「文献阅读」基于双刻蚀和锥形非对称定向耦合器的高性能超紧凑偏振分离旋转器

This article was last updated on 2022-08-16 23:08:39

文献概述

AbstractContent
TitleHigh-performance ultra-compact polarization splitter rotators based on dual-etching and tapered asymmetrical directional coupler
Published2021-9-13
PlatformSilicon
Functionthe incident light with orthogonal polarization (TE0 & TM0) can be separated and rotated on one of them (TM0 to TE0).
Footprinta total length of ~28μm
Resultmode extinction ratio is higher than 28 dB or 32 dB @1550 nm for the launched fundamental transverse magnetic or the transverse electric modes, while the corresponding insertion loss and polarization conversion loss are 0.33 dB and 0.18 dB.
an ER of more than 20 or 28 dB over 1510–1580 nm for TE0 or TM0 modes. For a launched TE0 mode, the insertion loss (IL) is less than 0.6 dB within the 70 nm bandwidth. For a launched TM0 mode, the polarization conversion loss (PCL) is less than 1 or 3 dB within the bandwidth of 45 or 70 nm, respectively.
PDFhere

结构图

Schematic diagrams of the two proposed PSRs. (a) PSR-1. (b) PSR-2. (c) Cross view.

笔记

1. Introduction

Photonic integrated circuits (PICs) fabricated in the silicon-on-insulator (SOI) platform are essential for massive applications in communications, military, and sensing. Due to its CMOS compatibility and ultra-high index contrast in the SOI platform, the PICs can be fabricated with low cost and compact footprint[1][2][3][4] .

Polarization splitter-rotators (PSRs) are one of the key components in PICs to overcome the highly polarization-dependent issue brought by the silicon waveguide.

Typically, the PSR combines the functions of the polarization beam splitter (PBS) and polarization rotators (PRs), so the incident light with orthogonal polarization can be separated and rotated on one of them.

Thus, the PSRs are widely used to meet significant requirements for polarization processing and multiplexing. Recently, various PSRs have been proposed with different structures, such as the mode-sorting asymmetric Y-junction[5][6] , multimode interferometer[7][8] , and asymmetric directional coupler (ADC)[9][10] .

Among them, the PSRs fabricated by the ADC structure have advantages on performance, footprint, and design flexibility. However, this type of PSR is sensitive to fabrication errors. To address this problem, several novel concepts have been incorporated into the ADC structures, such as subwavelength grating (SWG), hybrid plasmonic waveguide, and quasi-adiabatic couplers… However, the hybrid plasmonic is complicated to realize under current commercial tape-out processing conditions(流片).

2. Design

Schematic diagrams of the two proposed PSRs. (a) PSR-1. (b) PSR-2. (c) Cross view.

The top cladding of the PSR is specified as air to achieve a more compact footprint (the reason can be explained in this paper).

“ The through-port waveguides of two PSRs are both dual etched. The first stage is partially etched, and a tapered coupler is formed in the coupling region. An S-bend section is used to decouple at the end of the coupling region. Meanwhile, the etching width remains unchanged until arriving at the end of the S-bend. Then, the etched width is gradually increased, and it finally forms a full etching. Right now, the waveguide supports a single mode, and only the TE0 mode can be guided in this port. A reverse taper is used to recover the waveguide thickness. The cross-port waveguide might be an SWG or a nanowire, as shown in Figs. 1(a) and 1(b). ”

这里说的”Through-port”和”Cross-port”其实和微环谐振腔很类似,关于MRR之后需要弄一个专题去分析。在这里呢,对于从下端波导输入模式的情况而言,上波导为”cross”端,下波导为”through”端。

这个器件本质上是耦合器的原理,因此,文中也提到了phase matched的问题,即模式的相位匹配。换句话说,相互耦合的两个模式的有效折射率要尽可能接近(理想情况下是相等)。在这种耦合问题上,如果二者不满足相位匹配,还是可以发生耦合,只不过耦合效率很低,这种结果通过光场模拟仿真就可以看得很清楚。而如果二者严格满足相位匹配,理想情况下耦合系数可以达到100%。并且,耦合输出随着耦合长度呈现周期性变化,我印象中两端口的光强分别呈现$cos^2\theta$和$sin^2\theta$的变化。

3. Fabrication

A standard SOI wafer, has a 220 nm silicon layer and a 3 μm SiO2 substrate. Therefore, the whole height of core layer is 220 nm out of consideration for the fabrication process.

(a) SEM images of the reference waveguide and PSR. (b) Main structure of the PSR.

We designed two vertical coupling gratings for supporting the TE0 and TM0 modes, respectively. The period and filling factor of the TE0 grating coupler are designed as 620 nm and 45%, while 1050 nm and 47% are for the corresponding TM0 grating coupler.

4. Measurement

 Transmission response of the (a) TE0 and (b) TM0 vertical coupling gratings.

