應用於矽光子平台之高效率超寬頻多尖端型端面耦光器之研究

High-efficiency Ultra-broadband Multi-tip Edge Couplers for Silicon Photonics Platform

杜毅洲(Yi-Chou Tu)

指導教授 ： 黃定洧

國立臺灣大學/電機資訊學院/光電工程學研究所/博士(2019年)

https://doi.org/10.6342/NTU201902385

摘要

隨著光通訊領域在近年來蓬勃發展，絕緣體上矽(Silicon-on-Insulator, SOI)波導集成已成為此領域中之主流。其中，在光源和SOI矽波導晶片之間進行光訊號的直接耦合對於發送器和接收器至關重要。然而，將光訊號自微米尺度之光源耦合到幾百奈米尺度之矽波導中極具挑戰，因兩者間明顯的模態不匹配會造成顯著的耦合損耗。近年來，由於寬頻訊號傳輸為高速光通訊重要的技術，可寬頻操作的耦光器是下世代光通訊重要研發技術之一。此外，小尺寸耦光器可提高晶片的集積密度，以利大量生產、降低成本。因此，設計可直接將光源耦合至矽光子光路晶片的高效率、超寬頻、小尺寸耦光器具有極大應用價值。
目前一般商業用單一尖端逆向梯型(inverse taper)端面耦光器可以減少模態不匹配度並增加入射端面耦光效率，但其可調控自由度僅有尖端寬度，理論上入射端面耦光效率只能達到大約75%~85%。多尖端型端面耦光器增加了額外兩個自由度(尖端個數和間隔距離)，可進一步提高入射端面耦光效率趨近於95%。設計多尖端型端面耦光器主要挑戰在內部結構設計，因在目前製程廠技術可達到的最小線寬限制下，會造成耦光器內部結構形成不連續面，影響了耦光器內部傳輸效率，而無法達到理論上的最高整體耦光效率。
在本論文中，設計了二種類型可應用於矽光子平台的新穎多尖端型端面耦光器，其內部結構設計的線寬與間距皆符合實際製程廠的最小寬度限制，且同時可達到高效率內部傳輸。在設計分析過程中，利用了粒子群最佳化演算法( particle swarm optimization, PSO )結合有限差分特徵模態(finite-difference eigenmode, FDE)、特微模態展開方法(eigenmode expansion, EME)、三維時域有限差分法(three-dimensional finite-difference time-domain, 3D-FDTD) 等數值分析方法輔助設計。
第一類型為高效率超寬頻的四尖端型端面耦光器，應用於直接耦合分佈回饋型雷射(distributed feedback laser)的TE模態橢圓形光場，此耦光器結構由多尖端區段與連結區段組成，元件長度為九十微米。3D-FDTD的模擬結果顯示此耦光器耦光效率高達90.68% (0.4249 dB)，常用的光通訊波段1260 ~ 1675 nm (O-band, E-band, S-band, C-band, and L-band)都在其1-dB頻寬以內。
第二類型為高效率、小尺寸三尖端型端面耦光器，應用於耦合透鏡光纖(lensed fiber)。此耦光器結構為類三波導定向耦合器(triple-waveguide directional coupler-like)，元件長度僅五十微米，只有一般商業用端面耦光器長度的十分之一。3D-FDTD的模擬結果顯示此耦光器耦光效率在TE模態高達90.50% (0.4335 dB)，在TM模態達85.19% (0.6960 dB)，其1-dB頻寬涵蓋1300 ~ 1850 nm。若在此第二類型元件兩側搭配二氧化矽深蝕刻溝槽設計，可將TE模態耦光效率增加至93.33% (0.2998 dB)，且元件長度可縮小至僅四十微米。
本研究設計之兩種新穎高效率超寬頻耦光器完全符合現今製程廠規範，可直接整合於實際製程應用。相較於近年來已發表的幾種多尖端型端面耦光器，本論文提出之兩種類型耦光器具有耦光效率極高、操作頻寬最大、元件尺寸微小等優勢，展現其整合於光通訊晶片之極佳應用潛力。

