| 摘要: | 本論文提出一「矽光波導準TE模態至混合電漿波導準TM模態」之極化模態轉換器，其運作原理為於模態轉換區域引入金屬材料以劇烈破壞結構對稱性，使得極化模態轉換器中之準TE模態電場方向受其影響而旋轉，最後於極化模態轉換器之輸出端形成混合電漿波導之準TM模態。藉由分析轉換結構之極化消光比與傳播損耗，逐一完成轉換結構各段之設計，於波長1550 nm時，極化模態轉換器之最佳化設計為上層具四段金屬結構之「線性漸窄-直線-線性漸窄」介質波導，其長度僅為7 $\mu$m，寬度小於400 nm，而輸出端模態轉換效率、極化消光比與極化轉換效率分別為88.80$\%$、25.0929 dB、99.82$\%$，同時插入損耗僅有0.5108 dB。
研究顯示當金屬厚度愈薄時，其對結構對稱性之破壞程度較低，使得拉動輸入端介質波導之準TE模態電場旋轉之能力愈差，故於極化模態轉換器輸出端之準TM模態功率所佔傳播方向總功率比例減少，造成輸出端模態轉換效率愈低而傳播損耗與插入損耗愈高。此外，經最佳化設計之極化模態轉換器於光通訊波段內對波長之變化不甚敏感。以模態轉換效率高於80$\%$即模態轉換損耗低於0.9691 dB為基準，其傳輸頻寬可達103.8 nm (1490.9--1594.7 nm)；而以插入損耗低於1 dB為基準，其頻寬為105.8 nm (1490.0--1595.8 nm)。另一方面，對金屬結構容差之分析顯示，以模態轉換效率高於80$\%$為基準時，金屬厚度、直線區域金屬寬度與第一段線性漸寬金屬尖端寬度之容差範圍分別為70、75與100 nm。
未來於光子積體光路內，矽波導與混合電漿波導之混合應用為必要趨勢以達到高密度整合，故本研究之「光波至混合電漿波導」極化模態轉換器具前瞻性，並對光子積體光路領域具些許之貢獻。;In this research, a novel and ultracompact polarization mode converter (PMC) based on mode evolution that converts a quasi-TE$_{00}$ mode in a silicon strip waveguide to a quasi-TM mode in a hybrid plasmonic waveguide (HPW) has been proposed and numerically investigated. A four-section metal structure is introduced asymmetrically/symmetrically on top of a three-section dielectric (Si/SiO$_2$) waveguide to break the structural symmetry drastically. The dominant electric field of the quasi-TE$_{00}$ mode rotates as a result of the excitation of the hybrid plasmonic mode and eventually convert to the polarization state suitable for the quasi-TM mode of the HPW at the PMC output end. By analyzing the polarization extinction ratio (PER) and the propagation loss (PL) across the PMC, the preliminary design of each section is obtained followed by the finite-difference-time-domain method simulations with a mesh size of $\Delta x$ = 10 nm, $\Delta y$ = 4 nm, and $\Delta z$ = 10 nm for more accurate results. The footprint of the optimized PMC is $<$ 7 $\times$ 0.4 $\mu$m$^2$ and the corresponding mode conversion efficiency (MCE), PER, insertion loss (IL), and the polarization conversion efficiency (PCE) are 88.80$\%$, 25.0929 dB, 0.5108 dB and 99.82$\%$, respectively.
The effect of the top metal (Ag) layer is also investigated. The results show that as the Ag film thickness becomes smaller, its capability to rotate the quasi-TE$_{00}$ mode diminishes. Therefore, the ratio of the power of HP quasi-TM mode to the total $z$-directed power decreases at the PMC output end, leading to a smaller MCE and a higher IL. The optimized PMC is shown to be insensitive to wavelength over the span of entire C band. The 80$\%$ bandwidth for the MCE is 103.8 nm (1490.9-1594.7 nm) and the 1-dB bandwidth for the IL is 105.8 nm (1490.0-1595.8 nm). On the other hand, the fabrication tolerances of the Ag film thickness, the straight Ag strip width, and the Ag tip width are 70, 75, and 100 nm for MCE over 80$\%$, respectively.
In the future, it is expected that the combined Si waveguide and HPW would play a key role in achieving highly-integrated photonic circuits. Hence, the research work presented in this thesis may see great potentials and contributions to the field of integrated optics. |
