Ultra-fast fiber Bragg grating inscription in CYTOP polymer optical fibers using phase mask and 400 nm femtosecond laser

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 mu W. 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. (C) 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Automated summary

A new technique for producing high quality FBGs in CYTOP fiber using a femtosecond laser system and a phase mask is reported in this study, achieving rapid and efficient gratings with high reflectivity. The resonance wavelengths of FBGs in CYTOP fiber were confirmed by numerical computation, and the mode selection characteristic of FBGs was investigated. Temperature sensitivity of CYTOP FBG was measured, showing values independent of the mode group measured but with a small hysteresis.