Improving the sensitivity of WGM pressure sensors with Oxyfluoride glass microspheres
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Oxyfluoride glass microspheres doped Nd3+ were synthesized and calibrated to function as optical pressure sensors. When the microspheres were placed in a vacuum chamber and excited at 532 nm, Nd3+ typical emission spectrum was recorded. Due to the specific geometry of the microspheres, that allow them to act as an optical resonator, it was also possible to record Whispering Gallery Modes (WGM) superimposed to the Nd3+ typical emission spectrum. Furthermore, when decreasing pressure, changes in the Fluorescence Intensity Ratio (FIR) of the 2H9/2, 4F5/2→4 I9/2 (820 nm) and 4F3/2→4 I9/2 (890 nm) Nd3+ transitions as well as a red-shift of the WGM was observed. Both changes were calibrated as a function of pressure. Relative sensitivity and limit of detection were estimated for both techniques as a way of comparing their performance, also taking into account the previous results found in the literature. Due to the high thermal expansion and thermo-optical coefficient of oxyfluoride glasses and the high energy gap of Nd3+ thermally coupled levels, the sensor presented groundbreaking levels of relative sensitivity and limit of detection.