藍光零間隙式方向耦合器之研製

Abstract

隨著半導體技術的進步，藍光雷射可成功積體化於晶片上，因此積體光學應用波段亦可從1550nm縮短至藍光波長(473nm)。藍光波段可應用於軍事及商業之光通訊系統，亦可應用於光儲存以達高密度容量，未來頗有應用潛力，其光元件之研究與製作因此亦為重要課題。 操作於短波長的光學元件，所需製程精確定較高，單模態波導須要更精細的線寬。在研究中先針對方向耦合器模擬在不同誤差下，對元件功能所產生之影響，經過計算後，發現零間隙式方向耦合器的製程容忍度比有間隙式方向耦合器高。 實驗工作則在Z切鈮酸鋰基板上以鋅鎳共同擴散製作出藍光波段(473nm)下的單模態無間隙方向耦合器。由量測結果得知，於波導寬度1.6μm、擴散源鍍膜厚度鋅450Å、鎳150Å 時，耦合長度為508μm。在藍光波段下的無間隙方向耦合器，操作電壓與長度乘積為7.36V.cm，其輸出訊熄比為16.38dB，此為目前最佳數據。 實驗結果顯示零間隙式方向耦合器在藍光波長操作100分鐘後，其耦合分量變化僅有1.89%，顯示所製作之方向耦合器之功能，並未明顯受光折效應之影響。

Blue laser can be successfully integrated on a chip because of the modern advanced semiconductor fabrication technology, the range of wavelength for the application of integrated optics can be shrunk from 1550nm to blue-laser wavelength (473nm). Among them, optical communication system at blue-laser wavelength has been used for military and commercial application, and optical storage with high density can be realized. Thus, blue laser is of great potential for practical application in the near future. The study of optical devices at blue-laser wavelength is then important. Accuracy of fabrication is crucial for optical devices operating at short laser wavelengths owing to the need of narrower widths for single mode waveguides. In this work, the fabrication tolerances of directional coupler with various gaps are calculated. The results show that directional couplers without gaps are more tolerable than those with gaps. In experiment, zinc-and-nickel co-diffused single mode waveguides at blue-laser wavelength are used for the fabrication of proposed zero-gap directional couplers on a Z-cut lithium niobate substrate. The film thicknesses of diffused Zn and Ni are 450Å and 150Å, respectively. Experimental results show that the coupling length of the proposed directional coupler is 0.508mm for a waveguide of width 1.6μm. The measured operating voltage length product is 7.36V∙cm and the extinction ratio of signal output is 16.38dB, which are the best among those reported. Experimental results show the relative change of coupling ratio for 100 min test is as small as 1.89%, which indicates the photorefractive effect at blue-laser wavelength has no significant effect on the performance of the proposed directional coupler.