脊狀波導的洩漏模態

Abstract

中文摘要 光波導在傳輸過程中，若光場的侷限不佳，容易因繞射效應而向外洩漏，造成傳輸損耗。為探討此問題，我們利用 RSoft CAD 設計製作多層結構之脊脊狀波導（Rib Waveguide），並以 BeamPROP 進行數值模擬計算，找出該波導的模態及對應的場分佈，並分析入射波位置對耦合程度的影響。我們分別以疊代法（iterative method）與相關法（correlation method）求解波導模態，前者求得之基模並未收斂，意味著疊代法無法有效處理洩漏模，後者則進一步求得 m = 0、1、2 三種有效折射率含虛部的洩漏模態。模擬得到的場分佈顯示，光源置於脊狀波導中心時透過與基模耦合，系統的耦合效率最高，而光源位置適度偏移仍可藉由二階模態維持傳輸功率，證實此脊狀波導結構在低損耗與對準容忍度上具有應用價值。

Abstract During the propagation in optical waveguides, light that is not properly confined will leak outward due to diffraction, resulting in transmission loss. To address this issue, we used RSoft CAD to design a multilayer rib waveguide and employs BeamPROP to calculate the waveguide's modes and field distributions. The waveguide modes are solved using both the iterative method and the correlation method: the former yields a fundamental mode whose electric field dose not converge at the domain boundary, while the latter identifies three leaky modes (m = 0, 1, 2) whose effective indices possess non-zero imaginary parts. The calculated mode distribution indicates that the coupling efficiency is highest when the light is launched at the center of the rib waveguide to excite the fundamental mode. By coupling to the second-order mode, moderate shift of launching position does not change the transmitted power significantly. Out studies confirm the practical value of rib waveguide in achieving low loss and good alignment tolerance.