Dual parameter fiber-integrated sensor for refractive index and temperature measurement based on Fabry–Perot micro-resonators

We report on a miniature all-fiber dual parameter sensor capable of simultaneous measurement of the refractive index (RI) and temperature of fluids and gases. The high-sensitivity sensing element is comprised of two Fabry–Perot (FP) micro-resonators fabricated in a single-mode fiber and has a total length of <100  μm. The RI sensing cavity is formed by diamond blade dicing, whereas a thinner silicon inlay glued into it serves as a temperature sensor. The sensor’s performance was tested on sucrose solutions over a range of temperatures. For the evaluation of the backreflected FP spectra, phase tracking of the characteristic Fourier transform components was used. Good accuracy (0.01°C) and linearity of temperature measurement with Si inlay with sensitivity 0.097 rad/°C (85.2 pm/°C) were found, whereas the open cavity allowed for reliable temperature-compensated measurements of 10−3 RI steps with 290 rad/RIU (1130 nm/RIU) sensitivity.