基于光子晶体纳米梁腔的反射壁下载型电光调制器【增强内容出版】
With the development of optical interconnection and high-speed optical communication, electro-optic modulators have become a research hotspot. Silicon insulator materials have the advantages of compatibility with the complementary metal oxide semiconductor process, high integration, low power consumption, and high temperature resistance. There are several electro-optic modulators based on silicon materials. Electro-optic modulators with high modulation rate, compact size, and easy integration have been investigated previously, and the study on electro-optic modulators is crucial. Therefore, we design an electro-optic modulator with a reflective wall based on a one-dimensional photonic crystal nanowire cavity (PCNC). The modulator exhibits a high extinction ratio, large modulation bandwidth, and high modulation rate. Furthermore, it has a compact and simple structure and can easily to cascade other silicon photonic devices. With the development of integrated photonics in communication systems, the cascade of silicon photonic devices has a wider application prospect.
This study proposes a download-type electro-optic modulator with a reflective wall based on a silicon-on-insulator (SOI) one-dimensional PCNC. The main line waveguide, one-dimensional PCNC and download-type waveguide are used to form a download-type structure with a reflective wall. The duty cycle of the nanowire cavity decreases linearly from the center of the waveguide to the two ends, and doping is introduced at both sides of the modulator to form PN junctions. The finite difference time domain (FDTD) model in the optical simulation software Lumerical is used for simulation analysis. According to the free carrier dispersion effect in the silicon material, when the modulation voltage applied at both ends of the electro-optic modulator changes, the dielectric constant of the nanowire cavity material also changes. The refractive index change in the nanowire cavity produces a slight difference; hence, the resonant frequency of the cavity changes, i.e., the central wavelength of the electro-optic modulator shifts. Specifically, corresponding to the wavelength of 1550.01 nm, the addition or non-addition of the modulation voltage is equivalent to the “off” or “on” state of the modulator.
An electro-optic modulator with a reflective wall based on the SOI PCNC is proposed. The incident light is coupled into the one-dimensional PCNC after passing through the main line waveguide, and then coupled again to output through the download-type waveguide. The adjustments of the position and number of reflective circular holes in the main line waveguide and download-type waveguide are beneficial to improve the overall transmittance of the device. The nanowire cavity uses a gradual circular hole to confine the beam in the cavity. PN junction is generated by doping on both sides of the nanowire cavity, and a low bias voltage is applied to adjust the resonant wavelength of the nanowire cavity, to realize the “on” and “off” modulation of the optical signal at the working wavelength. 3D-FDTD is used to analyze the optical characteristics and electrical performance of the modulator. The results indicate that the electro-optic modulator can modulate the optical signal with the wavelength of 1550.01 nm, and the transmittances under the “off” and “on” states are 0.0037 and 96.34%, respectively (Fig.14). The modulation voltage is only 1.2 V, the insertion loss is 0.2 dB, the extinction ratio is 24 dB, and the size is only 54 μm2. The modulation frequency is 8.7 GHz, and the modulation bandwidth can reach 122 GHz, which implies that the proposed device has applications in optical communication and integrated photonics. In addition, after comparing the performances of the photonic crystal electro-optic modulators (Table 1), it is inferred that the proposed device exhibits excellent performance.
This study proposes a download-type electro-optic modulator with a reflective wall based on SOI one-dimensional PCNC. The downloadable structure of the reflection wall comprises a main line waveguide, one-dimensional PCNC, and downloadable waveguide. The doping method is introduced to form PN junctions at both sides of the modulator. Under the action of the modulation voltage, the refractive index of the silicon in the nanowire cavity changes, which triggers the migration of defect modes in the nanowire cavity; in addition, the “on” and “off” state modulations of the electro-optic modulator are realized. The electro-optical modulation is simulated and analyzed via the 3D-FDTD model in the Lumerical commercial simulation software. The simulation results demonstrate that compared with other electro-optical modulators based on nanowire cavity, the proposed electro-optical modulator has a higher extinction ratio, higher modulation bandwidth, higher modulation rate, compact and simple structure, and can easily be cascaded to other silicon photonic devices. The proposed electro-optic modulator exhibits a significant development and application value in the integrated photonics of optical communication.
