硅与铌酸锂异质集成电光调制器性能再创新高!

电光调制器作为光通信系统中的核心器件,承担着将电信号加载到光载波上的核心作用,其在大多数长距离陆地和海底光纤传输系统中扮演着至关重要的信息编码引擎角色。现如今,全世界已经使用了数百万个调制器,其中许多都是铌酸锂(LiNbO3)调制器(LiNbO3是一种在光通信领域广泛应用的人造晶体材料,具有良好的光学性质,在0.4-4μm的宽波长范围内光吸收损耗极低,同时具有非常强的非线性电光效应)。

尽管铌酸锂电光调制器已经广泛应用,但事实证明,其性能要满足光纤通信不断增长的带宽需求仍面临巨大的挑战。例如随着5G时代的来临,以及物联网、云计算、云存储等新兴技术的兴起,城市和数据中心的短距互联对光调制器提出了更高的要求—在满足更高集成度和更低功耗情况下要同时具备更大的带宽。

现如今,商用铌酸锂调制器能耗较高且体积较大,为了实现3.5V的半波电压往往需要器件的尺寸达到5 cm,而且关于其电光带宽是否能进一步提升始终没有找到确定的答案,这显然与未来光通信传输系统的需求背道而驰。

为了实现这种小型化和高性能的电光调制器,人们在硅、磷化铟、聚合物和等离子体等各种材料的研究上做了许多努力。其中,硅基光子学由于其具有独特的CMOS工艺兼容性、高集成度以及低成本等优势,在光通信领域已经成为了一个领先的平台。但是由于在纯晶体硅中并不存在泡克尔斯效应,因此不可能像在铌酸锂上一样去实现纯相位调制。

对于硅基调制器来说,其实现办法主要是依靠自由载流子色散效应,不幸的是,该效应本质上具有吸收性以及非线性,无法满足相干通信高阶调制中对线性度和纯相位调制的苛刻要求。此外,硅调制器往往需要在带宽、光损耗以及调制效率之间进行一系列权衡。因此,把硅和具有泡克尔斯效应的材料(如LiNbO3)进行异质集成可以为实现小型化和高性能的调制器提供解决方案。

近日,中山大学的蔡鑫伦教授课题组在Photonics Research 2020年第12期上( Shihao Sun, Mingbo He, Mengyue Xu, Shengqian Gao, Ziyan Chen, Xian Zhang, Ziliang Ruan, Xiong Wu, Lidan Zhou, Lin Liu, Chao Lu, Changjian Guo, Liu Liu, Siyuan Yu, Xinlun Cai. Bias-drift-free Mach–Zehnder modulators based on a heterogeneous silicon and lithium niobate platform [J]. Photonics Research, 2020, 8(12): 12001958)发表论文展示了一种高性能硅和铌酸锂混合集成调制器,其电光带宽大于70 GHz,半波电压为3 V,片上损耗小于1.8 dB,并成功演示了100 GB/s的OOK以及128 GB/s的PAM-4调制格式。

热调控制偏置点硅和铌酸锂异质集成调制器示意图

虽然该课题组去年已经演示了结构相似的异质集成调制器,该器件性能在这里得到了显著的提升。相较于之前的器件,该调制器在保持调制带宽不变的情况下,半波电压及损耗显著降低。此外,该器件还具有非常稳定的直流偏置点控制特性,这在实际应用中非常具有吸引力。

通常来说,LiNbO3调制器中存在直流偏置点漂移现象(偏置点会随时间变化而变化),该现象与直流电压作用下LiNbO3区域内电荷的流动和再分布有关。在调制器的实际应用中,往往需要一个实时偏置控制环路来稳定调制器的工作点,但这种环路将会增加系统的复杂性和引入更多的功耗。

该课题组利用硅波导以及侧顶部的钛加热薄膜电阻组成的热光移相器来替代传统的利用电光效应的偏置点控制器,在不影响调制器整体性能的情况下实现对偏置点的稳定控制。相较于铌酸锂上的电光移相器,硅的热光效应非常稳定,因此该方案可能只需要一个相对简单的控制环路就可以实现偏置点稳定功能。

蔡鑫伦教授认为,该异质集成方法的铌酸锂波导以及热光移相器是在SOI(Silicon-on-Insulator)芯片完成加工后通过高精度光刻套刻方法制作而得,这种制造过程具备高度的可扩展性。

由于硅光子平台的纯相位调制只能通过低速的热光效应来实现,虽然载流子色散效应允许高速相位调制但会引入与相位相关的损耗。因此,这里展示的能在硅光子平台有效的同时实现慢速和快速纯相位调制的方法,将有可能为通信及数据互联提供新一代的紧凑、高性能、非常稳定的电光调制器。这为许多对纯相位调制要求严格的应用场景例如量子光学、微波光子学等领域开辟新的实现途径。

Bias-drift-free Mach–Zehnder modulators based on a heterogeneous silicon and lithium niobate platform

Optical modulators provide the information encoding engines for most long-haul terrestrial and undersea fiber-optic transmission systems. The Internet that connects the people and businesses around the world can be possible because of the widespread of reliable, low-loss and high-speed optical modulators. As the trend toward ever-increasing bandwidths in fiber-optic communications continues, the optical modulator will remain an indispensable component for transmitting information. Today, hundreds of millions of modulators have been deployed worldwide, and many of them are lithium niobate (LiNbO3) modulators.

