介電泳致動之液態纖芯-液態包層光波導的光傳播損耗量測

Abstract

本論文延續先前本實驗室已建立的液態纖芯-液態包層光波導(Liquid-core/liquid-cladding optical waveguide, L2 WG)架構，在兩片濺鍍氧化銦錫(Indium Tin Oxide, ITO)導電電極之玻璃板間形成一個虛擬微流道，並利用介電泳(Dielectrophoresis, DEP)力在充滿包層液體：20 cSt矽油(Silicone oil, n = 1.401, □ = 2.50)的環境中驅動與之不相溶且介電係數較高的纖芯液體：□-丁內酯(□-Butyroluctone, GBL, n = 1.442, □ = 41.65)形成液態纖芯-液態包層光波導系統。利用介電泳力驅動之液態纖芯-液態包層光波導(DEP-driven liquid-core/liquid-cladding optical waveguide相較於一般在聚二甲基矽氧烷(Polydimethylsiloxane, PDMS)微流管道形成的液態纖芯-液態包層光波導，除了不需要實體流道之外，且不需額外補充液體和外加幫浦的設置。本論文試圖建構一套適用於本實驗室所建立的液態纖芯-液態包層光波導系統的光傳播損耗量測系統。因此為了方便觀測，在纖芯液體中溶入螢光染料-Rhodamine 6G(λexcitation = 510 ~550 nm/λemission = 590 nm)。將入射光源以垂直光波導系統平面的方向耦合光源，限制只有螢光被激發之激發光源波長於其間傳導，並沿著液態纖芯平行移動入射光源與偵測光強度儀器之相對位置，作為量測的基本架構。參考現今各式光波導光傳播損耗量測系統，並考量介電泳驅動之液態纖芯-液態包層光波導系統與光傳播損耗量測所需的儀器架設結合與方便性，本論文設計以下量測系統：(1)利用影像擷取單元(Charge-couple device, CCD)之散射光量測法，曝光時間9.9 s，量測直線光波導(寬 x 高= 150 □m x 100 μm)進行量測，得到1.85 dB/cm的傳播光損耗。然而影像擷取單元所量測的傳播光損耗並非只針對纖芯與包層液體間的散射偵測，可能包括ITO玻璃間的散射情形。(2)在光波導的尾端出口處放置光功率計(Power meter)或光譜儀(Spectrometer)偵測其相對入射光源位置的光強度變化，量測與上述方法相同條件的光波導，得到2.1 dB/cm的傳播光損耗。在本論文利用MATLAB模擬光波導尺寸對光傳播損耗的變化曲線，整理光譜儀之仿剪斷法的量測結果，符合模擬趨勢，並驗證改變入射光強度不會影響光傳播損耗量測結果。0.1 mM螢光濃度量測到的光傳播損耗最小，推估是在此濃度下螢光自吸收與散射的綜合影響最少所致。針對介電泳致動之液態纖芯-液態包層光波導設計的量測系統，基本上沒有問題，但量測的光傳播損耗結果仍是大於預期，推測其問題應是來自光波導系統架構的設計：此時部分的耦合光源無法被限制在纖芯液體中，光在纖芯與包層液體的界面經由Teflon與ITO洩漏至玻璃間傳導，耦合光源在玻璃間傳導不但使得液態纖芯-液態包層光波導系統的光傳播損耗增加而且增加光傳播損耗的量測困難度。

We measure the propagation loss of dielectrophoresis (DEP)-driven liquid-core/liquid-cladding optical waveguides (L2 optical WGs) previously studied in our laboratory. The tested L2 optical WGs consist of immiscible liquids core liquid, □-Butyroluctone (GBL, n = 1.442, □ = 41.65), and cladding liquid, 20 cSt Silicone oil (n = 1.401, □ = 2.50), sandwiched between two Indium Tin Oxide (ITO) glasses. In comparison to the most adopted L2 optical WGs formed by the laminar flow in a microfabricated polydymethylsiloxane (PDMS) microchannel, DEP-driven ones requires a limited volume of liquids actuated in a virtual microchannel without physical channel walls and external micropumps. Currently, optical property measurements, especially the propagation loss measurement, were developed based on PDMS L2 optical WGs systems. Here we investigate a proper propagation loss measurement method for the DEP-driven L2 optical WGs. A fluorescent dye, Rhodamine 6G (λexcitation = 510 ~550 nm/λemission = 590 nm), was added into the core liquid. A laser was shined from the bottom glass plate to excite the fluorescent dye in core liquid at the vertical direction. Two different measurement methods were investigated. (1) We first utilized a charge-couple device (CCD) for scattering loss measurement of a straight line WG (length x width = 40 mm x 150 μm). Under the CCD exposure time of 9.9 s, the measured propagation loss was 1.85 dB/cm. Because the CCD would detect the scattered light from not only the WG but also the whole device including the glass plates, the measurement variation was large. (2) Power meter or spectrometer was then used to replace the CCD and detect the output power intensity at the end of the L2 optical WGs while the exciting laser was shined at different locations of the WGs. The propagation loss was measured about 2.1 dB/cm with the same WG condition used in the CCD measurements. MATLAB software was used to simulate the influence of the WG size to the propagation loss. The propagation loss measurement results with a spectrometer accorded to the simulated size effects and also confirmed the input light intensity wouldn’t cause different measurement results. From the concentration of core liquid point of view, the propagation loss of 0.1 mM Rhodamine 6G was much lower than 1 mM and 0.01 mM, the reason might be the combined effect of self-absorption and scattering is reduced. Overall, our experiment results have shown our propagation loss measurement systems are able to perform the correct measurements of the DEP-driven L2 optical WG either with CCD or spectrometer. Nonetheless, the propagation loss of DEP-driven L2 optical WG is larger than we expected. We believe the major reason is the structure design of the optical WG. The current DEP-driven L2 optical WG is pretty much a two dimensional structure because the cladding liquid between the core liquid and the glass is too thin to be able to confine light effectively. Certain part of the coupled light wave would leak out from the core liquid through cladding, Teflon into the glass. The light wave propagates inside the glasses not only caused the bigger propagation loss of the optical WG but also caused great difficulties in the overall propagation loss measurements.