Fiber-Optic Fabry-Pérot Sensor Based on Periodic Focusing Effect of Graded-Index Multimode Fibers

In this letter, the periodic focusing effect of graded-index multimode fibers is used to improve the performance of fiber-optic Fabry–Pérot (FFP) sensors. The maximum fringe contrast of the reflective spectrum of the FFP sensor in air is
$>$
32 dB. By detecting the fringe contrast variation and wavelength shift of the reflective spectrum, such an FFP sensor is used to measure the refractive index and temperature with sensitivities of
$\sim$
45.05 dB/RIU and 11.5
$\hbox{pm}/^{\circ}\hbox{C}$
, respectively. It is also verified by the experiment that the simultaneous measurement of refractive index and temperature can be realized.