Evaluation of Thermo-Optical Properties of Selected Liquids by Photonic Crystal Fiber-Based Sensor ^†

Abstract

Thermo-optical properties of liquid materials are very important in many practical applications. The temperature dependence of the refractive index (RI) is usually sufficient to fully characterize a given material, however, for some applications an extinction coefficient (EC) temperature dependence has a great influence on the optical properties of the sensor’s transducer. Our previous studies have shown that light wave propagation properties in a large mode area (LMA) photonic crystal fiber (PCF) partially filled with liquid strongly depend not only on its RI, but also on the state of matter—either liquid or solid—which also determines its EC. Therefore, high absorption of the filling material significantly reduces the intensity of light passing through the PCF. In the presented work we studied commercial PCF (LMA-10) partially filled with liquid paraffin. Recorded light transmission within the temperature range of 20–120 °C allowed us to observe changes in the propagation conditions of such a hybrid PCF transducer, when the light transmission was supported either by index-guiding and bandgap-guiding mechanisms, depending on the paraffin’s RI. Furthermore, as the RI of the paraffin approached the RI of the PCF, we observed a lack of the transmission and then the propagation changed between the index-guiding and the bandgap-guiding conditions. The transmission in the bandgap-guiding region was reduced in comparison with the transmission in the index-guiding region. This can be caused by the difference of EC of paraffin in the two guiding regions. In our numerical simulations we analyzed the influence of this parameter on the transmission losses by changing the values of the EC to match them with the experimental results. Hence, we were able to obtain its value, which was not previously reported. The presented studies provide not only an insight into the material properties of paraffin, but also pave the way for PCF-based RI sensors.