Fiber-optic sensors provide remote access, are readily embedded within structures, and can operate in harsh environments. Nevertheless, fiber-optic sensing of liquids has been largely restricted to measurements of refractive index and absorption spectroscopy. The temporal dynamics of fluid evaporation have potential applications in monitoring the quality of water, identification of fuel dilutions, mobile point-of-care diagnostics, climatography and more. In this work, the fiber-optic monitoring of fluids evaporation is proposed and demonstrated. Sub-nano-liter volumes of a liquid are applied to inline fiber-optic micro-cavities. As the liquid evaporates, light is refracted out of the cavity at the receding index boundary between the fluid and the ambient surroundings. A sharp transient attenuation in the transmission of light through the cavity, by as much as 50 dB and on a sub-second time scale, is observed. Numerical models for the transmission dynamics in terms of ray-tracing and wavefront propagation are provided. Experiments show that the temporal transmission profile can distinguish between different liquids.

This paper proposes an all-optical-fiber sensor for continuous measurements of liquid levels. The proposed sensor utilizes an optically absorbing vanadium doped optical fiber, which is configured as a long-gauge, optically-heated, fiber-optic, Fabry-Perot interferometer that is immersed into the measured liquid. The sensor is excited cyclically by a medium-power 980 nm optical source, which induces periodic temperature variation and, consequently, optical path length modulation within the vanadium doped fiber. The amplitude of this path length variation depends on the liquid level and is measured by an interferometric approach. The relation between the liquid level and the amplitude of optical path length modulation caused by the fiber's temperature variation were investigated analytically, and the theoretical model proved to be in good agreement with the experimental results. Two versions of level sensors are demonstrated experimentally, the first with single-side optical heating power delivery and 0.45 m measurement range, and the second with dual-side power delivery and 1 m of operational measurement span. Experimental measurement level resolutions achieved for 0.45 m and 1m operational measurement span were approximately 2 and 3 mm, respectively. The simple and efficient design of sensor and signal interrogation system, the latter is based solely on a few widely available telecom components, provides straightforward opportunities for use of the proposed system in a variety of industrial applications.

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This paper presents a multiplexed fiber-optic sensor array suitable for detection of liquid intrusions, spills and leaks. The proposed detection systems is based on active heating and observation of temperature changes in short sections of optically absorbing (vanadium-doped) optical fibers that were interconnected into a sensing array by a standard single-mode optical fiber. The heating of the vanadium doped fiber sections was achieved by application of a common 980 nm pump laser source. The associated signal processing and integration utilizes a simple, custom-designed OTDR that relies on only a few low-cost standard telecom components.Index Terms-optical fibers, optical fiber sensors, Fabry-Perot sensors, multiplexed sensor array, vanadium doped fiber, water detection, liquid detection, optical signal interrogation

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