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Presented at the 7th International Workshop on Speciality Optical Fibres (WSOF), co-locating with the 24th Congress of the Australian Institute of Physics, the 2022 Conference on Optoelectronic and Microelectronic Materials and Devices (COMMAD), and the 2022 Australian and New Zealand Conference on Optics and Photonics (ANZCOP). The combined Congress was held in Adelaide, South Australia, fromSunday 11th - Friday 16th December 2022, with the Pre-Conference Workshop and Congress Reception taking place on Sunday, December 11th.

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.

CYTOP-XYLEX fibre Bragg grating (FBG) sensors’ response to relative humidity (RH) changes is investigated in this work. The intensive research on CYTOP polymer optical fibre sensors relies on their lower optical attenuation compared with the standard poly(methyl) methacrylate (PMMA) based polymer optical fibre sensors. However, CYTOP is a hydrophobic polymer and cannot be used as a standalone material in humidity-sensing applications. A hydrophilic material is required to be used as fibre over-cladding for RH detection; the water swelling occurring in the over-cladding material can strain the FBG structure located in the CYTOP core. In this work, a novel CYTOP fibre with a XYLEX over-cladding material is used to investigate the humidity responsivity of FBG sensors. A femtosecond laser system and the plane-by-plane inscription method were utilised to fabricate the FBG sensors through the fibre over-cladding. The FBG sensors were placed in a climatic chamber with controlled temperature and RH, and their response to RH changes was monitored. The RH sensitivity was evaluated for different temperatures. The results show that the RH sensitivity increases with temperature, whereas the reported RH sensitivity of PMMA-based sensors decreases with temperature. We also report that by applying a pre-strain on the fibre, the RH sensitivity can be eliminated for cross-sensitivity compensation purposes.

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.