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Control of polarization dependences of thermo-optic (TO) coefficients, i.e., the temperature dependence of birefringence (d(Δn)/dT), is critical issue for optical materials. Especially in active optical components like TO switches, polarization dependent behaviors are caused by d(Δn)/dT of materials. Origins of polarization dependence of TO coefficient were investigated for seven kinds of polyimide (PI) films in terms of main chain orientation and residual thermal stress. The values of d(Δn)/dT of PI films originated from chain orientation vary from nil to negative as the degrees of in-plane orientation increase. Magnitudes of d(Δn)/dT depend on intrinsic birefringences, second order orientation coefficients, and their temperature dependence. On the other hand, those of d(Δn)/dT originated from residual stress are negative for PI films exhibiting large in-plane coefficients of thermal expansion (CTE) and vary to zero as their CTE decreases due to reduced residual stress. d(Δn)/dT originating from chain or...

Polarization dependence of thermo-optic coefficients in polyimide films originating from chain orientation and residual thermal stress