Fiber-optic sensor measuring spatial distributions of refractive index and temperature

In this work, we present a fiber sensor designed to measure simultaneously spatial inhomogeneities of the refractive index and temperature in liquid media, for example, induced by biochemical reactions. The sensor's constituent elements are Fabry-Perot microresonators fabricated in standard single-mode optical fibers by diamond blade dicing. To allow simultaneous measurements of different refractive indices, the sensor comprises two open cavities approximately 2 mm apart. With a small Si inlay inserted into one of the resonators used for temperature measurements, the sensor allows for immediate compensation of crosstalk between temperature- and composition-induced fluids' refractive index changes. The measurements were evaluated by phase tracking of the characteristic Fourier transform components of the sensor's backreflected spectra. The temperature sensitivity of the Si inlay is 0.063 rad/°C (79 pm/°C), and an accuracy of 0.01°C is obtained. Meanwhile, the two refractive index sensing (open) cavities show a sensitivity of 1168 and 1153 nm/RIU for temperature-compensated measurements. Finally, the sensor performance to measure spatial distributions is demonstrated by measuring the diffusion behavior of sucrose in water, which allows precise monitoring of hydration effects and breaking of bonds at elevated temperatures.