Mid-infrared polarisation rotator based on an asymmetric Ge-strip-on-si waveguide

Polarisation rotators are of great importance for high-data rate integrated photonic circuits. In this study, the authors provide the design of a polarisation rotator based on an asymmetric germanium strip on a silicon substrate operating in the mid-infrared band which has recently received a strong interest because of its huge potential for various applications. Two different asymmetric geometries, the slant and the two-step waveguides, are presented and theoretically investigated. Both of them can achieve single-mode transmission and high conversion efficiency ( > 90%) in short polarisation rotation lengths (-60 μm or -10λ and -120 μm or -20λ) and wide operation bandwidth (-5 μm ≤ λ ≤ -7 μm). The optimisation process and spectral analysis are provided and the properties of these two geometries are compared.