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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