, Volume 34, Issue 11, pp 1378-1384

Refractive indices variation with temperature and humidity of optical adhesive

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Abstract

Refractive index and birefringence of optical adhesives is studied under different environmental exposures, such as temperature, humidity, and annealing. Refractive index of the adhesive decreases at a higher rate for the temperature treatment compared to the humidity exposure. A sudden decrease in the rate of refractive index (thermo-optic coefficient, dn/dT), from −2×10−4°C−1 to −4×10−4°C−1 was observed near the glass transition temperature. The adhesive’s refractive index (n) was studied during the moisture desorption cycle. Refractive index of the adhesive was found to increase at a constant rate with moisture. The change in refractive index of the adhesive upon moisture absorption was found to be irreversible. Birefringence (TE — TM) decreases with higher annealing temperature treatment of the adhesive, because the rate of increase in refractive indices for TE and TM modes is not similar. The refractive index in TM mode increases at a higher rate compared to that in the TE mode. This suggests that the material has become more isotropic due to annealing. The environmental changes in optical properties of the adhesive are discussed in light of Lorenz-Lorentz equations.