This paper reports on a multi-point optical fiber pressure sensor using fiber Bragg gratings (FBGs) written on an exposed core optical fiber (ECF) by femtosecond laser. The pressure sensing elements were constructed as Fabry-Perot (FP) interferometers of different cavity lengths using pairs of FBGs with identical resonant wavelength. In this fashion an interference pattern was formed within the FBG bandwidth with much narrower fringes, leading to better detection limit. Fast Fourier transform (FFT) was used to calculate the phase change of the FP interference pattern with respect to applied pressure. The pressure sensitivity was proportional to the cavity FP cavity length, and reached -0.672 rad/MPa for the case of FP with 9 mm cavity length. The proposed sensor has potential to measure pressure at very high temperature thanks to its single material configuration.
The ability to perform spatially resolved measurement of extreme temperatures, the order of 1000°C and above, would yield enormous benefit to many heavy industrial processes. While optical fibers can provide spatial information along their length through distributed and multi-point sensing techniques, operation at such temperatures is an area of ongoing research and development. A challenge is that conventional optical fibers, fabricated with a chemically doped core, suffer dopant diffusion at these high temperatures, ultimately limiting their operating lifespan. We can overcome this limitation by using specialty pure silica glass fibers, such as microstructured optical fibers. In this work we demonstrate the ability to use such fibers in a significantly multiplexed configuration with twenty fiber Bragg grating sensing elements written via femtosecond laser ablation.
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