Monitoring of Water Freeze–Thaw Cycle by Means of an Etched Single- Mode–Multimode–Single-Mode Fiber-Optic Refractometer

As an alternative to the different technologies that permit the detection of in situ ice formation on different surfaces, this contribution proposes the design of an etched single-mode–multimode–single-mode (E-SMS) fiber-optic-based structure as a multimode interference refractometer. This sensor provides enhanced properties with respect to a basic SMS structure, including a higher sensitivity and periodical interferometry bands that can measure surrounding refractive indices with repeatability and robustness. Since ice and water refractive indices are sufficiently different, this structure has been used to detect the freezing-thawing process of water taking place inside a freezer between −20 °C and +20 °C. Also, this work intends to show a proof of concept of a simple technology that can be applied in different situations, such as in smart cities, avionics, structural health monitoring (SHM) or even to avoid a cold chain breakage. Inside, novel developments to better understand the working operation of the E-SMS structure are shown, together with a study on how to correlate optical and thermal measurements from a refractive index point of view.