TFBGs in polyimide-coated optical fibers for multi-parameter measurements

Tilted fiber Bragg gratings (TFBGs) contain core and cladding modes and can be used to measure several environmental parameters. However, most of the research to date has focused on stripped single-mode fiber while the preservation of the coating can be highly relevant in practice, especially in harsh environments. In this paper, we use a femtosecond laser direct writing technique to inscribe a 12-degree tilted grating directly into a polyimide (PI)-coated single-mode fiber, where the PI coating keeps the fiber intact and robust. The protective coating layer also acts as a Fabry-Perot cavity and induces a periodic amplitude modulation of the cladding mode resonances. The latter behaves differently compared to mode resonances in response to different measurands. In this work, measurements of temperature, strain and surrounding refractive index are presented.