無極化相依性之SOI波導光柵耦合器之研究

本篇論文介紹許多以矽為基材的光子導線之發展與研究，並對無極化相依性光柵耦合器作進一步的理論分析及最佳化設計。不需要其他的製程步驟，光柵耦合器亦可被設計擁有其他的附加功能如聚焦和光分離。光柵耦合器所擁有的這個複合功能特色，可以減少或甚至移除多餘的光波導元件。光柵耦合器的一個重要限制是它們通常只設計給單一特定極化的入射光所使用。這是一個嚴重的缺點，因為在大部分狀況下由光纖出來的光，其極化是未知且很有可能是隨時間持續變化的。光柵耦合器一般來說對於極化有很高的敏感度，較不適合用在當只要設計規格不同就會有不同的極化光源的狀況，因此在實際的光纖系統中是不實用的。在本論文中，我們提出一光柵耦合器，在設計上同時針對兩個正交的入射光極化作最佳化處理，且入射光是打在一個二維的次波長光柵結構上。而我們的光柵結構在設計上縱向是由繞射性光柵組成，橫向則是由非繞射性次波長光柵結構所組成。因此橫向的結構可視為等效折射率之材質。而如此一個無極化相依性光柵耦合器可以藉由重疊兩個光柵耦合器而得，此兩個光柵耦合器是分別針對TE和TM兩種不同極化所設計。就結果顯示極化的反應會隨著二維光柵的縱向和橫向結構變化而呈高度的可調變性。因此無極化相依性是可以達成的，當光柵耦合器是設計來對兩個正交的入射光極化有相同的反應。我們使用的模擬軟體包含R-Soft及MathCAD。先利用MathCAD計算理論公式的推導過程，再將整個結構以時域有限差分（FDTD）進行數值模擬分析。設定光纖基礎模態為光源，向波導表面近似垂直傳播，經由光柵耦合後進入波導。最後我們使用微基因演算法進行最佳化設計以求得最佳的耦合效率。

關鍵字

積體光學；光纖；矽光子導線；光柵耦合器；非極化相依性

並列摘要

This thesis presents recent approaches in the development of a Si-based waveguides system for submicrometer devices. We focus on theoretical derivation and electromagnetic wave field analysis of the polarization-independent grating couplers to obtain the optimal design. Without additional steps in the fabrication of these devices, the grating couplers can also be designed to exhibit auxiliary functions such as focusing and splitting of the coupled light. The multifunctional capability of grating couplers may reduce or eliminate the need for additional optical waveguide components. An important limitation of these grating couplers is that they are designed solely for a specified polarization of the incident light. This may be a serious drawback that the polarization of the input light from an optical fiber is in most cases unknown and may even vary with time. Because the grating coupler is generally highly sensitive to the polarization, it will not work properly when input light having a polarization different from that given in the design specification, and thus it is impractical in a real optical fiber system. In this thesis we present the couplers designed by simultaneous optimization for two orthogonal polarization states of the light incident on the two-dimensional subwavelength gratings (SWGs). The grating structure is designed where the diffractive grating is formed in the longitudinal direction, whereas the nondiffractive SWG structure in the lateral direction acts as effective medium of the grating. Such a polarization independent grating coupler can be achieved by superimposing the grating couplers which were originally designed for coupling solely the TE and the TM modes. The results show that the polarization response is highly flexible as the longitudinal and lateral grating structures vary. Polarization independence can be achieved when the coupler is designed to have the same response for coupling the two orthogonal polarizations of the incident light into the TE and TM modes, respectively, of the waveguide. We use the software include R-Soft and MathCAD. First, MathCAD is used to calculate the theoretical derivation of the formulae. The entire structure is then analyzed numerically by using the finite-difference time-domain method (FDTD) and a fundamental mode of the optical fiber nearly incident toward the grating coupler and finally coupled into the SOI waveguide. Finally, we also used the micro genetic algorithm to optimize the coupling efficiency.

並列關鍵字

integrated optics ； fiber ； silicon photonic wire ； grating coupler ； polarization independence

參考文獻

[10] Tai Tsuchizawa, Koji Yamada, Hiroshi Fukuda, Toshifumi Watanabe, Jun-ichi Takahashi, Mitsutoshi Takahashi, Tetsufumi Shoji, Emi Tamechika, Sei-ichi Itabashi, and Hirofumi Morita, “Microphotonics Devices Based on Silicon Microfabrication Technology,” IEEE J. Quantum Electron., vol. 11, pp. 232–240, Jan./Feb. 2005.

[1] H. Nishihara, “Recent advancement on optical integrated circuits,” Conference on Computer and Communication Systems, 1990.

[2] R. Soref, “The Past, Present, and Future of Silicon Photonics,” Selected topics in quantum electronics, Vol. 12, No.6, pp. 1678–1687, 2006.

[3] M. Naydenkov, and B. Jalali, “Advances in Silicon-on-Insulator Photonic Integrated Circuit (SOIPIC) technology,” International SOI Conference, 1999.

[4] B. Jalali, and S. Fathpour, “Silicon Photonics” Lightwave tech., Vol. 24, 2006.

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無極化相依性之SOI波導光柵耦合器之研究

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