ni-indiffused ones because the diffusivity of Zn is lower than that of Ni. For practical applications, ZNI:LiNbO3 waveguides supporting single polarization are used as waveguide polarizers with polarization extinction ratios of 24dB and 29dB for extraordinary and ordinary waves, respectively, without any special design of buffer layer or waveguide structure. In addition, tunable polarization splitters, in which the output power ratio between two polarization components can be electrically tuned, are proposed and demonstrated with ZNI:LiNbO3 waveguides, leveraging both the fabrication-dependent polarization property and EO effect. Experiment results show polarization splitting ratio of 18dB and 23dB for extraordinary and ordinary waves, respectively, with mode conversion efficiency up to 60%. Advantages of applying ZNI:LiNbO3 waveguides to integrated optical applications are also addressed in this dissertation.