Simple Optical Fiber Interferometer for Dynamic Measurement of Refractive Index and Thickness of Polymer Films

Fiber optic-based refractometers is a thoroughly researched field, with many different configurations being used. However, most designs require external calibration using substances of known refractive index (RI) and their fabrication process might be impractical and time consuming, creating the need for a quick and accurate method of measuring RI of different substances. A simple method for simultaneous measurement in real-time of RI and thickness of polymer thin films is presented, allowing dynamic measurements in the presence of changing environmental parameters, such as temperature or humidity. This method, which does not require previous calibration, is based on an inline Fabry-Perot (FP) cavity, created by dipping the tip of a cleaved optical fiber (OF) in a polymer solution. The procedure consists of using the equations of the low finesse FP interferometers to directly extract information from the structure created, such as RI and cavity length, by working in the spectral window from 1500 to 1600nm. The method was validated by creating FP cavities with liquids of known RI, for which a typical precision of ${3}\times {10}^{-{3}}$ was achieved, along with errors lower than 0.6% and 1% for RI and cavity length determination, respectively. The procedure was then used to monitor three different curing processes, namely the temperature curing of Sylgard™ 184, the UV curing of Norland Optical Adhesives™ 65 and the mixing and curing of Ceys™ Araldite epoxy glue. Both RI and cavity length were compared to reference values, showing excellent agreement with the experimental results for a method that does not require external calibration.