Temperature and RH Response of Few-Mode Polymer CYTOP/XYLEX FBGs: Gamma-Radiation Tuning of Sensing Properties

We consider thermal and hygroscopic properties of polymer fiber Bragg gratings (PFBGs) inscribed in a few-mode perfluorinated CYTOP fiber with XYLEX protection overclad. The PFBGs were inscribed with a femtosecond laser using a plane-by-plane technique. The influence of γ-radiation treatment on PFBG temperature (T) and relative humidity (RH) sensitivities is presented. The RH sensitivity increases with received dose from 13.3 pm/%RH for pristine PFBG up to 56.8 pm/%RH for the dose of 520 kGy. The T sensitivity decreases from 19.6 pm/°C down to −38.9 pm/°C correspondingly. Irradiating the PFBGs with optimal γ-radiation dose (≈200 kGy) enables RH sensing with no T cross-sensitivity. The RH response time of PFBGs and its dependence on T and received γ-radiation dose is considered as well. Finally, the impact of RH on PFBGs reflection power is presented.