Controlled-chemical etching of the cladding in optical fibers for the design of analytical sensors

An optimized chemical method is developed for etching the cladding of an optical fiber without degrading its core. The goal is to obtain a smooth core surface that will be used further for sensor development. This technique consists of attacking the cladding with a buffer solution made of fluorinated salts. Three different concentrations in the buffer are used: 20%, 40%, and 60%. The bare core could then be used for sensing application thanks to interaction with the light signal. By following the evolution of the fiber diameter with the etching time, two different kinetics are observed for each concentration. These two parts of the curve correspond to the attack of the cladding and the core, and the transition point allows to find the time when the cladding is removed. With increased buffer concentration, this etching time is decreased. Concerning the evolution of a light signal transmitted throughout the fiber during the attack, three zones are observed on the transmitted signal: (i) a small linear decrease at the beginning of the attack (degradation of the cladding), (ii) a high drop in transmission (cladding is removed), and (iii) again a small linear decrease of the transmission (progressive degradation of the core). The more the buffer concentration is high, the higher is the transmitted signal loss. Finally, an optimized chemical condition is determined to obtain a good compromise between the time of etching and the resulting signal loss in transmission while limiting the chemical risks associated to the use of fluorinated salts.