Integrated Fiber-Optic Fabry-Pérot Interferometer Sensor for Simultaneous Measurement of Liquid Refractive Index and Temperature

In this paper, an integrated all-fiber Fabry–Pérot interferometer (FPI) sensor is proposed and fabricated for the simultaneous measurement of liquid refractive index (RI) and temperature. The sensor structure consists of an open micro-hole and a tiny section of single-mode fiber. Since, the micro-hole that is machined by 193 nm excimer laser, the micro-fiber, and the integrated micro-hole-fiber, respectively, act as different FPI, consequently, the regular interference spectrum is produced and used in simultaneous measurement based on analyzing wavelength shifts of interference fringes with Fourier band-pass filtering method. Experiments indicate the low-frequency interference fringes resulting from micro-hole cavity is sensitive to liquid RI and temperature changes, and the response sensitivities are 1143.0 nm/RIU and −0.1805 nm/°C, respectively. However, the high-frequency interference fringes of micro-fiber cavity is only sensitive to temperature change, and the obtained temperature sensitivity of 0.0115 nm/°C is obviously superior to that of integrated micro-hole-fiber cavity. Utilizing the different response coefficients, we not only construct a measurement matrix to realize simultaneous liquid RI and temperature measurement successfully, but also achieve resolutions of 0.1 °C and $1.4\boldsymbol {\times } 10^{-5}$ RIU. The excellent merits such as all-fiber, miniature size, simple structure, and wavelength-sensing structure make the FPI sensor promise for high precision bio/chemical sensing applications.