Fibre coupling to SiGe optoelectronic devices

Recent years have seen the rapid evolution of silicon-based optoelectronics. The absorption properties of Si_1-xGe_x alloys make them excellent materials for the realisation of photodetectors operating at the near-infrared wavelengths of 1.3 and 1.55 μm which are important for fibre optic communication systems. An important design consideration for such systems is the efficient coupling of optical fibres to optoelectronic integrated circuits. A Si _1-xGe_x rib waveguide spot-size transformer is presented that performs two essential roles: the transformation of the large spot fibre to a small spot waveguide mode, and subsequently the transfer of the mode from a passive Si waveguiding region to an active Si_1-xGe_x region where optical absorption may occur. This transformation is accomplished by the use of a tapered waveguide structure. An analysis technique based on the spectral index method is used to investigate the local modal behaviour of the structure and experimental results confirm its successful operation