A simple and low-cost cure monitoring system based on a side-polished plastic optical fibre

A novel cure monitoring system based on a side-polished interface on a plastic optical fibre is presented. Due to the mismatch of the refractive index of the fibre core and the external medium, part of the light beam is transmitted along the optical fibre by internal reflection and part is refracted in the sensitive zone. The phenomenon behind the principle of operation of the sensor is the reflection and transmission of light at an interface of two dielectric media traduced by Fresnel's equations. A good agreement between numerical and experimental results was achieved and real-time monitoring of the cure process was accomplished for a polyurethane resin and two different varnishes.