1 引言
随着信息技术的发展,光子作为一种新的信息载体,被用来开发越来越多的新的互联技术和集成方法。光子技术发展的远期目标是实现集成光路。所谓集成光路,即将各个光器件,例如半导体激光器[1-2]、集成光波导[3]、片上光调制器[4]、光开关[5]以及光探测器[6]等集成在同一块芯片上,使芯片能够对携带信息的光信号进行处理、发送以及接受。在集成光子学中,电光调制器作为一种重要的光通信器件,随着光互联和高速光通信的发展而成为研究热点,性能好、尺寸小以及集成度高成为调制器的重要发展方向。随着硅基光学技术的发展,硅绝缘体(SOI)材料凭借其优异的特性而得到广泛应用。SOI材料能够与互补金属氧化物半导体(CMOS)兼容,并且具有耐高温、损耗低和集成度高等优点[7],以其作为光子学材料[8]的纳米结构已经成为研究热点[9-10]。已有大量基于硅材料的电光调制器研究,Liu等[11]报道了基于硅材料的马赫-曾德尔干涉仪型电光调制器,随后作者采用反向PN结对原调制器进行改进,将其传输速度提升至40 Gbit/s[12]。
光调制器除了采用干涉原理实现光调制外,谐振腔是另一种常用方法。一维光子晶体纳米梁腔(PCNC)作为谐振腔,性能较为独特,它的模体积小,品质因子(Q)较高,与光波导易耦合集成[13]。目前,采用一维PCNC实现调制器的方式主要有两种。一种是将光波直接耦合到纳米梁腔中。2007年Schmidt等[14]首次在硅基一维PCNC上实现了超紧凑电光调制器,2014年Shakoor等[15]利用厚度为220 nm的SOI平台,制作了硅基纳米梁腔电光调制器。另一种设计调制器的方法是利用侧耦合结构,增添条形波导与纳米梁腔进行侧耦合,从而实现光调制。如2015年Pan等[16]提出了一种基于硅与石墨烯混合的一维PCNC电光调制器,2018年Liu等[17]提出了一种石墨烯/Al2O3多层堆叠(GAMS)的PCNC侧耦合电光调制器,这些器件存在功耗较大、消光比不高等缺点。2021年,徐文凯等[18]提出了一种基于硅绝缘体的侧耦合型一维PCNC电光调制器。该器件可实现1550.55 nm波长的光信号调制,调制电压仅为1.175 V,消光比为18.2 dB,调制速率为8.3 GHz,调制带宽可以达到90 GHz,其中消光比和调制带宽较低。可以看出,目前光子晶体纳米梁腔电光调制器仍存在消光比低以及调制带宽较小等不足,可以通过优化结构、增加反射壁来提高光子晶体纳米梁腔调制器的性能。
本文提出了一种基于SOI PCNC的改进型电光调制器,基于文献[18]的结构,在一维PCNC两侧增加空气孔,形成反射壁下载型结构,并在谐振腔两侧掺杂形成PN结。利用Lumerical仿真软件进行分析,结果表明,该器件的消光比与调制带宽高,性能优良,具有损耗小、功耗低、易于集成的优点。
2 结构模型与原理分析
本文设计的改进型电光调制器的三维结构如
图 1. 反射壁下载型电光调制器的三维结构
Fig. 1. Three-dimensional structure of download type electro-optic modulator with reflection wall
电光调制器在xz方向上的截面图如
调制器中反射圆孔和光子晶体纳米梁腔的结构与参数分别如
主线波导和下载波导中的反射圆孔周期数均为Nreflect=12,半径为rreflect=79 nm;纳米梁腔的镜面周期数Nmirror=6,渐变周期数Ntaper=26;圆孔半径由两端的rend=79 nm渐变增加到中心处的rcenter=109 nm。
本文设计的电光调制器属于带反射壁的下载型结构[19],建立的侧耦合结构理论模型如
式中:t表示时间;
式中:
图 5. 主线波导与谐振腔侧耦合结构的理论模型
Fig. 5. Theoretical model of side coupling structure between mainline waveguide and resonant cavity
当
图 6. 主线波导与谐振腔侧耦合结构理论透射谱
Fig. 6. Theoretical transmittance spectrum of side coupling structure between mainline waveguide and resonant cavity
通过在纳米梁腔上及其左右两侧掺杂Si薄层来制备PN结,如
图 7. 电光调制器硅基材料的掺杂示意图
Fig. 7. Doping diagram of electro-optic modulator silicon-based materials
按照硅材料中自由载流子色散(FCD)效应,即P型和N型载流子浓度(即空穴和电子浓度)发生变化时,基底硅的有效折射率也会发生变化[22]。当工作波长为1550 nm时,硅基材料的载流子浓度与相关参数之间的关系[21]为
式中:
接下来利用商业光学软件Lumerical的Device模块对器件进行电学设计,利用P+型重掺杂(掺杂载流子浓度
图 8. 调制电压为1.2 V时的载流子浓度分布。(a)N型载流子浓度;(b)P型载流子浓度
Fig. 8. Carrier concentration distributions when modulation voltage is 1.2 V. (a) N type carrier concentration;(b) P type carrier concentration
通过施加驱动电压,改变载流子浓度及谐振腔内材料的折射率。由
本文所设计的电光调制器调制原理为缺陷模迁移,当加在电光调制器两端的调制电压发生变化时,纳米梁腔的折射率将产生微小的变化,从而导致该腔的谐振波长发生变化,电光调制器的中心波长发生迁移。即对应于原中心波长,调制电压的加与不加可以使调制器实现光信号“断”和“通”调制。
3 参数优化和性能分析
3.1 电光调制器的结构参数优化
在本文提出的电光调制器中,纳米梁腔的中心渐变圆孔半径rcenter、最外侧圆孔半径rend、两侧反射圆孔周期数Nreflect以及光子晶体与两侧反射壁波导之间的距离间隔L对性能的影响最为明显。根据文献[24-25]中一维光子晶体微腔程式化设计方法,可以选取纳米梁腔的占空比,使其从波导的两端向中心线性增加,这样光在纳米梁腔中心处形成高斯镜像;并在圆孔最外侧额外增加数个相同大小的镜像圆孔,利用所形成的反射壁增强光的局域能力,从而提高透射率与Q值。同时,在设计谐振腔型电光调制器时,一般会对品质因子进行限制,使其最多略大于104[26]。
一般情况下,中心圆孔的占空比为0.2,最外侧圆孔的占空比为0.1,占空比计算公式[23]为
式中:
式中:
根据式(
为了提高器件的性能,在纳米梁腔结构之外,添加反射壁纳米线波导结构以形成异质结结构,此时需要研究反射壁结构参数对谐振波长的限制作用,以降低纳米梁腔与两侧波导的耦合损耗。纳米梁腔和两侧波导之间的耦合损耗与结构在传输方向上对谐振光波的限制作用有关,限制作用越弱,耦合损耗就越大。调制器的透射率与谐振波长随反射圆孔周期数Nreflect的变化曲线如
图 9. 透射率与谐振波长随Nreflect的变化
Fig. 9. Transmissivity and resonance wavelength versus Nreflect
最后,对主线波导与纳米梁腔之间的耦合间距L进行优化。当耦合间距L由200 nm减小到160 nm时,调制器的透射率和Q值随耦合间距L的变化曲线如
3.2 电光调制器的性能分析
优化结构参数后,光子晶体反射壁下载型电光调制器的主线波导、PCNC波导以及下载波导的宽度均为550 nm,PCNC与两侧波导之间的耦合距离为180 nm。一维光子晶体纳米梁腔圆孔周期数为26,镜面反射周期数为6,中心圆孔半径为109 nm,最外侧圆孔半径为79 nm,主线波导与下载波导的圆孔周期数为12,圆孔半径为79 nm。圆孔周期常数为a=330 nm。
仿真所得的调制器的光强分布如
图 12. 入射波长与谐振波长一致时稳定的光强分布
Fig. 12. Stable light intensity distribution when incident wavelength is consistent with resonant wavelength
光子晶体调制器的透射谱如
图 13. 调制器在“通”“断”状态下的透射谱
Fig. 13. Transmission spectra of modulator at “on” and “off” states
此时,该结构的消光比
调制速率V是指调制器单位时间内可调制的码元速率,单位为GHz,表征电光调制器“通”“断”状态的转换速度,与总响应时间
而系统总响应时间
仿真计算得到调制器“通”“断”状态的稳定时间,分别如
图 14. 模拟计算得到的调制器“通”状态的稳定时间
Fig. 14. Stable time at “on” state of modulator obtained by simulation calculation
图 15. 模拟计算得到的调制器“断”状态的稳定时间
Fig. 15. Stable time at “off” state of modulator obtained by simulation calculation
仿真计算得到的调制器3 dB带宽如
图 16. 模拟计算得到的调制器 3 dB 带宽
Fig. 16. 3 dB bandwidth of modulator obtained by simulation calculation
式中:
将本文所设计的调制器性能与文献[14-18]的调制器性能进行比较,结果如
表 1. 纳米梁腔电光调制器的性能对比
Table 1. Performance comparison of nanowire cavity electro-optic modulators
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4 结论
提出了一种基于SOI PCNC的反射壁下载型电光调制器,在一维PCNC两侧增加空气孔,形成反射壁下载型结构。根据缺陷模迁移理论,在谐振腔两侧通过掺杂形成PN结,通过施加驱动电压改变谐振腔内材料的折射率,从而谐振波长发生迁移,实现调制器对光信号的“通”与“断”调制。仿真结果表明,该电光调制器与其他纳米梁腔电光调制器相比,具有调制带宽及调制速率大、消光比高、结构简单、尺寸较小等优点,可以应用在双层光刻胶电子束曝光工艺中[29],而且易与其他硅光子器件级联,在高速大容量光通信系统和集成硅光子技术等领域中具有重要的应用价值。
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Article Outline
朱文玲, 陈鹤鸣, 徐文凯, 胡宇宸. 基于光子晶体纳米梁腔的反射壁下载型电光调制器[J]. 中国激光, 2023, 50(19): 1901005. Wenling Zhu, Heming Chen, Wenkai Xu, Yuchen Hu. Download Type Electro‑Optic Modulator with Reflective Wall Based on Photonic Crystal Nanowire Cavity[J]. Chinese Journal of Lasers, 2023, 50(19): 1901005.