| 摘要: | 本論文利用負型光阻SU-8做成一個光波導的微環形諧振器，由於諧振器能過濾特定波長的光波，因此可以透過使用多個微環形諧振器製造出高階數的光學濾波器，光學諧振器有小尺寸、低損耗、可整合光學系統的優點。除此之外，由於可以透過改變共振腔大小來改變共振波長，整個諧振器可以視為一個可調變的系統，這個特性可以用來製作某些力學、生物及化學感應器。本論文主要討論由SU8所製作之波導及其製程優化。研究的目的有三個，第一是以SU-8製程光波導的微環形諧振器測量本身波導的損耗與品質因子(quality factor, Q)，第二是當作其他材料製程光波導的微環形諧振器的擷取端(drop port)測量其損耗與品質因子。第三是SU-8波導可以設計為晶載的干涉儀，而可量測的範圍則受限於SU-8波導的色散。文中探討包括製程優化、光學量測、環形諧振器設計以及未來研究規劃，其中SU-8分別使用了SU-8 2000.5與SU-8 GM1040兩種不同的光阻來比較。本篇論文的模擬分析會在第二章製程當中討論，主要是探討不同厚度的SU-8與不同厚度的埋入氧化層對雷射通過波導後所產生的損耗，以及不同寬度的SU-8波導對能量的損耗，在單模的情況下，觀察其耦合的強弱。製程方面所使用的光刻技術為紫外光光刻技術(UV optical lithography)，不使用電子束(E-beam)的原因為其耗時過長，往往無法加快所需的產能與產量。光阻的選擇使用了SU-8 2000.5與SU-8 GM1040這兩種，由於兩者光阻型號不同，因此旋轉塗佈後能疊出的厚度也受到限制而會有所不同。曝光是以接觸式曝光(contact exposure)的方式，透過改變曝光時間，觀察光罩上波導寬度、間距與顯影後波導寬度、間距的差異，因為曝光存在著繞射效應的問題，因此本研究使用寬度尺寸為5μm的波導。量測的部分則是根據製程後的結果，探討雷射光耦合進入波導內的損耗以及產生穿透諧振。在橫向電場模態(Transverse Electric mode, TE mode)的情況下耦合進入微環形諧振器，主要探討的是100μm的環形半徑，所製造出來的微環形諧振器具有~2.24nm的自由光譜範圍(Free Spectral Range , FSR)，約為280 GHz。可以得到結論當SU-8的厚度越厚，光的損耗會越小；埋入氧化層的厚度越厚，也會使光的損耗越小。除此之外，透過光耦合進入波導並在不同的波長下量測，發現經過穿透端(through port)的波形有規律振盪，則可以得知該光波導具有諧振。在未來研究上，由於過去的研究中有擷取端設計之微環形諧振器，一般而言在腔內外累積之雷射能量可藉由耦合進入擷取端以量測諧振器行為，然而此方法會對諧振器內產生較多的損耗，因而造成較低的品質因子。因此本研究提供了利用後段製程的方式做出低損耗、可後製的SU-8光波導之可行性，這種可後製的SU-8光波導可以利用後段曝光製程，在量測完任意材料的微環形諧振器的品質因子後，再利用光阻去除的方式將SU-8光波導移除，同時達到量測諧振器內參數以及降低在偵測端對微環形諧振器之損耗。;This thesis discusses the measurement and fabrication process utilizing the negative photoresist SU-8 as an optical waveguide micro-ring resonator. Since the resonator can filter light of a specific wavelength, a high-order optical filter can be manufactured by using multiple micro-ring resonators. The optical resonator has the advantages of small size, low loss, and integration of optical systems. In addition, since the resonant wavelength can be changed by changing the resonator dimension, the resonator offers tunability for integrated photonics. This feature can be also used to make mechanical, biological, and chemical sensors. By utilizing the SU8 waveguide, there are a few potentials for integrated photonics. First, it provides a reconfigurable way to measure and to optimize the fabrication process of the optical waveguides. We discuss the fabrication process, waveguide loss, and quality factor of the micro resonators, providing the general waveguide parameters for the future reconfigurable design. Second, this study helps to build a reconfigurable in-line monitor platform for on-chip micro resonator (modulator). By fabricating a reconfigurable drop port and measuring the loss and quality factor of micro-ring resonators made of other materials (E.g. Silicon / silicon nitride), the SU8 waveguide can help to minimize the loaded effect on the micro resonator by later removing the drop port after the measurement. Last, the reconfigurable SU-8 waveguide provides the potential for on-chip interferometer by designing the proper resonator radius and the corresponding free-spectral range (FSR). In the thesis, we include the fabrication optimization of SU-8 waveguide, optical measurement, and preliminary results for a reconfigurable waveguide. In the experiments, two different photoresists of SU-8 2000.5 and SU-8 GM1040 are used for comparison. These photoresists provide different coating thicknesses, which strongly affect the waveguide behavior. Also, the simulation analysis of the propagated mode-profile will be discussed in the second chapter, by comparing the loss under different thicknesses of the SU-8 and of the buried oxide layer. In experiments, we will show the optical loss of the SU8 waveguide under different waveguide dimensions. Conventional UV optical lithography is used for waveguide fabrication, which provides better through-put for mass, rapid, and economic production for future usage. We use contact exposure method for UV patterning. By changing the exposure time, we observe the difference between the waveguide width and gap on the mask and the fabricated one after development. For optical measurement, we discuss the optical coupling, loss, and transmission resonance in the transverse electric mode (TE mode). By designing ring radius 100μm a free spectral range (FSR) of ~2.24nm is obtained corresponding to 280 GHz in frequency. We identify that lower optical loss can be achieved by either increasing the thickness of SU-8, or the thickness the buried oxide layer. The transmitted light is coupled into the waveguide and the corresponding transmission spectrum is measured in C-band. Resonance with Q up to 991 can be identified while the repetition rate (or FSR) is matched with our designed resonators. In the future study, this reconfigurable waveguide can be used in replacement of the regular drop-port waveguide, in which strongly degrades the quality factor in the resonator, or can be used as on-chip interferometry for optical frequency metrology. |
