| 摘要: | 偏振控制整合平台是矽光子中的重要元素。這些系統具有廣泛的應用，包括高速電信、量子技術以及感測。然而，大多數矽光子整合光路（PIC）依賴熱光效應（Thermo-Optics effect）進行相位調控，導致反應延遲與功耗升高。為了克服此類限制，研究者提出並探討了利用二階非線性材料的電光整合電路方法。其中，鈮酸鋰（Lithium Niobate）具有高二階非線性係數 χ^(2) 以及強電光（Electro-Optics）係數，使其成為整合至矽光子系統的理想材料。
在本研究中，我們探討了在一般鈮酸鋰（CLN）與薄膜鈮酸鋰（LNOI）波導平台上的偏振旋轉器或分光器。我首先描述在晶片上利用電光機制進行偏振調制。隨後的部分則闡述偏振分光器的漸近，並展示在鈮酸鋰上的其他應用。
首先，第一個元件是建立於鈦擴散週期性極化鈮酸鋰（PPLN）上的電光相位調制轉換器。該元件展現了約 20 V 的半波電壓以及約 2.6 nm 的頻寬轉換。偏振分光器在 1500 至 1640 nm 的光譜範圍內，對兩種不同的偏振模式（TE0 與 TM0）展現了超過 99% 的分光效率。
在第二個設計中，我利用基因演算法(GA)提升了轉換頻寬，並實現了一個非週期性極化鈮酸鋰（APPLN）電光偏振模式轉換器（EOPMC），其頻寬大於 13 nm。當其與自發參數下轉換（SPDC）整合時，元件突顯其在量子技術中的應用潛力。
第二階段是將技術轉移至 LNOI 基板，該基板提供更佳的光學侷限與更小的模態尺寸，因而縮小整體面積。同時也提升了整合密度、非線性響應以及電光轉換效率。在 LNOI 的實驗上相當成功，2.64 mm 的 PP-LNOI 元件能夠進行 TE/TM 模式的電光轉換，並展現超過 20 dB 的峰值效率。
在最終的設計中，我們展示了鈮酸鋰的多功能性，透過 LNOI 絕熱耦合器建立了一個不分偏振的分光器。我們提出在 LNOI 脊波導上實現不分偏振的分光器，其操作頻寬為 1500–1600 nm。該元件利用光束傳播模擬方法設計，並透過最佳化的黃光製程與蝕刻在 LNOI 上製作，隨後進行系統性的光學特性量測；TE模式的傳播損耗低至 1.63 dB/cm。實驗結果顯示，在所述波長範圍內具有穩定的偏振性能與分光行為，表明該平台適用於寬頻、偏振獨立的整合光子系統。此成果得益於對脊波導幾何結構的精確控制以及最佳化的乾蝕刻參數。
;Polarization Control Integrated Platforms are substantial elements in silicon photonics. These systems have an extensive variety of applications, which include high-speed telecommunications, quantum technologies and sensing. However, most of the silicon photonics integrated circuit (PIC) rely on the thermo‑optic (TO) effect for phase tuning, which leads to delayed response and elevated power consumption. To overcome such limitations, electro‑optic integrated‑circuit approaches utilizing second-order nonlinear materials are proposed and investigated. Among them, Lithium niobate possesses high second-order nonlinearity χ^((2) ) and a strong electro‑optic (EO) coefficient. These attributes make lithium niobate an attractive material for incorporation into silicon photonics systems.
In this study, we investigate polarization rotator/splitter in congruent Lithium Niobate (CLN) and Lithium Niobate on insulator (LNOI) waveguides platforms. I first describe the theory of Polarization modulation at the chip level utilizing electro‑optic mechanisms. The subsequent section will outline the adiabatic theory underlying polarization splitters and demonstrate its application in lithium niobate.
The initial component is an electro‑optic phase‑modulated converter developed on titanium‑diffused periodically poled lithium niobate (PPLN). The device exhibits a half‑wave voltage of approximately 20 V and a conversion bandwidth of about 2.6 nm. The polarization splitter demonstrates >99% splitting efficiency for two distinct polarization modes (TE0 and TM0) throughout the spectral window from 1500 to 1640 nm.
By utilizing a genetic algorithm in the second design, we enhanced the conversion bandwidth and realized an aperiodically poled lithium niobate (APPLN) electro‑optic polarization mode converter (EOPMC) with bandwidth greater than 13 nm. When integrated with spontaneous parametric down‑conversion (SPDC), the device yielded encouraging results, highlighting its applicability to quantum technologies.
The second stage of development involved transferring the technique to an LNOI substrate, which provides superior optical confinement and smaller mode dimensions, thereby reducing the overall footprint. This approach also enhances integration density, nonlinear response, and electro‑optic efficiency. Implementation on LNOI proved successful, with the 2.64‑mm PP‑LNOI device enabling electro-optics TE/TM‑mode conversions and demonstrating a peak efficiency greater than 20 dB.
In the final design, we demonstrate the versatility of lithium niobate by establishing a polarization‑insensitive beam splitter via an LNOI adiabatic coupler. We report a ridge lithium‑niobate on insulator (LNOI) waveguide implementation of a polarization‑insensitive beam splitter with a measured operational bandwidth of 1500–1600 nm. The device was designed using beam propagation simulation method and realized through optimized lithographic patterning and etching on LNOI, followed by systematic optical characterization; the transverse‑electric (TE) mode propagation loss was measured as low as 1.63 dB/cm. Experimental results demonstrate robust splitting behavior across the stated wavelength range with stable polarization performance, indicating the platform’s suitability for broadband, polarization‑independent integrated photonic systems. This outcome is enabled by precise control of ridge waveguide geometry together with optimized dry‑etch parameters. |
