Intrinsic Temperature Sensitivity of Fiber Bragg Gratings in PMMA-Based Optical Fibers

Thermal response of fiber Bragg gratings (FBGs) in polymethyl methacrylate-based optical fibers doped with trans-4-stilbenemethanol is studied at 0% relative humidity (RH) from -20^°C to 60^°C. The Bragg wavelength is found to negatively shift upon heating in a nonlinear manner. The higher the temperature, the larger the magnitude of wavelength shift is. The absolute value of calculated temperature sensitivity also increased with temperature. The sensitivity at room temperature is about -1 pm/^°C, which is one order of magnitude lower than that measured at constant RH for polymer FBGs and that of silica FBGs. Such low temperature sensitivity challenges the common notion of high temperature sensitivity of polymer FBGs. In addition, it is found that the dependence of Bragg wavelength of the polymer FBGs on RH is nonlinear. Below 30% RH, the Bragg wavelength shift upon RH increase is minimal compared to that above 30% RH.