Silicon photonic platform for mid-infrared applications
Compact carbon dioxide (CO2) gas sensor based on integrated mid-infrared (MIR) photonic circuit. The waveguide between the MIR source and detector is the component responsible for analyte interaction. The spiral configuration of the waveguide increases the interactive length and thus enhances the sensitivity.
Mid-infrared (MIR) wavelength, ranging from 2 to 20 μm, covers a lot of molecules' characteristic absorption spectra. This feature gives the opportunity to build up MIR photonic sensors used in environmental and industrial process monitoring, as well as bio-sensing. The atmospheric window in MIR range can also be utilized for free-space communication and light detection and ranging (LIDAR). Silicon photonic technology is an attractive solution for such applications, due to its advantages and successes exhibited at the near-infrared range.
A host of complementary metal-oxide-semiconductor (CMOS)-compatible platforms, including silicon-on-insulator (SOI), silicon-on-silicon nitride, germanium-on-silicon, germanium-on-SOI, germanium-on-silicon nitride, sapphire-on-silicon, silicon germanium alloy-on-silicon, and aluminum nitride-on-insulator are explored in order to realize low loss waveguides for different MIR wavelengths. Several passive and active MIR photonic devices based on these platforms have been demonstrated by different research groups over the world. The state-of-art achievements are comprehensively reviewed in Photonics Research, Volume 5, No. 5, 2017 (T. Hu, et al., Silicon photonic platforms for mid-infrared applications).
The authors' research and development effort on SOI and silicon nitride-on-insulator platforms for short MIR range are also introduced. Low loss MIR waveguides and basic passive components working at 2 and 3.8 μm have been demonstrated. To illustrate the application, the implementation of a CO2 gas senor is proposed. The theoretical analysis predicts that the spiral MIR waveguide based sensor can obtain a detection limit as low as 20 ppm when the propagation loss is 1.5 dB/cm. The detection sensitivity can be further improved if the propagation loss is reduced. Dr Patrick Guo-Qiang Lo, the Program Director of Nano-Photonics in Institute of Microelectronics (IME), Agency for Science, Technology and Research (A*STAR) Singapore, points out that the related work is of great importance in extending the applications of integrated photonics.
Further work should focus on the realization of high performance MIR passive devices and the integration of them with MIR sources and detectors, to demonstrate gas sensors with high sensitivity.
硅基光子集成回路在中红外波段的应用
基于中红外光子集成回路的紧凑型CO2气体传感器。中红外光源和探测器之间的光波导是与被探测物相互作用的元件，其螺旋结构可以增加相互作用的长度从而提高灵敏度。
中红外波段（2—20 μm）涵盖了很多分子的特征吸收谱。这一特性使得构建中红外光子传感器成为可能。这些传感器可应用于环境、工业的检测以及生物传感。此外，中红外波段含有大气高透射率窗口，可用于自由空间光通信和激光雷达系统。鉴于硅光子在近红外波段所表现出的优势，在中红外的应用中，它仍然是极具吸引力的技术方案。
许多互补金属氧化物半导体 (CMOS)工艺兼容的材料体系和技术平台被开发用于实现不同中红外波段的低损耗光波导。这些材料包括绝缘衬底上硅、氮化硅上硅、硅上锗、绝缘衬底上锗、氮化硅上锗、硅上蓝宝石、硅上硅锗合金以及绝缘衬底上氮化铝。基于这些材料，世界范围内的许多课题组制作出了中红外无源、有源光子器件。最新的相关研究成果综述发表在Photonics Research 2017年第5卷第5期上 (T. Hu, et al., Silicon photonic platforms for mid-infrared applications)
该文还报道了作者在短中红外波段所做的研究工作。他们设计并实现了基于绝缘衬底上硅和绝缘衬底上氮化硅的用于2 μm和3.8 μm的低损耗中红外波导及无源光子器件，提出了二氧化碳气体传感器作为中红外应用的例案。分析指出，使用传输损耗为1.5 dB/cm的螺旋光波导可以实现20 ppm的探测极限。如果波导传输损耗减小，探测灵敏度可以进一步提高。新加坡科技研究局微电子研究院纳米光子研究中心主任卢国强博士认为，相关研究对扩展集成光学在其他方面的应用具有重要意义。
下一步的工作重点是将高性能无源器件与中红外激光器、探测器集成，实现高灵敏度气体传感器。