Quantum Cascade Laser Integration with Mid-Infrared Photonic Integrated Circuits for Diverse Sensing Applications

Mid-IR gas sensing has revolutionized several applications ranging from industrial process control to environmental monitoring and healthcare. Characteristic absorption lines of many critical molecules lie in the mid-IR region (i.e., 2–25 μm) with sparse spectral distribution allowing selective spectroscopic detection in a fast, reliable and consumables or maintenance free operation. Quantum cascade lasers (QCLs) have enabled fast, and reliable sensing with higher signal-to noise ratio. Heterogeneous integration of QCLs on photonic integrated chips (PICs) allows to achieve low-cost and compact design with multiplexing of many different wavelengths in the individual devices opening wide possibilities with enhanced performances. For example, QCLs having relatively higher power output at different wavelengths can be fabricated on wafer scale and then integrated on separately fabricated PICs. Germanium's advantages like wide transparency in mid-IR range (up to 15 μm) and strong non-linear refractive index has favored its application in the mid-IR range as compared to the standard SiN and SOI platforms. This work presents different hybrid integration techniques to integrate multiple mid-IR lasers to butt-couple with respective Ge waveguides on individual Ge-on-SOI PICs in epi-up and epi-down orientation for several sensing applications.