Low-loss Ge-rich Si_0.2Ge_0.8 waveguides for mid-infrared photonics

We demonstrate low-loss Ge-rich Si_0.2Ge_0.8 waveguides on Si_1-xGe_x (x from 0 to 0.79) graded substrates operating in the mid-infrared wavelength range at λ=4.6  μm. Propagation losses as low as (1.5±0.5)dB/cm and (2±0.5)dB/cm were measured for the quasi-TE and quasi-TM polarizations, respectively. A total coupling loss (input/output) of only 10 dB was found for waveguide widths larger than 7 μm due to a good fiber-waveguide mode matching. Near-field optical mode profiles measured at the output waveguide facet allowed us to inspect the optical mode and precisely measure the modal effective area of each waveguide providing a good correlation between experiments and simulations. These results put forward the potential of low-index-contrast Si_1-xGe_x waveguides with high Ge concentration as fundamental blocks for mid-infrared photonic integrated circuits.