硫系材料光子集成支撑下一代2 μm波段宽带光互连新纪录

光纤通信系统的传输容量与延时在传统通信波段和传统石英光纤中的提升,已经逐渐接近极限,2 μm新波段通信逐渐受到研究人员的关注。空芯光纤具有超低延时、超低非线性、超大带宽和理论更低损耗的优点,为2 μm波段光通信提供了可靠的传输介质。此外,掺铥光纤放大器也在2μm波段提供了宽谱增益,保障了宽带传输性能。因此,2 μm新波段通信成为下一代超低延时超大容量光纤通信的重要潜力发展方向。目前,2 μm波段空芯光纤短距互连系统的最高单通道速率已达100Gbps。同时,面向2 μm片上互连的光子集成平台也是亟待发展的领域。

中红外波段的光子集成在多个材料平台上都有过研究和报道,如绝缘层上硅(SOI)、硅上锗、蓝宝石上硅、氮化硅、磷化铟、硫系玻璃(Chalcogenide)等。对于短波红外2 μm来说,SOI是更为常用的材料平台,然而相较传统的1550 nm波段,硅和二氧化硅在更长的2 μm波段处有更高的材料吸收损耗。

与硅材料相比,硫系材料在中红外波段具有很宽的透明窗口(0.8~20 μm),更低的双光子吸收(0.01~1×10-12 m/W),因此在2 μm波段具有更低的线性和非线性损耗。而且,硫系材料的折射率在2~3之间,用空气和二氧化硅做包层介质的硫系波导具有很好的集成度。此外,由于硫系材料折射率的色散曲线及波导有效折射率随波导尺寸的变化都更为平缓,硫系集成器件具有更宽的光学带宽,对波导尺寸的加工容限也更大。以上的优势都使得硫系材料为2 μm波段低损耗高集成度的光子集成器件提供了更优的平台。

近日,上海交通大学杜江兵研究员、何祖源教授团队,与中山大学张斌副教授、李朝晖教授团队,以及哈尔滨工业大学徐科副教授合作,在硫系材料平台上设计并制备了2 μm波段光子集成模分复用互连系统,并实现了3×80 Gbps的模分复用总传输速率(是已有报道结果的12倍),打破了2 μm波段光互连系统原有传输速率的记录。

研究团队在二氧化硅衬底上制备了600 nm厚的硫化砷(As2S3)条形波导,加工了多个在2 μm波段工作的关键无源器件。自主制备的硫系单模波导在2 μm波段具有1.4 dB/cm的低传输损耗。在此基础上,设计的标准结构垂直光栅耦合器具有4.3 dB的单端耦合损耗,且3 dB带宽为123.6 nm(几乎是SOI平台光栅耦合带宽的2倍),因此2 μm硫系平台可实现更宽带的波分复用。在此基础上配合宽带掺杂铥光纤放大,可以实现2 μm波段从光纤到芯片的宽带通信系统。

此外,研究团队还展现了其他无源器件的结果,包括布拉格光栅滤波器、功率分配器、马赫-曾德尔干涉仪,以及面向模分复用的模式转换器。实现两个高阶模耦合的模式转换器在宽谱范围内测得小于2 dB的插入损耗、低于-20 dB的模间串扰,并且加工误差在40 nm内可保持-1 dB以内的附加损耗,展现了优秀的加工可靠性。

杜江兵研究员认为,硫系材料光子集成为2 μm波段高速光互连和大容量光通信提供了一个新的高性能平台,也为面向中红外波段更宽广的应用提供了广阔的空间。较低的传输损耗、器件的宽带特性、较为宽松的加工误差容限、可靠的器件性能等,都使得硫系光子集成平台有能力同时支持宽带波分复用和模分复用,从而实现超高容量的2 μm光互连。此外,其他并未提及的关键器件,如面向宽带波分复用的阵列波导光栅,也是未来进一步提升2 μm光互连传输带宽需要解决的问题之一。

该工作发表在Photonics Research 2020年第9期(Weihong Shen, Pingyang Zeng, et al. Chalcogenide glass photonic integration for improved 2 μm optical interconnection[J]. Photonics Research, 2020, 8(9): 09001484.) ,上海交通大学沈微宏、中山大学曾平羊为共同第一作者,上海交通大学杜江兵研究员与中山大学张斌副教授、李朝辉教授为共同通讯作者。

基于硫系材料的2 μm光子集成平台

更大带宽和更宽加工容限的硫系光子集成

Chalcogenide glass photonic integration for improved 2 μm optical interconnection

In recent years, the 2 μm waveband has attracted increasing research for broad applications, including optical communication, optical sensing, and even the next generation of gravitational-wave observatories. Particularly, thanks to hollow-core fibers with low latency and low loss in an ultrawide bandwidth, and thulium (Tm) doped fiber amplifiers with a broad gain spectrum, 2 μm possesses the promising application of next-generation optical communication with ultralow latency and further higher capacity. Several 2 μm optical short-reach interconnections over hollow-core fibers have been carried out in our previous works, achieving a record single-lane speed of 100 Gbps. Meanwhile, the photonic integrated platform for 2 μm optical interconnection is also an essential field that needs to be exploited.

Till now, fabrications of mid-infrared (MIR) waveguides have been demonstrated in different platforms, including silicon-on-insulator (SOI), germanium-on-silicon, silicon-on-sapphire, silicon nitride, indium phosphide, and chalcogenide glasses (ChGs). In the range near 2 μm, the majority of integrated waveguides are still fabricated on SOI. However, chalcogenide (ChG) materials, with broadband infrared transparency (0.8~20 μm) and lower two-photon absorption α2 (0.01~1×10-12 m/W) than silicon, have extremely low linear and nonlinear propagation loss at 2 μm. With a relatively high refractive index of n ≈ 2-3, more compact on-chip integration can be realized on ChG. Thanks to the slower slop of refractive index v. s. wavelength and waveguide width, ChG components natively present a wider optical bandwidth and less sensitivity to the fabrication deviations. In a word, all of the merits mentioned above make the low loss at 2 μm and high-density integration of ChG optical devices possible.

Besides, a high nonlinear coefficient makes ChG realizable to obtain efficient nonlinear effect at lower pump threshold, like stimulated Brillouin scattering. As a result, ChG have been widely studied for on-chip all-optical signal processing, including frequency comb generation, supercontinuum generation, wavelength conversion and so on. In addition, since it's easy to deposit ChG films on many substrates like conventional silicon or polymer substrates at low temperature (<300℃), ChG can be part of hybrid waveguides for 2 μ- photonic integrated chips with other materials. Overall, holding various characteristics covering wide application range, ChGs present a significant and promising integrated platform, particularly for 2 μm optical communications.

ChG photonic integrated circuit for improved 2 μm optical interconnection was presented in Photonics Research, Volume 8, Issue 9, 2020 ( Weihong Shen, Pingyang Zeng, et al. Chalcogenide glass photonic integration for improved 2 μm optical interconnection[J]. Photonics Research, 2020, 8(9): 09001484.), under the cooperation between the research group of Prof. Jiangbin Du, Prof. Zuyuan He from Shanghai Jiao Tong University and that of Prof. Bin Zhang, Prof. Zhaohui Li from Sun Yat-sen University.

The researchers build up the ChG integrated platform with 600-nm-thick As2S3 strip waveguide on silicon dioxide substrate, and design several key passive components for 2 μm optical interconnection. The fabricated strip waveguide with single mode at 2 μm has low propagation loss of 1.4 dB/cm. The fiber-to-chip vertical coupling loss is optimized at 4.3 dB/facet with 123.6 nm 3 dB bandwidth around 2 μm, which is nearly 2 times wider than the grating bandwidth on SOI, so that further broadband wavelength division multiplexing (WDM) at 2 μm can be realized on ChG platform. Other essential components including Bragg grating filter, power splitter, Mach-Zander interferometer (MZI), and mode converters for on-chip mode division multiplexing (MDM), are also designed and fabricated at 2 μm with highly reliable performances. Mode converters for 2 higher-order modes present the insertion loss of less than 2 dB, crosstalk of less than -20 dB in a wide range, and the fabrication tolerance of waveguide width shows improved - 1 dB bandwidth of 40 nm. Finally, by means of the proposed As2S3 platform, they report a demonstration of on-chip MDM transmission at 2 μm, achieving total bit rate of 3×80 Gbps with error-free bit error rate lower than 3.8×10-3, which holds 12 times larger capacity than that of the previous results, and breaks the record of total capacity for 2 μm optical communication.

Prof. Jiangbing Du from Shanghai Jiao Tong University believes that, ChG platform draws the promising prospect for the future photonic integration and high-speed interconnection at 2 μm waveband. The low propagation loss and reliable devices with broadband operation and large fabrication tolerance make the ChG platform capable of holding broadband WDM and MDM simultaneously, so as to achieve ultrahigh-speed interconnection at 2 μm. Furthermore, other essential components such as arrayed waveguide grating for 2 μm WDM can be designed on the proposed ChG platform in the future, in order to further increase the total capacity of 2 μm optical interconnection.

ChG photonic integrated key components for 2 μm optical interconnection