During the performance tests, several TE0–TE0 and TM0–TM0 reference waveguides are fabricated, and their transmission responses are shown in Fig. 7. It can be found that coupling losses of TE0 and TM0 grating couplers are 7.7 dB/port and 11.4 dB/port at the center wavelength, respectively.

Transmission spectral responses: (a) TM0 mode launched and (b) TE0 mode launched.

The experiment and simulation results for PSRs are shown in Fig. 8, after being calibrated with the reference waveguide. The IL of the TE0 mode in through port is less than 0.6 dB within the bandwidth of 1510–1580 nm, while the ER is higher than 20 dB. The TM0–TE0 conversion efficiency reaches the maximum of 96% at λ = 1550 nm, with a PCL of 0.18 dB. In the 1535–1580 nm range, the PCL is less than 1 dB, and the ER is larger than 28 dB. (这里我比较疑惑PCL是怎么计算的

想法

这篇论文的表述很不错,Introduction部分写的很棒,值得借鉴。整个introduction都在循序渐进:PICs的应用——介绍PSRs的地位——其结构功能——用途——近期PSR的实现结构——among them 点出本文采用的这类结构的优点——指出这类结构的缺点——因此,结合实际的科学研究说明需要怎样提升——说出别人做的结构的缺点”Nevertheless, the footprint beyond 200 μm is too large to meet the stringent (严格) requirements for compactness.”——最终开始介绍our work(In this letter),很通。

本文提出了两种结构,二者其不同的地方在于与输入波导耦合的另一个波导的形状不同,一个是DC,一个是LPG。作者在设计部分花了大量的言语去说明自己的设计方案,其考虑了很多因素,包括耦合效率的优化、模拟模式转换损失的最小化、模拟器件转换效率以及后期对于工艺容差的考量(这一点可以学习一下)。

在测试的部分,作者说道“The TM0–TE0 conversion efficiency reaches the maximum of 96% at λ = 1550 nm, with a PCL of 0.18 dB.” 这个地方我咋越看越觉得是在1510 nm处效果最好捏,看结果肯定是(a)图,但这张图里面很明显频谱差在1510 nm处最大。

整体上说的话,这篇文章在设计部分写的有点仓促,尤其是没有说清楚模式是怎么转换的,有些细节性的东西,我在作者的另一篇文章中才看到。之后更新一下发表在AO上面的这一篇,里面详细的说明了作者的器件设计过程,比COL这一篇要详细得多。

参考文献

  1. X. Wang, G. Zhou, Z. Jin, L. Lu, G. Wu, L. Zhou, and J. Chen, “Wavelength-mode pulse interleaver on the silicon photonics platform,” Chin. Opt. Lett. 18, 031301 (2020). here
  2. Y. Zhang, Y. He, X. Jiang, B. Liu, C. Qiu, Y. Su, and R. A. Soref, “Ultra-compact and highly efficient silicon polarization splitter and rotator,” APL Photon. 1, 091304 (2016). here
  3. T. Tsuchizawa, K. Yamada, H. Fukuda, T. Watanabe, J.-I. Takahashi, M. Takahashi, T. Shoji, E. Tamechika, S. Itabashi, and H. Morita, “Microphotonics devices based on silicon microfabrication technology,” IEEE J. Sel. Top. Quantum Electron. 11, 232 (2005). here
  4. W. Bogaerts, R. Baets, P. Dumon, V. Wiaux, S. Beckx, D. Taillaert, B. Luyssaert, J. V. Campenhout, P. Bienstman, and D. V. Thourhout, “Nanophotonic waveguides in silicon-on-insulator fabricated with CMOS technology,” J. Lightwave Technol. 23, 401 (2005). here
  5. S. Keyvaninia, H. Boerma, M. Wössner, F. Ganzer, P. Runge, and M. Schell, “Highly efficient passive InP polarization rotator-splitter,” Opt. Express 27, 25872 (2019). here
  6. J. Wang, B. Niu, Z. Sheng, A. Wu, W. Li, X. Wang, S. Zou, M. Qi, and F. Gan, “Novel ultra-broadband polarization splitter-rotator based on mode-evolution tapers and a mode-sorting asymmetric Y-junction,” Opt. Express 22, 13565 (2014). here
  7. H. Liang, R. Soref, and J. Mu, “Compact polarization splitter based on a silicon angled multimode interferometer structure,” Appl. Opt. 58, 4070 (2019). here
  8. Ding, H. Ou, and C. Peucheret, “Wideband polarization splitter and rotator with large fabrication tolerance and simple fabrication process,” Opt. Lett. 38, 1227 (2013). here
  9. B. Bai, L. Pei, J. Zheng, T. Ning, and J. Li,“Ultra-short plasmonic polarization beam splitter-rotator using a bent directional coupler,” Chin. Opt. Lett. 18, 041301 (2020). here
  10. Z. Ying, G. Wang, X. Zhang, H.-P. Ho, and Y. Huang, “Ultracompact and broadband polarization beam splitter based on polarization-dependent critical guiding condition,” Opt. Lett. 40, 2134 (2015). here