液晶填充分佈式布拉格反射鏡波導; Liquid Crystal Infiltrated Waveguide with Distributed Bragg Reflectors

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题名:	液晶填充分佈式布拉格反射鏡波導;Liquid Crystal Infiltrated Waveguide with Distributed Bragg Reflectors
作者:	潘弘毅;Hung-yi Pan
贡献者:	光電科學研究所
关键词:	可調變式元件;液晶;中空光波導;tunable devices;liquid crystal;hollow waveguides
日期:	2011-07-27
上传时间:	2012-01-05 14:35:53 (UTC+8)
摘要:	在本論文中，我們提出了液晶灌注於布拉格反射鏡波導的結構，來製作一種可調變式的波導元件。藉由外加電壓的影響，來改變液晶分子排列與波導內部核心(core)的折射率變化。並且運用布拉格反射鏡的光波長選擇性，期望能將波導作為可電調控式的光開關與光學濾波元件。該元件中的波導包覆層(cladding)為布拉格反射鏡，乃經由二氧化矽(silicon dioxide, SiO2)與氮化矽(silicon nitride, Si3N4)兩種膜層堆所組成的高反射率之布拉格反射鏡，其原理是利用干涉光學將光侷限於液晶核心中。在膜層設計上，我們利用傳遞矩陣法(transfer matrix)來決定膜層結構，再藉由光束傳播法(Beam Propagation Method)模擬計算元件的侷限效果。在量測方面，(1)我們運用偏光顯微鏡架構來觀察不同電壓下波導中的液晶形態，藉由光偵測器來量化所看到的情形，進而臆測液晶傾角與電壓的關係。(2)我們量測在532nm波段的傳輸行為，發現不論任何角度偏振的光源耦合至波導，該波導輸出端的強度都會因外加電壓增加而衰減；當外加電壓為9Vrms時，輸出端的強度為最弱，且消光比(extinction ratio, ER)達到28dB，可作為光開關。(3)我們量測波導的可見光波段頻譜與電壓之影響，發現波導的傳輸頻譜會因為電場作用下而有了選擇性濾波的功能，並隨著電壓增加而輸出光強逐漸消逝。最後施加至30Vrms時，波導會變為可侷限全可見光範圍之傳輸頻譜的現象。我們的模擬及實驗結果，期望可幫助設計與製作以液晶灌注於布拉格反射鏡波導為基礎之可調式光電元件。 In this study, we propose the liquid crystal infiltrated waveguide with distributed Bragg reflector (DBR), which enables a dynamic control of tunable waveguide. The reorientation of the high susceptibility of liquid crystal (LC) molecular is successively achieved by varying external voltage to change the refractive index of the guiding core. Due to the electrical tuning of LC and the wavelength-selective property of DBR structure, the waveguide can be used as the tunable switching or the filter device. In the cladding of DBR structure, the multilayer stacks are formed by six pairs of Si3N4/SiO2, which is designed by the transfer matrix method. Also, we use the beam propagation method (BPM) to simulate the light behavior for different indices of LC in the guiding core. In experimental parts, we utilize three approaches to realize the characteristics of LC infiltrated waveguide with DBR. First of all, we use the polarized optical microscopy to observe and speculate the alignment of LC. Secondly, in the measurement of waveguide with different applied voltages, which is operating at 532nm, the waveguide can be served as an electrically optical switch with 28dB attenuation at 9Vrms. Finally, we measure the transmission spectrum of waveguides at visible wavelength range for varying applied voltages. The result shows that the LC in guiding core can change the transmission spectrum of the LC-filled waveguide. In the voltage-off state, the wavelength from 500nm to 660nm can be confined in the guiding core. As the applied voltage increases, the bandgap edge can be shift to the short wavelength. Specifically, while the external voltage is from 9Vrms to 20Vrms, the propagated light cannot be confined in the guiding core. At the external voltage of 30Vrms, we measured the all of visible wavelength that can be confined in the guiding core. The results of our simulation and the experiment measurement can help us to design and fabricate tunable devices based on the liquid crystal infiltrated waveguide with DBR.
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