利用粒子群優化演算法設計彎曲定向耦合型寬頻極化分離器

Design of Broadband Bent Directional Coupler Based Polarization Beam Splitters Using Particle Swarm Optimization Algorithm

范耿維(Keng-Wei Fan)

指導教授 ： 黃定洧

國立臺灣大學/電機資訊學院/光電工程學研究所/碩士(2019年)

https://doi.org/10.6342/NTU201904320

摘要

近年來隨著資料傳輸量需求的增加，用積體光路取代傳統電路來傳輸資料已成為新的趨勢，在積體光路中因為 SOI 光波導相容於現有成熟的 CMOS 製程技術而被廣泛使用，然而次微米等級的 SOI 光波導本身存在相當嚴重的極化相依問題，解決此問題較通用的做法為使用極化分集系統，而極化分離器為其中不可或缺的元件之一，為了同時滿足低製造成本與涵蓋寬頻帶，小尺寸寬頻極化分離器的設計成為一個關鍵的命題。彎曲定向耦合器因為其結構簡單且能有效讓兩種極化相位匹配條件顯著的不同，因此適合用來設計極化分離器，本論文將彎曲定向耦合器細分成若干小段，利用自訂的粒子群優化演算法優化每小段的結構參數，由於利用三維時域有限差分法進行優化相當費時，為了提高優化過程的效率，本論文使用轉移矩陣形式的彎曲波導中的耦合模態理論以及建立相關係數的查找表來快速求得元件的頻譜響應並進行元件參數的優化，優化後的元件再利用三維時域有限差分法對頻譜響應進行驗證，並且探討製程容忍度以及透過寬度漸變波導連接到單模波導之總體元件響應。
本論文針對兩種不同的頻寬標的 (100 nm 以及 200 nm) 的元件進行優化 (分別命名為元件一與元件二)，兩種元件的行經角度皆為 140 度，中心的曲率半徑為 20 μm，對應元件長度為 37.588 μm，對元件一而言，優化後在 1500–1600 nm 波長範圍內 TM 與 TE 的消光比皆可高於 21 dB，插入損耗皆可低於 0.11 dB，對元件二而言，優化後在 1450–1650 nm 波長範圍內 TM 與 TE 的消光比皆可高於 15.85 dB，插入損耗皆可低於 0.19 dB。此外，本論文也探討光行進方向上的波導寬度製程誤差，在製程造成的寬度誤差正負 10 nm 之內，元件一 TM 與 TE 的消光比皆可大於 16.27 dB，元件二 TM 與 TE 的消光比皆可大於 13.47 dB。為了相容於 IMEC 的標準元件庫，因此在設計的元件輸入與輸出端加上低插入損耗的寬度漸變結構做連接，並進一步分析其總體效能。在加上寬度漸變結構後，元件一 TE 與 TM 的消光比皆可高於 20.51 dB，插入損耗皆小於 0.12 dB，元件二 TE 與 TM 的消光比皆可高於 15.45 dB，插入損耗皆小於 0.2 dB。本論文所設計的元件與其他文獻相比，在更寬的操作頻譜範圍內仍有著高消光比，且較低的插入損耗。

關鍵字

粒子群優化演算法 ； 彎曲定向耦合器 ； 寬頻 ； 極化分離器 ； 最佳化

並列摘要

With the growth of data transmission demand in recent years, the data transmission by using optical interconnects has become an emerging trend. Among them, the silicon on insulator (SOI) waveguides have been widely used owing to the compatibility with the mature CMOS manufacturing technologies. However, the submicrometer-sized SOI waveguides are inherently highly polarization-dependent. To address this issue, a general solution is to utilize a polarization diversity system, in which the polarization beam splitters (PBS) are essential components. To achieve the required device performance, especially over a broad operation bandwidth with low manufacturing cost, the design of a compact and broadband PBS becomes a crucial issue. Bent directional coupler is suitable for designing PBS owing to simple device structure and providing significant different phase match conditions for the two polarized modes effectively. In this thesis, the bent directional coupler is segmented into short sections and the geometric parameters are optimized by a customized particle swarm optimization algorithm. To enhance the computational efficiency for the optimization process without carrying out the time-consuming three-dimensional finite-difference time-domain (3D FDTD) calculations, the spectral responses of the devices can be determined by using the coupled mode theory (CMT) in the transfer matrix form for bent waveguides. The lookup tables for the CMT-related parameters are also constructed to accelerate the optimization process. The spectral responses of the optimal devices are further verified using the 3D FDTD simulations. The fabrication tolerance and the overall responses of the optimal devices connected with standard single mode waveguides using tapers are also analyzed.
In this thesis, two different targets corresponding to the operating bandwidths of 100 nm and 200 nm are chosen for the optimal devices, named Device 1 and Device 2, respectively. These two devices have bent directional coupling sections of 140-degree arc angles and 20-μm-long central radius of curvature, resulting in 37.588-μm-long device lengths. For Device 1, the extinction ratio (ER) for both TM and TE modes is higher than 21 dB, and the insertion loss (IL) for them is lower than 0.11 dB for the operating wavelength range 1500 – 1600 nm. For Device 2, the ER for both TM and TE modes is higher than 15.85 dB, and the IL for them is lower than 0.19 dB for the operating wavelength range 1450 – 1650 nm. In addition, the variation of waveguide width along the propagation direction due to fabrication error is considered in this thesis. With the fabrication error of ±10 nm, the results show that the ER for both TM and TE modes of Device 1 is higher than 16.27 dB, and the ER for both TM and TE modes of Device 2 is higher than 13.47 dB. In order to be compatible with IMEC standard library, the low-loss tapers are connected to the input and output ports of the designed devices and their overall performances are further analyzed. The result shows that the ER for both TM and TE modes of Device 1 is higher than 20.51 dB and the IL for them is lower than 0.12 dB. The ER for both TM and TE modes of Device 2 is higher than 15.45 dB and the IL for them is lower than 0.2 dB. Compared to the polarization beam splitters shown in previous literatures, the devices presented in this thesis exhibit high extinction ratio and lower insertion loss in the broader operating bandwidth.

並列關鍵字

Particle Swarm Optimization Algorithm ； Bent Directional Coupler ； Broadband ； Polarization Beam Splitter ； Optimization

參考文獻

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