Enhancing Sensitivity of Fiber Bragg Grating-Based Temperature Sensors through Teflon Coating

As global warming and climate change persistently threaten our planet, especially in developing countries, environmental monitoring becomes more and more imperative. One of the key parameters in estimating ecological disturbance due to climate change is temperature. In this work, we design a temperature sensor based on a fiber Bragg grating (FBG) and investigate the effect on its sensitivity upon applying a polymeric coating, and contrast it with different metal coatings. The reflected spectrum of an FBG has a narrow peak, corresponding to the Bragg wavelength, which is contingent on the grating period, grating length and effective refractive index. We simulate an outdoor ambient environment and examine the change in the peak reflected wavelength on variation in temperature. The shift in the peak for a unit change in temperature can be defined as the sensitivity of the FBG sensor. To enhance the sensitivity, we apply a uniform coating of Polytetrafluoroethylene (PTFE), more popularly known as Teflon. We also compare the sensitivity obtained on using PTFE coating with that through other materials, such as zinc, aluminum and stainless steel. It is visibly evident from this assessment that the coefficient of thermal expansion (CTE) has a significant effect on the sensitivity, when other physical and mechanical parameters are maintained constant. Since the CTE value is much higher for polymers than it is for metals, PTFE is able to provide a sensitivity of 0.3 nm/ degrees C, which is impressive when compared to zinc, the metal offering the highest sensitivity of around 0.079 nm/ degrees C. However, the CTE of PTFE itself varies with temperature, which is why we predict a sudden nonlinearity in the temperature dependence of the reflected wavelength, in an experimental scenario. Investigating this deviation leads us to the conclusion that, in the temperature range of 19-25 degrees C, PTFE undergoes two phase transitions from a triclinic to a hexagonal crystal phase, and subsequently to a pseudo-hexagonal phase. We calculate the sensitivities for the various phases of PTFE, and conclude that the high numerical and simulated values make this technology a promising application for futuristic purely optical sensors.