Contaminants of insulating silicone oil in power transformers has been analyzed through Microwave absorption and Fourier Transform Infrared studies. Microstructured fiber has been proposed to quantify contaminants by optical studies. Finite element analysis is implemented to study mode pattern changes in oil infiltrated fiber due to the changes in the refractive indices of oil samples and the same is verified exp erimentally. Studies have been performed to sense even early, a very mild degradation using image processing. © 2020 Elsevier B.V., All rights reserved.