Modelling and characteristics of photoelastic waveguides in Si1-xGex/Si heterostructures

The modelling of the strain, refractive index and optical mode intensity profiles of photoelastic waveguide structures in Si_1-x Ge_x/Si heterojunctions are presented. Strain profiles due to the difference in the thermal expansion of Si_1-xGe_x and SiN_y are produced by depositing SiN_y stripes on the heterostructure surface. The corresponding strain-induced refractive index changes are modelled by finite element analysis. The photoelastic constants are calculated and are used to find the strain-induced refractive index profiles in the waveguides. It is shown that these photoelastic waveguides exhibit a low degree of birefringence because they are oriented along the [100] crystal axis. Optical mode profiles are generated from finite difference calculations which show that there are three distinct guiding regions under each SiN_y stressor stripe; these vary with stripe width. Experimental results show good agreement with the model, and both show that the modal characteristics of the photoelastic waveguides are determined by the SiN_y stripe width