具有次波長光柵結構之緊湊型寬頻分光器的優化設計

Abstract

矽光子積體迴路是目前正在蓬勃發展的一個技術，而在矽光子晶片中會使用大量的分光器作為其中作為訊號分配的元件，為了因應未來在分波多工的系統架構中同時會有多個波長在傳輸訊號經過這類分光元件，因此擁有寬頻和小尺寸的設計變得不可或缺。過去定向耦合器是最常被廣泛應用的，然而它卻對波長相依性非常敏感。本篇論文為了要改善波長相依性的問題，採用次波長光柵結構結合定向耦合器來做優化設計以解決此問題。 本論文所探討的次波長光柵結構結合定向耦合器之寬頻分光器，在設計階段先透過能帶結構的方法找到波長相依性較低的結構參數，再以二維等效時域有限差分法模擬矽波導與次波長光柵結構所設計之分光器的初估效能，以及使用三維時域有限差分法分析頻譜響應並進行驗證，最後利用參數掃描優化元件的結構參數。 本論文以極緊湊型寬頻 3-dB 分光器作為設計標的，其優化後元件之耦合長度為 3.9 μm，在中心波長為 1550 nm 的條件下，輸出波導穿透率在 3 ± 0.5 dB 的範圍內有 100 nm 的頻寬，且過量損耗皆小於 0.212 dB。所設計的優化元件在製程容忍度方面，包括傳播方向上光波導的寬度誤差 (450 ± 10 nm)、次波長光柵週期誤差 (200 ± 10 nm) 和高度誤差 (220 ± 5 nm) 對元件特性造成的影響也進行完整的探討。此元件在製程所造成的誤差範圍內，輸出波導穿透率在 3 ± 0.7 dB 的範圍內有 100 nm 的頻寬，且過量損耗皆低於 0.32 dB。相比於其他文獻，本研究所設計的元件有最短的耦合長度和極低的過量損耗，並在極緊湊的分光器尺寸仍然維持寬頻的效果。

The field of silicon photonic integrated circuits is currently undergoing rapid development. Within silicon photonic chips, a large number of power splitters are utilized as signal branching components. To accommodate future system architectures using wavelength division multiplexing (WDM), where multiple signals at different wavelengths are transmitted through such power-splitting devices, designs with broadband and compact sizes are becoming essential. In the past, directional couplers were the most commonly used, but they are highly sensitive to wavelength. To address this issue, this thesis employs subwavelength grating structure in a directional coupler to optimize the design and mitigate the wavelength dependence problem. For the design of the braodband power splitter, which consists of directional couplers combined with the subwavelength grating structure in this thesis, a band structure method was used to identify proper structural parameters with low wavelength dependence at first. Then, two-dimensional equivalent finite-difference time-domain (FDTD) simulations of silicon waveguides and the subwavelength grating structure are used to evaluate the approximate performance of the power splitter, while three-dimension FDTD analysis is applied to examine the frequency response and validate the design. Finally, sweep of parameters is conducted to optimize the structural parameters of the device. The target of this thesis is an ultra-compact broadband 3-dB power splitter. After optimization, the coupling length of the device is 3.9 μm, with a center wavelength of 1550 nm. The bandwidth of the transmitted signals at both output waveguides within 3 ± 0.5 dB is 100 nm, and the excess loss is less than 0.212 dB. In this thesis, the device’s fabrication tolerances, including errors in waveguide width (450 ± 10 nm), period of the subwavelength grating (200 ± 10 nm) and height (220 ± 5 nm) along the propagation direction, are also evaluated. Within the range of fabrication errors, the bandwidth of the transmitted signals at both output waveguides within 3 ± 0.7 dB is 100 nm, and the excess loss is lower than 0.32 dB. Compared to other studies, the splitter designed in this thesis has the shortest coupling length and the lowest excess loss while maintaining broadband performance in an ultra-compact size.