For more than two decades, LiNbO3 modulators have been the "bread and butter" for high speed and high fidelity electrical-to-optical conversion. LiNbO3 is a widely exploited crystal material that does not exist in nature. It has a trigonal crystal structure and has excellent physical properties, including large electro-optic coefficients and low optical absorption loss at a wide wavelength span from around 0.4 to 4 μm.

But in many ways, it has proven to be difficult to keep up with the trend toward ever-increasing bandwidths in fiber-optic communications. For example, the short reach links, such as metro and data-center interconnects, is a rapidly-growing field where the optical modulators must be operated in a compact space and featuring high performance in terms of power-consumption and speed. To date, the commercial LiNbO3 modulators are still bulky and power-consuming, with a moderate half-wave voltage (Vπ) of 3.5 V requiring device length of more than 5 cm, and with no clear route for further improving the modulation bandwidth (typically around 35 GHz), which limits their practical applications in the future optical links.

Many efforts have been made to realize compact and high-performance optical modulators in various material platforms, including silicon, indium phosphide, polymers and plasmonics. Among them, silicon photonics is apparently the leading platform due to the compatibility to the CMOS processing technology, the feasibility of dense integration, and the high-bandwidth and high-density I/O capability enabled by the compact waveguide dimensions. Because Pockels effect is absent in unstrained pure crystalline silicon, it is not possible to build a pure phase modulator in silicon in a similar way as the case in LiNbO3. Therefore, optical modulation in silicon mainly relies on free-carrier dispersion effect. Unfortunately, free-carrier dispersion is intrinsically absorptive and nonlinear. As a result, the silicon modulators normally suffer from the tradeoff among the bandwidth, optical losses and modulation efficiency. This is the reason why tremendous research efforts have been made to heterogeneously integrate silicon photonics with Pockels effect materials, like LiNbO3.

Prof. Xinlun Cai's group from the Sun Yat-sen University demonstrated a high-performance optical modulator by co-integrating LiNbO3 with silicon circuitry, in Photonics Research, Vol. 8, No. 12, 2020 (Shihao Sun, Mingbo He, Mengyue Xu, Shengqian Gao, Ziyan Chen, Xian Zhang, Ziliang Ruan, Xiong Wu, Lidan Zhou, Lin Liu, Chao Lu, Changjian Guo, Liu Liu, Siyuan Yu, Xinlun Cai. Bias-drift-free Mach–Zehnder modulators based on a heterogeneous silicon and lithium niobate platform [J]. Photonics Research, 2020, 8(12): 12001958). The present device shows a large modulation bandwidth (> 70 GHz), a low half-wave voltage ( 3 V) and low on-chip insertion loss ( < 1.8 dB). On–off keying (OOK) modulation up to 100 Gbit/s and PAM-4 up to 128 Gbit/s are successfully demonstrated.

Schematics of the bias-drift-free hybrid Si/LN modulator with silicon TOPS

Although the same approach has been demonstrated last year by our group, the performance here is boosted. Compared with the previous work, the present device shows much lower half-wave voltage while maintaining modulation bandwidth. Moreover, the present device exhibits a very stable DC bias point, which is very attractive for practical applications.

It's well known that the bias point in most LiNbO3 modulators change over time: this phenomenon is the so-called DC bias drift, which related to the flow and redistribution of electrical charges in LiNbO3 region under the application of the DC voltage. This requires a real time bias control loop to stabilize the operation point of the modulator. The present device utilize a thermo-optic phase shifter consisting of Ti heating film resistor on top of the silicon waveguide to control the DC bias point. The thermos-optic effect in silicon is very stable, leading to a much simpler control loop.

Prof. Cai believes that the demonstrated heterogeneous LN/SOI MZM achieve excellent optical modulation characteristics, featuring stable and substantially free of DC bias drift phenomena. LN phase modulation waveguides and the TOPS can be fabricated with lithographic precision and alignment accuracy in a back-end process after the SOI fabrication. This manufacturing procedure is highly scalable. As pure phase modulation capability in silicon photonic platform can only be achieved by utilizing slow thermal-optic effect, while carrier effects allow high-speed phase modulation accompanied by phase-dependent loss; the approach demonstrated here allows for the combination of both slow and fast pure phase modulation capabilities in silicon photonic platform. Therefore, the demonstrated approach potentially provides a new generation of compact, high-performance, and very stable optical modulators for telecommunications and data-interconnects, as well as opening up new avenues for many new applications, such as quantum photonics and microwave photonics, where pure phase modulation is crucial.