氮氧化矽在波導元件之應用與模擬

研究生：張智凱 指導教授：蘇炎坤、張允崇、莊文魁
國立成功大學光電科學與工程研究所

摘要
在本論文中，我們使用電漿輔助式化學氣相沉積系統(PECVD)成長氮氧化矽薄膜，由於氮氧化矽薄膜可調變的折射率範圍很大，光損失很小，很適合當作我們脊型波導的導光層，再經由模擬及實驗來驗證在特定的條件下的確可以達成單模態的輸出。
不同的氣體流量比的氮氧化矽薄膜，會有不同的光學特性，我們經由薄膜測厚儀，橢圓儀，還有棱鏡耦合的方式量測出薄膜的折射率以及厚度。並且使用傅立葉轉換紅外線光譜儀(FTIR)量測出不同鍵結的頻譜強度以及位置，可以發現當薄膜有較低的折射率時，薄膜層內的矽氧鍵結含量較高而矽氮鍵結、氮氫鍵結含量則相對較低，薄膜特性趨近於氧化矽薄膜。
光束傳播法(beam propagation method, 或簡稱BPM)是一套裝軟體，我們利用這套軟體模擬脊型波導結構的模態。由於脊型波導的優點在於可同時具備高折射率差異以及較大的波導單模傳播的截面尺寸，可使對光纖的耦合效率提高，導光能力上升。根據文獻，我們可以發現在特定的結構尺寸以及折射率的條件下，隨著傳播的距離的增加，高階模態會從脊型的兩側消散，形成單模傳輸。
經由BPM軟體的模擬結果，我們實作脊形波導，將1550nm以及633nm的雷射光經由光纖偶合器同時導入到波導的導光層，並觀察模態的圖形，並且量測經過波導的光損失。另外也利用透鏡將紅光雷射聚焦在脊型波導上，觀察模態的圖形，可以在屏幕上看到高斯分佈的單模圖案。
最後模擬當脊形波導達到單模傳輸時，進入更寬的截面積的脊型波導，形成多模干涉（Multi-Mode Interference，簡稱MMI）耦合器，並在未來加以實做量測之。

Silicon Oxynitride Based Waveguide Devices:
Numerical Simulation and Applications

Student : Chih-Kai Chang Advisor : Yan-Kuin Su Yun-Chung Chang Ricky Wenkuei Chuang

Institute of Electro-Optical Science and Engineering
National Cheng Kung University, Tainan701, Taiwan, R.O.C.

Abstract

We employ the plasma-enhanced chemical vapor deposition (PECVD) technique to controllably deposit SiON thin films by varying the ratio between oxygen and nitrogen. The advantages of using the SiON as the guiding layer include flexibility in adjusting the refractive index range and low optical propagation loss. On the device level, the single-mode rib waveguides are also successfully designed, fabricated and verified by simulation and experimental characterizations.
The SiON thin films with different compositions can be realized by selectively changing the gas ratio and their respective indices and thicknesses can be systematically determined using the combination of n&k spectroscopic analysis, ellipsometry, and prism coupling techniques. Furthermore, the Fourier Transform Infrared Spectroscopy (FTIR) are adopted in order to elucidate the chemical bonding characteristics of different SiON films.
For film with high refractive index, a FTIR spectrum similar to the one of SiO2 clearly exhibits the unique signature of Si-O stretching mode, while the Si-N and N-H stretching vibration modes are far less obvious.
We also use the beam propagation method (BPM) commercial-grade software package to design and simulate single-mode rib waveguide structures. The essences of this simulation rely on the fact that, under certain conditions involving selective rib waveguide dimensions and refractive index, the high order mode will gradually leak out laterally from the guiding rib into the slab region as the propagating length is increased. The field intensity thus obtained resembles a fundamental mode. The high index contrast and large cross section associated with SiON-based rib waveguides allow the single mode condition to be successfully achieved, which dramatically enhance the guiding ability and the coupling efficiency between the fiber and waveguide.
The simulation results mentioned previously are employed to design and fabricate a single-mode rib waveguide. The 1550 nm diode laser is subsequently fed into the rib waveguide using the optical lens and fiber coupler to observe and quantify the mode profile and the propagation loss. Finally, the aforementioned techniques are then applied to the design and fabrication of multi-mode interference (MMI) coupler. The characterization results performed thereafter would clearly demonstrate the power-splitting capability of this device.

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