改良式鈮酸鋰脊形光波導之特性與應用

Abstract

本論文以混合苯甲酸和己二酸作為質子酸源，在鈮酸鋰基板上進行質子交換溼式蝕刻法製作脊形光波導及元件。並針對其製程特性、蝕刻行為、質子交換氫離子擴散模型、以及相關的應用與優勢，進行深入而有系統的研究與分析。 在製程特性方面，於Z切晶片上，混合酸與傳統純苯甲酸的製程幾乎完全相同；主要差別在X切晶片的質子交換過程，本研究以碳酸鋰取代苯甲酸鋰來稀釋質子酸源，以求兩種酸酸度的下降比較平均。由蝕刻結果可以發現，在特定的己二酸莫耳濃度下且其他條件均相同時，以混合酸製作的脊形結構擁有較大的脊形深度，與較垂直的脊形側壁外觀。而且蝕刻出的表面仍相當平滑，適合用於製作脊形光波導。 在擴散模型方面，以二次離子質譜儀縱深分析，搭配數值方法反推出氫離子於鈮酸鋰晶體中的濃度相依擴散率。再以時域有限差分法，求解非線性的氫離子擴散微分方程式，並以實驗所得之蝕刻深度驗證擴散模型正確性。根據此法，本研究成功建立氫離子相對濃度的擴散模型，可用於後續之研究。 在元件的應用上，本研究以純苯甲酸和混合酸所製作的脊形結構，製作Ｓ形彎曲結構。實驗結果顯示因為脊形側壁所提供的光場橫向侷限性較傳統平面通道式波導為大，元件傳輸率因此明顯上升。而且以混合酸製作的脊形側壁較垂直，其Ｓ形彎曲波導的傳輸率較以純苯甲酸製作的樣本更高。這樣的脊形蝕刻法提供了縮短元件長度的可能性，在積體光學上，有一定的實用價值。

In this dissertation, by mixing benzoic and adipic acid as the source of proton exchange, wet etched ridge waveguides are successfully fabricated in LiNbO3. Systematic investigations are put on the fabrication characteristics, etching results, H+ diffusion behaviors, and advantages of relative applications. In Z-cut LiNbO3, the fabrication process with proposed joint proton source is almost identical to that with pure benzoic acid. In X-cut LiNbO3, however, there exists differentiation. To equally reduce the acidity of both acids, traditional lithium benzoate is no longer suitable for diluting joint proton source, and lithium carbonate is used instead. The etching results show that with certain mole percentages of adipic acid and other things being equal, the etched ridge structures possess larger ridge depths and more vertical ridge sidewalls. Besides, the etched surfaces are still smooth with respect to operating wavelengths, and suitable for producing optical ridge waveguide. To study diffusion behaviors, secondary ion mass spectrometer (SIMS) along with the Boltzmann-Matano inverse method is used to derive concentration-dependent diffusivity of H+ in Z-cut LiNbO3. With the derived diffusivity, FDTD-based diffusion simulator successfully solves the nonlinear diffusion equation numerically. The etching depths of the experimental results meet well with the simulations. The diffusion model of hydrogen ion in Z-cut LiNbO3 is built and applicable for future use. For realistic applications, the optical ridge waveguides can be used to fabricate S-bend structure. The experimental results reveal that the lateral confinement of optical field provided by sidewalls of ridge structures is much larger than that of traditional channel waveguides. Thus, the transmission of the S-bend structure can be enhanced. In addition, the sidewalls of samples made by join proton source are more vertical, transmission can therefore further improved. The proposed wet etching method gives an opportunity to shorten the device. In the realm of integrated optics, the practical use is possible.