在矽光子(Silicon Photonic)元件中，因波導長寬比及介電質材料本身產生的雙折射現象(Birefringence)會導致偏振相依損耗(PDL)和偏振模態色散(PMD)以及偏振相依波長特性(PDλ)，進而影響整個矽光子系統的表現。為了消除雙折射對損耗及頻寬的影響，我們提出以厚度240奈米氮化矽(Si3N4)與220奈米矽波導包覆於二氧化矽層組成之平台設計新穎的極化分集積體光學，通過兩個串連的多模干涉器(MMI)組成的馬克曾德爾-多模干涉型(MZI-MMI)極化分離器將TE及TM模態分光，再連接至模態耦合(mode-coupling)型極化旋轉器將TM模態旋轉成TE模態，確保系統不受偏振影響。
在本論文中實際做出的極化分離器元件，由兩個長度為675微米的多模干涉耦合器組成，連接其中的兩臂包含長度270微米的矽加熱器用於相位控制，於1260nm到1360nm的波段下量測到其極化消光比(PER)超過14.2dB，插入損失為8到12dB。

In order to maximize the advantages of photonics technology such as wide bandwidth and ultralow loss, we have to deal with the birefringence effects(BE) in optical waveguides. BE lead to polarization dependent loss (PDL) and polarization mode dispersion(PMD) which might decrease the performances of the devices. Therefore, we have proposed a polarization diversity circuit integrated on Si3N4-on-SOI platform. By cascading two optimized Si3N4 MMI couplers with length of 675μm simply construct a MZI-MMI polarization splitter. The two arms with an N^(++)-doped silicon region between the MMI structures play the function of phase controller.
The experimental result showed we have demonstrated a MZI-MMI polarization splitter with maximum polarization extinction ratio(PER) of 14.2dB and the insertion loss(IL) of 8-12dB in a bandwidth of 1260nm-1360nm.

參考資料
[1] A. Amari, "Nonlinear effects compensation for long-haul superchannel transmission system," 2016.
[2] K. Roberts et al., "Performance of Dual-Polarization QPSK for Optical Transport Systems," Journal of Lightwave Technology, vol. 27, no. 16, pp. 3546-3559, 2009, doi: 10.1109/jlt.2009.2022484.
[3] T. Barwicz et al., "Polarization-transparent microphotonic devices in the strong confinement limit," Nature Photonics, vol. 1, no. 1, pp. 57-60, 2006, doi: 10.1038/nphoton.2006.41.
[4] L. Chen, C. R. Doerr, and Y.-K. Chen, "Compact polarization rotator on silicon for polarization-diversified circuits," Optics letters, vol. 36, no. 4, pp. 469-471, 2011.
[5] J. Zhang, H. Zhang, S. Chen, M. Yu, G. Q. Lo, and D. L. Kwong, "A tunable polarization diversity silicon photonics filter," Optics express, vol. 19, no. 14, pp. 13063-13072, 2011.
[6] H. Fukuda, K. Yamada, T. Tsuchizawa, T. Watanabe, H. Shinojima, and S.-i. Itabashi, "Silicon photonic circuit with polarization diversity," Optics express, vol. 16, no. 7, pp. 4872-4880, 2008.
[7] H. Guan et al., "CMOS-compatible highly efficient polarization splitter and rotator based on a double-etched directional coupler," Opt Express, vol. 22, no. 3, pp. 2489-96, Feb 10 2014, doi: 10.1364/OE.22.002489.
[8] L. Liu, Y. Ding, K. Yvind, and J. M. Hvam, "Silicon-on-insulator polarization splitting and rotating device for polarization diversity circuits," Optics express, vol. 19, no. 13, pp. 12646-12651, 2011.
[9] W. D. Sacher, T. Barwicz, B. J. Taylor, and J. K. Poon, "Polarization rotator-splitters in standard active silicon photonics platforms," Opt Express, vol. 22, no. 4, pp. 3777-86, Feb 24 2014, doi: 10.1364/OE.22.003777.
[10] W. D. Sacher et al., "Polarization rotator-splitters and controllers in a Si3N4-on-SOI integrated photonics platform," Opt Express, vol. 22, no. 9, pp. 11167-74, May 5 2014, doi: 10.1364/OE.22.011167.
[11] Y. Xiong, J. G. Wanguemert-Perez, D. X. Xu, J. H. Schmid, P. Cheben, and W. N. Ye, "Polarization splitter and rotator with subwavelength grating for enhanced fabrication tolerance," Opt Lett, vol. 39, no. 24, pp. 6931-4, Dec 15 2014, doi: 10.1364/OL.39.006931.
[12] Y. Xiong, D. X. Xu, J. H. Schmid, P. Cheben, S. Janz, and W. N. Ye, "Fabrication tolerant and broadband polarization splitter and rotator based on a taper-etched directional coupler," Opt Express, vol. 22, no. 14, pp. 17458-65, Jul 14 2014, doi: 10.1364/OE.22.017458.
[13] H. Fukuda, K. Yamada, T. Tsuchizawa, T. Watanabe, H. Shinojima, and S.-i. Itabashi, "Ultrasmall polarization splitter based on silicon wire waveguides," Optics Express, vol. 14, no. 25, pp. 12401-12408, 2006.
[14] J. Feng and R. Akimoto, "Silicon nitride polarizing beam splitter with potential application for intersubband-transition-based all-optical gate device," Japanese Journal of Applied Physics, vol. 54, no. 4S, 2015, doi: 10.7567/jjap.54.04dg08.
[15] H. Fukuda, K. Yamada, T. Tsuchizawa, T. Watanabe, H. Shinojima, and S.-i. Itabashi, "Polarization rotator based on silicon wire waveguides," Optics express, vol. 16, no. 4, pp. 2628-2635, 2008.
[16] L. B. Soldano and E. C. Pennings, "Optical multi-mode interference devices based on self-imaging: principles and applications," Journal of lightwave technology, vol. 13, no. 4, pp. 615-627, 1995.
[17] J.-M. Fédéli et al., "TriPleX waveguide platform: low-loss technology over a wide wavelength range," presented at the Integrated Photonics: Materials, Devices, and Applications II, 2013.
[18] K. Wörhoff, R. G. Heideman, A. Leinse, and M. Hoekman, "TriPleX: a versatile dielectric photonic platform," Advanced Optical Technologies, vol. 4, no. 2, 2015, doi: 10.1515/aot-2015-0016.
[19] O. C. Zienkiewicz, R. L. Taylor, P. Nithiarasu, and J. Zhu, The finite element method. McGraw-hill London, 1977.
[20] T.-T. Le, "New Approach to Mach-Zehnder Interferometer (MZI) Cell Based on Silicon Waveguides for Nanophotonic Circuits," in Applications of Silicon Photonics in Sensors and Waveguides, 2018, ch. Chapter 3.
[21] T. Liang and H. Tsang, "Integrated polarization beam splitter in high index contrast silicon-on-insulator waveguides," IEEE photonics technology letters, vol. 17, no. 2, pp. 393-395, 2005.
[22] J. Van Roey, J. Van der Donk, and P. Lagasse, "Beam-propagation method: analysis and assessment," Josa, vol. 71, no. 7, pp. 803-810, 1981.
[23] K. S. Kunz and R. J. Luebbers, The finite difference time domain method for electromagnetics. CRC press, 1993.