本論文利用模擬退火法，計算出鈮酸鋰的非週期性晶疇反轉結構，並套用此結構在鈦擴散波導的電光式定向耦合器上。非週期結構定向耦合器，有別於傳統的週期結構定向耦合器，根據不同的排列情形，可獲得不同的耦合狀況，因而具有更大的自由度。本論文模擬出的非週期性結構，可使定向耦合器獲得更廣泛的工作範圍，根據不同結構，可以增加製程上的容忍度、工作頻寬或電壓調控的容忍度。本論文也實際製作出非週期結構定向耦合器晶片，證實模擬的正確性。
製程上以黃光微影製程及高溫擴散製程做出鈦擴散波導定向耦合器，並以高電場極化反轉製程做出非週期性結構，接著使用電子槍熱蒸鍍機台製作電極，最後進行端面拋光完成晶片。
晶片上有不同耦合區長度的定向耦合器，藉由計算出的非週期性極化反轉結構，使不同參數的各組定向耦合器，可以量測出同樣在30V附近產生crossover state，在50V附近產生straight-through state，平均開關電壓為17.85V。
藉由套用非週期結構，亦可增加定向耦合器的工作頻寬，套用非週期結構的定向耦合器，可容忍工作波長從1493.88nm~1643.05nm，工作頻寬達到149.17nm，相比於週期性結構定向耦合器的頻寬61.53nm為2.42倍。定向耦合器作為一個光通訊元件，頻寬增加後，其傳輸量也能隨之增加，寬頻的特性也成為本研究一大優勢。
未來可將非週期結構定向耦合器套用共平面波導(coplanar waveguide)電極，以行進波的方式對光進行調制，將其做成高速調制的光開關，或是進一步製作成積體光學邏輯閘。藉著本論文非週期結構的自由度，再加入快速調制的電極結構，電控式定向耦合器將獲得更多的應用性，增加元件競爭力。
本研究以題目“Electro-optic aperiodically poled Lithium Niobate directional couplers”，在澳洲CUDOS:2017workshop，獲得Best Student Poster Award。

In this study, we have used simulated annealing method to calculate the aperiodic domain inversion structure in LiNbO3 crystal. Moreover, we have applied this kind of structure on the electro-optically directional coupler. With the aperiodically poled LiNbO3 directional coupler (APPLNDC), it made the difference coupling result with periodically poled LiNbO3 directional coupler (PPLNDC). According to the difference arrangement of the poling structure, the directional coupler can get the difference coupling result, and thus APPLNDC can be more adjustable. In this study, our aperiodical design can make the APPLNDC get the higher fabrication tolerance, higher switched bandwidth and higher voltage tolerance. Furthermore, we fabricate the APPLNDC chip to verify our simulation result.
We used our standard lithography process and Titanium diffusion process to fabricate directional coupler and the aperiodically poled LiNbO3 was made by our standard CLN(congruent LiNbO3) poling process. After the poling process, we used the E-gun and Thermal machine to fabricate the electrode. Finally, we did the cutting and the end-face polishing process, then the APPLNDC chip was ready to be measured.
On the APPLNDC chip, there are different coupling length of the directional couplers. With the aperiodically poled LiNbO3, different coupling length of the directional couplers can have the same working voltage to reach crossover state at approximately 30V, and reach the straight-through state at approximately 50V. The average switched voltage is roughly 17.85V.
The aperiodically LiNbO3 also can enhance the working bandwidth of the directional couplers. By our measurement, the APPLNDC can work from 1493.88nm to 1643.05nm, so the bandwidth is approximately 149.17nm. Compare to PPLNDC’s bandwidth 61.53nm, APPLNDC is 2.42 times boarder than PPLNDC. Directional coupler as an optical communication component, the bandwidth increases, the transmission capacity can also be increased, the characteristics of broadband has become a major advantage of this study.
In the future, the APPLNDC can also be used to apply a coplanar waveguide electrode to modulate the light in a traveling wave, to make it a high-speed modulated optical switcher, or to further develop an integrated optical logic gate. By using the aperiodically poled LiNbO3 and then applying the high-speed modulated electrode, the electro-optically switched directional coupler will obtain more application and increase the competitiveness in the optical communication.
This study have got the Best Students Poster Award in CUDOS:2017workshop. I was the second author：
Hung-Pin Chung, Shih-Yuan Yang, Sung-Lin Yang, Tsai-Yi Chien, Kuang-Hsu Huang, Yen-Yu Chou, Kai Wang, Dragomir N. Neshev, and Yen-Hung Chen. “Electro-optic aperiodically poled Lithium Niobate directional couplers” CUDOS 2017 Annual workshop, Australia, Best Poster award of nonlinear quantum photonics, (2017).

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