Modal birefringence and power density distribution in strained buried-core square waveguides

The optical properties of engineered devices derive from the flow of energy within the structure, a flow that is governed by the interaction between device architecture and material properties. With evolving device and system sophistication, implementation of manufacturing processes becomes an exercise in global optimization. This task is addressed in the design phase by choosing metrics that correlate with critical operating parameters of the device. We illustrate two complementary metrics, arising from the same physical effect, that manifest at different length scales. By considering the thermoelastic properties of a doped-glass multilayer with an embedded waveguide, we reveal the impact of stress on modal birefringence and optical power distributions. The modal birefringence of a single-mode buried-core square waveguide irreducibly describes a discrete metric whose value derives from global strain contributions. It is an ideal metric for addressing polarization-dependent wavelength shifts in arrayed waveguide grating devices, which are particularly sensitive to material stresses over long length scales. By contrast, the power density distribution in the bound mode is a distributed metric dependent on local properties and is thus more suitable for devices whose operability is dependent upon shorter length scales. These complementary characterizations of stress-optical coupling are numerically assessed by finite element analysis. The material stresses are first found through solution of the structural problem, with subsequent solution of the the full-vector anisotropic Maxwell's equations as an eigenvalue problem. We found the modal birefringence to be primarily governed by the properties of the cladding material, the core properties having a negligible effect, while fine control may be achieved by varying the position of the interface between the lower and upper cladding. This demonstrates its suitability as a metric for devices primarily reliant on isolated waveguides. At shorter length scales, through contrast with the isotropic case, we showed that stress-optical coupling suppresses the in-plane symmetry axis of the logarithmic difference in spatial power densities, regardless of the degeneracy of the optical mode, with asymptotic behavior that effectively diverges. This revealed a metric of potential applicability for interferometric and evanescently coupled structures.