We describe a fiber-optic relative-humidity (RH) sensor comprising a moisture-sensitive overlay on a single-mode side-polished fiber. The hygroscopic polymeric material deposited was polyvinyl alcohol (PVA), which proved to have good adherence and stability. The film reached a fast equilibrium with atmospheric moisture (in less than 1 min), inducing changes in the output optical power of approximately 10 dB for the 70%-90% RH range. To yield a low-cost device, single-mode standard communication fibers were used; therefore all the components of the sensor can be commercial, mass-produced telecommunication devices. The experimental results obtained are consistent with the expected behavior of the system; the output power decreases because of losses in the polished region of the fiber as the refractive index of its external medium approaches the fiber core value. Because the external medium is PVA film, its refractive index changes in response to its water content.

Sensitive and versatile evanescent wave-sensing systems featuring polished optical fiber-based sensor designs with low-cost light sources have been developed for temperature, relative humidity, and pH measurements. The work herein contained describes the fabrication of three types of sensors based on standard silica, single-mode fibers previously subjected to a lateral polishing of the cladding. Temperature sensing through oils whose refractive index varied linearly with temperature showed applicability with up to 5 dB/ C for a 5 range. Polyvinyl alcohol films on the fibers showed almost 10-dB linear variation from 70% to 90% relative humidity. Sol-gel trapped dyes as thin films on the polished surface were capable of performing 15-dB output variation (although not linearly) for pH ranging from 2 to 11.

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The availability of sensors able to determine on-line the ageing (the evolution in the condition) of oils would be of grate interest in many industries. Oil finds applications mainly in three areas: (i) food processing (mainly frying) both in industrial and home environments, (ii) engines lubrication (cars, planes, boats) and (iii) insulation in transformers (and other power devices). In all cases the oils, complex mixtures of many chemical compounds, evolve with time and use leading, eventually, to a significant loss of its properties becoming necessary its replacement. In this paper we present a novel procedure to obtain physical information on the condition of oils in situ and in a relatively simple manner. Sensing procedure clearly distinguishes between different samples of fresh oils (olive, sunflower, etc.) and their mixtures and assesses the evolution with use of a particular oil. The proposed procedure is based on the combined measurement of the transmitted optical power through a small polished zone of a single mode optical fiber immersed in the oil and its temperature.