During the PIC fabrication process, materials with widely different thermal expansion coefficients are deposited and processed at high temperatures. They are later exposed to cooling down procedures, causing stress, which leads to strain (essentially, local anisotropic material expansion or contraction effects). This strain affects the dielectric tensor of the materials. This in turn will change the effective index of the waveguide modes – where different modes, especially different polarizations, are affected differently.
The Stress-Optic Simulation Module enables the calculation of the stress and strain induced by the interaction between the expansion or contraction of the materials, their Young’s modulus, and their geometry. It assumes a 2D cross-section, though bending of the wafer both in the cross-section plane and perpendicular to it is taken into account.
A full tensor model can be prescribed for all relevant physical parameters. The strain is calculated using a finite element model, and the resulting changes in the dielectric tensor of the materials in any location in the cross-section are used in a perturbation analysis on the modes of the waveguide, yielding effective index changes due to the stress.