We demonstrate elimination of temperature sensitivity for fiber Bragg gratings (FBGs) in polymer perfluorinated (CYTOP) fiber by gamma radiation treatment. Several FBG samples inscribed line-by-line in a few-mode fiber with 20-µm CYTOP core and 250-µm XYLEX overclad received gamma radiation doses in the range from 80 to 520 kGy. Initially positive value of temperature sensitivity (19.6 pm/℃) decreased with received dose with subsequent change of sign from positive to negative. Among the irradiated samples, the one that received 200-kGy dose demonstrated the closest to zero temperature sensitivity (≈ 1 pm/℃). Along with a decrease of temperature sensitivity, we observed an increase of RH sensitivity with received dose from 13.3 pm/%RH for pristine FBG up to 56.8 pm/%RH for the case of 520 kGy dose. Thus, gamma radiation treatment of CYTOP FBGs provides decrease of their temperature sensitivity and increase of RH sensitivity simultaneously. Correct selection of the irradiation dose allows to eliminate temperature sensitivity for RH sensing application.
Polymer fiber Bragg grating (FBGs) demonstrate a wider strain range and stronger temperature sensitivity compared to standard silica FBGs. Besides, their advantageous feature is sensitivity to humidity that enables FBG-based relative humidity (RH) sensing. However, practical realization of RH sensors requires temperature cross-sensitivity elimination. A certain optimal fiber pre-strain and gamma irradiation of perfluorinated (CYTOP) FBGs up to certain optimal dose are potential recently proposed solutions for this problem. In this work, we investigate temperature and RH response of FBGs inscribed line-by-line in a few-mode polymer fiber with 20-μm CYTOP core and 250-μm XYLEX overclad. We compare the cases of the pristine FBG sample and the sample received 200 kGy irradiation dose. The 200-kGy dose was previously confirmed to provide temperature sensitivity minimization at 40%RH. Here, we show the close-to-zero temperature sensitivity (≈1pm/°C) for 200-kGy dose at high RH value of 89%. Besides, we briefly analyze the stabilization process of FBGs response to strong and quick RH changes.
We report on the inscription of fiber Bragg grating in perfluorinated polymer optical fiber (CYTOP) by femtosecond pulses laser at 400 nm with phase mask technique and at 800 nm with two direct inscription methods: point-by-point and line-byline. We analyze the stability of the gratings after inscription and study their sensing properties. To discriminate between the properties of the CYTOP fiber and the influence of the over-clad, we consider gratings produced in CYTOP fiber with and without the over-clad, and we determine their sensitivities to temperature, humidity and strain.
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