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

Abstract

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 orientation decreases as the degree of chain orientation increases, but that from residual thermal stress increases, which is the reason that no apparent trend was observed for the d(Δn)/dT of PI films adhering to Si substrates. Based on these examinations, the d(Δn)/dT of on-substrate PI films were successfully decreased to the levels of peeled PI films by incorporation of alicyclic structures in the main chains and/or the use of PI substrates whose CTEs are close to those of PI films.