Methodology to study the effect of moisture on refractive index of optical adhesive

Optical adhesives (index-matching adhesive) serve a means of structural attachment as well as provide an optical path between connecting elements. Optical properties (such as refractive index and birefringence) of these polymeric adhesives are sensitive to environmental factors, which affects optical device performance. Thus this work attempts to develop a methodology to evaluate the impact of moisture on the refractive index of an index-matching adhesive. The return loss method was employed for monitoring the moisture dependence of refractive index of an acylate-based optical adhesive. The test was performed at a wavelength of 1.55 μm. At fixed time intervals, the return loss of the sample were measured with an optical time-domain reflectometer (OTDR) during moisture desorption. The amount of moisture present in the optical adhesive at different time was calculated with the help of desorption data. Thus, a correlation between time, the amount of moisture present and refractive index was obtained. At a constant temperature of 120°C, the refractive index of the adhesive decreases as moisture content increases. However, this decrease was more drastic for small amount of moisture absorbed at -1 × 10 ^-4 (% moisture content)^-1 compared to -7 × 10 ^-6 (% moisture content)^-1 when moisture absorption exceeds 10% of the dry sample weight.