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

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Abstract
The modelling of the strain, refractive index and optical mode intensity profiles of photoelastic waveguide structures in Si1−xGex/Si heterojunctions are presented. Strain profiles due to the difference in the thermal expansion of Si1−xGex and SiNy are produced by depositing SiNy 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 SiNy 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 SiNy stripe width.