Brillouin properties influenced by glass transition in plastic optical fibers

A temperature-dependence coefficient of Brillouin frequency shift (BFS) in plastic optical fibers (POFs) is known to start decreasing drastically at a certain "Brillouin critical temperature". Here, we study the relationship between the critical temperature and the core glass transition temperature (T_g) using five poly(2,2,2-trifluoroethyl methacrylate) samples with different T_g values. In all the samples, the BFS is found to start decreasing abruptly at a temperature lower than T_g by approximately 10 °C, which agrees well with the phase transition starting temperature measured by using a differential scanning calorimeter. Thus, the critical temperature in POFs is shown to be adjustable by controlling T_g, leading to the possible development of distributed temperature sensors with ultra-high sensitivity.