Surface-Grating-Coupled Low-Loss Ge-on-Si Rib Waveguides and Multimode Interferometers

Germanium-on-silicon is a highly promising platform for planar photonics for the midinfrared, due to germanium's wide transparency range. In this letter, we report Ge-on-Si waveguides with record low losses of only 0.6 dB/cm, which is achieved using a 2.9-mu m thick germanium layer, thus minimizing mode interaction with dislocations at the germanium/silicon interface. Using these waveguides, multimode interferometers with insertion losses of only 0.21 +/- 0.02 dB are also demonstrated.