Investigation of Hysteresis in the Temperature Response of Metal-Coated Optical Fibers

Fiber-optic sensors are widely used for measuring physical properties, such as strain, displacement, pressure, and temperature, during structural health monitoring. In this study, we analyzed the change in Brillouin frequency according to the temperature of metal-coated optical fibers installed in a distributed optical fiber sensor. We used aluminum (Al)- and copper (Cu)-coated optical fibers for the analysis. The existence of hysteresis was confirmed between the Brillouin frequency changes during the heating and cooling of the metal-coated optical fiber. In addition, the Al-coated optical fibers exhibited lower hysteresis against the temperature changes than the Cu-coated optical fibers. This was attributed to the thick Cu coating and the larger Young's modulus of Cu. The Brillouin frequency of the metal-coated optical fiber exhibited a rapid change at an initial temperature of approximately 5 °C; however, the changes remained constant thereafter. Therefore, the obtained results confirm that metal-coated optical fibers can be used as distributed fiber-optic sensors without hysteresis under continuous heating or cooling conditions.