關鍵字

矽光子學 ； 波導元件 ； 端面耦光器 ； 絕緣體上矽 ； 最佳化 ； 高效率 ； 寬頻帶 ； 小尺寸

並列摘要

With the flourish development of optical communications, the integration on the Silicon-on-Insulator (SOI) platform has become mainstream. Especially, directly coupling the light between signal sources and the SOI chips is crucial for both transmitters and receivers. However, the optical coupling between a micrometer-scale light source and a few hundred-nanometer waveguide presents challenges due to the large mode mismatch between the source and waveguide causing huge coupling loss. Other than that, the next-generation optical communications require the broadband and small footprint to increase the transmission speed and integration density. Hence, designing highly efficiency ultra-broadband small-footprint source-to-chip couplers for directly coupling the signals form light sources into silicon photonic circuits is valuable for practical applications.
The conventional inverse taper edge couplers can reduce the mode mismatch to enhance the coupling efficiency. In this arrangement, only the tip width can be adjusted to optimize the structure for greater mode match. The multi-tip edge couplers introduce additional degrees of freedom (the number of tips and the spacing between the tips) to increase the mode match and thus enhance the input interface coupling efficiency. One of the main challenge for designing the multi-tip edge couplers comes from the structural discontinuity resulted from the limitation of the fabrication process. It may decrease the mode evolution efficiency and cause an additional loss.
In this dissertation, two novel multi-tip edge couplers based on the SOI platform were proposed. The devices can achieve high mode evolution efficiency and the structural dimensions are at least as large as the minimum allowed waveguide width and gap for practical foundry fabrication. The particle swarm optimization in conjunction with numerical analysis techniques such as the finite-difference eigenmode method (FDE), the eigenmode expansion method (EME), and the three-dimensional finite-difference time-domain (3D-FDTD) method were used for designing and analyzing the devices.
The first proposed device is a high-efficiency ultra-broadband four-tip edge coupler for directly coupling the transverse-electric (TE) mode elliptic beam from a distributed feedback laser into the SOI chip. The device is composed of a multi-tip section and a combiner section, and the overall device length is 90 μm. The 3D-FDTD simulation results show that an overall coupling efficiency up to 90.68% (–0.4249 dB) with broadband operation from 1260 nm to 1675 nm, which covers entire commonly used optical communication bands from O-band, E-band, S-band, C-band, to L-band, with less than 1 dB extra loss.
The second proposed device is a high-efficiency broadband small-footprint dual wing edge coupler for coupling the circular beam from the lensed fiber into the SOI chip. The device is composed of an inverse taper waveguide and two side wing waveguides with equal waveguide width and symmetric spacing. Additionally, the device length is only 50 μm, which is only one-tenth the length of a typical commercial edge coupler. The 3D-FDTD simulation results show that an overall coupling efficiency for the TE mode is up to 90.50% (0.4335 dB) and that for the transverse-magnetic (TM) mode is as high as 85.19% (0.6960 dB). The device illustrates the broadband operation for the TE mode from 1300 nm to 1850 nm with less than 1 dB extra loss. Furthermore, with additional trench structures at the two sides of the dual wing edge coupler, the overall coupling efficiency can be enhanced to 93.33% (0.2998 dB) for the TE mode. Moreover, the device length can be further shrunk to only 40 μm.
The two novel high-efficiency edge couplers proposed in this dissertation can be directly integrated into practical applications. Compared to other state-of-the-art multi-tip edge couplers published in recent years, they possess outstanding advantages including extremely high coupling efficiency, largest operation bandwidth, and small device footprints. The proposed high-efficiency ultra-broadband small-footprint source-to-chip multi-tip edge couplers are promising as building blocks of the devices for high-performance, high-speed transmission, and high integration applications in the silicon photonics industry.

並列關鍵字

Silicon photonics ； Waveguide components ； Edge couplers ； Silicon-on-Insulator(SOI) ； Optimization ； High-efficiency ； Broadband ； Small-footprint

參考文獻

[3] L. Liu and Z. Zhou, “Refractive index engineering of high performance coupler for compact photonic integrated circuits,” Jpn. J. Appl. Phys., vol. 56, no. 4S, Apr. 2017, Art. no. 04CA01, doi: 10.7567/JJAP.56.04CA01.

[4] C. R. Doerr, P. J. Winzer, Y. K. Chen, S. Chandrasekhar, M. S. Rasras, L. Chen, T. Y. Liow, K. W. Ang, and G. Q. Lo, “Monolithic Polarization and Phase Diversity Coherent Receiver in Silicon,” J. Lightw. Technol., vol. 28, no. 4, pp. 520–525, Feb. 2010, doi: 10.1109/JLT.2009.2028656.

[5] P. Dumon, W. Bogaerts, D. Van Thourhout, D. Taillaert, R. Baets, J. Wouters, S. Beckx, and P. Jaenen, “Compact wavelength router based on a Silicon-on-insulator arrayed waveguide grating pigtailed to a fiber array,” Opt. Exp., vol. 14, no. 2, pp. 664–669, Jan. 2006, doi: 10.1364/OPEX.14.000664.

[6] G. Cong, Y. Maegami, M. Ohno, M. Okano and K. Yamada, “High-efficiency and high-speed silicon optical modulator based on amorphous silicon loaded SOI waveguide,” 2016 IEEE 13th Int. Conf. GFP, Shanghai, China, Aug. 2016, pp. 124–125, doi: 10.1109/GROUP4.2016.7739048.

[7] M. Niraula, J. W. Yoon, and R. Magnusson, “Single-layer optical bandpass filter technology,” Opt. Lett., vol. 40, no. 21, pp. 5062–5065, Nov. 2015, doi: 10.1364/OL.40.005062.