Fiber Bragg gratings (FBGs) in cyclic transparent fluoropolymer (CYTOP) optical fiber are the subject of a lot of research as they can be of interest for many applications, such as temperature, humidity, strain, and radiation sensing. We report here a new technique to produce high quality FBGs in CYTOP fiber. It uses a femtosecond laser system operating at 400 nm and a phase mask. In contrast to previously reported results, the gratings are obtained in a few seconds with a writing power as low as 80 μW. With this setup, 2 mm-long gratings with reflectivity up to 92 % and full width at half maximum bandwidth around 0.5 nm were obtained in less than 10 s. The resonance wavelengths of the FBGs are confirmed by numerical computation in the graded-index multimode CYTOP fiber, and the mode selection characteristic of FBGs in CYTOP is investigated. Finally, the temperature sensitivity of CYTOP FBG is measured in different mode groups for heating up and cooling down, showing values independent of the mode group measured, but with a small hysteresis.
We report the inscription of highly reflective fiber Bragg gratings in perfluorinated polymer (CYTOP) optical fibers using 800 nm femtosecond pulses and a line-by-line inscription method. We demonstrate that the energy for grating inscription without damage is below 27 nJ. After the writing process, we show that the grating reflectivity continues to rise for several hours. The obtained gratings show reflectivity degradation with temperature increase, while the Bragg wavelength undergoes a linear temperature dependence of 37.7 pm/°C when the humidity is controlled and the heating/cooling rate is relatively low (
≤
0.14
∘
C
/
m
i
n
). The humidity sensitivity at a fixed temperature (25°C) is 22.3 pm/%RH. Finally, the axial strain sensitivity is determined to be 1.35 pm/µɛ.
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.
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