Microsphere-based Fiber-Optic Sensors with ALD ZnO Coatings for Refractive Index and Temperature Measurements

Fiber-optic sensors have been developed and improved upon for over a decade. Due to their versatility, they are used in numerous fields, such as industry, science or medicine. Introduction of an optical microsphere with zinc oxide (ZnO) coating applied by Atomic Layer Deposition (ALD) on its surface, allows for constant monitoring of the integrity of the structure. This feature is especially important during long-term measurements and sensing in the remote and hard-to-reach places. This study presents preparation of the microsphere-based fiber-optic sensor, characterization of the ALD ZnO coatings and an experimental setup operating as an interferometer in reflective mode, which was used to obtain the results of refractive index and temperature measurements. The signal generated by a low-coherent light source passes through an optical coupler to a microsphere-based fiber-optic sensor head, placed in a measured medium, where it reflects and travels to a detector. Interferometric signal is used to control whether the microstructure is whole. The intensity of reflected signal is used to conclude changes in measured parameter. By changing the value of a measured parameter, either refractive index or temperature, the intensity of a reflected signal also changes. The R² coefficient of each of presented sensors indicates a good linear fit of over 0.99 to the obtained data. Sensitivity of the sensors, investigated in this study, equals 103.5 nW/°C and 384uW/RIU for temperature and refractive index measurements, respectively.