Birefringence of optical waveguides is closely related to the anisotropy of thermal stresses in the core. Closed-form solutions for estimating the thermal stresses in the cladding layers and in the core of an embedded channel waveguide are presented. The solutions are verified by finite-element simulation. It is found that the thermal stress in the core in the direction perpendicular to the wafer cannot be ignored, and one can tune the core stress anisotropy in the plane normal to the light-propagation direction by varying the thermal-expansion mismatch between the upper cladding layer and the substrate.
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