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.

In this work, we report the inscription of fiber Bragg grating through the over-clad in cyclic transparent fluoropolymer (CYTOP) optical fiber using a phase mask technique and a 400 nm femtosecond pulsed laser. 8 mm-long grating were obtained in less than 20 s with 500 $\mu$ W average beam power. To investigate the FBG properties and discriminate between the protective over-clad influence and the core-cladding effect, we inscribe FBGs in the fiber with and without the over-clad. We also show that the temperature sensitivity of CYTOP-FBGs with the over-clad depends on the relative humidity, whereas CYTOP-FBGs without the over-clad achieve almost zero sensitivity to humidity and sensitivity to temperature of 26.7 pm/ $^{\circ }$ C.

We propose and experimentally demonstrate a compact fiber-optic vibration sensor in which a tilted Bragg grating is inscribed in the core of a depressed cladding fiber. The sensor provides resolution up to 2×10^-2 m/s² and a linear acceleration response between 0.2 and 6.0 m/s², together with a flat frequency response between 1 and 120 Hz. The sensor takes advantage of a cost-effective interrogation scheme by monitoring the total reflection power of the fiber sensor. We use the calibrated output to eliminate the potential light source and fiber loss-induced power fluctuations. The proposed sensing system has been used to measure the vibration of a mechanical damper in electrical transmission lines and the results agree well with those from a commercial sensor.

We investigated the gamma radiation response of fiber Bragg gratings (FBGs) inscribed in a few-mode polymer optical fiber. The fiber had a graded-index CYTOP core of 20 µm and XYLEX overclad of 250 µm in diameter. Four FBGs were exposed to gamma radiation during four irradiation sessions at a 5.3 kGy/h dose rate. The FBGs showed a linear Bragg wavelength shift with the received dose with a mean sensitivity of −3.95 pm/kGy at 43 °C. The increased temperature provides a rise in the sensitivity: it reached −10.6 pm/kGy at 58 °C. After irradiation, the FBGs showed partial recovery, which increased with the received dose. Furthermore, the FBG’s reflection power decreased with the dose. This attenuation is mainly due to insertion losses caused by the radiation induced attenuation in the CYTOP fiber. Linear response to the received dose makes CYTOP FBGs attractive for gamma radiation dosimetry. However, temperature dependence of the sensitivity should be compensated in practical applications.