在本研究中，我們提出一個設計及優化環型共振腔的方法。首先，在環形共振腔外圍包裹一彎曲載波波導可以形成滑輪型耦合器並有效降低環形共振腔的彎曲損耗。滑輪型耦合器因具有較長的耦合區域可以減少環形共振腔的輻射損耗進而提高Q值。第二，載波波導與環形共振腔之間的傳遞常數愈大愈可以將光儲存在環形共振腔中。第三，可以藉由調整載波波導的寬度快速達到臨界耦合以增加Q值。根據以上幾點，我們比較其他種類的環形共振腔可以得知滑輪型環形共振腔可以提供較高的Q值。 此外，為了定義滑輪型耦合器的耦合長度，我們提出以保角映射法結合亥姆霍茲方程式來計算耦合長度。而此方法與有限時域差分法所計算出的耦合長度有很好的一致性。 為了實現高Q值的積體化元件，我們使用電子束微影及反應式離子蝕刻將滑輪式環形共振腔及光柵耦合器定義在SOI晶片上。其中光柵耦合器可藉由繞射原理將光引導至載波波導中，進而再由載波波導耦合至環形共振腔中。經由量測可以得到的Q值約173000。 目前文獻中，具有高Q值的滑輪型環形共振腔其半徑約在20 um，而Q值約落在20000 到 600000 之間，此外為了提高Q值，可以藉由photoresist reflow process 使波導側壁較為光滑以減少散射損耗或是藉由覆蓋一層包覆層減少元件的光損耗。然而就積體化而言，元件在能保持相對優良的性能之下，元件的體積能愈小愈好。因此在本文中提出具有高Q值(173000)和小體積(半徑4.43 um)的滑輪式環形共振腔，藉由改變載波波導的寬度可以在不增加環形共振腔半徑的前提之下而提高Q值。相較之下，本文提出滑輪式環形共振腔的半徑約為上述文獻的22.5%，且具有相當高的Q值，因此滑輪式環形共振腔是相當適合積體化的光學元件。 ;In this thesis, the microring resonators have been studied to propose the guidelines to design the high Q-factor microring resonators. First, by wrapping the curved waveguide around the microring to form the pulley coupler, the bending loss can be effectively decreased. The longer interaction length of coupling region such as the concentrically curved waveguides is helpful to decrease the radiation loss of microring to improve the Q-factor of resonator. Second, the large difference of propagation constant between the microring and the bus waveguide can maintain the propagating light in the microring. Third, the critical coupling can be optimized by adjusting the bus waveguide width to increase the Q-factor. The reasons mentioned above and the obtained results by comparing the performance of the different type ring resonators show that the pulley-type microring can provide higher Q-factor. Besides, in order to define the coupling length of the pulley coupler, the conformal transformation method based on Helmholtz equation is employed to estimate the coupling length of the concentrically curved waveguides. The coupling length obtained by the method proposed in this thesis has a good agreement with the finite-difference time-domain method. For demonstrating the performance of the designed high Q microring resonator, the pulley-microring resonator and the grating coupler are fabricated on the silicon-on-insulator substrate by using the E-beam lithography and the reactive-ion etching. The grating coupler is adopted to guide the light into the pulley-type microring resonator. The Q-factor can be obtained as high as 173000. In the literature, the microring resonator with the radius of 20 um constructed by the pulley coupler, such that pulley-type microring resonator can provide the high Q-factor from 20000 to 600000 by using the reflow process or covering the cladding layer. However, the radius of microring resonator is too long. In this thesis, the pulley-type microring resonator with high Q-factor of 173000 and with the smaller radius of 4.43 um is proposed and demonstrated. The high Q-factor pulley-type microring resonator is obtained by adjusting the bus waveguide width. This indicates that the pulley-type microring resonator is suitable for the miniaturization of the optical integrated circuit.
