A controllable coupling structure for silicon microring resonators based on adiabatic elimination

Adiabatic elimination-based coupler for the resonator-waveguide coupling control system.

Silicon photonics is one of the most promising ways for optoelectronic integration, ensuring CMOS fabrication compatibility and multitudinous production of low-cost devices. Optical microring resonator is one of the most important silicon-based devices and has been extensively employed in a wide range of physical studies and applications due to the resonance enhancement property. Incorporating coupling control of a microring resonator is necessary in many scenarios, but modifications are essentially added to the resonator and impairs the capability of optical enhancement. The researchers from Peking University proposed a flexible coupling structure based on adiabatic elimination that allowed low-loss active coupling control without any modifications to the resonators.

In the study, the research group experimentally demonstrated a flexible coupler based on the adiabatic elimination process so that the self-coupling coefficient can be monotonically or non-monotonically controlled. Through this coupler, some lossy electro-optical tuning mechanisms, such as the doping structure utilizing the plasma dispersion effect, can be employed to tune the self-coupling coefficient without any modifications to the resonators. The researchers derived the coupling coefficients in the coupling region under the adiabatic elimination condition and further established the transmittance function of the adiabatic elimination-based coupler, thus forming a consolidated analytical theory, which showed good agreement with numerical and experimental results. The tuning capabilities for coupling coefficients and the resulting coupling loss were analyzed, showing that the tuning in the adiabatic elimination-based coupler affected the coupling coefficients efficiently, but its loss was barely “felt” by the resonance system. The results have been published in Chinese Optics Letters, Vol 18, Issue 1, 2020 (Fenghe Yang, Pengfei Sun, Ruixuan Chen, Zhiping Zhou. A controllable coupling structure for silicon microring resonators based on adiabatic elimination[J]. Chinese Optics Letters, 2020, 18(1): 013601).

"It is very valuable to study how to incorporate coupling control of a microring resonator while maintaining the capability of optical enhancement," said the corresponding author Zhiping Zhou, "The proposed adiabatic elimination-based coupler gives a new possible way and we expect many new studies to begin with this paper."

In the proposed adiabatic elimination-based coupler, positive–negative (PN) junction can be implemented through CMOS fabrication processing, and the coupling coefficients would be dynamically modulated by electrical signal. In addition, the analysis result showed that the needed length of the PN junction was much shorter than the wavelength of the electrical driving signal, therefore, the active area of the adiabatic elimination-based coupler is equivalent to a lumped load, making the electro-optic bandwidth limited only by the resistor–capacitor (RC) constant and independent of the cavity photon lifetime. Based on the above considerations, high-speed electro-optic modulation with the proposed adiabatic elimination-based coupler is the follow-up work and this paper is the first step.

基于绝热消除的耦合可控硅微环谐振器

用于谐振器-波导耦合控制系统的基于绝热消除的耦合器

硅基光电子技术与互补金属氧化物半导体（CMOS）工艺相兼容，可大规模制备低成本光电子器件，将是最有前景的实现光电子全集成的技术手段。其中，光学微环谐振腔是最重要的硅基光电子器件之一，通过充分利用其谐振特性，光学微环谐振腔已经广泛地用于学术研究和实际应用。在研究和应用过程中，对微环谐振腔实施耦合控制是十分必要的，但这往往需要对谐振腔进行修饰，而这种修饰将破坏微环的谐振特性。北京大学周治平教授研究团队提出了一种灵活的耦合控制装置，该装置基于光学绝热过程，使得在不对微环进行任何修饰的条件下完成耦合控制成为可能。

在这项研究中，该团队成功制备了基于绝热消除过程的新型耦合器，可以在微环谐振系统中对耦合系数进行单调或者非单调的控制。此外，这种新型耦合器不需要对微环进行任何修饰，一些损耗性电光调谐装置，如等离子掺杂调控装置等，都可以应用于耦合器上，完成对微环谐振腔的耦合控制。该研究团队针对绝热消除状态下的微环耦合系统建立了完备的分析理论，包括耦合区内的耦合系数、传输函数等，整个理论分析结果与仿真和实验结果均十分吻合。他们还详细分析了该耦合器对耦合系数的调控能力和对系统损耗的影响情况，结果表明该新型耦合器可以有效的影响耦合系数，而自身的损耗对整个谐振系统几乎没有影响。研究结果发表于Chinese Optics Letters，2020年第18卷第1期 (Fenghe Yang, Pengfei Sun, Ruixuan Chen, Zhiping Zhou. A controllable coupling structure for silicon microring resonators based on adiabatic elimination[J]. Chinese Optics Letters, 2020, 18(1): 013601)。

该研究团队的周治平教授表示：“如何在不破坏微环谐振特性的同时实现耦合控制是十分有价值的研究，所提出的基于光学绝热过程的耦合器是一个可行的解决途径，我们期待这篇论文可以延伸出许多新的研究方向。”

文中所提出的新型耦合器可以通过CMOS工艺将PN结置于其中，因此可以对耦合系数实现电学调控。此外，文中分析结果表明所需要的PN结长度远小于电信号波长，因此，电调状态下的该耦合器等效于集总型负载，这使得电光响应带宽只受限于RC常数而不受微环谐振腔内光子寿命影响，有利于增大电光带宽。基于以上考虑，利用该新型耦合器实现微环的高速电光调制将是未来后续的研究内容。本文是开始的第一步。