Characterization of optical properties of acrylate based adhesives exposed to different temperature conditions

Optical adhesives combine the traditional function of structural attachment with a more advanced function of providing an optical path between optical inter-connects. This article aims to characterize refractive index and birefringence of such adhesives under environmental exposure to different temperature conditions. Optical time domain reflectometery (OTDR) and prism coupling methods were employed to measure optical properties of an optical adhesive. Thermo-optic coefficient (dn/dT) of the adhesive was observed to decrease noticeably from -2 × 10^-4°C^-1 to -4 × 10^-4°C^-1 around the glass transition temperature (T_g ∼ 78°C). It is observed that refractive indices for both T_E and T_M modes increase with increasing annealing temperature, but the birefringence (T_E - T_M) is decreasing. This suggests that the material has become more isotropic due to the annealing. The environmental changes in optical properties of the adhesive are discussed in the light of Lorentz-Lorenz equations.