Ge-rich SiGe-based wideband polarization insensitive photonic platform for mid-infrared free-space communications

The recent advances in the development of quantum cascade laser with room temperature operation in the mid infrared paved the way for the realization of wideband communication systems. Particularly, two mid-infrared atmosphere transparency windows lying between 3-5 μm and between 8-14 μm exhibit great potential for further implementation of wideband free space communications. Additionally this wide unregulated spectral region shows reduced background noise and low Mie and Rayleigh scattering. Despite the development of a plethora of photonic components in mid infrared such as sources, detectors, passive structures, less efforts have been dedicated to investigate polarization management for information transport. In this work, the potential of Ge-rich SiGe waveguides is exploited to build a polarization insensitive platform in the mid-infrared. The gradual index evolution in SiGe alloys and geometric parameter optimization are used to obtain waveguides with birefringence below 2×10-4 and an unprecedented bandwidth in both atmosphere transparency windows i.e. near 3.5 μm and 9 μm. Following waveguide birefringence optimization an ultra-wideband and polarization insensitive multimode interference coupler was designed. The optimized structure shows a 4.5 μm wide bandwidth in transverse electric and transverse magnetic polarization at 9 μm wavelength. The developed ultra-wideband polarization insensitive photonic building blocks presented in this work pave the way for further implementation of free space communication systems in the mid infrared spectral region.