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In this work, we investigate the influence of gamma radiation treatment on sensing properties of fiber Bragg gratings (FBGs) inscribed in polymer CYTOP fiber with line-by-line method and femtosecond laser pulses. Polymer FBGs are known to have a wider strain range and a stronger temperature sensitivity compared to silica FBGs. Also, they exhibit sensitivity to the relative humidity (RH) that is therefore an additional physical quantity possible to measure. However, in practical applications of RH sensing, temperature cross-sensitivity must be compensated. Irradiating CYTOP FBG samples with various doses (80, 160, 200, 280, and 520 kGy), we found that the gamma radiation treatment changes their climatic properties. Initially positive value of the temperature sensitivity (19.6 pm/℃) decreases with the received dose with subsequent change of the sign from positive to negative. This opens a possibility of making FBGs insensitive to temperature. Among the irradiated samples, the one received the dose of 200 kGy demonstrated the lowest temperature sensitivity (1.77 pm/℃). For higher dose (520-kGy), the sensitivity was found to be -38.9 pm/℃. Along with a decrease of temperature sensitivity, we observed an increase of RH sensitivity with the received dose from 13.3 pm/%RH for pristine FBG up to 56.8 pm/%RH for the case of 520 kGy dose. Thus, by correct selection of the irradiation dose, gamma irradiation of CYTOP FBGs is a promising pre-treatment technique to improve the RH sensitivity of CYTOP FBGs with eliminating the temperature effect.

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/℃) 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 investigate the effect of γ-radiation on temperature (T) and relative humidity (RH) sensitivities of polymer perfluorinated fiber Bragg gratings (FBGs). To this aim, different γ-radiation doses (80, 120, 160, and 520 kGy) were applied to a set of FBGs. We show that irradiated FBGs demonstrate an RH sensitivity rise with the received dose: from 13.3 pm/%RH for a pristine FBG up to 56.8 pm/%RH for a 520-kGy dose at 30℃. In contrast, T sensitivity decreases with radiation dose with a subsequent change of sign from positive to negative. Therefore, by experimental interpolation, T sensitivity can be eliminated at around a 160-kGy dose. This opens the possibility of designing an RH sensor with enhanced sensitivity, which at the same time is insensitive to T.

We investigate online response of a fiber Bragg grating (FBG) inscribed in a polymer optical fiber on gamma radiation. The fiber had a 20-μm graded-index CYTOP core and 250-μm XYLEX overclad. The FBG was inscribed by fs laser using the plane-by-plane technique. We irradiated the grating by 60Co sources at the dose rate of 5.3 kGy/h. The reflection peak (RP) of the grating was monitored before, during and after irradiation (online) using a commercial interrogator placed outside of the irradiation field. The FBG was connected with the interrogator by 10-m single-mode silica patchcord. We firstly applied a 40-kGy dose and then, after ≈90 hours of recovery, we applied the second dose of 80 kGy. The FBG demonstrated a blue shift of the RP during both sessions of irradiation. The speed of the RP change increased during receiving ≈15 kGy of the first irradiation, and then, the RP changed linearly with -3.5 pm/kGy speed. During the second irradiation, the RP changed linearly during the entire irradiation session with -4.13 pm/kGy speed. The linear FBG response to the received dose can be prospective for gamma radiation dosimetry., Fonds De La Recherche Scientifique – FNRS (T.0163.19 "RADPOF"). The research leading to these results has also received funding from the Horizon 2020 programme of the European Union (Marie Skłodowska-Curie Actions - Individual Fellowships) under REA grant agreement No. 844618 (project POSPORI). This work is also funded by the research project INTEGRATED/0918/0031 (LightSense Project) by the Republic of Cyprus through the Research and Innovation Foundation and European Development Fund and the Cyprus University of Technology.