近年來，矽光子學已被視為實現大規模光子積體電路的關鍵技術，包括用於數據中心的高速光連結、複雜的光信號處理、量子光子學和各種類型的傳感器。儘管以矽波導為主的光子積體電路有利於使系統晶片變得精小，但是它具有較大的傳播損耗和光纖邊緣耦合及封裝的困難。近來，氮化矽波導由於其低損耗和簡單的製造而受到越來越多的關注。但是，它缺乏諸如信號調制和檢測之類的高速主動功能。因此理想的光子積體電路應是可以整合矽和氮化矽波導的平台。
在這項研究工作中，我們提出了一種雙層光子集成電路的結構，其中頂層由氮化矽波導製成，而底層由矽線波導製成。我們通過具有可變錐度寬度和長度的同向耦合器將基於矽和氮化矽的光學結構結合在一起。 這種雙層光子結構提供了邊緣耦合（藉由氮化矽錐形波導）和表面耦合（藉由矽波導光柵）的功能。更重要的是，它有可能實現三維光子積體電路。

In recent years, silicon photonics has been considered as a key technology to implement large-scale photonic integrated circuits for many applications including high-speed optical links for data centers, sophisticated optical signal processing, quantum photonics and various types of sensors. Although Si waveguide is advantageous to make devices very compact, it suffers from relatively large propagation loss and difficulty for edge coupling. Recently, silicon nitride waveguide receives more and more attention because of low loss and simple fabrication. Nevertheless, it lacks of high-speed active functions like signal modulation and detection. Therefore, an ideal photonics platform will be an integration of both Si and Si3N4 waveguides which can be coupled together.
In this research work, we report a structure of bi-layer photonic integrated circuits, where the top layer is made by Si3N4 waveguides while the bottom layer is made by Si photonic wires. We combine Si and Si3N4-based optical structures via co-directional couplers with varying taper width and length. This bi-layer photonic structure provides capability of both edge coupling (through Si3N4 tapered waveguides) and surface coupling (through Si waveguide grating). More importantly, it potentially can implement a 3D photonic integrated circuit.

[1] R. K. Winn and J. H. Harris, “Coupling from Multimode to Single-Mode Linear Waveguides Using Horn-shaped Structures,” IEEE Transactions on Microwave theory and Techniques, vol. MTT-23, pp. 3012-3015 (1975).
[2] A. Fenner Milton and W. K. Burns, “Mode Coupling in Optical Waveguide horns,” IEEE Journal of Quantum Electronics, vol. QE-13, pp. 828-835 (1977).
[3] E. A. J. Marcatili, “Dielectric Tapers with Curved Axes and No Loss,” IEEE Journal of Quantum Electronics, vol. QE-21, pp. 307-314 (1985).
[4] G. R. Hadley, “Design of Tapered Waveguides for Improved Output Coupling,” IEEE Photonics Technology Letters, vol. 5, pp. 1068-1070 (1993).
[5] Nobuaki Hatori, “A Hybrid Integrated Light Source on a Silicon Platform Using a Trident Spot-Size Converter, “IEEE Journal of lightwave technology, vol. 32, pp. 0733-8724 (2014)
[6] Martin Papes, “Fiber-chip edge coupler with large mode size for silicon photonic wire waveguides. “OSA, vol.24, OE.24.005026 (2016)
[7] Takuya Oda et al., “Thermally Expanded Core Fiber with a 4-um Mode Field Diameter Suitable for Low-Loss Coupling with Silicon Photonic Devices. “OSA, pp. 060.2280 (2017)
[8] Takuya Oda et al. “High-Numerical-Aperture Optical Fibers for Low-Loss Coupling with Silicon Photonic Waveguides”, Fujikura Technical Review, (2017)
[9] P. Neutens et al. “Charactenents of PECVD silicon nitride photonic components at 532 and 900 nm wavelength.” Proc. Of SPIE, vol.9133.91331F (2014)
[10] Kerstin Worhoff and Rene G. Heideman, “TriPleX: a versatile dieletric photonic platform.” Review ArticleOpt.Techn, 4(2)189-207(2015)
[11] Carroll, L et al. “Photonic Packaging: Transforming Silicon Photonic Integrated Circuits into Photonic Devices.” Appl. Sci. Vol.6 12 ,10.3390 (2016)
[12] Takanori Shimizu et al.” Multichannnel and high-density hybrid integrated light source wuth a laser diode array on a silicon optical waveguide platform for interchip optical interconnection.” OSA, phoronics, vol.2, Issue3, pp A19-A24(2014)
[13] A. Moscoso-Martir et al.” Hybrid silicon photonics flip-chip laser intergration with vertical self-alignment.” OSA, Lasers and Electro, Opricla Society of America, paper s2069(2017)
[14] Wei-Ping Huang, “Coupled-mode theory for optical waveguides: an overview” J. Opt. Soc. Am. A, Vol. 11, No. 3(1994)
[15] H. A. Huas and W. Huang,” Coupled-mode theory”, IEEE Proceedings, Vol.79, No. 10, pp1505-1518(1991)
[16] A. Yariv” Coupled-Mode Theory for Guided-Wave Optics.” IEEE J Quantum electronics, Vol. 9, No.9, pp919-933(1973)
[17] A. Hardy, W. Strifier” Coupled Mode Theory of Parallel Waveguides”, IEEE J Lightwave of technology, Vol. 3, No.5, pp1135-1146(1985)
[18] L. Eckertova, and T. Ruzicka, “Diagnostics and Applications of Thin of Thin Films” Ch.1 & 2, Institute of Physics Publishing (1993).
[19] M. I. Alayo, I. Pereyra, and M. N. P. Carreno, “Thick SiOxNy and SiO2 films obtained by PECVD technique at low temperatures,” Thin Solid Films, 332, 40, (1998)
[20] M. S. Haque, H. A. Naseem, and W. D. Brown, “Characterization of High Rate Deposited PECVD Silicon Dioxide Films for MCM Applications,” J. Electrochem. Soc., 142, 3864, (1995)
[21] M. S. Haque, H. A. Naseem, and W. D. Brown, “Post-deposition processing of low temperature PECVD silicon dioxide films for enhanced stress stability,” Thin Solid Films, 308, 68, (1997)
[22] M. D. Feit and J. A. Fleck, Jr., “Light propagation in graded-index fiber,” Appl. Opt. 17,3990, (1978)
[23] Y. Chung, and N. Dagli, “Assesssment of finite difference beam propagation method,” IEEE J. Quantum Electron., Vol. QE-26, pp. 1335-1339, (1990)
[24] A. Splett, M. Majd, and K. Petermann, “A novel beampropagation method for large refractive index steps and largepropagation distance,” IEEE Photon. Technol. Lett., Vol. 3, pp. 466-468, (1991)
[25] D. Yevick, and B. Hermansson, “Efficient beam propagation techniques,” IEEE J. Quantum Electron., Vol. QE-26, pp. 109-112, (1990)
[26] Rsoft Design Group, BeamPROP user menu, Chapter 1, pp. 4-21.
[27] F. Bauters et al., "A comparison of approaches for ultra-low-loss waveguides," OFC/NFOEC, Los Angeles, CA, 2012, pp. 1-3. (2012)
[28] D. J. Blumenthal, R. Heideman, D. Geuzebroek, A. Leinse and C. Roeloffzen, "Silicon Nitride in Silicon Photonics," in Proceedings of the IEEE, vol. 106, no. 12, pp. 2209-2231. (2018)