Heterogeneously integrated widely tunable III-V-on-silicon Vernier lasers operating near 2.3 μm wavelength

Schematic of the widely tunable III-V-on-silicon laser.

Silicon photonics has initially been developed for ultrahigh-speed data transmission in computing systems and data centers, because of the manufacturing in a complementary metal oxide silicon (CMOS) fab and the high refractive index contrast of silicon-on-insulator (SOI) waveguides. However, the technology also enables low-cost, low-loss and ultra-compact silicon waveguide circuits in the mid-infrared wavelength range. These waveguides allow realizing high-performance building blocks for integrated mid-infrared laser sources, such as filters, beam combiners and gratings. The integrated lasers can be used as light sources for miniaturized spectroscopic sensors where the sensing is done either on the chip or in free space. The development of integrated laser sources operating in the 2-3 μm wavelength range is very interesting for trace gas detection and bio-sensing since this spectral region contains strong absorption lines of many important gases (e.g., CO2, CO and CH4) as well as an absorption band of blood glucose.

Recently, researchers from the Photonics Research Group of Ghent University-IMEC and the Walter Schottky Institute of Technical University of Munich demonstrated heterogeneously integrated widely tunable III-V-on-silicon lasers operating near 2.35 μm wavelength. In the device, an InP-based type-II epitaxial layer stack is heterogeneously integrated on a silicon photonic integrated circuit as the gain section to amplify the light. A Vernier filter consisting of two silicon micro-ring resonators is integrated in the laser cavity to select the lasing wavelength. The III-V-on-silicon Vernier lasers can provide a single mode tuning range over 30 nm in CW regime and ~50 nm in pulsed regime with a side mode suppression ratio higher than 35 dB over the whole tuning range. They also show that a wavelength tuning coverage over 70 nm can be achieved by combining two lasers with different distributed Bragg reflectors. Based on the tunable diode laser absorption spectroscopy technique, the laser integrated on silicon can be used to detect CO gas at different absorption lines. The experimental results are in good agreement with HITRAN database, which indicates that the III-V-on-silicon Vernier laser is suitable for optical sensing of gases over a broad wavelength range. Related results are published in Photonics Research, Vol. 6, Issue 9, 2018 (R. Wang et al., Widely tunable 2.3 µm III-V-on-silicon Vernier lasers for broadband spectroscopic sensing).

Prof. Gunther Roelkens from Ghent University-IMEC, the principal investigator of this project, believes that this work has significance for miniaturized spectroscopic sensing systems. The further work will focus on the tunable diode laser absorption spectroscopy of multi-species trace gases and on-chip sensing. In parallel, III-V-on-silicon photodetectors and spectrometers have been developed using the same technology for a fully integrated spectroscopic sensor system on silicon photonics.

可应用于光学传感的2.3 μm波段宽调谐硅上集成III-V激光器

宽调谐硅上集成III-V激光器示意图

得益于数据中心及计算机系统对高速数据传输日益增长的需求，硅基光电子学在过去的十五年迅速发展。近年来的研究表明，通过互补金属氧化物半导体（CMOS）工艺线的大规模稳定可靠加工，再配合基于绝缘体上硅（SOI）波导材料的高折射率差，研究人员可以获得低损耗、低成本并且极紧凑的中红外硅基光波导结构。这样的硅基光波导可用于实现片上集成的中红外激光器所需的各类高性能元件，例如滤波器、合束器以及光栅。而集成的中红外激光器是片上光传感系统的关键元件，对光学传感器小型化具有至关重要的作用。特别是在2-3 μm波段，由于大量的重要工业气体以及血糖等在都在这个波段有着很强的吸收，所以该波段的片上集成中红外激光器有非常好的应用前景。

最近，来自比利时根特大学、欧洲微电子研究中心(IMEC)和慕尼黑工业大学的研究小组研发出一种工作在的2.35 μm波段的宽调谐硅上集成III-V激光器。在这种激光器中，磷化铟(InP)基的二类量子阱外延结构被键合到硅基光波导上来实现片上集成的半导体光放大器。而激光器的工作波长可以通过调节两个硅基微环滤波器的重合共振峰来控制。在连续波条件下，该激光器具有超过30 nm的可调谐波长范围，而在脉冲条件下可以实现50 nm的调谐范围。在整个调谐范围内，激光器输出光谱的边模抑制比都超过了35 dB。他们的研究也表明调节分布布拉格反射镜（DBR）的光栅周期可以控制激光器的调谐范围。通过集成两个具有不同周期DBR的激光器，甚至可将调谐范围扩展到70 nm。基于可调谐激光吸收光谱技术，这种硅上集成的激光器可用于在不同的吸收峰探测CO气体，实验结果与HITRAN数据库非常吻合，这表明该激光器非常适合在较宽波长范围内进行光学传感探测。相关结果发表在Photonics Research 2018年第6卷第9期上 (R. Wang et al., Widely tunable 2.3 µm III-V-on-silicon Vernier lasers for broadband spectroscopic sensing)。

该项目的负责人Gunther Roelken教授认为这项研究对于实现硅基光电子传感器和小型化的传感系统具有重要意义。在未来的研究中，该团队将采用硅上集成的中红外激光器进行多种痕量气体的同时探测。与此同时，他的团队已经实现了中红外硅基光电子传感器所需的其他元件，例如硅上集成的III-V探测器和光谱仪。