鈮酸鋰側壁延伸式電極大角度可控制極化分離器

Abstract

本論文利用簡式同調耦合彎曲波導結構，以及側壁延伸式電極結構，在z切鈮酸鋰基板上，使用鋅鎳共同擴散式鈮酸鋰光波導，成功研製大角度極化分離器與側壁延伸式電極TE-TM模態轉換器，並將相位調變器、極化模態轉換器與大角度極化分離器整合成單一元件，實現可控制轉換與分離任意極化態輸入以產生任意TE與TM輸出光功率比的可控制極化分離器。 在極化分離器方面，本研究成功以簡式同調耦合彎曲波導結構，設計與製作出可滿足模態揀選效應，同時突破0.5°分岔角限制的大角度極化分離器，在兩單極化輸出間距為175 µm時，此大角度極化分離器較傳統Y分岔型極化分離器縮短了52%的元件尺寸，此極化分離器之極化訊息比不論TE或TM皆達到25dB以上。並透過與TE-TM模態轉換器之整合，證實了同調耦合彎曲結構並不影響此元件之操作機制，在此結構設計下，元件仍可以成功的對輸入之TE或TM模態進行轉換後經由極化分離器輸出。在側壁延伸式電極TE-TM模態轉換器方面，本研究成功的研製出脊形高6 µm之側壁延伸式電極。將此電極結構運用在TE-TM模態轉換器上，其模態轉換器外加電壓較傳統式脊形電極結構降低了52%。 最後，更利用簡式同調耦合彎曲與側壁延伸式電極結構，成功整合了極化控制與極化分離的功能在一16 mm的鈮酸鋰晶片上，研製出可控制轉換與分離任意極化態輸入產生任意TE與TM輸出光功率比的可控制極化分離器。經由實驗證實，原本對45° 線性極化輸入不具轉換功能之TE-TM極化模態轉換器，經由相位調製器偏壓控制，可在外加電壓27V時達成輸出TE或TM單極化光，對任意極化態輸入光，經由極化模態轉換器與相位調製器之適當控制，亦可輸出任意比例之TE與TM光功率。

A controllable Y-branch polarization splitter is successfully fabricated and demonstrated for the first time. The proposed device is an integration of a phase shifter, polarization converter, and polarization splitter on one chip. This device is to split a randomly polarized optical input into TE and TM outputs with arbitrary power ratio between these two polarizations. In order to design a compact device size, the simplified coherently coupled bent structure is applied to design the Y-branch polarization splitter with an effective full branching angle of 1.5°. This angle is three times larger than that of conventional Y-branch polarization splitter fabricated on lithium niobate. With this branch angle the device size can be reduced by 52% when the separation between TE and TM output branches is 175 µm. Moreover, the polarization splitter with this wide angle bent structure will still satisfy the mode sorting effect and possess more than 25 dB polarization extinction ratios for both TE and TM modes. To reduce the applied voltage of the polarization converter, the sidewall-extended electrodes are applied to increase the overlap integral of the converter. Experiment results show only 54V is needed to convert one single polarization state into the other, and the applied voltage can be reduced 52% than that with the conventional ridge-electrode structure. Finally, a phase shifter is integrated with a polarization converter with sidewall-extended electrodes, and a wide angle polarization splitter with simplified coherently coupled bent waveguide to form a 16 mm long controllable wide-angle polarization splitter. As a 45° linearly polarized input is lunched to this controllable polarization splitter, arbitrary power ration between two split polarization outputs can be achieved by biasing the phase shifter at proper voltage and applying a correct voltage to the converter. At a propoer bias, the single polarization output can be achieved by applying 27V to the converter. However, the same 45° linearly polarized input of the variable TE-TM polarization splitter without phase shifter can not be transferred to single polarization output. Moreover, experimental results show the proposed controllable polarization splitter can be used to transfer and split randomly polarized input to any output power ratio between TE and TM modes with suitable applied voltages for the phase shifter and the polarization converter.