Graded SiGe waveguides with broadband low-loss propagation in the mid infrared

Mid-infrared (mid-IR) silicon photonics is expected to lead key advances in different areas including spectroscopy, remote sensing, nonlinear optics or free-space communications, among others. Still, the inherent limitations of the silicon-on-insulator (SOI) technology, namely the early mid-IR absorption of silicon oxide and silicon at lambda similar to 3.6 mu m and at lambda similar to 8.5 mu m respectively, remain the main stumbling blocks that prevent this platform to fully exploit the mid-IR spectrum (lambda similar to 2-20 mu m). Here, we propose using a compact Ge-rich graded-index Si1-xGex platform to overcome this constraint. A flat propagation loss characteristic as low as 2-3 dB/cm over a wavelength span from. =5.5 mu m to 8.5 mu m is demonstrated in Ge-rich Si1-xGex waveguides of only 6 mu m thick. The comparison of three different waveguides design with different vertical index profiles demonstrates the benefit of reducing the fraction of the guided mode that overlaps with the Si substrate to obtain such flat low loss behavior. Such Ge-rich Si1-xGex platforms may open the route towards the implementation of mid-IR photonic integrated circuits with low-loss beyond the Si multi-phonon absorption band onset, hence truly exploiting the full Ge transparency window up to lambda similar to 15 mu m. (C) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement